Direct synthesis of imines from nitro compounds and biomass-derived carbonyl compounds over nitrogen-doped carbon material supported Ni nanoparticles

Article abstract of DOI:10.1039/d0nj05632d

The selective synthesis of imines from biomass-derived chemicals over heterogeneous non-noble metal catalysts is of great importance for organic transformation. Herein, non-noble heterogeneous nitrogen-doped carbon supported Ni catalysts (abbreviated as Ni/CN-MgO-T, whereTrepresents the pyrolysis temperature) have been facilely prepared from the simple pyrolysis of Ni precursors and biomass, and Ni/CN-MgO-600 with the smallest size of Ni nanoparticles demonstrated the highest catalytic activity. The reductive coupling of nitroarenes and carbonyl compounds could be performed under mild conditions (80 °C, and 10 bar H₂), affording structurally-diverse imines with high to excellent yields (84.2-98.1%). Thanks to the mild reaction conditions, the developed method showed good tolerance to other functional groups such as nitriles, halogen and vinyl groups.

Full text of DOI:10.1039/d0nj05632d

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Direct synthesis of imines from nitro compounds
and biomass-derived carbonyl compounds over
nitrogen-doped carbon material supported Ni
nanoparticles†
Cite this: New J. Chem., 2021,
45, 4464
Bo Li, Yanxin Wang, Quan Chi,* Ziliang Yuan, Bing Liu * and Zehui Zhang
*
The selective synthesis of imines from biomass-derived chemicals over heterogeneous non-noble metal
catalysts is of great importance for organic transformation. Herein, non-noble heterogeneous nitrogen-
doped carbon supported Ni catalysts (abbreviated as Ni/CN–MgO-T, where T represents the pyrolysis
temperature) have been facilely prepared from the simple pyrolysis of Ni precursors and biomass, and
Ni/CN–MgO-600 with the smallest size of Ni nanoparticles demonstrated the highest catalytic activity.
The reductive coupling of nitroarenes and carbonyl compounds could be performed under mild
conditions (80 1C, and 10 bar H2), affording structurally-diverse imines with high to excellent yields
(84.2–98.1%). Thanks to the mild reaction conditions, the developed method showed good tolerance to
other functional groups such as nitriles, halogen and vinyl groups.
Received 18th November 2020,
Accepted 15th January 2021
DOI: 10.1039/d0nj05632d
rsc.li/njc
activity for the hydrogenation of nitro compounds, which
Introduction
afforded amines with 99% yield at 140 1C and 25 bar H2.7
Hydrogenation reactions are of great importance in organic Recently, catalysis is aimed at cheaper, greener, and sustainable
transformations both in academic research and industry.1–3 methodologies. Therefore, the replacement of expensive and rare
Among various kinds of organic transformations, the hydro- noble metals with base metals is highly desirable towards
genation of nitro compounds represents one of the versatile organic transformations. As far as the hydrogenation of nitro
methods to access primary amines as well as their derivatives, compounds is concerned, the non-noble metal catalysts were
which are extremely important for the synthesis of various mainly based on the iron,8 nickel9 and cobalt.10
biological, agricultural and pharmaceutical compounds.4,5
The as-formed primary amines from the hydrogenation of
Therefore, there has been a continuous interest in the hydro- nitro compounds can also be further transformed into other
genation of nitro compounds to obtain primary amines as well derivatives. Imines are a class of nitrogen-based compounds
as their derivatives.
