Atomically dispersed Ni as the active site towards selective hydrogenation of nitroarenes

Article abstract of DOI:10.1039/c8gc03664k

Rational design of heterogeneous non-noble metal catalysts as highly efficient and selective catalysts for hydrogenation of nitroarenes with hydrogen as the reducing agent is currently a great challenge, which has attracted a great deal of attention. Herein, a new strategy for achieving atomic dispersion of Ni atoms on nitrogen-doped porous carbon (Ni-N-C) with a specific surface area of up to 810 m² g^-1 and nickel loading as high as 4.4 wt% is developed, yielding high activity, chemoselectivity, and reusability of catalysts in the hydrogenation of nitroarenes using hydrogen as the reductant with a turnover of number (TON) value of 84 and a turnover of frequency (TOF) value of 8.4 h^-1 for the first time. The Ni single atoms anchored on N-doped porous carbon by binding with nitrogen/carbon have been proved to be the active sites. Importantly, the Ni-N₃ active species is found to contribute more activity compared with Ni-N₂ and Ni-N₄. Density functional theory (DFT) calculations also reveal that the Ni-N₃ structure exhibits the highest activity according to the lowest adsorption energy and the longest elongation N-O bonds of nitrobenzene, which originated from the induced charge transfer. This work opens a new route for rational design and accurate modulation of nanostructured organic molecular transformation catalysts at the atomic scale.

Full text of DOI:10.1039/c8gc03664k

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DOI: 10.1039/C8GC03664K
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ARTICLE
Atomically dispersed Ni as the active site towards selective
hydrogenation of nitroarenes
Received 00th January 20xx,
Accepted 00th January 20xx
Fan Yang,†a Minjian Wang,†a Wei Liu,b Bin Yang,a Ying Wang,*c Jun Luo,*b Yushu Tang,a Liqiang
Hou,a Yun Li,a Zihui Li,a Bing Zhang,a Wang Yanga and Yongfeng Li*a
DOI: 10.1039/x0xx00000x
Rational design of heterogeneous non-noble metal catalysts as highly efficient and selective catalysts for hydrogenation of
nitroarenes with hydrogen as reducing agent is currently a great challenge, which has attracted a great deal of attention.
Herein, a new strategy for achieving atomic dispersion of Ni atoms on nitrogen-doped porous carbon (Ni-N-C) with a
specific surface area of up to 810 m2g-1 and nickel loading as high as 4.4 wt% are developed, yielding the high activity,
chemoselectivity, and reusability catalysts in hydrogenation of nitroarenes using hydrogen as reductant with turnover of
number (TON) value (84) and turnover of frequency (TOF) value (8.4 h-1) for the first time. The Ni single atoms anchored on
N-doped porous carbon by binding with nitrogen/carbon have been proven to be the active sites. Importantly, the Ni-N3
active specie is found to contribute more activity compared with Ni-N2 and Ni-N4. Density functional theory (DFT)
calculation also reveals that the Ni-N3 structure exhibits the highest activity according to the lowest adsorption energy and
the longest elongation N-O bonds of nitrobenzene, originated from the induced charge transfer. This work opens a new
route for rational design and accurate modulation of nanostructured organic molecular transformation catalysts at the
atomic scale.
www.rsc.org/
In recent years, many non-noble metal catalysis systems for
nitroarenes reduction reaction, such as Fe2O3, Ni-NiFeO4, Co-
Co3O4, carbon materials, and so on,5 have been developed.
1. Introduction
Synthesis of aniline derivatives have attracted much attention
owing to its the important intermediates for the manufacture of
pharmaceuticals, agrochemicals, pigments dyes and fine
chemicals.1 The traditional method is reduction of the
corresponding nitroarenes with stoichiometric reducing agents,
such as sulfides, Fe, and Zn.2 Such production processes show
low conversion rate and produce large amounts of waste acids
and residues. Although noble metal catalysts, such as Pd, Au,
and Ru, have been used in hydrogenation of nitroarenes to
synthesize functionalized anilines due to their extraordinary
activities,3 their high cost greatly limits their widespread
application, and it is a great challenge for chemoselectivity
reduction of nitroarenes bearing reducible groups (C-Br, C=O,
C=C, and so on).4 Therefore, it is necessary to develop the high
active and chemoselective non-noble metal catalysts.
