Cobalt nanoparticles anchoring on nitrogen doped carbon with excellent performances for transfer hydrogenation of nitrocompounds to primary amines and N-substituted formamides with formic acid

Article abstract of DOI:10.1016/j.catcom.2019.105747

Cobalt nanoparticles anchoring on nitrogen doped carbon derived from pyrolysis of a cobalt complex and chitosan were developed for reduction of nitrocompounds with neat formic acid to their corresponding amines or N-substituted formamides by switch of solvents. Characterization results revealed that most of the nitrogen atoms are present as graphitic N and pyridinic N as anchoring sites, and the cobalt nanoparticles are wrapped by nitrogen doped carbon layers, endowing the catalyst with excellent activity and superior reusability.

Full text of DOI:10.1016/j.catcom.2019.105747

Accepted Manuscript
Cobalt nanoparticles anchoring on nitrogen doped carbon with
excellent performances for transfer hydrogenation of
nitrocompounds to primary amines and N-substituted formamides
with formic acid
Yuecheng Zhang, Pengwei Cao, Hong-Yu Zhang, Guohui Yin,
Jiquan Zhao
PII:
DOI:
Article Number:
Reference:
S1566-7367(19)30209-2
https://doi.org/10.1016/j.catcom.2019.105747
105747
CATCOM 105747
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
2 April 2019
1 June 2019
27 June 2019
Please cite this article as: Y. Zhang, P. Cao, H.-Y. Zhang, et al., Cobalt nanoparticles
anchoring on nitrogen doped carbon with excellent performances for transfer
hydrogenation of nitrocompounds to primary amines and N-substituted formamides with
formic acid, Catalysis Communications, https://doi.org/10.1016/j.catcom.2019.105747
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ACCEPTED MANUSCRIPT
Cobalt Nanoparticles Anchoring on Nitrogen Doped Carbon with Excellent
Performances for Transfer Hydrogenation of Nitrocompounds to Primary
Amines and N-substituted formamides with Formic Acid
Yuecheng Zhang, Pengwei Cao, Hong-Yu Zhang, Guohui Yin, Jiquan Zhao *
School of Chemical Engineering and Technology, Hebei Provincial Key Lab of Green Chemical Technology & High Efficient Energy Saving,
Hebei University ofTechnology, Tianjin 300130, PR China
*Corresponding author,
E-mail addresses: zhaojq@hebut.edu.cn

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ABSTRACT
Cobalt nanoparticles anchoring on nitrogen doped carbon derived from pyrolysis of a cobalt
complex and chitosan were developed for reduction of nitrocompounds with neat formic acid to
their corresponding amines or N-substituted formamides by switch of solvents. Characterization
results revealed that most of the nitrogen atoms are present as graphitic N and pyridinic N as
anchoring sites, and the cobalt nanoparticles are wrapped by nitrogen doped carbon layers,
endowing the catalyst with excellent activity and superior reusability.
Keywords:
Cobalt nanoparticles; Nitrogen doped carbon; Formic acid; Amines; N-Substituted formamides

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1. Introduction
Amines are key starting materials for synthesizing dyes, drugs and so on [1-5]. Traditionally,
catalytic hydrogenation is the most selected process to produce amines owing to its atom-economy.
However, there are some risks in hydrogenation transportation and storage. N-Substituted
formamides (N-SFAs) are valuable substances broadly used as organocatalysts in organic
transformations [6,7] and raw materials for many purposes [8-10]. The traditional methods to
produce N-SFAs depend on the condensation of formic acid (FA) or its derivatives such as
formates with primary and secondary amines [11], suffering from the disadvantages of massive
waste generation and poor atom-economy. Newly developed protocols include N-formylation of
amines or one-pot N-formylation of nitro compounds to N-SFAs with FA or its salts [1,2,12-14],
and CO2/H2 as formyl sources catalyzed by noble-metal based heterogeneous catalysts [15], or
complexes with complicated structures [16-18]. However, the scarcity and high cost of noble
metals, and difficulty in recyclability of the complexes extremely hinder their practical
applications in large scale. Very recently, reductive N-formylation of nitroarenes with ammonium
formate to N-SFAs was achieved over a N,P-codoped carbon supported cobalt catalyst with
advantages of low toxicity and cost, and easy recyclability [19].
In fact, nitrogen doped carbon supported metal catalysts have found applications in various
reactions [3,20-24] since the pioneering work of Beller [25]. These catalysts can be easily
prepared from pyrolysis of neat metal complexes with nitrogen ligands [26] or the complexes
absorbed on some supports such as organic polymers and activated carbon [27,28], and metal
organic frameworks [29]. Beller [25] found that the selectivity and activity of a heterogeneous