with high reactivity due to the presence of unsaturated CQN
Previously, the reduction of nitro compounds was performed double bonds. Derivatives of imines have versatile uses in
using stoichiometric reducing agents such as NaBH4. Although the production of heterocyclic chemicals and pharmaceutical
these processes could proceed at room temperature, the use of compounds. Currently, the oxidation of primary amines has
stoichiometric reducing agents demonstrated some distinct received great interest for the synthesis of imines, which can
drawbacks such as the use of high cost reducing agents and even be carried out by using metal-free nitrogen-doped carbon
poor atomic efficiency with generation of toxic wastes. To over- materials. The oxidative method using nitro compounds
come these drawbacks, catalytic hydrogenation of nitrobenzene as the starting material for the synthesis of imines required
represents a sustainable way to access primary amines. Further- two steps: the first step was the hydrogenation of nitro
more, hydrogenation of nitrobenzene was performed over noble compounds into primary amines and the second step was
metal catalysts.6 For example, Corma and co-workers reported the oxidation of primary amines into imines. Obviously, this
that TiO2 supported gold catalysts demonstrated a high catalytic strategy required the separation of primary amines, which is
tedious, time-consuming and un-economical. Similar to the
Key Laboratory of Catalysis and Materials Sciences of Hubei,
nitro compounds, carbonyl compounds are readily available,
in that they can be obtained from renewable biomass
resources, and can undergo condensation with primary
South-Central University for Nationalities, Wuhan, 430074, P. R. China.
E-mail: liubing@mail.scuec.edu.cn, chiquanscuec@163.com,
ehuizh@mail.ustc.edu.cn; Fax: +86-27-67842572; Tel: +86-27-67842572
amines to produce imines in the presence of acid catalysts
or non-catalysts.10 Therefore, it is highly desirable to perform
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0nj05632d
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one-pot synthesis of imines directly from nitro compounds (Shanghai, China). Benzaldehyde was purchased from J&K
and carbonyl compounds without the separation of the
Chemical Co. Ltd, (Beijing, China). Ni(NO3)2ꢁ6H2O, chitosan,
primary amines. However, the selective synthesis of imines
from the coupling of nitro compounds with carbonyl com-
pounds is a great challenge. Supported noble metal catalysts
demonstrate high catalytic activity in the reduction of nitro
compounds, accompanied by further reduction of the CQN
bonds in imines into secondary amines. For example, Kim
and co-workers have reported that Fe3O4 supported AuPd alloy
nanocatalysts demonstrate exceeding activity in the synthesis
of secondary amines via the cascade reaction of nitro
reduction followed by reductive amination of the resulting
amine with an aldehyde under 1 atm of H2 at room tempera-
ture.11 Generally, the supported non-noble metal catalysts
demonstrated a relatively lower catalytic activity than the noble
metal catalysts in the reduction of nitro compounds, and
the harsh reaction conditions also resulted in the hydrogena-
tion of CQN bonds in imines.12,13 Therefore, the development of
base metal catalysts with high catalytic activity under mild condi-
tions would be an alternative route for the selective synthesis of
imines from the coupling of nitro compounds with carbonyl
compounds.
dicyandiamide and all other chemicals were purchased from
Aladdin Chemicals Co. Ltd (Beijing, China).
Preparation of the Ni/CN–MgO-T catalysts
Firstly, 0.2 g of Ni(NO3)2ꢁ6H2O, 0.6 g of Mg(NO3)2ꢁ6H2O, 0.3 g
of chitosan and 0.6 g of dicyandiamide were dissolved in 8 mL
of deionized water and stirred for 20 min to form a mix-
ture. Then, 300 mL of acetic acid was added into the above
mixture under vigorous stirring and the resulting solution
was dried overnight until the green precursor was obtained.
Afterwards, the as-formed solid was heated at 600 1C for 2 h
under a H2 atmosphere with a heating ramp of 2 1C minꢀ1 and
then cooled down to room temperature. The as-prepared
catalyst was labeled as Ni/CN–MgO-600. Similarly, Ni/CN-
MgO-500 and Ni/CN–MgO-700 were also prepared using the same
procedure at hydrogen reduction temperatures of 500 and 700 1C,
respectively.
