In Situ Synthesized Silica-Supported Co@N-Doped Carbon as Highly Efficient and Reusable Catalysts for Selective Reduction of Halogenated Nitroaromatics

Article abstract of DOI:10.1002/cctc.202000683

Silica-supported Co@N-doped carbon (Co@CN/SiO₂) catalysts were first prepared by a one-step impregnation with a mixed solution of cobalt nitrate, glucose and urea, followed by in situ carbonization and reduction. The Co@CN/SiO₂ catalysts were investigated for the selective reduction of nitro aromatics to the corresponding anilines using hydrazine hydrate. The Co@CN/SiO₂-500 carbonized at 500 °C exhibited the highest catalytic activity and excellent stability without any decay of activity after 6 cycles for the reduction of nitrobenzene. Both metallic Co atoms and Co?N species formed in the Co@CN/SiO₂ catalysts were active, but the Co?N species were dominant active sites. The high activities of the Co@CN/SiO₂ catalysts were attributed to the synergistic effect between the Co and N atoms, promoting heterolytic cleavage of hydrazine to form H⁺/H^? pairs. Representative examples demonstrated that the Co@CN/SiO₂-500 could completely transform various halogen-substituted nitro aromatics to the corresponding halogenated anilines with high TOFs and selectivity of '99.5 percent.

Full text of DOI:10.1002/cctc.202000683

The European Society Journal for Catalysis
Supported by
Accepted Article
Title: In Situ Synthesized Silica-Supported Co@N-Doped Carbon as
Highly Efficient and Reusable Catalysts for Selective Reduction
of Halogenated Nitroaromatics
Authors: Yao Sheng, Xueguang Wang, Shengnan Yue, Gonglin
Cheng, Xiujing Zou, and Xionggang Lu
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To be cited as: ChemCatChem 10.1002/cctc.202000683
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10.1002/cctc.202000683
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In Situ Synthesized Silica-Supported Co@N-Doped Carbon as
Highly Efficient and Reusable Catalysts for Selective Reduction
of Halogenated Nitroaromatics
Yao Sheng, Xueguang Wang,* Shengnan Yue, Gonglin Cheng, Xiujing Zou, Xionggang Lu*
Dr. Y. Sheng, Prof. X. Wang, Dr. S. Yue, Dr. G. Cheng, Dr. X. Zou, Prof. X. Lu
State Key Laboratory of Advanced Special Steel, School of Materials Science and Engineering,
Shanghai University,
99 Shangda Road, BaoShan District, Shanghai 200444, China
E-mail: wxg228@shu.edu.cn (X. Wang); luxg@shu.edu.cn (X. Lu)
Supporting information for this article is given via a link at the end of the document.
Abstract: Silica-supported Co@N-doped carbon (Co@CN/SiO2)
catalysts were first prepared by a one-step impregnation with a
mixed solution of cobalt nitrate, glucose and urea, followed by in situ
carbonization and reduction. The Co@CN/SiO2 catalysts were
investigated for the selective reduction of nitro aromatics to the
corresponding anilines using hydrazine hydrate. The Co@CN/SiO2-
the reactions.[21,22] N-doping can modify the electronic structures
of Co particles and improve the interactions between metal
particles and support to form metal–N sites,[23,24] which has been
reported as highly effective catalytic active sites. For example,
CoOx@N-doped carbon nanotubes, metal organic frameworks
derived Co-based catalysts, and single-atom dispersed Co–N–C
catalysts exhibit excellent catalytic properties for the selective
reduction of nitroaromatics.[25-28] Unfortunately, oxidation and
leaching problems of metallic Co active sites are commonly
found in the reaction process, which result in deactivation and
poor recyclability of the catalysts.[29-31] Recently, mesoporous
alumina and silica-supported Co nanoparticles and Ni(Co)-MoO3
catalysts modified with nitrogen-doped graphite-like carbon
layers, which were prepared by the two-step impregnation
approach using 1,10-phenanthroline as carbon and nitrogen
sources, have been reported to be highly efficient for the
reduction of nitro aromatics to the corresponding amines with
hydrazine hydrate (N2H4·H 2O), a green hydrogen donor which
only produces N2 and water as by-products in the catalytic
reductions.[32,33] The catalysts exhibit not only higher catalytic
activity than those catalysts supported on carbon materials
alone, but also more excellent anti-oxidation ability of metallic
Co, remarkably improving the stability and reusability of the
catalysts.
o
500 carbonized at 500 C exhibited the highest catalytic activity and
excellent stability without any decay of activity after 6 cycles for the
reduction of ntirobenzene. Both metallic Co atoms and Co–N
species formed in the Co@CN/SiO2 catalysts were active, but the
Co–N species were dominant active sites. The high activities of the
Co@CN/SiO2 catalysts were attributed to the synergistic effect
between the Co and N atoms, promoting heterolytic cleavage of
hydrazine to form H+/H– pairs. Representative examples
demonstrated that the Co@CN/SiO2-500 could completely transform
various halogen-substituted nitro aromatics to the corresponding
halogenated anilines with high TOFs and selectivity of > 99.5%.
