Efficient chemoselective hydrogenation of halogenated nitrobenzenes over an easily prepared γ-Fe2O3-modified mesoporous carbon catalyst

Article abstract of DOI:10.1039/c6gc03386e

Efficient chemoselective hydrogenation of halogenated nitrobenzenes using low-cost catalysts is an important research area of applied catalysis. In this work, a Fe metal organic gel (Fe-MOG) was prepared via metal coordination interaction between Fe(NO₃)₃·9H₂O and 1,4-naphthalenedicarboxylic acid. Fe-MOG was used as the precursor for the fabrication of a low-cost γ-Fe₂O₃-modified mesoporous carbon catalyst (γ-Fe₂O₃/MC) through carbonization under an N₂ atmosphere at high temperature. The obtained γ-Fe₂O₃/MC catalyst had high catalytic activity and selectivity for the hydrogenation of Cl^-, Br^- and I^- functionalized nitrobenzenes without any obvious dehalogenation. The hydrogenation reactions had a product yield and selectivity for the corresponding halogenated aniline of 100% when using hydrazine hydrate as the reducing agent. The whole hydrogenation reaction process was environmentally friendly because of its harmless byproducts (H₂O and N₂). In addition, the γ-Fe₂O₃/MC catalyst was recyclable because of the magnetism of the γ-Fe₂O₃ active sites. The catalytic activity of the γ-Fe₂O₃/MC catalyst was not obviously decreased after being recycled five times. Therefore, the γ-Fe₂O₃/MC catalyst has great potential for future applications in the chemoselective hydrogenation of halogenated nitrobenzenes.

Full text of DOI:10.1039/c6gc03386e

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Efficient chemoselective hydrogenation of halogenated nitrobenzenes over an
easily prepared γꢀFe2O3ꢀmodified mesoporous carbon catalyst
Meng Tian, Xueliang Cui, Man Yuan, Jin Yang, Jiantai Ma, Zhengping Dong*
State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical
Engineering, Gansu Provincial Engineering Laboratory for Chemical Catalysis, Lanzhou
University, Lanzhou 730000, PR China.
Eꢀmail: dongzhp@lzu.edu.cn (Zhengping Dong);
Fax: +86 931 8912582; Tel: +86 931 8912577.
Abstract
Efficient chemoselective hydrogenation of halogenated nitrobenzenes using lowꢀcost catalysts
is an important research area for applied catalysis. In this work, a Fe metal organic gel
(FeꢀMOG) which was prepared via metal coordination interaction between Fe(NO3)3·9H2O
and 1,4ꢀnaphthalenedicarboxylic acid. FeꢀMOG was used as the precursor for the fabrication
of a lowꢀcost γꢀFe2O3ꢀmodified mesoporous carbon catalyst (γꢀFe2O3/MC) through
carbonization under an N2 atmosphere at high temperature. The obtained γꢀFe2O3/MC catalyst
had high catalytic activity and selectivity for the hydrogenation of Cl-, Br- and I-
functionalized nitrobenzenes without obvious dehalogenation. The hydrogenation reactions
had product yield and selectivity for the corresponding halogenated aniline of 100% when
using hydrazine hydrate as the reducing agent. The whole hydrogenation reaction process was
environmentally friendly because of its harmless byproducts (H2O and N2). In addition, the
γꢀFe2O3/MC catalyst was recyclable because of the magnetism of the γꢀFe2O3 active sites. The
catalytic activity of the γꢀFe2O3/MC catalyst was not obviously decreased after being recycled
five times. Therefore, the γꢀFe2O3/MC catalyst has a great potential for future applications in
the chemoselective hydrogenation of halogenated nitrobenzenes.
