Enhanced catalytic performance of cobalt nanoparticles coated with a N,P-codoped carbon shell derived from biomass for transfer hydrogenation of functionalized nitroarenes

Article abstract of DOI:10.1039/c8gc00619a

The development of abundantly available base metal catalysts for organic transformations remains an important goal of chemical research. Herein, we report the first facile fabrication of active, inexpensive, and reusable cobalt nanoparticles (NPs) coated with a N,P-codoped carbon shell derived from naturally renewable biomass and earth-abundant, low-cost cobalt salt and PPh₃. The entire process is operationally simple, straightforward, cost-effective and environmentally benign and can be used in mass production for practical application. The resultant catalysts allow for highly efficient and selective transfer hydrogenation of functionalized nitroarenes to the corresponding anilines using formic acid or ammonium formate as the hydrogen donor. Uniformly incorporated N and P into the carbon lattices exhibited synergistic effects with the encapsulated Co NPs to engineer the structure and composition of the catalyst, thereby substantially boosting the catalytic efficiency. The most active catalyst Co@NPC-800 exhibited outstanding activity and exclusive selectivity for the reduction of functionalized nitroarenes to anilines, especially those decorated with readily reducible functional groups. The catalyst demonstrated high stability and can be easily separated by using an external magnet for successive reuses without significant loss in both activity and selectivity.

Full text of DOI:10.1039/c8gc00619a

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Enhanced Catalytic Performance of Cobalt Nanoparticles Coated
by a N,P-Codoped Carbon Shell Derived from Biomass for Transfer
Hydrogenation of Functionalized Nitroarenes
Received 00th January 20xx,
Accepted 00th January 20xx
Yanan Duan, Tao Song, Xiaosu Dong, and Yong Yang*
DOI: 10.1039/x0xx00000x
www.rsc.org/
The development of abundantly available base metal catalysts for organic transformations remains an important goal of
chemical research. Herein, we report the first facile fabrication of an active, inexpensive, and reusable cobalt nanoparticles
(NPs) coated by a N,P-codoped carbon shell derived from naturally renewable biomass and earth-abundant, low-cost
cobalt salt and PPh3. The entire process is operationally simple and straightforward, cost-effective and environmentally
benign and can be in mass production for practical application. The resultant catalysts allow for highly efficient and
selective transfer hydrogenation of functionalized nitroarenes to the corresponding anilines using formic acid or
ammonium formate as hydrogen donor. Uniformly incorporated N and P into the carbon lattices played synergistic effects
with the encapsulated Co NPs to engineer the structure and composition of the catalyst, thereby substantially boosting the
catalytic efficiency. The most active catalyst Co@NPC-800 exhibited outstanding activity and exclusive selectivity for
reduction of functionalized nitroarenes to anilines, especially for those decorating with readily reducible functional groups.
The catalyst demonstrated high stability and can be easily separated by using external magnet for successive reuses
without
significant
loss
in
both
activity
and
selectivity.
considerable attention because of their operational simplicity,
better control of chemoselectivity and good compatibility with
functional groups under mild conditions. As a consequence, a
number of well-defined homogeneous metal complexes and stable
heterogeneous metal-based catalysts have been developed for
catalytic transfer hydrogenation of nitroarenes over the last
decade.5 Obviously, from both economic and environmental
perspectives, non-noble metal-based heterogeneous catalysts
significantly stand out due to their integrated advantages such as
earth-abundant, low-cost and low-toxicity, stability, recyclability
and easy separation as well. In this regard, Beller and co-workers
Introduction
Anilines and its derivatives are an important class of compounds
used as building blocks in the manufacture of dyes, agrochemicals,
pigments, pharmaceuticals, and polymers.1 Owing to their immense
utility, the development of novel methods for the synthesis of
anilines continues to be an active area of research. Among all
known developed methods, direct reduction of nitroarenes
catalyzed by noble metal catalysts with molecular hydrogen
represents the most straightforward and atom-economic process.2
However, such process is usually conducted at high pressure and
temperature, which significantly restricts its wider application due
to the need for specialized equipment and potential safety issues
regarding H2 handling accompanying with high energy
consumption. More worse, the process generally demonstrates
poor chemoselectivity for hydrogenation of functionalized
nitroarenes bearing with diverse and readily reducible functional
groups,3 where the resulting anilines constitute key intermediates
for the synthesis of fine and bulk chemicals, especially in life science
applications.
