Well-constructed Ni@CN material derived from di-ligands Ni-MOF to catalyze mild hydrogenation of nitroarenes

Article abstract of DOI:10.1016/j.mcat.2020.110838

1,3,5-benzenetricarboxylate (BTC) and 4,4′-bipyridine (BIPY) are employed in the synthesis of dual ligands Ni-MOFs. The magnetic Ni@CN nanocatalyst is prepared by direct pyrolysis of Ni-MOFs in N₂ atmosphere, which exhibited excellent activity

Full text of DOI:10.1016/j.mcat.2020.110838

MolecularCatalysis485(2020)110838
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Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Well-constructed Ni@CN material derived from di-ligands Ni-MOF to
catalyze mild hydrogenation of nitroarenes
Haijun Pan, Yuanyuan Peng, Xinhuan Lu*, Jie He, Lin He, Chenlong Wang, Fanfan Yue,
Haifu Zhang, Dan Zhou, Qinghua Xia*
Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials & Ministry-of-Education Key Laboratory for the Synthesis and Application of Organic
Functional Molecules, Hubei University, Wuhan, 430062, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Ni-MOFs
Ni@CN
Selective hydrogenation
Nitroarenes
1,3,5-benzenetricarboxylate (BTC) and 4,4′-bipyridine (BIPY) are employed in the synthesis of dual ligands Ni-
MOFs. The magnetic Ni@CN nanocatalyst is prepared by direct pyrolysis of Ni-MOFs in N2 atmosphere, which
exhibited excellent activity in selective hydrogenation of nitrobenzene (NB) to aniline (AN) at 60 °C, 2.0 MPa H2.
The conversion of NB can reach 99.1 mol%, and the selectivity of AN is over 99 %. A series of characterizations
are obtained by XRD, Raman, XPS, SEM, TEM, TPR and TGA. It is found that the synergic effect of Ni species and
N contributes to the increase of catalytic activity of the catalyst. Cycling tests proved that the prepared catalyst
could be reused ten times without a visible reduction in catalytic activity of hydrogenation. The excellent sta-
bility and activity of Ni@CN nanocatalyst could be assigned to the porosity of carbon material doped with
nitrogen, derived from dual ligands Ni-MOFs.
Mild conditions
1. Introduction
[1,13–16] The use of these precious metal catalysts can greatly increase
the activity of the reaction, but the selectivity often does not perform
It is a great challenge to pay attention to the environment and re-
duce the energy consumption and chemical waste in chemical trans-
formation [1]. The selective hydrogenation of nitroarenes into corre-
sponding anilines is widely used in the manufacture of various
polymers, drugs, agricultural chemicals, dyes and pigments [2–4]. In
the traditional industry, functionalized AN is mainly obtained by large
amount of reducing agents such as sodium bisulfite, hydrazine hydrate
or some non-recyclable non-noble metal ions (such as tin, iron, zinc),
which will bring huge risks to the environment [5]. Therefore, there is
an urgent need to develop an efficient and environmentally friendly
way to produce AN. Catalytic hydrogenation with hydrogen as a hy-
drogen source is undoubtedly the best way to produce AN, owning to its
low cost, cleanliness and ease of production. Nevertheless, in the past,
most of the catalysts used in the catalytic hydrogenation reaction in-
evitably produced some by-products, such as azoxy derivatives, nitroso
and hydroxylamines [6–8]. This is because the strong adsorption of
intermediate products to catalysts increases the difficulty of selective
hydrogenation of substituents on nitroarenes with other reducible
groups (e.g., eOH, CC, CO, CN and Ce]e]eel) [9–12].
well. Although selectivity can be improved by adding transition metal
salts and additives to the reaction, the addition of these metal salts and
additives poisons the reactive sites and reduces the reactivity, and the
cost itself is high. Furthermore, it gives new problems to the environ-
ment [17,18]. Gold-silver based catalysts exhibit good selectivity for
nitro compounds, but lower conversion due to its poor capacity of hy-
drogen dissociation [19,20]. With the increasingly strict environmental
requirements in today's society, green chemistry is being called upon by
more and more people, and heterogeneous catalysts for hydrogenation
reactions play an important role in this process due to their easy se-
paration characteristics. Therefore, there is an urgent need to develop a
non-precious metal heterogeneous catalyst used for the hydrogenation
of nitro compounds under mild conditions [21,22].
