Iron oxide modified N-doped porous carbon derived from porous organic polymers as a highly-efficient catalyst for reduction of nitroarenes

Article abstract of DOI:10.1016/j.mcat.2020.111249

Fabrication of cost-effective non-noble metal-based catalysts is significant for heterogeneous catalysis. Here, we prepared a porous organic polymer (POP) through the facile condensation of p-phenylenediamine with ferrocene carboxaldehyde, and then the ferrocene-functionalized POP material was carbonized under inert atmosphere, obtaining the γ-Fe₂O₃ nanoparticles (NPs) modified N-doped porous carbon catalyst (γ-Fe₂O₃/NPC). Various characterizations (such as XRD, BET, TEM and XPS etc.) indicated that the obtained γ-Fe₂O₃/NPC catalyst exhibits unique structural properties with uniformly dispersed γ-Fe₂O₃ species, porous morphology, and N-doped defective amorphous carbon. The γ-Fe₂O₃/NPC catalyst shows excellent activity in the catalytic reduction of nitroarenes with hydrazine hydrate as reductant. The synergistic effect between γ-Fe₂O₃ NPs and N-doped porous carbon improve the hydrazine hydrate adsorption and activation for active hydrogen atoms production, which is beneficial for nitroarenes reduction. Moreover, the γ-Fe₂O₃/NPC catalyst could be easily recycled by using a magnet and reused without obviously loss of catalytic activity. Therefore, this work should provide a useful platform for designing and fabricating stable non-noble metal NPs based catalysts derived from POP materials that have potential for various catalytic applications.

Full text of DOI:10.1016/j.mcat.2020.111249

Molecular Catalysis 498 (2020) 111249
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.journals.elsevier.com/molecular-catalysis
Iron oxide modified N-doped porous carbon derived from porous organic
polymers as a highly-efficient catalyst for reduction of nitroarenes
,
,
,
Jing Lva 1, Zhengtang Liub 1,*, Zhengping Donga *
a Laboratory of Special Function Materials and Structure Design of the Ministry of Education, College of Chemistry and Chemical Engineering, Lanzhou University,
Lanzhou 730000, China
b College of Chemistry and Environment Engineering, Baise University, Baise 533000, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Fabrication of cost-effective non-noble metal-based catalysts is significant for heterogeneous catalysis. Here, we
prepared a porous organic polymer (POP) through the facile condensation of p-phenylenediamine with ferrocene
carboxaldehyde, and then the ferrocene-functionalized POP material was carbonized under inert atmosphere,
obtaining the γ-Fe2O3 nanoparticles (NPs) modified N-doped porous carbon catalyst (γ-Fe2O3/NPC). Various
characterizations (such as XRD, BET, TEM and XPS etc.) indicated that the obtained γ-Fe2O3/NPC catalyst ex-
hibits unique structural properties with uniformly dispersed γ-Fe2O3 species, porous morphology, and N-doped
defective amorphous carbon. The γ-Fe2O3/NPC catalyst shows excellent activity in the catalytic reduction of
nitroarenes with hydrazine hydrate as reductant. The synergistic effect between γ-Fe2O3 NPs and N-doped porous
carbon improve the hydrazine hydrate adsorption and activation for active hydrogen atoms production, which is
beneficial for nitroarenes reduction. Moreover, the γ-Fe2O3/NPC catalyst could be easily recycled by using a
magnet and reused without obviously loss of catalytic activity. Therefore, this work should provide a useful
platform for designing and fabricating stable non-noble metal NPs based catalysts derived from POP materials
that have potential for various catalytic applications.
N-doped porous carbon
Porous organic polymers
γ-Fe2O3/NPC catalyst
Reduction
Nitroarenes
Introduction
with high chemoselectivity is significant for nitroarenes reduction.
