Synergistic effect from Lewis acid and the Ni-W2C/AC catalyst for highly active and selective hydrogenation of aryl nitro to aryl amine

Article abstract of DOI:10.1039/c4ra01808g

This work presents a facile approach for clean and chemoselective synthesis of various functionalized arylamines from their corresponding substituted nitroarenes through the unexpected synergistic effect of a Lewis acid and the Ni-W₂C/AC catalyst, affording almost 100% arylamine yield. The results challenge the long-held axiom that the combination of Lewis acid and hydrogenation catalyst mainly enhances the transformation of nitrobenzene (NB) to p-aminophenol via Bamberger rearrangement of the formed intermediate phenylhydroxylamine (PHA) under catalytic hydrogenation conditions. X-ray diffraction (XRD) and FT-IR spectroscopy were employed to reveal the relationship between catalyst nature and catalytic performance, and a plausible reaction mechanism is also proposed. Reaction results demonstrate that the FeCl₃-Ni-W₂C/AC catalytic system shows comparable catalytic performance towards precious metals for chemoselective reduction of various aromatic nitro compounds, affording 100% yield for all substrates involved in this work (99.5% of isolated yield for model substrate). Moreover, it can be found that the catalyst could be easily recovered by filtration and recycled without visible loss of its catalytic activity. Therefore, the developed FeCl₃-Ni-W₂C/AC catalytic system in this work can be considered as a practical candidate for clean and highly-efficient synthesis of diverse functionalized arylamines. We believe this approach can be extended to the other hydrogenation reactions. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra01808g

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Synergistic effect from Lewis acid and the Ni–
W2C/AC catalyst for highly active and selective
hydrogenation of aryl nitro to aryl amine†
Cite this: RSC Adv., 2014, 4, 22669
*
Zhongkui Zhao, Hongling Yang and Yu Li
This work presents a facile approach for clean and chemoselective synthesis of various functionalized
arylamines from their corresponding substituted nitroarenes through the unexpected synergistic effect
of a Lewis acid and the Ni–W2C/AC catalyst, affording almost 100% arylamine yield. The results
challenge the long-held axiom that the combination of Lewis acid and hydrogenation catalyst mainly
enhances the transformation of nitrobenzene (NB) to p-aminophenol via Bamberger rearrangement of
the formed intermediate phenylhydroxylamine (PHA) under catalytic hydrogenation conditions. X-ray
diffraction (XRD) and FT-IR spectroscopy were employed to reveal the relationship between catalyst
nature and catalytic performance, and a plausible reaction mechanism is also proposed. Reaction results
demonstrate that the FeCl3–Ni–W2C/AC catalytic system shows comparable catalytic performance
towards precious metals for chemoselective reduction of various aromatic nitro compounds, affording
100% yield for all substrates involved in this work (99.5% of isolated yield for model substrate). Moreover,
it can be found that the catalyst could be easily recovered by filtration and recycled without visible loss
of its catalytic activity. Therefore, the developed FeCl3–Ni–W2C/AC catalytic system in this work can be
considered as a practical candidate for clean and highly-efficient synthesis of diverse functionalized
arylamines. We believe this approach can be extended to the other hydrogenation reactions.
Received 1st March 2014
Accepted 14th May 2014
DOI: 10.1039/c4ra01808g
www.rsc.org/advances
have oen failed to meet the dual requirements of activity and
selectivity. On one hand, the most common commercially
Introduction
available RANEY® nickel and palladium catalysts are widely
employed,15–18 though they show high activities for nitroben-
zene hydrogenation, the selectivity to the hydrogenation of
many functionalized nitroarenes, especially chloroni-
trobenzene, still remains low because of co-existing dehaloge-
nation reaction. On the other hand, platinum, rhodium and
ruthenium based catalysts have also been deeply investi-
gated,19–28 although they have shown interesting catalytic
performance under mild reaction conditions, they are not
suitable for an industrial expansion due to their high cost and
limited availability. Based on the above issues, there are
emerging numerous non-noble metal catalysts such as Fe, Co,
Ni, Cu and Zn based catalysts,29–41 whereas, the high hydroge-
nation ability of Ni based catalysts usually proceeds the
hydrogenation of other groups existing in the arene substrates,
leading to undesirable byproducts or the use of Ni–B, Ni–P,
NiPB and NiCoB metallic systems have been examined as
possible means of enhancing their selectivity.42–49 However, the
associated complexity and costs militate against the viability of
these approaches in terms of process scale up. Besides Ni based
catalysts, Cu based catalysts have also been found to be selective
for this reaction, but these catalysts were not active enough even
under harsh reaction conditions.37 Co based catalysts are
another choice, breakthrough progress has been made in recent
Functionalized arylamines serve as industrially important
organic intermediates for producing pharmaceuticals,
dyestuffs, functional polymers and rubbers.1–3 The chemo-
selective hydrogenation of diverse nitroarenes has been
considered as an efficient and clean strategy to synthesize the
corresponding functionalized arylamines, and is one of the
most active topics in applied and fundamental catalytic
research. For its importance, a number of methods, including
stoichiometric reducing agents,4–6 metal-catalyzed hydrogena-
tion featuring various reducing agents to reduce nitroarenes
have been developed.7–11 Among them, catalytic hydrogenation
is of particular interest owing to its environmental friendliness,
atomic efficiency, and compatibility with industrial
processes.12–14 The sustainable development demands highly
active and selective catalysts for the hydrogenation of
substituted nitroarenes. Unfortunately, the existing catalysts
State Key Laboratory of Fine Chemicals, Department of Catalysis Chemistry and
Engineering, School of Chemical Engineering, Dalian University of Technology, 2
Linggong Road, Dalian 116024, China. E-mail: zkzhao@dlut.edu.cn; Fax: +86-411-
84986354
† Electronic supplementary information (ESI) available: Degree of GC sensitivity,
XRD, H2-TPR, GC-MS, LC-MS and 1H-NMR analytic results. See DOI:
10.1039/c4ra01808g
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days, nevertheless, expensive ligand and high pressure are
essential for promoting the reaction to proceed smoothly,
recycling problems still exist in this catalytic system.50 Above all,
we can conclude that precious metals exhibit excellent activity
in the hydrogenation of nitroarenes compared with non-
precious metals, however, considering the limits of their
resources and high price, thus there is now a pressing need to
develop an inexpensive and efficient non-noble metal catalysts
replacing the noble metals to promote the selective hydroge-
nation of aromatic nitroarenes.