MOF is an inorganic-organic material composed of inorganic nodes
and organic linkers [23–25]. Compared with traditional porous mate-
rials, MOFs have many characteristics such as high specific surface area,
high porosity, and adjustable pore size [26,27]. Therefore, MOFs are
often used in many fields such as catalysis [28–31], gas adsorption and
separation [32,33], storage [34], fluorescence and molecular sensors
[35–38]. Especially in the field of heterogeneous catalysis [39,40],
MOFs materials and their derivatives are considered to be promising. At
Most of the literature on the production of aromatic amines is
heavily dependent on precious metals, such as Pd, Pt, Rh, and Ru.
⁎ Corresponding authors.
E-mail addresses: xinhuan003@aliyun.com (X. Lu), xiaqh518@aliyun.com (Q. Xia).
https://doi.org/10.1016/j.mcat.2020.110838
Received 10 December 2019; Received in revised form 16 January 2020; Accepted 10 February 2020
2468-8231/©2020ElsevierB.V.Allrightsreserved.

H. Pan, et al.
MolecularCatalysis485(2020)110838
Fig. 1. Powder XRD patterns of (a) Ni-MOF precursor (b) Ni@CN-x materials prepared at various treatment temperatures.
present, the preparation of porous carbon-containing metals by means
of pyrolysis of MOFs is a relatively simple method [41], and the ap-
plication of the MOFs derivatives obtained after pyrolysis in hydro-
genation reaction is also very common, e.g. in the hydrogenation of
nitrobenzene to aniline [42–47].
temperature, a series of black solid powders are obtained and desig-
nated as Ni@CN-x (namely Ni@CN catalyst, where x represents the
pyrolysis temperature).
2.3. Characterization of catalysts
Very recently, our group has published some works on the one step
hydrogenation of NB [13,48], the hydrogenation product is cyclohex-
ylamine. Herein, we report a simple preparation method for active N-
doped Ni@CN catalyst and its applications in the selective catalytic
hydrogenation of nitroarenes under mild conditions. The metal nano-
particles are uniformly distributed on the porous carbon, and the in-
teraction between the nitrogen atoms and the nickel facilitates the
formation of relatively electron-deficient and ultra-small sized nickel
clusters.
Powder X-ray diffraction (XRD) is carried out on a Bruker D8A25
diffractometer operating under conditions of CuKα radiation (λ is
1.54184 A) at 40 mA and 40 kV, ranging from 5 to 80° 2θ. Raman
spectra are obtained using a 512 nm argon ion laser (Renishaw
Instruments, England) at a range of 100 to 4000 cm−1. Scanning
electron images (SEM) are recorded on a JEOL (JSM6700 F) with an
accelerating voltage of 15 kV. Transmission electron microscopy (TEM)
images are collected under JEOL-135 2010 F Transmission Electron
Microscope at an accelerating voltage of 200 kV. X-ray photoelectron
spectra (XPS) are collected on a Perkin-Elmer PHI ESCA system.
Thermal gravimetric analysis (TGA) in N2 atmosphere is performed on a
PerkinElmer thermal analyzer. Temperature-programmed reduction
using H2 (H2-TPR) is carried out on the TP-5076 automatic gas sorption
analyzer.
2. Experimental
2.1. Materials
The chemicals used include Ni(NO3)2·6H2O (98 %, Sinopharm),
1,3,5-benzenetricarboxylate (BTC, Aladdin), 4,4′-bipyridine (BIPY,
Aladdin), DMF (98 %, Sinopharm), nitrobenzene (NB, 98 %,
Sinopharm). All other chemicals are analyzed for purity.
2.4. Selective hydrogenation
The catalytic reaction is carried out in a 25 ml stainless steel au-
toclave. Typically, (5.1–7.36 mol% [Ni]) of Ni@CN-x catalyst, 2 ml
deionized water and 0.5 mmol nitrobenzene (NB) are added in se-
quence. After sealing the reactor, the air content is purged by flushing
three times with 0.5 MPa H2. Then, the autoclave is heated to the given
temperature at an agitation rate of 400 rpm. The temperature is mon-
itored using a thermocouple inserted in the autoclave. The mixture in
the system is diluted with ethanol, and the resulting reaction mixture is
separated by centrifugation and analyzed by GC and GC–MS.
