Nitrogen-doped graphene-activated metallic nanoparticle-incorporated ordered mesoporous carbon nanocomposites for the hydrogenation of nitroarenes

Article abstract of DOI:10.1039/c8ra00761f

Herein, nanoscale metallic nanoparticle-incorporated ordered mesoporous carbon catalysts activated by nitrogen-doped graphene (NGr) were fabricated via an efficient multi-component co-assembly of a phenolic resin, nitrate, acetylacetone, the nitrogen-containing compound 1,10-phenanthroline, and Pluronic F127, followed by carbonization. The obtained well-dispersed nitrogen-doped graphene-activated transition metal nanocatalysts possess a 2-D hexagonally arranged pore structure with a high surface area (～500 m² g^-1) and uniform pore size (～4.0 nm) and show excellent activity for the selective hydrogenation-reduction of substituted nitroarenes to anilines in an environmentally friendly aqueous solution. The high catalytic performance and durability is attributed to the synergistic effects among the components, the unique structure of the nitrogen-doped graphene layer-coated metallic nanoparticles, and electronic activation of the doped nitrogen.

Full text of DOI:10.1039/c8ra00761f

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Nitrogen-doped graphene-activated metallic
nanoparticle-incorporated ordered mesoporous
carbon nanocomposites for the hydrogenation of
nitroarenes†
Cite this: RSC Adv., 2018, 8, 8898
a
ab
Yao Sheng,a Chenju Chen,b Xiujing Zou,b
*
Haigen Huang,
Xueguang Wang,
Xingfu Shangab and Xionggang Lu
ab
*
Herein, nanoscale metallic nanoparticle-incorporated ordered mesoporous carbon catalysts activated by
nitrogen-doped graphene (NGr) were fabricated via an efficient multi-component co-assembly of
a phenolic resin, nitrate, acetylacetone, the nitrogen-containing compound 1,10-phenanthroline, and
Pluronic F127, followed by carbonization. The obtained well-dispersed nitrogen-doped graphene-
activated transition metal nanocatalysts possess a 2-D hexagonally arranged pore structure with a high
surface area (ꢀ500 m2
g
ꢁ1) and uniform pore size (ꢀ4.0 nm) and show excellent activity for the selective
Received 25th January 2018
Accepted 13th February 2018
hydrogenation–reduction of substituted nitroarenes to anilines in an environmentally friendly aqueous
solution. The high catalytic performance and durability is attributed to the synergistic effects among the
components, the unique structure of the nitrogen-doped graphene layer-coated metallic nanoparticles,
and electronic activation of the doped nitrogen.
DOI: 10.1039/c8ra00761f
rsc.li/rsc-advances
expensive and have serious toxicity issues associated with them.
In the actual production, the use of organic solvents makes the
1. Introduction
separation process more complicated with higher energy
consumption. In addition, the reduction of aromatic nitro
compounds oen stops at an intermediate stage, yielding
hydroxylamines, hydrazines, azoarenes, or azoxyarenes as
byproducts.11–13 Obviously, the most environmentally benign,
atomic efficient, and compatible route for the production of
aromatic amines is the direct and quantitative hydrogenation of
nitroarenes over highly active and selective catalysts with
hydrogen molecules in the absence of a solvent or in environ-
mentally friendly solvents (an aqueous medium), which has
attracted widespread research interest.
Catalysis is a key technology for achieving more benign
processes in the chemical, pharmaceutical, and materials
industries. In terms of industrial applications, the careful
design of heterogeneous catalysts instead of homogeneous
catalysts is prevailing due to the ease of their separation and
recycling. Compared with the traditional bulk catalysts, nano-
catalysts are considered to be a bridge between homogeneous
and heterogeneous catalysts,14,15 which, in general, show high
catalytic activity and selectivity. However, for the reduction of
–NO2, especially in the presence of other reducible substituents,
such as aldehydes, ketones, nitriles, alkenes, and alkynes, noble
metals are oen not chemoselective, and this still remains
a challenge. The applications of the most common commer-
cially available palladium, platinum, ruthenium or rhodium
catalysts are hindered by these issues, and these catalysts must
Structurally diverse anilines constitute major building blocks
and key intermediates for the production of drugs, pharma-
ceuticals, pigments, dyes, and agrochemicals.1,2 The typical
route for their production is the selective reduction of the cor-
responding nitro-derivatives. The traditional non-catalytic
reduction processes (usually carried out using stoichiometric
reducing agents such as Fe/HCl, Zn, Sn, Al, and sulfur
compounds)3,4 require a stoichiometric excess of reagents,
which eventually produce a large amount of inevitable waste
and are therefore not considered to be environmentally benign
and economic processes. In comparison, catalytic reduction
using heterogeneous metal catalysts is a well-established tech-
nique and oen regarded as the method of choice for the
reduction of nitroarenes to anilines (which can be performed
over homogeneous or heterogeneous catalysts using various
reducing reagents such as hydrosilanes, sodium borohydride,
formic acid, hydrazine derivatives, and hydrogen).5–10 However,
the vast majority of catalytic reduction processes requires the
use of abundant volatile organic solvents such as methanol,
ethanol, toluene, tetrahydrofuran, and so on, which are
aState Key Laboratory of Advanced Special Steel, Shanghai University, Shanghai
200072, China. E-mail: wxg228@shu.edu.cn; luxg@shu.edu.cn
bShanghai Key Laboratory of Advanced Ferrometallurgy, Shanghai University, China
† Electronic supplementary information (ESI) available: Catalyst characterization
of Co/NGr@OMC-800 and Fe/NGr@OMC-800 catalysts; XRD, TGA, Raman
spectrum, N2 sorption, TEM and XPS spectra. See DOI: 10.1039/c8ra00761f
8898 | RSC Adv., 2018, 8, 8898–8909
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be modied with suitable additives to improve their selectivity, in catalysis for the selective hydrogenation of nitroarenes.