Transition-metal carbides, as Pt-like metals have attracted
much attention as promising material for their outstanding
bulk and surface physicochemical properties,51,52 therefore it's
eager to employ transition metal carbide catalyst in the hydro-
genation of nitroarenes with excellent activity and high selec-
tivity. According to the reported literature that transition metal
carbide had been attempted as additives or supports for
precious metal catalyzed hydrogenation of nitroarenes using
hydrazine hydrate as a hydrogen source, but the usage of
precious metals is indispensable.53,54 Many reports have
demonstrated that the supported metal carbide exhibits higher
catalytic performance compared with the unsupported carbide
catalysts, ascribed to high surface area, high sintering-tolerance
stability, and the possible metal–support interaction.55–59 In our
precious work, we have employed supported transition metal
molybdenum carbide catalysts in the hydrogenation of func-
tionalized nitroarenes using hydrazine hydrate as hydrogen
source,60 therefore, transition metal carbide has great potential
for the hydrogenation of nitroarenes under the condition of
hydrogen. From the viewpoint of economic and clean produc-
tion, to replace hydrazine hydrate by hydrogen is highly desir-
able. In this approach, the catalytic performance of metal
carbide catalysts for chemoselective hydrogenation of nitro-
arenes using hydrogen as reducing agent has been investigated.
It has been reported that the existing synergistic effect
between Lewis acid and various catalysts can notably enhance
the diverse hydrogenation reactions.61–69 However, in the pres-
ence of Lewis acid, due to the Bamberger rearrangement of the
formed intermediate phenylhydroxylamine (PHA) in process of
catalytic hydrogenation of nitrobenzene, the produced chemical
is p-aminophenol but not aniline.70–72 From the reference, the
existence of CO2 may enhance the catalytic performance of Ni
for the hydrogenation of nitrobenzene to aniline due to the dual
roles of Lewis acid and base of CO2.69 Due to the weak basic
properties of metal carbide,73,74 we speculate that the addition of
Lewis acid into the reaction systems to cooperate with transi-
tion metal carbides may exhibit high catalytic performance for
the hydrogenation of nitroarenes.
Scheme 1 A comparison of the Ni–W2C/AC@Lewis acid in this work
and traditional catalyst@Lewis acid for catalytic hydrogenation of
nitroaromatics.61–69
challenges the long-held axiom that the combination of Lewis
acid and hydrogenation catalyst mainly enhances the trans-
formation of nitrobenzene (NB) to aminophenol under hydro-
genation conditions (Scheme 1). The developed strategy of
Lewis acid–metal carbide synergistically catalyzed hydrogena-
tion of nitro group can be extended to various nitroarenes, can
be considered as an efficient and low-cost alternative to
precious metal catalysts for functionalized arylamine produc-
tion. The developed precious metal-free catalytic system can be
recycled eight times without visible loss in catalytic perfor-
mance and it is also extended to various substrates, suggesting
its industrial interest. This nding can pave a new avenue for
the use of synergism between catalyst and Lewis acid in the
other organic transformations.
Experimental section
General procedure for catalyst preparation
The as-received activated carbon derived from coconut shells
(Aladdin, special for catalyst support, China) was ground and
sieved into powders with grain size less than 62 mm for preparing
catalyst support. The surface modication process of activated
carbon via H2O2 oxidation was performed as following: a given
amount of ground activated carbon was immersed into a 35 wt%
H2O2 aqueous solution (10 ml gꢀ1 AC) at 5–20 C with continu-
ꢁ
ously stirring for 6 h. The sample was ltered, washed with
deionized water, and subsequently dried at 105 ꢁC overnight, and
then the AC supports were obtained. Aerwards, An aqueous
solution of M (M ¼ Fe, Co, Ni and Cu) nitric acid compounds and
ammonium tungstate solution containing desired M and W
contents were impregnated onto the AC support using incipient
wetness impregnation (IWI) method as reported in literatures,75,76
followed by drying overnight and calcinating at a certain
temperature in N2 atmosphere to obtain MOy–WOx/AC, subse-
quently, the series of MOy–WOx/AC were carburized in CH4–H2
mixture based on references,77–80 nally passivated in 1% O2–N2
when the temperature cooled to room temperature. The resultant
sample are denoted as M–W2C/AC catalysts.
In this work, we present the rst approach of the efficient
and chemoselective hydrogenation of nitrobenzene to aniline
over the Ni–W2C/AC catalyst on the basis of unexpected syner-
gistic effect (100% conversion and 100% selectivity). In this
approach, no rearrangement but only highly selective hydro-
genation products can be detected over Ni–W2C/AC catalyst in
the presence of Lewis acid, as well as obvious enhancing effect
Characterization
of adding Lewis acid can be observed due to the unexpected X-ray diffraction (XRD) patterns were collected from 10 to 80ꢁ at
synergism between Lewis acid and Ni–W2C/AC, which
a
step width of 0.02ꢁ using Rigaku Automatic X-ray
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Diffractometer (D/Max 2400) equipped with a CuKa source (l ¼ through evaporation method to the theoretical value based on