Non-noble metals, such as Co, Ni, Cu, and Fe-based heterogeneous
The catalytic reduction of nitroarenes is a remarkable valuable re-
action as the resulting anilines are important organic intermediates in
the industrial production of dyes, agrochemicals, pigments and phar-
maceuticals [1–3]. Traditionally, nitroarenes was reduced to anilines
under acidic reaction conditions using Fe and Zn as reducing agents; this
always produce a lot of pollutants, including waste acids and salt resi-
dues [4]. To make the nitroarenes reduction reaction process
environmentally-friendly and economical, noble metal-based heteroge-
neous catalysts including Pd, Pt, Au, Ru, and Rh nanoparticles (NPs)
based-catalysts were fabricated and used [5–9]. And some green
reducing agents, such as H2, sodium borohydride, and formic acid were
used for nitroarenes reduction [10–17]. Nonetheless, the high price and
complicated recovery procedure of noble metal-based catalysts often
limit their large-scale application, and the chemoselective is always very
poor when reducible groups (e.g. alkene, carbonyl, halides and nitrile)
are exist in nitroarenes. Therefore, fabrication of cost-effective catalysts
catalysts have attracted extensive attention for catalytic reduction of
nitroarenes, due to their abundant earth storage, economic price and
low toxicity [18–21]. However, the reaction conditions of these catalysts
catalyzed reaction usually very strict, including high temperature
(≥100 ◦C), high H2 pressure (≥1 MPa) and need additives (acid, base
and metal salts) [22,23]. It is worth mentioning that, the catalytic effi-
ciency of the above mentioned non-noble metal-based catalysts strongly
depended on the catalyst fabrication procedure, properties of supports
and metal active sites, nature of hydrogen sources, etc. [24]. For
example, in the wildly investigated N-doped porous carbon materials,
the electronic structure and atomic radius of nitrogen are different from
carbon atoms, which subsequently cause structural defects of the carbon
framework, endowing the supporting material with unique electronic
configuration [25–27]. Thus, the N atoms and the metal active sites
always show synergistic effects for enhancing the catalytic activity and
selectivity [28]. One of the useful strategies for the fabrication of
* Corresponding authors.
E-mail addresses: 672897488@qq.com (Z. Liu), dongzhp@lzu.edu.cn (Z. Dong).
1
Jing Lv and Zhengtang Liu equally contributed to this work.
https://doi.org/10.1016/j.mcat.2020.111249
Received 29 July 2020; Received in revised form 28 September 2020; Accepted 3 October 2020
Available online 21 October 2020
2468-8231/© 2020 Elsevier B.V. All rights reserved.

J. Lv et al.
Molecular Catalysis 498 (2020) 111249
Scheme 1. Synthetic route for the preparation of γ-Fe2O3/NPC-X (X represents the pyrolysis temperature).
heteroatoms (N, S and P)-doped porous carbon material is the carbon-
ization of the heteroatoms (N, S and P)-doped precursor materials
[29–33]. Especially the porous organic polymers (POPs) materials with
intrinsic heteroatoms have excellent skeletons and porous structures,
which can be transformed into porous carbon materials with hetero-
atoms well distributed in the porous carbon frameworks. However,
being different from the one-step pyrolysis of MOFs to prepare metal NPs
based porous carbon catalysts, POPs derived heteroatoms-doped porous
carbon materials always need another step for modification of metal NPs
active sites to further fabricate heterogeneous catalysts, facile one-pot
method is rare.
dimethylformamide (DMF) and Tetrahydrofuran (THF) were used as
received.
Preparation of γ-Fe2O3/NPC catalyst
The detailed synthetic procedure for the iron-containing POP mate-
rial is described as follows: first, Ferrocene carboxaldehyde (1.226 g,
6 mmol), p-phenylenediamine (0.649 g, 6 mmol) and DMSO (30 mL)
were added into a 100 mL single-necked round bottom flask that
equipped with stirrer and condenser, then immediately degassed by
argon. Under the protection of argon, the condensation reaction was
hold in a 180 ℃ oil bath for 12 h. Finally, the resulted black gel was
filtrated and washed successively with methanol, DMF and THF, dried at
70 ℃ for 6 h and ground into a powder. These powders are carbonized at
different temperatures for 2 h with a heating rate of 5 ℃ minꢀ 1 under
the protection of N2 to obtain target catalyst denoted as γ-Fe2O3/NPC-X
(X represents the carbonization temperatures).