but mostly at the cost of activity.16–19 A breakthrough obtained Herein, encouraged by recent investigations on the use of
by the Corma group, who reported a heterogeneous gold-based cobalt-based or iron-based catalysts,31,32 we report a protocol to
catalyst that can selectively hydrogenate nitroarenes with prepare novel nanoscale non-noble metal catalysts supported
functional groups, has provided new alternatives for the selec- on nitrogen-doped ordered mesoporous carbon via a chelate
tive hydrogenation of nitroarenes.20 However, it remains a great together with a nitrogen-containing compound-assisted multi-
challenge to realize commercialization because of the factors component co-assembly method. The obtained transition
affecting the supply and volatile price. Thus, there still exists metal-based catalysts possess a 2-D hexagonally arranged pore
a signicant interest in novel inexpensive, active, and selective structure with a high surface area, uniform pore volume, and
catalysts, especially in those based on non-noble metals, despite moderate pore size that is not affected by the addition of
numerous developments of hydrogenation catalysts. In terms of nitrogen. Signicantly, the catalysts present uniform, well-
the selective catalytic reduction of nitro compounds, nitrogen- dispersed metal nanoparticles that are partially embedded in
containing nanostructured carbon materials that possess the carbon framework and the remaining part is exposed to the
enhanced chemical, electrical, and functional properties are mesoporous channels. As expected, the pyrolysis of metal–phen
potentially of signicant technological interest for the devel- complexes leads to the formation of nanoscale metal particles
opment of catalytic systems. However, regarding the nitrogen- surrounded by a modied nitrogen-doped graphene layer (NGr)
doped activated carbon or carbon nanotubes (CNTs) without that are favorable for providing anchoring sites as well as
any loaded metals, which have been proven to be effective regulating the chemical and electronic properties of noble
catalysts for the reduction of 4-nitrophenol21 and nitroben- metals. Modication of the surface takes place and signicantly
zene,22 strong reductants, such as NaBH4 and hydrazine decreases the size of the metal particles of the phen-treated
hydrate, have been inevitably used rather than molecular materials as compared to the case of the material prepared
hydrogen, but these have inferior activity and limited reaction without phen; usually, remarkable differences in the catalyst
scope, which may be because carbon material with highly activity are observed. Therefore, using different nitrogen
graphitized sp2 carbon is too chemically inert to decompose ligands, the performance of the heterogeneous catalyst can be
molecular hydrogen.23,24 Furthermore, nitrogen-doped nano- tuned, and both the chelate and nitrogen-containing ligand
structured ordered mesoporous carbon materials M–N–C indirectly control the activity and selectivity of the given
(where M refers to transition metals, typically Ni, Fe, and reaction.
Co),25–30 a type of metal and nitrogen bi-modied carbon
material that usually evolves from the pyrolysis of metal
complexes with macrocyclic N4 ligands or a nitrogen-containing
2. Experimental
2.1. Chemicals and reagents
precursor, with an interpenetrated and regular mesopore
structure have recently attracted increasing attention because of
their potential cost and environmental advantages; moreover,
they exhibit highly selective hydrogenation of numerous struc-
turally diverse nitroarenes that is proceeded under industrially
viable conditions with high conversion rates. These materials
present fascinating features such as intrinsic electrical proper-
ties, good chemical and thermal stability, and functional
properties and have been widely investigated in heterogeneous
catalysis as potential alternatives to platinum and palladium.
However, it is still a difficult task to organize different nano
objects with tunable sizes, morphologies, and functions into
integrated and modied nanostructures for the development of
novel nanosystems that exhibit high performances in catalysis.
The poly(ethylene oxide)-block-poly(propylene oxide)-block-poly-
(ethylene oxide) triblock copolymer Pluronic F127 (Mw ¼ 12 600
g mol–1, PEO106PPO70PEO106) was purchased from Sigma-
Aldrich. Ni(NO3)2$6H2O, Fe(NO3)3$9H2O, Co(NO3)2$6H2O,
phenol, formalin solution (37 wt%), sodium hydroxide, hydro-
chloric acid, ethanol, 1,10-phenanthroline monohydrate
(phen), acetylacetone (acac), and nitroarenes were purchased
from Sinopharm Chemical Reagent Co., Ltd., which were of
analytical grade and used as received without purication.
Deionized water was used in all the experiments.
2.2. General procedure for catalyst preparation
Most of the catalysts prepared by pyrolysis at high temperatures Preparation of mesoporous carbon composites. Soluble
(600–900 ꢂC) are composed of exposed nanoparticles ranging phenolic resin precursors (20 wt% in ethanol) were prepared
from a few to tens of agglomerates that are easy to lose during using phenol and formaldehyde via a base-catalyzed process
the reaction; therefore, exploration of effective methods to according to a previously reported procedure.33 The nitrogen-
control the nucleation and growth of supported metallic parti- doped graphene-activated metallic nanoparticle-incorporated
cles and design size-variable and nitrogen-modied catalysts is ordered mesoporous carbon catalysts were synthesized by
crucial for the development of high-performance heteroge- a chelate together with a nitrogen-containing compound-
neous catalysts in industry.
assisted multicomponent co-assembly of phenolic resin,
To overcome the abovementioned limitations, we have nitrate, acetylacetone, a nitrogen-containing compound, and
speculated that the combination of N-doped ordered meso- Pluronic F127, followed by carbonization, as shown in Scheme
porous carbon and rst-row transition metals may lead to 1. The typical preparation process is as follows: 1.0 g of Pluronic
successful integration of the properties of the two components F127 was dissolved in 14.0 g of absolute ethanol at 40 ꢂC in
in new hybrid materials, which may present important features a water bath under constant stirring. Then, 5.0 g of the resol
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precursor solution (20 wt% in ethanol) was added to the JEM-2010F eld emission microscope. The sample was
abovementioned mixture and stirred for another 10 min. Aer prepared by placing a drop of the ethanol solution of a well-
this, appropriate amounts of nitrates (0.15 g Ni(NO3)2$6H2O, ground catalyst powder onto a carbon-coated copper grid fol-
Co(NO3)2$6H2O or 0.2 g Fe(NO3)3$9H2O was used) dissolved in lowed by evaporation of ethanol. XPS spectra were obtained
3.0 g of ethanol were added. Subsequently, 0.12 g (acac/nitrates using an ESCALAB 250Xi spectrometer equipped with mono-
were in a 2 : 1 molar ratio) of acac was added followed by the chromatized Al Ka radiation (hn ¼ 1486.6 eV) operated at ca. 1 ꢃ
addition of 0.10 g of 1,10-phenanthroline monohydrate (phen/ 10ꢁ9 torr. The spectra were calibrated using the binding energy
nitrates were in a 1 : 1 molar ratio). For comparison, samples of the C 1 s peak at 284.6 eV. The surface N/C molar ratios of the
were also prepared without loading a metal or without adding catalysts were obtained from the XPS spectra. The actual
the nitrogen containing compound 1,10-phenanthroline. Aer amounts of the C and N elements in the nal M/NGr@OMC-T
further stirring for 30 min, the mixture was cast into Petri catalysts were determined using the Perkin Elmer 2400 CHN
dishes, followed by the evaporation of ethanol for several hours elemental analyzer. The actual metal contents in the M/
at 40 ꢂC in a hood. The resulting sticky lms were transferred to NGr@OMC-T catalysts were measured by inductively coupled
an elecꢂtric thermostatic drying oven for thermopolymerization plasma atomic emission spectrometry (ICP-AES) conducted
at 100 C for 24 h. The obtained dark brown composite lms using a Perkin Elmer emission spectrometer. Thermogravi-
were scraped off, followed by pyrolysis in a tube furꢁn1ace at the metric analysis (TGA) curves were obtained using a thermogra-
dened temperature at the ramp rate of 1 ꢂC min (500 ꢂC, vimetric analyzer (Netzsch STA 4449 F3) from 25 ꢂC to 900 ꢂCꢁa1t
600 ꢂC, 700 ꢂC, 800 ꢂC or 900 ꢂC) for 3 h under a N2 atmosphere an air ow of 80 mL minꢁ1 at the heating rate of 5 C min
.
