Nitrogen-Doped Carbon-Supported Nickel Nanoparticles: A Robust Catalyst to Bridge the Hydrogenation of Nitriles and the Reductive Amination of Carbonyl Compounds for the Synthesis of Primary Amines

Article abstract of DOI:10.1002/cssc.201802459

An efficient method was developed for the synthesis of primary amines either from the hydrogenation of nitriles or reductive amination of carbonyl compounds. The reactions were catalyzed by nitrogen-doped mesoporous carbon (MC)-supported nickel nanoparticles (abbreviated as MC/Ni). The MC/Ni catalyst demonstrated high catalytic activity for the hydrogenation of nitriles into primary amines in high yields (81.9–99 %) under mild reaction conditions (80 °C and 2.5 bar H₂). The MC/Ni catalyst also promoted the reductive amination of carbonyl compounds for the synthesis of primary amines at 80 °C and 1 bar H₂. The hydrogenation of nitriles and the reductive amination proceeded through the same intermediates for the generation of the primary amines. To the best of our knowledge, no other heterogeneous non-noble metal catalysts have been reported for the synthesis of primary amines under mild conditions, both from the hydrogenation of nitriles and reductive amination.

Full text of DOI:10.1002/cssc.201802459

Accepted Article
Title: Nitrogen-doped carbon supported nickel nanoparticles: a robust
catalyst to bridge the hydrogenation of nitriles and the reductive
amination of carbonyl compounds
Authors: Hanmin Yang, Yangmin Zhang, Quan Chi, and Zehui Zhang
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10.1002/cssc.201802459
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Nitrogen-doped carbon supported nickel nanoparticles: a robust
catalyst to bridge the hydrogenation of nitriles and the reductive
amination of carbonyl compounds for the synthesis of primary
amines
Yangmin Zhang, Hanmin Yang*, Quan Chi, Zehui Zhang*
Abstract: An efficient method was developed for synthesis of
primary amines either from the hydrogenation of nitriles or
reductive amination of carbonyl compounds catalyzed by
nitrogen-doped mesoporous carbon (MC) supported nickel
nanoparticles (abbreviated as MC/Ni). The MC/Ni catalyst
demonstrated high catalytic activity for the hydrogenation of
nitriles into primary amines with high yields (81.9~99%) under
mild reaction conditions (80 oC and 2.5 bar H2). On the meanwhile,
the MC/Ni catalyst could also successfully promote the reductive
amination of carbonyl compounds for the synthesis of primary
amines at 80 oC and 1 bar H2. The hydrogenation of nitriles and
the reductive amination proceeded via the same intermediates for
the generation of primary amines. To the best of our knowledge,
no other heterogeneous non-noble metal catalysts have been
reported for the synthesis of primary amines under mild conditions
both from the hydrogenation of nitriles and reductive amination.
approaches (called transfer hydrogenation reduction) is not
environmentally benign with the release of over-
stoichiometric amounts of waste, which restrict their usage,
especially in industry. Obviously, molecular hydrogen as the
inexpensive and sustainable reducing agent in chemical
industry is preferred for the hydrogenation of nitriles with a
high atom efficiency of 100%.
The catalytic hydrogenation of nitriles with H2 has been
performed either in the presence of homogeneous catalysts
or heterogeneous catalysts. The homogeneous catalytic
systems are generally effective for the hydrogenation of
nitriles,[12,13] as the substrates can freely contact with the
active sites of the homogeneous catalysts. However, the
hydrogenation of nitriles by homogeneous catalysts
demonstrates distinct drawbacks in terms of the difficulty in
the recycling and reuse of the homogeneous catalysts as
well as the contamination of aim products with the trace
amount of metal from the homogeneous catalysts, which is
detrimental to the pharmaceuticals. To overcome the issues
of the homogeneous catalytic systems, a great deal of effort
has been devoted to the design of novel heterogeneous
catalytic systems for the hydrogenation of nitriles, [14-18] which
were mainly based on precious metals. Undoubtedly, the use
of expensive noble metal catalysts would result in a high cost
production in the practical applications. Therefore, the
replacement of precious metals by inexpensive and less
toxic non-noble metals has become one of the major goals
in modern chemistry. However, the heterogeneous non-
noble metal catalysts have been rarely studied for the
hydrogenation of nitriles into primary amines. [14] For example,
Beller and co-workers have prepared metal oxides supported
cobalt catalysts for the hydrogenation of nitriles into primary
amines, which were performed at 120~130 oC and 30 bar H2
pressure.[14] From the ecological and economic viewpoints, it
still has a large room to design active non-noble metal
heterogeneous catalysts for the selective hydrogenation of
nitriles under mild conditions.
Introduction
Primary amines have been emerged as significant
intermediates in the synthesis of
a vast number of
pharmaceuticals and agrochemicals.[1,2] Therefore, the
development of new catalytic routes for the effective and
sustainable synthesis of primary amines has received a great
interest for a long time. During recent decades, several novel
catalytic methods have been documented for the synthesis
of primary amines, such as the hydrogenation of amides,[3]
the hydrogenation of nitriles,[4,5] amination of aryl halides,[6]
reductive amination of carbonyl compounds,[7] and direct
amination of alcohols.[8] Although some of the reported
methods were effective for the synthesis of primary amines,
it is still necessary to develop new routes for the efficient and
selective production of primary amines under mild conditions.