developed
a novel heterogeneous catalyst in which cobalt
nanoparticles (NPs) are embedded in a N-doped carbon matrix by
direct carbonization of non-volatile molecularly defined Co-amine
ligated complexes for a general reduction of all kinds of nitroarenes
with outstanding catalytic performance in 2013.6 After their seminal
work, several beautiful N-doped carbon supported cobalt-based
catalysts have been successively developed for highly active and
selective hydrogenation of functionalized nitroarenes by several
independent research groups.7 These cobalt based N-doped carbon
catalysts were generally synthesized via pyrolysis of either
molecularly well-defined complexes together with inert supports
(e.g., SiO2, graphene, porous carbon, or carbon nanotubes) or pre-
prepared metal organic framework precursors (MOFs). In some
cases, the subsequent etching of SiO2 templates with acid or base
aqueous solution after pyrolysis is required to generate the desired
molecularly single active sites.4a,8 Nonetheless, intensive studies
disclosed that N-doped carbon supports not only can effectively
protect active cobalt NPs from harsh environments and strongly
resist NPs aggregation to enhance the durability, but also exert a
As a complement to catalytic hydrogenation with molecular
hydrogen, catalytic transfer hydrogenation of nitroarenes with
hydrogen-donor such as formic acid,4a,b sodium borohydride,4c
ammonia borane,4d alcohol,4e and hydrazine4f has received
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synergistic effect with cobalt NPs to substantially boast catalytic
activity and selectivity of these catalysts.
Despite such elegant achievements, the necessity of either the
use of sophisticated and expensive organic ligands, or tedious and
multi-step synthesis of sacrificial template materials accompanied
with uneconomical and inapplicable in mass production, or
emission of large amount of wastes in the preparation process Table 1. Catalytic performance with different catalystsa
H
constitutes a serious drawback. To solve these issues, attention is
being emergingly turned to the naturally renewable and available
biomass in combination with inexpensive and earth-abundant metal
salts to prepare the N-based carbon supported catalysts,9 which
represents an important aspect of sustainable chemical research.
Very recently, we developed a facile, green and eco-friendly
synthetic method to fabricate N-doped hierarchical porous carbon
using bamboo shoots as N and C sources, in which nearly 8 wt% of
N-containing compounds (e.g., proteins and amino acids) in its
composition can be ideally offered as N source as dopant. The
resulting carbon served as an excellent support for immobilization
of palladium nanoparticles to form a recyclable and efficient
heterogeneous catalyst for chemoselective semihydrogenation and
hydrosilylation of alkynes.10
NH2
NO2
N
H
Cat.
+
O
HCOONH4 (4 equiv)
THF/H2O (4.5/0.5, v/v)
120oC, 6 h
2a
1a
3a
Selectivity (%)b
Entry
Catalyst
Time (h) Conversion (%)b
2a
3a
1
Co@C-800
Co@NC-800
Co@NPC-700
Co@NPC-800
Co@NPC-900
AC-800
6(12)
6(12)
12
8(49)
41(95)
75
100(98)
100(97)
95
0(2)
0(3)
5
2
3
4
6(12)
12
97(100)
100
0.5
98(97)
89
2(3)
11
0
5
6c
7c
8c
9
12
100
Bearing these results in mind, we envision that complexation of
cobalt salt with bamboo shoots can be used for generation of cobalt
NPs dispersed on N-doped carbon. Meanwhile, to further modify
NC-800
12
6
100
0
NPC-800
12
22
100
0
no
12
0
-
-
and/or enrich the characteristics of carbon shell,
a more
10
Co(NO3)2
12
0.2
100
electronegative P atom was introduced simultaneously using cheap
and easily available PPh3 as P source during complexation for in-situ
growth of cobalt NPs encapsulated with N,P-dual doped carbon
matrix upon pyrolysis. To our surprise, we found that the second P-
dopant in the catalyst pronouncedly improve the reaction efficiency
compared with the counterpart with sole N-dopant. Herein, for the
first time, we report a novel nanostructured cobalt NPs coated by a
N,P-codoped carbon shell (hereafter referred to as Co@NPC-x,
where x denotes the pyrolysis temperature), which are synthesized
via a cost-effective and environmentally benign method using
naturally renewable and easily available biomass together with
earth-abundant and cheap cobalt salts. The obtained catalyst
exhibited outstanding activity and exclusive selectivity for catalytic
transfer hydrogenation of functionalized nitroarenes using formic
acid (FA) or ammonium formate as hydrogen donor. A broad variety
of functionalized nitroarenes can be efficiently reduced to their
corresponding anilines with excellent tolerance of functional
groups. The catalyst also demonstrates high stability and can be
easily separated using simple external magnet for at least 6
successive reuses without significant loss in both activity and
selectivity.