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Introduction
Halogenated anilines are a series of crucial intermediates and raw materials for the synthesis
of fine chemicals, such as pesticides, medicines, dyes, and photosensitive materials.1 Thus the
synthesis of halogenated anilines has attracted increasing attention. At present, halogenated
anilines are mainly obtained by the hydrogenation of the corresponding halogenated
nitrobenzenes.2, 3 Methods for the hydrogenation of halogenated nitrobenzenes in industry
mainly include metal/acid reduction, sulfide reduction, and catalytic hydrogenation. However,
production processes of the first two methods can generate hazardous substances, including
iron slag and sulfurꢀcontaining wastewater.4, 5 Catalytic hydrogenation is an environmentally
friendly method for the preparation of halogenated anilines.6 Until now, the catalysts that have
been investigated for the hydrogenation of halogenated nitrobenzenes have been mainly noble
metalꢀbased catalysts, such as PdꢀonꢀSi catalysts,7 RuꢀPt bimetallic nanocatalysts,8
Pd/γꢀFe2O3,9 PtꢀB/Al2O3,10 PdꢀNiO@SiO2,11 and Ru/NPC.12 However, there are still some
limitations that prevent largeꢀscale application of such catalysts. These include the slight
dehalogenation phenomenon that frequently occurs in the reaction when using noble
metalꢀbased catalysts and the scarcity and high cost of noble metals.9
To design of lowꢀcost catalysts for the chemoselective hydrogenation of halogenated
nitrobenzenes, the use of nonꢀnoble metalꢀbased catalysts with high catalytic activity that are
ecoꢀfriendly and easily recyclable is desirable. In this respect, the most widely used Raney Ni
catalyst has been used in the chemoselective hydrogenation of halogenated nitrobenzenes, but
inhibitors must be introduced into the reaction mixture to prevent the dehalogenation
phenomenon and to maintain the selectivity of the hydrogenation reaction.13 On the other
hand, nonꢀprecious metals supported on nanostructured porous materials for the catalytic
hydrogenation of halogenated nitrobenzenes can enhance the catalytic activity to some extent
because the nanostructured porous materials exhibit unique chemical, physical, and electrical
properties. Most recently, Hua Song et al. prepared carbon nanotubeꢀsupported CoꢀB
amorphous alloy catalysts for the hydrogenation of mꢀchloronitrobenzene (mꢀCNB) to
mꢀchloroaniline (mꢀCA) with high chemoselectivity under the reaction conditions of 413 K
and 2.0 MPa H2.14 The Niꢀsupported catalyst CN/Ni/Al2O3 has high chemoselectivity for
pꢀCNB under 1.0 MPa H2 and 393 K.15 Ni/TiO2 catalyst has also been used in the
2

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hydrogenation of halogenated nitrobenzenes with excellent catalytic performance.16
A
MOFꢀderived CoꢀCoO@Nꢀdoped porous carbon catalyst was reported by Jiang et al. for the
hydrogenation of nitro compounds with halogenated groups.17 The work of Lykakis et al.
proved that the γꢀFe2O3 nanoparticles (NPs) can catalyze the hydrogenation of nitro
compounds with reducing functional groups.18 The above reported nonꢀnoble metalꢀbased
catalysts are indeed lowꢀcost and have excellent chemoselectivity for the hydrogenation of
nitroaromatic compounds with or without halogenated groups. However, there are still some
drawbacks. In particular, the preparation processes of the reported catalysts were complicated,
the precursors acting as catalyst support sources were expensive, and most of the catalysis
processes required harsh conditions, including high H2 pressure.
Based on the above considerations and to address the limitations, we reported a simple
oneꢀpot pyrolysis method to fabricate a novel catalyst for the hydrogenation of a variety of
halogenated nitrobenzenes. First, a Fe metalꢀorganic gel (FeꢀMOG) was prepared through
coordination interaction between Fe(NO3)3·9H2O and 1,4ꢀnaphthalenedicarboxylic acid (NDC)
in ethanol in a simple method. Then the FeꢀMOG was carbonized under an inert atmosphere
at 500 oC to synthesize the γꢀFe2O3 NPꢀsupported mesoporous carbon catalyst (γꢀFe2O3/MC).
The active sites on γꢀFe2O3 NPs were inlaid uniformly in porous carbon. At atmospheric
pressure, this catalyst exhibited high yield and excellent chemoselectivity (typically 100%)
for the hydrogenation of halogenated nitrobenzenes to the corresponding halogenated anilines
using N2H4·H2O as the reductant. The whole reaction system did not generate byproducts or
harmful products. Moreover, the γꢀFe2O3/MC catalyst exhibited superior recyclability and
excellent stability after being reused at least five times. This work may promote the
investigation of lowꢀcost γꢀFe2O3ꢀbased catalysts for the highly chemoselective
hydrogenation of halogenated nitrobenzenes in the chemical industry.
Experiment
Materials
NDC and halogenated nitrobenzenes were purchased from Tianjin Heowns Biochem..
Fe(NO3)3·9H2O and N2H4·H2O (80 wt %) were supplied by Sinopharm Chemical Reagent Co.