Introduction
Aromatic amines are crucial intermediates for the synthesis of
various fine chemicals, such as pharmaceuticals, agrochemicals,
dyes, and pigments.[1,2] At present, most of aromatic amines are
prepared by catalytic reduction of aromatic nitro compounds.[3,4]
Particularly, heterogeneous catalytic reduction over supported
metal catalysts as an environment-friendly and efficient protocol
attracts much interest.[5,6] Pt-group-metal (Pt, Rh, Ru, Pd)
catalysts have been investigated extensively because of their
high intrinsic activities for the reduction of nitro aromatics.[7-9]
Nevertheless, most of these reported noble-metal catalysts are
poorly chemoselective for the reduction of nitro aromatics to the
corresponding aromatic amines with other reducible functional
groups, especially with halogens (F, Cl, Br, I).[10-12] Moreover, the
scarcity and volatile price of precious metals also limit their
large-scale applications in chemical industry.
Herein, porous silica-supported Co@N-doped carbon
materials (Co@CN/SiO2) were prepared by
a
one-step
impregnation, followed by in-situ carbonation and reduction at
o
500 C. The obtained Co@CN/SiO2 composites exhibited high
catalytic activity as well as excellent chemoselectivity for the
selective reduction of various halogenated nitro aromatics to the
corresponding haloanilines with hydrazine hydrate. More
importantly, there were no oxidation and loss in catalytic
performance observed after recycling at least 6 times for the
reduction of nitrobenzene, and it was revealed that the
Co@CN/SiO2 by the one-step method had significantly higher
stability than the one prepared by the two-step impregnation.
Supported catalysts based on non-noble metals (Fe, Co, Ni)
have been studied to substitute for noble metal catalysts.[13-17]
The Co-based catalysts attract more attention because they
frequently show not only high catalytic activity but also excellent
chemoselectivity of aromatic amines for the reduction of various
functionalized aromatic nitro compounds.[18-20] Nitrogen-doped
porous carbon materials have been demonstrated to be
excellent catalyst supports for the dispersion of Co species for
Results and Discussion
Catalyst characterization
The real contents of Co, C and N in the prepared materials were
measured by inductively coupled plasma atomic emission
spectroscopy (ICP-AES) and X-ray photoelectron spectroscopy
1
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Table 1. Contents of Co, C and N, and specific surface areas, pore volumes and mean pore sizes of the catalysts.
Co crystallite
size by XRD
(nm)
Co
particle
Specific surface
Pore volume
Mean
size (nm)
pore
Catalyst
Co (wt%)
C (wt%)
N (wt%)
size by TEM
(nm)
1
1
area (m2 g
)
(
cm3 g
)
SiO2



324
219
245
214
198
221
270
319
319
221
1.0
12.8
12.5
10.7
10.9
6.7


Co3O4/SiO2-500
Co@C/SiO2-500
CN/SiO2-500
13.5
12.5



0.68
0.54
0.59
0.33
0.36
0.41
0.50
0.49
0.52


19.7
28.3
24.3
25.6
27.9
30.1
22.8
27.5

7.0

7.0

7.3
11.0
8.2
6.3
5.0
3.2
6.1
Co@CN/SiO2-300
Co@CN/SiO2-400
Co@CN/SiO2-500
Co@CN/SiO2-600
Co@CN/SiO2-700
Co@CN/SiO2-imp-500
12.2
12.1
12.4
12.3
13.0
12.8


6.4
4.2
5.0
6.1
6.9
7.1
4.0
4.9
5.8
6.8
6.9
6.3
6.2
6.1
9.2
(XPS), respectively, and the results are given in Table 1. For the
Co@CN/SiO2-T (T referred to the carbonization temperature, T
= 300, 400, 500, 600, 700) samples, the Co contents were all
located at 12.5 ± 0.5 wt%, which were lower than that in the
Co3O4/SiO2-500 due to the addition of C and N. However, the
calcination temperatures remarkably affected the amounts of C
and N incorporated on the catalyst surfaces. The amounts of C
components gradually increased with increasing the
o
temperatures for 300 to 600 C, while the N contents showed a
decline trend in the temperature range. These results could be
explained by the fact that the C and N organic precursors were
progressively condensed on the surfaces, and meanwhile in this
process, parts of C–N bonds were cleaved to generate gas N-
containing compounds, the amounts of which increased with
elevating the temperature in the N2 atmosphere. When the
Figure 1. (a) N2 sorption isotherms and (b) BJH pore size distributions of the
SiO2,CN/SiO2-500, Co@CN/SiO2-T and Co@CN/SiO2-imp-500 catalysts.
o
temperature was further elevated to 700 C, the decomposition
condensation and/or decomposition of organic precursors to
generate porous amorphous N-containing carbon. The
decreases in the pore size with the temperatures were likely due
to the formation of more nonporous N-doped graphitic carbon on
the pore surfaces, which could be supported by the following
XPS spectra in Figure 4. The Co@CN/SiO2-imp-500 in Table 1
exhibited slightly smaller specific surface area, but larger pore
volume and pore size than the Co@CN/SiO2-500, probably
because the two-step impregnation route resulted in the
blockage of more micropores in the support.
and gasification of the C–N bonds were sped up, resulting in a
rapid decrease in the C and N contents on the catalyst surfaces.