˚
1.5406 A). H2 temperature-programmed reduction (H2-TPR) the amount of nitrobenzene.
experiments were performed in an in-house constructed system
equipped with a TCD (thermal conductivity detector) to
Results and discussion
Characterization of the catalyst
measure H2 consumption. A quartz tube was loaded with 100
mg of catalyst which was pretreated by calcination in Ar at
300 ꢁC for 30 min and then was cooled to ambient temperature
We prepared the supported metal carbides (Mo2C and W2C) and
the modied W2C (M–W2C/AC, M ¼ Fe, Co, Ni and Cu) catalysts
in Ar. Aer that, it was reduced with a 10 vol% H2–Ar mixture
(30 ml minꢀ1) by heating up to 800 C at a ramp rate of 10 C
minꢀ1. The FTIR was used to examine the interactions of
reacting species with catalytic system. The FTIR spectra of NB,
nitrosobenzene (NSB), and PHA with catalytic systems like
single Lewis acid, Ni–W2C/AC and the combined Lewis acid and
Ni–W2C/AC system (aer suffering from reaction temperature
and pressure but not adding H2) were collected with the same
spectrometer, as well as the FTIR spectra of pure liquids con-
taining NB, NSB or PHA, respectively were also collected for
comparison.
ꢁ
ꢁ
on AC according to the reported carbothermal reduction
method.81,82 XRD and H2-TPR experiments were performed to
further probe the roles of adding transition metals in improving
the catalytic performance. Fig. 1A presents the XRD patterns of
as-prepared W2C/AC catalysts with and without transition metal
modication. The diffraction peaks at 34.32, 37.79, and 39.40ꢁ
were detected, assigned to W2C phases with hexagonal closed
packed structure (PDF#20-1315), and the diffraction peaks
existing at 48.16ꢁ can be assigned to WC phase (PDF#65-8828).
The visible peaks towards Ni, Fe, Co, Cu on the modied,
associated with the no obvious shi for the peaks assigned to
W2C or WC, implying no obvious insertion of transition metal
Catalytic performance measurement
Catalytic measurements were performed in a 50 ml stainless
steel autoclave batch reactor. In a typical experiment, 50 mg of
the as-prepared catalyst, 15 g of the solvent (cyclohexane,
Tianjin Fuyu Chemicals Corp., Ltd) and 4.1 mmol of nitroben-
zene (Tianjin Beilian Fine Chemicals Corp., Ltd), and the
desired amount of Lewis acids were put into the reactor and
purged 3 times with N2 to replace the air in the reactor and
purged 3 times with H2 to replace the N2 for 3 times at room
temperature respectively. In our experiments, all the reagents
were analytical grade and used without further purication. The
reaction was then carried out at 240 ꢁC and a H2 pressure of 2.5
MPa for 2.5 h under vigorous stirring (1000 rpm), the pressure
in the reactor gradually dropped, indicating the conversion of
nitrobenzene. Aer appropriate reaction time, the temperature
was lowered quickly in an ice-water bath and the hydrogen
pressure was released, then the catalysts were separated from
the products by easily ltration, washed thoroughly with
cyclohexane three times, and then reused in the next catalytic
cycle under the same conditions. The ltrate and washing
liquid were combined to obtain the mixture for analysis. The
mixture was quantitatively analyzed by Gas Chromatography
(GC) and Liquid Chromatography (LC). The molecular struc-
tures of the obtained series of arylamines were conrmed by
Mass Spectrometer (MS) and 1H NMR (Fig. S5 and S6, in ESI†).
The conversion was calculated by weight percent of the
consumed aromatic nitro compounds in the added total
amount; the selectivity to arylamines was calculated by weight
percent of the desired product in the total products. The yields
included in this paper are the GC or LC yield, except for the
scaled-up experiment. The separated and washed catalyst was
dried and reused for recyclability measurement without extra
catalyst being added. In order to probe the perspective of
practical application in industrial production towards the
developed catalyst, the scaled-up experiment by factor 10 was
performed on the basis of above procedure, and the isolated
Fig. 1 (A) XRD patterns and (B) H2-TPR patterns of modified and
yield was calculated by the percentage of the separated aniline unmodified W2C/AC catalysts.
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Table 1 Hydrogenation of nitrobenzene over various catalystsa
into metal carbide matrix. The supported W2C catalyst on AC
can be prepared via carbothermal reduction method,81 we per-
formed the H2-TPR experiments on the oxide state precursors of
the diverse transition metals modied supported W2C catalysts
to investigate the effect of the addition of transition metals on
the redox of supported WO3 by using the same amount of
samples. The proles are presented in Fig. 1B. There are two or
three peaks on the proles of the modied catalysts. The peaks
at low temperature can be assigned to the reduction of high
valent W species to low valent ones, and the reduction of tran-
sition metal oxides to metals; as well as the last peak can be
indexed as the reduction of low valent W oxide to zero valent
metal. The latter is crucial for the formation of metal carbide.
From Fig. 1B, the last peaks corresponding to the reduction of
lower WOx to metal W shows a shi to lower temperature in
some degree while the diverse transition metals except for Cu
were added, suggesting the addition of Fe, Co or Ni enhances
the reduction of WOx, possibly arising from the transition
metals promoted H2 dissociation to assist the reduction of
WOx.81 As a result, the formation of W2C and WC phases was
enhanced, consistent with the previous reports.81,83 This
promoting process can be further proved by the XRD in Fig. 1A,
and the more sharp and strong peaks towards carbide on the
modied W2C/AC than on the unmodied one can be observed,
suggesting that the addition of transition metals doesn't change
the dispersity of W2C on the support. The promotion effect of
adding transition metals on the formation of carbide mainly
resulted from the enhanced H2 dissociation on transition
metals.84 This kind of promoting effect can efficiently improve
the catalytic performance of supported W2C catalyst on AC for
chemoselective hydrogenation of nitro groups. Moreover,
although the reduction peak corresponding to NiO on the NiO–
WO3/AC sample shis to higher temperature in comparison of
that on 5% NiO/AC, the added Ni also can promote the hydro-
genation of nitro compounds, since the Ni is active for the H2
dissociation. That is to say, the addition of transition metals,
especially Ni can synergistically promote the W2C/AC catalyzed
hydrogenation reaction through the two possible aspects: one is
to promote the formation metal carbide, the other is its
inherent catalysis on this reaction.
