Stable Ni catalyst encapsulated in N-doped carbon nanotubes for one-pot reductive amination of nitroarenes with aldehydes

Article abstract of DOI:10.1016/j.apcata.2021.118230

A novel strategy involving a popping process and carbothermal reduction was developed to create a kind of stable nickel catalyst (Ni-NC). The popping process of the mixture being composed of carbon nitride (C₃N₄) and nickel nitrate decomposed the nickel nitrate into nickel (oxide) nanoparticles that afterwards functioned as catalyst to grow N-containing carbon nanotubes with carbon nitride as N-containing carbon source. Finally, the nickel catalyst possessed a special structure of nanoparticles encapsulated in N-doped carbon nanotubes. This special structure is helpful to prevent nickel nanoparticles from being oxidized in air for months so that the catalyst exhibits high stability in air atmosphere. As a practical application, this encapsulated nickel catalyst exhibited excellent catalytic activity and stability in one-pot cascade reaction involving nitro-reduction and reductive amination of nitroarenes.

Full text of DOI:10.1016/j.apcata.2021.118230

Applied Catalysis A, General 622 (2021) 118230
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Applied Catalysis A, General
journal homepage: www.elsevier.com/locate/apcata
Stable Ni catalyst encapsulated in N-doped carbon nanotubes for one-pot
reductive amination of nitroarenes with aldehydes
,
,
Yuzhu Xua, Yaru Liua, Penglei Cuib, Ningzhao Shangb, Chun Wangb *, Yongjun Gaoa *
a Key Laboratory of Analytical Science and Technology of Hebei Province, College of Chemistry and Environmental Science, Hebei University, Baoding 071002, China
b College of Science, Hebei Agricultural University, Baoding 071001, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
A novel strategy involving a popping process and carbothermal reduction was developed to create a kind of
stable nickel catalyst (Ni-NC). The popping process of the mixture being composed of carbon nitride (C3N4) and
nickel nitrate decomposed the nickel nitrate into nickel (oxide) nanoparticles that afterwards functioned as
catalyst to grow N-containing carbon nanotubes with carbon nitride as N-containing carbon source. Finally, the
nickel catalyst possessed a special structure of nanoparticles encapsulated in N-doped carbon nanotubes. This
special structure is helpful to prevent nickel nanoparticles from being oxidized in air for months so that the
catalyst exhibits high stability in air atmosphere. As a practical application, this encapsulated nickel catalyst
exhibited excellent catalytic activity and stability in one-pot cascade reaction involving nitro-reduction and
reductive amination of nitroarenes.
Nickel
Carbon nanotubes
Reductive amination
One-pot reaction
1. Introduction
reported to catalyze this one-pot reductive amination of aldehydes with
nitroarenes [13]. To avoid the hydrogenolysis of product, second metal
Green and sustainable chemistry points out that synthetic strategy or
methodology for target product should be simple and concise in syn-
thetic chemistry [1,2]. Therefore, chemists usually expect to achieve
some multistep reactions for synthesizing fine chemicals to be
substituted by a simplified one-pot cascade strategy, which avoids
tedious and time-consuming separation/purification processes so that it
is an efficient method to reduce the cost of final product [3]. Synthesis of
secondary amines represents one of the most important and active fields
in modern synthetic chemistry because these compounds have wide
application in the field of pharmaceuticals, pesticides, dyes and fine
chemicals [4,5]. Several synthetic methodologies have been developed
to synthesize secondary amines, such as N-alkylation of primary amine
with alcohol or alkyl halides [6,7], Buchwald-Hartwig reaction [8],
was introduced to adjust the catalytic ability of palladium so that Pd-Ag
alloy [14,15], AuPd@Fe3O4 nanoparticle catalysts [16] were developed
for this one-pot cascade reaction. RuCl3/PPh3 with glycerol as hydrogen
source can also achieve the one-pot reductive amination of nitro com-
pounds to amines [17]. On the other hand, a series of non-precious such
as cobalt-based catalytic systems with carbon monoxide, HCOOH or H2
as reductive agent, were also researched [18]. Molybdenum sulfide with
lots of S vacancies [19] or molybdenum sulfide clusters [20] can also
promote the one-pot reductive amination smoothly under high-pressure
hydrogen atmosphere. However, there is no report corresponding to
heterogeneous and stable nickel-based catalytic system to promote this
one-pot cascade reaction.
