MOF-derived Ni?NC catalyst: Synthesis, characterization, and application in one-pot hydrogenation and reductive amination

Article abstract of DOI:10.1039/c9cy00734b

MOF-derived catalysts with nickel nanoparticles (NPs) uniformly embedded in nitrogen doped carbon shells were developed by pyrolysis of nickel-based metal-organic frameworks (MOFs), in which amino groups were introduced to act as the nitrogen source and possible anchoring sites for Ni NPs. The resulting materials were characterized systematically. In the architecture of the catalysts, a core-shell structure was discovered, in which Ni NPs with a diameter of 6-7 nm were surrounded by N doped graphitic layers, indicating that the introduction of amino groups into precursors is beneficial to the dispersion of NPs. One-step hydrogenation and reductive amination (OHRA) is a promising route to produce secondary amines, which avoids tedious separation of intermediates. Accordingly, we applied the prepared composites as catalysts to OHRA of benzaldehyde with nitrobenzene. Among them, the catalyst pyrolyzed at 600 °C for 1.5 h exhibited the best catalytic performance (conversion: >99%, selectivity for N-benzylaniline: 97.96%) and excellent recyclability. N-doping remarkably promotes the dispersion and stability of Ni NPs, thus improving their catalytic activity and selectivity. Furthermore, the Ni-N species as well as synergism of Ni NPs and adjacent pyridinic N may also facilitate the activation of H₂. And its excellent stability and recyclability can be attributed to the core-shell structure. The achieved success in MOF-derived nanocomposites may pave the way for further industrial applications.

Full text of DOI:10.1039/c9cy00734b

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DOI: 10.1039/C9CY00734B
ARTICLE
MOFs-derived Ni@NC catalyst: synthesis, characterization, and
application in one-pot hydrogenation and reductive amination
Jiayi Li, a# Bowei Wang, abc# Yutian Qin,a Qin Tao and Ligong Chen*abc
Received 00th January 20xx,
Accepted 00th January 20xx
MOFs-derived catalysts with nickel nanoparticles (NPs) uniformly embedded in nitrogen doped carbon shells were
developed by pyrolysis of nickel-based metal-organic frameworks (MOFs)，in which amino groups were introduced to act
as the nitrogen source and possible anchoring sites for Ni NPs. The resulting materials were characterized systematically. In
the architecture of the catalysts, the core-shell structure was discovered, in which Ni NPs with the diameter of 6-7 nm were
surrounded by N doped graphitic layers, indicating the introduction of amino groups in precursors is beneficial to the
dispersion of NPs. One-step hydrogenation and reductive amination (OHRA) is a promising route to produce secondary
amines, which avoids tedious separation of intermediates. Accordingly, we applied the prepared composites as catalysts
into OHRA of benzaldehyde with nitrobenzene. Among them, the catalyst pyrolyzed at 600 oC for 1.5 h exhibited the best
catalytic performance (conversion: >99%, selectivity for N-benzylaniline: 97.96%) and excellent recyclability. N-doping
remarkably promotes the dispersion and stability of Ni NPs, so improves their catalytic activity and selectivity. Furthermore,
the Ni-N species as well as synergism of Ni NPs and adjacent pyridinic N may also facilitate the activation of H 2. And its
excellent stability and recyclability can be attributed to the core-shell structure. The achieved success in MOF-derived
nanocomposites may pave the way for further industrial applications.
DOI: 10.1039/x0xx00000x
performance of these materials, which are greatly influenced by the
surface properties of support.13, 14 Generally, small particles endow
the materials better catalytic activities.15 Herein, the design and
preparation of catalysts for OHRA with desired selectivity and good
stability is still a great challenge.