In the case of the Co@CN/SiO2-imp-500, there were similar
amounts of Co,
C and N to the Co@CN/SiO2-500 as
summarized in Table 1.
N2 sorption measurements were applied to analyze the
textural properties of the materials. As illustrated in Figure 1a for
the SiO2, Co@CN/SiO2-T (T = 300, 400, 500, 600, 700),
CN/SiO2-500, and Co@CN/SiO2-imp-500 materials, all the
samples exhibited typical type IV isotherms with clear hysteresis
loops, which were an indication of mesoporous structure, similar
to pure silica support. This result demonstrated that the
mesoporous structure of silica was still kept after being
incorporated with Co, C and N components. The similar pore
size distributions of the samples in Figure 1b also proved this
point, and the pore sizes were primarily in the range of 5–20 nm.
However, as seen in Table 1, all the catalysts showed a
remarkable decline in specific surface area, pore volume and
pore size due to the incorporation of Co, C and N, compared
with pure silica. In the case of the Co@CN/SiO2-T (T = 300, 400,
500, 600, 700) samples, in general, the specific surface areas
and pore volumes gradually increased with increasing the
calcination temperature. This result could be attributed to the
Powder X-ray diffraction (XRD) patterns of the CN/SiO2-500,
Co@CN/SiO2-T (T
=
300, 400, 500, 600, 700), and
Co@CN/SiO2-imp-500 materials are presented in Figure 2. All
the samples showed a broad diffraction peak at 2θ = ~22.5o,
which was assigned to amorphous silica and/or graphitic carbon.
Because the diffraction patterns of amorphous silica and carbon
were overlapped, the carbon structures in the catalysts were
analyzed by Raman spectra (not shown). All the Co@CN/SiO2
catalysts exhibited a defect (D) band at ~1356 cm−1 and a
graphite (G) band at ~1580 cm−1, which were attributed to the
disordered graphitic carbon and the graphitization degree,
respectively.[19,31] Generally, the graphitization degrees
increased with elevating the calcination temperature. For the
2
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Co(111)
Co@CN/SiO2-imp-500
T = 700
T = 600
T = 500
T = 400
T = 300
CN/SiO2-500
10 20 30 40 50 60 70 80 90
2θ/o
Figure 2. XRD patterns of the CN/SiO2-500, Co@CN/SiO2-T and
Co@CN/SiO2-imp-500 samples.
Co@CN/SiO2-T samples, no peak associated with Co species
was found for the Co@CN/SiO2-300 sample, indicating that Co
oxide species were highly dispersed in the CN/SiO2 matrix, and
that there was no metallic Co particles formed in the calcination
o
process. When the temperature was elevated to 400 C, a new
diffraction peak at 2θ = 44.2o was observed, corresponding to
cubic metallic Co (111) reflections (PDF 15–0806). This result
indicated that the Co oxides could be reduced to generate
metallic Co crystallites with glucose and/or urea under the N2
atmosphere. As the calcination temperature was further
increased, the intensity of the diffraction peaks for metallic Co
was strengthened, but the peak width became narrower. This
result implicated that more Co oxides were reduced to form
metallic Co crystallites, and the Co crystallite sizes increased
with the calcination temperature. Table 1 gives the mean sizes
of the Co crystallites obtained using the peak width of Co (111)
by the Scherrer equation. The Co crystallite sizes were
enhanced from 4.2 to 6.9 nm when the temperatures were
elevated from 400 to 700 oC. As expected, the Co@CN/SiO2-
imp-500 exhibited stronger diffraction peaks and narrower peak
widths for metallic Co than the Co@CN/SiO2-500, which was
indicative of the reduction of more Co oxides to metallic Co and
the presence of larger Co crystallites (7.1 nm). It was noteworthy
that the larger Co crystallites (7.0 nm) were also formed on the
Co@C/SiO2-500 sample in Table 1. This result implied that the
use of urea precursor or the addition of N improved the
dispersion of metallic Co on the catalyst surface.
Figure 3. TEM images and particle size distributions of the materials. (a)
Co@CN/SiO2-300, (b) Co@CN/SiO2-400, (c) Co@CN/SiO2-500, (d)
Co@CN/SiO2-600, (e) Co@CN/SiO2-700 and (f) Co@CN/SiO2-imp-500.
elevated from 400 to 700 oC, the Co particle sizes increased
from 4.0 to 6.8 nm. In terms of the Co@CN/SiO2-imp-500, both
the density and the mean size of the Co particles were larger
than those for the Co@CN/SiO2-500. These results further
confirmed that the calcination temperature and preparation route
had significant effects on the reduction degree of the Co oxide
species with organic precursors, and on the sizes of metallic Co
crystallites or particles formed, as revealed by the XRD results in
Figure 2 and Table 1. It could be seen from Table 1 that the Co
particle sizes counted by TEM were approximately equal to the
mean sizes of Co crystallites by XRD. This meant that the Co
particles existed in the single crystalline form, which was also
proved by the typical HR-TEM images of Co particles (not
shown).