Traditionally, supported metal nanoparticles were prepared
by the reduction of the respective metal salts on heterogeneous
supports using traditional impregnation methods, and the
metal nanoparticles tended to grow large in size or aggregate
during the preparative process, which was caused by the weak
interaction between the metal nanoparticles and the supports.14
In recent years, the active non-noble metal catalysts can be
prepared via the structure controlling templates, such as metal–
nitrogen complexes or structure-controlling templates.15,16 In this
respect, metal organic frameworks (MOFs) built from metal ions
and different organic linkers can be used as self-sacrificing
compounds for the preparation of carbon-supported non-noble
metal catalysts with high catalytic activity for chemical reactions
via their direct pyrolysis.17
Complementary to these materials, most recently, we have
described the use of magnesium nitrate hexahydrate and nickel
nitrate hexahydrate as precursors for the preparation of supported
nanoparticles and single Ni atoms. Among them, biomass such as
chitosan is used as the carbon source, and dicyandiamide is used
as the nitrogen source which exhibit excellent catalytic activity for
reductive aminations. In continuation of our efforts to develop
cost-efficient materials for sustainable catalysis, herein, we
describe the simple preparation of a Ni carbon nitrogen catalyst,
which forms NiO/Ni particles. The resulting nanoparticles are
supported on carbon, which creates stable and reusable catalysts
for the selective hydrogenation of aliphatic and aromatic nitriles
and nitro compounds.
Catalyst characterization
Transmission electron microscope (TEM) images of the sam-
ples were obtained using a FEI Tecnai G2-20 instrument.
The sample was firstly dispersed in ethanol and dropped onto
copper grids for observation. X-ray powder diffraction (XRD)
patterns of the catalysts were obtained using a Bruker
Advanced D8 powder diffractometer (Cu Ka). All XRD patterns
were collected with the 2y range of 10–801 at a scan rate of
0.0161 sꢀ1
. Micropore and mesopore surface area and
pore size measurements were performed with N2 adsorp-
tion/desorption isotherms at 77 K using a V-Sorb 2800P
instrument. Before measurements, the samples were degassed
at 100 1C for 12 h. X-ray photoelectron spectroscopy (XPS)
was conducted using a Thermo VG scientific ESCA MultiLab-
2000 spectrometer with a monochromatized Al Ka source
(1486.6 eV) at a constant analyzer pass energy of 25 eV. The
binding energy was estimated to be accurate within 0.2 eV.
All binding energies were corrected with reference to the C1s
(284.6 eV) peak of the contamination carbon as an internal
standard.
General procedure for the one-pot reductive amination of
nitrobenzene and benzaldehyde
The reductive reactions were conducted in a 50 mL autoclave.
Typically, 1 mmol of nitrobenzene, 2 mmol of benzaldehyde,
and 10 mL of solvent were added into the reactor and sealed
under 10 bar H2. Then, the reactor was heated from room
temperature to 80 1C and kept at 80 1C for 4 h with magnetic
stirring at 1000 rpm. After being cooled down to room tem-
perature, the Ni/CN–MgO-600 catalyst was collected by centri-
fugation, and the liquid solution was analyzed using Agilent
Experimental
Materials
Acetic acid, Mg(NO3)2ꢁ6H2O, nitrobenzene and all the solvents
were purchased from Sinopharm Chemical Reagent Co., Ltd GC 7820A.
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Equation for selectivity calculation:
Paper
nProduced aniline
SelectivityAniline
¼
nConverted nitrobenzene
nProduced N-benzylideneaniline
SelectivityN-benzylideneaniline
¼
nConverted nitrobenzene
nProduced N-phenylbenzylamine
nConverted nitrobenzene
SelectivityN-Phenylbenzylamine
¼
Equation for carbon balance calculation:
ꢀ
C¼
Fig. 1 TEM images of Ni/CN–MgO-T catalysts: (a) and (b) Ni/CN–MgO-
500, (c) and (d) Ni/CN–MgO-600, and (e) and (f) Ni/CN–MgO-700.
nProduced anilineþnProduced N-benzylideneanilineþnProduced N-phenylbenzylamine
nConverted nitrobenzene
Results and discussion
Catalyst preparation and characterization
In this study, three Ni/CN–MgO-T catalysts were prepared using
a one-step pyrolysis method, which was eco-friendly and easy to
prepare compared with other catalysts (Table S5, ESI†).
Ni(NO3)2ꢁ6H2O and Mg(NO3)2ꢁ6H2O served as the precursors
of Ni nanoparticles and MgO support. Chitosan and dicyan-
diamide were used as the carbon and nitrogen sources of the
nitrogen-doped carbon materials. First, the metal salts and
dicyandiamide were dissolved in water to give a clear solution,
and chitosan was homogeneously dispersed in the solution.