2.2. Synthesis
2.2.1. Synthesis of Ni-MOF
Nickel nitrate hexahydrate (Ni(NO3)2•6H2O, 0.876 g, 3 mmol) as
the metal source, BTC (2.270 g, 10.8 mmol) and BIPY (1.406 g, 9 mmol)
as ligands, DMF as solvent are added in the PTFE liner. The ratio of
metal to each ligand is (3 mmol: 10.8 mmol: 9 mmol = 1: 3.6: 3). When
different ligand ratios are changed, other conditions such as tempera-
ture, time and synthesis method are remained the same. The mixture is
stirred at room temperature for about 30 min and sealed into an au-
toclave, which is placed in an oven that has been preheated to certain
temperature (60–150 °C) for some time (12–72 h). After the completion
of crystallization, the content is cooled down to ambient temperature,
recovered by filtration and washed with deionized water and DMF,
followed by drying in 100 °C oven for 6 h to obtain green powder
crystal, which is named as Ni-MOF precursor (a di-ligands Ni-MOFs
material).
3. Results and discussion
3.1. Characterization of Ni@CN-x materials
Fig. 1(a) shows thus-synthesized Ni-MOF precursor and the simu-
lated XRD patterns. The powder XRD pattern of the Ni-MOF matched
well with the standard XRD pattern. The XRD patterns of Ni@CN-x
catalysts obtained by calcining Ni-MOF precursor at different tem-
peratures is shown in Fig. 1(b). The XRD patterns of the Ni@CN-x na-
nocatalysts exhibited three diffraction peaks at around 44.5°, 51.8° and
76.4°, corresponding to the characteristic diffractions of Ni [111], [200]
and [220] lattice planes of metallic Ni, respectively (PDF# is 04-0850).
The mean size of Ni NPs in Ni@CN-400, Ni@CN-500, Ni@CN-600,
Ni@CN-700 and Ni@CN-800 is respectively calculated to be 8.0, 7.2,
2.2.2. Preparation of Ni@CN-x catalysts
The precursor material obtained above is placed in a porcelain boat
set in a vacuum tube furnace and pyrolyzed at 400−800 °C. Under the
inert N2 atmosphere, the furnace is heated to the targeted temperature
with 2 h pyrolysis at 1 °C/min heating rate. After being cooled to room
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H. Pan, et al.
MolecularCatalysis485(2020)110838
MOF is a regular hexagonal structure. It can be seen from Fig. 3(b) and
(c) that as the pyrolysis temperature rises, the surface of the material
begins to become rough, meaning that the active sites are slowly ex-
posed. However, when the temperature exceeds 500 °C, the shrinkage
and collapse of the surface of the material occurs clearly. By the pyr-
olysis temperature rising to 800 °C, the surface agglomeration of the
material occurred severely. As the temperature increases, the Ni-MOFs
precursor begins to transform to Ni@CN composite.
The morphology images of the Ni@NC-x catalyst is detected by TEM
(Fig. 4, Table 1). It can be seen that the morphology of Ni-MOF is a
hexagonal block structure. Its side length is about 4 μm and the
thickness is about 0.6 μm. It is found that the Ni@CN-400 nickel na-
noparticles are well distributed with small particle size. As shown,
during the pyrolysis process nickel nanoparticles and elongated carbon
tubes are gradually formed in the Ni@CN-500 catalyst. When the
temperature exceeds 600 °C, the nickel nanoparticles are clearly ob-
served. The particle size ranges from 1 to 40 nm, which increases with
the increase of pyrolysis temperature. According to statistic results, the
average particle sizes of Ni@CN-400, Ni@CN-500, Ni@CN-600,
Ni@CN-700 and Ni@CN-800 are 8.6, 10.1, 15.7, 17.3 and 36.9 nm,
respectively, consistent with XRD results. TEM images show that carbon
nanotubes are formed at 500, 600, and 700 °C, but the pyrolysis tem-
perature at 800 °C will cause the carbon nanotubes aggregated. At the
same time, it can be clearly see from Fig. S1 that when the pyrolysis
temperature is 800 °C, the material shows a very obvious core-shell
structure and the active metal Ni is surrounded by a thick carbon shell.
