Selective hydrogenation of nitroarenes under mild conditions by the optimization of active sites in a well defined Co?NC catalyst

Article abstract of DOI:10.1039/d0gc01835j

The catalytic hydrogenation of aromatic nitro compounds containing multiple functional groups into amino compounds with high conversion rates, selectivity, and stability under mild conditions is a great challenge. Herein, a well defined catalyst (Co?NC) is prepared through the pyrolysis of the Co-centered metal-organic framework (MOF) at the optimized temperature. The as-synthesized catalyst exhibits a high conversion rate and selectivity for the hydrogenation of 12 aromatic nitro compounds with different competing groups into desired amino compounds with hydrazine hydrate under mild conditions (80 °C, 30 min, and 1 atm). The catalyst also shows excellent stability and can be reused over 20 times without considerably losing its activity. It is found that the Co-Nx site is the main active site for catalytic hydrogenation, and the Mott-Schottky effect between the surface Co NPs and N-doped carbon can further promote the hydrogenation reaction. EXAFS, TEM, XPS, and Raman analyses confirm that cobalt nanoparticles (NPs) are properly encapsulated by the N-doped carbon matrix at the optimized temperature, and the Co species maintain a high spin state after the catalysis, which may be responsible for the high performance of Co?NC. This work demonstrates not only a highly efficient catalyst for hydrogenation under mild conditions, but also provides insight into the active sites in Co-based catalysts for hydrogenation.

Full text of DOI:10.1039/d0gc01835j

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Selective hydrogenation of nitroarenes under mild
conditions by the optimization of active sites in a
well defined Co@NC catalyst†
Cite this: DOI: 10.1039/d0gc01835j
Shuo Chen,‡ Li-Li Ling,‡ Shun-Feng Jiang and Hong Jiang
*
The catalytic hydrogenation of aromatic nitro compounds containing multiple functional groups into
amino compounds with high conversion rates, selectivity, and stability under mild conditions is a great
challenge. Herein, a well defined catalyst (Co@NC) is prepared through the pyrolysis of the Co-centered
metal–organic framework (MOF) at the optimized temperature. The as-synthesized catalyst exhibits a
high conversion rate and selectivity for the hydrogenation of 12 aromatic nitro compounds with different
competing groups into desired amino compounds with hydrazine hydrate under mild conditions (80 °C,
30 min, and 1 atm). The catalyst also shows excellent stability and can be reused over 20 times without
considerably losing its activity. It is found that the Co–Nx site is the main active site for catalytic hydro-
genation, and the Mott–Schottky effect between the surface Co NPs and N-doped carbon can further
promote the hydrogenation reaction. EXAFS, TEM, XPS, and Raman analyses confirm that cobalt nano-
particles (NPs) are properly encapsulated by the N-doped carbon matrix at the optimized temperature,
and the Co species maintain a high spin state after the catalysis, which may be responsible for the high
performance of Co@NC. This work demonstrates not only a highly efficient catalyst for hydrogenation
under mild conditions, but also provides insight into the active sites in Co-based catalysts for
hydrogenation.
Received 30th May 2020,
Accepted 27th July 2020
DOI: 10.1039/d0gc01835j
rsc.li/greenchem
of commonly used precious metal catalysts for the hydrogen-
ation of nitroarenes limits the widespread use of these cata-
1. Introduction
Amines are an important chemical substance in pesticides, lysts. Therefore, low-cost, highly selective, and highly stable
medicines and dyes. The hydrogenation of corresponding catalysts for the selective reduction of nitroarenes to corres-
nitroarenes to functionalized amines, important chemicals for ponding amines are urgently needed.6
agrochemicals, pharmaceuticals, and dyes and pigments, is a
In recent years, transition metal nanoparticles (NPs) were
typical reduction reaction,1 which usually depends on metal- widely reported to replace precious metal catalysts in hydro-
based heterogeneous catalysts. However, most organic com- genation reactions.7,8 For instance, nickel-based and copper-
pounds contain unsaturated functional groups, such as CvC, based catalysts are widely used in the hydrogenation of nitro-
CvO, and C–Cl, apart from nitro groups. These functional benzene. Unfortunately, owing to the high surface energies,
groups are easily hydrogenated to saturated functional groups small metal NPs with high catalytic performance are usually
during the hydrogenation of the nitro groups, producing unde- prone to re-aggregation during recycling, thus damaging the
sired byproducts,2 particularly when noble metal catalysts (Pd, stability of catalysts.9 Confining metal NPs inside heteroatom-
Au, and Pt) with high hydrogenation catalytic activity are doped porous materials can not only tune the catalytic activity
used.3–5 Thus, selectively reducing the nitro groups on nitro- but also significantly improve the stability of catalysts.10,11 For
arenes containing other unsaturated functional groups to instance, uniform thin Co nanoclusters supported on N-doped
amino groups is still a big challenge. In addition, the high cost carbon were reported for the hydrogenation of nitro com-
pounds to anilines;12 owing to the N coordination, the
obtained ultra small Co nanoclusters were intercalated within
CAS Key Laboratory of Urban Pollutants Conversion, Department of Applied
graphitic carbon layers and thereby greatly improved the cata-
lytic performance.13,14 However, the preparation of the Co
Chemistry, University of Science and Technology of China, Hefei 230026, China.
E-mail: jhong@ustc.edu.cn
nanoclusters supported on N-doped carbon is still laborsome,
and the stability of the as-prepared catalyst needs to be
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0gc01835j
improved.