ꢂ
to decompose the triblock copolymer templates, carbonize the Raman spectra were obtained at ambient temperature using
resol precursors and the nitrogen-containing ligand, and in situ a 785 nm HPNIR excitation laser via a Renishaw Raman spec-
generate metallic nanocrystallites surrounded by nitrogen- trometer equipped with an Olympus microscope and a CCD
doped graphene. All the materials were designated as M/ detector. The laser power on the sample was 15 mW, and a total
NGr@OMC-T (where M refers to the corresponding metallic of 20 acquisitions were taken for each spectrum.
ꢂ
species; T represents the calcination temperature, C).
2.4. Catalytic reactions and product analyses
2.3. Catalyst characterization
2.4.1. General procedure for the hydrogenation of nitro-
arenes. All hydrogenation experiments were carried out in
100 mL autoclaves. To avoid unspecic hydrogenation caused
by the reaction devices, all catalytic reactions were carried out
either in glass vials, which were placed inside the autoclave, or
in Teon vessel-tted autoclaves to avoid direct contact between
the reactants and reaction devices. The magnetic stirring bar
and the corresponding nitro compound (1 mmol) were trans-
ferred into the glass vial, and then, 2 mL deionized water was
added. Aer this, 20 mg of catalyst was added, and the vial was
tted with a septum, cap, and needle. The autoclave was ushed
with hydrogen twice at a 30 bar pressure, and then, it was
pressurized to a 50 bar hydrogen pressure. During the reaction,
the inside temperature of the autoclave was measured to be
Powder X-ray diffraction (XRD) was performed via a Rigaku D/
MAX-2200 apparatus using Cu Ka radiation (l ¼ 0.1542 nm)
operated at a voltage of 40 kV and a current of 40 mA. Moreover,
2q scans were performed from 0.5ꢂ to 5ꢂ at 0.5ꢂ minꢁ1 and from
10ꢂ to 90ꢂat 8ꢂ minꢁ1. The crystallite sizes of the metal nano-
particles were calculated using the Scherrer equation. N2 sorp-
tion was carried out using
a Micromeritics ASAP 2020
sorptometer at ꢁ196 ꢂC. The sample was degassed in vacuum at
200 ꢂC for 8 h before the measurement. The specic surface area
(SBET) was evaluated using the Brunauer–Emmett–Teller (BET)
method. Pore size distribution was analyzed using the adsorp-
tion branch and the Barrett–Joyner–Halenda (BJH) method. The
pore size (Dp) was obtained from the maximum of the pore
distribution curve. The pore volume (Vp) was taken at the P/P0 ¼
0.990 single point. TEM images were obtained using the JEOL
Fig. 1 (a) Low-angle XRD patterns and (b) wide-angle XRD patterns of
the nickel nanoparticle-incorporated nitrogen-doped ordered mes-
oporous carbon nanocomposites synthesized at different
temperatures.
Scheme 1 Preparation of M/NGr@OMC catalysts.
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Table 1 Textural properties of the prepared materials
SBET (m2 gꢁ1
)
Vp (cm3 gꢁ1
)
Dp (nm)
WT (nm)
a
b
Sample
d100 (nm)
a0 (nm)
NGr@OMC
10.0
11.5
11.2
10.7
10.6
10.5
10.0
10.6
10.7
11.5
13.3
12.9
12.3
12.2
12.1
11.5
12.2
12.3
269
444
453
483
520
444
439
486
445
0.16
0.34
0.35
0.35
0.37
0.36
0.30
0.36
0.39
3.7
4.7
4.5
4.5
4.1
4.1
4.1
4.1
4.2
7.8
8.6
8.4
7.8
8.1
8.0
7.4
8.1
8.1
Ni/NGr@OMC-500
Ni/NGr@OMC-600
Ni/NGr@OMC-700
Ni/NGr@OMC-800
Ni/NGr@OMC-900
Ni/OMC-800
Co/NGr@OMC-500
Fe/NGr@OMC-500
a
a0 is the cell parameter calculated from the SXRD patterns (a0 ¼ 2d100/O3). b Wall thickness (WT) values calculated from WT ¼ a0 ꢁ Dp.
ꢂ
100 C, and this temperature was used as the reaction temper-
3. Results and discussion
3.1. Structure identication of the M/NGr@OMC-T catalysts
ature. Aer the completion of the reaction, the autoclave was
immediately cooled down to room temperature by owing cold
water. The remaining hydrogen was discharged, and the
samples were removed from the autoclave. To individual vials,
n-hexadecane (100 mL) was added as an external standard, and
the reaction mixture from the vial was completely transferred to
a beaker. Aer this, the mixture was extracted and diluted with
ethyl acetate (10–15 mL) followed by ltration and then
analyzed by gas chromatography and gas chromatography-mass
spectrometry (GC-MS, Shimadzu GCMS-QP2010 Plus). Conver-
sion and yields were determined by GC-FID (Varian CP-3800)
with a capillary column (column VF-1 ms, 15 m, 0.25 mm,
0.25 mm) and a ame ionization detector (FID). Quantitative and
qualitative analysis of all the amines were conducted by GC and
GC-MS analysis.
2.4.2. Procedure for catalyst recovery and utilization.
Under similar experimental conditions as described in Section
2.4.1, aer completion of the reaction, the autoclave was
cooled down to room temperature. The remaining hydrogen
was discharged completely, and the substrate was removed
from the autoclave. The catalyst from the reaction mixture was
ltered off, washed with water and then ethyl acetate followed
by drying without any other treatment, and then used for the
next cycle.