Entry
Catalyst
Sel./Yieldb (%)
1
None
100/4.9
2
AC
100/4.9
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
25% Mo2C/AC
25% W2C/AC
15% W2C/AC
20% W2C/AC
100/17.3
100/35.0
100/22.0
100/28.0
100/32.0
100/43.7
100/45.0
100/52.0
100/39.7
100/36.0
100/48.4
100/50.7
100/47.0
100/19.0
100/42.0
30% W2C/AC
5% Fe 25% W2C/AC
5% Co 25% W2C/AC
5% Ni 25% W2C/AC
5% Cu 25% W2C/AC
3% Ni 25% W2C/AC
4% Ni 25% W2C/AC
6% Ni 25% W2C/AC
7% Ni 25% W2C/AC
5% Ni/AC
5% Ni/AC + 25% W2C/AC
a
Reaction conditions: 50 mg catalyst, 4.1 mmol nitrobenzene, P ¼ 2.5
H2
MPa, T ¼ 240 ꢁC, t ¼ 2.5 h. Detected by GC-MS and 1H-NMR.
b
supported metal carbide catalysts on AC, especially Ni modied
W2C/AC to be promising catalysts for the chemoselective
hydrogenation of nitrobenzene with 100% of selectivity. Only
4.9% of yield in the blank experiment or over bare AC support
conrm the essential catalytic role of W2C/AC in this trans-
formation. 25% of optimal W2C loading is required for
achieving high yield. However, the catalytic activity is depressed
if the supported carbide is solely employed, which is consistent
with the reported results.60 According to the previous reports,
Fe, Co, Ni are active catalysts for the hydrogenation of nitro-
benzene, although the selectivity is not satisfactory,7,37,42 as well
as the Fe, Co and Ni are demonstrated to promote the formation
of molybdenum carbides,82,83 which suggest the possibility of
synergistic interaction between Fe, Co or Ni and Mo2C. There-
fore, we added some transition metals and investigated the
promoting effect on the Mo2C/AC catalyst. Interestingly, we
illustrate that the addition of a small amount of non-precious
transition metals, especially Ni can dramatically improve the
conversion from 35% to 52% (100% selectivity). The Ni loadings
were optimized to improve the catalytic performance by
diversing the loading from 3 to 7% (entries 10–15, Table 1), and
5% of optimum Ni loading is indispensable to achieve 52% of
higher yield. In order to elucidate the promoting effect of the
added Ni on the catalytic performance of the supported W2C
catalyst on AC for the chemoselective hydrogenation of nitro-
arenes, the two samples 25% W2C/AC and 5% Ni/AC were also
tested. In comparison of the 25% W2C/AC or 5% Ni/AC, the
incorporated catalyst (5% Co–25% W2C/AC) exhibits unexpect-
edly better catalytic activity, even much better than the physi-
cally mixed 25% W2C/AC or 5% Ni/AC, implying the existence of
synergistic effect between Ni and W2C for chemoselective
hydrogenation.
Optimization of reaction
The nitrobenzene was used as a probe molecule to evaluate the
catalytic performance of the obtained catalysts for hydrogena-
tion of nitroarenes. The selectivity and yield were calculated
based on GC analytical results. Table 1 illustrates the catalytic
properties of various catalysts without the addition of Lewis
acid. As we all know, the possible existing hydrogenation
intermediates like nitrosobenzene (NSB) or PHA in reaction
mixture may be highly carcinogenic, and therefore to establish
high sensitive analysis method and to investigate the possible
existence of trace nitroso or phenylhydroxylamine are very
important issues. In our previous report,60 it has been estab-
lished that our analysis method is sensitive enough to exclude
the existence of the carcinogenic nitroso and phenylhydroxyl-
amine intermediates. From Table 1, it can be observed that the
Through adding Ni into W2C/AC catalyst, the catalytic
performance of supported carbide catalyst has been notably
improved. However, the only 52% of yield can be obtained. The
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improvement in catalytic activity of Ni–W2C/AC for chemo- which are signicantly higher than those on Ni–W2C/AC (52%,
selective hydrogenation is highly desirable. The synergistic entry 2 in Table 2) or only in the presence of FeCl3 (9.2%, entry 1
effect between Lewis acid and hydrogenation catalyst on effi- in Table 2), which suggests the existence of strong synergism
ciently promoting the other hydrogenation process but not between Lewis acid and Ni–W2C/AC. According to the reported
nitro group hydrogenation have been reported,61–69 we proposed literatures, the iron complex catalysts may be efficient for
the addition of Lewis acid into the hydrogenation reaction hydrogenation of nitroarenes,85 we investigated the catalytic
systems over Ni–W2C/AC catalyst may signicantly promote the performance of the catalytic systems containing Ni–W2C/AC
hydrogenation of nitroarenes. Once the improved chemo- and non-Lewis acidic Fe(NO3)3 (or FeCl2), but only 64 or 74% of
selective hydrogenation of nitro on the basis of synergistic effect yield can be obtained, respectively. In combination of the above
between Lewis acid and Ni–W2C/AC catalyst can be successfully results, the unexpected synergism between Ni–W2C/AC and
established, the concept for Lewis acid promoting Bamberger Lewis acid especially FeCl3 signicantly enhances the catalytic
rearrangement of the formed intermediate phenylhydroxyl- hydrogenation of nitrobenzene. More interestingly, by using the
amine (PHA) in process of catalytic hydrogenation of nitro- catalytic system including Ni–W2C/AC and FeCl3, the aniline is
benzene to produce p-aminophenol but not aniline,70–72 would the only product, no p-aminophenol can be detected, which
be broken.
challenges the long-held axiom that the combination of Lewis
Under the same conditions, the catalytic hydrogenation acid and hydrogenation catalyst mainly enhances the trans-
reactions of nitrobenzene over Ni–W2C/AC in the presence of formation of nitrobenzene (NB) to p-aminophenol under
Lewis acid like FeCl3, AlCl3 or SnCl2 were performed, and the hydrogenation conditions. That is to say, by using the developed
100, 97 or 97.3% (entry 2, 3 and 4, respectively, Table 2) of Ni–W2C/AC catalyst, the addition of Lewis acid notably
unexpectedly high yield with 100% selectivity can be obtained, enhances the chemoselective hydrogenation of nitro group, but
not Bamberger rearrangement of the formed intermediate PHA.