‡These authors contributed equally to this work.
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Metal–organic frameworks (MOFs) represent a class of crys- (MeIm, purity 99%), hydrazine hydrate (N2H4·H2O), hydro-
talline porous materials, which feature tailorability, structural chloric acid (37%), ethanol (>99%), methanol, Co powder,
diversity, and ultrahigh surface areas for potential applications nitrobenzene, nitrobenzaldehyde, nitrostyrene, and different
in many fields.15 MOFs as host materials with a confinement aromatic or halogen nitro compounds were of analytic grade
effect can stabilize fine metal NPs during pyrolysis, and the and used without further purification.
MOF-obtained carbon support can improve electron transfer
2.2. Synthesis of ZIF-67 and ZIF-8
during catalytic hydrogenation.16 Therefore, MOFs are con-
sidered as a promising candidate for synthesizing catalyst car- The synthesis method of ZIF-67 is shown in Fig. 1a. Briefly,
riers. For instance, Sun et al. prepared a Co–N–C material by 2.933 g of Co(NO3)2 and 6.489 g of imidazole were dissolved in
high-temperature carbonization of ZIF-67 and cobalt carbide a mixed solution of 160 mL of methanol and 160 mL of
salt and used it in chemoselective hydrogenation of nitroaro- ethanol, respectively. Then, the solution of Co(NO3)2 was
matics. However, high performance is achieved only under the poured into the solution of imidazole to form a mixed solu-
conditions of 110 °C, 3 MPa H2, and 3 h, which need to be tion, and the mixture was incubated at room temperature for
considerably improved.17 In addition, cobalt-based catalysts 48 h. Then, the resulting purple materials were collected by
are often used in gas-phase hydrogenation, and their catalytic centrifugation. For the synthesis of ZIF-8, 1.68 g of Zn (NO3)2
activity will decrease after several cycles due to the high temp- and 3.7 g of imidazole were dissolved in 80 mL of methanol,
erature and high pressure reaction conditions.18
respectively. The imidazole solution was then transferred into
Besides the severe conditions for selective hydrogenation of a Zn (NO3)2 solution, and the mixture was stirred at room
nitroaromatics, the exact hydrogenation active sites in the temperature for 24 h. Then, the obtained white material was
cobalt-based catalyst need further investigation. For instance, collected by centrifugation and further dried at 80 °C.
Hu et al. studied the interaction between Co particles and N
2.3. Preparation of Co@NC, NC, Co@NC(al), Co–NC and Co/
AC
species in the nitroaromatic hydrogenation system of Co@NC
material derived from ZIF-9, but the effect of different Co
species on catalytic hydrogenation has not been explored.19 The ZIF-67 was heated in a tube furnace under N2 flow with a
Liu et al. found that the Co@C catalyst had no detectable ramp rate of 3 °C min−1, maintained at a set temperature
hydrogenation activity after acid leaching, and concluded that (800 °C) for 3 h and then cooled to room temperature, and the
the hydrogenation activity of the Co@C catalyst was attributed black catalyst was obtained named Co@NC. As a comparison,
to the metal Co NPs supported on the catalyst surface.20 ZIF-67 was calcined at 600 °C and 1000 °C under a N2 atmo-
However, Cheng et al. proposed that exposed Co NPs do not sphere to obtain Co@NC-600 and Co@NC-1000, respectively,
play a role in hydrogenation activity, and the real active site is while maintaining the same heating rate and calcination time.
a highly dispersed Co–Nx species located in the carbon In order to remove Co NPs on the catalyst surface and only
substrate.21
retain the Co–Nx structure, Co@NC was immersed in 6 M HCl
Thus, the main objectives of this study were to prepare a solution for 24 h, and then washed several times with water to
high performance Co-based catalyst for selective hydrogen- neutrality. After centrifugation, it was dried under vacuum at
ation of nitroaromatics under mild conditions and elucidate 60 °C, and the product was named Co@NC(al). In order to syn-
the exact active sites in the catalyst for hydrogenation. For thesize a material with a similar NC (N-dope carbon) structure
these purposes, we obtained well-confined Co NPs in N-doped as well as the same content and size of metal Co without a
carbon (Co@NC) by facile pyrolysis of a metal–organic frame- package structure, a ZIF-8 material with the same configur-
work (MOF) precursor ZIF-67 at an optimized pyrolytic temp- ation as ZIF-67 was calcined at 800 °C for 3 h to obtain NC,
erature. The conditions for hydrogenation reactions were opti- and the NC was uniformly mixed with Co (NO3)2 in the same
mized. The conversion rate and selectivity in the hydrogen- proportion by the dipping method. After centrifugal drying, it
ation of 12 aromatic nitro compounds with different compet- was calcined at 800 °C for 3 h at a heating rate of 3 °C min−1
ing groups in N2H4·H2O and Co@NC catalytic systems are under a N2 atmosphere, and finally Co–CN was obtained. In
detailedly investigated. The stability of the catalyst is also eval- addition, the same proportion of activated carbon AC and Co
uated after 20 cycles. Furthermore, we have conducted in- (NO3)2 were uniformly mixed and calcined under the same
depth research on the active sites of Co@NC and their effects conditions to obtain Co/AC.
on hydrogenation by X-ray absorption near edge structure
(EXAFS), transmission electron microscopy (TEM), X-ray photo-
2.4. Material characterization
electron spectroscopy (XPS), and Raman analyses.