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Ltd (China). All other reagents used in this work were of analytical grade and did not require
further purification.
Catalyst preparation
Based on recent research,19 we prepared the FeꢀMOG by coordination interaction of iron and
NDC. 5 mmol of NDC was dissolved in 50 mL of ethanol at 343 K. Then, 100 mL of
Fe(NO3)3·9H2O solution (75 mM) was added to the hot solution. The mixture was stirred at
ambient temperature for 48 h. The FeꢀMOG was obtained by suction filtration, washed with
ethanol, and dried in a vacuum oven at 40 oC overnight. The product was then calcined at a
certain high temperature (400 oC, 500oC, 600oC or 700 oC) for 30 min with a heating rate of
10 oC minꢀ1 under a N2 atmosphere. Finally, the 0.75ꢀXꢀγꢀFe2O3/MC catalyst was prepared (X
represents the carbonization temperature).
Hydrogenation of halogenated nitrobenzenes
In
a typical experiment, 0.5 mmol of halogenated nitrobenzenes, 15 mg of
0.75ꢀ500ꢀγꢀFe2O3/MC catalyst, and 5 mL of ethanol were ultrasonically dispersed in a 25 mL
twoꢀneck roundꢀbottom flask. Then 2 mmol of N2H4·H2O was added to the reaction system,
and the mixture was stirred vigorously at 80 oC for approximately one hour with a reflux
condenser. The reaction products were analyzed by gas chromatographyꢀmass spectroscopy
(GCꢀMS, Agilent 5977E).
Catalyst characterization
Transmission electron microscopy (TEM, Tecnai G20 F30) was used to characterize the
morphology of the γꢀFe2O3/MC catalysts. Powder Xꢀray diffraction (PXRD) was recorded
with a Rigaku D/maxꢀ2400 diffractometer using CuꢀKα radiation. This was used for
investigating the physical properties of the catalyst. Xꢀray photoelectron spectroscopy (XPS,
PerkinꢀElmer PHIꢀ5702) was used to measure the electronic states of the surface of the
γꢀFe2O3/MC catalysts. The Fe content of the prepared nanocatalyst was determined by
inductively coupled plasma atomic emission spectroscopy (ICPꢀAES). A Quantum Design
Vibrating sample magnetometer (VSM) was employed for measuring the magnetism of the
catalyst. FTIR spectra of the FeꢀMOG and γꢀFe2O3/MC were obtained on a Fourier transform
infrared spectrometer (FTIR). Raman spectroscopy (Jobin Yvon Lab Ram HR evolution) and
4

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a micromeritics ASAP 2010 (USA) were used to observe the physical properties of the
catalyst. Elemental composition of γꢀFe2O3/MC was performed using energy dispersive Xꢀray
spectroscopy (EDS). The conversion and chemoselectivity of the hydrogenation reactions
were investigated by GCꢀMS (Agilent 5977E).
Scheme 1. Schematic illustration of the synthesis and application of γꢀFe2O3/MC catalyst.
Results and Discussion
Characterization of the catalyst
The catalyst precursor FeꢀMOG was synthesized by the simple mixing of Fe(NO3)3·9H2O and
NDC in ethanol solution, and the γꢀFe2O3/MC catalyst was prepared by carbonization of the
FeꢀMOG under a nitrogen atmosphere. The preparation process of the catalyst γꢀFe2O3/MC is
illustrated in scheme 1. The morphologies of all the prepared samples characterized using
TEM are shown in Fig. 1. Fig. 1a shows the sheet structure of the FeꢀMOG. When the
FeꢀMOG was carbonized in different annealing temperatures (400oC, 500oC, 600oC, and 700
oC), the obtained materials showed slight differences in morphology. For example, the porous
structure was more evident, and the γꢀFe2O3 NPs were uniformly dispersed in the carbon
framework after calcination at 500 oC, and the lamellar structure of the 0.75ꢀ500ꢀγꢀFe2O3/MC
catalyst was almost unchanged (Fig. 1c). It can be seen from the lattice that the interplanar
spacing of 0.25 nm can be attributed to the (311) crystal face of γꢀFe2O3 (Fig. 1c insert).20
However, in the sample of 0.75ꢀ400ꢀγꢀFe2O3/MC, the γꢀFe2O3 NPs did not form inside of the
porous carbon under the insufficient temperature of 400 oC (Fig. 1b). In contrast, at high
o
o
annealing temperatures (600 C and 700 C), the 0.75ꢀ600ꢀγꢀFe2O3/MC (Fig. 1d) and
0.75ꢀ700ꢀγꢀFe2O3/MC (Fig. 1e) catalysts, had obviously larger γꢀFe2O3 NPs due to
agglomeration at high temperature. The γꢀFe2O3 particle size may influence the catalytic
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activity of the γꢀFe2O3/MC catalysts. The EDS spectrum shows that γꢀFe2O3/MC catalyst was
composed of carbon, oxygen, and iron (Fig. 1f).