A plausible reaction mechanism is proposed (Scheme 2).
Table 2 Hydrogenation of nitrobenzene over 5% Ni–25% W2C/AC
Metal carbide is most active catalyst for hydrogen-concerned
catalysts with and without the addition of different promoting
reactions,51–54 Ni–W2C/AC can activate H2 to produce active H
additivesa
for the reaction. The Lewis base (Ni–W2C/AC)53,54 and acid
(FeCl3) may interact with the substrate and the intermediates to
weaken the NO2, N]O and N–O bonds (can be illustrated by the
peak shi in the FTIR, Fig. S1†), and then they are hydrogenated
by the active H. As a result, the chemoselective hydrogenation of
nitroarenes can be efficiently enhanced by the catalytic systems
containing Ni–W2C/AC catalyst and Lewis acid. From our
previous report,60 it can be proposed that the NSB and PHA are
possible intermediates in the reduction process of nitrobenzene
by Lewis acid promoted Ni–W2C/AC catalytic system. In details,
rstly, the N–O of NO2 is activated by the coordination of Ni–
W2C/AC with N atom and the Lewis acid with O, and the acti-
vated H by Ni–W2C/AC interacts with the activated N–O
bond, and as a result, the intermediate Ar–NO is formed. The
Entry
Catalyst
Additives
Sel./Yieldb (%)
1
2
3
4
5
6
7
8
9
—
Anhydrous FeCl3
—
100/9.2
5% Ni 25% W2C/AC
5% Ni 25% W2C/AC
5% Ni 25% W2C/AC
5% Ni 25% W2C/AC
5% Ni 25% W2C/AC
5% Ni 25% W2C/AC
5% Ni 25% W2C/AC
5% Ni 25% W2C/AC
100/52.0
100/100
100/97.0
100/97.3
100/64.0
100/74.0
100/100c
100/100d
Anhydrous FeCl3
Anhydrous AlCl3
Anhydrous SnCl2
Anhydrous Fe(NO3)3
Anhydrous FeCl2
Anhydrous FeCl3
Anhydrous FeCl3
a
Reaction conditions: 50 mg catalyst, 0.02 g additives 4.1 mmol
b
nitrobenzene, PH ¼ 2.5 MPa, T ¼ 240 ꢁC, t ¼ 2.5 h. Detected by GC-
2
MS and 1H-NMR. The nitrosobenzene being used as substrates.
c
d
The phenylhydroxylamine being used as substrates.
Scheme 2 Plausible mechanism of the dual activation in nitrobenzene hydrogenation by Ni–W2C/AC and Lewis acid.
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Table Summary on the chemoselective hydrogenation of Table 3 (Contd.)
Paper
3
substituted nitroarenes over the developed catalytic system containing
Ni–W2C/AC catalyst and Lewis acida
FeCl3 PH
Time Sel./Yieldb
2
Entry Substrate
Product
(g)
(MPa) (h)
(%)
FeCl3 PH
Time Sel./Yieldb
2
Entry Substrate
Product
(g)
(MPa) (h)
(%)
14
0.04
3.0
3.0
4.5
4.5
100/100
1
0.02
2.5
2.5
2.5
2.5
100/100
15
0.04
0.04
100/100
100/100
2
0.02
0.02
0.04
0.04
100/100c
100/99.5d
99.5/98.5
99/99.0
3
2.5
3.0
3.0
2.5
5.0
5.0
16
3.0
4.5
4e
5e
a
Reaction conditions: 50 mg catalyst, 4.1 mmol substrates, T ¼ 240 ꢁC.
b
Detected by GC-MS and 1H-NMR. c Scaled up by factor 10. d Separated
e
yield for c. T ¼ 220 ꢁC.
Ni–W2C/AC and Lewis acid synergistically catalyze the above
process. Secondly, the Ar–NO is further activated and hydroge-
nated to Ar–NHOH, and then the formed Ar–NH–OH is further
activated and hydrogenated to produce the nal product Ar–
NH2 though the similar activation process as above. In order to
further shed light on the above mechanism, the control exper-
iments by using nitrosobenzene and phenylhydroxylamine
intermediates as substrates in the same reaction conditions as
those for chemoselective reduction of nitrobenzene were per-
formed, and the reaction results are listed in Table 2 (entries 8
and 9). The reaction results demonstrate that the aniline is the
only product (Fig. S2–S4†) for the nitrosobenzene, phenyl-
hydroxylamine or nitrobenzene as substrate of chemoselective
reduction. Correlated to the former mechanism analysis and
reference, the control experimental results further conrm the
above proposed reaction route for the chemoselective reduction
of nitro group over the developed catalytic system containing
Ni–W2C/AC catalyst and Lewis acid in this work.