Transmission electron microscopy (TEM) images of the
The XPS spectra were used to study the elemental
compositions, chemical states and the interactions among the
elements on the surfaces. The analysis of the XPS survey
spectra revealed that for all the Co-containing catalysts in Table
2, the surface Co compositions were much lower than those
determined by ICP in Table 1. This result demonstrated that the
Co species might be situated mainly below the surface and
subsurface of the matrices. In the case of the Co@CN/SiO2-T (T
= 300, 400, 500, 600, 700), the Co contents on the surfaces
Co@CN/SiO2-T (T
=
300, 400, 500, 600, 700) and
Co@CN/SiO2-imp-500 materials were shown in Figure 3. No
metallic Co particles were found on the Co@CN/SiO2-300;
however, the Co@CN/SiO2-400 exhibited sparse darker metallic
Co particles with a mean size of ca. 4.0 nm. When the
calcination temperatures were continuously increased, both the
densities and the average sizes of the Co particles were
increased. As seen in Table 1, when the temperature was
3
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The N 1s XPS spectra of the prepared catalysts are illustrated
in Figure 4b. The N 1s XPS spectra of the Co@CN/SiO2-T (T =
300, 400, 500) could be fitted into two components peaked at
the BEs of 398.5 eV and 399.5 eV, which were attributed to
pyridine-type N, pyridinic N for the Co–N species and/or pyrrole-
type N, respectively.[38-41] When the temperature was elevated to
Table 2. Total Co contents and Co compositions in individual Co species on
the catalyst surface by XPS.
Total surface
Co (wt%)
Metallic
Co (wt%)
Co3O4
(wt%)
Co–N
(wt%)
Catalyst
Co3O4/SiO2-500
6.3
5.9
5.4
4.8
3.5
2.2
2.8
0
6.3
4.8
3.7
1.3
0.9
0.5
0.7
0
o
600 C, graphitic N was observed at the BE of 401.0 eV,[42,43]
Co@CN/SiO2-300
Co@CN/SiO2-400
Co@CN/SiO2-500
Co@CN/SiO2-600
Co@CN/SiO2-700
Co@CN/SiO2-imp-500
0
1.1
1.2
2.2
1.1
0.5
0.5
and its relative peak intensity was strengthened with further
increasing the temperature, and in the meanwhile, the relative
peak intensity for pyridinic N was found to be decreased.
However, the peak intensities for the pyridinic N for the Co–N
and/or pyrrole-type N showed no obvious change (Table S2).
These results also suggested that graphitic N atoms should be
formed predominantly by the transformation of the pyridinic N
atoms in the matrices. It was noteworthy that the CN/SiO2-500
exhibited a clear N 1s XPS spectrum from graphitic N, but a
weaker relative intensity of pyridinic N compared with the
Co@CN/SiO2-T (T = 300, 400, 500). This result indicated that
the presence of Co could suppress the transformation of
pyridinic N to graphitic N probably by the formation of Co–N
during the pyrolysis process of organic precursors. In terms of
0.5
1.3
1.5
1.2
1.6
decreased with elevating the calcination temperature mainly
because of the combination of two factors of the increases in C
contents and particle sizes of metallic Co on the catalyst
surfaces. Furthermore and for the same reason, the surface Co
composition on the Co@CN/SiO2-500 was higher than on the
Co@CN/SiO2-imp-500, but smaller than on the Co3O4/SiO2-500
as shown in Table 2.
the Co@CN/SiO2-imp-500, more pyridinic
N
atoms were
transformed into graphitic
Co@CN/SiO2-500.
N atoms, compared with the
Figure 4a displays the Co 2p XPS spectra of the Co3O4/SiO2-
500, Co@CN/SiO2-T (T = 300, 400, 500, 600, 700) and
Co@CN/SiO2-imp-500 samples. It could be found that the
Co3O4/SiO2-500 sample exhibited a symmetric Co 2p3/2 primary
spectrum peaked at the binding energy (BE) of 780.7 eV, which
was assigned to Co3O4.[34,35] For the Co@CN/SiO2-T samples, a
broadened non-symmetric Co 2p3/2 peak was observed, which
could be deconvoluted into two or three components. The
Co@CN/SiO2-300 had two peaks positioned at the BEs of 780.7
eV and 782.5 eV, corresponding to Co3O4 and Co−N species,
respectively.[23,24,26,36] When the calcination temperature was
o
increased to 400 C, a new peak at the BE value of 778.2 eV
appeared for metallic Co,[26,37] and its peak intensity was
strengthened with the increase in the temperature relative to the
total Co species. These results demonstrated that Co oxides
o
could be reduced to metallic Co above 400 C, and the relative
amounts of metallic Co were enhanced with the temperature
(Table S1), which was in agreement with the XRD and TEM
results shown in Figure 2 and Figure 3. In terms of the Co−N
species formed on the surfaces, the relative peak intensity to
total Co species increased with elevating the calcination
temperature, but exhibited a maximum at 500 oC (Table S1).
This phenomenon could be attributed to the formation of larger
Co particles and the reduction in N content on the catalyst
surfaces at higher temperatures as listed in Table 1. The Co
percentage contents in the individual Co species on the catalyst
surfaces were calculated and summarized in Table 2. It could be
seen that for the Co@CN/SiO2-T catalysts, the Co3O4 contents
on the surfaces always decreased with the calcination
temperature, however, the content of metallic Co for the
Co@CN/SiO2-600 and the Co content in the Co–N species for
the Co@CN/SiO2-500 showed a maximum value, respectively,
due to the combination factors including total Co contents, Co
particle sizes and N contents on the catalyst surfaces. As for the
Co@CN/SiO2-imp-500, more Co oxide was reduced into metallic
Co, but significantly smaller amounts of Co–N species were
formed, compared with the Co@CN/SiO2-500.