Powder X-ray diffraction (XRD) patterns were recorded using a
nickel-filtered Cu Kα radiation source (30 kV/160 mA, λ =
0.154056 nm) (Mxpahf, Rigaku, Japan). Scanning electron
microscopy (SEM) and TEM measurements were carried out by
using a Sirion 200, FEI electron optics company (USA) and a
JEOL (FE-TEM, JEM-2100F, Japan), respectively. XPS was used
2. Experimental section
2.1. Chemicals
Cobalt nitrate hexahydrate (Co(NO3)2·6 H2O, >99%), zinc to determine the surface properties and chemical state of cata-
nitrate hexahydrate (Zn(NO3)2·6 H2O, 98%), 2-methylimidazole lysts in an ESCALAB250 system (Thermo Fisher Inc., USA). The
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Fig. 1 (a) The preparation of Co@NC from ZIF-67. The representative TEM images (b, c) of ZIF-67 and (d) HRTEM image of Co@NC. (e–g) TEM
images of (e) Co@NC-600, (f) Co@NC, and (g) Co@NC-1000. (h–j) Co particle size distribution histogram at different temperatures: (h)
Co@NC-600, (i) Co@NC, and ( j) Co@NC-1000.
Barrett–Emmett–Teller (BET) method was used to detect the 1 × 1010 photons per s. Co loading of the catalyst was obtained
specific areas and pore volumes of the catalysts in by determining the Co concentration in the concentrated
Micromeritics Gemini apparatus at 77 K (ASAP 2020 M + C, nitric acid extraction solution of the sample using inductively
Micromeritics, USA). Raman experiments were conducted on a coupled plasma-optical emission spectrometry (ICP-MS,
LABRAMHR spectrometer (France) using the green line of an Optima 7300 DV, PerkinElmer Corporation, USA).
argon laser (λ = 514.5 nm) as the excitation source. The elec-
2.5. Hydrogenation reaction
tronic structure and chemical state of catalysts electrodepos-
ited onto N-doped carbon were investigated by X-ray spec-
troscopy (ESCALAB250, Thermo-VG Scientific, UK). The XANES
soft X-ray absorption spectroscopy (sXAS) spectra were
measured at the photoemission end-station at the beamline
BL10B in the National Synchrotron Radiation Laboratory
(NSRL) in Hefei, China. A bending magnet is connected to the
beamline, which is equipped with three gratings covering
photon energies from 100 to 1000 eV. In this experiment, the
samples were kept in the total electron yield mode under an
ultra-high vacuum at 5 × 10−10 mbar. The resolving power of
the grating was typically E/ΔE = 1000, and the photon flux was
10 mg of the catalyst and 1 mmol nitrobenzene were dissolved
in 5 mL ethanol in a round-bottom flask for thorough mixing
at a stirring rate of 500 rpm, then 6 equivalents of N2H4·H2O
were added into the above solutions and the mixture was
refluxed under strong stirring for the given reaction time. Next,
the resulting reaction mixture is filtered and liquids are deter-
mined by gas chromatography with flame ionization detection
(GC-FID) and gas chromatography-mass spectrometry
(GC-MS). The reaction product was quantitatively determined
by an external standard method. As a comparison, the catalytic
activity of Co@NC(al) and Co–NC was also measured under
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the same experimental conditions. The stability of the catalyst (Fig. 1e–j). Two reasons can explain the increase in Co NP size.
in the hydrogenation reaction was checked by the following First, increasing the calcination temperature promotes the
method. Considering the catalyst loss in the cycle experiment, growth of Co NPs.22 Second, several adjacent metal particles
the amount of reactants and the catalyst was increased by 5 fuse after the melting of the surface, thereby increasing the
times during the cycle experiment. After each reaction, the particle size.23 Given that large Co NPs are unfavorable to the
catalyst was collected using a simple external magnet, and the catalytic reaction, 800 °C was adopted in the preparation of
collected catalyst was dried for the next experimental cycle. Co@NC for hydrogenation.
The conversion rate and selectivity were evaluated based on
the amount of nitrobenzene (Text S1†).
The XRD pattern in Fig. 2a shows the composition of
Co@NC. Three significant peaks at 44.1°, 51.6°, and 75.9° can
be attributed to the (111), (200), and (220) reflections of metal-
lic Co, respectively. The presence of these peaks suggests that
the Co ions were successfully reduced into a metallic state
after carbonization.24,25 The remaining peak at approximately
25° can be attributed to the (002) peak of graphitic carbon
materials with a low degree of graphitization. These results
agree with the TEM images.
Further insights into the structural and electronic pro-
perties of the materials can be obtained from Raman spec-
troscopy. Fig. 2b shows two sharp peaks at 1351 and
1591 cm−1, which are attributed to the D and G bands, respect-
ively. The relative ratio of G bands to D bands (IG/ID) can be
used in the evaluation of the crystallization degree of graphitic
carbon structures. IG/ID increased with pyrolysis temperature
from 600 °C to 1000 °C, indicating that high pyrolysis tempera-
ture can lead to ordered graphitic structures. The Raman shift
regions at 515, 609, and 671 cm−1 can be attributed to the
metallic Co and indicated that Co was successfully incorpor-
ated into the carbon matrix.26
3. Results and discussion
Co@NC was synthesized as shown in Fig. 1a. Fig. 1b and c
show that the precursor ZIF-67 is a nearly regular hexagon and
is evenly distributed, indicating the successful synthesis of
ZIF-67. The morphology and structure of Co@CN were investi-
gated by transmission electron microscopy (HRTEM). Fig. 1d
shows a typical metallic Co with crystal lattices of 0.205 nm,
which is assigned to the (111) interplanar spacing of metallic
Co. The lattice spacing outside of the graphite carbon is
0.347 nm, which can be assigned to C(200), suggesting that
the metallic Co was encapsulated by a well-ordered graphitic
layer.