In this work, we use a facile one-pot condensation reaction for
preparation of an iron-containing POP material using p-phenylenedi-
amine and ferrocene carboxaldehyde as the precursors, obtaining the
ferrocene-functionalized POP. Then the as-prepared POP was pyrolyzed
under argon atmosphere, and transformed to a N-doped porous carbon
(NPC) material with γ-Fe2O3 NPs highly dispersed in the carbon
framework. The obtained γ-Fe2O3/NPC catalyst shows excellent cata-
lytic activity in the reduction of functionalized nitroarenes using hy-
drazine hydrate (N2H4⋅H2O) as reducing agent, taking advantage of the
synergistic effect between the γ-Fe2O3 NPs and NPC for N2H4⋅H2O
adsorption and activation for active hydrogen atoms production. The
γ-Fe2O3/NPC catalyst also presents outstanding recyclability due to its
ferromagnetism, and can be facilely reused with high stability.
Catalytic reduction of nitroarenes
Typically, 20 mg of catalyst was added into a two neck round bottom
flask contains the mixture of 1 mmol of nitrobenzene and 5 mL of
ethanol, followed by refluxing in an 80 ℃ thermostatic oil bath. Then,
500 μL N2H4⋅H2O was added into the reaction mixture using an injection
Experimental
syringe, and the reaction was started. The conversion and selectivity of
the reaction were monitored by n-decane as an internal standard for gas
chromatography-mass spectrometry (GC–MS) analysis.
Material
To investigate the general applicability of the γ-Fe2O3/NPC catalyst,
the reduction of nitroarenes with different substituents was carried out
under similar reaction conditions as mentioned above. To test the
reusability of γ-Fe2O3/NPC catalyst, the used γ-Fe2O3/NPC catalyst was
separated by a magnet, washed with ethanol for three times, and then
dried in an oven for 6 h at 60 ℃, and reused for next catalytic run.
Ferrocene carboxaldehyde and p-phenylenediamine were purchased
from Sinopharm Chemical Reagent Co., Ltd. and Tianjin Heowns
Biochemical Reagent Co., Ltd., respectively. Nitroarenes and N2H4⋅H2O
(80 wt%) were supplied by Shanghai Zhongqin Chemical Reagent Co.,
Ltd.. Dimethyl sulfoxide (DMSO), methanol, ethanol, N, N-
Fig. 1. (a) TGA curve of the POP precursor and (b) FT-IR spectra of the POP precursor and γ-Fe2O3/NPC catalyst.
2

J. Lv et al.
Molecular Catalysis 498 (2020) 111249
Fig. 2. (a) N2 adsorption-desorption isotherm plots, (b) pore size distribution profiles of the γ-Fe2O3/NPC-X catalyst.
Characterizations
Table 1
Elemental analysis, BET surface area, and pore structure characterization pa-
rameters of γ-Fe2O3/NPC at different carbonization temperatures.
The transmission electron microscopy (TEM) was carried out on a
FEI Tecnai G2 F30, 200 KV to investigate the morphology of the ob-
tained samples. The N2 adsorption-desorption measurements (Micro-
meritics ASAP 2010) were employed to obtain the pore size distribution,
and BET special surface areas of the catalysts. Powder X-ray diffraction
Entry
Catalyst
Surface
Area
pore
N
C
H
Fe%
volume
(%)
(%)
(%)
(ICP)
(m2 gꢀ 1
)
(cm3
gꢀ 1
)
(PXRD, Rigaku D/max-2400 with monochromatized Cu-Kα radiation λ
1
2
3
4
5
γ-Fe2O3/
NPC-600
γ-Fe2O3/
NPC-700
γ-Fe2O3/
NPC-800
γ-Fe2O3/
NPC-900
γ-Fe2O3/
NPC-1000
381.6
302.4
415
0.310
0.260
0.371
0.417
0.447
3.27
1.59
1.91
1.51
1.21
61.0
64.3
67
1.05
0.67
0.4
20.1
21.0
34.9
30.5
24.9
=1.5406 Å) was used to determine the crystal structure of the prepared
catalyst. To determine the chemical composition and electronic struc-
ture, X-ray photoelectron spectroscopy (XPS, PHI-5702 instrument,
Electro Physics Corporation, US) was used. Fourier transform spectros-
copy (FT-IR, VERTEX 70, Bruker) was recorded to analyze the functional
groups of the samples. Thermogravimetric analysis (TGA) of POP ma-
terial was performed on a TA-Q50 equipment under N2 atmosphere with
a heating rate of 10 ℃ minꢀ 1. Raman analysis were recorded on Jobin
336.7
315.3
68.5
69.0
0.36
0.26
–
Yvon LabRam HR spectrometer. GC MS (Agilent 5977E) was employed
for monitoring the reaction process.