In recent years, carbon nitride (C3N4) has been a glaring star in the
field of photocatalysis [21]. In the territory of heterogeneous catalysis,
carbon nitride also exhibited diverse applications as catalyst or support
for active components [22,23]. In our laboratory, we found that the
mixture of carbon nitride with cupric nitrate has the popping property
during the heating process, which could not only promote the decom-
position of cupric nitrate but also contribute to the well-distribution of
copper-based species [24]. Besides, it was reported that carbon nitride
would be decomposed when the calcinating temperature is higher than
–
Ullman-type C N coupling reaction [9] and reductive amination [10,
11]. Among these methods, the key substrates are amines, which are
usually achieved from the reduction of nitro compounds [12]. So, it is
possible to transform nitro compounds to secondary amines with alde-
hydes directly in one-pot strategy, which is in accordance with the cri-
terions of green and sustainable chemistry. To realize this one-pot
cascade reaction, several multifunctional catalysts have been developed.
For instance, gum-acacia-stabilized palladium nanoparticles was
* Corresponding authors.
E-mail addresses: wangchun@hebau.edu.cn (C. Wang), yjgao@hbu.edu.cn (Y. Gao).
https://doi.org/10.1016/j.apcata.2021.118230
Received 11 December 2019; Received in revised form 22 April 2021; Accepted 24 May 2021
Available online 26 May 2021
0926-860X/© 2021 Elsevier B.V. All rights reserved.

Y. Xu et al.
Applied Catalysis A, General 622 (2021) 118230
Fig. 1. a) XRD patterns of C3N4 and Ni(NO3)2@C3N4; b) XRD patterns of Ni-NC-300, Ni-NC-700 and Ni-NC-G-700; c) FTIR spectra of C3N4, Ni(NO3)2@C3N4, Ni-NC-
300, Ni-NC-700 and Ni-NC-G-700; d) magnetization curve of Ni-NC-G-700 at room temperature.
600 ◦C [22]. Under the illuminations of these two properties, we expect
to fabricate a nickel catalyst with special structure so that the catalyst is
stable under air atmosphere and has excellent catalytic performance in
the one-pot reductive amination of aldehydes with nitroarenes.
Herein, we report our achievement that a catalyst of nickel nano-
particles encapsulated in N-doped carbon nanotubes has high stability
and catalytic activity for the one-pot reductive amination of aldehydes
with nitroarenes. It was found that the popping process promoted the
decomposition of nickel nitrate into nickel (oxide) nanoparticles which
afterwards functioned as a catalyst to generate N-containing carbon
nanotubes with carbon nitride as N-containing carbon source. The metal
nickel catalyst was air-tolerant due to the encapsulation of carbon
nanotubes and could be reused at least five times without obvious
decrease of catalytic activity.
Nimol = 100:1) was evenly added on the powder drop-by-drop. After
dried at 60 ◦C in oven, the powder containing Ni-NC-300 and glucose
was filled into a quartz tube and heated to 700 ◦C with a ramp of 10 ◦C
under nitrogen atmosphere (40 mL/min) and held for one hour. Then
product was achieved and labelled as Ni-NC-G-700. According to ICP-
OES measurement, the content of nickel in Ni-NC-G-700 is 43.5 wt %.
As control experiments, if Ni-NC-300 was directly heated to 700 ◦C with
a ramp of 10 ◦C under nitrogen atmosphere (40 mL/min) and held for
one hour, the product is named Ni-NC-700 and nickel content is 44.4 wt
% according to ICP-OES analysis. In addition, if 5 mol % and 10 mol % of
glucose were introduced into Ni-NC-300 before annealed at 700 ◦C, the
final products were labelled as Ni-NC-5G-700 and Ni-NC-10G-700.