The pyrolytic temperature had a substantial effect on the
formation and size distribution of Co NPs. The particle sizes
of the NPs increased with calcination temperature, and the dis-
tribution of the pore size of resultant products indicated that
the average diameter of the Co NPs gradually increased from
8 nm to 51 nm at a pyrolysis temperature of 600–1000 °C
The polymerization and carbonization of the imidazolate
framework can produce substantial meso/macropores, leading
Fig. 2 (a) XRD patterns of the fresh Co@NC. (b) Raman spectrum of Co@NC at different pyrolysis temperatures. (c) Nitrogen adsorption–desorption
isotherms of Co@NC at 77 K. The inset in (c) is the pore size distribution. The Co 2p (d), N 1s (e) and C 1s (f) XPS spectra of the Co@NC.
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to a pore structure different from that of the ZIF-67 precursor. > Co@NC-1000 (Table S2†). The conversion of the reactant and
The nitrogen adsorption–desorption isotherm in Fig. 2c shows product distributions with reaction time revealed that nitro-
a representative IV curve with a typical H4 hysteresis loop. benzene can be smoothly converted into aniline under mild
There are no data points in the low relative pressure range to conditions. No evident by-products were detected in the cata-
verify the existence of micropores. A hysteresis loop is found in lytic system (Fig. 3a), suggesting that a direct route may be
the high-pressure region, suggesting the formation of meso- involved in the hydrogenation of nitrobenzene to aniline.31,32
and macropore structures in Co@NC and confirming the pres- The carbon (C) balance of nitrobenzene hydrogenation by
ence of a hierarchical pore structure in Co@NC. The pore size Co@NC is 94.3% according to the external standard method.
distribution derived from the adsorption curve (Fig. S1†) also In addition, in order to verify whether ethanol can act as a
demonstrates the existence of mesopores and macropores. The reaction partner through hydrogen transfer, we performed a
specific surface area of Co@NC is 288 m2 g−1, and the volume nitrobenzene hydrogenation experiment without adding hydra-
of mesopores is 0.129 cc g−1. The porosity structure allows easy zine hydrate. Under the same reaction conditions, the conver-
penetration of reactants and helps increase the exposed active sion rate of nitrobenzene is 0 (Fig. S2†), indicating that
sites for hydrogenation.27
ethanol cannot provide a hydrogen source.
The surface chemistry and composition of the as-syn-
In terms of the reusability of the catalyst, recycling experi-
thesized Co@NC were investigated through XPS. The high- ments for the selective hydrogenation of nitrobenzene were
resolution Co 2p spectrum (Fig. 2d) shows two peaks at performed using Co@NC under identical conditions in succes-
binding energies of 796.5 and 785.9 eV, which are attributed to sive runs (Fig. 3b). In 20 consecutive runs, the catalytic activity
Co 3p1/2. Meanwhile, the binding energies of Co 2p3/2 are cen- was well retained, and the conversion rate remained
tered at 786.6 and 780.6 eV. The N 1s core-level spectrum of unchanged, but the chemical selectivity decreased slightly.
Co@NC can be deconvoluted into three peaks at binding ener- The stability was compared with that of the reported catalysts
gies of approximately 398.9, 399.8, and 399.8 eV, which can be used in other nitrobenzene hydrogenation studies. It can be
attributed to pyridinic, graphitic, and Co–N nitrogen species, seen from Table 2 that Co@NC shows excellent stability in the
respectively (Fig. 2e). The C 1s spectrum of Co@NC in Fig. 2f nitrobenzene hydrogenation reaction system with different
can be deconvoluted into four peaks at approximately 284.4, hydrogen sources (N2H4·H2O, H2, PMHS, and NaBH4). In
284.8, 285.4, and 286.7 eV, which can be attributed to C–C, addition, the catalyst is easy to recover due to a magnetization
CvC, CvN, and C–O, respectively.28 The nitrogen-doped of 70 emu g−1 (Fig. S3†).
carbon layers ensured the high activity and stability of the Co
particles for hydrogenation without suffering from aggregation we prepared a sample in which Co NPs were deposited on the
and leaching.29,30
outer surface of NC (denoted as Co–NC) for comparison (see
To confirm the importance of the encapsulated structure,
To explore the catalytic performance of Co@NC, we selected the XRD pattern in the ESI, Fig. S4†). Co–NC can also selec-
the hydrogenation of the benchmark substrate nitrobenzene tively hydrogenate nitrobenzene to aniline. However, in terms
as a model reaction. The conversion and selectivity of nitro- of recyclability of the catalyst, Co–NC exhibited a remarkable
benzene as a function of the reaction temperature are pre- inactivation in the process of reuse, and only 40% of yield
sented in Table 1. The results show that the conversion and remained after five cycles (Fig. 3c).
selectivity of nitrobenzene steadily increased with the increase
A thermal filtration test was carried out to verify the hetero-
of the reaction temperature and did not change evidently at geneous catalysis of the Co@NC catalyst. After 5 minutes of
80 °C. Notably, even at 30 °C, 98% yield and 83% selectivity reaction, the reaction mixture was filtered from the mixture by
are achieved. When the reaction temperature was 80 °C, the centrifugation. No further aniline products were detected even
nitrobenzene was completely converted and the selectivity of after 2 h of reaction under optimal conditions, indicating that
aniline reached 97%. Furthermore, Co@NC hydrogenated the dissolved Co species (if any) was noneffective in hydrogen-
nitrobenzene with a turnover frequency of 91.1 × 10−4 (s−1), ation and the process was heterogeneous (Fig. S5†). Therefore,
which surpasses that of reported non-noble metal catalysts the role of leached Co NPs or other Co species in the catalytic
(Table S1†).