In this context, the catalytic hydrogenation of nitriles and
the reductive animation of carbonyl compounds represent
two interesting means of access to primary amines, because
of the readily available starting materials as well as the high
atom efficiency. The catalytic hydrogenation of nitriles can be
performed by the use of different kinds of the reducing agents
including aliphatic alcohols,[9] formic acid salts,[10] and
inorganic hydride. [11] Synthesis of primary amines by these
In recent years, nitrogen-doped carbon materials have
received great attention for the immobilization of metal
nanoparticles, owing to their high surface area, thermal
stability, and porous surface.[19, 20] Herein, a nitrogen-doped
carbon material with mesoporous structure was prepared for
the immobilization of nickel nanoparticles, and the as-
prepared catalyst demonstrated a high catalytic activity for
the hydrogenation of nitriles under mild conditions (80 oC and
2.5 bar H2). Furthermore, the as-prepared nickel catalyst was
also found to be active for the synthesis of primary amines
via reductive amination.
Y.M. Zhang, H.M. Yang, Q. Chi, Z.Z. Zhang
Key Laboratory of Catalysis and Materials Sciences of the Ministry of
Education, South-Central University for Nationalities, Wuhan, 430074, P.
R. China.
Tel. +86-27-67842572. Fax: +86-27-67842572. E-mail:
yhm20181011@163.com & zhangzehui@mail.scuec.edu.cn
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Results and Discussion
Transmission electron microscopy (TEM) image of the
MC/Ni catalyst is shown in Fig. 2. Nickel nanoparticles were
clearly observed on the surface of the carbon materials.
These results suggested that this method was effective for
the construction of MC/Ni catalyst. However, some
aggregation of nickel nanoparticles were also visible on the
TEM image of the MC/Ni catalyst. The average size of the
as-formed nickel nanoparticles was calculated to be 21 nm.
In addition, TEM images also clearly indicated the highly
porous nature of the material, which are caused by the gel
structure of the polymer.
Catalyst preparation and characterization
Fig. 1 Schematically illustration of the preparation of MC/Ni catalyst.
Procedure of the preparation of the MC/Ni catalyst
involves a hydrothermal process, an ion-exchange, and a
pyrolysis/reduction as well as a mild oxidation step (Fig. 1).
Firstly, the polymer gel was prepared by the polymerization
MC/Ni
PDF #01-1258
of
2,4-dihydroxybenzoic
acid
(DA)
with
hexamethylenetetramine (HMT) and ethylenediamine by the
use of triblock copolymer (P123) as the soft template at 130
oC for 4 h under hydrothermal conditions. Then, nickel
ammine complexes were embedded into the polymer
framework via ion exchange. Next, the polymer containing
the nickel precursor was pyrolyzed at 500 oC under hydrogen
atmosphere to form the mesoporous carbon. During the
pyrolysis process, the nickel precursor was simultaneously
reduced to form nickel nanoparticles, which were supported
on the mesoporous carbon to give rise to the MC/Ni catalyst.
Finally, the MC/Ni catalyst was treated by 1% O2 in nitrogen
atmosphere for 2 h at room temperature. The nickel content
in the as-prepared MC/Ni catalyst was determined to be
38.4% by ICP-AES.
10
20
30
40
50
60
70
80
2Theta (degree)
Fig. 3 XRD patterns of the MC/Ni catalyst.
The crystal structure of the as-prepared MC/Ni catalyst
was characterized by an X-ray Powder Diffractometery. In
the wide-angle XRD pattern of the MC/Ni catalyst (Fig. 3),
three peaks at 2θ = 44.1°, 51.7° and 76.2° with strong
intensity appeared, which corresponded to (111), (200), and
(220) reflections of metallic Ni (PDF number 01-1258).[21] The
XRD results clearly indicated that nickel cations were
successfully reduced to nickel nanoparticles by hydrogen
under the prepared conditions. In addition, there were no
characteristic XRD peaks for the NiO species, indicating the
mild treatment of the MC/Ni catalyst in 1% O2 and 99% N2
atmosphere did not change the crystal phase of metallic
nickel nanoparticles, albeit the metallic nickel on the surface
of nickel nanoparticles was oxidized to the high state as
characterized by XPS in the following.
35
20.7 nm
30
25
20
15
10
5
ID/IG = 1.3
0
5
10
15
20
25
30
35
40
45
Dimeter (nm)
500
1000
1500
2000
2500
Wavelength (nm)
Fig. 2 TEM image of the MC/Ni catalyst and the particle size distribution of
nickel nanoparticles.
Fig. 4 Raman spectrum of the MC/Ni catalyst.
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Raman spectrum were used to probe for the
carbonaceous structure of the MC/Ni catalyst. Two peaks
with the wavelength at 1351 and 1588 cm-1 were observed in
the Raman spectrum of the MC/Ni catalyst (Fig. 4), which are
called G and D bands, respectively. The D bond and G bond
are associated with structural defects of lattice symmetry and
the characteristic of the sp2 hybridization of carbon. [22] In
addition, the intensity ratio of D band to G band was
calculated to be 1.3, indicating abundant defects were
present in the graphitic network of the MC/Ni catalyst.