2.3. Catalytic reaction
The one-pot cascade reductive amination of aldehydes with nitro-
arenes was carried out in a stainless autoclave (GaoYao instruments,
Baoding, China). Typically, nitroarene (0.5 mmol), aldehyde
(0.7 mmol), catalyst(4.2 mg), ethanol (3 mL) were added into the auto-
clave in turn and then the autoclave was sealed. The autoclave was
purged with hydrogen twice to remove the air in it and charged to
2 MPa. Then the autoclave was placed in an oil bath maintained at
120℃. After reaction, n-dodecane (50 u L) was introduced into the
mixture as internal standard. The mixture was diluted with ethyl acetate
(10 mL) and stirred fully. The catalyst was filtered out and the filtrate
was analyzed by gas chromatograph (GC, Agilent 7820A) with a HP-5
capillary column so that the conversion of nitroarenes and the yield of
target product could be achieved. The structure of the product was
further determined by Gas chromatography–mass spectrometry (Agilent
7890B-5977A GC/MSD). To achieve the isolated yields of secondary
amines, the reaction product was further separated from the reaction
system by column chromatograph on silica gel (petroleum ether/ethyl
acetate). The structures of secondary amines were further confirmed by
H NMR.
2. Experimental section
2.1. Preparation of C3N4
Urea (10.0 g) was placed in a covered crucible and then heated to
500 ◦C in muffle furnace with a ramp of 10 ◦C/min. After the furnace
was held at 500 ◦C for one hour, the furnace was powered off and cooled
down. The light-yellow C3N4 was achieved.
2.2. Preparation of Ni-NC catalyst
The mixture of Ni(NO3)2∙6H2O (0.5047 g) and C3N4 (0.5 g) was fully
ground in an agate mortar and lyophilized. The content of nickel in the
sample 20 wt % and the sample is termed as Ni(NO3)2@ C3N4. Then the
mixture powder was loaded into a quartz tube and heated in a tube
furnace under nitrogen atmosphere (40 mL/min). The powder was
rapidly popped into the light-yellow powder at about 300 ◦C due to the
rapid pyrolysis. The product was named Ni-NC-300 and ICP-OES de-
termines that the content of nickel is 18.5 wt %. The sample Ni-NC-300
was then collected and a small amount of glucose solution (glucosemol
/
2

Y. Xu et al.
Applied Catalysis A, General 622 (2021) 118230
Fig. 2. a) TEM image of Ni-NC-300; b) HRTEM image of Ni-NC-300; c) Elemental mapping analysis of Ni-NC-300; d) TEM image of Ni-NC-700; e) TEM image of Ni-
NC-G-700; f) Elemental mapping analysis of Ni-NC-G-700.
3. Results and discussion
some diffraction peaks of nickel nitrate hexahydrate (PDF# 25-0577),
indicating there is no obvious phase change during the grinding pro-
cess. After the popping process, XRD pattern of Ni-NC-300 points out
that the characteristic peaks for nickel nitrate disappear and a group of
new peaks being ascribed to metal nickel with hexagonal structure
appear, suggesting that nickel nitrate has been decomposed into metal
nickel during this instantaneous high-temperature process. In addition,
weak peak locating at 27.3◦ still remains, suggesting that the structure of
C3N4 wa only partly destroyed. When Ni-NC-300 was directly heated to
700 ◦C and held at this temperature for one hour, the diffraction peaks at
44.5◦, 51.8◦ 76.4◦ corresponding to (111), (200) and (220) lattice planes
3.1. Structure of catalyst
We used XRD to follow the changes of phases during the fabrication
firstly. As shown in Fig. 1a, C3N4 derived from urea possesses the
characteristic diffraction peaks at 27.3◦ corresponding to the diffraction
of (002) facet [25]. The characteristic diffraction of (100) and (111)
facets locating at 13◦ and 44◦ are too weak to be identified. The mixture
of carbon nitride with nickel nitrate Ni(NO3)2@C3N4 presents the
characteristic diffraction of carbon nitride and nickel nitrate except
3