benzene ring; whereas, these peaks were not observed in γ-Fe2O3/NPC
– –
sample. In addition, some new broad peaks corresponding to C C, C
N
groups stretching vibration at 1035, 1082 and 1153 cmꢀ 1 were
appeared in γ-Fe2O3/NPC sample [36,37]. This result confirms that
high-temperature carbonization greatly pyrolyzes the covalent bonds of
POP precursor to form stable N-doped porous carbon material. Besides, a
new peak emerges at 605.5 cmꢀ 1 is assigned to the vibration of the Fe–O
band [38,39].
Results and discussion
Catalyst characterization
In this study, the POP material was synthesized by one-step poly-
merization of p-phenylenediamine and ferrocene carboxaldehyde. The
p-phenylenediamine is firstly reacted with ferrocene carboxaldehyde to
give a mono-imine product, which in turn reacts with another p-phe-
nylenediamine to give the aminal-based POP material [34]. The ob-
tained ferrocene-functionalized POP precursor has Fe-containing
monomers in its matrix, Thus, through carbonization, the Fe-based
BET measurements were conducted at 77 K to examine the porous
properties of the γ-Fe2O3/NPC materials with the influence of carbon-
ization temperatures. Fig. 2a shows that all samples basically presenting
type H4 isotherms [40,41], and a hysteresis loop above P/P0 = 0.4~0.8
and continuously quantity adsorbed increased within this relative
pressure range, which is characteristic of the presence of abundant
mesopores in the γ-Fe2O3/NPC materials. Furthermore, these curves
have all increased in different degrees of significance under relatively
high pressures (P/P0 > 0.8) as the pyrolysis temperature increases,
indicating that the excessive pyrolytic temperature will introduce the
change of some pore channels collapse to form macropores. Fig. 2b
displays the effect of carbonization temperatures on the distribution of
pore size. The mesopores with the average diameter of 3.8 nm are the
main pores in the prepared materials. This porous structure provides
channels for the mass transfer of reactants. From the N2
adsorption-desorption analysis, the obtained γ-Fe2O3/NPC-800 catalyst
has a surface area of 415 m2 gꢀ 1 and pore volume of 0.371 cm3 gꢀ 1, the
active
sites
can
be
well-dispersed
[35].
Here,
the
ferrocene-functionalized POP precursor was carbonized to form
γ-Fe2O3/NPC-X catalysts (Scheme 1). TGA measurement was used to
determine the thermal stability of POP precursor (Fig. 1a). According to
the TGA results, the total weight was reduced by 55 % when the
carbonization temperature was rose from 25 to 1000 ℃. Notably, when
the carbonization temperature reached above 711.2 ℃, the weight of
the sample slowly decreases. Thus, the carbonization over 711.2 ℃
should obtain stable samples with abundant pore structure. FT-IR
analysis was performed to identify the chemical bonds and functional
groups of the POP precursor and γ-Fe2O3/NPC catalysts. Fig. 1b shows
the peaks for POP precursor located at 1512, 1615 and 1664 cmꢀ 1
–
belonged to C C stretching vibration and the skeleton vibration of the
–
3

J. Lv et al.
Molecular Catalysis 498 (2020) 111249
Fig. 3. The TEM image of (a) γ-Fe2O3/NPC-600, (b) γ-Fe2O3/NPC-700, (c) γ-Fe2O3/NPC-800, (d) γ-Fe2O3/NPC-900, (e) γ-Fe2O3/NPC-1000, (f) the element mapping
images of γ-Fe2O3/NPC-800.