284.6(81%)
C1s
285.6(19%)
X-ray photoelectron spectroscopy (XPS) was used to
examine the oxidation states of elements on the surface of
the MC/Ni catalyst. The XPS survey spectrum confirms the
presence of mainly Ni, C, N, and O in the catalyst (Fig. 5). It
is surprised to note that nitrogen atoms were also present on
the surface of the MC/Ni catalyst. Although the polymer
framework did not contain nitrogen-containing unit, the
nitrogen atoms in nickel ammine complexes during the
pyrolysis process should be introduced in the carbon layer of
the MC/Ni catalyst. Typically, three peaks can be fitted into
the N 1s XPS spectra of the MC/Ni catalyst with the binding
energies at 398.5, 399.5 and 400.8 eV, and the three peaks
can be assigned to pyridinic N (N-1), pyrrolic N (N-2) and
graphitic N (N-3), respectively (Figure 5). [23] As shown in
Figure 5, the C 1s XPS spectra of the MC/Ni catalyst show
one main peak at 284.6 eV, corresponding to sp2- hybridized
graphitic carbon (C=C). In addition, the other weak peak with
the binding energy at 285.6 eV should be attributed to
C−O/C=N species.[24]
292
290
288
286
284
282
Binding energy (eV)
2p3/2
Ni2+
2p1/2
Satellite
Ni
Ni2+
Ni
850
860
870
880
890
Binding Energy (eV)
Fig. 5 XPS spectra for the MC/Ni catalyst.
C KLL
As shown in Fig. 5, the peaks of binding energies at
8152.6 eV of Ni 2p3/2 and 870.8 eV of Ni 2p1/2 are assigned
Ni 2p
1/2
Ni 2p
∼
∼
3/2
to the metallic Ni, while the peaks with the binding energies
at ~854.0 eV (Ni 2p3/2) and 876.1 eV (Ni 2p1/2) can be
attributed to the divalent valence state Ni2+.[25] In addition, a
peak with the binding energy of 859.6 eV was also observed,
which is the satellite peak of nickel. [25] Obviously, most of the
nickel nanoparticle were present in its oxidation state as
detected by XPS, which revealed that the metallic nickel on
the surface of nickel nanoparticle was majorly oxidized into
its oxidation state either by the treatment with 1% O2 in
nitrogen atmosphere or by the air during the storage.
Ni LMM
O 1s
C 1s
N 1s
400
Ni 3p
1000
800
600
200
0
Binding Energy (eV)
N2 adsorption–desorption of the MC/Ni catalyst shows an
H1 type hysteresis loop and a type IV isotherm (Figure 6),
which is a characteristic isotherm for mesoporous material.
However, the sorption isotherm does not level out in a
plateau at relative pressures p/p0>0.9, indicating the
existence of macrospores. Pore size distribution in Figure 6
also showed three types of macrospores, mesoporoes and
microspores were present MC/Ni catalyst. The macrospores
and mesoporoes had a large size distribution, while the
microspores had narrow size distribution with an average
size of 0.54 nm, respectively. The mesoporous surface area
and the microporous surface area of the MC/Ni catalyst were
determined to be 316.1 m2/g and 79.8 m2/g, and the
mesoporous volume and the microporous volume were
calculated to be of 0.35 cm3/g and 0.10 cm3/g. The large
surface area together with abundant porous structure was
N1s
N-1
N-2
N-3
404
402
400
398
396
Binding Energy (eV)
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due to the introduction of P123 as the soft template, which
would be very helpful for the chemical reactions, as the
reaction molecules and products can move freely between
the reaction solvent and the active sites on the surface of the
MC/Ni catalyst.
model reaction, which was performed under mild conditions
(60 oC and 2.5 bar H2). Initially, the influence of the reaction
solvents on the hydrogenation of benzonitrile was assessed
(Table 1). Because different solvents have different
properties such as polarity, acidic-basic properties, which
should affect the catalytic performance of the MC/Ni catalyst.
Table 1. Catalytic hydrogenation of benzonitrile in different solvents.a
300
250
200
150
100
50
Selectivity (%)
Conversion
Entry
Solvent
(%)
1
0
2
3
0
1
2
Hexane
Toluene
Dichloromethane
Ethyl acetate
THF
26.8
10.1
16.7
18.1
25.4
41.9
48.6
55.4
96.1
0
97.4
100
100
99.4
72.0
77.3
77.5
79.1
47.2
-
0
0
0.2
0.4
0.6
0.8
1.0
68
2
relative pressure (p/p0)
3
0
0
4
0
0
26.1
5
28.0
22.7
22.5
20.9
14.4
-
0
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
6
i-PrOH
0
7
EtOH
0
8
MeOH
0
9
H2O
19.4
-
4.2
10b
11c
MeOH
MeOH
0
-
-
-
a Reaction Conditions: benzonitrile (1 mmol), MC/Ni catalyst (20 mg),
solvent (10 mL), 60 °C, 2.5 bar H2, 4 h.
3
5
8
14
22
40
b No catalyst was added.
pore diameter (nm)
c 20 mg of MC was used as the catalyst.
As shown in Table 1, the polarity of the solvents showed
a great effect both on the conversion of benzonitrile as well
as the selectivity of the products. The hydrogenation of
benzonitrile over the MC/Ni catalyst in none-polar or weak
polar solvents produced much lower conversions of
benzonitrile than the reaction in the alcohol solvents with
stronger polarity (Table 1, Entries 1~5 vs 6~8). For different
kinds of alcohols, the conversion of benzonitrile increased
with an order of iso-propanol, ethanol and methanol (Table
1, Entries 6~8). In addition, a high benzonitrile conversion of
96.1% was obtained in water (Table 1, Entry 9). The high
conversion of benzonitrile in alcohols and water was possibly
caused by the fact that alcohols or water can active the
substrate or stabilize the product via the hydrogen bonds. In
addition, the selectivity of the products was also affected by
the reaction solvents. For these reactions in non-polar or
weak polar solvents, N-benzylidenebenzylamine was the
only detected product (Table 1, Entries 1~4), which was
considered to be the intermediate during the transformation
of benzonitrile. [26] While both the aim product of benzylamine
and the N-benzylidenebenzylamine intermediate were
produced in tetrahydrofuran (THF) and alcohols (Table 1,
2.5
2.0
1.5
1.0
0.5
0.0
0.53
0.6
0.8
1
pore diameter (nm)
Fig. 6 BET adsorption–desorption isotherm and pore size distribution of the
MC/Ni catalyst.