The effects of different pyrolysis temperatures were then
hydrogenation reaction was excluded.
In order to understand the catalytic active sites of the
briefly examined. The order of the conversion and selectivity of Co@NC material, a series of control experiments were per-
nitrobenzene in the tested systems was Co@NC > Co@NC-600 formed (Table 3). The transformation of nitroarenes has not
been observed in the blank group and ZIF-67 as the catalyst
under the same reaction conditions, indicating that the active
species were formed during the pyrolysis. In addition, the
comparative NC sample synthesized from ZIF-8(Zn) had no
Table 1 Catalytic hydrogenation of nitrobenzene at different reaction
temperatures
catalytic activity, while the synthesized Co@NC catalyst
showed good hydrogenation activity and high selectivity for
aniline, indicating that cobalt-containing materials were
involved in the reaction and played a key role in the catalytic
process. Next, the catalytic hydrogenation of nitroarenes by
Entry
Catalyst
T (°C)
Time (min)
Yield (%)
Sel. (%)
1
2
3
Co@NC
Co@NC
Co@NC
30
50
80
30
30
30
98
100
100
83
95
97
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Fig. 3 (a) The product distribution during the hydrogenation of nitrobenzene. Reaction conditions: 1 mmol of nitrobenzene, 10 mg of catalyst, 5 mL
of ethanol, 400 μL of N2H4·H2O. Recycling tests of (b) Co@NC and (c) Co–NC. Reaction conditions: 5 mmol of nitrobenzene, 50 mg of Co@NC and
Co–NC, 25 mL of ethanol, 2 mL of N2H4·H2O, a temperature of 80 °C, and a reaction time of 30 min.
Table 2 Comparison of the cycle stability of nitrobenzene hydrogen-
Table 3 Catalytic hydrogenation of nitrobenzene with different
ation in this system with other representative works
catalysts
Reaction
conditions
Yield
Entry
Catalyst
T (°C)
Time (min)
Yield (%)
Sel (%)
No. Catalyst
Recycles (%) Ref.
1
2
3
4
5
6
—
ZIF-67
NC
Co powder
Co/AC
Co@NC
80
80
80
80
80
80
30
30
30
30
30
30
—
—
—
13
68
100
—
—
—
70
95
97
1
2
Pd/C
N2H4·H2O, 80 °C 5th
N2H4·H2O, 80 °C 4th
80
90
33
34
g-Fe2O3/
mCN-900-20
B-OLC
3
4
N2H4·H2O, 100 °C 5th
Nano-sized nickel H2 1.65 MPa,
102 °C
78
46
35
36
13th
Reaction conditions: 1 mmol of nitrobenzene, 10 mg of catalyst, 5 mL
of ethanol, 400 μL of N2H4·H2O.
5
6
7
8
Pd/PV
NAP-Mg–Au(0)
CoPc
H2 2.0 MPa, 45 °C 7th
NaBH4, rt
PMHS, 100 °C
N2H4·H2O, 80 °C 20th
74
75
59
37
38
39
5th
4th
Co @ NC-800
87.4 This study
CvO (nitrobenzaldehyde). Co–NC (externally loaded Co NPs)
and Co@CN(al) (acid washing to remove Co NPs and still
commercially available cobalt powders and Co/AC was con- retain the Co–Nx structure) were prepared. ICP-MS analysis
ducted under the same conditions and both of them showed (Table S3†) shows that the Co contents in Co@NC, Co–NC,
lower catalytic activity, indicating that a synergistic effect and Co@NC(al) samples were 36.8%, 32.1%, and 14.7%,
existed between the metal Co and NC. The relationship respectively. The mass ratio of each element calculated from
between the active sites of Co nanoparticles and NC formed the atomic ratio in the XPS spectrum (Table S4†) is shown in
during the pyrolysis process was continued to be explored as Table S5.† The Co mass percentage of Co@NC determined
follows.
from the XPS results (20.2%) is less than that determined from
It is important to explore the active sites of Co@NC and the the ICP results (36.8%), demonstrating that the metal Co was
effect of active sites on the selectivity of CvC (nitrostyrene) or wrapped by the NC structure. The mass percentage of Co of
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Co–NC determined by the XPS result (28.5%) is basically con-
The experimental results show that Co@NC and Co@NC(al)
sistent with the mass percentage of Co determined by the ICP catalysts have almost no selectivity for CvC (p-nitrostyrene) or
result (32.1%), indicating that most of the Co in Co–NC as Co CvO (p-nitrobenzaldehyde), and the selectivity of the Co–NC
NPs existed on the catalyst surface. At the same time, the mass catalyst is 79% and 64%, respectively. The reaction products of
percentage of Co on the surface of Co@NC(al) is 2.6%, indicat- the hydrogenation reaction catalyzed by Co–NC were detected
ing that the Co NPs on the surface of Co@NC(al) were basically by GC-MS and NMR spectra (Fig. S7–S10†). A little amount of
removed after pickling.