Y. Xu et al.
Applied Catalysis A, General 622 (2021) 118230
Fig. 3. a) Nitrogen adsorption-desorption isotherms of Ni-NC-300, Ni-NC-700 and Ni-NC-G-700; b) pore-size distribution of Ni-NC-300, Ni-NC-700 and Ni-NC-G-700
based on adsorption isotherms.
of cubic nickel arise and the characteristic diffraction of C3N4 disappears
(Fig. 1b), indicating the structure of nickel components transformed and
carbon nitride decomposed during the annealing process. Besides, a
small and broad peak corresponding to nickel oxide can also be distin-
guished, illustrating that the nickel was partly oxidized in air atmo-
sphere or nickel was not totally reduced. To eliminate the nickel oxide
species which maybe impact the catalytic activity, extra glucose (1 mol
% of Ni) was introduced into Ni-NC-300 before annealing at 700 ◦C.
Satisfactorily, there is no diffraction peaks of nickel oxide in the pattern
of Ni-NC-G-700, which implies that the extra carbon derived from
glucose was conducive to fully reduce nickel oxide and protected the
nickel from being oxidized in air. In the FTIR spectra (Fig. 1c) of C3N4, Ni
(NO3)2@C3N4 and Ni-NC-300, the peaks at 808 cmꢀ 1 and
1200~1600 cmꢀ 1 are ascribed to the characteristic vibrations of
nanoparticles can be observed on the support of C3N4 (Fig. 2a, b), further
illustrating nickel nitrate decomposed during popping procedure.
Elemental mapping analyses (Fig. 2c) pointed out that nickel element
was well-distributed on carbon nitride. To our surprise, nickel nano-
particles encapsulated in carbon nanotubes can be observed in the im-
ages of Ni-NC-700 and Ni-NC-G-700 (Fig. 2d, e). Why does the simple
annealing process promote carbon nanotubes to grow up? As described
in previous reports, C3N4 would be decomposed when annealing tem-
perature was higher than 600 ◦C. In addition, supported nickel nano-
particles can be used as catalyst to prepare carbon nanotubes due to
their excellent carburizing capacitor [26]. Therefore, we deduced that
the nickel oxide nanoparticles could be reduced firstly by carbon species
decomposed from C3N4 and then functioned as a seed to grow carbon
nanotubes. Because the decomposed products from C3N4 contain ni-
trogen element, the carbon nanotubes covered on nickel nanoparticles is
a kind of N-doped carbon nanotubes. This point can be demonstrated by
elemental mapping analysis. As shown in Fig. 2f, carbon and nitrogen
elements are well distributed in the carbon nanotubes and the nano-
particles in the nanotube are composed of nickel component. However,
there is no obvious difference between the TEM images of Ni-NC-700
and Ni-NC-G-700 (Fig. 2d, e), further indicating that carbon for
growing carbon nanotubes comes from carbon nitride but NOT glucose.
The function of glucose is just to help carbon nanotubes to improve the
stability of nickel nanoparticles in air.
–
s-triazine ring and C N heterocycles in carbon nitride, respectively.
–
This result further demonstrates that the main structure of C3N4 in the
sample Ni-NC-300 remains. In the spectra of Ni-NC-700 and
Ni-NC-G-700, the characteristic peak locating at 808 cmꢀ 1 almost dis-
–
appears and the vibrations C N heterocycles situating at 1200~1600
–
cmꢀ 1 remains, suggesting that structure of C3N4 was destroyed and the
structure of N-doped carbon generated at high calcination temperature.