Introduction
Reductive amination is an effective and important method to
produce secondary amines, which are widely applied in the
production of medicine, pesticides and functional materials as
common building blocks in modern chemistry.1 Although one-step
hydrogenation and reductive amination avoids the tedious
separation of intermediates, the traditional strategies with
conventional reductants, such as sodium borohydride and lithium
aluminum hydride, always present serious pollution. Even though
catalytic hydrogenation avoids environmental problems, the
catalysts are generally based on precious metals, such as palladium,2
platinum,3, 4 rhodium5 and gold.6 Unsatisfactorily, the drawbacks of
high cost, low reserves and poor durability greatly limit their
industrial application.7 Catalysts with non-precious metals such as
iron,8 cobalt9 and nickel10, have attracted lots of attention, which
have demonstrated a high efficiency in hydrogenation or reduction
of nitroarenes. Although there is some progress, these catalysts
typically suffer from low selectivity towards secondary amines in
reductive amination.11, 12 It has been reported that the size and
distribution of NPs play an dominant role to the catalytic
In 2013, Beller et. al16 developed a novel heterogeneous Co-N-
C catalyst with core-shell structure by the pyrolysis and it exhibited
outstanding activity for the selective reduction of nitrobenzene. The
doped N species modifies the charge dispersion on the surface of
carbon improving its catalytic performance.17 And in the M-N-C
catalyst, it indicates that the N species may interact with supported
metal forming M-N species as possible active sites. Subsequently,
many attentions have been drawn to M-N-C materials and the
19
applications ranged from oxygen reduction reaction (ORR),18,
hydrogen evolving reaction (HER),20 photocatalysis,21 and
reduction22-26 of organic molecules. Merging rationally-designed
precursor with cheap transition metals is a promising way to develop
active, selective and cheap catalytic materials. Moreover, in order to
eliminate the aggregation of metallic NPs during pyrolysis procedure,
the precursor should be rationally designed. MOF is a reasonable
candidate, in which highly ordered metal ions are isolated by ligands
and firmly confined to the cages.27, 28 The pore and channels formed
in pyrolysis may also hinder the agglomerating of NPs endow them
better stability.29 Furthermore, in view of the high metal content, it
will provide more active sites after successful transformation by
pyrolysis. Recently, some materials derived from MOFs were
reported and successfully applied in some reactions,30, 31 such as
catalytic hydrogenation30-35 and oxidation,13 but most of them need
to be etched by acid36, 37 or the particle sizes are normally large (>10
nm) with uneven distribution.38, 39 So far, only a few applications of
a. School of Chemical Engineering and Technology, Tianjin University, Tianjin
300350, P. R. China.
b. Collaborative Innovation Center of Chemical Science and Engineering (Tianjin),
Tianjin 300072, P. R. China.
c. Tianjin Engineering Research Center of Functional Fine Chemicals, Tianjin, P. R.
China.
# These authors contributed equally to this work.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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the above materials in reductive amination have been reported.40 The transmission electron microscopy (TEM) and high-
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Consequently, further efforts are still expected to develop more resolution transmission electron mDiOcrI:o1s0c.1o0p3y9/C9(CHYR0T07E3M4B)
stable and selective catalysts with superior dispersion derived from observations were carried out on a Tecnai G2 F20 high
suitable MOFs.
resolution analytical electron microscope operating at an
Based on the consideration mentioned above, we developed a acceleration voltage of 200 kV. Scanning electron microscope
facile strategy to obtain novel and robust Ni@NC catalysts with well (SEM) images were obtained by Hitachi S-4800 scanning
dispersed NPs. For one thing, we choose Ni-MOFs with highly electron microscope. Powder X-ray diffraction patterns (XRD)
ordered metal ions fixed in cages as precursor to provide enough were recorded on Panalytical X’Pert Pro using a Cu Kα X-ray
active sites and resist the agglomeration of Ni NPs during pyrolysis. source (40 kV, 40 mA) in the range of 5–80o and operated at a
For another, we modified the Ni-MOFs with amino groups to stabilize scan rate of 4o/min. X-ray photoelectron spectroscopy (XPS) was
NPs and further promote metal dispersion. Meanwhile, the amino performed using X-ray photoelectron spectrometer (Thermo
groups introduced into ligands are designed to dope carbon lattice ESCALAB 250Xi). All of the binding energies were calibrated by
without extra nitrogen source. To explore the structure in detail, the using the adventitious carbon (C1s=284.6 eV) as a reference.
composites were characterized by powder X-ray diffraction (XRD), N2 Elemental analysis (EA) was carried out on a Vario EL cube
adsorption–desorption, elemental analysis (EA), scanning electron elemental analyzer. The inductively coupled plasma optical
microscopy (SEM), transmission electron microscopy (TEM), and X- emission spectrometer (ICP-OES) was used to confirm Ni
ray photoelectron spectroscopy (XPS). All catalysts were applied in content in materials. Nitrogen adsorption-desorption
OHRA of benzaldehyde with nitrobenzene. Furthermore, the scope experiments were carried out on a NOVA 2000e analyzer
of substrates and the stability of the catalyst were also explored.
