Nitrogen-doped carbon supported iron oxide as efficient catalysts for chemoselective hydrogenation of nitroarenes

Article abstract of DOI:10.1039/c6ra18935k

Chemoselective hydrogenation has been widely used in the production of fine chemicals, and developing heterogeneous catalysts with high activity and chemoselectivity is always a challenging topic. Herein, we report a new type of catalysts synthesized from biomass-derived chitosan and non-noble iron, which is denoted as nitrogen-doped carbon supported iron (Fe/N-C). TEM and XRD characterization indicate the presence of iron species. Interestingly, the Fe/N-C catalysts exhibited excellent catalytic performances in the hydrogenation of nitroarenes, and excellent yields of target aniline products could be obtained under industrially viable conditions.

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Nitrogen-doped Carbon Supported Iron Oxide as Efficient
Catalysts for Chemoselective Hydrogenation of Nitroarenes
Shaodan Xu,*a,b Deqing Yu,a Shangfu Liao,a Tao Ye,a,c and Huadong Sheng*a
Received 00th January 20xx,
Accepted 00th January 20xx
The chemoselective hydrogenation has been widely used in the production of fine chemicals, and developing
heterogeneous catalysts with high activity and chemoselectivity is always a challengeable topic. Herein, we reported a new
type of catalysts synthesized from biomass-derived chitosan and non-noble iron, which is denoted as nitrogen-doped
carbon supported iron (Fe/N-C). TEM and XRD characterization indicate the presence of iron species. Interestingly, the
Fe/N-C catalysts exhibited excellent catalytic performances in the hydrogenation of nitroarenes, and excellent yields of
DOI: 10.1039/x0xx00000x
www.rsc.org/
target
aniline
products
could
be
obtained
under
industrially
viable
conditions.
production of agrochemicals and pharmaceuticals.31-35 Such
process has great limitation for the catalysts especially more
than one reducible group are present in the nitroarene
molecule, which easily leads to the hydrogenation of these
substituted groups (C=O, C-Cl and so on), forming undesired
products.36 In order to solve this problem, many efforts have
been focused on enhancing the chemoselectivity of noble
metal catalysts. More recently, Beller and co-workers firstly
found that the combination of nitrogen-doped carbon with Fe
or Co oxides could produce a new type catalyst which could
solely catalyze the hydrogenation of nitro-groups.33,37
However, it should be noted that the preparation of these
catalysts need expensive 1,10-phenanthroline (phen) ligand as
carbon precursor and high pyrolysis temperature at as high as
Introduction
Chemoselective hydrogenation of easily available molecules
into desired products using molecular hydrogen is always an
important topic in the field of heterogeneous catalysis.1-7
Many Pd, Au, Rh, Ir and Pt metals have been regarded as
highly efficient catalysts for the hydrogenation, and already
used in industrial scales.8-14 Notably, the high price and limited
availability of these metals make these applications high-cost,
therefore, much interest has been attracted in developing
earth-abundant alternative metal catalysts. Many earth-
abundant metal salts coupled with soluble organic ligands
have exhibited high activity in the hydrogenation reaction
which used to be performed over noble metals previously.15-19
However, the homogeneous feature makes these catalysts
difficult in separation and regeneration from the reaction
system, which strongly limited their wide applications in the
future.20-24 Recently, the Fe, Cu, Ni and Co-based
heterogeneous catalysts have been found to be active for the
hydrogenations, which have good feature in catalytic stability
and recyclability, but their catalytic activity and
chemoselectivity could not satisfy the application of these
catalysts in chemoselective hydrogenations.25-30 Therefore,
developing efficient heterogeneous catalysts from earth-
abundant metals is challengeable for chemoselective
hydrogenations.
800
°C. It is also found that other cheap ligands and low-
temperature (400-600
°
C) calcination leads to catalysts with
poor activity.27 Furthermore, the no-porosity and bulky metal
oxides leads to relatively low frequency of the active sites in
the hydrogenation.
In this work, we report a new type of nitrogen-doped
mesoporous carbon supported iron catalysts (Fe/N-C-x, x
denotes the calcination temperature). It is interesting to found
that the Fe/N-C sample obtained at 500 °C (Fe/N-C-500) gives
the highest activity and chemoselectivity in the hydrogenation
of nitrobenzene. Importantly, the Fe/N-C-500 catalyst could
catalyze the hydrogenation of
a series of substituted
nitroarenes to the corresponding anilines in good activities and
chemoselectivities. More importantly, the Fe/N-C-500 catalyst
is stable and can be easily reused for several recycles without
any lost in activity and selectivity. The features of easy
available, high activity, excellent chemoselectivity, and good
recyclability make the Fe/N-C-500 catalyst potentially useful
for the applications in the chemoselective hydrogenation of
substituted nitroarenes in the future.