Catalytic hydrogenation of benzonitrile in different
solvents
The catalytic performance of the MC/Ni catalyst was
evaluated by the hydrogenation of nitriles to primary amines.
The hydrogenation of benzonitrile with H2 was used as a
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Entries 5~8). However, the reaction in water produced a side
increasing the reaction temperature to 80 oC, the side
product of dibenzylamine was also produced, which was
product
of
dibenzylamine
besides
the
benzylidenebenzylamine intermediate and the aim product of
benzamine (Table 1, Entry 9). Considering both the
conversion and the selectivity into consideration, methanol
should be the best solvent for the hydrogenation of nitriles
over the MC/Ni catalyst. A blank reaction was carried out in
the absence of the MC/Ni catalyst, and no reaction
proceeded (Table 1, Entry 10). The reaction also did not
occur in the presence of the MC, which was prepared as the
same procedure of the Ni/C catalyst without the addition of
nickel complex. These results suggested that nickel
nanoparticles were the active sites for the hydrogenation of
nitriles. Generally, metallic nanoparticles have been
accepted to show the ability to activate molecular hydrogen
in chemical reduction reactions.14-18
generated
benzylidenebenzylamine. These results suggested that the
hydrogenation of N-benzylidenebenzylamine into
dibenzylamine had a higher activation energy than the
transformation of N-benzylidenebenzylamine into
from
the
hydrogenation
of
N-
benzylamine, which required a higher reaction temperature.
Effect of hydrogen pressure and ammonia amount on
the hydrogenation of benzonitrile
The effect of hydrogen pressure on the hydrogenation of
benzonitrile was also studied at the reaction temperature of
80 oC (Figure 8). It was noted that the conversion of
benzonitrile slightly increased with an increase of hydrogen
pressure from 78.1% at 1 bar H2 to 100% at 5 bar H2. This is
due to the fact that the concentration of hydrogen in the
reaction solution enhanced with an increase of the hydrogen
pressure. Only the aim product of benzylamine and the
intermediate of N-benzylidenebenzylamine were produced at
80 oC and 1 bar H2 pressure. Increasing the hydrogen
pressure can simultaneously accelerated the transformation
of the intermediate N-benzylidenebenzylamine into the
benzylamine and the side product of dibenzylamine. These
results suggested that the increase of hydrogen pressure
can both promote the positive and side reactions.
Effect of reaction temperature on the hydrogenation of
benzonitrile
Selectivity of Benzylamine
Selectivity of Dibenzaylamine
Selectivity of N-benzylidenebenzylamine
Conversion
100
80
60
40
20
0
Selectivity of Benzylamine
Selectivity of Dibenzylamine
Selectivity of N-benzylidenebenzylamine
Conversion
100
80
60
40
20
0
40
50
60
70
80
Temperature (oC)
Fig. 7 Effect of reaction temperature on the hydrogenation of benzonitrile.
Reaction conditions: benzonitrile (1 mmol), the MC/Ni catalyst (20 mg), MeOH
(10 mL), 2.5 bar H2, and 4 h.
The effect of the reaction temperature on the
hydrogenation of benzonitrile over the MC/Ni catalyst was
also studied. As shown in Figure 7, the hydrogenation of
benzonitrile was observed to be sensitive to the reaction
temperature. The conversion of benzonitrile was observed to
greatly increase with an increase of the reaction temperature.
The MC/Ni catalyst was even active for the hydrogenation of
1
2.5
5
Pressure (bar)
Fig. 8 Effect of hydrogen pressure on the hydrogenation of benzonitrile.
Reaction conditions: benzonitrile (1 mmol), the MC/Ni catalyst (20 mg), MeOH
(10 mL), 80 °C, and 4 h. Note：The reaction at 1 bar was performed with balloon.
o
benzonitrile at 40 C albeit the conversion was only 17.4%
As discussed above, N-benzylidenebenzylamine was
also identified to be the intermediate for the hydrogenation of
benzonitrile. It is reported that the transformation of N-
benzylidenebenzylamine into benzyl amine involves the
addition of one NH3 molecule and the subsequent
after 4 h under 2.5 bar H2 pressure. To the best of our
knowledge, no such a supported non-noble metal catalyst
has been reported to be so active under such mild reaction
conditions. Prolonging the reaction time to 36 h, a high
conversion of 98.6% was obtained, and the selectivity of
benzylamine was 100%. The conversion of benzonitrile
remarkably increased from 17.4% at 40 oC to 55.4% at 60 oC,
and further increased to 93.5% at 80 oC. As far as the product
distribution, benzylamine was the only determined product at
the low temperature of 40 oC. Increasing the reaction
temperature to 60 oC, both benzylamine and the intermediate
of N-benzylidenebenzylamine were observed with the
selectivity being 20.9% and 79.1%, respectively. Further
[27]
hydrogenation process as depicted in Scheme 1.
Therefore, the effect of the amount of NH3 was also studied
for the hydrogenation of benzonitrile at 80 oC and 2.5 bar H2
(Figure 9). Indeed, the addition of NH3 can promote the
hydrogenation of benzonitrile. Full benzonitrile conversion
was obtained after 4 h by the use of 50 μL of 26.5 wt.%
ammonia solution (0.7 mmol), while that was 93.5% without
the addition of ammonia solution under the same conditions.