6e
0.04
2.8
4.0
100/100
7
8
0.05
0.06
3.2
3.2
5.0
5.0
100/100
100/100
9
0.04
0.04
3.2
3.0
5.0
5.0
100/100b
100/100
10e
Scope of the substrates
To investigate the scope of the developed catalytic system in this
work, the hydrogenation reactions of structurally diverse nitro
compounds were performed, and the optimized reaction results
are presented in Table 3. From Table 3, the Ni–W2C/AC catalyst–
Lewis acid system has excellent selectivity and yield for the
hydrogenation of various functionalized nitroaromatics under
the optimized reaction conditions. Interestingly, halogen
substituted nitrobenzenes were reduced to corresponding
chloroanilines in high yields and without any dehalogenation
(Table 3, entries 4–6). The reducible functional groups in reac-
tion substrates like OH, OCH3, COOC2H5 and COOH remained
totally unaffected under the reaction conditions (Table 2,
entries 7–11). Notably, the hydrogenation reactions of nitro-
aniline also occurred selectively to give the corresponding
11
12
13
0.04
0.04
0.04
3.5
3.0
3.0
5.0
5.0
5.0
100/100
100/99
100/100
22674 | RSC Adv., 2014, 4, 22669–22677
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diphenylamines with a 100% selectivity (Table 2, entries 12 Therefore, we investigated the recyclability of our developed
and 13). The multiamino aromatic compounds, o-toluidine, catalytic for chemoselective hydrogenation reaction by using
m-toluidine, p-toluidine and nitronaphthalene, as key frag- nitrobenzene as a model substrate. The separated spent catalyst
ments in dyes can also be efficiently and selectively produced by was used only by washing with solvent and drying before use, as
the chemoselective hydrogenation of corresponding nitroarenes well as no extra fresh catalyst was added. Fig. 2 presents the
using Ni–W2C/AC catalyst–Lewis acid system (Table 2, entries 14– experimental results for recycling eight times. From the reac-
18). The above results exhibit the developed Ni–W2C/AC catalyst– tion results, we can observe that the developed catalytic system
Lewis acid system in this work is a promising catalyst for containing Ni–W2C/AC and Lewis acid can be reused by simply
the clean and efficient production of functionalized arylamines separation without losing its activity for more than eight runs of
via chemoselective hydrogenation reaction from their corre- the used catalyst, and there is no loss in the yields of aromatic
sponding nitroarenes, the vulnerable moieties in the various anilines, suggesting the potential catalyst for the chemo-
substrates were well tolerated during the reaction process in this selective hydrogenation of nitroarenes in practical applications.
approach.
Conclusions
Scaled-up catalytic properties
In summary, we present an interesting and practical approach
As we all know that the scaled-up catalytic properties and iso-
for the clean synthesis of various functionalized arylamines
lated yield are very important for a practical catalyst, and
from corresponding nitroarenes by the synergistically enhanced
thereby, using nitrobenzene as a probe substrate, the scaled-up
catalysis of Ni–W2C catalyst and Lewis acid whose role is
anomalous in our catalytic system, as the traditional catalytic
hydrogenation of nitrobenzene is to produce phenylhydroxyl-
amine (PHA) in acid medium and further rearrange to p-ami-
experiment by factor 10 was performed, and the GC yield and
isolated yield are presented in Table 3 (entries 2 and 3). Reac-
tion results show that no difference in selectivity and GC yield
take place if the reaction is scaled-up by factor 10, as well as
nophenol (PAP), during the process of analyzing the nal
99.5% of isolated yield can be obtained, which is very close to
reaction mixture, no PAP was detected. Our developed catalytic
the GC yield. In our previous report,60 it was found that the
system containing Ni–W2C catalyst and Lewis acid demon-
transition metal modied carbide on AC is so stable in the
strated excellent catalytic properties, ascribing to the remark-
reduction systems that no leaching may be detected. In this
ably synergistic effect between Ni–W2C catalyst and Lewis acid.
work, without adding extra fresh catalyst, the separated catalyst
can be reused leaching of Ni or W is negligible. From above, the
Separation of the product from the catalyst–Lewis acid system is
simple, and the catalytic system can be reused directly. This
non-precious without loss in catalytic performance, and there-
catalytic system has great potential for industrial application. In
fore we think the metal Ni–W2C/AC catalysed chemoselective
hydrogenation could be a practical approach for the production
view of the importance of functionalized arylamines in the
industrial manufacture. The developed catalytic system in this
of functionalized arylamines.
work, once complete developed, may open an avenue for the
production of valuable amine-containing chemicals from nitro
Recyclability
groups. In addition, the substitution of precious metal catalysts
In industrialized production process, the catalyst recycling is
with less-expensive transition metal carbides in various trans-
denitely important in the heterogeneous catalytic reactions.
formation reactions will be of great importance due to the
limited availability and high cost of noble metals. This
approach can provide some guidance of developing Lewis acid
promoted catalytic systems in the other applications.
Acknowledgements
This work is nancially supported by the Joint Fund of Coal, set
up by National Natural Science Foundation of China and
Shenhua Co., Ltd. (U1261104), the Chinese Ministry of Educa-
tion via the Program for New Century Excellent Talents in
University (NCET-12-0079) and the Fundamental Research
Funds for the Central Universities (DUT12LK51).
Notes and references
1 A. M. Tafesh and J. Weiguny, Chem. Rev., 1996, 96, 2035.
2 H.-U. Blaser, C. Malan, B. Pugin, F. Spindler, H. Steiner and
M. Studer, Adv. Synth. Catal., 2003, 345, 103.
Fig. 2 Recyclability test of the developed 5% Ni 25% W2C/AC catalyst–
Lewis acid catalytic system for chemoselective hydrogenation of
nitrobenzene to yield aniline. Reaction conditions: 50 mg catalyst,
0.02 g FeCl3, 4.1 mmol nitrobenzene, PH ¼ 2.5 MPa, T ¼ 240 ꢁC, t ¼
3 H.-U. Blaser, H. Steiner and M. Studer, ChemCatChem, 2009,
2
2.5 h.
1, 210.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 22669–22677 | 22675

View Article Online
RSC Advances
Paper
4 A. Saha and B. Ranu, J. Org. Chem., 2008, 73, 6867.
5 X. Wang, D. Liu, S. Song and H. Zhang, Chem.–Eur. J., 2013,
19, 5169.
31 W. Y. Wang, D. S. Wang, X. W. Liu, Q. Peng and Y. D. Li,
Chem. Commun., 2013, 49, 2903.
´
32 M. Baron, E. Metay, M. Lemaire and F. Popowycz, Green
¨
6 G. Wienhofer, I. Sorribes, A. Boddien, F. Westerhaus,
Chem., 2013, 15, 1006.