the fully hydrogenated product (p-aminophenylethane) was
The GC spectra of Co@CN, Co–NC and Co@NC(al) for the determined in the p-nitrostyrene hydrogenation reaction cata-
catalytic hydrogenation of substituted nitroarenes (p-nitroben- lyzed by Co–NC, and p-nitrobenzaldehyde formed a fully
zaldehyde and p-nitrostyrene) are given in Fig. S6.† It is found hydrogenated product (p-aminobenzyl alcohol) and then the
from Table 4 that the conversion rate of Co@NC was similar to product underwent dehydration condensation with H to form
that of Co–NC, both of which were higher than that of Co@NC p-aminobenzenemethane. In addition, the main by-products
(al), suggesting that both Co NPs and Co–Nx were involved in in the Co–NC system are attributed to the coupling reaction of
the hydrogenation reaction. However, after Co NPs were the product and the group (–CHO and –NH2, –OH, –CH3, and
removed by acid washing (i.e., Co@NC(al)), the conversion –OCH2CH3 in ethanol), and a small proportion of by-products
rates of nitrobenzene, nitrostyrene, and nitrobenzaldehyde are azo compounds formed by coupling reactions of various
were maintained at 81%, 74%, and 79%, respectively, indicat- nitroaromatics. The decrease in the chemical selectivity of Co–
ing that the Co–Nx sites were the main active sites for catalytic NC is due to the catalytic coupling of the group and the azo
hydrogenation.
compound, which is due to the excessive Co NPs (Table S4†)
on the surface of Co–NC leading to the co-adsorption of two or
more groups and nitroaromatic compounds and then further
producing a coupling reaction.40
Table 4 Catalytic hydrogenation of substituted nitroarenes with
Co@CN, Co–NC and Co@NC(al)
The Mott–Schottky base effect can cause the redistribution
of electrons at the interface of cobalt and N-rich carbon.41,42
According to the Mott–Schottky base effect, N-rich carbon has
higher potential energy than metallic Co materials, and thus
N-rich carbon will accept electrons from Co NPs until the
Fermi level reaches equilibrium. XPS survey can be used to
explain the activity and selectivity of the catalysts. Typical Co
2p XPS peaks are shown in Fig. 4a. The binding energies of Co
2p peaks of Co@NC and Co–NC are obviously higher than
those of Co@NC(al), while the N 1s binding energies of Co–NC
and Co@NC are obviously lower than that of Co@NC(al), indi-
cating that electron transfer occurred at the interface of Co
NPs and the N-doped carbon support through the Mott–
Schottky heterojunction.42,43 The N-doped carbon support is
in the electron-rich region due to the electrons lost by the Co
NPs on the surface, and –NO2 is an electron-deficient group
Entry Substrate Product Catalyst
Time (min) Yield (%) Sel (%)
1
Co@CN
Co–NC
30
30
100
100
81
97
93
98
Co@NC(al) 30
2
Co@CN
Co–NC
90
90
100
97
74
95
79
99
Co@NC(al) 90
3
Co@CN
Co–NC
180
180
100
99
79
97
64
99
Co@NC(al) 180
Reaction conditions: 1 mmol of reactant, 10 mg of catalyst, 5 mL of
ethanol, 400 μL of N2H4·H2O and a reaction temperature of 80 °C.
Fig. 4 (a) The Co 2p and (b) N 1s XPS spectra of the Co–NC, Co@NC, and Co@NC(al).
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Table 5 Chemoselective hydrogenation of various substituted
nitroarenes
that is more easily attracted to the electron-rich carbon sub-
strate, enhancing the mass transfer rate of the hydrogenation
reaction.44,45 In addition, Co NPs can act as a hydrogen accep-
tor because their surface is in an electron-deficient state,
thereby promoting the cleavage of hydrogen bonds. Similarly,
the Co NPs on the Co@NC (al) surface were removed by pick-
ling, so there was almost no Mott–Schottky effect between Co
NPs and the N-doped carbon support which resulted in the
relatively low conversion rate by Co@NC(al). However, when
there are excess Co NPs on the surface of Co–NC (Table S4†),
the catalytic coupling of azo compounds may occur due to the
co-adsorption of two or more nitroaromatic hydrocarbons,
resulting in a decrease in the chemical selectivity.44 Therefore,
Co@NC with suitable surface Co NPs and internally encapsu-
lated Co–Nx showed the best activity and selectivity in the
N2H4·H2O hydrogenation system. Co–Nx is the main active
site, and the Mott–Schottky effect between the surface Co NPs
and N-doped carbon can further promote the hydrogenation
reaction.
Entry
1
Substrate
Product
Time (min)
30
Yield (%)
98
Sel (%)
98
2
3
4
60
100
100
100
100
100
98
120
90
5
150
100
91
For the verification of the general applicability of Co@NC
in tandem catalysis, various nitro compounds were investi-
gated under identical conditions. Different aromatic or
halogen nitro compounds were converted into corresponding
amines with excellent yields and selectivity (Table 5). All ortho-
, meta-, and para-group nitrobenzenes were converted rapidly
and completely, revealing that slight space steric hindrance to
the hydrogenation of the nitro group can be well tolerated.