(Quantachrome, USA) at a liquid nitrogen temperature.
Experimental Section
Results and Discussion
Catalyst preparation
Materials characterization
Preparation of Ni-MOF-NH2. The Ni-MOF-NH2 was prepared as the The required Ni-MOFs was prepared by solvothermal method
precursor of Ni@NC composite.41 Typically, 2-aminoterephthalic acid according to reported literature,41 and the amino groups were
(NH2-H2BDC, 0.27 g, 1.5 mmol) and polyvinylpyrrolidone (PVP, 0.05 g) introduced to modify the precursor. Ni@NC-T-t (T=400-700 oC, t=1-2
were dissolved in N,N-dimethyl- formamide (DMF, 5 mL), and nickel h) and Ni@C-600-1.5 (T=600 oC, t=1.5 h) materials were obtained by
nitrate hexahydrate (0.44 g, 1.5 mmol) was dissolved in deionized the pyrolysis of Ni-MOF-NH2 andNi-MOF, respectively. The details are
water (60 mL), respectively. Then, the DMF solution was slowly added summarized in scheme 1.
into the aqueous solution dropwise under vigorous string at room
temperature. After stirring for 1h, the mixture was transferred into a
100 mL Teflon coated stainless steel autoclave and maintained at 160
oC for 8 h. The resultant green deposits were collected by
centrifugation, washed by DMF and ethanol for 4 times, and then
dried at 80 oC in vacuum overnight. The Ni-MOF was prepared using
p-phthalic acid in the same way.
Preparation of Ni@NC composite. The obtained green powder Ni-
MOF-NH2 was placed in a quartz boat and pyrolysed in a tubular
furnace up to setting temperature (400-700 oC) for a certain time (1-
2 h) under argon atmosphere at a rate of 5 oC min-1. The resulting
product from Ni-MOF-NH2 is denoted as Ni@NC-T-t (T represents the
pyrolysis temperature, t represents the pyrolysis time) and the
resultant sample from Ni-MOF at 600oC for 1.5 h is named as Ni@C-
600-1.5.
Catalytic test
Scheme 1. Preparation of Ni@NC-T-t.
The XRD peaks of Ni-MOF-NH2 and Ni-MOF match well with the
previous literatures, indicating the successful synthesis of Ni-MOF-
NH2 and Ni-MOF (Fig. S1). Fig. 1 presents the XRD patterns of Ni@NC-
T-t and Ni@C-600-1.5. After pyrolysis, the peaks ascribed to MOFs
disappeared and new diffraction peaks at 44.5o, 51.8o and 76.4o were
observed, corresponding to (111), (200) and (220) crystal planes of
metallic Ni, respectively. That manifests the framework was
destroyed and Ni2+ was reduced in pyrolysis. Besides, no diffraction
peak ascribed to NiO was observed in Fig. 1. Comparing the XRD
spectra of the obtained materials pyrolyzed at different temperatures,
only Ni@NC-700-1.5 displays a small diffraction peak at 26.5o, which
The OHRA of carbonyl compounds with nitro compounds was
performed in a 30 mL autoclave. In a typical run, nitrobenzene
(0.5 mmol), benzaldehyde (1 mmol), catalyst (10 mg),
tetrahydrofuran (2 mL) and deionized water (2 mL) were added
into the autoclave. The autoclave filled with 2 MPa of H2 was
o
heated in an oil bath at 100 C for 4 h under magnetic stirring.
After reaction, 1ml ethanol was added into the mixture to make
liquid phase homogeneous. Then the solid-liquid mixture was
filtrated and analyzed by gas chromatography (GC).