Further XPS measurements of Ni@CN-400, Ni@CN-500, Ni@CN-
700 and Ni@CN-800 are shown in Fig. 5. When the temperature ranges
from 500 to 800 °C, significant binding energy transfer occurs in Ni 2p
spectra of Ni@CN-x, which means that Ni species in Ni-MOF may be
reduced to metal Ni particles in situ during thermal decomposition
(Table 1). For example, when the decomposition temperature is 400 °C,
the content of NiO is 44.6 %; while the temperature continues to rise to
500, 700 or 800 °C, the content of Ni° is increased to 54.5, 58.5 or 62.8
%, respectively. According to previous reports, if the reduction poten-
tial of metal ions is -0.27 eV or higher (for example, Cu2+, Co2+ and
Ni2+), they are easily reduced to metal nanoparticles in situ through a
pyrolysis process under an inert atmosphere. At the same time, the
peaks in Fig. 5(c) at 398.9 eV, 400.4 eV and 401.4 eV are classified as
pyridinic N, pyrrolic N and graphitic N [49,50]. In Ni@CN-400, pyr-
idinic N and pyrrolic N account for 50.1 and 49.9 % in total N atoms.
However, in Ni@CN-500, pyridinic N proportion is 67.3 % and the
Fig. 2. Raman spectra of (a) Ni@CN-400, (b) Ni@CN-500, (c) Ni@CN-600, (d)
Ni@CN-700 and (e) Ni@CN-800.
13.7, 20.4 and 27.0 nm according to the Scherer analysis based on the
XRD results. The mean size of nanoparticles increases with an increase
in the calcination temperature. When the calcination temperature is
risen to 600, 700, and 800 °C, a new diffraction peak appears around
26.1°, attributed to the characteristic peak of graphite carbon.
Fig. 2 is the Raman spectra of Ni@CN prepared at different pyrolysis
temperatures. The D band at ∼1350 cm−1 represents the disordered
carbon with structural defects, while the G band at ∼1590 cm−1 re-
presents the highly ordered graphitic carbon with sp2 in-plane vibra-
tion. A higher ID/IG value indicates that the Ni@CN sample has more
carbon matrix defects. The rise of the heating temperature from 400 to
500 °C leads to increase of ID/IG value from 0.804 to 1.071. The ID/IG
value drops from 1.071 to 1.015 as the temperature is increased from
500 °C to 800 °C. This result indicates that there are more structural
defects in the catalyst treated at 500 °C, confirmed by the catalyst ac-
tivity in the hydrogenation of nitrobenzene.
The morphology of the dual ligands Ni-MOFs and Ni@CN-x cata-
lysts are shown in Fig. 3. Fig. 3(a) shows that the morphology of Ni-
Fig. 3. SEM images of (a) Ni-MOF precursor, (b) Ni@CN-400, (c) Ni@CN-500, (d) Ni@CN-600, (e) Ni@CN-700 and (f) Ni@CN-800.
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H. Pan, et al.
MolecularCatalysis485(2020)110838
Fig. 4. TEM images of (a) Ni-MOF precursor, (b) Ni@CN-400, (c) Ni@CN-500, (d) Ni@CN-600, (e) Ni@CN-700 and (f) Ni@CN-800.
temperature may enhance the thermal stability of Ni@CN-x within a
certain temperature range. The Ni@CN-500, Ni@CN-600, Ni@CN-800
have similar thermal stability, and the weight loss before 470 °C is at-
tributed to the decomposition of DMF molecules and ligands. The
weight loss after 470 °C is attributed to the pyrolysis of CHx. The spe-
cific loss amount corresponding to the temperature is shown in Table
S1.
Fig. 7 presented that the H2-TPR profiles of Ni@CN materials. In
general, the interaction of metal species with carriers can create new
surface compounds or alter their chemical states, further change re-
duction temperatures. The different reduction temperature regions re-
present the metal reducible ability or the strength of the metal-support
interaction. As shown in the Fig. 7 that Ni@CN-400 has two signal
peaks at 201 and 531 °C. The first peak is led by pyrolysis of the pre-
cursor into Ni° particles, and the second signal peak indicates that Ni°
has a strong effect on the support. Ni@CN-500 has three signal peaks at
223, 348 and 568 °C, respectively. The front two signal peaks are led by
the reduction of NiO particles to Ni°, while the signal peak at 568 °C
indicates that Ni° has strong interaction with the carrier. The three
signal peaks on Ni@CN-700 have the same chemical behaviors as
Ni@CN-500 at 199, 362 and 617 °C. The relatively weak peak on
Ni@CN-800 indicates that the Ni sample is higher dispersion on the CN.