Fig. 1. TEM images of FeꢀMOG (a), 0.75ꢀ400ꢀγꢀFe2O3/MC (b), 0.75ꢀ500ꢀγꢀFe2O3/MC (c),
0.75ꢀ600ꢀγꢀFe2O3/MC (d) and 0.75ꢀ700ꢀγꢀFe2O3/MC (e). EDS spectrum of the 0.75ꢀ500ꢀγꢀFe2O3/MC
(f).
In order to study the thermal stability and composition of the sample, the TGA curve was
recorded in a nitrogen atmosphere, and the temperature control program was set to rise from
o
25 C to 900 oC at a heating rate of 10 oC minꢀ1 (Fig. 2a). The TGA curve of the FeꢀMOG
sample during annealing shows a gentle incline corresponding to weight loss between 25 oC
and 380 oC resulting from the removal of micromolecules. When the calcination temperature
o
reached 380 C, the weight of the sample began to decrease abruptly, probably because of
pyrolysis of the organic framework. The curve tended to be stable between 480oC and 635oC,
and the target catalyst sample was successfully obtained. The weight again sharply dropped in
the range from 635 oC to 680 oC due to the loss of carbon and oxygen, which could be caused
by the reduction reaction of the structure carbon and iron oxide.
XRD spectra of the prepared catalysts were used to further investigate the crystalline
phases and structure of γꢀFe2O3/MC. Fig. 2b shows the XRD spectra of 0.75ꢀ400ꢀγꢀFe2O3/MC,
0.75ꢀ500ꢀγꢀFe2O3/MC, 0.75ꢀ600ꢀγꢀFe2O3/MC, 0.75ꢀ700ꢀγꢀFe2O3/MC, 0.5ꢀ500ꢀγꢀFe2O3/MC
(prepared using 5 mmol of NDC and 5 mmol of Fe(NO3)3·9H2O), and the reused catalyst of
0.75ꢀ500ꢀγꢀFe2O3/MC. The characteristic peaks of γꢀFe2O3 were clearly observed, and
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corresponded to the (220), (311), (400), (511), and (440) crystal faces (JCPDS No.
39ꢀ1346).21 However, the spectrum of 0.75ꢀ700ꢀγꢀFe2O3/MC is different from the others, and
a new diffraction peak at 44.7o was observed, indicating the formation of Fe(0) (JCPDS No.
6ꢀ696).22 This is because the γꢀFe2O3 NPs were reduced by the structure carbon of the MC in
the high temperature range (635oC to 680 oC), which is consistent with the results from the
TGA analysis.
Fig. 2. TGA curve of 0.75ꢀ500ꢀγꢀFe2O3/MC (a), XRD patterns for γꢀFe2O3/MC (b).
Fig. 3. XPS wideꢀscan spectrum of the 0.75ꢀ500ꢀγꢀFe2O3/MC (a), XPS spectrum of Fe2p (b).
The XPS wideꢀscan spectrum of the catalyst also shows the existence of iron (Fig. 3a). To
further understand the valence information of the iron element, the fine spectrum of Fe 2p
was investigated. As shown in Fig. 3b, two broad peaks were observed at 721.5 eV and 707.9
eV, and these corresponded to the Fe 2p1/2 and Fe 2p3/2, respectively. Furthermore, the satellite
peak between 721.5 eV and 707.9 eV suggests that the iron oxide is γꢀFe2O3 NPs.20, 23 The
XPS results demonstrate the existence of γꢀFe2O3 and agree with the XRD analysis.
Additionally, there are almost no other crystal forms of iron oxides.