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More importantly, it was observed that the addition of 0.7
equiv. of NH3 greatly inhibited the formation of the side
product of dibenzylamine, which decreased from 29.5%
without NH3 to 11.5% by the use of 0.7 equiv. of NH3. On the
meanwhile, the selectivity of benzylamine increased from
47.4% without NH3 to 61.4% in the presence of 0.7 equiv. of
NH3. The selectivity of dibenzylamine further decreased with
an increase of the amount of NH3, while the selectivity of
benzylamine increased. These results suggested that NH3
promoted the transformation of the intermediate N-
benzylidenebenzylamine into benzylamine, and thus the side
reaction of the hydrogenation of N-benzylidenebenzylamine
into dibenzyl amine was inhibited. No side products were
observed when the amount of 26.5 wt.% ammonia solution
was beyond 200 μL (2.8 equiv.). Therefore, the appropriate
amount of NH3 was 2.8 equiv. (200 μL, NH3·H2O) for the
hydrogenation of benzonitrile without the formation of the
side product of dibenzylamine.
100
80
60
40
20
0
Benzonitrile
Benzylamine
N-benzylidenebenzylamine
0
1
2
3
4
5
6
Time (h)
Fig. 10 Time course of the product distribution during the hydrogenation of
benzonitrile. Reaction conditions: benzonitrile (1 mmol), the MC/Ni catalyst (20
mg), MeOH (10 mL), 80 °C, 2.5 bar H2 and 200 μL of NH3∙H2O.
Selectivity of Benzylamine
Selectivity of Dibenzylamine
Selectivity of N-benzylidenebenzylamine
Conversion
Substrate scope
100
With the optimal conditions in hand, we would like to
study the general applicability of the developed methods for
the hydrogenation of structurally-diverse nitriles into primary
amines, and these results are shown in Table 2. Most of the
substrates can be successfully converted into the primary
80
60
40
20
0
o
amines at 80 C and 2.5 bar H2 pressure with 2.8 equiv. of
NH3∙H2O. However, the activity of the aromatic nitriles was
affected by the substituted groups. Generally speaking, the
substrates with electron-withdrawing substituted groups
proceeded more sluggishly than the substrates with electron-
releasing substituted groups (Table 2, Entries 1~5 vs 7, 8),
which required a long reaction time to get high yield. For
example, 4-(Trifluoromethyl)benzonitrile with the strongest
electron-withdrawing groups required the longest reaction
0
50
100
150
200
250
Amount of ammonia (L)
Fig. 9 Effect of amount of ammonia on the hydrogenation of benzonitrile.
Reaction conditions: benzonitrile (1 mmol), the MC/Ni catalyst (20 mg), MeOH
(10 mL), 80 °C, 2.5 bar H2 and 4 h.
o
time of 18 h to accomplish the hydrogenation at 80 C and
2.5 bar H2 pressure in the presence of 2.8 equiv. NH3∙H2O
(Table 2, Entry 1). More importantly, the developed method
was also tolerant to other functional groups such as the
halogen group and the ester group. It is observed that the
position of the substituted groups also showed a great effect
on the activity of the substrates. By contrast, ortho-
substituted benzonitriles required much longer reaction time
to attain a high yield of the corresponding primary amine than
the ortho and para- substituted ones, which should be
caused by the steric hindrance of the methyl group at the
neighborhood of the nitrile groups (Table 2, Entries 7, 8 vs
9). The hydrogenation of fused-ring aromatic nitriles and
hetero-aromatic also proceeded smoothly, affording the
corresponding primary amines with high yields (Table 2,
Entries 10 & 11).
Time course of the hydrogenation of benzonitrile was
studied at 80 oC and 2.5 bar H2 in the presence of 2.8 equiv.
NH3. It was noted that benzonitrile was fast consumed at an
early reaction stage of 2 h due to its high concentration and
then it disappeared after 4 h. N-benzylidenebenzylamine
was the only detected intermediate by the gas
chromatography, and its percentage first increased from zero
to 4 h and then decreased from 4 h to 6 h. The yield of
benzylamine increased gradually during the reaction process,
and it reached a high yield of 100% after 6 h at 80 oC and 2.5
bar H2 in the presence of 2.8 equiv. NH3. To the best of our
knowledge, there were no heterogeneous non-noble metal
catalysts that were reported for the hydrogenation of nitriles
under such mild conditions.
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Table 2. Substrate scope of the hydrogenation of nitrile over the MC/Ni
amines with high yields by slightly increasing the reaction
temperature to 100 oC (Table 2, Entries 14 &15).
catalyst. a
Synthesis of primary amines via reductive amination of
carbonyl compounds
The ability of the MC/Ni catalyst towards the
hydrogenation of nitriles into primary amines inspired us to
carry out the synthesis of primary amines via the reductive
amination with carbonyl compounds, which involves the
condensation of the carbonyl group with NH3 to generate
imines and the subsequent hydrogenation. The reductive
amination of carbonyl compounds has also been considered
to be environmentally-friendly method for the synthesis of
primary amines, because of readily available carbonyl
compounds as well as the clean process with water as the
only byproduct.
Yield (%)
Time
(h)
Con.
(%)
Entry
Substrate
1
2
3
0
1
18
100
95.6
4.4
2
3
14
12
100
100
95.1
87.5
4.9
0
0
12.5
4
12
100
81.9
5.5
0
140
Benzaldehyde
5
6
15
6
100
100
88.8
>99
5.5
0
0
0
Benzylamine
N-benzylbenzylideneamine
120
100
80
60
40
20
0
4
.