Table 1
Structural properties of Ni@CN-x.
Catalysts
Particle size (nm)
Relative atomic
percentage (%)a
Relative atomic
percentage (%)
XRD
TEM
N1
N2
N3
Ni0
N2+
Ni@CN-400 8.0
Ni@CN-500 7.2
Ni@CN-600 13.7
Ni@CN-700 22.4
Ni@CN-800 32.0
8.6
50.1
67.3
–
49.9
32.7
–
–
–
–
32.3
33.8
44.6
54.5
–
55.4
45.5
–
10.1
15.7
17.3
36.9
46.4
43.0
21.3
23.2
58.5
62.8
41.5
37.2
a: N1 ― Pyridnic N, N2 ― Pyrrolic N, N3 ― Graphitic N.
pyrrolic N proportion is 32.7 % in total N atoms. Remarkably, as the
temperature increases, graphitic N begins to appear. For example, when
the pyrolysis temperature is 700 °C, graphitic N accounts for 32.3 %; at
800 °C graphitic N amount is increased to 33.8 %. This indicates that
the transition metal can induce the generation of graphitic carbon at a
high temperature. The C 1s peak of Ni@CN-x (Fig. 5d), located at ap-
proximately 284.5 eV with one tail peak at 288.4 eV, is assigned to C–Ni
bonding and the electron-deficient carbons bound to nitrogen (CeN),
respectively [51]. With increasing the pyrolysis temperature, this CNe
signal is weakened significantly, suggesting that the pyridine-like or
aromatic N is partly transformed into graphitic N in the carbon ske-
leton, this result is consistent with XRD and XPS characterization re-
sults. Too high temperature further weakens this signal. According to
the analysis of ICP-AES, the metal Ni contents in Ni@CN-400, Ni@CN-
500, Ni@CN-700 and Ni@CN-800 are 37.88, 39.89, 33.81 and 34.12 %,
respectively.
As displayed in thermogravimetric analysis (TG) curves, both Ni-
MOF and Ni@CN-x are stable up to 250 °C (Fig. 6). The weight loss at
30–250 °C is mainly due to the evaporation of the moisture and DMF
molecules on the surface of the material. The weight loss at 250–470 °C
is mainly attributed to the decomposition of BTC and BIBY, which in-
dicates that the MOF skeleton has collapsed down. The weight loss at
470–660 °C is mainly due to the reduction of Ni species caused by
pyrolysis, and further rise of the temperature does not result in an
obvious weight loss. It is worthwhile mentioned that the weight loss
rate of Ni@CN-500, Ni@CN-700 and Ni@CN-800 is slower than that of
Ni@CN-400. This indicates that the increase of the pyrolysis
3.2. Catalytic performance
Fig. 8 shows the effect of different ligand ratios on catalyst hydro-
genation (1: 3.6: 3.0=Ni: BTC: BIPY). On condition that the total
amount of the catalysts is kept the same in the NB hydrogenation re-
action, the catalyst derived from the dual ligands Ni-MOF with appro-
priate ligands ratios show much higher NB conversion than ones from
the single ligand Ni-BTC and Ni-BIPY. As-prepared Ni-BTC-500 pyr-
olyzed by Ni-BTC shows very low conversion of NB (52.6 %), indicating
that Ni-BTC-500 derived from single ligand precursor is not an efficient
catalyst for NB hydrogenation. This is due to the lack of the N element
that can interact with the nickel nanoparticles to facilitate the reaction.
Compared to Ni@CN-500 (di-ligands), Ni-BIPY-500 also exhibits a low
activity in the NB hydrogenation (78.9 mol% of NB conversion). This
may be due to the weak coordination ability between BIPY and Ni in the
absence of BTC (low yield of Ni-BIPY), resulting in low catalytic ac-
tivity. Combining the above two factors, we adopt the di-ligands
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H. Pan, et al.