Fig. 4. (a) Raman spectroscopy and (b) XRD patterns of catalysts at different pyrolytic temperatures.
specific parameters of other catalysts are summarized in Table 1.
TEM characterization was carried out at the same resolution (50 nm)
to investigate the morphology and the influence of pyrolysis tempera-
tures on the obtained materials. γ-Fe2O3/NPC-800 shows obvious
porous structure, γ-Fe2O3 NPs with an averge diameter of about 18 nm
are uniformly anchored in the carbon matrix (Fig. 3c). Porous structure
in the material has not yet been fully formed at low pyrolysis temper-
atures of 600 ℃ (Fig. 3a) and 700 ℃ (Fig. 3b), while higher pyrolysis
temperatures like 900 ℃ (Fig. 3d) and 1000 ℃ (Fig. 3e) led to the ag-
gregation of γ-Fe2O3 NPs to large particles. Apparently, the small sized
Fig. 5. (a) Wide-range XPS spectra, (b) N 1s spectra and (c) Fe 2p spectra of γ-Fe2O3/NPC-800.
4

J. Lv et al.
Molecular Catalysis 498 (2020) 111249
Table 2
The catalytic reduction of nitrobenzene under different conditions.a.
Entry
Catalyst
Catalyst
Temp.
Time
Conv.
(%)
Sel.
(%)
dosage (mg)
(℃)
(min)
1
γ-Fe2O3/
NPC-600
γ-Fe2O3/
NPC-700
γ-Fe2O3/
NPC-800
γ-Fe2O3/
NPC-900
γ-Fe2O3/
NPC-1000
γ-Fe2O3/
NPC-800
γ-Fe2O3/
NPC-800
γ-Fe2O3/
NPC-800
γ-Fe2O3/
NPC-800
γ-Fe2O3/
NPC-800
γ-Fe2O3/
NPC-800
γ-Fe2O3/
NPC-800
γ-Fe2O3 nano
powder
20
20
20
20
20
20
20
20
30
25
15
10
10
80
80
80
80
80
60
40
20
80
80
80
80
80
80
80
80
80
80
80
80
80
60
60
80
80
80
75
89
100
56
47
99
77
28
99
98
93
71
91
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
2
Scheme 2. Mechanism of γ-Fe2O3/NPC-800 catalytic reduction of
3
nitrobenzene.
4
that are attributed to pyridinic N (398.6 eV), pyrrolic N (400.1 eV),
graphitic N (401.1 eV) and oxidized N (402.8 eV) [51,52], an apparent
strong binding energy could be observed and the largest fraction is
pyrrolic N peaks, followed by graphitic N 30.91 %, pyridinic N 26.29 %
and oxidized N 7.45 % (Fig. 5b). Many researchers have reported that
the higher pyrrolic N atoms can exist at the edge and vacancy of carbon
layer, contribute to the good electron conductivity and ion permeability
like Lewis basic sites to improve catalytic activity [53]. The binding
energies of Fe 2p3/2 and Fe 2p1/2 centered at 711.0 and 724.6 eV,
respectively [54,55]. Fig. 5c shows that the Fe 2p3/2 peak is narrower
and stronger than Fe 2p1/2 and the area of Fe 2p3/2 peak is greater than
Fe 2p1/2 because in spin–orbit (j–j) coupling. More importantly, the
present of characteristic satellite peak (718.2 eV) further demonstrates
that iron species evolved into γ-Fe2O3 NPs.
5
6
7
8
9
10
11
12
13
Catalytic reduction of nitroarenes
a
Reaction conditions: 1 mmol nitrobenzene, 20 mg of catalyst, 500
N2H4⋅H2O, and 10 mL of ethanol.