Among the above catalysts, great efforts have been devoted to
studying the heteroatom-doped carbon materials due to their
cheap, stable, biocompatible, environment-friendly and readily
available properties.5d Although N-doped graphene, carbon
nanotubes (CNTs) and N-doped CNTs have been used in
reduction of nitroarenes, the strong reduction agents, such as
NaBH4 and hydrazine hydrate, are inevitably used.5e,6 It
remains a challenge for selective hydrogenation of nitroarenes
in using the cost-effective H2 by designing heteroatom doped
carbon materials. Based on these considerations, employing N-
doped carbon as dispersion for abundant no-noble metal
catalytic active site is the promising candidate to enhance the
catalytic activity. The M-N-C (M refers to transition metal)
single-atom catalysts (SACs) have emerged as a new frontier in
the field of heterogeneous catalysis due to their remarkable
catalytic performances and maximized atom utilization.7 The
M-N-C SACs have exhibited superior activities in CO2
reduction, oxygen reduction reaction (ORR), hydrogen
evolution reaction (HER) and other important reactions.8 To
date, the M-N-C SACs used in hydrogenation of aromatic nitro
compounds have never been reported yet. Moreover, the
structure of activity site in M-N-C SACs for such reaction is
ambiguous, thereby hindering the further application of these
catalysts in organic molecular transformation reactions.
a. State Key Laboratory of Heavy oil Processing, China University of Petroleum
(Beijing), Beijing 102249, China. E-mail: yfli@cup.edu.cn.
b. Center for Electron Microscopy and Tianjin Key Lab of Advanced Functional
Porous Materials, Institute for New Energy Materials & Low-Carbon Technologies,
School of Materials Science and Engineering, Tianjin University of Technology,
Tianjin 300384, China. E-mail: jluo@tjut.edu.cn.
c. State Key Laboratory of Rare Earth Resource Utilization Changchun Institute of
Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China. E-
mail: ywang_2012@ciac.ac.cn
Herein, we propose a strategy to facilitate construction of an
effective catalysis system based on atomically dispersed Ni on
† These authors contributed equally.
Electronic
Supplementary
Information
(ESI)
available:
See
DOI: 10.1039/x0xx00000x
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N-doped porous carbon (Ni-N-C) for hydrogenation of MgO) was washed by 1 mol/L hydrochloric acid for 1 h and 1
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o
DOI: 10.1039/C8GC03664K
nitroarenes to arylamine using H2 as reduction agent. As a mol/L H2SO4 at 80 C for 2 h, followed by thoroughly washing
result, a catalyst with the Ni loading of up to 4.4 wt%, a huge with deionized water and ethanol. For comparison, Nitrogen-
specific surface area (810 m2g-1) and lamellar hexagonal porous doped porous carbon (labeled as N-C) with the same procedure
structure is derived from the pyrolysis of a mixture of a was prepared except for the absence of NiCl2·6H2O.
coordinatively saturated NiCl2 with 1,10-phenanthroline 2.4 Synthesis of Ni-NC Catalysts
organometallic compound and inexpensive porous MgO
Porous carbon was prepared according to the literature
powder at 700 oC. It is denoted as Ni-N-C-700. Extended X-ray
previously reported by our research group.10 Firstly, 5 g MgO
absorption fine structure (EXAFS) spectra illustrate that such a
template and 3 g TPM were first mixed together in ethanol
facile method is powerful to tune the coordination numbers of
under magnetic stirring at room temperature. The obtained
Ni with N of the Ni-N-C catalysts through controlling the
calcination temperature without immensely influencing the
morphology. The Ni-N-C-700 catalyst also shows good
was obtained by removed the MgO template with the
functionalized group tolerance in contrast to noble metal
catalysts, high chemoselectivity in the presence of reducible
material with mixed acid of sulfuric acid and nitric acid. To
groups, excellent stability with no significant decrease in
activity after 5th consecutive runs, which exhibits superior
carbon (Ni-NC), 200 mg of oxidized porous carbon material
performance than those of previously reported no-noble metal
catalysts under mild conditions with molecular hydrogen as
addition of 100 mg of NiCl2·6H2O. After ethanol was removed
reductant. The control experiments have been prove that Ni
single atoms anchored on N-doped porous carbon by binding
mg of melamine into a homogeneous mixture, followed by a
with nitrogen/carbon is the active sites. Density function theory
(DFT) caculation reveals the sequence of Ni-N1, Ni-N2, Ni-N3,
The Ni-NC was obtained by cooling to room temperature.
and Ni-N4 activities in current hydrogenation reaction. Our
o
MgO/TPM composite was pyrolyzed at 700 C for 2 h under
argon. After cooling to room temperature, the porous carbon
hydrochloric acid. Then, oxidation treatment of porous carbon
prepare metallic Ni-nanoparticle-decoratednitrogen-doped
was first dispersed in ethanol by ultrasonication, followed by
by rotary evaporators, the dried product was ground with 500
o
pyrolyzed process at 700 C for 2 h under argon atmosphere.
2.5 Catalytic hydrogenation of nitrobenzene
strategy may open up an effective routine for general
controllable construction of atomic-scale M-N-C active sites for
stainless steel reactor equipped with a magnetic stirred.
organic molecular transformation reaction.
The catalytic hydrogenations were carried out in a 30 mL
Typically, 0.25 mmol nitroarenes, 5 mL ethanol and an
appropriate amount of catalyst were charged into the reactor.
