Recyclable and Selective Nitroarene Hydrogenation Catalysts Based on Carbon-Coated Cobalt Oxide Nanoparticles

Article abstract of DOI:10.1002/cctc.201501265

Co/CoO nanoparticles coated with graphene layers (Co/CoO@Carbon) have been developed through direct heating treatment of cobalt oxide precursors incipiently deposited over nanographite materials. Cobalt oxides are partially reduced to active zero-valent metal species and the simultaneous formation of carbon layers over the nanoparticles protects them from oxidation and deactivation. This nanocatalyst performs excellently in chemoselective hydrogenation of some challenging nitroarenes with reducible functionalities to the corresponding anilines. The catalyst is kept in active and selective performance in a ten-run recycling test.

Full text of DOI:10.1002/cctc.201501265

DOI: 10.1002/cctc.201501265
Full Papers
Recyclable and Selective Nitroarene Hydrogenation
Catalysts Based on Carbon-Coated Cobalt Oxide
Nanoparticles
Bingfeng Chen, Fengbo Li,* Zhijun Huang, and Guoqing Yuan*[a]
Co/CoO nanoparticles coated with graphene layers
(Co/CoO@Carbon) have been developed through direct heat-
ing treatment of cobalt oxide precursors incipiently deposited
over nanographite materials. Cobalt oxides are partially re-
duced to active zero-valent metal species and the simultane-
ous formation of carbon layers over the nanoparticles protects
them from oxidation and deactivation. This nanocatalyst per-
forms excellently in chemoselective hydrogenation of some
challenging nitroarenes with reducible functionalities to the
corresponding anilines. The catalyst is kept in active and selec-
tive performance in a ten-run recycling test.
Introduction
Many important industrial conversions of functional groups in-
volve the use of molecular hydrogen.[1] If hydrogen is added
across a p-bond, the process is typically called hydrogenation.
It is generally defined as hydrogenolysis when hydrogen is
added across a s-bond. Homogeneous[1b] or heterogeneous[2]
metal catalysts are crucial to the conversion efficiency of both
processes. Homogeneous catalysis can lead to high selectivity
of the targeted product through well-tailored ligands and
metal complexes, but there are the inevitable drawbacks of re-
cycling and recovering expensive ligands and metal species.
Heterogeneous reaction systems with solid catalysts actually
dominate practical operations in the petrochemical industry
and other industrial organic conversions.
tivity and recyclability. Other base metals (Co, Cu, Fe) have the
same drawbacks.[6] Great efforts have been put into replacing
precious metal catalysts and this gives an impetus to develop-
ing base metal catalysts with improved selectivity and stabili-
ty.[7] The development of nanoscience offers specific solutions
to improve catalytic performances through harnessing nano-
structured or nanoscale materials as catalysts.[8] Supported
metal nanoparticles and metal species dispersed over nanoma-
terials are general nanocatalysts that afford specific properties
from quantum size effect.[9] However, nanoparticles of base
metals (Ni, Co, Cu, Fe) are not air-stable and fast deactivation
takes place under the practical operation.[10] The group of Mat-
thias Beller has reported the preparation of iron oxide and
cobalt oxide nanoparticles coated by nitrogen-doped gra-
phene layers though the pyrolysis of metal acetates and nitro-
gen ligands and the form of FeNx or CoNx centers gives the
nanocatalysts high selectivity in nitroarene hydrogenation.[11]
In this work, cobalt oxide precursors are introduced onto
nanographite materials prepared by a mechanical method (ball
Hydroprocessing of petroleum includes hydrogenation (hy-
drodearomatization) and hydrogenolysis (hydrodesulfurization,
hydrodenitrogenation, hydrocracking, hydrodeoxygenation, hy-
droisomerization). Hydroprocessing catalysts[3] are Mo (W)-con-
taining supported catalysts promoted by Co or Ni, which work
under relatively severe conditions (above 623 K). In organic
synthesis, precious metals (Pt, Pd, Rh, Ru, Ir, Os, Re) in most
cases show high activity and selectivity under mild conditions.
However, they are very expensive because of the low abun-
dance and substantial costs during the required recovery and
recycle processes.[4] Nickel is the preferred catalyst for hydro-
treatment of many functional groups under higher reaction
temperature and hydrogen pressure,[5] but it shows poor selec-
[a] Dr. B. Chen, Dr. F. Li, Dr. Z. Huang, Prof. G. Yuan
Beijing National Laboratory of Molecular Science
Laboratory of New Materials
Institute of Chemistry, Chinese Academy of Sciences
Beijing, (P.R. China)
Fax: (+86)10-62559373
E-mail: lifb@iccas.ac.cn
yuangq@iccas.ac.cn
Supporting information and ORCID(s) from the author(s) for this article
are available on the WWW under http://dx.doi.org/10.1002/
cctc.201501265.
