Chemoselective hydrogenation of functionalized nitroarenes using MOF-derived co-based catalysts

Article abstract of DOI:10.1016/j.molcata.2016.04.008

The synthesis, characterization, and application of nitrogen-doped carbon supported Co catalysts in selective hydrogenation of nitroarenes are described. The cobalt-based catalysts are prepared by simple pyrolysis of ZIF-67, a typical MOF material, under inert atmosphere. Physicochemical properties of the Co/C-N catalysts have been investigated by X-ray diffraction, elemental analysis, atomic absorption spectroscopy, scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The Co-based materials were found to be highly efficient in the chemoselective hydrogenation of nitroarenes. A broad range of substituted nitroarenes are converted to the corresponding anilines in excellent yields under industrially viable conditions with other reducing groups remaining intact. In situ ATR-IR and XPS characterizations reveal that the Co-N centers present in the catalyst favor the preferential adsorption of nitro groups, leading to this unique chemoselectivity. The kinetic parameters of 4-nitrostyrene hydrogenation over the Co/C-N catalyst were investigated.

Full text of DOI:10.1016/j.molcata.2016.04.008

Accepted Manuscript
Title: Chemoselective hydrogenation of functionalized
nitroarenes using MOF-derived co-based catalysts
Author: Xi Wang Yingwei Li
PII:
S1381-1169(16)30130-3
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2016.04.008
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Journal of Molecular Catalysis A: Chemical
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Accepted date:
19-3-2016
12-4-2016
12-4-2016
Please cite this article as: Xi Wang, Yingwei Li, Chemoselective hydrogenation of
functionalized nitroarenes using MOF-derived co-based catalysts, Journal of Molecular
Catalysis A: Chemical http://dx.doi.org/10.1016/j.molcata.2016.04.008
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Chemoselective Hydrogenation of Functionalized
Nitroarenes Using MOF-Derived Co-Based Catalysts
Xi Wang, Yingwei Li*
School of Chemistry and Chemical Engineering, South China University of
Technology, Guangzhou 510640, China
*Corresponding author. Email: liyw@scut.edu.cn

Graphical Abstract
A novel MOF-derived metallic cobalt-based catalyst is developed for chemoselective
hydrogenation of substituted nitroarenes. A broad range of substituted nitroarenes are converted
to the corresponding anilines in 93-99% yields under industrially viable conditions with other
reducible groups remaining intact.
Highlights
●
Co/C-N was prepared by simple pyrolysis of MOFs.
●
intact.
●
Nitroarenes are converted to anilines with other reducing groups remaining
Co-N centers present in the catalyst lead to the unique chemoselectivity.
●
The reaction showed a first-order dependence on H pressure.
2

Abstract
The synthesis, characterization, and application of nitrogen-doped carbon
supported Co catalysts in selective hydrogenation of nitroarenes are described. The
cobalt-based catalysts are prepared by simple pyrolysis of ZIF-67, a typical MOF
material, under inert atmosphere. Physicochemical properties of the Co/C-N catalysts
have been investigated by X-ray diffraction, elemental analysis, atomic absorption
spectroscopy, scanning electron microscopy, transmission electron microscopy,
Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The
Co-based materials were found to be highly efficient in the chemoselective
hydrogenation of nitroarenes. A broad range of substituted nitroarenes are converted
to the corresponding anilines in excellent yields under industrially viable conditions
with other reducing groups remaining intact. In situ ATR-IR and XPS
characterizations reveal that the Co-N centers present in the catalyst favor the
preferential adsorption of nitro groups, leading to this unique chemoselectivity. The
kinetic parameters of 4-nitrostyrene hydrogenation over the Co/C-N catalyst were
investigated.
Keywords: heterogeneous catalysis, cobalt, metal-organic frameworks, amines,
hydrogenation