The reactor was flushed three times with H2 and N2 before the
required H2 pressure was introduced. The reactors were then
placed into an oil bath, heated to desired temperature and the
2. Materials and methods
2.1 Materials
All the chemicals are analytical grade and without further
purification. Aromatic nitro compounds, 1,10-phenanthroline,
triphenylmethane (TPM), (J&K Scientific), MgO powder
(Sinopharm Chemical Reagent Beijing Co., Ltd), silica gel
(Alfa Aesar, Johnson Matthey Company), HCl, NiCl2·6H2O,
H2SO4, (Beijing Chemical Works).
reactions were stirred for 10 h. After completion of the reaction,
the mixture was cooled to room temperature. Filtered the
catalyst, the organic phase was concentrated, and the residue
was purified by column chromatography (petroleum ether/ethyl
acetate mixture) to give the isolated yield.
2.6 Characterization
2.2 Preparation of MgO template
The morphology and structure of the samples were
characterized using a Hitachi SU8010 scanning electron
microscope (SEM) operated at 3.0 kV and JEM-2100
transmission electron microscope (TEM) operated at 120.0 kV.
The TEM images and SAED patterns were taken on JEM-2100
operated at 120.0 kV. The HAADF-STEM images and EDS
maps were taken on an FEI microscope of Titan Cubed Themis
G2 300 operated at 200.0 kV. X-ray photoelectron spectroscopy
(XPS) measurements was carried out on Thermo Fisher K-
Alpha American with an Al Ka X-ray source. Powder X-ray
diffraction patterns of samples were recorded on Bruker D8
Advance Germany with Cu Kα radiation. BET specific surface
area, pore radius and volume are analyzed with N2
adsorption/desorption isotherms at 77 K on a SSA4200
instrument (Beijing Builder Electronic Technology Co., Ltd.).
The Ni concentration of the sample was conducted on the
Inductively Coupled Plasma Optical Emission Spectrometry
(ICP-OES Optima 7300V). Raman spectra were recorded on
HORIBA Xplora PLUS Raman microscope system with Ar
Porous MgO layers were prepared according to the
previously reported literature.9 The MgO powder (30 g) was
mixed with deionized water (300 mL) accompanied with
ultrasonic agitation. The mixture was boiled for 48 hours in a
reflux apparatus, and then the white solid was filtered and dried
o
at 80 C overnight. The obtained bulk was then smashed into
fine powder and heated at 500 oC for 2 h.
2.3 Synthesis of atomically nickel-dispersed Catalysts
The procedure for the preparation of the catalysts was
described as follows: 1 mmol NiCl2·6H2O and 3 mmol 1,10-
phenanthroline were dissolved in 100 ml ethanol for 60 min
under constant stirring. Then MgO template (1 g) was added
and the mixture was stirred overnight. After ethanol was
removed by rotary evaporators, the green solid was ground into
fine powder and pyrolyzed at 600 oC, 700 oC and 800 oC for 2 h
under argon. The samples prepared were appointed as Ni-N-C-
600, Ni-N-C-700 and Ni-N-C-800 respectively. After cooled to
room temperature, obtained black powder (labeled as Ni-N-C-
2 | J. Name., 2012, 00, 1-3
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laser excitation at 532 nm laser. The X-ray absorption fine atomically dispersed Ni sites may exist in VietwheArticcleaOrbnloinne
structure spectra were performed at the beamline 1W1B station framework, which are further confiDrmOIe:d10.1b03y9/Ca8bGeCrr0a3t6i6o4nK-
of the Beijing Synchrotron Radiation Facility (BSRF), China. corrected high-angle annular dark field scanning transmission
The energy of the storage ring electron beam was 2.5 GeV with electron microscopy (HAADF-STEM) images (Fig. 1e and f).
a
ring current of ~250 mA. The hard X-ray was The monodispersed isolated bright dots in the carbon matrix are
monochromatized with Si(111) double-crystal monochromator corresponding to single Ni atoms, because Ni is heavier than N
and the detuning was done by 30% to remove harmonics. The and C. Meanwhile, energy-dispersive X-ray spectroscopy (EDS)
data collection was carried out in transmission mode using mapping with STEM shows that Ni and N are uniformly
ionization chamber for Ni foil, NiPc, and in fluorescence distributed in the entire architecture (Fig. 1g). Therefore, it is
excitation mode using a Lytle detector for Ni-N-C materials. confirmed that atomically dispersed Ni sites on N-doped porous
All spectra were collected in ambient conditions. The acquired carbon (NC) are successfully obtained. For comparison, Ni-N-
EXAFS data were processed and fitted according to the C-600 and Ni-N-C-800 samples were synthesized by the same
o
o
standard procedures using the Athena and Artemis procedure with 600 C and 800 C, and the NC-supported Ni
implemented in the IFEFFIT software packages.