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catalyst can be controlled by the ligand in the complex and the support before conversion into the
active heterogeneous catalysts. The nitrogen numbers coordinated to center metal in a complex is
crucial to afford highly active species in the catalyst preparation. Besides, the suitable combination
of a complex and a support is also of importance in determining the catalysis of a catalyst. As for
metals, cobalt and iron are the most selected due to their lower toxicity, more abundance and low
cost. Some cobalt or iron based catalysts derived from the pyrolysis of iron and cobalt complexes
or the complexes absorbed on various supports including some biomasses were found good
performances in reduction of nitro compounds to amines [4,5,28,30,31].
Inspired by aforementioned results, we prepared several nitrogen-doped carbon supported
cobalt catalysts CoNC-T (where T represents the pyrolysis temperature) from pyrolysis of a
multidentate nitrogen ligand coordinated cobalt complex Co(bbp)2 absorbed on chitosan at
different temperatures. The catalyst CoNC-700 showed high activity, excellent selectivity, and
outstanding recyclability in reduction and one-pot reductive N-formylation of nitroarenes with FA.
2. Experimental
The experimental procedures were given in Supplementary information.
3. Results and Discussion
3.1. Catalyst preparation and characterization
As depicted in Scheme S1, the cobalt complex Co(bbp)2 was absorbed on the chitosan to give
o
the catalyst precursor Co(bbp)2-chitosan, after which being pyrolyzed at 600, 700, 800 and 900 C
respectively to give the catalysts CoNC-T in view of TGA analysis (Fig. S1). The cobalt contents
of CoNC-600, CoNC-700, CoNC-800 and CoNC-900 were 2.97%, 3.16%, 3.51% and 3.85%,

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respectively, determined by ICP (Table S2).
The TEM image revealed that no cobalt nanoparticles were existed in the CoNC-600 sample
(Fig. S2a). In combination with TGA analysis, this result indicated that the required catalyst was
not generated at 600 oC. However, cobalt nanoparticles with average diameter of 24.9 nm were
clearly observed in the image of the CoNC-700 sample (Fig. S2b), indicating the transformation of
o
precursor to catalyst at 700 C. The cobalt nanoparticles were also observed in CoNC-800 and
CoNC-900 with respective average diameters of 34.4 nm and 78.0 nm (Fig. S2c, S2d), indicating
increase in nanoparticle diameter with increase of temperature. Besides, HRTEM images of the
CoNC-700 sample clearly manifest lattice spacing of 0.205 nm and 0.34 nm (Fig. S2f),
corresponding to metallic cobalt (111) and graphitic carbon (002) planes [26], respectively. It is
also observed that cobalt nanoparticles are covered by graphitic carbon layers, effectively
protecting cobalt nanoparticle from aggregation or being oxidized (Fig. 1 and Fig. S2e, S2f).
HAADF-STEM images (Fig. S2g) and corresponding elemental mapping results show
homogeneous dispersion of C, N, and Co.
Graphitic
carbon layer
Cobalt
nanoparticle
5nm
5
Fig. 1 HRTEM images of catalyst CoNC-700 with structures of covering cobalt nanoparticles by graphitic
carbon layers.
XRD and XPS were performed to disclose the compositions and valence states of as prepared

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catalysts. As shown in Fig.2a, typical diffraction peaks of metallic cobalt at 2θ= 44.2o, 51.5o, 75.8o
(PDF 15-0806) are found in all catalysts except for CoNC-600. The large broad peak located at
about 26o is assigned to C (002) plane diffraction [26]. No diffraction peaks related to Co3O4 are
found, indicating the protection of aforementioned carbon layers on Co0 species from oxidation
during XRD measurement. Besides, no peaks related to the metallic cobalt are observed in the
XRD patterns of CoNC-600, probably due to the failure of formation of cobalt nanoparticles
pyrolyzing at 600 oC. XPS analysis revealed the presence of elements C, N and Co in the catalyst
samples (Fig. 2b). Besides, a peak at around 500 eV ascribed to the Auger Peak of sodium are
found in the survey spectra of all the fresh catalysts except for CoNC-900. The sodium might
come from the contamination of cobalt complex Co(bbp)2 by Na2CO3 in the preparation process.
The peak disappeared in the survey spectra of CoNC-900, and the reused CoNC-700, which could
o
be ascribed to sodium evaporation at 900 C, and washing away of sodium by water during
recovery procedures.
The total nitrogen content decreased with increase in carbonization temperature (Table S2),
being agreement with elemental analysis results. Surprisingly, no peaks representing N and Co
species were found in the survey spectrum of CoNC-900, which seems contradict to the ICP and
XRD characterization results. These could be ascribed to the evaporation of the surface N and
transformation of Co into bulk phase at 900 oC. The cobalt nanoparticles in the bulk phase could
not be detected by XPS [32]. The N 1s XPS spectrum shown in Fig.2c is fitted into three binding
energies, corresponding to graphitic N (401.2 eV), pyrrolic N or CoNx (399.8 eV) and pyridinic N
(398.7 eV), respectively [3,21,24]. The approximate contents of the three nitrogen species were
calculated from the areas under the fitted curves in N 1s region (Table S3). The results revealed