MolecularCatalysis485(2020)110838
Fig. 5. XPS spectra of Ni@CN-x (a), Ni2p (b), N1 s (c) and C1 s (d).
Fig. 6. TGA curves of (a) Ni-MOF precursor, (b) Ni@CN-400, (c) Ni@CN-500,
(d) Ni@CN-700 and (e)Ni@CN-800.
Fig. 7. H2-TPR profiles of Ni@CN materials: (a) Ni@CN-400, (b) Ni@CN-500,
(c) Ni@CN-700 and (d) Ni@CN-800.
strategy to overcome these two defects. The coordination capability of
BIPY is enhanced and the N element is also introduced. Upon in-
troducing the dual ligands into the precursor of Ni@CN-500, the
activity of the resulting Ni@CN-500 increases much, appreciably higher
than those of the mono-ligand led catalyst. Over the Ni@CN-500
5

H. Pan, et al.
MolecularCatalysis485(2020)110838
Fig. 9. The effect of pyrolysis temperatures on the catalytic performance of
Ni@CN in NB hydrogenation (Reaction conditions: 5.1-7.36 mol% [Ni] catalyst,
0.5 mmol NB, 2.0 ml H2O, 60 °C, 2 h, 2.0 MPa H2.).
Fig. 8. Effect of different coordination ratios on catalyst performance in hy-
drogenation of NB (Reaction conditions: Ni@CN-500 catalyst (7.36 mol% [Ni]),
0.5 mmol NB, 2.0 ml H2O, 60 °C, 2 h, 2.0 MPa H2.).
NB is decreased from 99.1 to 26.1 mol%, and the selectivity of AN is
decreased from > 99 to 82.4 %. These results indicate that the content
and variation of N dopant has played important roles in the hydro-
genation of NB (Fig. 5) As can be seen from Fig. 2, Ni@CN-500 has the
highest ID/IG. And, Ni@CN-500 shows smaller particle size and more
uniform distribution (Fig. 4). These characterizations can be combined
to explain the good catalytic activity of Ni@CN-500. When the pyrolysis
time of Ni@CN-500 is 2 h, the conversion of NB is 99.1 mol%, with AN
selectivity of > 99 % (Fig. S3). With increasing the pyrolysis time from
1 to 2 h, the conversion is increased to 99.1 mol% in NB, and the se-
lectivity of AN is lifted from 95.6 to > 99 %. As the pyrolysis time
continues to increase (from 2 to 4 h), the conversion of NB is main-
tained at about 99.1 mol%, but the selectivity of AN is slightly de-
creased from > 99 to 97.4 %.
The influence of reaction temperature on catalytic activity is in-
vestigated under 30–80 °C at hydrogen pressure of 2.0 MPa, as shown in
Fig. S4. At low hydrogen pressure of 2.0 MPa, even though the tem-
perature raise is slightly varied from 30 to 60 °C, the AN yield is greatly
increased from 13.3 % at 30 °C to 99.1 mol% at 60 °C, meaning sig-
nificant effect of reaction temperature on reaction effect over Ni@CN-
500 catalyst. When the reaction temperature is continuously increased
from 60 °C to 80 °C, the conversion of NB is basically kept above 99 mol
%. H2 pressure has a large impact on catalytic activity (Fig. S5) and the
observed conversion at 2.5 MPa (100 mol%) is more than 2.0 times
higher than at 0.5 MPa (49.2 mol%). The selectivity towards AN also
increases considerably from 85 % to > 99 % with increase of H2 pres-
sure from 0.5 to 2.0 MPa, respectively. Therefore, the hydrogenation of
NB over Ni@CN-500 is more sensitive to H2 pressure and reaction
temperature.
Under the same conditions, we investigate the effect of solvent on
the hydrogenation of NB (Fig. 10). The hydrogenation results show that
the change in solvent significantly affects the reaction efficiency.