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The Raman spectrum of the 0.75ꢀ500ꢀγꢀFe2O3/MC catalyst is shown in Fig. S1. The D
band and G band appearing in the Raman spectrum are the characteristic peaks of the carbon
crystal. The peak at 1336 cmꢀ1 is the D band, indicating a lattice defect of the carbon crystal,
while the G band located at 1581 cmꢀ1 represents an inꢀplane stretching vibration of sp2
hybrid carbon atoms.24 Generally, the degree of disorder of the carbon material is measured
by the intensity ratio of the Dꢀpeak to Gꢀpeak (ID/IG). The value of ID/IG of
0.75ꢀ500ꢀγꢀFe2O3/MC is 0.87, indicating that the carbon crystal has fewer defect sites and
tends to be ordered. From the FTIR spectra of the FeꢀMOG and 0.75ꢀ500ꢀγꢀFe2O3/MC (Fig.
S2), it can be seen that the annealed catalyst undergoes a significant change in structure
compared to before the heat treatment (Fig. S2a). As shown in Fig. S2b, a prominent peak
appears at 592 cmꢀ1, which is the characteristic absorption peak of the ironꢀoxygen bond.
Furthermore, from the ICPꢀAES results, the Fe content of the prepared 0.75ꢀ500ꢀγꢀFe2O3/MC
catalyst is 30.33%. The mesoporous material γꢀFe2O3/MC exhibits the typical IV type
isotherm, due to the presence of mesoporous structure. The surface area of the γꢀFe2O3/MC is
223 m2 gꢀ1 and average pore size of is 5 nm as can be seen from the Fig. S3. Fig. S4 shows the
magnetization curves of 0.75ꢀ500ꢀγꢀFe2O3/MC. The magnetic hysteresis loop was clearly
observed, which indicated the magnetic nature of the 0.75ꢀ500ꢀγꢀFe2O3/MC catalyst. The
saturation magnetization was approximately 14.9 emu g−1, and the catalyst was easily
adsorbed on the centrifuge tube wall by a magnet (Fig. S4, insert). Therefore, the desirable
magnetic properties of the catalyst are favorable for the recycling and reuse of the
0.75ꢀ500ꢀγꢀFe2O3/MC catalyst.
Hydrogenation of halogenated nitrobenzenes
The prepared γꢀFe2O3/MC catalysts were studied for the liquidꢀphase hydrogenation of
halogenated nitrobenzenes to the corresponding halogenated anilines. The main side reaction
is dehydrohalogenation, which leads to aniline as the byproduct.25 As shown in Table 1, the
reaction conditions and catalyst were screened using the hydrogenation of pꢀCNB to pꢀCA as
the model reaction. Initially, the catalysts were prepared by calcination at 400oC, 500oC, 600
o
oC, and 700 C, and the catalytic performance was investigated at the optimal reaction
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conditions (using 4 equivalents of N2H4·H2O as the reducing agent, ethanol as the solvent,
atmospheric pressure, and 80oC for the reaction temperature). The results show that excellent
yield and selectivity of reaction reach up to 100% in one hour by using 0.75ꢀ500ꢀγꢀFe2O3/MC
as the catalyst. However, other catalysts did not afford the desired catalytic effect (Table 1,
entries 1ꢀ4). This was probably because the active sites γꢀFe2O3 had almost not been formed
when annealed at 400 oC. In contrast, annealing at higher temperatures of 600 oC and 700oC,
the γꢀFe2O3 NPs tended to aggregate and were further reduced to Fe NPs. Low loading of the
catalyst (0.5ꢀ500ꢀγꢀFe2O3/MC) exhibited 97.4% conversion and 100% selectivity at two hours
(Table 1, entry 5). Table 1, entries 6ꢀ8 shows that the conversion of pꢀCNB to pꢀCA decreased
when the reaction temperature decreased to 60, 40, and 25 oC. The reaction has almost no
yield when N2H4·H2O was not added or when H2 was used as the reducing agent (Table 1,
entries 9, 12). Furthermore, the reaction effect was undesirable when using two or one
equivalents of N2H4·H2O as the reducing agent (Table 1, entries 10, 11). The reaction yield
was only 67.3% when water was used as a solvent, mainly because the
water (Table 1, entry 13). The use of the Pd/C catalyst for the reaction resulted in poor
selectivity for the hydrogenation of ꢀCNB to ꢀCA, and a high proportion of dechlorinated
pꢀCNB is insoluble in
p
p
aniline product was produced (Table 1, entry 14). Thus, it can be seen that the γꢀFe2O3ꢀbased
γꢀFe2O3/MC catalysts exhibit much more excellent chemoselectivity for the hydrogenation of
pꢀCNB to pꢀCA than the noble metalꢀbased catalyst (Pd/C). Furthermore, the reaction system
was investigated without a catalyst, and the yield was only 1.23% (Table 1, entry 15). As
shown in entry 16, using a nonꢀheat treated iron gel (FeꢀMOG) as the catalyst, the conversion
of the
by the hydrogenation of the γꢀFe2O3/MC catalyst was also investigated for the hydrogenation
of ꢀCNB, the yield of ꢀCA was found to be 32.3% (Table 1, entry 17). The XRD spectrum
pꢀCNB was 8.8% under the same reaction conditions. In addition, the Fe/MC obtained
p
p
of the Fe/MC catalyst consisted with the spectrum of the 0.75ꢀ700ꢀγꢀFe2O3/MC is displayed
in Fig. S5, which was contained a little iron oxide. From the above results, the
0.75ꢀ500ꢀγꢀFe2O3/MC catalyst showed good catalytic activity and selectivity for the
hydrogenation of
pꢀCNB to
pꢀCA at the optimized reaction conditions.