The low-angle XRD patterns corresponding to the samples
calcined at different temperatures are shown in Fig. 1a. Most
materials retained their particularly ordered structure, showing
three diffraction maxima (100), (110), and (200), indicating an
ordered 2D hexagonal mesoporous structure (space group
p6mm). It should be noted that no diffraction peaks were
observed for the samples without loaded metal nanoparticles;
this indicated the disorder of their mesoporous structure;
moreover, in this case, phen was used as a ligand without acac
(not shown here). This result demonstrated that the addition of
metal ion as well as acac strongly inuenced the co-assembly
self-organization of resol and F127 with the ligands in the
solvent evaporation process. The formation of metal nano-
particles with semi-exposed morphology in the nitrogen-doped
ordered mesoporous composites was mainly attributed to the
chelating effect of the acac molecules and phen molecules with
the metal ion. It should be noted that an increase in the calci-
nation temperature from 500 ꢂC to 900 ꢂC resulted in a diffrac-
tion maximum position shi to higher 2q. In contrast, the Ni/
NGr@OMC-500 sample showed an apparent diffraction peak
at 2q ¼ 0.77ꢂ with a d-spacing of 11.5 nm. This diffraction peak
could be indexed to the (100) reection of the hexagonal
structure. With a further increase in the calcination tempera-
ture to 800 ꢂC, the intensity of the (100) reection shied to the
low diffraction angle of 0.83ꢂ, and the higher order weak (110)
and (200) reections were also observed. Correspondingly, the
d100-spacing was decreased from 11.5 to 10.6 nm, which corre-
sponded to the cell parameters (a0) of 13.3 nm and 12.2 nm,
respectively (Table 1). The higher d value for the Ni/NGr@OMC-
500 sample indicates a lower carbon network contraction
degree during carbonization, which can be attributed to the
presence of nickel metallic nanoparticles embedded into the
carbon matrix.
The N2 sorption analysis (Fig. 2a) of M/NGr@OMC-T showed
typical type IV isotherms with clear hysteresis loops and narrow
pore size distributions, attributed to the existence of well-
developed meso- and micropores formed by the removal of
the F127 template and burn-out of C, H, and O from the
phenolic resin framework during the pyrolysis procedure.34
Fig. 2 (a) N2 sorption isotherms and (b) BJH pore size distributions of
the nickel nanoparticle-incorporated nitrogen-doped ordered mes-
oporous carbon nanocomposites synthesized at different
temperatures.
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partially penetrate into the mesopore walls and are partially
exposed to the mesoporous channels; this was also evidenced by
the TEM observations. As listed in Table 1, the M/NGr@OMC-T
composites displayed high BET surface areas (SBET) of 269–520
m2 gꢁ1, large pore volume (Vp) of 0.34–0.40 cm3 gꢁ1, and pore
sizes (Dp) of 3.5–4.5 nm.
3.2. Characterization of metal dispersion and component
contents
To more accurately understand the composition of the nickel
nanoparticles (NPs) in the catalyst, wide angle X-ray diffraction
(XRD) investigations were carried out (Fig. 1b). Herein, two
remarkable peaks at 2q ¼ 26ꢂ and 43ꢂ corresponding to the
(002) and (1 0) diffraction peaks of graphitic carbons were
observed in the wide-angle XRD patterns of the materials Ni/
NGr@OMC-800 and Ni/NGr@OMC-900 (Fig. 2b), which indi-
cated the crystallization of the carbon frameworks in the
resulting products and incorporation of graphite carbon inside
the mesochannels. On the other hand, when the calcination
ꢂ
temperature was lower than 800 C, these peaks disappeared,
showing a signicant difference. This demonstrates that
different temperatures have a signicant inuence on the
crystalline form of graphitic carbons. It should be noted that the
much weaker peaks at 2q ¼ 26ꢂ and 43ꢂ were observed for the
sample without added phen; this indicated that the combined
pyrolysis of phen strongly promoted the formation of graphitic
carbon, which played a key role in the hydrogenation of
Fig. 3 TEM images of the nickel nanoparticle-incorporated nitrogen-
doped ordered mesoporous carbon nanocomposites synthesized at
different temperatures: (a) NGr@OMC-800, (b) Ni/NGr@OMC-500, (c)
Ni/NGr@OMC-600, (d) Ni/NGr@OMC-700, (e) Ni/NGr@OMC-800, (f)
Ni/NGr@OMC-900, (g) Ni/OMC-800, and (h) HRTEM of Ni/
NGr@OMC-800.
These results suggest that the obtained M/NGr@OMC-T
samples possess similar pore structures and features. The pore
size distribution curves (Fig. 2b) derived from the adsorption
branches using the BJH model revealed the presence of rela-
tively uniform mesopores with mean mesopore sizes ranging
from 3.8 to 4.3 nm, which slightly declined with an increase in
the calcination temperature. The calculated values of pore wall
thicknesses (WT) were in the range of 7.4–8.6 nm, less than the
sizes (15–30 nm) of Ni particles as calculated by XRD. Conse-
quently, it is reasonable to deduce that these Ni particles
Fig. 4 (a) TGA curves, (b) Ni 2 p XPS spectra, (c) N 1 s XPS spectra, and
(d) Raman spectra of the nickel nanoparticle-incorporated nitrogen-
doped ordered mesoporous carbon nanocomposites synthesized at
different temperatures.
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nitroarenes. The growth of particles within the Ni/NGr@OMC part of the nanoparticle being partially exposed to pore chan-
series was also supported by the wide angle XRD and TEM nels and the other part being tightly trapped in the carbon
results. Wide-angle XRD patterns of the materials aer being framework. In addition, the connement effect of the channels
pyrolyzed at 500–900 ꢂC showed three resolved diffraction peaks within OMC may help to limit the migration distance of metal
(Fig. 1b), which could be assigned to the (1 1 1), (2 0 0), and (2 2 elements, inhibit their aggregation, and promote the formation
0) reections of metallic nickel. This result indicates that the of Ni (Fe or Co)–NGr moieties during the heat-treatment
main form of nickel in the catalyst is metallic nickel. Further- process.
more, as estimated from the line broadening of the (1 1 1)
TGA curves obtained in air for the nitrogen-doped ordered
reection by the Scherrer equation, the sizes of the nano- mesoporous carbon/nanoparticle metal composites display
crystalline Ni particles were 11.7, 15.4, 18.4, 21.5, and 24.8 nm similar curves, except for their distinct weight losses (Fig. 4a).