μL
The catalytic reduction of nitrobenzene was chosen as a benchmark
reaction to investigate the catalytic performance of γ-Fe2O3/NPC sam-
ples in the presence of N2H4⋅H2O as a hydrogen source and ethanol as a
solvent. The reason for choosing ethanol as reaction solvent was that,
nitrobenzene and N2H4⋅H2O both can be efficiently dissolved in ethanol
so that they can effectively contact with γ-Fe2O3/NPC catalysts and
promote the reduction reaction. Moreover, ethanol is also a cost-
effective and green solvent which can be effectively recycled in many
organocatalysis reactions. Table 2 illustrates the reduction effects of
nitrobenzene under different reaction conditions. First, γ-Fe2O3/NPC-
800 catalyst gives the reaction conversion and selectivity up to 99 % and
100 %, respectively, indicating 800 ℃ is the best one among all
carbonization temperatures for the preparation of the γ-Fe2O3/NPC
catalysts (entries 1–5). Entries 6–8 shows that the higher temperature of
the reaction system remarkably boosted the reaction efficiency, oper-
ating conditions below 80 ℃ gave weak reaction efficiency. Addition-
ally, the increase of catalyst dosage brought better reaction performance
(entries 9–12), various comprehensive factors determine 20 mg as the
most appropriate dosage of γ-Fe2O3/NPC-800 catalyst. Moreover, when
use 10 mg of γ-Fe2O3 nano powder as catalyst, the yield of the aniline is
about 91 %, that is a little lower than the γ-Fe2O3/NPC-800 catalyst in
the same reaction conditions (entry 13). The reason may be due to the
magnetic γ-Fe2O3 nano powder is easily aggregated on the surface of the
magnetic stir bar during the reaction process. From the above reaction
conditions screen experiment, it can be seen that γ-Fe2O3/NPC-800
catalyst can effectively catalyze reduction of nitrobenzene using
N2H4⋅H2O in ethanol solvent.
γ-Fe2O3 NPs in γ-Fe2O3/NPC-800 resulted in a significant increment of
the exposed active sites, and enhancing the catalytic activity, in addi-
tion, the fluffy porous NPC structure allowed high dispersion of γ-Fe2O3
NPs. Moreover, a high-angle annular dark-field scanning transmission
electron microscopy (HAADF-STEM) image was collected for observing
the homogeneous distribution of Fe and N atoms within the catalyst, the
images reveal the enrichment of Fe signal, confirming γ-Fe2O3 species
was successfully inserted into the NPC sample (Fig. 3f).
Raman spectroscopy is an efficient, local structural probe for con-
firming the detailed carbon skeletons by analyze the peak area ratio of
the D and G bands of samples at different pyrolysis temperatures [42,
43]. The D band (1360 cmꢀ 1) represents the defective and disordered
carbon layers, while the G band (1580 cmꢀ 1) represents the well-defined
sp2 bonded carbon [44,45]. So, the intensity ratio (ID/IG) of the two
composites reveals the relative concentration of defect sites [46]. As
shown in Fig. 4a, the ID/IG value of γ-Fe2O3/NPC-800 reached maximum
intensity than catalysts pyrolyzed at other temperatures, which suggests
more defects on γ-Fe2O3/NPC-800 catalyst that can expose more active
sites. To our knowledge, the incorporation of heteroatom in carbon
framework can cause adjacent carbon to produce more defects, the
higher ID/IG value also explained that γ-Fe2O3 and N species was suc-
cessfully doped into γ-Fe2O3/NPC-800. The phase structure of the pre-
cursor and the as-transformed samples at different pyrolysis
temperatures were examined by powder XRD (Fig. 4b). The peaks
located at 30.2◦, 35.64◦, 43.3◦, 57.3◦ and 62.8◦ are attributed to the
(220), (311), (400), (511) and (440) of characteristic XRD peaks of
γ-Fe2O3 (JCPDS NO. 39-1346) [47–49], and 2θ = 44.5◦ is assigned to
Scheme 2 shows the reaction mechanism of the γ-Fe2O3/NPC-800
catalyzed reduction of nitrobenzene. The porous structure of the
γ-Fe2O3/NPC-800 catalyst provides a large amount of surface area for
the reactants. First, N2H4⋅H2O and nitrobenzene were adsorbed on the
catalyst surface, N2H4⋅H2O is decomposed by γ-Fe2O3 activation into
N2H2, 2H+ and electrons. 2H+ and electrons are transferred to the sur-
rounding nitrobenzene, then nitrobenzene is hydrogenated directly to
nitrosobeenzene, and the unstable nitrosobeenzene intermediate further
reduced into aniline. Finally, N2H4⋅H2O is decomposed into N2 and H2O
ε
-Fe2O3 [50]. The peak area increases with the increase of carbonization
temperature, these information also indicate the successful introduction
of γ-Fe2O3 into the amorphous carbon material.