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Table 1. Screening the reaction conditions using the catalytic hydrogenation of p-CNB to p-CA as the
probe reaction.a
Cl
NO2
Cl
NH2
Temperature (oC
)
Entry
Catalyst
Time (min)
Conv. (%)
Sel. (%)
1
2
0.75ꢀ400ꢀγꢀFe2O3/MC
0.75ꢀ500ꢀγꢀFe2O3/MC
0.75ꢀ600ꢀγꢀFe2O3/MC
0.75ꢀ700ꢀγꢀFe2O3/MC
0.5ꢀ500ꢀγꢀFe2O3/MC
0.75ꢀ500ꢀγꢀFe2O3/MC
0.75ꢀ500ꢀγꢀFe2O3/MC
0.75ꢀ500ꢀγꢀFe2O3/MC
0.75ꢀ500ꢀγꢀFe2O3/MC
0.75ꢀ500ꢀγꢀFe2O3/MC
0.75ꢀ500ꢀγꢀFe2O3/MC
0.75ꢀ500ꢀγꢀFe2O3/MC
0.75ꢀ500ꢀγꢀFe2O3/MC
Pd/C
80
80
80
80
80
60
40
25
80
80
80
80
80
25
80
80
80
180
60
23.3
100
13.9
27.2
97.4
66.7
26.5
13.1
0.2
100
100
100
100
100
100
100
100
100
100
100
100
100
34.7
100
100
100
3
120
120
120
180
300
300
300
60
4
5
6
7
8
9b
10c
11d
12e
13f
14
15
16
17
49.1
72.6
0.2
60
60
60
67.3
99.6
1.23
8.8
40
None
300
60
FeꢀMOG
Fe/MC
60
32.3
Reaction conditions: a p-CNB (0.5 mmol), catalyst (15 mg), 2 mmol of N2H4·H2O, enthanol (5 mL).
f
b Without reductant. c 1 mmol of N2H4·H2O. d 0.5 mmol of N2H4·H2O. e Using H2 as the reductant.
Uing water as solvent.
To investigate the general applicability of the 0.75ꢀ500ꢀγꢀFe2O3/MC catalyst, the
hydrogenation of a variety of halogenated nitrobenzenes was carried out. As shown in Table 2,
the hydrogenation of
o (m, p)ꢀCNB, o (m, p)ꢀbromonitrobenzene, and 1ꢀiodoꢀ4ꢀnitrobenzene
(
pꢀINB) were all fully converted to the corresponding haloanilines using the
0.75ꢀ500ꢀγꢀFe2O3/MC catalyst. Both the yield and selectivity reached 100% in an hour or in
70 minutes (entries 1ꢀ7). Hydrogenation of 1ꢀnitroꢀ2,4ꢀdichlorobenzene still achieved a high
selectivity of 98.3%, while only 1.7% of
of the oꢀposition Cl (Table 2, entry 8). Besides, the hydrogenation of
ꢀFNB) has been investigated using 0.75ꢀ500ꢀγꢀFe2O3/MC catalyst. As can be seen from the
mꢀCNB was formed because of the dehalogenation
pꢀfluoronitrobenzene
(
p
entry 9, the reaction has the conversion of 82% and selectivity of 96.6% in the 3 h. Substrate
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and product concentration ratio curves for the hydrogenation of
p
ꢀCNB,
p
ꢀbromonitrobenzene
ꢀCNB to
pꢀIA) increased linearly for the first 40 min of reaction, and
(
pꢀBNB), and
pꢀINB are shown in Fig. S6. The conversion for the reduction of
p
p
ꢀCA and ꢀINB to
p
p
ꢀiodoaniline (
then the curves increased gradually up to 60 min. The figures only have two curves of
substrates and target products, which implies that there were almost no byproducts formed in