Scheme 1. Preparation procedure of the Co/CoO@Carbon catalyst.
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milling). Through further heating treatment, carbon-coated
nanoparticles are formed and cobalt oxides are partially re-
duced to zero-valent metal states (Co/CoO@Carbon) (as shown
in Scheme 1). Nanographite layers are the carbon source for
formation of graphene layers over nanoparticles and the re-
ductant for cobalt oxide species. Coating of graphene layers
leads to excellent recyclability. The formation of zero-valent
cobalt centers and their synergistic effects with cobalt oxide
species endow the supported nanoparticles with unexpected
selectivity and activity in hydrogenation of nitroarenes to aro-
matic amines. Sustainability of chemical processes is readily im-
proved through the development of this well-designed cata-
lyst.
Results and Discussion
Development of Co/CoO@Carbon catalysts
Figure 1. a,b) TEM images of supported Co/CoO nanoparticles coated with
graphene carbon layers. c) HAADF image of the Co/CoO@Carbon catalyst
particles with element mapping for the selected particle. d) EDX analysis of
the element mapping.
Figure 1 shows transmission electron microscopic images of
monodispersed supported nanoparticles. All nanoparticles are
coated with several layers of graphene carbon (Figure 1a, b),
which provide protection against oxidation and deactivation.
High-angle annular dark-field (HAADF) imaging is used to view
the full-size spectra of the selected particle and the related ele-
ment mapping shows that it has a core of cobalt species with
some oxygen distributions and a carbon shell (Figure 1c). The
local energy dispersion X-ray (EDX) analysis reveals the same
element mapping (Figure 1d).
X-ray photoelectron spectroscopy (XPS) measurement was
performed to investigate the chemical state evolution of
cobalt oxide nanoparticles during heating treatment (Figure 2).
Cobalt oxide precursors are incipiently introduced onto nano-
graphite materials and metallic cobalt species are gradually
formed with the elevation of treatment temperature. The XPS-
Figure 2. a) XPS survey spectra of the selected catalyst samples (A: nanographite deposited with cobalt oxide precursors; B: the sample treated at 873 K for
2.0 h in argon; C: the sample treated at 1073 K for 2.0 h in argon). b) Fitting of Co XPS peaks of A, B, and C samples. c) XPS survey spectra of the sample pre-
pared by replacing nanographite with silica and treated at 1073 K for 2.0 h in argon (the inset shows the Co XPS peaks).
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ed on kieselguhr, Raney copper, and nickel sulfide supported
by alumina.[13] These processes are generally operated at high
temperature (>523 K) and show poor chemoselectivity for pro-
ducing substituted aromatic amines. Homogeneous complexes
of platinum[14] or modified supported platinum[15] show im-
proved performances in chemoselective hydrogenation of the
nitro group in the presence of other sensitive moieties. Howev-
er, they remain a great challenge. In Table 1, experimental re-
sults of the chemoselective hydrogenation of nitro groups
over partially reduced cobalt oxide nanoparticles coated with
carbon layers (Co/CoO@Carbon) are listed. The catalyst exhibits
excellent performance in chemoselective hydrogenation of
twenty nitroarenes with electron-withdrawing or electron-do-
nating substituents. The substrates with methyl and methoxyl
are converted with excellent yields. Nitroarenes with halogen
substituents show good selectivity. Dehalogenation and cou-
pling side reactions are avoided in the hydrogenation of nitro-
arenes with iodine and bromine (Table 1, entry 5, 6). Fluoroani-
line is prepared with a yield of 99% (entry 7). The presences of
protic substituents (hydroxyl and amino) show no negative ef-
fects on the catalytic performances (entry 8–11). Carboxylic de-
rivative substituents, nitrile, ester, and amide, are kept stable
during the hydrogenation of the nitro group (entry 15–17).
Supported gold catalysts[16] have been reported to show che-
moselectivity in the presence of olefinic bonds. Some sub-
strates with highly reducible moieties (entry 18–20) are con-
verted to their corresponding aromatic amines with high yield
(90–99%).
Figure 3. a,b) TEM images of the sample prepared by replacing nanogra-
phite with silica and treatment at 1073 K for 2.0 h in argon. c,d) TEM images
of the sample treated at 873 K for 2.0 h in argon. e,f) TEM image of the
sample treated at 1273 K for 2.0 h in argon.