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that the content of pyrrolic N decreased with increase in pyrolysis temperature, accompanying the
increase of the contents of graphitic N and pyridinic N, indicating higher graphitization degree and
embedment of nitrogen into the carbon framework to form N doped carbon layers [31]. High
contents of pyridinic N and graphitic N are important, because they are anchoring sites for Co
species in the catalysts [21]. Careful deconvolution of Co 2p3/2 XPS spectra revealed four peaks at
the binding energies of 778.4, 779.9, 782.0, and 786.0 eV, ascribed to the ones of metallic Co,
Co2+, CoNx and Co2+ satellite peak, respectively [31-33]. Deconvolution of Co 2p1/2 spectra shows
binding energies of about 794 eV, 796 eV and 803 eV, ascribed to metallic Co, Co2+ and satellite
peak of Co2+. The prominent satellite peaks at 786 eV and 803 eV indicated that CoO was the
dominating oxide [34,35]. Combined with the fitted N 1s XPS spectrum, we can conclude that
partial Co species exist in the form of CoNx, which means cobalt coordinating with doped
nitrogen, and at least some metallic Co species rooted in the CoNx. No metallic Co species were
detected by XPS, being consistent with the XRD analysis.

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Co (111)
C (002)
a
b
O 1s
Co 2p
C 1s
N 1s
Co (200)
CoNC-900
CoNC-800
CoNC-700-used
Co (220)
CoNC-900
Na Auger
CoNC-700
CoNC-600
CoNC-800
CoNC-700
CoNC-600
1000
800
600
400
200
0
10
20
30
40
50
60
70
80
2θ degree
Binding energy (eV)
metallic Co
Pyrrolic N or Co-Nx
Graphitic N
Pyridinic N
Co2+
c
d
CoNx
Co2+ satellite peak
CoNC-900
CoNC-900
CoNC-800
CoNC-800
CoNC-700
CoNC-700
CoNC-600
CoNC-600
404
402
400
398
396
394
810
805
800
795
790
785
780
775
Binding energy (eV)
Binding energy (eV)
Fig.2 (a) XRD patterns (b) XPS survey scan, (c) high resolution XPS analysis in N 1s region, and (d) detailed
Co 2p spectra of catalyst samples.
N2 adsorption-desorption analysis revealed that all catalysts display type-IV isotherms (Fig.
S3), indicating the presence of mesoporous in the catalysts. As shown in Table S4, the specific
areas of the catalysts increased with the pyrolysis temperature and reached 82.703 m2g-1 at 800 oC,
futher increasing pyrolysis temperature led to decrease of specific areas. The average pore
diameters were calcuated to be 20.41, 19.50, 14.78 and 8.7 nm respectively, indicating pore
diameter decrease with the increase of pyrolysis temperature. Larger pore diameter facilitates mass
transfer and thus endows with CoNC-700 higher catalytic activity.
3.2. Catalytic performance