0
aReaction conditions: nitrobenzene (0.5 mmol), HCOONH4 (2.0
mmol), catalyst (4.8 mol% of Co), THF/H2O (5 mL, 4.5/0.5, v/v),
120oC. bdetermined by GC-FID using dodecane as an internal
standard.c40 mg of the supports was used.
Results and discussion
The catalysts Co@NPC-x were prepared in
a sequential
hydrothermal and pyrolysis procedure (Figure 1, see details in the
Supporting Information). Briefly, the fresh bamboo shoots were
firstly cut into slices, dried and ground into powder followed by the
hydrothermal process to get the brown solids. The resulting solids
were homogeneously mixed with Co(NO3)2 and PPh3 followed by
pyrolysis under N2 atmosphere at varying temperature. The Co
content in catalysts was determined to be 2.14 ~ 4.04 wt% by the
coupled plasma optical emission spectrometry (ICP-OES) (Table S7,
Supporting Information). For reference, the catalyst Co@NC with
sole N-dopant, Co@C without N and P dopant by direct pyrolysis of
the mixture of activated carbon and cobalt salt, and the bare
supports NPC and NC without metal loading were also prepared
with similar procedure, respectively.
The obtained catalysts were investigated for catalytic transfer
hydrogenation of functionalized nitroarenes in the presence of
various hydrogen donors. Transfer hydrogenation of nitrobenzene
was chosen as benchmark reaction, and upon intensively screening
of a set of factors including solvent, temperature, amount of
catalyst loading, type of hydrogen donors (e.g., FA, ammonium
formate, NaBH4, or hydrosilane (PhMe2SiH)) and their amounts
employed with respect to nitrobenzene (see Table S1-6 in
Supporting Information), we found that the best catalytic
performance was achieved with complete conversion and excellent
selectivity to aniline (up to 97%) using THF/H2O (5 mL, 4.5/0.5, v/v)
as solvent, 4 equivalent of ammonium formate at 120oC in the
presence of 40 mg of Co@NPC-800 within 12 h. Notably, both FA
and ammonium formate, which are easily accessible from biomass
processing with high hydrogen capacity and attractive features of
Figure 1. Schematic illustration of the preparation of Co NPs coated
by N,P-codoped carbon shell.
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convenient storage and delivery, can serve as ideal hydrogen addition, such unique nanostructured cobalt NPs encapsulated in
donors for the reduction, while ammonium formate distinctly the heteroatom-doped carbon shells is the essential for this transfer
exhibited superior efficiency to FA under identical conditions. hydrogenation reaction. The reaction completely shut down with
Control experiment reveals that the reaction proceeded quite either the absence of the catalyst or cobalt salt e.g. Co(NO3)2 as
smoothly to achieve 96% conversion of nitrobenzene in the first 6 h, catalyst (entries 9 & 10).
while it took another 6 h for complete conversion.