Catalyst characterization
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is ascribed to the (002) plane of graphitic carbon, due to its higher The SEM images of Ni-MOF-NH2 (Fig. 2a) and Ni@NC-600-1.5 (Fig. 2b)
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graphitization degree. Moreover, the diffraction peaks of Ni become are displayed in Fig. 2. Obviously, the Ni-MDOOFI-:N1H0.210e3x9h/iCb9itCsYr0e0g7u3l4aBr
sharper with the rise of pyrolysis temperature and time. As calculated rhombic flakes and the thickness is less than 40 nm. Although there
by Debye-Scherrer Equation, the size of Ni NPs in Ni@NC-T-1.5 are is no peak indexed to Ni-MOF-NH2 in XRD patterns, Ni/CN-600-1.5
within a range from 5.5 nm to 11 nm and the Ni@C-600-1.5 displays also keeps the regular shape. However, in comparison with the Ni-
an average diameter of 66 nm (Table S1). In addition, Ni@NC-600-t MOF-NH2, the surface of Ni@NC-600-1.5 is obviously rougher and
showed the similar trend, and the average diameters of Ni NPs there are some mesopores on the surface according to Fig. 2b.
increase from 5.9 nm to 30.9 nm as the pyrolysis time extends.
Besides, the surface morphology of the composites derived from
pyrolysis at different temperatures are evidently different (Fig. S2).
With the rise of processing temperature, the surface of materials
becomes rougher. When the temperature is above 600 oC, there are
apparent mesopores probably ascribed to collapse of skeleton and
gas effusion. And the appearance of spherical particles on the surface
of Ni@NC-700-1.5 is possibly attributed to the aggregation of metal.
Fig. 1. XRD patterns of Ni-based composite (a) Ni@NC-T-1.5, (b)
Ni@NC-600-t, (c) Ni@C-600-1.5.
The physical and textural properties of the samples were
characterized by EA and N2 adsorption-desorption isotherms (Table
S2). The results of EA suggest that Ni@NC-T-1.5 nanomaterials
derived from Ni-MOF-NH2 mainly contains Ni, C, H and N elements,
while no N element is detected in Ni@C-600-1.5 derived from Ni-
MOF. Besides, the pore volumes of Ni@NC-T-1.5 pyrolyzed at 400 oC,
500 oC, 600 oC, 700 oC are 0.44 cm3g-1, 0.30 cm3g-1, 0.28 cm3g-1, 0.23
cm3g-1, respectively, and the BET surface areas are 240.14 m2g-1,
257.97 m2g-1, 213.41 m2g-1, 184.43 m2g-1, correspondingly. It can be
found that the pore volumes of resultants descend with the increase
of pyrolysis temperature due to the collapse of skeleton.
Nevertheless, their specific surface area is characterized by a slight
rise first and then a significant decrease. This phenomenon may be
caused by the damage of the 3D microporous structure and the
emerging pores formed by effusion of gas during the pyrolysis. It’s
worth noting that the pore volume and BET surface area of Ni@C-
600-1.5 are only 0.04 cm3g-1 and 36.61 m2g-1 respectively, much
smaller than the Ni@NC composites. Since there is no amino group
in the precursor Ni-MOF, Ni species may exhibit poor stability and
aggregate easily in pyrolysis. Therefore, the extremely aggregated Ni
and the serious framework collapse may be the reason for the poor
pore volume and low specific surface area.
Fig. 3. (a) TEM images with size distribution and (b) HRTEM of
Ni@NC-600-1.5; TEM images of (c)Ni@NC-500-1.5 (d)Ni@NC-600-
1.5 (e)Ni@NC-700-1.5 (f)Ni@C-600-1.5; (g) HAADF-STEM images
and the corresponding elemental mappings of Ni@NC-600-1.5.
Fig. 2. SEM images of (a) Ni-MOF-NH2 and (b) Ni@NC-600-1.5.
This journal is © The Royal Society of Chemistry 20xx
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To further understand the materials structure and Ni distribution, O, respectively. (Fig. 4c) The peak of C-N/C-O further reveals the
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DOI: 10.1039/C9CY00734B
TEM characterization was employed. The TEM images of Ni@NC- inclusion of nitrogen atoms into the carbon lattice.