The recyclability of Co/CoO@Carbon catalyst
The recyclability of the Co/CoO@Carbon catalyst was investi-
gated through an eleven-run recycling test of nitrobenzene hy-
drogenation (Figure 4). After the ninth recycling batch, there is
a tiny loss in the aniline yield. In the eleventh test, deactivation
is observed and the aniline yield decreases to 44.2%. As re-
vealed by XPS measurements, the loss of catalytic activity is re-
lated to the gradual oxidation of active metallic cobalt species
(Figure 5a). Carbon layers over Co/CoO nanoparticles are grad-
ually abraded by the inevitable intergranular friction, the colli-
determined atomic concentration of metallic cobalt reaches
37.1% in the sample obtained at 873 K and the value increases
to 53.3% with the elevation of the treatment temperature to
1073 K (Figure 2b). The control experiment was performed
over fumed silica deposited with cobalt oxide precursors. No
metallic species are detected by XPS measurement after heat-
ing treatment (Figure 2c) and no carbon layers over supported
nanoparticles are observed by TEM (Figure 3a, b). Heat-driven
transfer of carbon species form nanographite to cobalt oxide
nanoparticles leads to the formation of carbon layers coating
the particles (Figure 3c, d). A further increase of treatment
temperature to 1273 K causes the embedment of cobalt oxide
nanoparticles in thick carbon layers (Figure 3e, f).
Selective nitroarene hydrogenation over Co/CoO@Carbon
Nitrobenzene hydrogenation is the principal process for pro-
ducing anilines and related derivatives, which are key building
blocks for the synthesis of polyurethanes, rubber chemicals,
agriculture products, dyestuffs, photographic chemicals, and
drugs, as well as various other chemicals.[12] Industrial hydroge-
nation is performed in the vapor phase in tubular reactors
over copper chromites, copper oxide, or nickel oxide support-
Figure 4. Recycling test of catalytic nitrobenzene hydrogenation over the
Co/CoO@Carbon catalyst.
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Table 1. Screening experiments of nitroarene hydrogenation over Co/CoO@Carbon.
Entry
1
Product
Yield [%] (t [h])
>99 (4)
Entry
11
Product
Yield [%] (t [h])
>99 (4)
2
3
>99 (4)
12
13
>99 (24)
>99 (4)
97 (4)
4
5
95 (4)
14
15
>99 (6)
>99 (4)
81 (4)
6
7
81 (24)
16
17
>99 (4)
>99 (4)
>99 (12)[a]
8
>99 (4)
18
>99 (4)
9
92 (4)
98 (4)
19
20
95 (4)
10
90 (4)[b]
Reaction conditions: 1.0 mmol substrate, 5.0 mL THF/0.2 mL H2O, 393 K, 3.0 MPa Hydrogen. [a] The solvent is H2O (5.0 mL). [b] 1.0 MPa hydrogen. Yields
were determined by GC.
sion with the stirrer and autoclave wall, and the leaching effect
of the solvent (Figure 5b, c). The vanishing of protection coat-
ing directly leads to the oxidation of the active metallic cobalt
species.
drawback is the poor efficiency of depositing the metal precur-
sors over them.
Conclusions
Carbon layers should be kept in appropriate thickness to
balance protection coating and surface accessibility to the re-
actant molecules. Co/CoO nanoparticles can be overloaded
with carbon layers through treatment at elevated temperatures
(1273 K, see Figure 3e, f) and the resulting materials show rela-
tively poor catalytic activity (Figure 6a). Typical oxides are used
as the supports. The corresponding catalysts show very low
yields of hydrogenation products. Other carbon supports, such
as activated carbon and bulk graphite powder, seem not suita-
ble for the formation of appropriate carbon-coating layers (Fig-
ure 6b). Nanocarbon materials, such as nanotubes and gra-
phene, are costly to prepare the practical catalysts and another
Co/CoO nanoparticles coated with appropriate carbon layers
(Co/CoO@Carbon) were facilely developed though heating
treatment of nanographite supports deposited with cobalt
oxide precursors. Nanographite layers were the catalyst sup-
port, the carbon source for the formation of graphene layers
over nanoparticles, and the reductant for cobalt oxide species.
The Co/CoO@Carbon catalyst displayed excellent performance
in the chemoselective hydrogenation of nitroarenes. Some
most challenging nitroarenes with highly reducible functionali-
ties are readily hydrogenated to the corresponding anilines
with the yield above 90%. The catalyst was kept in highly
active and selective performance in a ten-run recycling test.