Further studies showed that the amount of PPh3 added for
pyrolysis to obtain the catalyst Co@NPC had a profound influence
on the reduction efficiency (Figure 2). Upon addition of 1 wt% of
PPh3, the conversion of nitrobenzene was boosted substantially
from 41 to 60%, further increase of the added amounts led to
gradual improvement in activity without significant influence on
selectivity to desired aniline. However, an optimal amount to be 20
wt% existed and a decrease in catalytic performance was observed
with further more increase of the amount of PPh3. For clarity, all
catalysts for characterization and catalytic test are based on 20 wt%
of PPh3 thereinafter unless otherwise noted.
Given such impressive findings, great efforts were devoted to
gain profound insight into the structure of the catalyst Co@NPC and
to correlate the relationship between structure and catalytic
performance. X-ray diffraction (XRD) characterization disclosed that
the metallic cobalt was predominantly generated after pyrolysis
with characteristic diffraction peak appearance at 44.2, 51.5, and
75.9o, assignable to the (111), (200), and (220) facets of structured
metallic cubic cobalt (JCPDS No. 15-0806). Besides, a characteristic
bump diffraction peak at around 25o corresponding to the (002)
plane of graphitic carbon was also observed (Figure 3a). The almost
same XRD pattern was observed for the catalyst Co@NC-800. These
observations are critically different from the previously reported
examples,7 in which the mixed cobalt phases including metallic Co,
CoO, and Co3O4 were formed simultaneously. Such discrepancy
clearly indicates that this pyrolysis reduction strategy directly
derived from biomass as C and N sources could manufacture
metallic cobalt with nice crystallinity. Notably, for the catalysts with
N-doping or N,P-codoping, in particular for the cases of Co@NPC-x
with variable pyrolysis temperature, tiny diffraction peak at 40.6o
appeared (Figure S1, Supporting Information). Similarly, the tiny
peak at 40.6o was also detected for the catalysts Co@NPC-800 with
different PPh3 added amounts, especially for the catalysts with
larger than 5 wt% amounts (Figure S2, Supporting Information).
However, such peak is not observed for the sample Co@C-800,
suggesting the interaction between cobalt NPs and heteroatoms in
the carbon matrix to most likely form Co-P, or Co-Nx, or their
mixture phases.11
Field emission scanning electron microscopy (FE-SEM) and
transmission electron microscopy (TEM) observation of the
Co@NPC-800 demonstrated that the formation of uniform spherical
particles with a diameter of around ca. 23 nm (Figure 3b-f). Further
high-resolution (HR) TEM analysis revealed a highly integrated
nanostructure of the layered graphitic carbon-coated cobalt NPs
(Figure 3f). The well-resolved lattice spacing of 0.205 and 0.338 nm
corresponding to the (111) and (002) plane of the metallic cobalt
and graphitic carbon can be perceived, respectively, which is in
good line with the XRD observation. A high-angle annular dark-field
scanning transmission electron microscopy (HAADF-STEM) images
with sub-Ångström resolution further confirmed that uniform
cobalt NPs were formed (Figure 3g), and the corresponding energy-
dispersive X-ray (EDX) maps showed that Co, N, P, and C atoms
were distributed homogeneously within the catalyst (Figure 3h-k).
Magnetic measurements also confirmed the formation of metallic
cobalt NPs (Ms = 6.2 emu/g, T = 300 K), which is particularly helpful
for simple separation with external magnet after reaction (Figure
S10).
Figure 2. The catalytic performance as a function of the added
amount of PPh3 in the catalyst Co@NC-800.
Reaction conditions: nitrobenzene (0.5 mmol), HCOONH4 (2.0
mmol), constant cobalt contents (4.8 mol% of Co) in the selected
catalysts, THF/H2O (5 mL, 4.5/0.5, v/v), 120oC, 6 h under nitrogen
atmosphere.