2.7 Computational details
nanoparticles (Ni-NC) were also prepared. These control
samples have the same morphologies as Ni-N-C-700 (Fig. S2,
S3 and S4). Ni-N-C-600 contains only atomically dispersed Ni
sites and no nanoparticles exist in it (Fig. S2), which illustrates
that the Ni nanoparticles (Ni NPs) can be efficiently removed
by acid leaching. But Ni-N-C-800 contains not only atomically
dispersed Ni sites but also Ni NPs (Fig. S3). The reason for Ni-
N-C-800 contain Ni NPs whereas Ni-N-C-600 and Ni-N-C-700
inexistence Ni NPs is due to that high crystallinity carbon layer
All the electronic structure and energy calculations were
performed by the spin-polarized density functional theory (DFT)
by using Vienna ab initio simulation package (VASP).11 PAW
potentials were used to describe ion cores and valence electrons
interactions.12 The generalized gradient approximation (GGA)
with the Perdew-Burke-Ernzerhof13 was selected to describe the
exchange-correlation functional. A kinetic energy cut off of 400
eV was used with a plane-wave basis set. The integration of the
Brillouin zone was conducted using a 1×1×1 Monkhorst-Pack
grid.14 The convergences of the force and the total energy were
set as 0.05 eV/Å and 1.0×10-5 eV/atom, respectively.
o
on the surface of some Ni NPs is formed at 800 C, which can
prevent the acid leaching from washing away the Ni NPs.15 Fig.
S4 shows the Ni nanoparticles are dispersed on the surface of
Ni-NC.
The typical Ni-Nx (x=1-4) substrates were established
according to the previous studies based on 5 × 5 graphene. All
atoms were allowed to relax. vdW interaction was employed in
our calculations by a semiempirical DFT-D2 force-field
approach.
To study the stability of nitrobenzene and H2 on Ni-Nx
surface, the adsorption energy was defined as,
Eads=Esubstrate+n–En–Esubstrate
Eads(co-adsorption)=Esubstrate+H2+n–EH2–En–Esubstrate
(1)
(2)
where Esubstrate+n, Esubstrate+H2+n, En, EH2 and Esubstrate correspond
to the total energies of substrate and nitrobenzene molecule,
substrate and nitrobenzene and H2 molecule, a gas phase
nitrobenzene, a gas phase H2 molecule and an isolated substrate,
respectively. A negative value indicates an exothermic
chemisorption.
3. Results and discussion
3.1 Characterizations of Ni-N-C catalysts
Fig. 1 (a) Schematic illustration for synthesis of Ni-N-C catalysts. (b) SEM image, (c) TEM
image (inset shows the corresponding SAED pattern), and (d) high-magnification TEM
image of Ni-N-C-700. (e), (f) Aberration corrected HAADF-STEM images of Ni-N-C-700,
in which single Ni atoms are clearly seen. Some of them are highlighted by red circles.
(g) HAADF image and its corresponding EDS maps revealing that the Ni and the N
elements are homogeneously distributed in Ni-N-C-700.
The single Ni atom catalyst was achieved by pyrolysis of
coordinatively saturated NiCl2 with 1,10-phenanthroline
dispersed on the surface of hexangular lamella porous MgO at
o
700 C under Ar atmosphere and acid leaching (Fig. 1a). The
SEM and TEM images of Ni-N-C-700 catalyst (Fig. 1b and c)
show laminated hexagonal appearance, which is derived from
the MgO template (Fig. S1a and b). Fig. 1d shows that
abundant mesopores are located on the layers of the carbon
sheets. Notably, no metal particles can be observed, and the
sample has a poor crystallinity as proven by the selected-area
electron diffraction (SAED) pattern. These findings suggest that
The as-synthesized catalysts are also studied by X-ray
diffraction (XRD), Raman spectroscopy, and Brunauer-
Emmett-Teller (BET) measurement. As shown in Fig. 2a, only
the (002) and (004) planes of carbon are observed at 26.2o and
44.0o in Ni-N-C-600 and Ni-N-C-700 samples.7e,8d In contrast,
the metallic Ni diffraction peaks are distinctly observed in Ni-
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N-C-800 and Ni-NC. These results are consistent with the TEM results, as summarized in Table S2. Fig. 2f and FVigie.wSAr7tibclesOhnolinwe
DOI: 10.1039/C8GC03664K
and SAED results. The BET adsorption-desorption isotherms that the high-resolution XPS N1s spectrum deconvoluted into
also demonstrate that the Ni-N-C-600, Ni-N-C-700, and Ni-N- five nitrogen configurations, originating from pyridinic-N
C-800 catalysts possess a mesoporous structure with specific (398.5 ± 0.2 eV), Ni-N (399.4 ± 0.2 eV), pyrrolic-N (400.6
surface area (SSA) of 750, 810 and 790 m2g-1, respectively (Fig. ±0.2 eV), graphitic-N (401.3 ± 0.2 eV) and N-oxidized (> 403
S5). Notably, the 3D porous structure of catalysts with high eV), respectively.20 The presence of Ni-N suggests that these
SSA not only is beneficial to the adsorption reactant and mass nitrogen atoms bind to Ni atoms directly, and the binding
transfer but also gives accessibility to the active sites for the energy of Ni-N is similar to that of in NiPc (Fig. S7c).21
hydrogenation reaction. Moreover, the Raman spectra (Fig. S6) Moreover, the contents of different nitrogen configurations are
show that two conspicuous peaks are attributed to the D band at summarized in Table S3. The ratio of Ni-N specie to Ni is
about 1350 cm-1 for the disordered carbon, edge defects, and decreased with increasing the pyrolysis temperature from 600
o
other defects and the G band at about 1580 cm-1 for E2g to 800 C, indicating that some Ni-N coordination bonds are
vibrational mode in the sp2-bonded graphitic carbon, broken, which neatly supports EXAFS fitting results.