Figure 4. XPS spectra of the Co3O4/SiO2-500, Co@CN/SiO2-T, Co@CN/SiO2-
imp-500, and CN/SiO2-500 samples.
4
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Table 3. Selective reduction of nitrobenzene to aniline over various catalysts[a]
.
It has been reported that the pyrrolic and pyridinic N sites not
only provide strong N coordination but also act as anchoring
sites for the formation and stabilization of Co particles.[44,45] The
Co@CN/SiO2-500 and Co@CN/SiO2-imp-500 samples were
analyzed by XPS after exposure to air for more than 3 months
(Figure S1). It was found that the Co 2p spectra of the
Co@CN/SiO2-500 exhibited no change, while that of the
Co@CN/SiO2-imp-500 demonstrated that all of metallic Co was
re-oxidized on the surface. This result revealed that the
Co@CN/SiO2-500 catalyst possessed superior anti-oxidation
ability in air.
Entry
Catalyst
Conv. (%)
Sel. (%)
1
CN/SiO2-500
0

2
Co3O4/SiO2-500
0.1
100
100
100
100
100
100
100
100
100
100
100
100
100
3
Co3O4/SiO2-500-H2
Co@CN/SiO2-300
Co@CN/SiO2-400
Co@CN/SiO2-500
Co@CN/SiO2-600
Co@CN/SiO2-700
Co@CN/SiO2-500
Co@CN/SiO2-imp-500
Fe@CN/SiO2-500
Ni@CN/SiO2-500
Pt@CN/SiO2-500
Pd@CN/SiO2-500
3.0
4
32.1
41.2
65.3
20.3
14.7
3.5
5
6
Catalytic reaction
7
Nitrobenzene was first applied as a model compound to
compare the catalytic activities over the prepared catalysts for
the selective reduction of nitro aromatics to the corresponding
anilines with hydrazine hydrate and the reaction results are
summarized in Table 3. The CN/SiO2-500 and Co3O4/SiO2-500
catalysts (entries 1 and 2) showed no activity, while the fresh
Co3O4/SiO2-500-H2 obtained by the H2 reduction at 500 oC
8
9b
10
11
12
13
14
12.0
5.0
without exposure to air exhibited
a 3.0% nitrobenzene
9.5
conversion under the identical conditions (entry 3). However,
when the Co3O4/SiO2-500-H2 was exposed to ambient air for
more than 24 h, it became almost completely deactivated due to
the surface oxidization of metallic Co. These results
demonstrated that metallic Co atoms on the surfaces were
33.4
20.8
[a] Reaction conditions: 150 mmol nitrobenzene, 25 mg catalyst, 20 mL
ethanol, 600 mmol N2H4·H 2O, 70 oC, 30 min. [b] Reaction conditions: 150
mmol nitrobenzene, 25 mg catalyst, 20 mL ethanol, 2.0 MPa H2, 120 oC.
active to
a certain extent. All the Co@CN/SiO2-T and
Co@CN/SiO2-imp-500 catalysts (entries 4–9) exhibited
significantly higher conversions of nitrobenzene. This result
implicated that the presence of N-doped carbon from glucose
and urea precursors could greatly improve the performance of
the Co@CN/SiO2 catalysts for the selective reduction of nitro
aromatics with hydrazine.
nitrobenzene conversion of 12.0% (entry 10), due to smaller
amounts of Co–N active species on the catalyst surface as
shown in Table 2, compared with the Co@CN/SiO2-500. Entries
11–14 in Table 3 also demonstrated that the catalytic activity of
the Co@CN/SiO2-500 for the reduction of nitrobenzene was not
only higher than non-noble metal Fe, Ni@CN/SiO2-500, but also
more excellent than noble metal Pt, Pd@CN/SiO2-500 catalysts
by the same one-step impregnation method. In Table 3, all the
catalysts exhibited an aniline selectivity of 100% for the selective
reduction of nitrobenzene.
The reduction of nitrobenzene was further used to investigate
the stability and recyclability of the Co@CN/SiO2-500 and
Co@CN/SiO2-imp-500 for comparison. The reaction conditions
for ca. 88.3% nitrobenzene conversion at 60 min over the
Co@CN/SiO2-500 were selected for the first reaction, where the
reduction of nitro groups was controlled by chemical kinetics.
The spent catalyst after every reaction was recovered by
centrifugation, washing with ethanol and drying at 80 oC. As
presented in Figure 5, the nitrobenzene conversions were
almost kept constant over the Co@CN/SiO2-500 for the 6-time
cycles. In contrast, the nitrobenzene conversions over the
Co@CN/SiO2-imp-500 declined from 20.4% for the first reaction
to 13.1% after 6 cycles. After every cycling reaction, the Co
contents in the product solutions were determined by ICP-AES.