Meanwhile, several other features are noticeably observed upon
reaction conditions optimization (Table 1). Firstly, the pyrolysis
temperature for the resulting catalysts Co@NPC had a considerable
impact on the catalytic performance. The catalyst Co@NPC-700
gave inferior reaction efficiency than that of Co@NPC-800, while an
appreciable lower selectivity to aniline was achieved in the
presence of Co@NPC-900 (entries 3-5). The catalyst Co@NPC-800
showed maximum performance in terms of both activity and
selectivity. Secondly, heteroatoms doping in the carbon framework
of the catalysts remarkably boosted the reaction efficiency. The
catalysts Co@C-800 and Co@NC-800 were respectively employed
with identical molar percentage of cobalt (equals to Co@NPC-800)
for the reduction (entries 1 & 2). Under otherwise identical
conditions, only 8% conversion was achieved with the catalyst
Co@C-800 after 6 h, while the catalyst Co@NC-800 gave 41%
conversion and is 4 times higher than that of Co@C-800. More
impressively, nearly complete conversion of nitrobenzene was
observed for the catalyst Co@NPC-800. Apparently, the reaction
efficiency follows the order of Co@NPC-800 > Co@NC-800 > Co@C-
800. Such results clearly indicate that heteroatom N or N,P in the
carbon framework exerted a vital role for the reduction. This is
indeed the case and was further verified by the reaction by using an
inert activated carbon (AC), NC-800, and NPC-800 as catalyst,
respectively (entries 6-8). The reaction did not proceed in the
presence of AC, while NC-800 and NPC-800 gave 6 and 22%
conversion of nitrobenzene to aniline, respectively. A sharp increase
in activity was distinctly observed for these carbon supports with
heteroatoms modification while their efficiency was not
comparable to the level of their analogues with Co NPs loading. In
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The formation of graphitic carbon was further proved by the samples was observed, indicating the coordination interaction
Raman spectrum (Figure 4a), in which the G band at 1590 cm-1 between Co NPs and N in the carbon framework, which is believed
indicates the in-plane vibration of the sp2 carbon atoms, while the D to improve the catalytic activity. The XPS 2p spectrum (Figure 4d) in
band at 1350 cm-1 is a defect-induced nonperfect crystalline Co@NPC-800
structure of the material.12 The IG
/ID (ID and IG signify the intensity of shows deconvoluted peaks at 129.3, 130.2, 132.4, corresponding to
D and G band) ratio of the Co@NC-800 to be 1.03 is slightly higher the P 2p3/2, P 2p1/2, P-C bond, and a peak at 133.4 eV could be
than that of Co@NPC-800 (1.01), indicative of more structural assigned to the P-O oxidized species of phosphide due to the
defects and disordered graphitic carbon due to N and P atoms inevitable oxidation in the storage.16 The Co 2p XPS spectrum was
dually incorporated into the resulting carbon lattices.13 Such trend fitted into four peaks at 778.5, 780.3, 782.7, and 785.4 eV (Figure
agreed well with the observation for the Co@NPC-800 with varying 4e). The peaks at 778.5 and 129.3 eV are close to the binding
P
energies of Co and P in CoP,17 and the BE of Co 2p3/2 has positive
shift and P 2p3/2 has negative shift, respectively, compared with the
BE of metallic Co
Figure 3. a) X-ray diffraction patterns for the catalysts Co@NC-800
and Co@NPC-800; b, c) SEM images of Co@NPC-800. The magnified
image in (c) clearly reveals the uniformly sized nanospheres. d, e, f)
HR-TEM image of Co@NPC-800 with different maganitute, showing
the carbon shells and coated metal nanoparticles. Inset (f): cystal
(111) plane of the metallic Co and (002) plane of the carbon. g-k)
HAADF-STEM image and corresponding EDX maps of Co@NPC-800
for C (h), N (i), P (j), and Co (k).