respectively.16
To confirm the chemical state and coordination of the Ni
atoms in Ni-N-C catalysts, the Ni K edge X-ray absorption near
the edge structure (XANES), extended X-ray absorption fine
structure (EXAFS), and X-ray photoelectron spectroscopy
(XPS) have been performed, as shown in Fig. 2c-2f and Fig.
S7-S8. The near edge of Ni-N-C-600, Ni-N-C-700 and Ni-N-C-
800 are between those of Ni foil and NiPc, suggesting that Ni in
the pyrolysis products is in the intermediate valence state.8c,8d,17
This is due to that the neighboring N atoms partially deplete Ni
free electrons through the valence bond.8c It is evident that the
Ni-N-C-800 near-edge absorption is closer to Ni foil, due to the
presence of metallic nickel in Ni-N-C-800. As shown in Fig. 2d,
Ni metal exhibits a typical first shell Ni-Ni pair at ~2.18 Å,
while Ni-N interaction in NiPc locates at ~1.47 Å. The peak
located at ~1.4 Å for all Ni-N-C samples corresponds to the Ni-
N scattering path, indicating that the Ni atoms are atomically
dispersed and bonded with N atoms in the catalysts.8c,18 Ni
nanoparticles coexistence with single Ni atoms in Ni-N-C-800
has been further identified due to the presence of Ni-Ni
scattering. The quantitative structure of Ni species investigation
was carried out by the EXAFS fitting (Fig. S8). The
Fig. 2 (a) XRD patterns of Ni-N-C-600, Ni-N-C-700, Ni-N-C-800 and Ni-NC. (b) Nitrogen
adsorption isotherms of Ni-N-C-600, Ni-N-C-700, and Ni-N-C-800 measured at 77 K. (c)
Ni K-edge XANES of the Ni-N-C catalysts and Ni foil and NiPc as the references, (d)
Fourier transformed EXAFS spectra of Ni K-edge for Ni-N-C catalysts and corresponding
reference samples. (e) Ni 2p high-resolution XPS surveys of Ni-N-C catalysts, Ni-NC and
NiPc. (f) N1s XPS spectra of Ni-N-C.
coordination numbers of Ni in Ni-N-C-600, Ni-N-C-700, and
Ni-N-C-800 are 3.7, 3.3, and 2.6, respectively (Table S1).
These results demonstrated that Ni atoms are atomically
dispersed in the N-doped porous carbon, which are mixed with
two-fold, three-fold and four-fold coordinated by N atoms.
Moreover, the coordination numbers of Ni-N is reduced along
with the temperature increased.19 This result illustrates that
some Ni-N bonds are broken with increasing the pyrolysis
temperature. It also explains that the Ni-N bonds further breaks
3.2 Hydrogenation of nitroarenes
The catalytic activity of as-synthesized samples are initial
evaluated using the hydrogenation of 4-chloronitrobenzene with
5 equiv. sodium borohydride as reducing agent (Table S4). The
Ni-N-C-700 catalyst exhibits higher activity compared to that
of Ni-N-C-600 and Ni-N-C-800, which is superior to other
carbon catalysts (Table S5). We also extend the Ni-N-C-700
catalyst to various nitroarenes, and the corresponding aromatic
aniline is formed smoothly in excellent yield (Table S6).
Considering the high catalytic activity of Ni-N-C-700, the
weaker reductants H2 is used to examine the hydrogenation of
4-chloronitrobenzene (Table 1). It is interesting to find that the
reaction can complete by using 5 mg Ni-N-C-700 catalyst to
catalyze 0.25 mmol of 4-chloronitrobenzene with 3 MPa H2 in
o
with the pyrolysis temperature up to 800 C, which tend to the
formation of the Ni NPs and the high crystallinity carbon layer
on the surface of some Ni NPs. X-ray photoelectron
spectroscopy (XPS) has been further used to demonstrate the
above results. The binding energy of Ni 2p3/2 in pyrolysis
products are around 855.0 eV, higher than that of metallic Ni0
(~853.0 eV) and lower than that of Ni2+ in NiPc (~855.7 eV),
showing the valence states of Ni species in pyrolysis products
between 0 and +2 (Fig. 2e),8c,8d,17 being in agreement with the
XANES results. The Ni contents in Ni-N-C-600, Ni-N-C-700,
Ni-N-C-800, and Ni-NC measured by inductively coupled
plasma optical emission spectroscopy (ICP-OES) are 4.9, 4.4,
3.6, and 10.3 wt%, respectively, which are consistent with XPS
o
ethanol at 120 C for 8 h (entry 1). When decreasing the H2
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pressure, catalyst amount, and reaction temperature, the efficiency of Ni-N-C-700 with that of knownViecwatAartliyclsetOsnliinne
DOI: 10.1039/C8GC03664K
conversion of 4-chloronitrobenzene is incomplete (entries 2-4). literatures (Table S7). We found that Ni-N-C-700 catalyst
However, this process can be accomplished with prolonging the
reaction time (entry 5). Other solvents, such as H2O, EtOAc,
Table 1 Evaluation of reaction conditions for the reduction of 4-chloronitrobenzenea
and THF, are inferior to EtOH in current reaction (entries 6-8).