Among the solvents tested, water is the optimal one, giving 99.1 mol%
conversion of NB and > 99 % selectivity of AN. When the reaction
solvent is ethyl acetate, the conversion of NB is 100 mol%, but the
selectivity of AN is only 86.4 %. Toluene seems to be the worst one,
which achieves the conversion of only 7.1 mol%, may be due to the low
adsorption performance of the catalyst in toluene (Fig. 10). Obviously,
the catalytic performance of NB hydrogenation reaction in polar sol-
vents is much better than in non-polar solvents. The possible explana-
tion could be that Ni@CN-500 catalyst with N doping usually possesses
high hydrophilic properties, which results in homogeneous dispersion
catalyst (the ratio of BTC: BIPY is 3.6:3.0), 99.1 mol% of NB conversion
is achieved, which is the highest activity. However, with the decrease in
the ratio of BTC: BIPY, NB conversions are obviously declined to 67.0
and 43.1 % (based on the ratio of BTC: BIPY is 3.6:1.8 or 1.8:1.8),
respectively. In terms of the selectivity, the ratio of BTC: BIPY = 3.6:3.0
is the optimal beneficial to the formation of AN (> 99 % selectivity).
Whereas the decrease in the ratio of BTC: BIPY to 3.6: 1.8 or 1.8: 1.8
will result in the decrease of cyclohexylamine selectivity. Therefore, the
selection of the ligand ratio is very important. When the ratio of BIPY is
small or the ratio of BTC and BIPY is reduced simultaneously, the
conversion of the substrate and the selectivity of the product will be
reduced. Moreover, the Ni@CN-x catalysts not only exhibits higher
nitrobenzene conversion and selectivity for aniline, but also better
catalytic activity under the conditions of H2O as green reaction solvent,
much low reaction temperature and low H2 pressure than other Ni-
based catalysts (Table S2) [13,42–45].
The effect of crystallization temperature and time on the catalytic
hydrogenation of thus-prepared Ni@CN-500 in NB is presented in Fig.
S2(a) and (b). The optimal crystallization temperature and time is 100
°C and 48 h. When the crystallization temperature is increased from 60
to 100 °C, the conversion of NB is increased from 9.7 to 99.1 mol%, and
the selectivity of AN is increased from 75.3 to > 99 %. If the tem-
perature is risen to 150 °C, the conversion of NB and the selectivity of
AN are decreased to 15.6 mol% and 92.1 %. It is found that when the
crystallization time of the precursor is 48 h, the conversion of NB is 99.1
mol%, with AN selectivity of > 99 %. As the crystallization time is risen
from 12 to 48 h, the conversion of NB and the selectivity of AN are
increased from 43.1 to 99.1 mol%, and from 84.8 to > 99 %. When the
time is further increased to 72 h, the conversion of NB is decreased from
99.1 to 83.7 mol%, along with the decrease of AN selectivity from > 99
to 87.8 %.
Fig. 9 studies the effect of different pyrolysis temperatures on the
hydrogenation of the catalyst. The blank reaction (without any catalyst)
did not show any activity in this system. All the as-synthesized Ni@CN-
x catalysts are highly active for this hydrogenation. It is quite clear that
the catalysts prepared at suitable pyrolysis temperatures exhibit good
catalytic activity. It can be seen that the Ni@CN-400 catalyst only gives
77.2 mol% conversion of NB with > 99 % selectivity to AN (Fig. 9),
while Ni@CN-500 exhibits the highest conversion of 99.1 mol%
and > 99 % selectivity to the target product AN. When the pyrolysis
temperature is continuously risen from 500 to 800 °C, the conversion of
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H. Pan, et al.
MolecularCatalysis485(2020)110838
of the catalyst in water so as to improve the catalyst exposure to the
reactants, consequently increasing the catalytic performance.
3.3. Stability and reusability of Ni@CN-500 catalyst
In terms of the reusability of the catalyst, we performed the selective
hydrogenation of NB using Ni@CN-500 at 60 °C and 2.0 MPa in suc-
cessive runs and compiled the results in Fig. 11. Due to the magnetic
property of Ni-based material, it is easily separated from the solution
using a magnet. The specific steps are as follows. After the reaction
mixture is sampled, one magnet is placed at the bottom of the reactor
until the catalyst is completely adsorbed to the bottom; when the so-
lution becomes clear, a dropper is used to suck off the supernatant. The
above steps are repeated, and the solid catalyst is washed three times
with ethanol, followed by air-drying at room temperature. The re-
covered catalyst is applied in the next reaction under the same reaction
conditions. Therefore, the stability and reusability of the Ni@CN-500
catalyst is proven in the hydrogenation of NB, in which the catalytic
activity is maintained at almost a constant level. XRD patterns don’t
detect observable difference between fresh catalyst and 10 runs reused
one (Fig. S6). The particle size distribution of the Ni@CN-500 after 10
cycles showed that the mean particle size is ∼10.0 nm, almost the same
with the fresh catalyst (Fig. S7).