for the samples Ni/NGr@OMC-500, Ni/NGr@OMC-600, Ni/ Weight loss observed below 200 ꢂC was caused by desorption of
NGr@OMC-700, Ni/NGr@OMC-800, and Ni/NGr@OMC-900, the physisorbed water. Signicant weight losses occurred in the
respectively. This is contrary to the case of the M–Nx/C mate- range of 400–600 ꢂC, independent of the content of Ni, sug-
rials prepared by the simplied pyrolysis of metal–phen or gesting carbon combustion. Slight weight-increasing
pyrolysis of metal–phen on supports such as Al2O3 or active phenomena were visible above 600 ꢂC, resulting from the
carbon reported in literature, which have a wide size distribu- oxidation of metallic Ni nanoparticles in air with the NiO
tion of particles or agglomerates in the range 20–80 nm and residue. The mass ratios of the residual NiO composites to the
even larger structures up to 800 nm.10a,b Furthermore, it is materials were determined to be 14.0, 7.9, 8.4, 10.4, 10.5, and
difficult to uniformly incorporate the precursor compounds 14.0 wt% for the samples Ni/OMC-800, Ni/NGr@OMC-500, Ni/
into the inner pores of the carbon support by this method, and NGr@OMC-600, Ni/NGr@OMC-700, Ni/NGr@OMC-800, and
active metal–nitrogen (M–Nx) moieties are usually formed Ni/NGr@OMC-900, respectively, which were consistent with the
exclusively on the external surface of the support, resulting in mass ratios of Ni to composites, calculated to be 12.2, 6.9, 7.3,
insufficient catalytic activity for hydrogenation. TEM images of 8.9, 9.2, and 11.7 wt%, respectively, based on the Ni content
all Ni/NGr@OMC nanocomposites showed stripe-like and obtained from the ICP measurement (Table 3). It should be
hexagonally arranged pore morphology over large domains, noted that the combustion temperature of carbons shied to
conrming an ordered mesostructure with a 2-D hexagonal pore higher temperatures for the Ni/NGr@OMC-500, Ni/NGr@OMC-
symmetry (Fig. 3b–g). As shown in the TEM image of the sample 600,
Ni/NGr@OMC-700,
Ni/NGr@OMC-800,
and
Ni/
Ni/NGr@OMC-500 series (Fig. 3b), uniform Ni nanoparticles, NGr@OMC-900 samples because a higher degree of graphiti-
with a mean diameter of ꢀ12 nm, were highly dispersed in the zation in the carbon walls hindered the burning of carbon.35 It
ꢂ
carbon matrix. When the temperature was increased to 800 C was noted that a similar phenomenon was found when the
for the Ni/NGr@OMC-800 sample, uniform Ni nanoparticles of sample Ni/OMC-800 was compared with the sample Ni/
about 22 nm were obtained without aggregation of large parti- NGr@OMC-800.
cles. In comparison, agglomerates in the range 20–80 nm and
even larger structures up to 200 nm were present for Ni/OMC-
800 (Fig. 3g). With the enlargement of the particle size, these
3.3. XPS characterization
Ni nanoparticles can penetrate the carbon walls and get stuck in Due to the superposition of C and N signals, no conclusive
two and even three mesopores, but are still conned in the information on the location of nitrogen in the samples could be
carbon matrix. High-resolution TEM (HRTEM) images (Fig. 3h) obtained by XRD or TEM analysis. Thus, to obtain further
clearly show that the well-crystallized Ni nanoparticles are insight into the structure of the catalyst and especially the role
embedded in the graphite carbon matrix and surrounded by of nitrogen from the organic ligand, XPS investigations on the
NGr with a homogeneous dispersion. Furthermore, TEM bonding of nitrogen and nickel were carried out (Fig. 4b and c).
images show that the Ni nanoparticles in the obtained The calcination temperature plays a key role in determining the
composites have unique semi-exposure morphology, with one type and status of the N species. In the NGr@OMC-800 sample,
without any loaded metal, two N1s peaks occurred at 397.3 eV
and 400.1 eV, which were assigned to pyridinic nitrogen and
graphite N centers, respectively.36 The former peak might be
Table 2 Surface elemental analysis of the materials calculated by XPS
a reection of N within the free ligand, whereas the latter arises
from graphitization of the ligand N atoms. On the other hand,
N/metal
Sample
C (wt%)
N (wt%)
Ni (wt%)
(mol molꢁ1
)
in Ni/NGr@OMC-500 and, to a more pronounced extent, in the
most active catalyst Ni/NGr@OMC-800, these peaks are shied
to higher binding energies (398.4 eV and 401.1 eV). The N 1 s
peak 401.1 eV, which can be assigned to nitrogen in a graphite-
like structure, gains intensity and undergoes a weak shi to
a higher binding energy aer pyrolysis at higher temperatures.
This result indicated that N was incorporated into the support
lattice. Compared to the NGr@OMC-800 sample without any
loaded metal, a new N1s peak at ꢀ399.3 eV was observed for
NGr@OMC
87.9
80.0
81.3
81.8
79.6
80.6
80.0
79.5
84.3
1.7
3.6
3.3
2.6
2.4
1.6
—
—
—
Ni/NGr@OMC-500
Ni/NGr@OMC-600
Ni/NGr@OMC-700
Ni/NGr@OMC-800
Ni/NGr@OMC-900
Ni/OMC-800
3.3
4.1
5.0
6.6
8.8
8.8
6.3
4.3
4.5
3.9
2.2
1.9
1.5
—
Co/NGr@OMC-800
Fe/NGr@OMC-800
1.8
1.7
1.2
3.1
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nitrogen species in contact with the deposited Ni atoms on top. OMC-800, the ID/IG value showed an increase from 2.5 to 2.6
It was assigned to the so-called distorted NiNx center,37 in which for Ni/NGr@OMC-800; this indicated the formation of many
the Ni–N distances were slightly different at different calcina- more structural defects on the N-doped OMC. Moreover, the
tion temperature; this gave rise to peak splitting and shiing to decrease in the intensity ratios of the D and G bands (ID/IG)
lower binding energies with an increase in the calcination suggest that the heat treatment at elevated temperatures
ꢂ
ꢂ
temperature. It is probable that similar NiNx centers are formed (from 500 C to 900 C) will increase the graphitization degree
with the progressing pyrolysis on the surface of the emerging and decrease the N defect density.
nickel particles. Furthermore, the Ni XPS spectra presented a Ni
By utilization of the similar effective complexation between
2p3/2 primary peak corresponding to Ni2+, as indicated by the acac, phen, and Co2+ or Fe3+ ions, a series of size-tunable Co-
binding energy of 855.7 eV and the presence of a satellite peak, based and Fe-based nanoparticle-incorporated nitrogen-
which strongly coordinated with the N atoms in the NGr layer, doped mesoporous carbon nanocomposites can be synthe-
and a shi to higher values with the increasing calcination sized using cobalt nitrate and iron nitrate at a constant ratio as
temperature was also observed. Loss of nitrogen was observed the metallic source and conducting calcination at 800 ꢂC in N2.