Catalyst, H2
Cl
NO2
Cl
NH2
Yieldb
Generally, the hydrogenation activity of nitrobenzene exhibits a
positive relation with the solvent polarity. However, both
dispersibility of the catalyst and solubility of the reactant
performances are very important for selecting solvent of
heterogeneous catalysis reaction system. In such system, the
low catalytic activity of hydrogenation of nitrobenzene reaction
with water as solvent is probable due to the poor dispersibility
performance of catalyst and the low solubility of nitrobenzene
in water.22 In order to identify the active species derived from
the Ni-N-C catalysts, a series of control experiments, such as
using NiCl2, NiCl2 and 1,10-phen, N-C and Ni-NC as catalysts
have been carried out. The reaction cannot proceed in the cases
of using NiCl2, NiCl2 and 1,10-phen as catalysts (entries 10-11),
and the N-C and Ni-NC catalysts show extremely low activities,
affording 4-chloroaniline in 21% and 38%, respectively (entries
12-13). It is demonstrated that the N doping and the Ni
nanoparticles are contribute little to such reaction. Moreover,
the Ni-N-C-600 and Ni-N-C-800 catalysts show lower activity
compared with Ni-N-C-700, giving 4-chloroaniline in 82% and
63%, respectively (entries 14-15). However, when addition of
NaSCN to the reaction system, the yield of 4-Chloroaniline
decreases dramatically to 28% (entry 16). These results indicate
that Ni single atoms bonded to N atoms in the porous N-doped
carbon framework are critical active sites for impelling this
reaction. More importantly, the Ni-N3 active species probably
show better activation performance than Ni-N2 and Ni-N4 due
to the fact that the higher content of Ni-N3 exists in Ni-N-C-700
compared with that of Ni-N-C-600 and Ni-N-C-800.
Additionally, in order to predict the possible path of reaction,
the two intermediates, nitrosobenzene and azoxybenzene, have
been observed by CC-MS after the reaction carried out in 3 h
(Fig. S9). Therefore, we can conclude that the reaction
mechanism probably works in hydroxylamine and diazene
oxide two pathways (Fig. S10).
EtOH
Usage
(mg)
Entry
Catalyst P (Mpa)
T (h)
T (oC)
(%）
1
2
3
4
5
6
7
8
9
Ni-N-C-700
3
2
3
3
3
3
3
3
3
3
5
5
5
4
4
4
4
4
4
4
8
8
8
8
10
10
10
10
10
10
120
120
110
120
120
120
120
120
120
120
99
78
82
81
Ni-N-C-700
Ni-N-C-700
Ni-N-C-700
Ni-N-C-700
Ni-N-C-700
Ni-N-C-700
Ni-N-C-700
MgO
NiCl2
NiCl2, 1,10-
phen
N-C
Ni-NC
99
85c
43d
13e
NRf
NRf
10
11
3
4
10
120
NRf
12
13
14
15
16
3
3
3
3
3
4
4
4
4
4
10
10
10
10
10
120
120
120
120
120
21
38
82
63
28g
Ni-N-C-600
Ni-N-C-800
Ni-N-C-700
[a] Reaction conditions: 0.25 mmol 4-nitrochlorobenzene in 5 ml ethanol. [b]
isolated yield. [c] 5 ml of H2O was used as solvent. [d] 5 ml of Ethyl Acetate was
used as solvent [e] 5 ml of THF was used as solvent. [f] NR: No Reaction. [g] 4-
nitrochlorobenzene of hydrogenation in the presence of NaSCN.
shows higher TOF values (8.4 h-1) than the majority of other
reported Ni catalysts in the comparative reaction conditions.
More importantly, the 100% selectivity of our catalysis
hydrogenation reaction is superior to the most of Ni catalysts.
Compared with other non-precious metals catalysts, such as Fe
and Co, it also shows the comparative catalytic activity and
selectivity.
Table 2 Catalytic reduction of nitroarenes into amines by H2 catalyzed by Ni-N-C-700a
Various nitroarenes have been further extended in catalysis
by Ni-N-C-700 catalyst under the optimized reaction condition.