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Figure 5. a) Fitting of Co XPS peaks of the catalyst during the recycling test. b) TEM image of the used catalyst after the sixth recycling test run. c) TEM image
of the used catalyst after the eleventh recycling test run.
pressure was approximately 3”10À9 mbar. The binding energies
were referenced to the C1s line at 284.8 eV from adventitious
Experimental Section
Materials
carbon. The Eclipse V2.1 data analysis software supplied by the VG
ESCA-Lab200I-XL instrument manufacturer was used to manipulate
the acquired spectra. TEM was performed on a JEOL 2010 TEM
equipped with an attachment for local EDX analysis. The accelerat-
ing voltage was 200 kV, and the spot size was 1 nm. High-angle an-
nular dark-field scanning transmission microscopy (HAADF–STEM)
was performed on the carbon-coated Co bimetallic nanoparticles
catalysts with JEOL JEM-2100F microscope in a scanning transmis-
sion electron microscopy (STEM) mode operated at 200 kV.
Organic substrates were purchased from Acros Organics. Pd/C (Pd:
10 wt%), Pt/C (Pt: 5 wt%), Ru/C (Ru: 5 wt%), Rh/C (Rh: 5 wt%) and
Raney Ni was obtained from Alfa Aesar. Co(NO3)2·6H2O was pur-
chased from Sinopharm Chemical Reagent Co., Ltd (Shanghai,
China). Nanographite powder (D50<600 nm, 99.95% metal basis,
CAS: 7782-42-5) was purchased from Aladdin Reagent Limited
Company (Shanghai, China).
Preparation of Co/CoO@Carbon catalyst
Catalytic hydrogenation test
The Co/CoO@Carbon catalyst was prepared through a two-step
method. Cobalt tartrate (6.25 mmol) was dissolved in 30 mL deion-
ized water and then mixed with 50 mL glycerol. Nanographite ma-
terials (0.5 g) were added and dispersed by ultrasonic treatment
for 1.0 h. The mixture was sealed in a hydrothermal kettle and kept
at 423 K for 12 h. The solid was obtained by centrifugal separation,
washed with ethanol three times, and dried at 373 K for 3.0 h. The
resulting solids were further treated at 1073 K in pure argon for
2 h.
In 100 mL stainless steel autoclave with Teflon liner were added ni-
troarene (1.0 mmol), solvent (5.0 mL THF/0.2 mL water), and Co/
CoO@Carbon catalyst (20 mg). The sealed autoclave was flushed
with hydrogen for four times. The reaction was performed under
3.0 MPa hydrogen at the selected temperature for the given time.
After the reaction, the reaction mixture was analyzed by GC and
GC–MS. In recycling tests, the Co/CoO@Carbon catalyst was collect-
ed by a magnet and washed by ethanol. After drying in air at
333 K, the catalyst was recycled into the next batch test.
The resulting Co/CoO@Carbon catalyst (20 mg) was dissolved in
a HNO3/HCl mixture (4.0 mL, volume ratio=1:3) and kept at 373 K
for 30 min. Carbon solids were filtrated out and the aqueous solu-
tion was diluted to 500 mL. The Co concentration determined by
inductively coupled plasma mass spectrometry is 11.66 mgLÀ1. The
load of cobalt was calculated to be 29.15 wt%.
The products were analyzed by GC and GC–MS. GC was performed
by using a GC-2014 instrument (Shimadzu) with a high-tempera-
ture capillary column (MXT-1, 30 m, 0.25 mm ID) and flame ioniza-
tion detector. GC–MS was performed by using a GCT Premier in-
strument (Waters) with a capillary column (DB-5MS/J&W Scientific,
30 m, 0.25 mm ID). NMR data of some new nitroarene hydrogena-
tion products are listed in the Supporting Information.
Characterization methods
XPS data were obtained with an ESCALab220i-XL electron spec-
trometer from VG Scientific using 300 W AlKa radiations. The base
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Figure 6. a) Catalytic activity of nitrobenzene hydrogenation over the Co/
CoO@Carbon catalyst prepared at different treatment temperatures. b) Cata-
lytic activity of nitrobenzene hydrogenation over the Co/CoO@Carbon cata-
lyst prepared over various cobalt oxide depositing supports (NG: nanogra-
phite; BG: bulk graphite powder; AC: activated carbon).
Acknowledgements
This work was supported by the Natural Science Foundation of
China (21403248, 21174148, 21101161, 21304101).
Keywords: cobalt · heterogeneous catalysis · graphene ·
hydrogenation · nanoparticles
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Received: November 17, 2015
Published online on February 4, 2016
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