with the increasing pyrolysis temperature in both the near- The catalysts were characterized by XRD, XPS, TGA, Raman
surface region and the bulk, the surface N/Ni ratio decreased spectroscopy, BET, and TEM (Fig. S1–S8†). The results
from around 4.5 in the catalyst Ni/NGr@OMC-500 to 1.5 in Ni/ demonstrated that the catalysts had similar ordered meso-
NGr@OMC-900 (Table 2), and the bulk N/Ni ratio decreased porous structures and components; however, compared with
from around 1.9 in the catalyst Ni/NGr@OMC-500 to 0.4 in Ni/ those in the Ni/NGr@OMC-800 sample, the metallic nano-
NGr@OMC-900, (Table 3). Signicantly, in the less active cata- particles in the Co/NGr@OMC-800 and Fe-NGr@OMC-800
lyst Ni/NGr@OMC-900, the characteristic NiNx peak centred samples were slightly smaller. Specically, TEM images of
around 399.3 eV decreased in intensity, leaving behind signals the Co/NGr@OMC-800 and Fe/NGr@OMC-800 samples
with higher proportions of pyridinic (398.6 eV) and graphite conrmed that cobalt nanoparticle species and iron nano-
enclosed nitrogen (401.1 eV). These results indicated decom- particle species with a uniform size were well dispersed in the
position of the NiNx centers during pyrolysis when the calci- nitrogen-doped ordered mesoporous carbon matrix (Fig. S6†).
nation temperature was over 900 ꢂC, accompanied by partial Moreover, the TEM images showed that the cobalt or iron
sublimation and incorporation of nitrogen into the support nanoparticles were homogeneously embedded in the meso-
structure. It should be noted that no Ni–N centers are found porous carbon frameworks and partially exposed to the pore
when the catalyst is prepared in the same way but without any channels; this was similar to the case of the Ni/NGr@OMC-800
phenanthrene, in which the nanoparticles with a larger and sample. The wide-angle XRD patterns showed that the well-
uneven size distribution are formed. A comparison of the resolved diffraction peaks could be assigned to cobalt for the
characterization and catalytic results suggests that these sample Co/NGr@OMC-800 (Fig. S1b†), whereas the mixed
particular NiNx centers formed in a narrow range of pyrolysis phases of Fe, a-Fe2O3, g-Fe2O3, and Fe3C were found for the
temperatures around 800 ꢂC govern the unique catalytic activity. sample Fe/NGr@OMC-800 (Fig. S2b†), and among them, iron
This general composition is in agreement with the elemental was the dominant component.
analysis. The observed carbon layers might be generated from
the metal precursor (acac) and/or the ligand (phen), whereas
Table 4 Catalytic performance of catalysts for the chemoselective
hydrogenation of nitrobenzenea
carbonization of nitrogen-containing ligands should lead to
nitrogen-doped graphene-type structures (NGr). The Raman
spectra of the Ni-based catalysts are presented in Fig. 4d, in
which the G-band at ꢀ1590 cmꢁ1 indicates in-plane vibration of
sp2 carbon atoms, whereas the D band at ꢀ1350 cmꢁ1 is
a defect-induced Raman feature representing the imperfect
crystalline structure of carbon.37b Compared with that of Ni/
Entry Catalyst
Reducing agent Conv. (%) Sel. (%)
Table 3 Actual elemental content of the materials measured by
elemental analysis
1
2
No catalyst
Ni–phen
H2
H2
0
0
0
0
3
4
5
6
7
8
9
10
11
12
Ni–phen-resol-F127 H2
0
0
N/metal
NGr@OMC-800
H2
H2
H2
H2
H2
H2
H2
H2
H2
<1
33
32
38
45
86
62
66
61
>99
>99
>99
>99
>99
>99
>99
>99
>99
Sample
C (wt%)
N (wt%)
Ni (wt%)
(mol molꢁ1
)
Ni/OMC-800
Ni/NGr@OMC-500
Ni/NGr@OMC-600
Ni/NGr@OMC-700
Ni/NGr@OMC-800
Ni/NGr@OMC-900
Co/NGr@OMC-800
Fe/NGr@OMC-800
NGr@OMC
82.0
75.3
78.6
79.1
81.0
82.4
84.2
80.3
79.7
0.8
3.0
2.2
1.6
1.2
0.9
—
—
—
Ni/NGr@OMC-500
Ni/NGr@OMC-600
Ni/NGr@OMC-700
Ni/NGr@OMC-800
Ni/NGr@OMC-900
Ni/OMC-800
6.5
6.9
8.5
8.6
10.7
11.2
6.3
6.0
1.9
1.3
0.8
0.6
0.4
—
a
Co/NGr@OMC-800
Fe/NGr@OMC-800
1.3
1.1
0.9
0.7
Reaction conditions: catalyst (20 mg), nitrobenzene (1 mmol), water
(2 mL), 100 ꢂC, 1 h. Reducing agent: 5 MPa H2.
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The formation of metal nanoparticles with the semi-exposed
morphology incorporated with NGr in the ordered mesoporous
composites was mainly attributed to the chelating effect of the
acac and phen molecules. This feature seemed plausible to be
explained by the following aspects: in the ethanol solution of
resol, F127, and metal ions, the resol molecules with several
phenolic hydroxyl groups not only interacted with the EO
segments of F127 through the formation of hydrogen
bonds,16a,24 but also coordinated with metal ions to form metal–
phenol complexes such that the resol precursors could not
undergo simple thermopolymerization around the triblock
copolymers to generate a rigid hydrocarbon network with three-
connected benzene rings via the formation of covalent bonds.
Upon addition of acac and phen, metal ions were chelated with
acac and phen due to strong complexation, as shown in Scheme
1. The resol molecules could be smoothly assembled and ther-
mopolymerized around the surfactant micelles during solvent
evaporation without perturbation from Ni(II) ions to generate
(acac)x–M–(Phen)y/resol/P127 composites of
a long-range
ordered array structure. During pyrolysis above 500 ꢂC in N2,
the F127 template molecules were rst decomposed, leaving
highly ordered aligned mesopores, and the phenolic resin was
then carbonized into a rigid carbon framework accompanied by
the in situ growth of metal nanoparticles via slow decomposi-
tion of (acac)x–M–(Phen)y complexes; this resulted in the
formation of metal nanoparticles accompanied by the forma-
tion of nitrogen-doped graphene-incorporated mesoporous
carbon materials. Without the addition of acac and phen
molecules, the metallic ion species in the phenolic resin can
readily migrate, aggregate, and grow during both thermosetting
and pyrolysis treatments because of the low melting points of
the nitrates, leading to the formation of large particles up to
with a wide range from 10–80 nm. With the addition of acac and
phen, the nanoclusters of metal oxides formed at the early stage
of pyrolysis can be capped with the acac molecules that prevent
their fast growth; on the other hand, the in situ formed nitrogen-
doped graphene also helps to inhibit the growth and migration
of metal nanoparticles. The promoting effect of nitrogen atoms,
leading to a smaller particle size and basicity provided by NGr,
may be responsible for the higher catalytic activity.