The catalyst shows good activity for the hydrogenation of both
Catalyst (4mg)
NO2
NH2
EtOH, 120 o
C
R
R
NH2
Cl
NH2
NH2
NH2
Br
NH2
NH2
NH2
electro-donation
and
electro-withdrawing
substituted
nitroarenes (Table 2). It is noteworthy that the chloro, bromo
substituted nitroarenes can be reduced without undergoing
dehalogenation (entries 2-3). The hydrogenation of nitrile, keto,
carboxyl and ester substituted nitroarenes can take place
selectivity without reduction of these reducible functional
groups in aromatic nitro substrates (entries 8-11). The
hydrogenation of nitrostyrene reaction is further carried out to
evaluate the selectivity of Ni-N-C-700. Only 4-nitrostyrene and
4-vinylaniline were detected by GC-MS in the filtrate after the
reaction (Fig. S11), and 91% of 4-vinylaniline isolated yield is
obtained (entry 12). These results illustrate that Ni-N-C-700
catalyst exhibits high activity for hydrogenation of nitroarenes
and great chemoselectivity. We have also compared the
1, 99 %
4, 98 %
7, 96 %
2, 99 %
5, 99 %
8, 97 %
3, 99 %
6, 99 %
9, 99 %
H2N
NH2
NH2
H3C
HO
H3CO
HOOC
N
C
H3COC
NH2
NH2
H3COOC
NH2
12, 91 %
10, 96 %
11, 99 %
[a] Reaction conditions: 0.25 mmol 4-nitrochlorobenzene in 5 ml ethanol, 4 mg
catalyst, 120 oC, 3 MPa H2, 10 h . [b] isolated yield.
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J. Name., 2013, 00, 1-3 | 5
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The most outstanding advantage of the heterogeneous Å), especially for Ni-N3 substrates. The inducedVlieownAgreticslte ONn-linOe
catalyst is its capability for easy separation and recycling. The bond indicates the highest catalytic abiliDtyO.I:F1i0g.1.033e9-/Cf 8dGeCp0ic3t66th4Ke
recyclability of Ni-N-C-700 has been evaluated by using 4- most stable co-adsorption configurations of nitrobenzene and
chloronitrobenzene as a test substrate (Fig. S12). After H2 molecules on Ni-Nx. Followed H2 co-adsorption, the
completion of the reaction, the catalyst was filtered from the adsorption energies and the length of N-O are slightly increased,
reaction and washed with ethyl acetate for the next run. promising the subsequent nitrobenzene reduction reaction occur
Although, the chemical selectivity is maintained perfect easily. Fig. S17 shows the most stable nitrobenzene and co-
throughout the process, the yield of 4-chloroaniline gradually adsorption structures of nitrobenzene molecules and H2 on the
decreased along with the cycle times. However, the other optimized Ni-N3 structures. We can find that the Ni-N3 in
hydrogenation process can complete by extending the reaction Fig. 3c possesses the highest catalytic ability according to the
time. This result demonstrates that the Ni-N-C-700 catalyst lowest adsorption energy and longest N-O bond length, which
possesses good recyclability. After five times recycles, various is consistent with the experimental observations.
experiments have been carried out to examine the recycled
catalyst. The SEM images show that the more fragmented is
observed compared with the fresh catalyst (Fig. S13a). No any
nickel-containing nanoparticles are found in TEM Image (Fig.
S13b). HAADF-STEM images reveal that single atom nickel
remains in Ni-N-C-700 after cycle reaction (Fig. S13c). EDS
mapping result shows that the elements of Ni, N and C are
uniformly distributed in the recycled catalyst (Fig. S13d). These
results illustrated that the microstructure of catalyst is slightly
damaged. However, no aggregation phenomenon was occurred
during the reaction. Moreover, we have analysed the leached
liquid after the first time reaction by ICP-OES. Trace of nickel
(0.8 ppm) can be detected in the reaction solution. In addition,
Fig.
3 DFT calculations. The investigated most stable structures of (a-d)
the content of Ni in the five times recycled catalyst was 2.8 wt%
determined by ICP-OES (Table S8). Therefore, we proposed
the decrease of the catalysis activity is due to the catalyst
structure damage and active species Ni leaching. In order to
demonstrate the reaction proceed in a heterogeneous way, the
leaching test has been also carried out (Fig. S14). When the
catalyst is removed from the reaction, the yield of 4-
chloroaniline is not increased. When rejoin the Ni-N-C-700
catalyst, the reaction can be accomplished.
nitrobenzen adsorption and (e-h) nitrobenzen and H2 co-adsorption on Ni-Nx (x =
1-4), as well as the adsorption energy (Eads) and N-O and H-H bond length. The
gray, blue, light blue, red, and white balls stand for C, N, Ni, O, and H atoms,
respectively.