Figure 4. a) Raman spectroscopy for the catalysts Co@NC-800 and
Co@NPC-800; b) N2 sorption isotherms of the catalysts Co@NC-800
and Co@NPC-800 at 77 K, and the inset is corresponding pore size
distributions calculated using a nonlocal density function theory
(NLDFT) method; d) Magnetic hysteresis loops of Co@NPC-800 at
300 K, and the inset is the illustration of magnetic separation of the
catalyst from solution with external magnet; c) N 1s, d) P 2p, and e)
Co 2p spectra of Co@NPC-800 catalyst.
contents (Figure S5). Similarly, increasing the pyrolysis temperature
(700, 800, and 900oC) for the Co@NPC-x led to relatively higher
degree of graphitization (Figure S4). N2 adsorption/desorption
measurements clearly demonstrated that the catalysts prepared in
this strategy possess hierarchically micro-, meso-, and macro-pores
with high specific surface areas and large pore volumes as shown in
Figure 4b, Figure S3, and Table S7 in the Supporting Information,
which are expected to favor the rapid mass-transfer and can
provide sufficient active sites exposure for catalysis. Remarkably,
the catalyst Co@NPC-800 showed a relatively higher surface area
than Co@NC-800, indicating that the second P-doping is
significantly beneficial to the pores formation during pyrolysis,14
thereby leading to a great increase in surface area.
X-ray photoelectron spectroscopy (XPS) was carried out to
further investigate the surface compositions and chemical state of
the Co@NPC (Figure 4c-e). The XPS survey spectrum clearly showed
the obvious signals of N, P, C, O, and Co elements without other
impurities. Four types of N, including graphitic (401.5 eV, 21.4%),
pyrrolic (400.2 eV, 31.5%), Co-Nx (399.7 eV, 12.6%), and pyridinic
(397.8 eV, 34.5%), could be distinguished from N 1s spectrum
(Figure 4c).15 Pyrrolic-N and pyridinic-N species in Co@NPC-800
catalyst are the dominant peaks in the N 1s XPS spectrum, which
not only are favorable for the reduction, but also serve as anchoring
sites for formation of Co-Nx sites. Notably, the Co-Nx species in all
(778.1 eV) and elemental phosphorus (130.0 eV). Such results
suggest that Co and P in catalyst Co@NPC-800 have a partial
positive charge (δ+) and negative charge (δ-), respectively, which
results from the electron transfer from Co to P.18 In contrast, only
metallic Co and oxidized Co peaks were observed in Co 2p XPS
spectrum for the catalyst Co@NC-800 (Figure S9). In addition, the
BE of 132.4 eV for P-C further demonstrates the doping of
additional P into N-doped carbon lattices. Note that the content
percentage of P 2p peaks assignable to CoP and P-C species
gradually increased when the amount of PPh3 was enhanced or the
pyrolysis temperature increased (Figure S6-8), suggesting more P
atoms incorporate into the carbon lattices, thereby increasing the
contents of Co-P species in the catalysts.
Taken all characterization results together, such a geometric
confinement of Co NPs within N,P-codoped graphitic carbon shells
is believed that: 1) N and P heteroatoms in the carbon framework
can serve as active sties beneficial for interacting with
functionalized nitroarenes substrate and activating FA or
ammonium formate, which was truly reflected by the difference in
catalytic performance among the supports of activate carbon, NC-
800, and NPC-800 as catalyst, respectively (Table 1, entries 6-8); 2) a
synergistic effect between Co NPs and N,P-dopants in the carbon
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framework to form Co-P, Co-Nx, or their mixture species, integrally of the halide substituent did not induce any significant differences
allows it to significantly boost the reaction efficiency in combination in reactivity, and no dehalogenation processes are observed for all
Table 2. Substrate scope of functionalized nitroarenes.a
Reaction conditions: nitro compound (0.5 mmol), HCOONH4 (2.0 mmol), Co@NPC-800 (40 mg, 4.8 mol% of Co), THF/H2O (4.5/0.5,
v/v, mL), 120oC, 12 h. Conversion and selectivity were determined by GC-FID using dodecane as an internal standard. The product
was structually confirmed by using an authentic sample.
with the N,P-dopants as active sites.