0
7
8
9
6
6
6
100
100
100
>96.0
>99
0
0
0
0
94.7
5.3
0
2
4
6
8
10
12
10
11
12
6
87.9
94.5
87.9
94.5
0
0
0
0
Time (h)
Fig. 10 Time course of reductive amination of benzaldehyde. Reaction
conditions: benzonitrile (1 mmol), the MC/Ni catalyst (20 mg), MeOH (10 mL),
80 °C, and 200 μL NH3∙H2O.
12
13
14
15
16
8
98.4
98.2
100
90.8
98.2
87.8
85.6
92.1
8.4
0
0
0
0
0
0
The reduction amination of benzaldehyde was used as
the model reaction, which was performed at 80 C and 2.5
o
18
18
18
18
bar H2 pressure by the use of 26.5 wt.% NH3·H2O (200 μL of
26.5 wt.% NH3∙H2O, 2.8 mmol). Although benzaldehyde
conversion reached 100% after 4 h, the aim product of
benzylamine was only attained in a yield of 67.0%. Besides
benzylamine, the intermediate of N-benzylidenebenzylamine
and the side product of dibenzylamine were also produced in
the yields of 24.8% and 8.2%, respectively (Table 3, Entry 1).
To inhibit the side reaction of the hydrogenation of N-
benzylidenebenzylamine, the reductive amination of
carbonyl compounds were performed by lowering the
hydrogen pressure. As expected, the side reaction of the
12.2
14.4
0
100
92.1
a Reaction conditions: 80 °C, MC/Ni catalyst (20 mg), MeOH (10 mL),
36 wt.% NH3∙H2O (200 μL, 2.8 mmol), 2.5 bar H2; b The reaction
temperature was lifted to 100 °C and 5 bar H2, and 500 μL NH3∙H2O
was added.
hydrogenation
of
N-benzylidenebenzylamine
was
completely inhibited under 1 bar hydrogen pressure at 80 oC
(Table 3, Entry 2). As shown in Figure 10, benzaldehyde was
Compared with aromatic nitriles, the non-aromatic nitriles
were less active, which required either longer reaction times
or harsh reaction conditions. For example, the hydrogenation
of non-aromatic nitriles such as valeronitrile and benzyl
cyanide required a long reaction time of 18 h to afford the
corresponding primary amines with good yields (Table 2,
Entries 12 & 13). The hydrogenation of aliphatic nitriles with
a large steric hindrance at 80 oC was sluggish. To our delight,
it can be hydrogenated into the nonresponding primary
o
fully converted within 2 h at 80 C under 1 bar H2 pressure.
N-benzylidenebenzylamine was identified to be the sole
intermediate during the reaction process, which has the
highest yield of 69.8% after 2 h, and then decreased to zero
after 12 h. Benzylamine was not detected until the
disappearance of benzaldehyde, because benzylamine
easily reacted with benzaldehyde to give the intermediate of
N-benzylidenebenzylamine. Benzylamine was attained in a
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10.1002/cssc.201802459
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high yield of 99.1% after 12 h at 80 oC under 1 bar H2
pressure (Figure 10). It is very exciting to note that the
hydrogenation of benzonitrile can be performed under
atmospheric hydrogen pressure over the MC/Ni catalyst. We
can use the common glass reactor to promote the
hydrogenation of nitriles without the use of high-pressed
autoclaves, which is safe and economic in the industry
chemistry.
underwent the second hydrogenation step to give rise the
final product of benzylamine (Path A, Scheme 1). However,
phenylmethanimine
was
not
observed
by
gas
[28]
chromatography, which should be due to its instability.
Besides the direct hydrogenation of phenylmethanimine into
benzylamine, the as-formed benzylamine can also react with
phenylmethanimine to generate the intermediate of N-
benzylidenebenzylamine (Scheme 1) by the release of one
NH3 molecule. As listed in Fig. 9, the addition of ammonia
Then the reductive amination by the use of some
representative substituted benzaldehyde was also studied at
solution
promote
the
transformation
of
benzylidenebenzylamine into benzylamine, which was
reported to involve the addition of one NH3 molecule to N-
benzylidenebenzylamine to generate the instable geminal
diamine, and the subsequent hydrogenation of geminal
diamine to produce the final product of benzylamine (Path B,
Scheme 1). [27] The addition of one hydrogen molecule to N-
benzylidenebenzylamine gave rise to the side product of
dibenzyl amine, which was observed at a relative high
reaction temperature or a relative high hydrogen pressure or
(Figure 7 & 8).
o
80 C and 1 bar hydrogen pressure in the presence of 200
μL NH3∙H2O. As expected, the developed method was
efficient for the reductive amination of carbonyl compounds,
affording the corresponding primary amines with high yields
(Table 3, Entries 3~9).
Table 3. The results of the reductive amination of aldehydes for the
synthesis of primary amines. a
As far as the reductive amination of benzaldehyde, the
first step should be the condensation of benzaldehyde with
NH3 to generate phenylmethanimine. Similar as the reaction
pathway of the hydrogenation of benzonitrile, the generated
phenylmethanimine during the reductive amination of
benzaldehyde also underwent the hydrogenation step to give
rise to benzylamine. Benzylamine can readily react with
Pres
sure
(bar)
Yield
2
Con.
(%)
Entry
Structure
3
4
8.