The solutions for the Co@CN/SiO2-500 hardly contained the Co
element (< 1 ppm), while for the Co@CN/SiO2-imp-500, the Co
contents in the solutions were always above 20 ppm.
The reaction results in entries 4–8 of Table 3 revealed that the
calcination temperatures significantly affected the activity of the
Co@CN/SiO2-T for the selective reduction of nitrobenzene. The
nitrobenzene conversions increased with elevating the
o
temperature, and showed a maximum value of 65.3% at 500 C.
Turnover frequency (TOF) estimated on the basis on the total
moles of Co in the Co@CN/SiO2-500 by ICP reached 4282 h-1
(Table 4), which, to the best of our knowledge, was the highest
TOF over various non-noble metal and metal-free catalysts ever
reported for the selective reduction of nitrobenzene with
hydrazine,[46] sodium borohydride,[47] formic acid[48] or gas
hydrogen[49] as
a reducing agent under similar reaction
conditions (Table S3). When the temperature was further
elevated, the nitrobenzene conversion showed a clear decline,
for instance, 20.3% and 14.7% for Co@CN/SiO2-600 and
Co@CN/SiO2-700, respectively. These variations matched well
with the compositions of Co−N species on the catalyst surfaces
shown in Table 2. It could be concluded that the Co atoms with
the strong interaction with the N atoms, i.e., Co−N species, were
dominantly responsible for the high catalytic activity of the
Co@CN/SiO2-T
catalysts.
However,
the
nitrobenzene
o
conversion was only 3.5% even at 120 C for the Co@CN/SiO2-
500 catalyst with hydrogen as the reducing agent. This
phenomenon implied that the nitrobenzene reduction was not
performed through the decomposition hydrogenation route,
although hydrazine was easy to be decomposed into N2 and
H2.[50,51] The Co@CN/SiO2-imp-500 only showed a lower
The spent catalysts were characterized by XRD, TEM, XPS,
and ICP. It could be found that XRD patterns and TEM images
of the spent Co@CN/SiO2-500 and Co@CN/SiO2-imp-500
catalysts had no observable variations in phase structure and
Co particle size after 6 cycles (Figure S2 and S3). Figure 6 also
5
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10.1002/cctc.202000683
ChemCatChem
FULL PAPER
revealed that the Co@CN/SiO2-500 before and after cycling
exhibited almost the same XPS Co 2p spectra; however, for the
spent Co@CN/SiO2-imp-500, the peak for metallic Co
disappeared after the cycles, indicating that the metallic Co
atoms on the surface were re-oxidized into Co oxides. These
results further confirmed that the Co@CN/SiO2 catalysts by our
one-step impregnation and in-situ carbonization method had
more excellent anti-oxidation ability for the reactions. ICP
analysis demonstrated that the total Co content in the spent
Co@CN/SiO2-500 was 12.1%, very close to that before the
reaction in Table 1; and the Co content of the spent
Co@CN/SiO2-500-imp was 8.5 wt%, obviously lower than that
before the reaction.
The halogenated anilines were an important group of key
intermediates and raw materials in the pharmaceutical industry,
such as chlorfluazuron and hexaflumuron.[52-54] However, over
traditional noble metal catalysts the selective reduction of the
halogenated nitro aromatics to the halogenated anilines always
made some unwanted dehalogenated side-products which are
difficult to separate, seriously reducing the product quality.[55-57]
The reductions of some representative halogen-substituted nitro
aromatics were examined over the Co@CN/SiO2-500 catalyst
using hydrazine hydrate and the results are given in Table 4. To
our great surprise, all the halogenated nitro aromatics could
smoothly be transformed into the corresponding halogenated
anilines with a selectivity of > 99.5%. The halogen atoms on
benzene rings exhibited a great and complicated influence on
the catalytic activity. However as a whole, the catalytic activity
for the reduction of the halogen-substituted nitro aromatics with
a halogen atom at the same position could be found to follow the
following order：Br > I > Cl > F (entries 2–11), which was
different from that on Pt-based catalysts (F > Cl > Br > I)
reported in the previous document;[51,58] This phenomenon
meant that the reductions of the halogenated nitro aromatics
over the Co@CN/SiO2 catalysts might have different reaction
mechanism from those over the Pt-based catalysts. For the
reduction of the halogen-substituted nitro aromatics with the
same halogen atom at the different positions, the catalytic
activity followed the order: ortho- > meta- > para-. Entries 21 and
22 in Table 4 revealed that the halogenated N-heterocyclic nitro
aromatics could also be smoothly and quantitatively converted to
the corresponding halogenated N-heterocyclic aromatic amines
with high TOFs.
100
Co@CN/SiO2-imp-500
Co@CN/SiO2-500
Table 4. Selective reducitons of various halogenated nitroarenes over
80
60
40
20
0
Co@CN/SiO2-500[a]
.
Time
(min)
Conv.
(%)
Sel.
(%)
TOF[b]
(h-1)
Entry
Substrate
1
2
70
100
100
>99.5
>99.5
4282
552
480
1
2
3
4
5
6
Recycle run
3
4
5
240
150
180
100
100
100
>99.5
>99.5
>99.5
971
Figure 5. Reuse tests of the Co@CN/SiO2-500 and Co@CN/SiO2-imp
catalysts. Reaction conditions: 150 mmol nitrobenzene, 25 mg catalyst, 600
mmol N2H4H2O, 20 mL ethanol, 70 oC, 60 min.