With careful observation, the FTIR spectrum of Ni-NC-G-700 is simpler
than Ni-NC-700, indicating that Ni-NC-G-700 possesses high degree of
graphitization with the help of extra glucose at high annealing tem-
perature. Interestingly, the sample Ni-NC-G-700 possesses excellent
magnetic property because the metal nickel exists, which was investi-
gated by using a magnetic property measurement system at room tem-
perature (Fig. 1d). The saturation magnetization value of Ni-NC-G-700
reached to 30.5 emu/g. Besides, the hysteresis loop and remanence
cannot be identified, pointing out that Ni-NC-G-700 was super-
paramagnetic at room temperature so that it was easily recycled when
Ni-NC-G-700 was used as catalyst.
BET surface area of Ni-NC-300 based on nitrogen adsorption-
desorption experiment is about 196 m2/g and only possesses meso-
porous structure with 0.65 cm3/g total pore volume (Fig. 3). Ni-NC-700
and Ni-NC-G-700 have similar shape of nitrogen adsorption-desorption
isotherms and almost same BET surface area of about 260 m2/g. The
two samples both have micro (~0.65 nm) and mesoporous (~2 nm)
structures with about 0.9 cm3/g total pore volume. This result illustrates
that the introduction of small amount of glucose do not influence the
structure of final product.
The morphology of Ni-NC-300, Ni-NC-700 and Ni-NC-G-700 was
studied by TEM (Fig. 2). For the sample Ni-NC-300, small nickel oxide
The difference of surface composition of C3N4, Ni-NC-300, Ni-NC-G-
4

Y. Xu et al.
Applied Catalysis A, General 622 (2021) 118230
Fig. 4. a) XPS full spectra of C3N4, Ni(NO3)2@C3N4, Ni-NC-300, Ni-NC-700 and Ni-NC-G-700; b) XPS Ni2p spectra of Ni(NO3)2@C3N4, Ni-NC-300, Ni-NC-700 and Ni-
NC-G-700; c) XPS C1s spectra of C3N4, Ni(NO3)2@C3N4, Ni-NC-300, Ni-NC-700 and Ni-NC-G-700; d) XPS N1s spectra of C3N4, Ni(NO3)2@C3N4, Ni-NC-300, Ni-NC-
700 and Ni-NC-G-700.
Table 1
Catalytic performance of different nickel catalyst in the one-pot reductive amination of nitrobenzene with benzaldehyde.
sel. / %
entry
catalyst
temperature/ ◦
C
solvent
con./ %
a
b
c
1
–
120
120
120
120
120
120
80
ethanol
ethanol
ethanol
ethanol
ethanol
ethanol
ethanol
ethanol
water
0
0
0
0
2
Ni-NC-300
Ni-NC-G-700
Ni-NC-700
Ni-NC-5G-700
Ni-NC-10G-700
Ni-NC-G-700
Ni-NC-G-700
Ni-NC-G-700
Ni-NC-G-700
Raney Ni
18.5
100
100
100
78
2.3
99.7
22.4
32.5
19
90.8
0.3
50.7
56.3
67
6.9
0
3
4
26.9
11.2
14
5
6
7
31.1
100
100
50.2
100
21.9
23.4
89
75.1
6
1.5
5
8
100
120
120
120
120
9
82.7
1.7
60.5
8
9.9
64
7.4
34.3
38.2
5.9
10
11
12
acetonitrile
ethanol
ethanol
1.3
86.1
Raney Co
Reaction conditions: nitrobenzene (0.5 mmol), benzaldehyde (0.7 mmol), catalyst (4.2 mg, mole ratio of Ni/nitrobenzene is 1/15), H2 (2 MPa), time (8.5 h).