the whole reaction process (Figs. S6a and c). This was also confirmed by the GCꢀMS
monitoring process. Similarly, the reduction of
reaction time that was only ten minutes longer (Fig. S6b). The comparison of the reaction
conditions, conversions and selectivity in the hydrogenation of ꢀCNB to ꢀCA using
pꢀBNB had the same reaction system with a
p
p
0.75ꢀ500ꢀγꢀFe2O3/MC catalyst and other reported noble metals and nonꢀprecious metalꢀbased
catalysts were listed in Table 3. The noble metals based catalysts 0.12PdꢀSi7 and
Ru5/NPC(NB)12 can hydrogenate the
pꢀCNB to
p
ꢀCA with high selectivity of 99.9% and 98%,
11
respectively, but they need high H2 pressure and reaction temperatures. The PdꢀNiO@SiO2
26
and Pd@Ptꢀ1/0.25/Al2O3 catalysts although can catalyze hydrogenation of
pꢀCNB under
mild reaction conditions, the selectivity of
pꢀCA is low. Other reported nonꢀprecious
metalꢀbased catalysts CN/Ni/Al2O315 and 12% Ni/AlꢀSBAꢀ1527 also need high H2 pressure for
hydrogenating the
pꢀCNB to pꢀCA with selectivity lower than 100%. The reported
Co0.2Fe2.8O4 catalyst with the same reaction conditions of this work, has a
pꢀCA selectivity of
99.6%.28 Although the reported 5wt% Pt/CMKꢀ329 and 5wt% Ru/CMKꢀ330 have very high
selective hydrogenation to pꢀCNB, the 5wt% Pt/CMKꢀ3 also need high H2 pressure and the
5wt% Ru/CMKꢀ3 has only 49.9% of conversion in 1 h. Thus, it was concluded that the
0.75ꢀ500ꢀγꢀFe2O3/MC catalyst exhibited excellent catalytic activity for the hydrogenation of a
variety of halogenated nitrobenzenes with almost 100% yield and selectivity.
Table 2. Hydrogenation of various halogenated nitrobenzenes to corresponding halogenated aniline
using 0.75ꢀ500ꢀγꢀFe2O3/MC catalyst.
NO2
NH2
X
X
Time Yield Sel.
Entry
1
Substrate
Product
(min)
(%)
(%)
60
100
100
Cl
NH2
NO2
Cl
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2
3
4
5
6
70
60
70
60
70
100
100
100
100
100
100
100
100
100
100
NH2
NH2
NO2
Br
Br
NO2
NO2
I
I
Cl
Cl
NH
2
Br
Br
NO2
NH2
Cl
Cl
NO2
Br
NH2
Br
7
60
100
100
NH2
Cl
NO2
Cl
Cl
Cl
98.3
1.7
82
Cl
NH2
8
9
180
180
98.3
NO2
NH2
NH2
NO2
F
F
96.6
Reaction
conditions:
substrate
(0.5
mmol),
0.75ꢀ500ꢀγꢀFe2O3/MC nanocatalyst (15 mg), N2H4·H2O (2
mmol), solvent (5 mL), reaction temperature (80 oC). Yield
Table 3. The comparison of the catalytic performance over different catalysts in the catalytic
hydrogenation of p-CNB to p-CA.
Substrates
Dosage
Active Sites
Loading
T/
oC
Conv.
(%)
Sel.
(%)
Entry Catalyst & Dosage
Time
P/MPa
Ref.