2
1b
2b
3
2.5
1
100
100
100
100
100
100
67.0
64.2
99.1
98.2
96.8
94.8
24.8
benzaldehyde
to
produce
the
intermediate
N-
35.8
0
0
0
0
0
0
benzylidenebenzylamine,
a
stable intermediate of the
reductive amination of benzaldehyde.
1
4
1
1.8
3.2
5.2
5
1
6
1
7
1
100
94.6
5.4
0
3.
9
8
9
1
1
100
100
83.2
88.4
12.9
10.2
0.
8
Scheme 1. Proposed mechanism for the synthesis of primary amines over the
MC/Ni catalyst.
a
Reaction conditions: 1mmol substrates, 20 mg catalyst, 10 mL
Recycling experiments of the MC/Ni catalyst
b
MeOH, 36 wt.% NH3∙H2O (200 μL, 2.8 mmol), 80 °C,12 h, 1 bar H2
reacting for 4 h, other conditions remain the same.
Finally, the reusability of the MC/Ni catalyst was
investigated. The hydrogenation of benzonitrile was used as
the model reaction, which was performed at 80 C and 2.5
o
Reaction pathways for the synthesis of primary amines
bar H2 in the presence of 200 μL of 26.5 wt.% ammonia
solution. After the first run, the MC/Ni catalyst after reaction
was collected via the centrifugation, which was washed with
water and ethanol, respectively. Then the spent catalyst was
dried in a vacuum oven, and followed by drying and
treatment under H2/Ar at 300 oC for 2 h to remove residues.
As shown in Figure 12, the MC/Ni catalyst was highly stable
As discussed above, N-benzylidenebenzylamine was
identified to be the same intermediate both for the
hydrogenation of benzonitrile and the reductive amination of
benzaldehyde over the MC/Ni catalyst. As far as the
hydrogenation of benzonitrile, the first step should be the
hydrogenation of nitrile group by one hydrogen molecule to
give the intermediate of phenylmethanimine, which
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10.1002/cssc.201802459
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without the loss of its catalytic activity during the recycling
experiments.
Materials
Pluoronic P123, (PEO20PPO70PEO20, molar mass 5800
g·mol−1) was purchased from Sigma-Aldrich. 2,4-dihydroxybenzoic
acid (98%), hexamethylentetramine (99%) and ethylenediamine
(99%) were purchased from Aladdin Chemicals Co. Ltd. (Beijing,
China). Nickel (II) nitrate hexahydrate (Ni(NO3)2·6H2O) was
purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai,
China). All other chemical and solvents were purchased from
Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China).
Conversion
Selectivity
100
80
60
40
20
0
Preparation catalyst
In the first step, a homogeneous solution was prepared by
dissolving 2,4-dihydroxybenzoic acid (3.08 g), ethylenediamine (0.6
g), hexamethylentetramine (HMT, 0.934 g) and Pluronic P123 (3.5 g)
in H2O (80 mL). Then the mixture was transferred into a teflon-lined
stainless steel autoclave, and the temperature of the autoclave was
kept at 130 °C for 4 h. Afterwards, the autoclave was allowed to cool
down to room temperature. The as-prepared polymer gel product
was mashed and washed three times with deionized water. Then the
polymer gel was further dispersed in 120 mL of solution (96 mL of
H2O and 24 mL of ammonium hydroxide solution (28.0-30.0%)
containing 4.88 mmol (1.419 g) of Ni(NO3)2·6H2O, which was stirred
at 50 °C for 6 h. After the introduction of nickel complex into the
polymer gel, the polymer gel was collected by filtration, washed three
times with deionized water and dried at 50 °C under vacuum for 8 h.
Finally, the polymer gel was subjected to the pyrolysis under
hydrogen atmosphere. The detail pyrolysis process was described
as follows: the sample was heated to 400 °C with a rate of 2 °C min-
1
2
3
4
5
6
No.
Fig. 12 The recycling experiments of the MC/Ni catalyst towards the oxidative
dehydrogenation of benzaldehyde. Reaction conditions: Benzaldehyde (1
mmol), MC/Ni (20 mg), MeOH (10 mL), 200 μL of 26.5 wt.% NH3∙H2O, 80 °C,12
h and 1 bar H2.
Conclusions
In conclusion, we have developed an effective and
environmentally-friendly method for the synthesis of primary
amines via the hydrogenation of nitriles and the reductive
amination in the presence of the MC/Ni catalyst. The MC/Ni
catalyst can be easily prepared by the ion-exchange of nickel
precursor into the polymer framework, followed by the
pyrolysis process. The as-prepared MC/Ni catalyst showed
high catalytic activity towards the hydrogenation of nitriles. A
variety of (hetero)aromatic and aliphatic nitriles were
successfully hydrogenated into primary amines with yields
1
and kept at that temperature for 3 h, then heated to 500 °C with a
rate of 1 °C min-1 and kept at that temperature for 2 h. Afterwards,
the sample was allowed to cool to room temperature and was treated
in a flow of 1% oxygen under nitrogen atmosphere for 2 h at room
temperature. The as-prepared mesoporous carbon supported nickel
nanoparticles were abbreviated as MC/Ni.
Catalyst characterization
o
from 81.9 to 99% at 80 C and 2.5 bar H2 with ammonia as
the additive. On the meanwhile, the as-prepared MC/Ni
catalyst was also active for the synthesis of primary amines
via the reductive amination of carbonyl compounds under
mild conditions (80 oC and atmospheric hydrogen pressure).
Time course of the products distribution of the hydrogenation
of nitriles and the reductive amination was studied, and an
overall reaction pathway was proposed for the synthesis of
primary amines by the two methods over the MC/Ni catalyst.