halogenated substrates. Gratifyingly, nitroarenes bearing amine,
Subsequently, we investigated the substrate scope to explore the hydroxyl, cyano, or ester, amide substituents were all tolerated by
general applicability of this catalytic transfer hydrogenation this protocol, giving the corresponding anilines in >99% yield
protocol under the optimized conditions (Table 2). In general, the without any signs of side products. Particularly, the nitroarenes
catalyst Co@NPC-800 enabled the reduction of a diverse array of decorated with the most challenging reducible functional groups,
functionalized
nitroarenes,
consistently
providing
the such as aldehyde, ketone, nitrile, and C=C, were successfully
corresponding anilines in high yields with exclusive selectivity. reduced to the aniline
Electron-donating and electron-withdrawing substituents at the o-,
m-, p-positions on the phenyl ring of nitroarenes all gave high yields
to their corresponding anilines, suggesting that the electronic
and/or steric effect of substituents on aromatic rings is negligible.
More importantly, for halogenated nitrobenzenes, full conversion is
achieved, giving excellent yields of the target product. The position
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confirmed by ICP-OES analyses that the amounts of Co that leached
into the supernatant after the first run were not detectable (below
the detection limit). These tests rule out the role of any leached Co
nanoparticles or other Co-species in catalyzing catalytic transfer
hydrogenation reaction.
To shed insight into the possible pathway of the catalytic transfer
hydrogenation of nitroarenes, additional experiments were carried
out (Scheme S1, Supporting Information). The reduction of
nitrobenzene in the presence of an atmosphere of molecular
hydrogen instead of formic acid or ammonium formate as hydrogen
donor didn’t proceed under otherwise identical conditions,
indicating that nitrobenzene is reduced by in-situ generated cobalt
hydride species rather than molecular hydrogen. No hydrogen was
detected out for the headspace gas after reaction by GC-TCD
analysis, in further support of the cobalt hydride formation. It was
widely accepted that the nitrogen atoms in the metal modified N-
doped carbon catalysts act as base sites.7b,19 To confirm this
assumption, the reaction was carried out with identical equivalent
of HCOONH4 and H3PO4. In this case, 51% conversion of
nitrobenzene with exclusive selectivity to 2a was observed; while
97% conversion with 98% selectivity to 2a was achieved without
addition of H3PO4 under otherwise identical conditions. This result
clearly indicates that H3PO4 as a stronger acidity more readily
neutralizes the nitrogen atoms to form NH+, thus partially
deactivating the Co@NPC-800 catalyst. Accordingly, the catalyst
Co@NC-800 was treated with the same manner for the benchmark
reaction and only gave 7% conversion of 1a, which is comparable to
the catalytic performance of the catalyst Co@C-800 without N
doping under identical conditions. We further prepared the
catalysts Co@NC-800 by pyrolysis of the mixture of activated
carbon and CoCl2 with exogenous addition of melamine as N source
at 800oC under N2 atmosphere. Their respective catalytic
performance for benchmark reaction shows a high dependence on
N content and superior catalytic efficiency is obviously achieved
with higher N content. These results firmly indicate that N atoms
indeed played an essential role in catalysis.
Meanwhile, to distinguish the role of P dopant in the catalyst, we
prepared the catalyst Co@PC-800 by direct pyrolysis of the mixture
of activated carbon, CoCl2 and PPh3 (20 wt%, an optimal P content
in the catalyst Co@NPC-800) as P source with the same preparation
procedure. The resultant catalyst showed 79% of conversion of 1a
with 100% selectivity to 2a under standard conditions, which is
inferior to that of Co@NPC-800 while considerably higher than that
of Co@C-800. This result further confirms the boosting role of P
atoms for the reaction efficiency. Furthermore, head-to-head
comparison demonstrates that the catalysts Co@C-800, Co@NC-
800, and Co@NPC-800 are significantly higher in activity than their
corresponding supports AC-800, NC-800 and NPC-800, clearly
verifying the critical role of Co NPs. Taken all these results into
consideration, we strongly believe that it is the synergistic effect of
multi-components among N, P, and Co NPs rather than either the
heteroatoms or Co nanoparticles solely facilitate the reaction.