One great discovery in catalytic hydrogenations using H2
is the chemoselective reduction of nitroarenes into anilines,
which are industrially valuable intermediates for the
a.Zhejiang Institute of Quality Inspection Science, Hangzhou 310018, China
E-mail: shaodanxu@zju.edu.cn; shd761002@163.com
b.Department of Chemistry, Zhejiang University, Hangzhou 310027, China
c. MOE Key Laboratory of Macromolecular Synthesis and Functionalization,
Department of Polymer Science and Engineering, Zhejiang University, Hangzhou
310027, China
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Experimental
Results and discussion
Sample preparation
The Fe/N-C catalysts were directly obtained from the
hydrothermal treatment of biomass-derived chitosan as
nitrogen/carbon precursor and iron(III) acetylacetonate as iron
precursor in the absence of any surfactants or templates,
followed by calcination (Scheme 1).
All commercial chemicals are directly used without further
purification.
Synthesis of Fe/N-C-x
As a typical run, 1g of chitosan was dissolved in 22 ml of water
and 0.5 g of acetic acid, then 0.7 g of borax, iron(III)
acetylacetonate (~ 50 mg of Fe), and 5 ml of water were added.
After stirring at room temperature for over night, the
homogeneous gel was transferred into autoclave and heated
OH
O
N
N
OH
O
OH
O
Hydrothermal
treatment
O
HO
HO
O
OH
HO
HO
N
N
NH2
NH2
NH2
N
n
N
N
High-temperature
Pyrolysis in N2
O
O-
Fe3+
Fe species
at 200 °C for 12 h. Then the obtained solid was filtrated,
washed, and calcined at different temperatures in N2 flow for
4 h to obtain the final Fe/N-C-x catalyst.
Scheme 1. Synthesis of Fe/N-C-x samples.
Sample characterization
Table 1. Structural parameters of various samples.
FEI Tecnai G2 F20 microscope equipped with an EDAX detector
was employed to acquire TEM images. The microscope was
operated at an accelerating voltage of 200 kV. The samples
were ultrasonically dispersed in ethanol, and then a drop of
the solution was deposited on a holey C/Cu TEM grid to be
used for Transmission Electron Microscopy (TEM)
characterization and elemental mapping. Rigaku powder X-ray
diffractometer using Cu Kα (λ = 1.5418 Å) radiation. X-ray
photoelectron spectra (XPS) were performed by Thermo
ESCALAB 250, and the binding energy was calibrated by C1s
peak (284.9 eV). Nitrogen isotherms at the temperature of
liquid nitrogen were measured using a Micromeritics ASAP
Tristar. The samples were outgassed for 10 h at 150 ºC before
the measurement. Pore-size distribution was calculated using
Barrett-Joyner-Halenda (BJH) model. The content of Fe was
determined from inductively coupled plasma (ICP) with a
Perkin-Elmer plasma 40 emission spectrometer. Scanning
electron micrograph (SEM) images were performed on Hitachi
SU-1510 electron microscopes.
Sample
SBET
Fe
C/N
molar ratio
8.1
Fe
formc
Ps(nm)d
2.8-6.8
3.0-7.0
(cm2/g)a (wt%)b
Fe/N-C-400
Fe/N-C-500
287
309
6.4
6.7
Fe2O3
Fe3O4,
FeNx
8.3
9.7
Fe3O4,
FeCx
Fe/N-C-600
320
6.5
3.0-9.0
Fe3O4,
FeCx
Fe/N-C-800
270
6.9
16.4
4.0-9.4
a BET Surface area, by N2 sorption; b Fe Loading, by ICP analysis; c By
XRD patterns; d By counting ~80 nanoparticles in TEM images.
Figure 1A shows the scanning electron microscopy (SEM)
images of the Fe/N-C-400, Fe/N-C-500, Fe/N-C-600, and Fe/N-
C-800 samples. All samples exhibit an angular morphology, and
the surface areas were established at 270-320 cm2/g (Table 1),
indicating the rich porosity. Figure 1e shows the scanning
transmission electron microscopy (STEM) image of Fe/N-C-500,
the Fe species and carbon support could be easily
distinguished from each other by the image contrast because
of the different atomic number of C and Fe. In the STEM
image, the Fe species with bright contrast are highly dispersed
on the carbon support with relatively dark contrast.38
Furthermore, the Fe elemental EDX mapping analysis give the
observation of the distribution of Fe element (Figure 1g),
which is in good agreement with the distribution of Fe species
in the STEM image (Figure 1f). By analyzing about 80
nanoparticles, more than 90% of the nanoparticles gave a
small size distribution at 3-7 nm (Figure 1h). These results
indicate the successful synthesis of nanoporous carbon
supported small-sized FeOx particles.