In addition, recycling experiments revealed that the MC/Ni
catalyst was highly stable without the loss of its catalytic
activity. Compared with the already disclosed synthesis
methodologies, this novel catalytic system hold multiple
advantages, including cost effectiveness (using nonprecious
metals as catalyst), environmental safety (under mild
reaction condition in water), simplicity (facile magnetic
separation of the catalyst) and a broad substrate scope. To
the best of our knowledge, this is the first case that a
supported non-noble metal catalyst can promote both the
hydrogenation of nitriles and reductive amination for the
synthesis of primary amines under such mild conditions.
Transmission electron microscope (TEM) images of the samples
were obtained on a FEI Tecnai G2-20 instrument. The samples were
firstly dispersed in ethanol with an assist of ultrasonication and
dropped onto copper grids for observation. Micropore and mesopore
surface area and pore size measurements were performed with N2
adsorption/desorption isotherms at 77 K on a V-Sorb 2800P
instrument. Before measurements, the samples were degassed at
100 °C for 12 h. X-ray powder diffraction (XRD) patterns of samples
were determined with a Bruker advanced D8 powder diffractometer
(Cu Kα). The XRD patterns were collected with a scanning rate of
0.016o/s. X-ray photoelectron spectroscopy (XPS) was conducted on
a Thermo VG scientific ESCA MultiLab-2000 spectrometer with a
monochromatized Al Kα source (1486.6 eV) at constant analyzer
pass energy of 25 eV. The binding energy was estimated to be
accurate within 0.2 eV. All binding energies were corrected
referencing to the C1s (284.6 eV) peak of the contamination carbon
as an internal standard. The nickel content in the catalysts was
quantitatively determined by inductively coupled atomic emission
spectrometer (ICP-AES) on an IRIS Intrepid II XSP instrument
(Thermo Electron Corporation). Raman spectra were measured on a
confocal laser micro-Raman spectrometer (Thermo Fischer DXR)
equipped with a diode laser of excitation of 532 nm (laser serial
number: AJC1200566). Spectra were obtained at a laser output
Experimental Section
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10.1002/cssc.201802459
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power of 1 mW (532 nm), and a 0.2 s acquisition time with 900
lines/mm grating (Grating serial number: AJG1200531) in the
Reference
wavenumber range of 50–3500 cm−1
.
[1]
FW Zhang, C Zhao, S Chen, H Li, HQ Yang, XM Zhang, J.
Catal. 2017, 348, 212-222.
Catalytic hydrogenation of nitriles
[2] M. Largeron, K. M. H. Nguyen, Synthesis-Stuttgart. 2018, 49,
241–253.
In a typical run, benzonitrile (1 mmol), methanol (10 mL), the
MC/Ni catalyst (20 mg), 26.5 wt. % aq. NH3 solution (0.2 mL), and a
magnet bar were placed in were charged into 50 mL autoclave. The
autoclave was flushed with nitrogen atmosphere and then tightly
closed. After being sealed, the autoclave was charged with 2.5 bar
H2 at room temperature. The experiment was performed at 60 °C
under magnetic stirring of 1000 rpm. After that the reaction mixture
was cooled in cold water and the gas carefully released. The reaction
mixture was analyzed by GC-MS and GC with n-hexadecane as an
internal standard. The selectivity was calculated from the following
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9352.
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nProduced benzylamine
SelectivityBenzylamine
=
nConverted benzonitrile
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72, 5988-5993.
nProduced N−benzylidenebenzylamine
SelectivityN−benzylidenebenzylamine = 2 ∗
nConverted benzonitrile
[7] R. V. Jagadeesh, K. Murugesan, A. S. Alshammari, H.
nProduced dibenzylamine
Neumann, M. M. Pohl, J. Radnik, M. Beller, Science. 2017
,
SelectivityDibenzylamine = 2 ∗
nConverted benzonitrile
358, 326-332.
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Reductive amination of carbonyl compounds
Synthesis of primary amines was also conducted by the
reductive amination. These procedures on the reductive amination of
carbonyl compounds were almost similar as the hydrogenation of
nitriles. The selectivity of each product was calculated from the
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SelectivityBenzylamine
=
Organometallics. 2017, 36, 4864–4882.
nConverted benzaldehyde
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nProduced N−benzylidenebenzylamine
SelectivityN−benzylidenebenzylamine = 2 ∗
7215
nConverted benzaldehyde
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SelectivityDibenzylamine = 2 ∗
nConverted benzaldehyde
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autosampler and a flame ionization detector. The products were separated
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rate of 20 °C min-1 to 220 °C. Products were identified by the comparison
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The Project was Supported by the Special Fund for Basic
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Keywords: Primary amines; Hydrogenation of nitriles; Reductive
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Entry for the Table of Contents
Nitrogen-doped mesoporous carbon
(MC) supported nickel nanoparticles
demonstrated high catalytic activity for
the hydrogenation of nitriles into
primary amines with high yields
(81.9~99%) under mild reaction
Yangmin Zhang, Hanmin Yang*, Quan
Chi, Zehui Zhang*
Page 1. – Page 10.
Nitrogen-doped carbon supported
nickel nanoparticles: a robust catalyst
to bridge the hydrogenation of nitriles
and the reductive amination of
o
conditions (80 C and 2.5 bar H2). On
the meanwhile, the as-prepared
catalyst could also promote the
reductive amination of carbonyl
compounds for the synthesis of
primary amines at 80 oC and 1 bar H2.
carbonyl
compounds
for
the
synthesis of primary amines
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