Fig. 10. Effect of reaction solvent on the hydrogenation of NB (Reaction con-
ditions: Ni@CN-500 catalyst (7.36 mol% [Ni]), 0.5 mmol NB, 2.0 ml solvent, 60
°C, 2 h, 2.0 MPa H2.).
Fig. 11. Recycling tests of the catalyst Ni@CN-500 (Reaction conditions: 7.36 mol% [Ni] catalyst, 0.5 mmol NB, 2.0 ml H2O, 60 °C, 2 h, 2.0 MPa H2.).
7

H. Pan, et al.
MolecularCatalysis485(2020)110838
Table 2
Ni@CN-500-catalyzed hydrogenation of different substituted nitroarenes.
Entry
1
Substrate
Product
Reaction time (h)
2
Conversion (%)
Selectivity (%)
> 99
99.1
92.4
96.3
2
3
3
2
> 99
96.9
4
5
2
3
100
100
96.6
> 99
6
7
3
3
96.2
92.4
> 99
> 99
8
9
3
4
95.2
100
88.2
85.4
10
11
12
13
3
3
2
2
90.0
94.0
74.6
73.4
> 99
92.1
> 99
91.6
14
2
62.4
> 99
15
16
3
4
73.4
80.8
> 99
89.1
3.4. Chemoselective hydrogenation of various substituted nitroarenes
2.0 ml H2O, 60 °C, 2.0 MPa H2.
To explain the suitability of Ni@CN-500 catalysts better, we explore
a series of other substituted nitroarenes. Expectedly, this catalyst has
shown high conversion and selectivity for most nitroarenes. When the
substituents on the aromatic ring of NB are methyl, hydroxy or amino
group (Table 2, entry 2–7), the overall conversion efficiency is slightly
better than that of halogenated nitroaromatics. This may be because
these groups are electron-donating group, quite favorable for the se-
lective hydrogenation on the benzene ring. Importantly, halogen-sub-
stituted nitrobenzenes can be reduced to halogenated anilines, known
as an important agrochemical intermediate with high yield and no
dehalogenation (entry 8–13). However, for multi-halogen substituted
nitrobenzene like 2,3,5-trifluoronitrobenzene (entry 14), the reaction
only can reach 62.4 mol% for 2 h. In addition, aldehydes as sensitive
functional groups are well preserved in our catalytic systems (entry
15–16), among which nitrobenzaldehyde spends 4 h to obtain the
conversion of only 80.8 mol%, with 89.1 % selectivity of O-amino-
benzaldehyde.
4. Conclusion
The dual-ligands Ni-MOF is synthesized by hydrothermal method,
where BTC and BIBY provide carbon source and nitrogen source, which
is then pyrolyzed under N2 atmosphere to receive Ni@CN-x catalysts.
The best crystallization and pyrolysis conditions are discussed, followed
by a series of structural characterizations and catalytic tests. The effects
of reaction solvent, temperature, time and H2 pressure on the hydro-
genation reaction are investigated systematically. In the hydrogenation
of NB, the conversion of NB is up to 99.1 mol%, and the selectivity of
AN is > 99 %. This high activity is due to the strong interaction be-
tween Ni nanoparticles and N species. Thus-prepared catalysts have
excellent stability and recyclability.
Author contributions
All authors contributed to the writing of this manuscript and ap-
proved the final version of the manuscript.
Reaction conditions: 7.36 mol% [Ni] catalyst, 0.5 mmol substrate,
8

H. Pan, et al.
MolecularCatalysis485(2020)110838
Declaration of Competing Interest
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Acknowledgments
Financial support from the National Natural Science Foundation of
China (21673069, 21503074, 21571055), Hubei Province Students
Innovation and Entrepreneurship Training Foundation of China
(201710512045), Hubei Province Outstanding Youth Foundation
(2016CFA040), and Applied Basic Frontier Project of Wuhan Science
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