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Nitrobenzene was chosen as a model substrate to evaluate the catalysis of as-prepared
catalysts in the reduction of nitroarenes with FA in acetonitrile. As shown in Table S1, no reaction
was detected in the presence of chitosan-800 or absence of catalyst, indicating its necessity of
cobalt in the catalysts. CoNC-600 has no activity, too. Catalysts CoNC-700, CoNC-800 and
CoNC-900 showed moderate activity to afford N-phenylformamide (N-PFA) as a main product
from the reductive N-formylation of nitrobenzene, and aniline as a minor product from the direct
reduction of nitrobenzene. And CoNC-700 showed best performance in view of its activity and
selectivity towards N-PFA.
TEM, XPS and XRD analysis disclosed that no metallic cobalt species existed in CoNC-600,
which might be the reason of its inactivity. Metallic cobalt species were observed in all the other
three catalysts. Elementary analysis revealed that the cobalt content increased with the pyrolysis
temperature in the preparation of catalysts, but the nitrogen content decreased (Table S2). Besides,
CoNC-700 and CoNC-800 had similar abundances of graphitic and pyridinic nitrogen as exhibited
in Fig. 2c. Therefore, there are more metallic Co species rooted in the CoNx in CoNC-700. The
CoNx species and the metallic Co species growing on them could exhibit synergistic effect in the
reduction of nitroarenes with FA [31], endowing CoNC-700 with the highest activity. CoNC-900
also had good performance, though its surface compositions disclosed by XPS were very different
from those of CoNC-700 and CoNC-800. This could be ascribed to that CoNC-900 had enough N
and Co species present in its bulk phase (Table S2), and similar textural properties with CoNC-700
and CoNC-800 (Fig. S3, Table S4).
Next, several solvents were tested in the reaction (Table S5). In all the tested solvents,
nitrobenzene was almost completely converted to aniline and N-PFA. In the cases of acetonitrile

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and 1,4-dioxane as solvents, the major product was N-PFA. Acetonitrile gave the highest
selectivity of 75.6% towards N-PFA. On the contrary, more aniline was formed in methanol and
ethanol. A nitrobenzene conversion as high as 98.3% with aniline selectivity 93.1% was received
in methanol. So, either N-PFAor aniline can be selectively obtained over this catalyst by switch of
solvents.
The effects of reaction conditions on the reaction were investigated and the results are given
in Supplementary information (S3). Based on the results, the optimized reaction conditions were
obtained, which were 10 equivalent formic acid, catalyst loading 1 mol%, reaction temperature
110 oC, reaction time 5 h, solvent acetonitrile.
The CoNC-700 catalyst was then applied to the reductive N-formylation of various
nitroarenes under the optimized reaction conditions, and the results are exhibited in Table 1. All
nitroarenes either with electron donating or electron withdrawing substituent were converted with
full conversion to their corresponding N-SFAs, except for o-nitrotoluene and p-nitrobenzonitrile.
In the case of o-nitrotoluene as substrate, o-toluidine was the major product, obviously owing to
the steric hindrance of o-methyl preventing formylation of o-toluidine with FA.
p-Nitrobenzonitrile did not afford the corresponding N-SFA, but p-cyanoaniline as a major
product, at least partly due to the electron withdrawing effect of cyano group preventing further
formylation of p-cyanoaniline. Importantly, nitroarenes with other unsaturated functional groups
including carbonyl and ester could tolerate the conditions and afford the corresponding N-SFAs in
excellent yield, manifesting the excellent chemoselectivity of CoNC-700. More importantly,
dehalogenation was not found for halogenated nitrobenzenes, generally occurring in catalytic
hydrogenation processes. This catalytic reaction system was also applied to the reductive

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N-formylation of aliphatic nitro compounds. All the tested nitroalkanes, including
nitrocyclohexane, afforded their N-SFAs in moderate yields. The low yields in these cases might
be ascribed to the low stability of reduction intermediates of nitroalkanes compared to the ones of
aromatic substrates. Compared with the catalyst Co@NPC-800 [19], CoNC-700 exhibited much
good performance. Especially, only FA as both hydrogen and formyl sources was required to
transform nitroarenes to the corresponding N-SFAs over CoNC-700, without needing any
auxiliaries.
Table 1 One-pot reductive N-formylation of various nitro substances
Entry
Substrate
Conversion (%)
Selectivity (%)a
Yield (%)a
1
2
94.8
85.8
70.5
79.8
56.9
59.2
3
4
84.3
99.9
84.2
99.9
60.7
95.4
5
99.9
99.9
96.2
6
7
8
99.9
98.7
99.8
36.1
99.8
99.8
23.2
94.5
95.2
9
99.9
99.8
95.7