K. Junge, H. Junge, R. Llusar and M. Beller, J. Am. Chem. 33 U. Sharma, N. Kumar, P. K. Verma, V. Kumar and B. Singh,
Soc., 2011, 133, 12875. Green Chem., 2012, 14, 2289.
7 K. Junge, B. Wendt, N. Shaikh and M. Beller, Chem. 34 S. K. Mohapatra, S. U. Sonavane, R. V. Jayaram and
Commun., 2010, 46, 1769. P. Selvam, Tetrahedron Lett., 2002, 43, 8527.
8 Y. J. Jang, S. Kim, S. W. Jun, B. H. Kim, S. Hwang, I. Song, 35 U. Sharma, P. Kumar, N. Kumar, V. Kumar and B. Singh, Adv.
B. M. Kim and T. Hyeon, Chem. Commun., 2011, 47, 3601.
9 P. P. Sarmah and D. K. Dutta, Green Chem., 2012, 14, 1086.
10 H. Lin, L. Wang, H. Sun, J. Ni, Y. Cao, H. Y. He and K. N. Fan,
Angew. Chem., Int. Ed., 2009, 48, 9538.
Synth. Catal., 2010, 352, 1834.
36 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.
11 G. Wienh, I. Sorribes, A. Boddien, F. Westerhaus, K. Junge,
Chem., 2013, 5, 537.
H. Junge, R. Llusar and M. Beller, J. Am. Chem. Soc., 2011, 37 T. Shi, H. Li, L. Yao, W. Ji and C.-T. Au, Appl. Catal., A, 2012,
133, 12875. 425, 68.
12 J. Glckler, S. Ktzke, W. Meyer-Zaika, A. Reller, F. J. Garca- 38 W. Lin, H. Cheng, J. Ming, Y. Yu and F. Zhao, J. Catal., 2012,
Garca, H.-H. Strehblow, P. Keller, E. Rentschler and
W. Klui, Angew. Chem., Int. Ed., 2007, 46, 1164.
13 H. Duan, D. Wang, Y. Kou and Y. Li, Chem. Commun., 2013,
49, 303.
291, 149.
39 V. Mohan, C. V. Pramod, M. Suresh, K. H. P. Reddy, B. David
Raju and K. S. Rama, Catal. Commun., 2012, 18, 89.
40 J. Relvas, R. Andrade, F. G. Freire, F. L. P. Araujo, M. J. Pinho,
C. P. Nunes and F. R. Ribeiro, Catal. Today, 2008, 133, 828.
14 R. F. DVries, M. Iglesias, N. Snejko, S. Alvarez-Garcia,
E. Gutierrez-Puebla and M. A. Monge, J. Mater. Chem., 41 R. Xu, T. Xie, Y. Zhao and Y. Li, Nanotechnology, 2007, 18,
2012, 22, 1191. 55602.
15 J. M. Nadgeri, N. S. Biradar, P. B. Patil, S. T. Jadkar, A. Garade 42 H. Li, Y. Xu, H. Yang, F. Zhang and H. Li, J. Mol. Catal. A:
and C. V. Rode, Ind. Eng. Chem. Res., 2011, 50, 5478. Chem., 2009, 307, 105.
16 M. Heckenroth, V. Khlebnikov, A. Neels, P. Schurtenberger 43 H. Li, Q. Zhao, Y. Wan, W. Dai and M. Qiao, J. Catal., 2006,
and M. Albrecht, ChemCatChem, 2011, 3, 167. 254, 251.
17 C. Liang, J. Han, K. Shen, L. Wang, D. Zhao and 44 M. Lin, B. Zhao and Y. Chen, Ind. Eng. Chem. Res., 2009, 48,
H. S. Freeman, Chem. Eng. J., 2010, 165, 709. 7037.
18 P. Sangeetha, K. Shanthi, K. S. R. Rao, B. Viswanathan and 45 G. Xie, W. Sun and W. Li, Catal. Commun., 2008, 10, 333.
P. Selvam, Appl. Catal., A, 2009, 353, 160.
46 B. Zhao and Y. Chen, Mater. Chem. Phys., 2011, 125, 763.
´
¨
19 F. Cardenas-Lizana, C. Berguerand, I. Yuranov and L. Kiwi- 47 G. Xie, Z. Lu and G. Wang, Mater. Chem. Phys., 2009, 118,
Minsker, J. Catal., 2013, 301, 103.
281.
20 Y. Wu, S. Cai, D. Wang, W. He and Y. Li, J. Am. Chem. Soc., 48 H. Li, Q. Zhao and H. Li, J. Mol. Catal. A: Chem., 2008, 285, 29.
2012, 134, 8975.
21 V. Pandarus, R. Ciriminna, F. Beland and M. Pagliaro, Adv.
Synth. Catal., 2011, 353, 1306.
22 S. Cai, H. Duan, H. Rong, D. Wang, L. Li, W. He and Y. Li,
ACS Catal., 2013, 3, 608.
49 Y. Liu, C. Huang and Y. Chen, Ind. Eng. Chem. Res., 2006, 45,
62.
50 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. Chem., 2013, 5, 537.
´
23 I. Nakamula, Y. Yamanoi, T. Imaoka, K. Yamamoto and 51 A. M. Alexander and J. S. J. Hargreaves, Chem. Soc. Rev., 2010,
H. Nishihara, Angew. Chem., Int. Ed., 2011, 50, 5830. 39, 4388.
24 A. Corma, P. Serna, P. Concepcion and J. J. Calvino, J. Am. 52 V. Presser, M. Heon and Y. Gogotsi, Adv. Funct. Mater., 2011,
Chem. Soc., 2008, 130, 8748. 21, 810.