After the addition of acetic acid, chitosan underwent the self-
crosslinking process and a homogeneous sol–gel was formed.
The as-formed sol–gel was dried in an oven at 100 1C overnight
and then subjected to pyrolysis under a H2 atmosphere. During
the pyrolysis process, Mg(NO3)2ꢁ6H2O was transformed into
Fig. 2 XRD patterns of the Ni/CN–MgO-T catalysts.
MgO, while Ni(NO3)2ꢁ6H2O was first transformed into NiO, (JCPDS no. 74–1225). Three peaks at 44.61, 51.91 and 76.51 can
which was further reduced into metallic Ni after pyrolysis under be assigned to the (111), (200) and (220) lattice planes of
a H2 atmosphere. The as-prepared catalysts were denoted as metallic Ni, respectively.18,19 However, the peak of MgO and
Ni/CN–MgO-T catalysts, in which T represents the pyrolysis the peak of Ni overlapped, the half-width of nickel of Ni/CN–
temperature. Firstly, the morphology and structural features MgO-500 was affected during the calculation, which lead to the
of the as-prepared Ni/CN–MgO-T materials were characterized increase in the size of nickel in Ni/CN–MgO-500. This phenom-
using the transmission electron microscope (TEM) technique. enon can be observed from the XRD pattern. The calculated
As shown in Fig. 1, Ni nanoparticles were homogeneously average particle sizes of Ni nanoparticles from line broadening
dispersed onto the surface of the Ni/CN–MgO-T catalysts; it were 15.1, 15.8 and 19.8 nm for Ni/CN–MgO-500, Ni/CN–MgO-600,
was noted that the average particle size was greatly influenced by and Ni/CN–MgO-700 catalysts, respectively, which was consistent
the pyrolysis temperature. The Ni nanoparticles grew into particles with the TEM results. As shown in Fig. 2, the characteristic peaks
of larger size with an increase in the pyrolysis temperature, and of Ni became sharper, demonstrating that the crystallinity of Ni
the average size of the Ni nanoparticles increased from 7.5, 11.1 to increased with the increase in the pyrolysis temperature.19
16.5 nm for Ni/CN–MgO-500, Ni/CN–MgO-600 and Ni/CN–MgO-
700 (Fig. S1, ESI†), respectively.
X-ray photoelectron spectroscopy (XPS) was further conducted
to study the valence states of the surface elements in the Ni/CN–
XRD patterns of the Ni/CN–MgO-T catalysts are shown in MgO-T catalysts. The XPS survey scan (Fig. S3, ESI†) of the
Fig. 2. A wide peak at about 261 was observed in the XRD Ni/CN–MgO-600 catalyst showed several peaks with different
pattern of the Ni/CN–MgO-500 catalyst, and this peak was binding energies, corresponding to the Ni, C, N, O, and Mg
assigned to the (002) plane of the graphitic carbon. The peaks elements, which suggested that these five elements were success-
at 36.81, 42.81, 62.21, 74.51 and 78.41 were attributed to the fully induced into the Ni/CN–MgO-600 catalyst.19,20 High resolu-
(111), (200), (220), (311) and (222) reflections of the MgO phase tion spectra of Ni 2p are demonstrated in Fig. 3. The peaks with
4466 | New J. Chem., 2021, 45, 4464ꢀ4471
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Fig. 4 The N2 adsorption–desorption curves of (a) Ni/CN–MgO-500,
(b) Ni/CN–MgO-600, and (c) Ni/CN–MgO-700 and (d) enlarged Ni/CN–
MgO-500 isotherm curve.
to 181.0 and 171.2 m2 gꢀ1 for Ni/CN–MgO-600 and Ni/CN–MgO-
700 catalysts, which were much larger than that of Ni/CN–MgO-
500. In addition, the pore size distribution was wide for each of
the catalysts, which tallied with the isotherm curves (Fig. S8,
ESI†).