600-1.5 in Fig. 3a clearly illustrate the uniform distribution of metal
particles in regular rhombic flakes. Most NPs exhibit small size with
typical diameter of 6-7 nm, according to the histogram of particle size
distribution presented in Fig. 3a. This result agrees well with the
calculated diameters by Debye-Scherrer Equation. Moreover, as
shown in Fig. S3 (a-c), the ascending trend of NPs size in materials
with increasing pyrolysis temperature is also corroborated by the
results from XRD patterns. The core-shell structure of composite is
further revealed by HRTEM images in Fig. 3b, in which Ni NPs are
surrounded by graphitic layers. The number of carbon layers in
Ni@NC-500-1.5 is approximately 3-4, smaller than Ni@NC-600-1.5
and Ni@NC-700-1.5 with majority of carbon shell composed of 5-8
carbon layers (Fig. 3c-e). Meanwhile, the insert figure in Fig. 3b allows
to measure particle lattice fringe of 0.201 nm and 0.342 nm, which
are attributed to the (111) plane of metallic Ni and (002) plane of
carbon shell, respectively. Moreover, the elemental mapping is
carried out to explore the distribution of elements in the material.
According to Fig. 3g, nickel exists in the form of NPs, which are
surrounded with N-doped carbon layers.
In addition, the TEM images of Ni@C-600-1.5 (Fig. S3d) show inferior
morphology with uneven distribution of carbon and severe
aggregation of metal particles compared with Ni@NC-T-1.5 materials.
The size of particles in Ni@C-600-1.5 is within a range from hundreds
of nanometers to several nanometers and particles with diameters
less than 10 nm are exiguous. Even worse, there are only a few
particles presenting complete core-shell structure, which may cause
oxidization of Ni NPs by atmosphere due to the lack of protection
from graphitic layers (Fig. 3f). Moreover, the thickness of outer
carbon shells in Ni@C-600-1.5 is also complicated. The thick parts
present 9-10 layers while the thin ones show 2-3 layers. In
comparison to Ni@NC-600-1.5, Ni@C-600-1.5 evidently display large
NPs and uneven carbon shells, revealing that the precursors can
significantly affect the morphology and properties during pyrolysis.
As there is no amino group in Ni-MOF, no extra anchoring site is
provided to stabilize Ni particles. When it comes to Ni-MOF-NH2, the
amino group interacts with Ni species, preventing it from gathering
at high temperature. More than that, the introduction of amino
groups can endow doping nitrogen species to the carbon support in
the pyrolysis, which also make contribution to impede further
aggregation.
Fig. 4. XPS spectra of Ni@NC-T-1.5 series composite. (a) Ni 2p, (b) O
1s, (c) C 1s and (d) N 1s regions of Ni@NC-600-1.5; N 1s regions of
(e) Ni@NC-500-1.5 and (f) Ni@NC-700-1.5.
The XPS spectrum of N1s (Fig. 4d) shows the presence of Ni-N (397.6
eV), pyridinic N (398.6 eV), pyrrolic N (400.0 eV), graphitic N (401.0
eV), and N-O (403.3 eV). The relative atomic ratios of each type of
nitrogen species are summarized to further understand the N-doped
mode in these materials (Table S3). Their relative contents in Ni@NC-
600-1.5 are 5.79%, 37.68%, 8.98%, 44.04% and 3.51% respectively.
Noticeably, with the increase of temperature from 500 to 700oC, the
proportion of graphitic nitrogen shows a rise from 33.22% to 50.28%,
indicating that the N doping mode can be altered by pyrolysis
temperature.42, 43 Meanwhile, the content of Ni-N species in Ni@NC-
500-1.5 (0%, Fig. 4d) and Ni@NC-700-1.5 (2.86%, Fig. 4e) is obviously
less than that in Ni@NC-600-1.5 (5.79%, Fig. 4f), proving more
interactions between Ni and N in Ni@NC-600-1.5. During pyrolysis
process, Ni originally coordinated with oxygen of terephthalic acid in
Ni-MOF-NH2 is gradually liberated and reduced, tending to
agglomerate during pyrolysis. At a certain temperature, the Ni-N
species formed due to the interaction of N species and Ni NPs. The
doped N atoms, especially pyridinic N atoms, form defective sites in
outer graphitic carbon layers and then interact with metallic Ni NPs,
which can not only modulate the charge distribution but also act as
anchoring sites for Ni NPs to resist aggregation.44 Moreover, the
decrease in Ni-N content of Ni@NC-700-1.5 implies the Ni-N species
was not stable and may decompose at higher pyrolysis temperature.