These results again demonstrate that when N2H4·H2O provides
a hydrogen source through dehydrogenation coupling, the
Co@NC catalyst is highly efficient and chemoselective for the
reduction of diverse nitro compounds to the corresponding
amines.
6
7
90
90
100
100
100
100
8
9
30
100
100
100
93
180
The Raman spectra result of Co@NC reused 20 times shows
typical D- and G-bands of carbon and the ratio of IG/ID did not
change substantially compared with that of fresh ones, which
indicates that graphite carbon remained stable after being
recycled (Fig. S11a†). The XRD patterns of fresh and reused
Co@NC (Fig. 5a) show that the catalyst after 10 cycles has
hardly changed compared to the original catalyst. After 20
cycles of the Co@NC catalyst, the diffraction peaks of CoO and
the crystalline state of Co species did not change substantially,
which indicates the retention of the catalytic activity after
recovery. In addition, the diffraction peak of Co3O4 or CoO was
probably due to the slight oxidation of the catalyst surface
after being heated, which resulted in a slight decrease in cata-
lyst selectivity after 20 cycles. On the contrary, the XRD pat-
terns of fresh and 5th reused Co–NC have obvious differences
(Fig. 5b). Compared with the original catalyst, the Co species
10
120
100
100
11
12
120
150
100
100
95
97
Reaction conditions: 1 mmol of reactant, 10 mg of Co@NC, 5 mL of
ethanol, 400 μL of N2H4·H2O and a reaction temperature of 80 °C.
of Co–NC changed significantly after 5 cycles, the Co(220) process. The formation of a graphitic structure can efficiently
reflection corresponding to 75.9° in the spectrum almost dis- facilitate electron transfer and prevent the leaching of acidity
appeared and the surface of the catalyst showed significant oxi- or the poisoning of Co NPs, thereby improving the catalytic
dation. These reasons lead to a significant decrease in the activity. The TEM image of Co–NC (Fig. S11b†) shows that no
hydrogenation yield of nitrobenzene after 5 cycles of the Co– packaging structure was formed on the outer surface of Co–
NC catalyst.
NC, and Co NPs were dispersed on the surface of the catalyst
The nitrogen-doped carbon matrix is an important reason and had obvious agglomeration due to sintering. The TEM
for maintaining relatively high activity during the cycling image of Co@NC (Fig. S11c†) shows that the Co-active species
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Fig. 5 The XRD patterns of catalysts before and after cycling: (a) Co@NC and (b) Co–NC and (c, d) metal particle size distribution histogram of 20th
reused Co@NC and 5th reused Co–NC.
were surrounded by a graphene layer, and a well-defined core/ probably because the surface of the catalyst was partially oxi-
shell structure was formed in the mesoporous carbon frame- dized by air during storage.
work. In addition, the packaging structure can also prevent the
Co L-edge XANES involving the excitation of 2p electrons
growth of metal, which is conducive to improving its cycle into the unoccupied states of 3d character was employed to
stability. The particle size distribution of 20th Co@NC shows probe the valence and the coordination environment of
that the particle size was ∼12.1 nm even after 20 cycles elemental Co (Fig. 6b). Fresh Co–NC and Co-@NC had two
(Fig. 5c). However, for Co–NC, evident particle aggregation main peaks at the edge of L3 (780 eV) and L2 (795 eV) due to
occurred, which grew from ∼15.2 nm (Fig. S12†) to ∼19.4 nm the electronic transition of Co 2p 3/2 and 2p 1/2 core electrons:
(Fig. 5d). Thus, we conclude that the structure of nitrogen- a lower energy L3 peak (2p 3/2 → 3d) and a higher energy L2
doped carbon coated Co-active species is essential to the peak (2p 1/2 → 3d).46,47 In addition, the L2 and L3 peak posi-
cycling stability of Co@NC catalytic performance compared tions corresponding to 5th Co–NC and 20th Co@NC were
with Co–NC.
shifted to the right, indicating that the valence state of Co led
X-ray absorption near edge structure (XANES) is a powerful to an increase in energy. The increase of 5th Co–NC energy
spectroscopy technique for specific elements, involving elec- was substantially higher than that of 20th Co@NC, indicating
tron excitation from the core level to the empty state, which that the degree of oxidation of 5th Co–NC was considerably
can effectively characterize complex systems and obtain struc- higher than that of 20th Co@NC, which is consistent with XPS
tural information of different species. To gain insight into the results. The L3/(L3 + L2) branching ratio of 5th Co–NC deter-
considerable cycling stability of Co@NC caused by the package mined in our spectrum was 0.64, and the L3/(L3 + L2) branch-
structure from the perspective of electronic level transitions, ing ratio of 20th Co@NC was 0.71. According to the literature,
we performed XANES measurements on five cycles of the Co– the typical high- and low-spin branching ratios are ∼0.7 and
NC catalyst and 20 cycles of the Co@NC catalyst which were ∼0.6, respectively.48,49 These results indicate that in the oxygen
used to analyze the local coordination geometry and the elec- content range of 20th Co@NC, cobalt maintained a high spin
tronic state sensitivity of an element. To clearly understand the state, whereas cobalt in 5th Co–NC maintained a low spin
compositional changes of the catalyst elements before and state,50 which may be the high stability of Co@NC. Therefore,
after the cycle, the surface properties of the catalyst were first the catalytic performance of the Co–NC material deposited on
investigated by XPS analysis. The XPS measurement spectrum the outer surface of the NC was substantially reduced after five
(Fig. 6a) shows that the four groups of catalysts consisted of cycles, whereas the Co@NC material with the package struc-
Co, C, O, and N, and no other impurities were observed. The ture maintained high cycle and catalytic performance after 20
fresh catalyst had a weak oxygen characteristic peak, which is cycles, reflecting the importance of the package structure.