Fig. 3 XPS spectra of (a) Ni 2p, (b) C 1s, (c) N 1s, and (d) O 1s in the Ni/NC–
MgO-600 catalyst.
the binding energies at 853.1/869.9 eV, 855.6/873.5 eV and 861.7/
880.5 eV were attributed to Ni, NiO and the satellite peaks of Ni
2p3/2 and Ni 2p1/2, respectively.21 The presence of NiO in the
Ni/CN–MgO-T catalysts was due to the oxidation of the surface
Catalyst screening for the one-pot reductive amination of
nitrobenzene
metallic Ni nanoparticles during storage in the air. The N 1s spectra The one-pot reductive amination of nitrobenzene and benzal-
can be assigned to four peaks: pyridinic N (398.5 eV), pyrrolic N dehyde was used as a model reaction to study the catalytic
(400.0 eV), graphitic N (401.2 eV) and oxidized N (405.5 eV).4,22 activity of the as-prepared Ni/CN–MgO-T catalysts (Table 1),
However, no oxidized N species were observed in the Ni/CN–MgO- which were performed at 90 1C and 10 bar H2. Initially, the
600 and Ni/CN–MgO-700 catalysts, which indicated that the Ni/CN–MgO-500 catalyst possessed the lowest catalytic activity
oxidized N species were reduced at higher pyrolysis temperature for the reduction amination of nitrobenzene and benzaldehyde,
by H2. As shown in Table S2 (ESI†), the relative peak area which afforded a nitrobenzene conversion of only 34.5% after
percentage of pyridinic
N decreased for Ni/CN–MgO-500, 6 h (Table 1, entry 1). Then, the conversion of nitrobenzene
Ni/CN–MgO-600 and Ni/CN–MgO-700, indicating that pyridinic greatly increased to 90.2% under the same conditions for the
N transformed into pyrrolic N and graphitic N with the increase Ni/CN–MgO-600 catalyst (Table 1, entry 2), and it then
in the pyrolysis temperature. As detected using XPS, the total atom decreased to 41.6% for the Ni/CN–MgO-700 catalyst (Table 1,
percentage of N gradually decreased with the increase in the
pyrolysis temperature (Table S2, ESI†). These results suggested
that nitrogen atoms in the carbon layer were gradually destroyed
with the increase in the pyrolysis temperature. The C 1s spectra
Table 1 The influence of different catalysts in the reductive amination
reactiona
can be convoluted into four peaks: CQC (284.6 eV), CQN/C–O
(284.9 eV), C–N (285.9 eV) and O–CQO (288.4 eV).23 The content of
sp2 CQC increased with the increase in the pyrolysis temperature
(Table S1, ESI†), demonstrating that the nitrogen-doped carbon
gradually changed into graphene. The peaks in the O 1s spectra
can be deconvoluted into Ni–O (529.8 eV), Mg–O (530.5 eV),
CO(OH) (531.8 eV) and C–CQO (532.9 eV).24 As the pyrolysis
temperature increased, the percentage of Mg–O lattice oxygen
increased and Ni–O decreased because of the reduction of H2.
The N2 adsorption–desorption experiment was performed to
detect the specific area and pore structure of the catalysts. As
shown in Fig. 4, all isotherm curves of the catalysts possessed
IV-type isotherm curves with an H4 hysteresis loop, indicating
that the catalysts had a mesoporous structure. The specific area
Selectivity (%)
Entry
Samples
Conversionb (%)
3
4
5
1
2
3
4
5
6
Ni/CN–MgO-500
Ni/CN–MgO-600
Ni/CN–MgO-700
Ni/CN-600
Ni/MgO-600
CN–MgO-600
34.5
90.2
41.6
49.2
13.5
0
0
100
84.3
80.0
78.9
85.2
0
0
0
0
0
0
0
15.7
20.0
21.1
14.8
0
a
Reaction conditions: 1 mmol nitrobenzene, 1.5 mmol benzaldehyde,
10 mL ethanol, 10 bar H2, 90 1C and 6 h. Conversion was calculated
by the consumption of nitrobenzene through the internal standard
b
of the Ni/CN–MgO-500 catalyst was 35.5 m2 gꢀ1, and increased method.