Additionally, combined with the surface element contents of
materials analyzed by XPS (Table S4), the content of pyridinic N
atoms is the highest inNi@NC-600-1.5 and the lowest in Ni@NC-700-
1.5, which are easy to interact with Ni NPs.45 Hence, more Ni-N sites
can be provided on the surface of Ni@NC-600-1.5.
XPS was measured to gain information on the surface state of
Ni@NC-600-1.5 and the analytic results are shown in Fig. 4. The
global spectrum shows that the composite consists of Ni, C, O and N
elements, which is consistent with the result of EA (Fig. S4). The high-
resolution XPS spectrum of Ni 2p (Fig. 4a) proves the existence of Ni0
Ⅱ
Ⅱ
and Ni at 852.1 eV and 854.2 eV respectively. The peak of Ni
illustrates the interaction between Ni and oxygen or nitrogen.
However, there are no diffraction peaks corresponding to NiO or
Ni(OH)2 according to the XRD pattern (Fig. 1a). Simultaneously, the
O1s spectrogram (Fig. 4b) shows the presence of C-O (529 eV), -
OH(531.2 eV) and C=O(533.3 eV), while there is no peak assigned to
Ⅱ
Ni-O. Hence, the Ni may be attributed to the interaction between
Ni and nitrogen. The C 1s spectrogram can be fitted with two peaks
at about 284.7 eV and 285.9 eV, which are ascribed to C-C and C-N/C-
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Catalytic properties for one-pot hydrogenation and reductive content of pyridinic N to some extent. The similar proposal have been
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amination
reported in studies of cobalt-based materialDsO.47I: B10e.s10id3e9s/,Ci9tCisY0w0i7d3e4lBy
The prepared catalysts were evaluated for OHRA of benzaldehyde recognized that the doped N can alter the electron distribution of
with nitrobenzene (Table 1). Three products, including aniline (1), N- adjacent metal cluster and promote the interfacial reaction by a
benzylideneaniline (2), and N-benzylaniline (3), were detected in the synergetic effect.48, 49 And the active H atoms on the surface of metal
catalytical system. In Table 1, the conversions of nitrobenzene over may be transferred to carbon matrix in forms of C-H or N-H.50
all Ni@NC catalysts were nearly 100% (Table 1, Entry 1-6). However, Subsequently, there is a distinct decline in the catalytic activity of
the Ni@NC-600-1.5 showed the highest catalytic performance, Ni@NC-700-1.5 (Table 1, Entry 4) primarily due to its low surface area
achieving 97.96% selectivity for N-benzylaniline (Table 1, Entry 3). and agglomeration of Ni NPs, indicating that the well dispersed Ni
This is a promising candidate, to our knowledge, that possesses the NPs in small size play a critical role in catalytic activation. Moreover,
extremely high catalytic performance compared with the reported the conversion of Ni@C-600-1.5 is 40.64% and the yield of N-
catalysts in the OHRA under such mild conditions (Table S5).
benzylaniline is only 11.66%, further confirming the importance of
the particle size and N-doped carbon shells in catalysts. N atoms
acted as defect sites in carbon lattice provide lone pair electrons to
Table 1. Comparisons of catalytic performance for the one-pot
hydrogenation and reductive amination over various catalysts.
coordinate with Ni, anchoring the Ni NPs and promoting the metal-
52
support interactions.51,
The absence of amino groups in its
precursor Ni-MOF may be responsible for the serious agglomeration
of Ni NPs in Ni@C-600-1.5. And it has been proposed that N-doping
can lower the dissociation energy of H-H derived from DFT
calculations.53 Meanwhile, its fragmentary core-shell structure fails
to prevent further aggregation and the poor porosity possibly
impedes mass transfer.