700 and Ni-NC-700 was further determined by high-resolution XPS
analysis, as shown in Fig. 4. Except that there is no signal of nickel in the
spectrum of C3N4, all the three samples display four characteristic peaks
of carbon, nitrogen, oxygen and nickel at binding energies of 285 eV,
400 eV, 532 eV and 855 eV (Fig. 4a). The contents of nickel on the sur-
face of Ni-NC-300, Ni-NC-G-700 and Ni-NC-700 were 2.45 atom%, 0.83
atom% and 0.91 atom% respectively according to XPS analysis. How-
ever, ICP-OES demonstrated that the contents of nickel in Ni-NC-300,
Ni-NC-G-700 and Ni-NC-700 were 18.5 wt %, 43.5 wt % and 44.4 wt
%, which were extremely higher than the results presented by XPS
analysis. This difference attests that most of nickel in Ni-NC-G-700 and
Ni-NC-700 is coated by carbon during the thermal annealing process and
5

Y. Xu et al.
Applied Catalysis A, General 622 (2021) 118230
Fig. 5. a) Catalytic performance of Ni-NC-G-700 in recycling experiment, Reaction conditions: nitrobenzene (0.5 mmol), benzaldehyde (0.7 mmol), catalyst (mole
ratio of Ni/nitrobenzene is 1/15), H2 (2 MPa), time (8.5 h); b) Products distribution along with the reaction time in the one-pot reductive amination of nitrobenzene
with benzaldehyde, Reaction conditions: nitrobenzene (0.5 mmol), benzaldehyde (0.7 mmol), catalyst (mole ratio of Ni / nitrobenzene is 1/15), H2 (2 MPa).
Table 2
Comparison of catalytic activity of Ni-NC-G-700 with other reported catalysts.
entry
catalyst
reductive agent
time/h
T/ ºC
Solvent
Yield/%
Ref.
1
Ni-NC-G-700
GA-Pd
H
2 2 MPa
8.5
6
120
r.t.
ethanol
99
88
93
96
50
99
92
99
98
83.5
90
This
[13]
[16]
[18]
[35]
[36]
[37]
[38]
[39]
[19]
[20]
2
H2 1atm
H2 1atm
HCOOH
methanol
methanol
ethyl acetate
methanol
ethanol
3
AuPd-Fe3O4
Co-Nx
3
r.t.
4
10
8
150
90
5
Ni/NiO@C
Co-MoxC@CN
Co/GS@C
H
H
2 2 MPa
2 1MPa
6
4
70
7
H2 4MPa
CO 4MPa
NaBH4
24
10
~5
19
18
120
170
r.t.
t-BuOH
8
Co/N-C-600
Fe-Ni/MMT
MoS2
H2O
9
methanol
ethanol
10
11
H2 2 MPa
H2 2 MPa
120
70
[Mo3S4Cl3(dmen)3](BF4)
tetrahydrofuran
Scheme 1. Proposed reaction steps of reductive amination of nitrobenzene with benzaldehydes.
the trace nickel detected by XPS can be attributed to the nickel nano-
particles uncovered or covered by thin carbon layers. The Ni2p XPS
spectra of Ni-NC-300, Ni-NC-G-700 and Ni-NC-700 all display two main
peaks at binding energies of 855 eV and 872 eV corresponding to nick-
elous oxide (Fig. 4b) [27]. The small peaks locating at 861 eV and
879 eV can be ascribed to satellite peaks of nickelous oxide [28]. No
metal nickel is detected in the three samples, illustrating that the un-
covered nickel or nickel coated by thin graphitic layers is oxidized in air.