1
. Introduction
Aniline and its derivatives, with more than 4 million tons produced per year, are
known as major feedstocks and key intermediates for the synthesis of pharmaceuticals,
dyes, functional polymers, agrochemicals, and fine chemicals [1,2].Generally, anilines
are prepared by reduction of corresponding nitro substrates. However, the traditional
non-catalytic processes require stoichiometric amounts of reducing agents (e.g., Fe,
Sn, Zn and metal sulfides) that generate a large number of wastes [3]. Similarly,
catalytic routes utilizing various reducing agents are often troubled by the violent
reduction (e.g., NaBH
4
), toxic (e.g., N
2
H
4
•H O), or potentially explosive (e.g., silicon
2
hydride) issues [4-17]. As the most environmentally benign and cost-effective
reducing agent, hydrogen would be the preferred choice in the catalytic reduction of
nitroarenes. Nevertheless, the selective hydrogenation of a nitro group in the presence
of other reducible substituents, e.g., carbonyl, cyano, benzyloxy, and alkenic groups,
is challenging. Therefore, it is crucial to develop chemoselective catalysts for the
reduction of nitro groups.
Noble-metal catalysts (e.g., Au, Ag, Pt, Pd, Rh, and Ru) have been deeply
investigated for the catalytic hydrogenation of nitro groups [18-46]. However, the
problem often arising is that the highly active noble metals can activate nitro groups
as well as carbonyl and double bonds, leading to unselective reductions. It is generally
accepted that commercially available Pd or Pt catalysts cannot be used to hydrogenate
substituted nitrobenzenes, since the selectivity still remains low. So these catalysts
must be modified by alloying or poisoning with other metal oxides or molecules that

improve selectivity but at the expense of activity [42-46]. More recently, Corma et al.
approached this issue by controlling the coordination of metal surface atoms and
selecting an appropriate support [18,28]. Despite high activity achieved with noble
metals, the high cost and limited availability of these metals is a big issue which leads
researchers to develop more economical and bio-relevant alternatives.
In this context, catalytic reductions based on non-noble metals (e.g., Fe, Co, Ni,
Cu, and Zn) have been investigated intensively [47-64]. The commercially available
Raney-nickel catalyst also suffers from the unselective problems mentioned above.
Modification of the catalyst with B, P, Ni-B, Ni-P, or Ni-P-B have been investigated
as possible means to enhance the selectivity [62-64]. Cu-based catalysts have also
been found to be selective for this reaction, but these catalysts require relatively harsh
reaction conditions [59]. Cobalt catalysts are scarcely utilized for this transformation
[
12,43,44]. Zhao et al. inserted Co into the Mo C matrix to promote the formation of
2
molybdenum carbide crystal phase, achieving an improvement in reactivity for
catalytic reduction of nitroarenes [12]. Raja et al. reported Co nanoparticles supported
on mesoporous silica for the hydrogenation of nitroarenes [43]. High reactivity was
obtained even with large-scale substrates. But the chemoselectivity over these Co
catalysts was limited due to the absence of challenging substrates bearing reducible
groups, e.g., olefins, and alkynes. More recently, important advancements have been
reported by the group of Beller, who disclosed heterogeneous catalytic systems based
on carbonized cobalt– or iron–phenanthroline complexes that displayed exciting
selectivity for numerous structurally diverse nitroarenes [47,50]. However, the

catalysts require relatively expensive nitrogen-containing precursors as well as high
pressure (50 bar). Therefore, the development an efficient and cost-effective
H
2
non-noble metal catalyst system for chemoselective reduction of all kinds of
nitroarenes is highly desirable.
Metal-organic frameworks (MOFs) are an emerging class of ordered porous
materials built with metal ions and organic ligands. Recently, thermal decomposition
of MOFs has been demonstrated as a facile route to synthesize metal containing
porous carbon materials [65-68]. In our previous study, we demonstrated that
pyrolysis of a cobalt-containing MOF yielded highly dispersed metallic Co
nanoparticles embedded in N-doped carbon (Co/C-N) [69]. The excellent redox
property of Co-based materials drove us to explore the catalytic activity of Co/C-N
for the hydrogenation of nitroarenes. Here, we show that these MOF-derived
materials represent excellent catalysts for the chemoselective hydrogenation of a
variety of functionalized nitroarenes to anilines with other reducible groups untouched.
The superior catalytic performance can be attributed to the presence of Co-N centers
which favor preferential adsorption of the nitro groups. Moreover, these Co-based
catalysts are easily separated and can be conveniently reutilized, which is an
important aspect in the development of practical and cost-effective catalytic
hydrogenation processes.