The wide-range XPS shows that four peaks of Fe 2p, C 1s, O 1s and N
1s can be observed in the sample of γ-Fe2O3/NPC-800 (Fig. 5a). The C 1s
peak at 284.8 eV was used to calibrate the binding energy. The high-
resolution spectrum of N 1s can be deconvoluted into four pairs peaks
5

J. Lv et al.
Molecular Catalysis 498 (2020) 111249
metal-based catalysts such as Pt@PNIPAM [56], Pd/γ-Al2O3@ASMA
[57], and Au/P[VBTACl]-Al [58] in the catalytic hydrogenation of
halo-nitrocompounds showed low selectivities due to their dechlorina-
tion effect. In addition, the catalyst also shows excellent chemo-
selectivity in the reduction of substituted nitroaromatics with quite
challenging functional groups, such as methyl, amido and hydroxy
(entries 8–13), giving the corresponding anilines derivatives without
any by-product. These remarkable results indicate that the
γ-Fe2O3/NPC-800 catalyst exhibits advantages including cost effective
and excellent chemoselectivity in the catalytic reduction of substituted
nitrocompounds. And the γ-Fe2O3/NPC catalysts may be used as po-
tential candidates for catalytic reduction of nitrocompounds.
Table 3
Reduction of different nitroarenes by γ-Fe2O3/NPC-800 catalyst.a.
Entry
Substrate
Product
Time
Conv.
(%)
Sel.
(%)
(min)
1
2
60
45
100
100
>99
>99
3
4
5
60
45
60
100
99
>99
>99
>99
The magnetic properties of the fresh and reused γ-Fe2O3/NPC-800
were obtained by a vibrating sample magnetometer (VSM) system at
room temperature. As shown in Fig. 6a, with the field sweeping from
-20,000 to +20,000 Oe, both catalysts exhibited good super-
paramagnetic, the measured magnetic saturation (Ms) value are
approximately 13.8 and 10.3 emu gꢀ 1, respectively. The magnetic
property endows the recovery of catalysts using external magnets. The
inset of Fig. 6a shows a simple separation process of catalyst by magnet
adsorption. The catalyst can be attracted in a short time by using an
external magnet, and washed three time with ethanol before the next
test. As shown in Fig. 6b, the γ-Fe2O3/NPC-800 catalyst showed excel-
lent catalytic efficiency in ten consecutive experiments, without obvious
decrease in its activity. TEM showed that γ-Fe2O3 active sites were still
widely dispersed in the reused γ-Fe2O3/NPC-800 catalyst (Fig. S1), and
XRD results also confirmed that the structure of reused γ-Fe2O3/NPC-
800 is similar to the fresh catalyst (Fig. S2). In addition, the XPS spectra
of the reused γ-Fe2O3/NPC-800 also shows satellite peak which assigned
to γ-Fe2O3 NPs (Fig. S3). Thus, it is sufficient to indicate that the
γ-Fe2O3/NPC catalyst exhibit high chemical and thermal stability.