1959
1266
Co3O4
Co0
Co@CN-SiO2-500
Satellite
Co-N
6
180
150
150
180
100
120
100
100
100
100
100
100
>99.5
>99.5
>99.5
>99.5
>99.5
>99.5
1239
2267
2062
1371
3547
2501
7
Co@CN-SiO2-imp-500
8
9
776 778 780 782 784 786 788 790
Binding energy (eV)
10
11
Figure 6. Co 2p XPS spectra of the spent Co@CN/SiO2-500 and
Co@CN/SiO2-imp-500.
6
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10.1002/cctc.202000683
ChemCatChem
FULL PAPER
Metal nitrides have widely been studied and exhibited
excellent catalytic performance for hydrazine decomposition to
H2 and N2.[61-63] It could be assumed that the dissociation of
hydrazine on the surface Co−N species of the Co@CN/SiO2
catalysts was also performed through the similar mechanism of
hydrazine to on metal nitride for the selective reduction of nitro
groups. Therefore, it was proposed that hydrazine molecule
might be first adsorbed on the metallic Co atoms; the synergistic
effect between the Co and pyridinic N atoms in the Co-N species
promoted heterolytic cleavage of hydrazine hydrate to form H–
species and split-over H+ species in a Co–H– form and in a N–H+,
respectively.[50,64,65] The nitro groups on the catalyst surface
interacted with the adjacent H+/H– pairs to form the nitroso
intermediates. These nitroso compounds were further reduced
to hydroxylamine, which was directly hydrogenated to benzene
amine (Scheme 1).
Besides, our experiments demonstrated that the fresh
Co3O4/SiO2-500-H2 showed no activity and the Co@CN/SiO2-
500 generated small amounts of benzene for the reaction of
chlorobenzene alone in ethanol with hydrazine hydrate; however,
for the ethanol solution of mixed chlorobenzene and
nitrobenzene, the Co@CN/SiO2-500 was completely deactivated
for the dechlorination of chlorobenzene, but still remained almost
unchanged activity for the reduction of nitrobenzene. This result
indicated that nitro and Cl groups should be competitively
adsorbed on the catalyst surface for the reactions. It was
concluded that the high selectivity for the selective halogenated
nitro aromatics over the Co@CN/SiO2 could be attributed to the
following aspects: 1) lower activity of the catalyst itself for the
dehalogenation of halogenated nitro aromatics; 2) competitive
adsorption of nitro groups on the catalyst surface inhibiting the
dehalogenation of halogenated nitro aromatics.
12
13
14
15
70
100
100
100
100
>99.5
>99.5
>99.5
>99.5
4162
4052
3437
3547
70
100
100
16
17
18
19
20
21
22
100
190
150
80
100
100
100
100
100
100
100
>99.5
>99.5
>99.5
>99.5
>99.5
>99.5
>99.5
3405
1246
2343
3851
3741
3362
1869
80
100
150
[a] Reaction conditions: 30 mmol substrate, 5 mg catalyst, 4 mL ethanol, 120
mmol N2H4·H 2O, 70 oC. [b] Based on total metal by ICP and initial substrate
conversions at 10 min.
Reduction mechanism of nitro aromatics over Co@CN/SiO2
catalysts
It is widely believed that the reduction of nitro aromatics to the
corresponding aniline proceeds by the two reaction pathways,
i.e., direct and indirect. The direct way includes two consecutive
steps of nitro group to the nitroso, and then to the hydroxylamine,
and finally the hydroxylamine is reduced into the amine;
whereas the indirect way involves the azo intermediates
condensed
by
the
nitroso
and
hydroxylamine
intermediates.[38,59,60] For the reduction of nitrobenzene as the
model reaction over the Co@CN/SiO2 catalysts, no
intermediates including nitrosobenzene, phenylhydroxylamine
and azoxybenzene were detected in the product solutions. In
order to make clear the reaction routes of nitro aromatics with
hydrazine hydrate, some separate experiments were designed
over the Co@CN/SiO2-500. It was found that the reduction rate
of phenylhydroxylamine to aniline was significantly faster than
that of nitrobenzene with hydrazine; while the reduction of
azoxybenzene could hardly proceed under the identical reaction
conditions. These results clearly demonstrated that the reduction
of nitro aromatics to the corresponding anilines was performed
over the Co@CN/SiO2 catalysts through the direct route.
Scheme 1. Possible reduction mechanism of nitroarenes with hydrazine.