2
. Experimental Section
Co(NO
A.R.), ethyl acetate (A.R.), and isopropyl alcohol (A.R.) were purchased from the
Sinopharm Chemical Reagent Co., Ltd. 2-Methylimidazole (98%),
,4-benzenedicarboxylic acid (98%), polyvinylpyrrolidone (PVP, M.W. = 30000),
NaBH (98%), and nitroarenes (>99%) were purchased from Alfa Aesar. Commercial
3
)
2
·6H
2
O (A.R.), methanol (A.R.), ethanol (A.R.), DMF (A.R.), THF
(
1
4
samples of 5% Pd, Pt, Ru, and Rh on active charcoal and pure active charcoal were
purchased from Sigma-Aldrich. Deionized water with conductivity <1 μS/cm was
obtained from the ultrapure water polishing system made by Shanghai Xunhui
Environment Technology Co., Ltd. All chemicals were used without further
purification.
2
.1. Materials preparation
In a typical synthesis of ZIF-67 [69], cobalt nitrate hexahydrate (0.45 g, 1.55
mmol) was dissolved in 3 mL of deionized water (0.52 mol/L), and
-methylimidazole (5.5 g, 67 mmol) was dissolved in 20 mL of deionized water (3.35
2
mol/L). The two solutions were mixed and stirred for 6 h at room temperature. The
resulting purple precipitates were collected by centrifugation and filtration, washed
with water and methanol subsequently for 3 times, and finally dried under vacuum at
8
0 °C for 24 h. In the synthesis of Co-MOF-71 [70], a solid mixture of
Co(NO •6H O (0.4 g, 1.37 mmol) and 1,4-benzenedicarboxylic acid (230 mg, 1.37
mmol) was dissolved in a single binary solution of DMF (10 mL) and ethanol (2.5
mL). The resulting solution was transferred into a Pyrex tube, frozen in a liquid N
3
)
2
2
2

bath, evacuated (200 mTorr), flame-sealed and then heated at 100 °C for 15 h. The
resulting pink/purple crystals of Co-MOF-71 were collected by filtration, washed with
DMF subsequently for 3 times, and finally dried overnight under vacuum at 150 °C.
Co/C-N catalyst was synthesized by direct pyrolysis of ZIF-67 (Yield, ca. 60
wt%). In a typical preparation, 1 g of pre-synthesized ZIF-67 was pyrolyzed at a
particular temperature for 10 h with a heating rate of 1 ºC/min under argon
atmosphere. The prepared catalyst was denoted as Co/C-N-X, where X indicated the
MOF pyrolysis temperature. Co/C-X was prepared following the same procedures as
above except using Co-MOF-71 as sacrificed templet.
For comparison, an activated carbon (Sigma-Aldrich) supported metallic Co
sample was also prepared by using a simple colloidal deposition method [71]. Firstly,
1
0 mL aqueous solution of Co(NO
3
)
2
•6H O (740 mg, 2.54 mmol) was prepared, and
2
PVP (2.82 g, 25.4 mmol) was added as a protecting agent. The obtained solution was
placed in an ice bath and vigorously stirred for 1 h. 5 mL freshly prepared aqueous
solution of NaBH (96 mg, 2.54 mmol) was added under vigorous stirring to obtain a
4
dark brown colloidal dispersion. The preformed Co:PVP hydrosol was subjected to
sonication for 20 s, and during the sonication, 10 mL aqueous solution of activated
carbon (500 mg) was added. The suspension was stirred at 0 °C for 4 h, followed by
washing thoroughly with deionized water. The powder was dried under vacuum at
1
00 °C for 2 h and then heated at 200 °C in H
2
for 2 h to obtain the Co/C. Ni/C was
•6H O as the metal precursor.
also prepared by the same recipe except using Ni(NO
3
)
2
2

2
.2. Materials characterization
Powder X-ray diffraction patterns of the samples were obtained on a Rigaku
diffractometer (D/MAX-IIIA, 3 kW) employing Cu Kα radiation (λ = 0.1543 nm) at
4
3
0 kV, 30 mA at room temperature. 2θ scans were performed from 20° to 100° at
°/min. X-Ray photoelectron spectroscopy (XPS) measurements were performed on a
−9
Kratos Axis Ultra DLD system with a base pressure of 10 Torr. The size and
morphology of materials were investigated by high-resolution scanning electron
microscopy (HR-SEM, S-3700N of HITACHI) equipped with an energy dispersive
X-ray spectroscopy (EDS) and high-resolution transmission electron microscopy
(HR-TEM, C/M300 of Philips) operated at 30 kV and 200 kV respectively.
2
.3. General procedure for hydrogenation
The hydrogenation of nitroarenes was carried out in a Teflon-lined stainless steel
autoclave equipped with a pressure gauge and a magnetic stirrer. Typically, a mixture
of 0.5 mmol nitroarene, 15 mol% Co/C-N-X catalyst, 100 μL n-hexadecane and 2 mL
solvent was introduced into the reactor at room temperature. Air in the autoclave was
purged several times with H . Then, the reaction began by starting the agitation (600
2
r/min) when hydrogen was regulated to 1 MPa after the reaction temperature was
reached. After reaction, the solid was isolated from the solution by centrifugation. The
products in the solution were quantified and identified by GC-MS analysis (Shimadzu
1
GCMS-QP5050A equipped with a 0.25 mm × 30 m DB-WAX capillary column). H
1
3
NMR and C NMR data were obtained on Bruker Avance III 400 spectrometer using