Given all control experiments, we tentatively proposed the
plausible reaction mechanism as shown Scheme 1. In the first step,
N atoms on the catalyst, as basic sites with remarkable role in
facilitating proton transfer, can capture H+ from the formic acid to
generate NH+ and form the Co-formate intermedidate. The formate
anions tend to adsorb on the surface of Co NPs, as the formate
anions could be coordinate with the empty d orbitals of Co owing to
the low d-electron density of Co. In the second step, the Co-formate
intermediate can produce a CO2 molecule and in-situ form CoH-.
Due to the P doping, as a more electronegative atom and stronger
Figure 5. a) Recycling and b) hot filtration test of the catalyst
Co@NPC-800 for catalytic transfer hydrogenation of nitrobenzene
to aniline.
Reaction conditions: nitrobenzene (0.5 mmol), HCOONH4 (2 mmol),
Co@NPC-800 (40 mg, 4.8 mol% of Co), THF/H2O (5 mL, 4.5/0.5, v/v),
120oC, 12 h.
products with exclusive selectivity and maintained the reducible
groups untouched, highlighting the excellent chemoselectivity of
the nanostructured core-shell cobalt NPs catalyst and the
remarkable advantage compared to that of noble metal-based
catalysts. In addition, heteroatom-containing nitroarenes can also
be efficiently reduced to their corresponding anilines, which are in
particular important intermediates in the pharmaceutical and
agrochemical industries.
Durability/recyclability of
a catalyst is critical for practical
applications. Upon completion of the reduction of nitrobenzene,
the catalyst Co@NPC-800was recollected using simple external
magnet, washed, and dried for subsequent cycles. As shown in
Figure 5a, the activity and selectivity remained with negligible
changes after five recycling experiments, demonstrating the high
durability of this catalyst. The heterogeneity of Co@NPC-800 was
further confirmed by hot filtration test (Figure 5b). We carried out
the reduction of nitrobenzene and removed the catalyst from the
reaction mixture by hot filtration at approximately 45% conversion
of nitrobenzene. After removal of the Co@NPC-800 catalyst, the
filtrate was again held at 120oC for continuing reaction. In this case,
no additional conversion increment of nitrobenzene was observed,
indicating that leached Co NPs or other Co-species from the catalyst
(if any) are not responsible for the observed activity. It was
6 | J. Name., 2012, 00, 1-3
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coordinating capacity, P can strongly coordinate with Co NPs, even
to form CoP species, which has been elucidated by XPS
characterizations. Such interaction resulted in the Co has a partial
positive charge (δ+) due to the electron transfer from Co to P, which
is beneficial for the preferential adsorption of the oxygen atom in
the NO2 group and formate anions.19b In the third step, when
nitroarenes is added in the catalytic system, two parts of the
hydrogen in NH+ and CoH- prefer to react with the nitro groups to
generate an aniline with no hydrogen detected, thereby completing
the entire catalytic cycle.
Y6710619KL) and 13th-Five Key Project of the Chinese
Academy of Sciences (Grant No. Y7720519KL). We thank
Professor Xiufang Chen (QIBEBT) for generous supply of
bamboo shoots.
Notes and references
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Scheme 1. Proposed mechanism for transfer hydrogenation
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Conclusions
In conclusion, we developed a heterogeneous, inexpensive, and
active cobalt NPs encapsulated with a N,P-codoped carbon shell
derived from the biomass. The as-prepared catalysts possess large
surface area, high pore volume with hierarchical micro-, meso-, and
macro-pores structure. The synergism of N and P dopants on the
graphitic carbon and confined Co NPs produces extremely high
active sites for catalytic transfer hydrogenation of functionalized
nitroarenes using formic acid or ammonium formate as hydrogen
donor. The most active catalyst showed outstanding activity and a
broad variety of functionalized nitroarenes can be efficiently
reduced to their corresponding anilines with exclusive selectivity
and great tolerance of functional groups. The catalyst also
demonstrates high stability and can be easily separated using
simple external magnet for at least 6 successive reuses without
significant loss in both activity and selectivity. Hence, this study
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
We gratefully acknowledge the start-up financial support from
Qingdao Institute of Bioenergy and Bioprocess Technology
(QIBEBT), Chinese Academy of Sciences (Grant NO.
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