Sel[b ]( %)
Conv[b]
Entry[a] Catalyst
(%)
1
2
3
Moreover, no obvious conversion was detected in the presence
of precursor Ni-MOF-NH2 or Ni-MOF (Table 1, Entry 8-9) indicating no
active species in the parent MOFs. The series of materials reduced by
NaBH4 present invisible activity (Table 1, Entry 11-13). Without the
N-doped carbon shells, the metallic Ni may be oxidized or
agglomerated, suggesting the necessity of core-shell structure. This
structure can protect Ni NPs efficiently and endow material with
unique metal-support interaction, which improve the ability to
activate H2 by reducing the H2 dissociation energy.54
1
Ni@NC-400-1.5
Ni@NC-500-1.5
Ni@NC-600-1.5
Ni@NC-700-1.5
Ni@NC-600-1
Ni@NC-600-2
Ni@C-600-1.5
Ni-MOF-NH2
Ni-MOF
>99
7.12
5.60
0.98
2.04
34.4
25.56
1.41
15.85
87.28
89.73
97.96
46.26
51.40
96.45
28.70
2
>99
9.93
3
>99
<0.01
19.34
23.04
2.13
4
>99
5
>99
6
>99
7
40.64
55.44
8
-
-
-
-
-
-
9
10[c]
11[c]
12[c]
13[c]
Ni/MOF-NH2
Ni/MOF
Ni/SiO2
Ni/AC
[a]
Reaction conditions: catalyst (10 mg), nitrobenzene (0.5 mmol),
benzaldehyde (1 mmol), tetrahydrofuran (2 mL) and deionized water (2
mL). 100 oC, 2 MPa H2 ,4 h.
[b] Determined by GC using n-dodecane as an internal standard.
[c] The catalysts were prepared by soaking the support (Ni-MOF-NH2, Ni-
MOF, SiO2, active carbon) in solution of Ni(NO3)2 and then reducing with
NaBH4.
Fig. 5. Varying effect on the one-pot hydrogenation and reductive
amination:(a) Pressure of H2; (b) Temperature.
Notably, the yield of N-benzylaniline (Table 1, Entry 1-3) is
The effect of pressure and temperature on conversion and
selectivity were studied. The conversion of nitrobenzene and yield of
N-benzylaniline (3) are improved with the increase of pressure and
temperature as shown in Fig. 5. Moreover, when H2 pressure exceeds
2 MPa, the conversion and yield flatten out (Fig. 5a). Besides, the
influence of temperature in the conversion and yield is more
prominent (Fig. 5b). And there is a sudden change in yield of N-
o
promoted with the increase of pyrolysis temperature below 600 C.
As generally believed, metallic NPs produce active H atoms by
facilitating homolytic dissociation of H2 for subsequent steps.
However, the Ni@NC-500-1.5 exhibits poorer performance than
Ni@NC-600-1.5 despite their similar metal content and smaller
particle size of Ni@NC-500-1.5. As shown in XPS, the Ni@NC-600-1.5
possesses the most surface content of Ni-N and pyridine N.
Accordingly, we speculate that the Ni-N species as well as the
synergism of Ni NPs and their adjacent pyridine N play a momentous
role in facilitating the heterolytic dissociation of hydrogen. It was
reported that the metal-nitrogen (M-N) may serve as high active sites
for catalytic hydrogenation by facilitating heterolytic cleavage of
hydrogen.46 And the Ni NPs may cooperate with pyridinic N atoms
nearby to activate H2, thus its catalytic activity is related to the
o
o
benzylaniline from 90 C to 100 C, where the yield of secondary
amine rose from 0 to 97.96%. As the hydrogenation of the C=N bond
in N-benzylideneaniline is much harder, there is higher demand for
reaction conditions. Consequently, the reduction of Schiff base is the
rate controlling step in OHRA. Additionally, the reaction of aniline (1)
and benzaldehyde to produce N-benzylideneaniline (2) is reversible,
so its accumulation inhibits the reaction proceed. Thus, when the
temperature meets the demand of rate-controlled reaction, there is
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Journal Name
a mutation in both the conversion and the yield of N-benzylaniline.