However, there is no characteristic peak of nickelous oxide in the XRD
patterns because nickelous oxide is amorphous and tiny proportion in
the samples. The C1s XPS spectra clearly reflect the changes of carbon
structure during the preparation of samples (Fig. 4c). For pure C3N4, Ni
(NO3)2@C3N4 and Ni-NC-300, the C1s XPS spectra presented the char-
acteristic sp2 N-C = N and C = C species of the s-triazine ring and aro-
matic rings of C3N4 at binding energies of 288.0 eV and 284.8 eV,
respectively [29]. However, the proportion of sp2 C in aromatic rings in
Ni-NC-300 is higher than in C3N4, indicating that the popping process
partly destroys the structure of C3N4 and promotes the generation of sp2
C = C component. This result is accordance with the analysis resulted
from XRD analysis. When Ni-NC-300 was heated to 700 ◦C and held for
one hour in nitrogen atmosphere, graphitic carbon became into main
carbon species presenting characteristic peak with binding energy at
284.8 eV. The samples Ni-NC-G-700 and Ni-NC-700 express similar C1s
2
–
–
–
– –
XPS spectra, involving characteristic sp C C, C N, C O, C O and
–
COOH at binding energies of 284.8 eV, 285.9 eV, 286.8 eV, 287.8 eV and
289.3 eV [30]. Similarly, the N1s XPS spectra also show the change of
nitrogen species in these samples (Fig. 4d). The N1s spectrum of pure
C3N4 can be deconvolved into four peaks situating at 398.2 eV, 399.0 eV,
400.4 eV and 404.5 eV, which are attributed to sp2 N (N
C
N) in
–
–
–
3
–
– –
triazine rings, sp -hybidized N atoms (N( C)3), C
N H and delo-
calized
π electron [31,32]. In the N1s spectrum of Ni(NO3)2@C3N4, a
new peak ascribing to nitrates is appeared at about 408 eV [33,34]. After
popping process, the peak corresponding to nitrates disappears and a
new peak at 404.8 eV arises in N1s spectrum of Ni-NC-300, further
proving that nickel nitrate was decomposed during the popping process.
When Ni-NC-300 was further annealed at 700 ◦C in nitrogen atmo-
sphere, pyridinic N became into the main nitrogen species.
6

Y. Xu et al.
Applied Catalysis A, General 622 (2021) 118230
Table 3
Reductive amination of nitroarenes with different aldehydesa.
entry
Carbonyl compounds
Nitro compounds
Time/h
8
Products
Con./%
100
Isolated yieldb/%
72
1
2
8.5
100
87
3
4
8
100
100
79
88
8.5
5
6
8
100
100
69
85
10
7
8
8
9
100
100
93
79
9
14
100
100
88
0
10
a
8.5
Reaction conditions: nitroarenes (0.5 mmol), aldehydes (0.7 mmol), catalyst (4.2 mg, mole ratio of Ni/nitrobenzene is 1/15), ethanol (3 mL), reaction temperature
(120 ◦C), H2 2 MPa, time (8ꢀ 14 h).
b
Yields of product are isolated yield and the structures of amines are determined by HNMR (Supporting information).
3.2. Catalytic performance
because the carbon generated from glucose is helpful to reduce nickel
oxide to metal nickel in the annealing process at 700 ◦C and improve the
stability of the nickel catalyst in air. However, when we increased the
amount of glucose from 1 mol % to 5 mol % and 10 mol % in the prep-
aration of catalyst and the achieved catalysts were named Ni-NC-5G-700
and Ni-NC-10G-700, the catalytic activity decreased (Table 1, entry
5–6). We think that more glucose can generate thicker carbon covering
on metal nickel nanoparticles, impacting the efficient interaction be-
tween active sites and substrates. TEM images of Ni-NC-5G-700 and Ni-
NC-10G-700 in Fig. S1 confirmed our conclusion. Therefore, a moderate
amount of glucose is desirable, which can reduce and protect nickel
components successfully, but not influence the catalytic activity. We
also evaluated the catalytic performance of Ni-NC-G-700 under different
reaction conditions. In general, lower reaction temperature, such as
80 ◦C and 100 ◦C, slowed down the conversion of nitrobenzene and
decreased the selectivity to N-benzyl aniline (Table 1, entry 7 and 8).
With other solvents such as acetonitrile or water, catalytic performance
also decreased (Table 1, entry 9 and 10), indicating that the protonic
solvents favored the one-pot cascade reaction. As commercial hydro-
genation catalysts, Raney Ni and Raney Co were also selected as
As a green and sustainable strategy to obtain secondary amines, the
key point of one-pot reductive amination of nitroarenes with aldehydes
is to develop a multifunctional catalyst possessing catalytic activities to
nitro-reduction and reductive amination. We evaluated the catalytic
activity of various nickel catalysts, and the results were listed in Table 1.