CDCl or DMSO-d6 as solvent and tetrmethylsilane (TMS) as an internal standard.
3
The pure product in the scale-up experiment was obtained by flash column
chromatography (petroleum ether and ethyl acetate).
For the recyclability test, the reactions were performed maintaining the same
reaction conditions as described above, except using the recovered catalyst. At the end
of catalytic reaction, the mixture was centrifuged and the solid was recovered, which
was thoroughly washed with methanol, and then reused in a next run.
To determine whether there was any leaching of Co during the reaction, the solid
catalyst was removed from the reaction solution by hot filtration. The metal contents
of the filtered solution were determined quantitatively by atomic absorption
spectroscopy (AAS) on a HITACHI Z-2300 instrument.
2
.4. In situ ATR-IR spectroscopy
ATR-IR spectra were recorded on a Thermo Fisher iS10 equipped with a liquid
nitrogen cooled MCT detector. The spectra were obtained by averaging 32 scans at a
-1
resolution of 1 cm . The thin film of catalyst powder deposited on the ZnSe element
for ATR-IR spectroscopic study was prepared as follow. A suspension of ca. 50 mg of
catalyst powder in 2 mL of methanol was stirred overnight to eliminate any serious
agglomeration in solution. Then, the slurry was dropped onto a ZnSe internal
reflection element (IRE) and dried out at room temperature, which was subsequently
dried in a vacuum oven for a complete evaporation of methanol.

3
. Results and Discussions
3
.1. Optimization of reaction conditions
Nanoscale Co-based catalysts were prepared by pyrolysis of a Co-containing
MOF (ZIF-67) under a typical temperature ranging from 600 °C to 900 °C.
Disappearance of obvious peaks ascribed to the characteristic diffractions of ZIF-67
in the range of 5-20 degrees (Fig. S1) suggested the complete collapse of the ordered
pristine structure after the pyrolysis process. The as-synthesized nanoparticles were
identified as face-centered-cubic (fcc)-structured cobalt (JCPDS 15-0806) from the
powder XRD patterns, where the major diffraction peak could be indexed to the (111)
diffraction plane (Fig. S2). The TEM images (Figs. 1 and S3) of Co/C-N-600 revealed
an average particle size of ~9 nm in spheric shape. Typical scanning electron
microscopy (SEM) images were displayed in Fig. S4. As can be seen from the
corresponding elemental mapping images, Co, C, and N were homogeneously
distributed in Co/C-N.
We investigated the catalytic activities of the freshly prepared nanocatalysts for
the selective hydrogenation of nitro groups by using 4-nitrostyrene (denoted as NS) as
a benchmark substrate. As can be seen from Table 1, the parent ZIF-67 gave no
conversion of 4-nitrostyrene, suggesting that cobalt ions coordinated with
2
-methylimidazole were not active for reduction of nitroarenes under the investigated
conditions (entry 1). However, ZIF-67 pyrolyzed materials showed great performance
in this transformation, and significantly, Co/C-N-600 exhibited the highest conversion
and 97% selectivity to the target product 4-aminostyrene (Table 1, entries 2-5) under

the investigated reaction conditions. Both conversion and selectivity decreased with
an increase in pyrolysis temperature of the MOF. As the nitrogen content (e.g., 12.0
wt% at 600 °C, and 3.3 wt% at 800 °C) in the Co/C-N also decreased with increasing
temperature (Table S1), these results suggested the presence of nitrogen might play an
important role in this catalytic system. A complete conversion could be obtained by
simply prolonging reaction time without any significant reduction in product
selectivity (Table 1, entry 6).
To identify the active site for 4-nitrostyrene hydrogenation, we examined the
catalytic efficiency of Co/C-N-600 after removing the metallic Co or carbon under
identical conditions. First, the as-synthesized Co/C-N-600 was immersed in aqua
regia for 24 h, and the yielded material was denoted as Co/C-N-600-C. As illustrated
in Figs. S5 and S6, after acid treatment, the Co nanoparticles were completely
removed from the carbon matrix. The Co/C-N-600-C was not active in the
hydrogenation of 4-nitrostyrene, demonstrating the requirement of a metal to catalyze
the reaction (Table 1, entry 7). On the other hand, the Co/C-N-600 material was
subjected to heating at 400 °C for 1 h in air and then for 1 h in H . The carbon
2
composite was almost burned off after this treatment, and the resulting material was
denoted as Co/C-N-600-Co. Co/C-N-600-Co showed poor activity and produced
4
-aminostyrene in only 4% yield (Table 1, entry 8). Furthermore, we investigated the
activity of Co nanoparticles supported on activated carbon (Co/C) [71]. Interestingly,
this materials exhibited no reactivity (Table 1, entry 9).
In order to gain insight into the role of nitrogen in this catalytic system, we