3.4. Catalytic reaction
All the prepared materials were tested for their hydrogenation
activity towards the industrially important substrate nitroben-
zene in an aqueous solution under industrially feasible condi-
tions. Interestingly, the reaction rate of hydrogenation is
dependent on the amount of water present. When we used dry
organic solvents, for example, tetrahydrofuran (THF) or
ethanol, the reaction times had to be prolonged to achieve full
conversion. This stimulated us to perform the reaction simply
in water without any additional organic solvent present. To our
delight, the catalyst activity was even higher in pure water than
that in THF or ethanol. Parameters, such as pyrolysis temper-
ature of the catalysts, type of ligands, and type of supported
metals, were systematically investigated (Table 4). A blank
experiment with almost no detectable products could exclude
Fig. 5 Catalytic results for the selective hydrogenation of p-CNB over
(a) Ni/OMC-800, (b) Ni/NGr@OM-800, (c) Co/NGr@OM-800, and (d)
Co/NGr@OM-800. Reaction conditions: catalyst (20 mg), p-CNB (1
mmol), water (2 mL), and 100 ꢂC. Reducing agent: 5 MPa H2.
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the device itself that catalysed the reaction (Table 4, entry 1).
Neither the homogeneous nickel complexes (Table 4, entry 2)
nor the precursor gel containing incorporated Ni2+-phen
without pyrolysis were active (Table 4, entry 3). However, aer
pyrolysis, the resulting catalysts showed different activities. The
carbon carrier itself did not show any activity; this indicated
that the metal nanoparticles acted as catalytic active centers
(Table 4, entry 4). Using the benchmark substrate Ni/
NGr@OMC-800, nitrobenzene was hydrogenated to obtain
aniline in an excellent yield (86%) (Table 4, entry 9); thus, Ni/
NGr@OMC-800 exhibited almost triple the activity of the
sample Ni/OMC-800 without nitrogen-doped activated gra-
phene (Table 4, entry 5); it must be noted that the lower catalytic
activity of Ni/OMC-800 may be caused by the much larger
particle size and the absence of nitrogen-doped activated gra-
phene. These observations also appear to indicate that the
doped nitrogen atoms play a crucial role in our system. The
versatile nitrogen can provide more surface nucleation sites and
promote the formation of some individual sections around the
N-rich sites,38 allowing efficient anchoring of metal NPs.39 The
pyrolysis temperature also plays a critical role in the activity. It
is interesting to note that the Ni/NGr@OMC-800 catalyst with
a lower N content shows better activity than the sample with
pyrolysis temperature in the range from 500 to 700 ꢂC (Table 4,
entry 6–8); this better activity is probably due to the higher
degree of crystallinity of graphite carbon in Ni/NGr@OMC-800,
which also has a higher graphite N content, and not caused by
the defect sites on the surface of catalysts since the sample with
Table 5 Selective hydrogenation of various substituted nitroarenes to
the corresponding anilines over Ni/NGr@OMC-800
Entry Substrate
Product
Time (h) Conv. (%) Sel. (%)
1
2
2
>99
>99
>99
>99
2.5
3
4
5
6
2
>99
>99
>99
>99
>99
>99
>99
>99
2
2.5
2
7
4.5
6
>99
>99
>99
>99
>99
>99
>99
>99
8
9
5
ꢂ
10
6
a lower pyrolysis temperature in the range from 500 to 700 C
with a higher content of defect sites (Fig. 4b) is not the best
catalyst. These results indicate that graphite N in NGr of the
catalysts is critical for impelling the reaction rather than the
amount of N atoms in nitrogen-doped graphene-activated
catalysts, in accordance with the XRD and XPS results. Upon
further increasing the pyrolysis temperature to 900 ꢂC, the
activity of the resulting catalyst decreased (Table 4, entry 10);
this might have been caused by metal particle aggregation to
provide less active sites. Obviously, it is generally accepted that
the dispersion and the mean size of NPs may greatly affect the
catalytic activity.40 However, the yield of aniline was still higher
as compared to that obtained in the corresponding reaction
using Ni/OMC-800 or the sample with a pyrolysis temperature in
the range from 500 to 700 ꢂC as catalysts. These results strongly
demonstrate that NGr in the catalysts is indispensable for the
hydrogenation of nitroarenes. In comparison, the Co/
NGr@OMC-800 and Fe/NGr@OMC-800 catalysts showed
slightly lower activity than Ni/NGr@OMC-800 (Table 4, entry 11
and 12).
It should be noted that the chemoselective hydrogenation of
substituted nitroarenes with halogen groups is highly desired.
For example, the unwanted C–Cl bond hydrogenolysis is diffi-
cult to fully avoid. Herein, substituted nitrobenzenes with –Cl
(an electron-withdrawing substituent) at the para position were
chosen as the reactants. The kinetic prole of the reaction
revealed that 4-chloronitrobenzene (p-CNB) was smoothly con-
verted into 4-chloroaniline without a signicant formation of
dangerous intermediates, e.g., hydroxylamine or nitroso
11
12
7
7
>99
>99
>99
>99
13
14
15
9
2
3
>99
>99
>99
>99
>99
>99
16
17
5
4
>99
>99
>99
98.6
18
19
10
11
>99
>99
>99
>99
20
12
>99
>99
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Aer the successful hydrogenation of the benchmark reac-
tion, we extended Ni/NGr@OMC-800 to various substituted
nitroarenes to demonstrate the generality of this reaction under
the optimized conditions, and the results are summarized in
Table 5. It can be seen that the chemoselective reduction of all
the functionalized nitroarenes can be completed at 100 ꢂC in an
aqueous solution with perfect chemoselectivity (>99%). The
substituted nitrobenzenes, having electron-donor or electron-
acceptor groups, such as –CH3, –CF3, CH3O–, –OH, –NH2,
–NO2, and so on (Table 5, entries 1–15), were also furnished
with excellent yields. The Ni/NGr@OMC-800 catalyst also had
a high activity towards catalyzing the reduction of esters,
alkynes, and nitriles with hydrogen. This feature is advanta-
geous because it can realize the selective reduction of the nitro-
group in the presence of other reducible functionalities (Table
5, entries 16 and 17). Notably, the catalyst could also promote
heterocyclic nitroaromatic reduction in yields of >99% without
being deactivated by N (Table 5, entries 18 and 19). For example,
8-aminoquinolines, which are important nitrogen-containing
heterocycles widely used in bio-active molecules and agro-
chemicals and have been developed as signicant bidentate
directing groups or as auxiliary ligands in various kinds of
organic reactions, especially in the area of C–H bond func-
tionalization,43 can be efficiently and selectively reduced to the
corresponding aniline product in an aqueous solution, where
green production is critical for the requirements of product
Fig. 6 A possible reaction mechanism for the hydrogenation of
nitroarenes catalyzed by the M/NGr@OMC-T catalyst.