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Paper
Table 2 The influence of different solvents for the reductive amination
entry 3). Generally speaking, the catalytic activity of Ni nano-
particles decreased with an increase in the size of its particles,
because the Ni nanoparticles with a small size would expose much
more surface sites to make contact with the substrates. Although
the particle size of the Ni nanoparticle in the Ni/CN–MgO-600
catalyst was larger than that in the Ni/CN–MgO-500 catalyst, it
demonstrated a much higher catalytic activity. These results
demonstrated that the catalytic activity of the Ni/CN–MgO-T
catalysts was not only depended on its size. According to the
results from the N2 adsorption–desorption curves, the surface area
of the Ni/CN–MgO-500 catalyst was much lower than that of the
Ni/CN–MgO-600 catalyst (35.5 m2 gꢀ1 vs. 181.0 m2 gꢀ1). Thus, the
low catalytic activity of the Ni/CN–MgO-500 catalyst should be
mainly caused by its low specific surface area, as the surface was
crucial for the mass transfer of the substrates from the reaction
solution to the catalyst surface. As the surface area of the Ni/CN–
MgO-600 catalyst was similar to that of the Ni/CN–MgO-700
catalyst, the lower catalytic activity of the Ni/CN–MgO-700 catalyst
reactiona
Selectivity (%)
Entry
Solvent
Conversion (%)
3
4
5
1
2
3
4
5
6
7
Methanol
Ethanol
Toluene
THF
Acetonitrile
DMF
85.1
81.4
19.2
12.7
12.1
28.6
13.1
24.6
21.1
16.7
25.2
0
38.5
22.9
69.6
78.9
83.3
74.8
100
61.5
77.1
5.8
0
0
0
0
0
0
Hexane
a
Reaction conditions: 1 mmol nitrobenzene, 2 mmol benzaldehyde,
10 mL solvent, 10 bar H2, 90 1C, 4 h and 20 mg Ni/CN–MgO-600.
should be due to the larger particle size of Ni nanoparticles. was also studied. As shown in Table 2, the reaction solvents
Therefore, both the surface area and the particles size of the Ni greatly affected the activity of the catalyst and the products
nanoparticles were crucial for the reduction amination of nitro- selectivity. Protic solvents, ethanol and methanol, produced
benzene and benzaldehyde over the Ni/CN–MgO-T catalysts. The much higher conversions of nitrobenzene (Table 2, entries
Ni/CN–MgO-600 catalyst with a medium size of Ni nanoparticles 1 and 2 vs. entries 3–7), and the possible reason should be that
and large surface size demonstrated the highest catalytic activity. methanol and ethanol could activate nitrobenzene to some
In all cases, N-benzylideneaniline was the major product, which degree by the formation of hydrogen bonds. However, low
was produced by the condensation of benzaldehyde with the selectivity was observed from methanol. The possible reason
in situ formed aniline. In all cases, further hydrogenation of the was that the polarity and dielectric constant of methanol were
CQN bond in N-benzylideneaniline was not observed. Thus, the larger than those of ethanol, which made it easier for the catalyst
Ni/CN–MgO-T catalyst provided a mild and effective way for the to activate hydrogen and promote the hydrogenation of nitro-
selective synthesis of imines from carbonyl compounds and nitro benzene and imine.27 The reaction became slow in other sol-
compounds.
vents including acetonitrile, N,N-dimethylformamide (DMF) and
To investigate the function of each of the components in the THF, and the possible reason should be the coordination ability
catalyst, the Ni/CN-600 catalyst was also prepared using the same of oxygen or nitrogen atoms in the solvents with the Ni sites.15
method as the Ni/CN–MgO-600 catalyst but without the addition The poor conversion of nitrobenzene in n-hexane should be due
of Mg(NO3)2ꢁ6H2O, which showed a much lower activity in com- to the non-polarity of hexane, which resulted in a poor disper-
parison with that of the Ni/CN-600 catalyst (Table 1, entry 4), sion of the Ni/CN–MgO-600 catalyst as well as the limited
demonstrating that MgO played a role in the reduction amination solubility of the substrates in n-hexane. Ethanol was selected to
reaction. MgO might serve as the catalytic basic site to promote be the best solvent for the reductive amination of nitrobenzene
the hetero-cleavage of H2 over the Ni nanoparticles.25
with benzaldehyde over the Ni/CN–MgO-600 catalyst.