Catalytic tests
The hydrogenation reactions were performed in a 60-ml high
pressure autoclave with
a magnetic stirrer (1200 rpm).
Typically, the catalysts, substrates, and solvent were mixed in
the reactor. Then, the reaction system was filled with H2 and
heated to a desired temperature. The temperature was
measured with a thermometer in an oil bath. After reaction,
the product was taken out from the reactor and analyzed by
gas chromatography (GC-2014C, Shimadzu, using a flame
ionization detector) with a flexible quartz capillary column
coated with FFAP. The dodecane or decane was used as
internal standard in the GC analysis. The recyclability of the
catalyst was tested by separating from the reaction system by
centrifugation, washing with a large quantity of methanol and
water, and drying at 80 oC for 6 h.
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Figure 1. SEM images of (a) Fe/N-C-400, (b) Fe/N-C-500, (c)
Fe/N-C-600, and (d) Fe/N-C-700; (e and f) STEM images of
Fe/N-C-500, (g) the corresponding Fe elemental EDX mapping,
and (h) FeOx particle size distribution.
Figure 2. (A) Raman spectra, (B) XRD patterns of (a) Fe/N-C-400,
(b) Fe/N-C-500, (c) Fe/N-C-600, and (d) Fe/N-C-800 samples
Figure 2A shows the Raman spectra of Fe/N-C-400, Fe/N- (Square: Fe2O3; Heart-shape: Fe3O4; Pentagon: FeNx; Cycle:
C-500, Fe/N-C-600, and Fe/N-C-800 samples, which have G FeCx) and (C) N1s XPS spectra of Fe/N-C-500.
band at about 1589 cm-1, indicating the in-plane vibration of
sp2-derived aromatic carbons. The D band at about 1353 cm-1
presents the sp2-derived carbons in the unperfected aromatic
The study on the catalytic performances starts from
structure. Notably, the signal of D band is very strong, which is employing the hydrogenation of nitrobenzene as a model
only slightly weaker than the G band.38 In this work, this reaction. Table 2 shows the catalytic reaction in different
phenomenon should be due to the presence of rich nitrogen solvents of ethyl acetate, toluene, ethanol, and mixed THF-
species on the carbon structure, where the C/N atomic molar water, which were reported as favorable solvents for the
ratio could be estimated by elemental analysis. As shown in nitroarene hydrogenation reactions in various reaction
Table 1, the C/N molar ratios of Fe/N-C-400, Fe/N-C-500, and systems. Interestingly, the Fe/N-C-500 catalyst exhibit full
Fe/N-C-600 is lower (8.1-9.7) than that of Fe/N-C-800 (16.4), conversion of nitrobenzene to aniline as sole product in ethyl
which is reasonably attributed to the leaching of N atoms acetate. In contrast, under the same reaction conditions, the
during high-temperature pyrolysis at 800 °C. Figure 2B shows nitrobenzene conversions could only reach 95.0, 58.0, and
the XRD patterns of Fe/N-C-400, Fe/N-C-500, Fe/N-C-600, and 72.1% in toluene, ethanol, and mixed THF-water solvent,
Fe/N-C-800 samples. Notably, the peaks are wide and weak in respectively. These results indicate that ethyl acetate was a
intensity, associating with the small size of Fe species, which is suitable solvent for obtaining high catalytic activity.
in good agreement with the results of STEM characterization. Additionally, the other features including low price, low boiling
In the XRD pattern of Fe/N-C-400, typical peaks associated point, and easy separation with aniline also suggest the great
with Fe2O3 are presented. For Fe/N-C-500, the Fe species are potentiality of ethyl acetate for wide application.
mainly presented as Fe3O4. Notably, a peak at 42.4° is also
observed, associated with FeNx species, which might be due to Table 2. Hydrogenation of nitrobenzene to aniline in different
the Fe-N interaction on the surface of Fe3O4 nanoparticles. solvents.a
Furthermore, higher pyrolysis temperature of 600 and 800 °C
could produce FeCx species. Figure 2C gives the N1s XPS
spectra of Fe/N-C-500, which exhibited two peaks at 398.3 and
400.2 eV, assigning to pyridinic nitrogen and FeNx species,
indicating the presence of interaction between Fe and N,
which is in agreement with the observations of FeNx species in
the XRD pattern.
Entry
Solvent
ethyl acetate
toluene
Ph-NO2 Conv. (%)
>99.0
Ph-NH2 Sel. (%)
>99.0
1
2
95.0
>99.0
3
4
ethanol
58.0
72.1
>99.0
>99.0
THF-water b
a Reaction conditions: 0.5 mmol of nitrobenzene, 50 mg of Fe/N-C-500, 4
ml of solvent, 120
°
C, 4 MPa of H2, 15 h; b 2 ml of THF and 2 ml of water.