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10
11
12
13
14
99.9
99.9
99
99.8
97.2
34.2
97.6
99.7
96.2
93.8
20.4
94.1
95.7
99.9
99.9
15
16
17
99.9
99.9
99.9
93.6
99.8
95.4
90.1
95.2
91.2
18
19
99.8
99.7
44.7
99.8
31.6
95.2
Reaction conditions: Acetonitrile 5 mL, nitroarenes 0.5 mmol, formic acid 5 mmol, cat 9.5 mg (1 mol%
Co),110 oC, 5 h 1 bar N2. Conversion and selectivity were determined by GC analysis.
a The selectivity and isolated yield of a
Direct reduction of nitrocompounds to amines with FA in methanol was finally performed. As
exhibited in Table 2, all aromatic substrates with various functional groups were converted to
amines with excellent yields. The catalyst showed high tolerance to substrates with easily reduced
groups such as carbonyl, ester and nitrile groups, and high efficiency in the reduction of
2-nitronaphthalene to 2-naphthylamine. o-Substituted aromatic substrates also afforded the
corresponding amines in excellent yields, indicating little effect of steric hindrance on the

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reduction of nitro compounds to amines. Same as in reductive formylation, nitroalkanes afforded
their corresponding amines with moderate yields, due to the lower stability of the intermediates
than the ones in the reduction of aromatic substrates.
Table 2 Chemoselective reduction of various nitro substances to amines
Entry
Substrate
Conversion (%)
Selectivity (%)
Yield (%)a
1
2
75.8
73.7
99.7
99.4
56.7
52.4
3
4
80.5
99.8
99.7
99.7
70.1
94.6
5
99.7
99.8
95.4
6
7
8
99.8
99.8
99.8
99.6
99.7
99.4
94.2
96.2
94.1
9
99.8
99.7
99.8
99.8
99.8
99.8
99.8
99.8
95.7
94.3
95.6
97.2
10
11
12

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13
14
99.8
99.8
99.8
99.9
96.2
95.6
15
16
17
99.8
99.8
99.9
99.7
99.7
98.4
96.1
95.6
94.3
18
19
99.7
99.8
99.5
99.8
95.4
95.7
Reaction conditions: Methanol 5 mL, nitroarenes 0.5 mmol, formic acid 1.75 mmol, cat 29.5 mg (3 mol%
Co),160 oC, 5 h 1 bar N2. Conversion and selectivity were determined by GC analysis.
a Isolated yield.
Hot filtration experiment was performed to evaluate the heterogeneity and stability. As shown
in Fig. S4a, the conversion of nitrobenzene keeps constant after the catalyst was removed,
indicating no cobalt leaking during reaction. The reusability of CoNC-700 was examined for five
recycles (Fig. S4b). No obvious decrease in activity was found in the recycles. TEM and HRTEM
images (Fig. S5) of catalyst sample revealed that the particle sizes did not severely changed and
metallic cobalt species were still existed. In the survey XPS spectrum of the used CoNC-700 the
signals related to Co, O, N and C species were still clearly observed (Fig. 1b). Besides, the Co 2p
signal is more pronounced than that of the fresh CoNC-700, indicating exfoliation of some surface
compositions of the catalyst during catalytic reaction or recovery procedures. However, almost no
peaks related to Co species were observed in the XRD patterns of the sample used five times (Fig.

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S6), which might be due to the decrease of crystallinity of the Co species during catalytic run.
Anyway, the catalyst CoNC-700 had good stability and reusability in view of both the catalytic
and characterization results.
4. Conclusion
In conclusion, cobalt nanoparticles anchoring on nitrogen doped carbon were readily
prepared via pyrolysis of a multidentate-nitrogen ligand cobalt complex supported on chitosan at
o
temperatures higher than 600 C. In these materials, the cobalt nanoparticles anchor on the
graphitic N and pyridinic N generated in situ during pyrolysis, and are embedded in the nitrogen
doped graphitic carbon layers, preventing cobalt nanoparticles from aggregation. These materials,
o
especially the one obtained by pyrolysis at 700 C, exhibited excellent performances in reduction
and reductive N-formylation of nitroarenes with neat FA to their corresponding amines or N-SFAs
with ease of recovery and good reusability.
Acknowledgement
This work was supported by the National Natural Science Foundation of China (Grant NO.
21776056) the Natural Science Foundation of Hebei Province (CN) (Grant No. B2018202253).
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Graphical abstract

ACCEPTED MANUSCRIPT
Highlights
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CoNC-T catalysts were prepared through pyrolysis of a cobalt complex on chitosan.
Graphitic and pyridinic N sites are present to anchor and stabilize cobalt species.
Cobalt nanoparticles were wrapped by nitrogen doped carbon layers.
N-Substituted formamides and amines were obtained quantitively with CoNC-700.
Formic acid serves as both hydrogen and formyl sources.