Furthermore, the Ni/MgO-600 catalyst was prepared without the
With Ni/CN–MgO-600 as the best catalyst and ethanol as the
introduction of chitosan and dicyandiamide during the catalyst best solvent in hand, the influence of reaction temperature on
preparation process, and only a 12% yield of N-benzylideneaniline reduction amination was investigated (Fig. 5). It was noted that
was obtained (Table 1, entry 5). TEM image of the Ni/MgO-600 the conversion of nitrobenzene was sensitive to the reaction
catalyst revealed that it had much larger sized Ni nanoparticles of temperature. The reaction at 60 1C produced a low nitrobenzene
15.3 nm (Fig. S2, ESI†), thus it demonstrated a much lower catalytic conversion of 25.8% after 4 h under 10 bar H2, and the conver-
activity. These results suggested that the nitrogen atoms could sion increased to 39.3% at 70 1C. To further improve the catalytic
prevent the growth of Ni nanoparticles during the reduction effciency, a one-pot reductive amination was performed at higher
process, because the nitrogen atom can coordinate with Ni reaction temperatures. Nitrobenzene conversion increased to
nanoparticles.26 The CN–MgO-600 catalyst was prepared to inves- 66.2% and 81.4% after 4 h at 80 1C and 90 1C, respectively.
tigate the function of Ni nanoparticles towards the reduction The increase in temperature promoted the movement of the
amination reaction (Table 1, entry 6). As expected, no catalytic reaction molecules, and thus much more effective molecules
activity of CN–MgO-600 was observed, indicating that H2 cannot be were present around the active sites, leading to the higher
activated in the absence of Ni nanoparticles.
conversion of benzene. As for the products, the hydrogenation
Furthermore, the effect of the reaction solvents on the of N-benzylideneaniline was not observed in all cases. Aniline
catalytic performance of the Ni/CN–MgO-600 catalyst toward was not observed for the reactions at the reaction temperatures
the reductive amination of nitrobenzene with benzaldehyde of 60 and 70 1C with low nitrobenzene conversion.
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5 bar to 10 bar was due to the increase of hydrogen concen-
tration in the solution, which resulted in the generation of
much more active hydrogen species for the hydrogenation of
nitrobenzene. On further increasing the hydrogen pressure from
10 bar to 15 bar, the nitrobenzene conversion slowly increased
from 66.2 to 72.5% H2. These results suggested that the solubility
of hydrogen gradually reached saturation beyond 10 bar, resulting
in the slow increase of the conversion of nitrobenzene. Again, the
hydrogenation of N-benzylideneaniline was not observed at 80 1C
under high hydrogen pressure.
The time course of the reduction amination of nitrobenzene
and benzaldehyde was performed at 80 1C under 10 bar H2
pressure over the Ni/CN–MgO-600 catalyst (Fig. 7). The
Fig. 5 The influence of different reaction temperatures on the reductive
amination reaction.
Table 3 Substrate scope for the one-pot reductive aminationa
Substrate
Nitro
Entry compound
Time Con.
Sel.
(%)
Aldehyde Product
(h)
(%)
1
2
3
4
5
6
12
97.3 95.4
13
13
17
13
16
100
98.1
95.1 91.6
95.3 91.7
95.4 95.6
93.4 90.1
Fig. 6 The influence of different H2 pressures on the reductive amination
reaction.
The influence of H2 pressure on the reduction amination of
nitrobenzene and benzaldehyde was also examined over the
Ni/CN–MgO-600 catalyst (Fig. 6). A low nitrobenzene conversion
of 36.5% was obtained at 80 1C under 5 bar H2 after 4 h with a
N-benzylideneaniline yield of 31.6%. Then, the conversion of
nitrobenzene greatly increased to 66.2% under 10 bar H2
pressure. The great increase of nitrobenzene conversion from
7
8
9
17
16
17
96.1 93.6
94.5 95.6
96.0 94.3
10
17
95.2 92.3
11
16
19
20
19
19
98.2 95.6
97.3 93.6
95.3 90.9
98.1 94.7
12b
13b
14b
15b
100
95.4
Fig. 7 Time course of the reduction amination of benzaldehyde with nitro-
benzene. Reaction conditions: 1 mmol nitrobenzene, 2 mmol benzaldehyde,
10 mL ethanol, 10 bar H2, 80 1C, 0–12 h and 20 mg Ni/CN–MgO-600.
a
Reaction conditions: 1 mmol nitro compound, 2 mmol aldehyde,
b
10 mL ethanol, 80 1C, 10 bar H2 and 20 mg Ni/CN–MgO-600. 100 1C.