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Table 3. Hydrogenation of nitrobenzene to aniline over Fe/N-C
catalysts obtained at different pyrolysis temperature.a
°C) to form FeOx species with Fe-N interaction on the surface.
In contrast, the biomass-derived chitosan has rich oxygen and
nitrogen content, which could easily form FeOx with Fe-N
interaction at relatively low pyrolysis temperature (500 °C).
Pyrolysis
Entry
Ph-NO2 Conv. (%)
Ph-NH2 Sel. (%)
>99.0
Furthermore, the rich porosity of Fe/N-C-500 is favorable for
the high dispersion of the metal nanoparticles and access with
the reactants, thus leading to excellent catalytic performances.
Table 4 gives the recycle data of Fe/N-C-500 catalyst, we can
easily draw the conclusion that the Fe/N-C-500 catalyst is
reusable. After each run, it can be easily recycled by filtration
with negligible Fe leaching as confirmed by ICP analysis.
Consequently, it exhibited constant catalytic performance
during continuous recycles. When used for the hydrogenation
of nitrobenzene, for example, it gives stable conversion of
nitrobenzene (~97.5%) and aniline selectivity (>99.0%) in five
reaction runs (Table 4). These results demonstrate that the
Fe/N-C-500 catalyst has good recyclability, which is a very
important parameter for a heterogeneous catalyst.
temperature (
°C)
1
2
3
4
400
88.0
>99.0
>99.0
72.0
500
600
800
>99.0
92.0
95.1
a
Reaction conditions: 0.5 mmol of nitrobenzene, 50 mg of Fe/N-C
catalyst, 4 ml of ethyl acetate solvent, 120 °C, 4 MPa of H2, 15 h.
Table 4. Recycle data of Fe/N-C-500 catalyst in the
hydrogenation of nitrobenzene to aniline.a
Table 5. Hydrogenation of various substituted nitroarenes to
the corresponding anilines over Fe/N-C-500 catalyst.a
Runs
Ph-NO2 Conv. (%)
>99.0
Ph-NH2 Sel. (%)
>99.0
Substrate
Product
Time (h)
15
Conv. (%)
>99.0
Sel. (%)
97.5
1
2
3
>99.0
>99.0
1
2
98.1
>99.0
18
>99.0
>99.0
4
5
98.5
97.3
>99.0
>99.0
3
15
21
98.5
>99.0
97.0
a
Reaction conditions: 0.5 mmol of nitrobenzene, 50 mg of Fe/N-C-
500 catalyst, 4 ml of ethyl acetate solvent, 120 °C, 4 MPa of H2, 15 h.
4 b
>99.0
Table 3 shows the catalytic data of various Fe/N-C
catalysts obtained from different pyrolysis temperature. The
Fe/N-C catalysts with different type of iron species are all
active for the reaction. Notably, the Fe/N-C-500 and Fe/N-C-
600 catalysts exhibited higher nitrobenzene conversion
(>99.0%) than Fe/N-C-400 and Fe/N-C-800 catalysts (72.0-
5
6
14
12
>99.0
>99.0
>99.0
>99.0
88.0%).
Additionally,
Fe/N-C-500
exhibited
higher
chemoselectivity to aniline (>99.0%) than Fe/N-C-600. The
results indicate the good catalytic performance of the catalyst
7
19.5
>99.0
>99.0
calcined at 500 °C. This phenomenon is reasonably related to
the formation of FeNx species on Fe/N-C-500 observing from
the XRD patterns, which is reported as efficient catalytically
active and selective sites for the hydrogenation reactions.
Interestingly, the catalytic performance of Fe/N-C-500 catalyst
is even more excellent than conventional nitrogen-doped
a Reaction conditions: 0.5 mmol of nitrobenzene, 50 mg of Fe/N-C-500 catalyst,
4 ml of ethyl acetate solvent, 120 °C, 4 MPa of H2, 15 h. b 75 mg of catalyst.
carbon supported FeOx catalyst obtained at 500 °C, and
comparable with that obtained at 800 C. This phenomenon
°
A series of substituted nitroarenes as substrates have also
might be attributed to the employment of 1, 10- been investigated. From table 5, we can know that the Fe/N-C-
phenanthroline as carbon precursor in the conventional 500 could efficiently catalyze the conversion of substituted
catalyst, which required high pyrolysis temperature (e.g. 800 nitroarenes to the corresponding substituted aniline products.
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Nitrogen-doped carbon supported iron oxide as efficient catalysts for chemoselective
hydrogenation of nitroarenes.
Conv. >99.0%, Sel. >97.0%