25 I. Nakamula, Y. Yamanoi, T. Yonezawa, T. Imaoka, 53 Y. J. Jang, S. Kim, S. W. Jun, B. H. Kim, S. Hwang, I. K. Song,
K. Yamamoto and H. Nishihara, Chem. Commun., 2008, 5716.
26 X. Cui, F. Shi and Y. Deng, ChemCatChem, 2012, 4, 333.
27 M. Liu, X. Mo, Y. Liu, H. Xiao, Y. Zhang, J. Jing, V. L. Colvin
and W. W. Yu, Appl. Catal., A, 2012, 439, 192.
B. M. Kim and T. Hyeon, Chem. Commun., 2011, 47, 3601.
54 G. Horanyi and G. Vertes, J. Chem. Soc., Perkin Trans. 1, 1973,
1400, 827.
55 A. R. S. Darujati and W. J. Thomson, Appl. Catal., A, 2005,
296, 139.
28 D. G. Galvez, P. Lara, O. Rivada-Wheelaghan, S. Conejero,
B. Chaudret, K. Philippot and P. W. N. M. Leeuwen, Catal. 56 S. S.-Y. Lin, W. J. Thomson, T. J. Hagensen and S. Y. Ha, Appl.
Sci. Technol., 2013, 3, 99. Catal., A, 2007, 318, 121.
¨
29 G. Wienhofer, I. Sorribes, A. Boddien, F. Westerhaus, 57 S. Naito, M. Tsuji and T. Miyao, Catal. Today, 2002, 77, 161.
K. Junge, H. Junge, R. Llusar and M. Beller, J. Am. Chem. 58 S. Naito, A. Takada, S. Tokizawa and T. Miyao, Appl. Catal., A,
Soc., 2011, 133, 12875.
2005, 289, 22.
30 S. Park, I. S. Lee and J. Park, Org. Biomol. Chem., 2013, 11, 59 T. Christofoletti, J. M. Assaf and E. M. Assaf, Chem. Eng. J.,
395.
2005, 106, 97.
22676 | RSC Adv., 2014, 4, 22669–22677
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
60 Z. K. Zhao, H. L. Yang, Y. Li and X. W. Guo, Green Chem., 74 I. Sorribes, G. Wienhçfer, C. Vicent, K. Junge, R. Llusar and
2014, 16, 1274. M. Beller, Angew. Chem., Int. Ed., 2012, 51, 7794.
61 H. Liu, T. Jiang, B. Han, S. Liang and Y. Zhou, Science, 2009, 75 Z. K. Zhao, X. L. Lin, R. H. Jin, Y. T. Dai and G. R. Wang,
326, 1250. Catal. Commun., 2011, 12, 1448.
62 M. P. Boone and D. W. Stephan, J. Am. Chem. Soc., 2013, 135, 76 Z. K. Zhao, R. H. Jin, T. Bao, X. L. Lin and G. R. Wang, Appl.
8508.
Catal., B, 2011, 110, 154.
63 J. Paradies, Synlett, 2013, 24, 777.
64 Y. Jiang, B. Schirmer, O. Blacque, T. Fox, S. Grimme and
H. Berke, J. Am. Chem. Soc., 2013, 135, 4088.
65 M. Chen, N. Maeda, A. Baiker and J. Huang, ACS Catal., 2012,
2, 2007.
77 J. B. Claridge, A. P. E. York, A. J. Brungs and M. L. H. Green,
Chem. Mater., 2000, 12, 132.
78 T. C. Xiao, A. P. E. York, V. C. Williams, H. A. Megren,
A. Hanif, X. Y. Zhou and M. L. H. Green, Chem. Mater.,
2000, 12, 3896.
66 A. S. Gowda, S. Parkin and F. T. Ladipo, Appl. Organomet. 79 A. Hanif, T. C. Xiao, A. P. E. York, J. Sloan and M. L. H. Green,
Chem., 2012, 26, 86. Chem. Mater., 2002, 14, 1009.
67 B. Xu, G. Kehr, R. Frhlich, B. Wibbeling, B. Schirmer, 80 N. M. Schweitzer, J. A. Schaidle, O. K. Ezekoye, X. Q. Pan,
S. Grimme and G. Erker, Angew. Chem., Int. Ed., 2011, 50,
S. Linic and L. T. Thompson, J. Am. Chem. Soc., 2011, 133,
7183.
23787.
68 B. Zhao, C. Chou and Y. W. Chen, Ind. Eng. Chem. Res., 2010, 81 N. Ji, T. Zhang, M. Y. Zheng, A. Q. Wang, H. Wang,
49, 1669.
X. D. Wang and J. G. G. Chen, Angew. Chem., Int. Ed., 2008,
69 X. Meng, H. Cheng, Y. Akiyama, Y. Hao, W. Qiao, Y. Yu,
F. Zhao, S. Fujita and M. Arai, J. Catal., 2009, 264, 1.
70 C. V. Rode, M. J. Vaidya and R. V. Chaudhari, Org. Process
Res. Dev., 1999, 3, 465.
47, 8510.
82 A. C. Lausche, J. A. Schaidle and L. T. Thompson, Appl.
Catal., A, 2011, 401, 29.
83 J. M. Cheng and W. Huang, Fuel Process. Technol., 2010, 91,
71 T. Komatsu and T. Hirose, Appl. Catal., A, 2004, 276, 95.
185.
72 J. M. Nadgeri, N. S. Biradar, P. B. Patil, S. T. Jadkar, 84 C. H. Liang, F. P. Tian, Z. L. Li, Z. C. Feng, Z. B. Wei and C. Li,
A. C. Garade and C. V. Rode, Ind. Eng. Chem. Res., 2011, 50,
Chem. Mater., 2003, 15, 4846.
5478.
85 A. N. Mahajan, D. S. Ozarde, R. M. Deshpande and
R. V. Chaudhari, US 2006/0100463 A1, 2006.
´
´
73 F. Cardenas-Lizana, D. Lamey, S. Gomez-Quero, N. Perret,
L. Kiwi-Minsker and M. A. Keane, Catal. Today, 2011, 173, 53.
This journal is © The Royal Society of Chemistry 2014
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