conducted another control experiment using the material Co/C-X derived from
another cobalt-containing MOF without nitrogen element (Co-MOF-71). Co/C-600
possessed the similar structure as Co/C-N-600 except the absence of N element (Figs.
S3, S7 and Table S1). Interestingly, poor conversion and unsatisfactory selectivity
(50%) were observed over the Co/C-600 material (Table 1, entry 10). The above
control experiments suggest the important synergic interactions between the N-doped
carbon composite and Co nanoparticles for the catalytic performance of Co/C-N-600
in the hydrogenation of 4-nitrostyrene. Reactions catalysed by typical supported
transition-metal catalysts were also conducted (Table 1, entries 11-15). All the
supported catalysts except Ni/C exhibited high activities towards the hydrogenation of
4
-nitrostyrene with complete conversion under the same conditions. However, the
product was 4-ethylaniline with the alkenic group hydrogenated. The selectivity to the
target product 4-aminostyrene were 0%. These results are in accordance with the data
reported in the literature [18,22], demonstrating the speciality of the present materials
for selective hydrogenation of nitroarenes.
As the solvent strongly affects the activity and chemoselectivity of the reaction
[13], the effect of solvents was also investigated. Results of the hydrogenation showed
that changing the solvent indeed influenced reaction efficiency dramatically. However,
no apparent change of product selectivity was observed. Reductions in protonic
solvents gave better yields as compared to aprotic solvents (Table S2, entries 1-8).
Taking THF as an example, only 23% conversion, which was the lowest one among
the solvents we investigated, was obtained (Table S2, entry 2). It was interesting to

note that a big improvement in conversion was observed when the reaction was
conducted in a mixed solvent in which only a small amount of H O was added (Table
S2, entry 4), suggesting that H O, as a strong protonic solvent, might have a positive
effect on the reaction. However, H O seemed to be unable to promote the reaction to a
satisfactory conversion solely, probably due to poor solubility of the substrate in it
2
2
2
(Table S2, entry 3). The reaction under N
2
atmosphere suggested that the reductant
(
H
2
) was crucial for the reduction process (Table S2, entry 9).
3
.2. Stability and reusability of Co/C-N-600
Recyclability is definitely significant for a heterogeneous catalytic system.
Therefore, the stability and reusability of the Co/C-N-600 catalyst were investigated
in the hydrogenation of 4-nitrostyrene. The catalyst could be easily recycled by
centrifugation, sedimentation, or even by using a magnet due to the magnetic property
of the Co-based material. The separated catalyst was washed with methanol and then
reused under identical reaction conditions without decay in activity for at least 5
cycles (Fig. 2a). PXRD patterns showed that no apparent difference was observed
between the fresh catalyst and the reused one up to five runs (Fig. S2). TEM images
of the recycled catalyst indicated that there was no obvious change in particle shape
and average size (ca. 9 nm) as compared to the fresh one (Fig. 1).
In order to demonstrate the synthetic utility of our catalyst system for potential
practical application, the hydrogenation of 4-nitrostyrene was scaled-up by 10 under
the investigated conditions. An excellent isolated yield of 95% could be achieved,

suggesting that Co/C-N-600 could be a practical catalyst for the reduction of
nitroarenes to anilines. An important issue that can not be ignored at large-scale
production of arylamines is the formation of hydroxylamines. These compounds,
which are often less reactive as compared to the starting nitroarenes, are known as
carcinogens and are potentially explosive at higher concentrations due to their thermal
instability [48,72,73]. To our delight, no such intermediates like hydroxylamines were
observed in our experiments with 4-ethylaniline as the only byproduct. Additionally, a
leaching experiment was performed to further check the stability of the Co
nanoparticles. The reaction with the solution after hot filtration at approximately 25%
conversion essentially stopped (Fig. 2b) and no Co absorbance was observed to the
detectable limits of AAS in the filtrate after reactions, strongly suggesting that metal
leaching did not occur and the reaction was predominantly heterogeneous.
3
.3. In situ ATR-IR spectroscopic study of single-component adsorption
To gain more in-depth information about the selective hydrogenation reaction, in
situ ATR-IR spectroscopy was employed and 4-nitrostyrene was chosen as substrate
to gain some molecular insight into the surface adsorption property of the catalysts.
The Co/C-N-600 powder showed no absorbance, while three characteristic bands at
-1
1
116, 1418, and 1450 cm were observed for methanol in the range of 1050−1950
-1
cm (Fig. S8). The ATR-IR spectra of pure 4-nitrostyrene in the liquid-phase and in
-
1
methanol were also recorded (Fig. S9). The band at 1595 cm could be assigned to
-1
the stretching vibration of the conjugated C=C group. The peaks at 1511 cm and