No product was detected in blank reaction (Table 1, entry 1). Ni-NC-300
displayed poor catalytic ability for the one-pot cascade reaction because
the nickel components in the sample mainly existed in oxidized state
(Table 1, entry 2). Ni-NC-G-700 exhibited excellent catalytic activity for
the nitro reduction and reductive amination reactions, which can ach-
ieve 100 % of conversion and 99 % of selectivity to N-benzylaniline
(Table 1, entry 3). For the sample Ni-NC-700 under same reaction
conditions, although nitrobenzene could be fully converted to hydro-
genated products, the selectivity to N-benzyl aniline was only 22.4 %
and imine was the main product (Table 1, entry 4). In addition, 26.7 %
selectivity to aniline was obtained, which clarified that catalytic activity
of Ni-NC-700 was lower than Ni-NC-G-700. This result illustrates that a
small amount of glucose is a key factor during the preparation of catalyst
7

Y. Xu et al.
Applied Catalysis A, General 622 (2021) 118230
comparison catalysts to promote this cascade reaction. Although nitro-
benzene was hydrogenated over Raney Ni under the same conditions,
the selectivity to N-benzyl aniline was only 60.5 % (Table 1, entry 11).
Benzyl alcohol hydrogenated from benzaldehyde was detected, sug-
gesting that the selectivity of Raney Ni for nitrobenzene hydrogenation
is not ideal in the presence of benzaldehyde. As a result, the residual
benzaldehyde was not enough to react with aniline hydrogenated from
nitrobenzene, resulting to the selectivity of 38.2 % to aniline. Raney Co
exhibited poor catalytic activity for the reaction because the conversion
was only 21.9 % and N-benzylideneaniline was the main product
(Table 1, entry 12). It is thus clear that Ni-NC-G-700 is an excellent
catalyst for the one-pot reductive amination of nitrobenzene with
aldehyde. Significantly, the catalyst Ni-NC-G-700 was easily recycled
due to its magnetic property with little compromise of catalytic per-
formance after the fourth cycles (Fig. 5a). It can be observed that the
decrease of catalytic activity is accompanied with the increase of
selectivity to aniline. Based on the reported mechanism, the reaction
between aniline and benzaldehyde is a reversible reaction. If the cata-
lytic ability to hydrogenation decreases so that imine cannot be hydro-
genated to N-benzyl amine, N-benzylideneaniline will decompose to
aniline and benzaldehyde. The XRD pattern of recycled Ni-NC-G-700
indicated that the metal nickel in the catalyst was not oxidized to
nickel oxide (Fig. S2a). However, TEM image of Ni-NC-G-700 showed us
that the structure of carbon nanotubes was destroyed and nickel nano-
particles agglomerated (Fig. S2b), which resulted to the slight decrease
of catalytic activity of hydrogenation. Compared to the other reported
catalytic systems listed in Table 2, the catalytic performance of Ni-NC-G-
700 was competitive in the one-pot reductive amination of nitrobenzene
with aldehyde.
and stability in the cascade reactions containing nitro-reduction,
coupling and hydrogenation. This investigation provides a new strat-
egy to fabricate air-tolerant transition metal catalysts for hydrogenation.
CRediT authorship contribution statement
Yuzhu Xu: Writing - original draft. Yaru Liu: Data curation. Penglei
Cui: Methodology. Ningzhao Shang: Writing - review & editing. Chun
Wang: Supervision. Yongjun Gao: Methodology, Supervision, Funding
acquisition.
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgements
Thanks for the financial support of the following funders: National
Natural Science Foundation of China (No. 21773053), the One Hundred
Talent Project of Hebei Province (No. E2016100015). Laboratory
opening project of Hebei University (No. sy201832).
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