To investigate the essential reason for the highest catalytic
activity of Ni-N3, we also plot the charge difference with two
kinds of reference, as shown in Fig. 4a-b. From Fig. 4a we can
see that most of the charge transferring occurred in H2 self and
small amount on the Ni (yellow part), therefore, we can
conclude that the catalyst of Ni-N3 induced polarization of non-
polarized H2 molecule, which is responsible for the slightly
decreased adsorption energy by H2 co-adsorption, as shown in
Fig.3. Furthermore, if we take nitrobenzene and Ni-N3 (Fig. 4b)
as the references, more charges are transferred between N or Ni
in Ni-N3 and N or O in nitrobenzene, which is regarded as the
essential reason for the highest catalytic activity of Ni-N3.
3.3 DFT calculations
To understand the nature of the high hydrogenation of
nitroarenes reactivity of atomically dispersed Ni catalyst,
density-functional theory (DFT) calculations were performed.
As shown in Fig. 3, four Ni-N structures with Ni atoms
coordinated with 4 N atoms (Ni-N4, Ni-N2+2), 3 N atoms (Ni-N3,
Ni-N3V, Ni-N3C, Ni-N3C2), 2 N atoms (Ni-N2C1, Ni-N2C2, Ni-
N2, Ni-N2V2) and 1 N atoms (Ni-N1C2, Ni@G) have been
considered to represent different Ni-N coordination conditions,
where V stands for coordination vacancy. Also, some other
typical Ni-Nx (x=1-4) structures are selected, as shown in Fig.
S15. To investigate the interaction between nitrobenzene
molecule and Ni-Nx, many initial adsorption structures (parallel
or vertical to Ni-Nx ) were involved into these simulations. Fig.
3a-d show the most stable adsorption structures of nitrobenzene
molecule on Ni-Nx and the others are shown in Fig. S16. It is
clearly seen that nitrobenzene molecule is adsorbed parallel on
the substrates with the AB stacking-like morphology. Also, the
adsorption energies (Eads) of nitrobenzene molecule on three
substrates are all negative (-0.59 ~ -2.22 eV), suggesting a
stable adsorption, and all the N-O bonds are activated with the
longer N-O bond length compared to the gas phase (N-O = 1.24
Fig. 4 The charge difference of (a) H2 vs. Nitrobenen-adsorbed Ni-N3; (b)
nitrobenzen vs. Ni-N3. Isovalue=0.005 a.u.
6 | J. Name., 2012, 00, 1-3
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Green Chemistry
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Journal Name
ARTICLE
Li, ACS Catal. 2013, 3, 608-612; (e) S. El-Hout, S. El-Sheikh, H.
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4. Conclusions
View Article Online
In summary, atomically dispersed Ni sites on N-doped
porous carbon are synthesized by the pyrolysis of
coordinatively saturated NiCl2 with 1,10-phenanthroline using
porous MgO template. The structure of as-synthesized Ni-N-C
catalysts has been characterized by HAADF-STEM, and the
coordination numbers of Ni with N is reduced along with the
temperature, as indicated by XAFS and XPS characterization.
4
5
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8750-8754.
o
The Ni-N-C catalyst prepared at 700 C exhibits high activity
for hydrogenation of nitroarenes, great chemoselectivity, and
good recyclability. The outstanding catalytic performance
originates from the induced change transfer and the active sites
are the atomic dispersed single Ni with high Ni-N3 content,
which is proven by control experiments and DFT calculations.
This study may offer a new strategy for rational design and
application of single-atom catalysts in organic molecular
transformation reactions.
6
7
Author contributions
Fan Yang and Minjian Wang conceived and designed the
experiments. Minjian Wang and Bin Yang performed
laboratory experiments. Jun Luo, Wei Liu and Yushu Tang
carried out TEM. Liqiang Hou, Yun Li, Zihui Li, Bing Zhang,
Wang Yang conducted part of the characterization analysis of
the material. Ying Wang provided theoretical calculation and
discussion. Yongfeng Li and Fan Yang were responsible for
planning and supervising the project.
8
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
9
T. Wang, L. X. Wang, D. L. Wu, W. Xia, D. Z. Jia, Sci. Rep.
2015, 5, 9591.
We gratefully thank for the National Natural Science
Foundation of China (Nos. 21776302, 21576289, 21776308,
21673220), the Science Foundation of China University of
Petroleum, Beijing (Nos. 2462017BJB04, 2462015YQ0306,
2462017YJRC051, C201603), and Thousand Talents Program.
10 F. Yang, X. X. Fan, C. X. Wang, W. Yang, L. Q. Hou, X. W. Xu, A.
D. Feng, S. Dong, K. Chen, Y. Wang, Carbon 2017, 121, 443-
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8 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C8GC03664K
A graphic abstract
Noble-metal-free catalytic hydrogenation of nitroarenes using H2 as reductant is achieved
through the rational design of atomically dispersed Ni sites on N-doped porous carbon. The
outstanding activity of the catalyst originates from the atomic dispersion of single Ni active sites
with high Ni-N3 content, proven by control experiments and DFT calculations.