100
6
7
80
40
95
99
>99
>99
8
9
120
120
999
100
>99
>99
10
110
100
>99
11
12
120
100
99
99
>99
>99
13
150
99
>99
Conclusion
a Reaction conditions: 1 mmol of substrate, 20 mg of γ-Fe2O3/NPC-800, 500
N2H4⋅H2O, and 10 mL of ethanol.
μL
In summary, we fabricated a heterogeneous, high stability, magnetic
retrievable, and highly efficient γ-Fe2O3 NPs embedded in N-doping
porous carbon catalyst via the facile carbonization of a ferrocene-
functionalized POP material. The γ-Fe2O3/NPC-800 catalyst shows
excellent catalytic performance in catalytic reduction of functionalized
nitro compounds under mild reaction conditions with superior chemo-
selectivity. The high catalytic activity of γ-Fe2O3/NPC-800 catalyst
should be attributed to the synergistic effect between γ-Fe2O3 NPs and
N-doped porous carbon, which can improve the hydrazine hydrate
adsorption and activation and thus is beneficial for nitroarenes reduc-
tion. The catalyst can be facilely recycled with a magnet and can be
reused at least ten times without reducing catalytic activity and selec-
tivity. This work demonstrates a new platform for the further studies of
high-activity metal/metal oxide species encapsulated in N-doped porous
carbon to improve the catalytic performance in various catalytic
which are green byproducts for the environment.
Encouraged by the excellent performance of the γ-Fe2O3/NPC-800
catalyst in nitrobenzene reduction, the catalytic reduction of various
functional nitroaromatic was investigated to explore the general appli-
cability of γ-Fe2O3/NPC-800 under the optimized reaction conditions.
It’s clear to observe from Table 3 that, even the o-, m- and p- positions on
the phenyl ring of nitroarenes are replaced by electron donor and
electron acceptor functional groups, the catalyst shows excellent cata-
lytic activity with satisfactory conversion and selectivity. Which suggest
that the electronic and steric effect of substituent on phenyl rings is
negligible. For example, the reaction times of halo-nitrocompounds
were obviously shortened and no dehalogenation was found in the
reduction process (entries 1–7). However, some other reported noble
Fig. 6. (a) The hysteresis loop of γ-Fe2O3/NPC-800 and reused γ-Fe2O3/NPC-800 (inset, the catalyst is simply separated by magnet); (b) Reuse of γ-Fe2O3/NPC-800
for reduction of nitrobenzene.
6

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Molecular Catalysis 498 (2020) 111249
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CRediT authorship contribution statement
[17] Parisa Fakhri, Mahmoud Nasrollahzadeh, Babak Jaleh, Graphene oxide supported
Au nanoparticles as an efficient catalyst for reduction of nitro compounds and
Suzuki–Miyaura coupling in water, RSC Adv. 4 (2014) 48691–48697.
[18] Dinesh Mullangi, Debanjan Chakraborty, Anu Pradeep, Vijay Koshti, Chathakudath
P. Vinod, Soumendranath Panja, Sunil Nair, Ramanathan Vaidhyanathan, Highly
stable COF-Supported Co/Co(OH)2 nanoparticles heterogeneous catalyst for
reduction of Nitrile/Nitro compounds under mild conditions, Small 14 (2018),
https://doi.org/10.1002/smll.201801233.
Jing Lv: Data curation, Writing - original draft. Zhengtang Liu:
Formal analysis, Writing - review & editing. Zhengping Dong: Super-
vision, Conceptualization, Methodology.
Declaration of Competing Interest
[19] Y.M. Qu, G.D. Xu, J.H. Yang, Z.S. Zhang, Reduction of aromatic nitro compounds
over Ni nanoparticles confined in CNTs, Appl. Catal. A-Gen. 590 (2020), 117311.
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magnetic nanoparticles from Co, Ni, and Fe chloride salts and their catalytic
performances on 4-nitrophenol reduction, Inorg. Chem. Commun. 116 (2020),
https://doi.org/10.1016/j.inoche.2020.107930.
There are no conflicts of interest to declare.
Acknowledgements
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Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.mcat.2020.111249.
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