-1
1
337 cm corresponded to -NO
2
stretching vibrationin 4-nitrostyrene.
Next, the adsorption of 4-nitrostyrene in Ar-saturated methanol on different
catalysts was investigated. The Co/C-600 prepared from thermolysis of
nonnitrogenous Co-MOF-71 was choosen as a reference catalyst to explore the
potential role of nitrogen element in the adsorption of 4-nitrostyrene. Initially, the
ATR-IR spectra of 4-nitrostyrene in Ar-saturated methanol at room temperature were
obtained (Figs. 3a and 3b). Interestingly, no obvious change in intensity or position of
the bands for ν(C=C) and ν(-NO ) was observed after 2 h, implying a weak adsorption
2
of 4-nitrostyrene on both catalysts under the condition.
However, when the temperature was elevated to 100 °C, apparent changes in
peak intensity were observed, implying a chemical adsorption character. It was worth
mentioning that the adsorption rates in the ATR-IR experiments were obviously
slower than that obtained in a batch reactor, likely due to the mass-transfer restriction
without stirring. The spectra of adsorbed 4-nitrostyrene on both Co/C-N-600 and
Co/C-600 at different time are displayed in Figs. 3c and 3d. For Co/C-N-600, the
−1
band at 1595 cm related to C=C stretching vibrations of both the phenyl group and
the conjugated C=C bond in 4-nitrostyrene, showed no obvious change in intensity or
band shift with reaction time. Comparably, the characteristic absorption at 1511 cm-1
-1
and 1337 cm for the -NO stretching vibration of 4-nitrostyrene gradually became
2
-1
-1
weaker and slightly blue-shifted (by ca. 6 cm and 7 cm respectively). The above
observation suggested that 4-nitrostyrene might adsorb vertically on the Co(111)
surface via only the nitro group[74-76].

Thus, selective reduction of the nitro group was expected under the
hydrogenation condition. However, for Co/C-600, the characteristic absorption
intensities for both olefin and nitro groups decreased with time, implying
4
-nitrostyrene might adsorb parallelly onto the Co(111) surface of Co/C-600 by the
two groups, leading to simultaneous reduction of both groups in the hydrogenation.
These results demonstrated that nitrogen played an important role in the selective
adsorption of nitro group onto the catalyst surface.
This speculation was further confirmed by the XPS characterization. XPS
investigations on the bonding of cobalt and nitrogen were carried out. Two distinct
peaks were observed in the N 1s spectra (Fig. 4a) of Co/C-N-600 with an
electron-binding energy of 399.0 eV and 401.1 eV, respectively. The peak at 399.0 eV
(purple line) could be assigned to pyridinic nitrogen coordinated to cobalt [77]. The
latter (red line) could be attributed to a pyrrole-type nitrogen doped into the carbon
matrix [78]. Deconvolution revealed that about 67% of all nitrogen atoms were
bonded to Co. Statistically, around 1.7 nitrogen atoms were coordinated to one
surface cobalt atom. Carbon atoms could also bond to Co, thus the surface of an
individual Co nanoparticle could be a mixture of partial cobalt nitride and partial
cobalt carbide.
The contribution of electrons from nitrogen to cobalt might have a major
influence on the chemical environments of Co and N in the near-surface region.
Consequently, we compared the Co 2p spectra of Co/C-N-600 with Co/C-600. As can
be seen from Fig. 4b, in Co/C-600, three peaks were observed at 779.3 eV (red line),