compounds were undetectable (Fig. 5); this was because these
steps for the conversion of the two intermediates were relatively
fast. The p-CNB conversion via hydrogenation with H2 in the
absence of a catalyst was below the detection limit. Compared
with the Ni/OMC-800 catalyst (Fig. 5a), the Ni/NGr@OMC-800
catalyst (Fig. 5b) modied with nitrogen-doped graphene dis-
played a higher conversion activity as well as higher selectivity
for the hydrogenation of p-CNB to generate the corresponding
amine product. Nickel and platinum have been proven to have
poor selectivity for the hydrogenation of halogenated nitro
compounds. Dehalogenation occurs for both the Ni/OMC-800
catalyst and Ni-NGr@OMC-800 catalyst, but the selectivity of
the Ni/OMC-800 catalyst (88.2%) is much lower than that of the
Ni-NGr@OMC-800 catalyst (95.6%). To our great delight, when
the Co-NGr@OMC-800 catalyst (Fig. 5c) and Fe/NGr@OMC-800
catalyst (Fig. 5d) were used for the hydrogenation of p-CNB, the
unwanted by products in p-CNB hydrogenation, which included
aniline (produced by dehalogenation), N-ethyl chloroaniline,
bis-dichlorophenyldiazene, and dichloroazoxybenzene (caused
by coupling reactions), were undetectable in the process of the
reaction or aer the reaction.41 During the reaction, the –Cl
bond remained intact, thereby avoiding the undesired hydro-
dechlorination. The Ni/NGr@OMC-800 catalyst exhibited
higher conversion as well as higher selectivity towards chlor-
onitrobenzene reduction as compared to the Ni/OMC-800
catalyst without modication by the NGr species, avoiding C–
Cl bond scissions even at a total conversion, and no further
decrease in the yield of chloroaniline products was observed,
even upon extending the reaction duration aer the complete
conversion of chloronitrobenzene. The electron-enriched
chemical state of nitrogen-doped graphene species, which was
proved by results that the XPS position of the Ni species shied
to higher binding energies by increasing the calcination
temperature, was caused by the synergistic effect with transition
metal species and nitrogen-doped graphene species, which
inhibited dechlorination. However, chemisorption of hydrogen
on N-doped graphene inhibited the active hydrogen species
from attacking the C–X bond or produced specic absorption
toward C–X groups, which therefore inhibited the dehalogena-
tion of halonitrobenzene. In this process, the dihydrogen
molecule is formally cleaved into a hydride and a proton. The
hydride atom is bound to the metal-based nanoparticle, Fig. 7 (a) Reuse of Ni-OMC-800, 1 h up to the 7th cycle; (b) reuse of
Ni-NGr-OMC-800, 1 h up to the 7th cycle. Reaction conditions:
whereas the proton is attached to the basic N atoms in close
proximity to the nanoparticle in the liquid phase.32,42
catalyst (20 mg), nitrobenzene (1 mmol), water (2 mL), 100 ꢂC, and
5 MPa H2.
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quality. A possible reaction mechanism for the hydrogenation
of nitroarenes catalyzed by the M/NGr@OMC-T catalyst is
proposed in Fig. 6. The nitro group is rst reduced to the nitroso
group, followed by reduction by two H* to form the hydroxyl-
amine intermediate; then, hydroxylamine is further reduced to
the amino group. During the process, NGr plays a positive role
in improving the catalytic activity; this is ascribed to metal
nanoparticles being activated by the surrounded NGr as the
absorption of reactants on the surface of NGr is a critical step
for catalyzing the reactions. This reaction mechanism is anal-
ogous to that reported in literature.6b,9b,44
Finally, the recyclability of catalysts was explored (Fig. 7). In
the case of the Ni/OMC-800 catalyst (Fig. 7a), a slight decrease in
the conversion was detected in the rst to fourth runs, whereas
for the Ni-NGr@OMC-800 catalyst (Fig. 7b), aer an initial slight
decrease in the rst run, the activity remained almost constant.
ICP analysis of the liquid phase aer each run revealed no metal
leaching for both catalysts (detection limit < 1 ppm), which,
coupled with ICP analyses, clearly demonstrated that the cata-
lysts maintained their original composition even aer recycling.
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Herein, we have demonstrated a facile chelate-assisted multi-
component co-assembly for the synthesis of ordered meso-
porous carbons with embedded and well-dispersed transition
metal nanoparticles modied with nitrogen-doped graphene as
novel catalysts for the reduction of various functionalized
nitroarenes to the corresponding aniline in an aqueous solu-
tion. The recycling tests and characterizations demonstrate that
the catalysts are stable and can be reused for the hydrogenation
of nitroarenes. The excellent catalytic properties of the catalysts
can be attributed to the remarkably synergistic effect between
the metal nanoparticles and nitrogen-doped graphene. This
study can stimulate signicant interest in the development of
novel non-noble catalysts for other important hydrogenation
reactions.
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Conflicts of interest
¨
10 (a) F. A. Westerhaus, R. V. Jagadeesh, G. Wienhofer,
M. M. Pohl, J. Radnik, A. E. Surkus, J. Rabeah, K. Junge,
H. Junge, M. Nielsen, A. Bruckner and M. Beller, Nat.
There are no conicts to declare.
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Chem., 2013, 5, 537–543; (b) F. Chen, A. E. Surkus, L. He,
M. M. Pohl, J. Radnik, C. Topf, K. Junge and M. Beller, J.
Am. Chem. Soc., 2015, 137, 11718–11724.
Acknowledgements
This research was supported by the Open Project of State Key
Laboratory of Advanced Special Steel of Shanghai University
(SKLASS2015-Z052), the National Basic Research Program of
China (973 Program, No. 2014CB643403), the National Natural
Science Foundation of China (No. 51574164), and the Basic
Major Research Program of Science and Technology Commis-
sion Foundation of Shanghai (No. 14JC1491400).
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