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Fig. 6 (a) XPS spectrum of fresh Co–NC, 5th-Co–NC, fresh Co@NC, and 20th-Co@NC and (b)–(d) XANES Co L-edge spectra, O K-edge spectra
and C K-edge spectra of fresh Co–NC, 5th-Co–NC, fresh Co@NC, and 20th-Co@NC respectively.
Fig. 6c shows the O K-edge XANES spectrum of the two cata- graphitic nitrogen with three C neighbors.58–61 Peak N4 was
lysts before and after the cycle. The characteristic peaks at independent of any particular species, and it was assigned to a
529.8 eV (labeled A) and 539.9 eV (labeled C) indicate that the general conversion from the N 1s core level to the C–Nσ*
O 2p state produced by the hybridization of the Co 3d state state.62 The results further show no substantial change in the
and the Co 4sp state was well matched.51 A 532 eV character- structure and combination of N after the two catalysts were
istic peak (labeled A) can be assigned to the π* excitation of recycled.
various C–O groups, and a 544 eV characteristic peak (labeled
C) can be assigned to the σ* excitation of various C–O
groups.52,53 The O-characteristic peak corresponding to 5th
Co–NC showed an evident enhancement, indicating that Co
4. Conclusions
NP deposition was considerably oxidized on the outer surface
of NC, consistent with XPS results.
In summary, we demonstrate that the Co@NC catalyst has an
excellent catalytic activity and good stability for the chemo-
selective hydrogenation of nitrobenzene under mild conditions
(80 °C, 30 min, and 1 atm). The excellent catalytic performance
is attributed to the following reasons: (1) the confinement
effect of the MOF at an optimum pyrolytic temperature ren-
dered the size of Co NPs uniform after annealing. (2) The
encapsulation structure by graphite carbon layers can enor-
mously protect Co particles from growing during the catalytic
process. (3) The doped N can improve electron transfer during
the hydrogenation reaction. (4) The Mott–Schottky effect exists
between Co NPs and the N-doped carbon support, which
further improves the hydrogenation reaction activity. In
addition, the large surface area and the mesoporous pores
enabled access to catalytically active sites. In the nitrobenzene
hydrogenation system, Co–Nx plays a more important role
than Co NPs.
Fig. 6d shows a comparison of the C K-edge XANES spectra.
The spectra show three distinct characteristic peaks A, B, and
C at about 286, 289.0, and 293 eV, respectively, and the spectral
shape was well reported in the previous literature.54,55
According to the literature, feature A is explicitly assigned as
the π* excitation of the C–C bond in the C structure, feature C
represents the σ* excitation of the C–C bond,56 and feature B is
converted to the sp3 hybrid state due to the oxidizing group
such as C–O or CvO.57 The change of the B characteristic
peak intensity of 5th Co–NC was substantially higher than that
of 20th Co@NC which further changed from C–O or CvO to
the electronic level of the sp 3 hybrid state to prove a consider-
able change in the nature of 5th-Co–NC.
XANES spectra at the N K-edge of the fresh and reused cata-
lysts are shown in ESI Fig. S13.† Four well-resolved formants at
N1 (∼397 eV), N2 (∼398.2 eV), N3 (∼399.8 eV), and N4 (∼406 eV)
were observed. The peak N1 at 397 eV in the N K-edge corre-
sponded to π*-transition in the aromatic C–N–C portion of the
pyridinic site, and the sharp peaks at 399.8 eV 1s-to-p* can be
attributed to graphitic N that can be assigned to sp2-bonded
Conflicts of interest
There are no conflicts to declare.
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18 Z. Wei, S. Mao, F. Sun, J. Wang, B. Mei, Y. Chen, H. Li and
Acknowledgements
Y. Wang, Green Chem., 2018, 20, 671–679.
The authors gratefully acknowledge financial support from the 19 A. Hu, X. Lu, D. Cai, H. Pan, R. Jing, Q. Xia, D. Zhou and
National Natural Science Foundation of China (21677138 and Y. Xia, Mol. Catal., 2019, 472, 27–36.
21876166) and the Key Special Program on the S&T for the 20 L. Liu, P. Concepción and A. Corma, J. Catal., 2016, 340, 1–
Pollution Control and Treatment of Water Bodies (No. 9.
2017ZX07603-003). We thank the Photoemission Endstations 21 T. Cheng, H. Yu, F. Peng, H. Wang, B. Zhang and D. Su,
(BL10B) in National Synchrotron Radiation Laboratory (NSRL)
for help in characterization.
Catal. Sci. Technol., 2016, 6, 1007–1015.
22 Y. Lu, Y. Wang, H. Li, Y. Lin, Z. Jiang, Z. Xie, Q. Kuang and
L. Zheng, ACS Appl. Mater. Interfaces, 2015, 7, 13604–13611.
23 M. G. Naseri, E. B. Saion, H. A. Ahangar, M. Hashim and
A. H. Shaari, Powder Technol., 2011, 212, 80–88.
24 Y. Hou, Z. Wen, S. Cui, S. Ci, S. Mao and J. Chen, Adv.
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