0
2+
7
80.1 eV (blue line), and 781.6 eV (purple line) that could be ascribed to Co , Co
and Co-OH species, respectively [79]. However, in Co/C-N-600, these peaks were
shifted to lower binding energies (i.e., 778.7 eV, 779.7 eV, and 781.0 eV). These
results suggested that the Co-N centers formed in the ZIF-67 pyrolysis process (Fig.
S10), in which the Co and N atoms were tightly bonded enabling an adjustment of the
electron distribution on the catalyst surface, played a critical role in the selective
adsorption and hydrogenation of nitroarenes. It is notworthy that there could be a
small fraction of surface Co not in contact with N because the reaction results
indicated that the selectivity to 4-aminostyrene was not 100%. However, as shown in
Table 1 (entries 9-10), such Co species showed a low activity under the investigated
conditions. Therefore, these Co atoms not in contact with N should not be dominant
because of the low fraction and reactivity. The Co nanoparticles were partially coated
and isolated by the carbon support, which was believed to play an important role in
preventing metal aggregation during catalytic reactions and hence enhancing the
catalyst stability. Beller and co-workers first developed this kind of Co(Fe)−N−C for
fine chemical synthesis, and they claimed that the Co
x
O
y
particles or FeN species in
x
the size range of 20−80 nm were responsible for the excellent catalytic performances.
More recently, Zhang et al. proposed Co single atoms bonded with N within graphitic
sheets to be active sites in the Co-N-C catalyst for C-C coupling reactions [80].
However, the high cobalt loading and long calcination time would eliminate the
existence of Co single atoms on the surface of our catalysts.

3
.4.Effect of kinetic parameters on the catalytic performance of Co/C-N
Since the stirring speed could play an important role in the external mass transfer
of a liquid-phase catalytic reaction and might affect the reaction rate and product
selectivity, the hydrogenation of 4-nitrostyrene (NS) over the Co/C-N-600 catalyst
was carried out under identical conditions (i.e., 1 MPa H , 100 °C, and 2 h), except
2
using different stirring rates. As shown in Fig. S11, the conversion of NS increased
when the stirring speed was enhanced from 400 rpm to 600 rpm. Further increaseing
the stirring rate, there was no obvious improvement of NS conversion, indicating that
the stirring speed was sufficiently high for negligible diffusion limitation. Therefore,
the stirring rate of 600 rpm was chosen for all the hydrogenation reactions in this
work.
Kinetic experiments were carried out over the Co/C-N-600 catalyst under the
same conditions (i.e., 1 MPa H
Four different initial NS concentrations were investigated, i.e., 0.05, 0.15, 0.25, and
.35 M, respectively. It can be seen from Fig. S12a that the initial NS concentration
NS,0) seemed to have no influence on the reaction rate. Neither did on the product
distribution during the overall reaction process. To rule out the dependence of the
reaction on NS concentrations, the reaction rate (r ) at each initial concentration was
determined from the concentrations versus time curves. Plot of lgr
2
, and 100 °C), except for the initial NS concentration.
0
(
C
0
0
versus lgCNS,0
confirmed our former observation (Fig. S12b). The slope of the straight line for the
four NS concentrations was about zero. This result suggested that the reaction would
be zero-order dependence on NS concentrations.

In another set of experiments, the effect of H
performance was investigated by varying the H pressures over the range of 0.6-1.2
MPa. As can be seen in Fig. S13a, increasing the H pressure enhanced the reaction
2
pressure on the hydrogenation
2
2
rate, while the selectivity to 4-aminostyrene was still maintained at >96% for each run
up to complete conversion of NS. It is plausible that a high pressure could improve
the solubility of hydrogen in the reaction solution, rendering more hydrogen
molecules accessible to the catalyst surface.
Plot of lgr
evolution of NS concentration for each run is presented in Fig. S13b. The fitting data
indicated that lgr was a good linear function of lgPH2 with a slope of ca. 1.02,
suggesting that the reaction order with respect to H pressure was first-order.
0
against lgPH2 with the initial reaction rates determined from the time
0
2
The effect of reaction temperature on the catalytic hydrogenation was
investigated at various temperatures in the range of 60-120 °C under 1 MPa H . The
2
results are summarized in Fig. 5a. As expected, increasing the reaction temperature
from 60 °C to 120 °C led to continuous shortening of reaction time for complete
conversion of NS. For example, the time to reach the maximum conversion was 7 h at
6
0 °C but only 1.5 h at 120 °C. Notably, the highest yield of 4-aminostyrene with
selectivity >90% was achieved at the time of just reaching a complete conversion of
NS for the reaction conducted at 120 °C. A further reaction at prolonged time would
lead to overhydrogenation to 4-ethylaniline.
Plot of lnk against the inverse of reaction temperature with the rate constants
determined from the time evolution of NS concentration for each run is presented in
2
Fig. 5b, which gave a straight line as lnk = -3.377(1000/T) + 12.91 with a R of 0.99.