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nitrobenzene was consumed quickly during the first three high activity and selectivity for the reduction amination of nitro-
hours, affording 57.3% conversion. Then, the reaction rate benzene and benzaldehyde. The Ni content in the reaction filtrate
gradually decreased at the late reaction stage, because of the was detected using the inductively coupled plasma–atomic emis-
gradual decrease in the nitrobenzene concentration during the sion spectrometry (ICP–AES), and the content of Ni was under the
reaction process. The conversion of nitrobenzene reached limit of detection. The stability of Ni nanoparticles should be due to
97.3% after 12 h and N-benzylideneaniline was produced in the strong interaction of nitrogen in the Ni/CN–MgO-600 catalyst
92.8% yield. During the entire reaction process, the hydrogenation with the Ni nanoparticles.
of N-benzylideneaniline into N-phenylbenzylamine was not
observed. In addition, aniline was present in a low content
during the reaction process, suggesting that the condensation
of aniline with benzaldehyde was a fast step reaction.
Conclusions
In conclusion, we developed a new catalytic system for the
The reduction amination of different carbonyl compounds
selective synthesis of imines from the reduction amination of
with nitro compounds was then investigated over the Ni/
nitro compounds with carbonyl compounds. The hetero-
CN–MgO-600 catalyst (Table 3). At first, various nitro compounds
geneous Ni/CN–MgO-600 catalyst possessed high activity and
were used to aminate benzaldehyde, and the corresponding
selectivity for the reaction, affording the corresponding imines in
imines were produced in the yields from 84.2% to 98.1% under
high to excellent yields from 84.2% to 98.1%. Both the surface
the reaction conditions. It was noted that the electron-donor
area as well as the particle size of Ni nanoparticles were found to
substituted nitrobenzene were more active than the electron-
play crucial roles in the catalytic activity. In addition, MgO and
withdrawing group substituted nitrobenzene (Table 3, entries
nitrogen-doped carbon material were also important to achieve a
2–5 vs. 6–11). In addition, no dehalogenation was observed in
high catalytic activity of the Ni/CN–MgO-600 catalyst. Further-
the halogen-substituted nitro compounds (Table 3, entries 6–8). In
more, the heterogenous Ni/CN–MgO-600 catalyst retained its
addition, o-nitrotoluene suffered from a steric hinderance effect,
activity after six runs with good stability.
which was much less active than p-nitrotoluene and m-nitrotoluene
(Table 3, entries 4 vs. 2–3 and 5). Next, the amination of diverse
carbonyl compounds with nitrobenzene was studied. Electron-
donor and electron-withdrawing group substituted benzaldehydes
Conflicts of interest
all demonstrated lower activity at 100 1C after long reaction time There are no conflicts to declare.
compared with benzaldehyde at 80 1C after 12 h (Table 3, entries
12–15 vs. entry 1). However, the selectivity of the Ni/CN–MgO-600
catalyst was found to be well maintained for all the substrates.
Acknowledgements
This work was supported by the National Natural Science Foun-
dation of China (no. 21872175 & 21922513) and Fundamental
Research Funds for the Central Universities, South-Central Uni-
versity for Nationalities (CZY 17004).
Catalyst recycling experiment
Finally, a recycling experiment was performed to investigate
the stability of the Ni/CN–MgO-600 catalyst. Thanks to the magnetic
properties of Ni nanoparticles, the Ni/CN–MgO-600 catalyst could
be easily recovered from the reaction mixture using an external
magnet. As shown in Fig. 8, the Ni/CN–MgO-600 catalyst retained
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