0.12Pd–Si
50 mg
Ru5/NPC(NB)
10 mg
Pd–NiO@SiO2
0.19 g
Pd@Ptꢀ1/0.25/Al2O3
0.1 g
CN/Ni/Al2O3
50 mg
12%Ni/AlꢀSBAꢀ15
15mg
Co0.2Fe2.8O4
0.1 mmol
5wt% Pt/CMKꢀ3
25 mg
1
2 mmol
1 mmol
1 g
Pd/0.12wt%
Ru/4.4wt%
Pd/0.36wt%
Pt/0.25wt%
/
36 h
2.5 h
2.5 h
3 h
100
140
30
P
H2=1
>99.9
98
>99.9
98
7
2
P
P
P
H2=1.5
H2=0.1
H2=0.1
12
11
26
15
27
28
29
30
3
100
98.5
96.8
100
100
77.7
49.9
100
82.7
82.1
>99.9
97
4
2 g
45
5
50 mg
10 h
7.5 h
120
110
80
PH2=1
PH2=2.5
N2H4·H2O
6
1 mmol
10 mmol
21 mmol
8.1 mmol
0.5 mmol
Ni/12wt%
/
20
min
10
7
99.6
>99
>99
100
8
Pt/5wt%
Ru/5wt%
25
PH2=4
min
5wt% Ru/CMKꢀ3
20.9 mg
0.75ꢀ500ꢀγꢀFe2O3/MC
15 mg
9
1 h
1 h
30
N2H4·H2O
N2H4·H2O
γꢀFe2O3/43
wt%
This
work
10
80
Scheme 2 shows the mechanism for the hydrogenation of halogenated nitrobenzenes to
12

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DOI: 10.1039/C6GC03386E
halogenated anilines using the γꢀFe2O3/MC catalyst. First, Fe3+ was reduced to Fe2+ by
N2H4·H2O and released nitrogen and the hydrogen ion. Subsequently, the nitroso compound
was generated by the reduction of the nitrocompound in which Fe2+ supplied electrons and
was oxidized to Fe3+. Then the nitroso compound reacted with Fe2+ to form
31
phenylhydroxylamine.18,
Finally, phenylhydroxylamine was further reduced to the
arylamine compound. During the reaction procedure, negligible halogenated
phenylhydroxylamine has been detected by the GCꢀMS. However, the halogenated
phenylhydroxylamine should be quickly transformed into halogenated anilines, thus the final
products are the halogenated anilines, H2O, and N2.
Scheme 2. Mechanism for hydrogenation of halogenated nitrobenzenes over 0.75ꢀ500ꢀγꢀFe2O3/MC
catalyst.
The stability and recyclability of catalysts are of great importance in practical application.
In this work, to further explore the lifetime and stability of the catalyst, the conversion rate of
each reused reaction was recorded at 15 minute intervals. The catalyst can be separated from
the reaction system using an external magnetic field because of the magnetic property of
γꢀFe2O3. Fig. 4 shows that the catalyst still maintained excellent catalytic activity after five
repetitions. The reused catalyst was further characterized. As shown in Fig. 2b, the XRD
result revealed that the crystalline phase and structure of the reused 0.75ꢀ500ꢀγꢀFe2O3/MC
catalyst did not change as compared with the fresh prepared catalysts. The FTIR and TEM
results (Fig. S2b, Fig. S7, respectively) also shows that the morphology and the structure of
the catalyst had no obvious changes. Therefore, the 0.75ꢀ500ꢀγꢀFe2O3/MC catalyst exhibit
superior reusability and stability.
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Fig. 4. Recyclability tests of 0.75ꢀ500ꢀγꢀFe2O3/MC catalyst for hydrogenation of p-CNB to p-CA.
Conclusions
In conclusion, we have demonstrated a concise and efficient method to synthesize the
γꢀFe2O3/MC catalyst for the hydrogenation of halogenated nitrobenzenes. The catalysts were
prepared by an accurate annealing procedure of FeꢀMOG which was generated by the mixing
of ferric nitrate and NDC. The γꢀFe2O3 NPs were uniformly encapsulated in a carbon structure,
which exhibited dramatically high catalytic activity and specific selectivity for the
hydrogenation of a variety of halogenated nitrobenzenes to the corresponding halogenated
anilines. The obtained catalyst was readily recycled because of its intrinsic magnetic property
and could be reused at least five times without decrease of catalytic activity and selectivity.
Therefore, such a highly efficient, lowꢀcost, strongly stable, and ecoꢀfriendly catalyst provides
a versatile platform and has potential application in other catalytic reduction reactions.
Acknowledgements
This work is supported by the Fundamental Research Funds for the Central Universities
(lzujbkyꢀ2015ꢀ21).
Notes and references
Supplementary data associated with this article can be found, in the online version.
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Graphical Abstract
The easily prepared costꢀeffective γꢀFe2O3/MC catalyst was used for the
hydrogenation of halogenated nitrobenzenes with 100% selectivity for the
corresponding halogenated anilines.