Conclusion
Silica-supported Co@N-doped carbon catalysts (Co@CN/SiO2)
have been developed by the one-step impregnation and in situ
carbonization. The Co@CN/SiO2 catalysts could completely
transform various halogen-substituted nitro aromatics to the
corresponding aromatic amines with a TOF of 4282 h-1 and a
selectivity of > 99.5% with hydrazine hydrate. The Co–N species
formed in the Co@CN/SiO2 catalysts were dominant active sites
for the reduction of nitro aromatics. The synergistic effect
7
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10.1002/cctc.202000683
ChemCatChem
FULL PAPER
specific surface areas were calculated by the Brunauer–Emmett–Teller
(BET) method and the single-point pore volume was calculated from the
amount of nitrogen adsorbed at a relative pressure of 0.990. The mean
pore sizes were obtained by the BJH method. The pore size distributions
were calculated using the adsorption branches of the N2 sorption
isotherms by the Barrett–Joyner–Halenda (BJH) method. Raman
measurements were carried out at room temperature on a Renishaw
2000 Ramascope with an Ar+ ion laser (514.4 nm) as the excitation
source. Prior to each measurement, the spectrometer was calibrated with
a silicon wafer. TEM analysis was performed on a JEOL JEM-2010F field
emission microscope at an acceleration voltage of 200 kV. The samples
were prepared by drying an ethanolic dispersion of the well-ground
catalyst powder on a holey-carbon-coated copper grid.
between the Co and N atoms promoted heterolytic cleavage of
hydrazine hydrate to form H+/H– pairs, resulted in higher activity
of the Co@CN/SiO2 catalysts. The recycling tests indicated that
the Co@CN/SiO2-500 catalyst was highly stable and could be
reusability for the selective reduction of nitroarenes without any
loss of catalytic performance and leaching of metal components
due to its high anti-oxidation ability. The present study can be
extended to design other transition metal-based catalysts for
various catalytic reactions.
Experimental Section
Catalytic reaction and product analyses
Chemicals
The chemoselective reduction of aromatic nitro compounds was carried
out in a two-necked flask of 200 mL with a water-cooled condenser tube,
which was placed in a thermostatic water bath with a magnetic stirrer. In
a typical experiment, 150 mmol of nitro aromatics, 25 mg catalyst, 20 mL
of ethanol, and 1 mL of n-decane as an internal standard were added
into the flask, and kept stable at 70 oC for 30 min. Then 600 mmol of
hydrazine hydrate was rapidly added into the reaction solution under
vigorous stirring at a stirring rate of 800 rpm. After the reaction, the liquid
solution was separated from the reaction mixture by centrifugation or
magnet for analysis. Each reaction was repeated more than three times
to reach the carbon balance of more than 98%. The solutions were dried
with anhydrous Na2SO4. The qualitative and quantitative analyses of the
products were done by gas chromatography-mass spectrometry (GC-
MS) (Shimadzu GCMS-QP 2010 Plus), and GC (Varian CP-3800) with a
capillary column (column HP-5, 30 m length, 0.32 mm internal diameter,
0.25 μm film thickness) and a flame ionization detector, respectively.
Commercial SiO2 was purchased from Huating (Shanghai) Nano Science
& technology Co., Ltd. The SiO2 powder was washed and calcined at 550
oC for 6 h before being used as the support. Glucose, urea, metal salts,
hydrazine hydrate aqueous solution (80 wt%) and all nitro compounds
were bought from Sinopharm Chemical Reagent Co., Ltd. All chemicals
and solvents were used as received without further treatment. The gases
(H2, N2) used for catalyst preparation and reaction were ultra-highly pure.
Catalyst preparation
The Co/CN@SiO2 catalysts were prepared by
a facile one-step
impregnation method, followed by in situ carbonization and reduction in a
flow of N2. In a typical process, 1 g of commercial SiO2 powder was
added into a mixed aqueous solution of 20 mL containing 1 g of
Co(NO3)2·6H2O, 0.5 g of glucose, and 1.5 g of urea at room temperature
under stirring for 2 h. Then, the mixture was evaporated under stirring at
60 oC. The obtained solid was calcined in a flow of high-purity N2 (30 mL
min−1) at 300–700 oC for 1 h at a heating rate of 2 oC min−1. The obtained
catalyst was denoted as Co@CN/SiO2-T (T referred to the carbonization
temperature).
For comparison, CN/SiO2-500, Co@C/SiO2-500, Fe@CN/SiO2-500,
and Ni@CN/SiO2-500 were prepared by the identical one-step route with
the aqueous solutions containing the same mass percentages of metal
and/or carbon and nitrogen sources; Pt@CN/SiO2-500, and
Pd@CN/SiO2-500 containing 2 wt% noble metals were also prepared by
the same approach, except that H2PtCl6 and PdCl2 were used as Pt and
Pd sources, respectively; The Co3O4/SiO2-500 was prepared by the
same way as the Co/CN@SiO2-500 with the Co(NO3)2·6H2O aqueous
solutions without carbon and nitrogen sources. The Co3O4/SiO2-500-H2
Acknowledgements
This research was supported by National Natural Science
Foundation of China (U1860203, 51574164) and CAS
Interdisciplinary Innovation Team.
Keywords: Heterogeneous catalysis • Halogenated nitroarenes
• Chemoselective reduction • N-doped carbon • Co nanoparticles
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ChemCatChem
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10.1002/cctc.202000683
ChemCatChem
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The Co@CN/SiO2 catalysts prepared by a one-step impregnation route exhibit high catalytic activity and excellent stability for the
reduction of nitro aromatics with hydrazine hydrate. Representative examples demonstrate that the Co@CN/SiO2 catalysts can
completely transform various halogen-substituted nitro aromatics to the corresponding halogenated anilines with high TOFs and
selectivity of > 99.5%.
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