Under the optimal conditions for OHRA, the substrate scope of
the developed catalytic method was studied and summarized inTable
2. Firstly, the reaction scope was explored employing nitro
compounds as substrates. Despite the different electronic nature and
position of the substituent groups, the nitro compounds were
reduced smoothly and preferable yields of secondary amines were
obtained (Table 2, Entry 1-7). Although there was slight
dehalogenation, the reaction of nitro compounds with halogen
substituents exhibited favorable yields of corresponding secondary
amines (Table 2, Entry 1-2). Moreover, the aliphatic nitro compound
1-nitrocyclohexane showed excellent yield in our catalytic method,
proving the substrate scope is not limited to nitro aromatic
compounds (Table 2, Entry 8). And then, the substrate scope of
carbonyl compounds was explored. (Table 2, Entry 9-17). The
electronic effects of substituents in aromatic carbonyl compounds
have no significant effect on the conversion and selectivity (Table 2,
Entry 11-12). Meanwhile, it illustrates the minimal impact of steric
effect in aromatic carbonyl compounds on the applicability of the
catalytic method (Table 2, Entry 13-15). Despite slight
dehalogenation (Table 2, Entry 9-10), there are still favorable yields
of secondary amines obtained with halogen group substituted
aromatic carbonyl compounds. In addition, the aliphatic compounds
cyclohexanecarboxaldehyde and isobutyraldehyde also reacted
smoothly to produce corresponding secondary amines in high yields,
proving that the aliphatic carbonyl compounds are also effective
substrates (Table 2, Entry 16-17). These results suggest the catalyst
developed exhibits excellent yield with wild substrate scope under
mild conditions and makes great contributions to producing
secondary amines simply. Furthermore, the catalyst Ni@NC-600-1.5
also showed preeminent catalytic activity and selectivity in the
synthesis of quinoline from o-nitrocinnamic aldehyde, indicating its
promising potential in other hydrogenation reactions.
View Article Online
DOI: 10.1039/C9CY00734B
>99
96.18
6
>99
>99
97.99
91.70
7
8
>99
>99
>99
94.06
88.95
97.51
9
10
11
>99
>99
92.67
97.96
12
13
>99
>99
>99
>99
>99
97.34
95.63
99.43
14
15
16
17
18
The main text of the article should appear here with headings
as appropriate.
89.7
>99
Table 2. One-pot hydrogenation and reductive amination with
substituted nitro and carbonyl compounds.
Nitro
compounds
Carbonyl
compounds
Conv.[b]
(%)
Entry[a]
Sel. [b] (%)
92.21
[a] Reaction conditions: Ni@NC-600-1.5 (10 mg), nitro compound
(0.5 mmol), carbonyl compound (1 mmol), tetrahydrofuran (2 mL)
and deionized water (2 mL). 100 oC, 2 MPa H2 ,4 h.
>99
>99
>99
1
[b] Determined by GC using n-dodecane as an internal standard.
Finally, the recycling test was carried out. The magnetic
hysteresis loop shown in Fig. S7 confirms its magnetism, which allows
the Ni@NC catalysts efficiently recovered from reaction mixture and
recycled by external magnet. Under the identical condition, the yield
90.07
98.60
2
3
4
>99
>99
97.54
99.76
5
6 | J. Name., 2012, 00, 1-3
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Journal Name
ARTICLE
4.
5.
6.
7.
F. G. Cirujano, A. Leyva-Perez, A. Corma and F. Xamena,
of secondary amine was upto 94.74% after five times as shown in Fig.
6, illustrating the outstanding recyclability. The XRD patterns (Fig. S8)
of recycled catalyst displayed the same diffraction patterns as fresh
catalyst, confirming good stability of Ni@NC-600-1.5 catalyst owing
to the core-shell structure in which N-doped carbon shell protects Ni
NPs from leaching and aggregation.55, 56
View Article Online
ChemCatChem, 2013, 5, 538-549.
DOI: 10.1039/C9CY00734B
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Fig. 6. Recycle experiment for one-pot hydrogenation and reductive
amination using Ni@NC-600-1.5.
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In summary, novel Ni@NC materials were successfully
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carbon shells. And the particle size and thickness of carbon shells in
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Conflicts of interest
There are no conflicts to declare.
Nat. Commun., 2017, 8, 13.
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This work was supported by the National Natural Science
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