According to the Arrhenius equation,
ln k = ln A – E /RT
the apparent activation energy (E
determined from the slope and intercept of the straight line for the four temperatures
a
a
) and pre-exponential factor (A) were
5
-1 -1 -1
-1
covered, which was ca. 28.1±2.0 kJ/mol, and 4.04±1.90×10 mmol h g Co MPa ,
respectively. The activation energy measured in this work is similar to (or even
slightly lower than) the values reported on precious metal-based catalysts (e.g., 30.8
kJ/mol for the nitrobenzene hydrogenation with Au/TiO catalyst [26], 33 kJ/mol for
2
the nitrobenzene hydrogenation with Ru/Iron oxide catalyst[81], and 31.2 kJ/mol for
the reduction of 4-nitrophenol with Au NPs[82]). The low activation energy is
probably related to the unique structrue of the Co/C-N materials.
3
.5.Selective hydrogenation of nitroarenes
Under the optimized reaction conditions, the general scope of the novel
Co/C-N-600 system was extensively investigated for the hydrogenation of various
functionalized nitro compounds. From a synthetic point of view, it is useful but
challenging that reducible moieties remained unaffected. Interestingly, halogen
substituted nitrobenzenes were reduced to halogenatedanilines, which are known as
key agrochemical intermediates, in excellent yields without any dehalogenation
(Table 2, entries 1-4). Notably, even labile bromide- and iodo-nitroarenes were
selectively hydrogenated in 99% yields, which are rarely reported in the
hydrogenation catalysed by noble metals.
Moreover, sensitive functional groups such as aldehydes, ketones, nitriles, esters,
amides, olefins, and even alkynes were also well tolerated in our catalytic system
(Table 2, entries 5-18). In the presence of olefinic bonds, the yields of the desired
anilines were obtained in 97-99% yields (Table 2, entries 5-7), which are comparable
to the results reported in the literature [33,47,50].
Selective hydrogenation of the substrates bearing alkynes is most challenging
and thus is rarely achieved [28,46,47,50]. In this respect, we are delighted to find our
catalyst was also capable of reducing nitro groups in the presence of alkynes with full
conversion and excellent selectivity (Table 2, entry 8). It is noteworthy that the

amino-benzonitriles, which constitute common building blocks for the chemical
industry, could be obtained in up to 99% yield (Table 2, entry 9). An excellent yield
was also gained although a prolonged reaction time was needed for the conversion of
di-nitrile substituted nitroarenes (Table 2, entry 10).
The position of aldehydic substituent did not affect the reactivity significantly
(Table 2, entries 11-12). Moreover, only the ortho- nitro group was reduced when
nitro groups were present on benzaldehyde at both ortho- and para- positions (Table 2,
entry 13). Interestingly, even prolonging the reaction time did not result in the
reduction of the para- nitro group either. Furthermore, substrates that contained
sensitive keto, amide, carboxylic acid, or ester substituents were also hydrogenated
smoothly and selectively into the corresponding anilines in almost quantitative yields
(Table 2, entries 14-18).
The amino-substituted N-heterocycles are important intermediates in
pharmaceutical and agrochemical industries. Finally, we tested the hydrogenation of a
variety of heterocyclic nitro compounds (Table 3). Again, all corresponding
heteroaromatic amines were achieved in 95-99% yields. The above results suggest
that the Co/C-N-600 material would be a promising catalyst for the clean and efficient
production of functionalized arylamines via a chemoselective reduction of their
corresponding nitroarenes with other reducible functional groups remaining intact.
4
. Conclusions
In summary, we have develped a stable, inexpensive, and reusable Co/C-N
catalyst for chemoselective hydrogenation of substituted nitroarenes to anilines with
other reducible functional groups remaining intact. The novel cobalt-based catalyst is
prepared by simple pyrolysis of ZIF-67, a typical MOF material, under inert
atmosphere. It allows for highly selective hydrogenation of numerous structurally
diverse nitroarenes in excellent yields under industrially viable conditions. In situ
ATR-IR and XPS characterizations reveal that Co-N centers may be responsible for

the preferred adsorption and selective hydrogenation of nitro group as compared to
other functional groups in nitroarenes. The kinetic study indicates a zero-order
dependence on 4-nitrostyrene while a first-order dependence on H pressure. Due to
2
their facile operation on synthesis, separation, and reutilization, the stable and
cost-effective materials would hold the potential for broad applications in green and
economical synthesis of fine chemicals.
Acknowledgements
This work was supported by the NSF of China (21322606, 21436005, and
2
1576095), the Doctoral Fund of Ministry of Education of China (20120172110012),
FRFCU (2015ZP002 and 2015PT004), and Guangdong NSF (2013B090500027).
Appendix A. Supplementary data.
Catalyst characterization, reaction results, and spectra data for the products. This
material is available free of charge via the Internet at http://.

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