Cobalt-modified molybdenum carbide as an efficient catalyst for chemoselective reduction of aromatic nitro compounds

Article abstract of DOI:10.1039/c3gc42049c

This work presents a facile and clean transformation for synthesizing diverse functionalized arylamines through chemoselective reduction reaction of their corresponding substituted nitroarenes catalyzed by the supported cobalt-promoted molybdenum carbide catalyst on modified activated carbon (Co-Mo₂C/AC, AC is denoted as the modified activated carbon by H ₂O₂ oxidation treatment). Various characterization techniques including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), inductively coupled plasma (ICP) and H₂ temperature-programmed reduction (H₂-TPR) were employed to reveal the relationship between catalyst nature and catalytic performance, and the plausible reaction mechanism is also proposed. The characterization results suggest that the addition of a small amount of transition metals, especially cobalt could significantly promote the formation of a perfect molybdenum carbide crystal phase, resulting in the improvement in catalytic properties of the supported molybdenum carbide catalyst. Reaction results demonstrate that the optimized Co-Mo₂C/AC catalyst 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 (99.3% of isolated yield for model substrate). Moreover, it can be found that the catalyst could be easily recovered by filtration and recycled without obvious loss in its catalytic properties. Therefore, the developed Co-Mo₂C/AC catalyst in this work can be considered as an industrially viable and cheap candidate for clean and highly-efficient production of diverse functionalized arylamines.

Full text of DOI:10.1039/c3gc42049c

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Cobalt-modified molybdenum carbide as an
efficient catalyst for chemoselective reduction of
aromatic nitro compounds†
Cite this: DOI: 10.1039/c3gc42049c
Zhongkui Zhao,* Hongling Yang, Yu Li and Xinwen Guo
This work presents a facile and clean transformation for synthesizing diverse functionalized arylamines
through chemoselective reduction reaction of their corresponding substituted nitroarenes catalyzed by
the supported cobalt-promoted molybdenum carbide catalyst on modified activated carbon (Co–Mo2C/
AC, AC is denoted as the modified activated carbon by H2O2 oxidation treatment). Various characteriz-
ation techniques including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), inductively
coupled plasma (ICP) and H2 temperature-programmed reduction (H2-TPR) were employed to reveal the
relationship between catalyst nature and catalytic performance, and the plausible reaction mechanism is
also proposed. The characterization results suggest that the addition of a small amount of transition
metals, especially cobalt could significantly promote the formation of a perfect molybdenum carbide
crystal phase, resulting in the improvement in catalytic properties of the supported molybdenum carbide
catalyst. Reaction results demonstrate that the optimized Co–Mo2C/AC catalyst shows comparable cata-
lytic performance towards precious metals for chemoselective reduction of various aromatic nitro com-
pounds, affording 100% yield for all substrates involved in this work (99.3% of isolated yield for model
substrate). Moreover, it can be found that the catalyst could be easily recovered by filtration and recycled
without obvious loss in its catalytic properties. Therefore, the developed Co–Mo2C/AC catalyst in this
work can be considered as an industrially viable and cheap candidate for clean and highly-efficient pro-
duction of diverse functionalized arylamines.
Received 2nd October 2013,
Accepted 8th November 2013
DOI: 10.1039/c3gc42049c
www.rsc.org/greenchem
commercially available Ni or Pt catalysts cannot be used to
hydrogenate substituted nitrobenzenes with H2, since they
Introduction
Functionalized arylamines, that serve as industrially important lack the necessary chemoselectivity.2t Because of this lack of
organic intermediates for producing pharmaceuticals, selectivity and limited availability of the noble metals, Fe-
dyestuffs, functional polymers, etc.,1 are mainly produced by mediated catalysts have also been well developed, demonstrat-
chemoselective hydrogenation of the corresponding nitro- ing high activity and selectivity for the synthesis of arylamines
arenes over metallic catalysts including noble metals such as Pt, and realizing magnetic separation. However, adaptation for
Pd, Ru, Rh and non-noble metals such as Fe, Co, Ni, Cu and nitro-containing substrates and the reusability of the catalysts
Zn catalysts.2 In most cases, the precious metal catalysts were need to be improved. Moreover, sometimes harsh conditions
reported to be efficient for this transformation. In particular, are required for the reaction to proceed smoothly,2m–p and
the highly-efficient and magnetically recyclable Rh–Fe3O4 cata- therefore the catalytic activity needs to be further improved. Co
lyst has been reported.2a It is generally accepted that is an alternative chemoselective catalyst for the production of
functionalized arylamines.2r–t However, KOH as an additive is
required to promote the reaction on Co(II) substituted meso-
porous aluminophosphate; although Co(II) phthalocyanine has
State Key Laboratory of Fine Chemicals, Department of Catalysis Chemistry and
Engineering, School of Chemical Engineering, Dalian University of Technology,
high activity it was found to have poor reusability. The on-line
prepared supported Co catalyst on carbon exhibited exciting
catalytic activity, but the expensive ligands are indispensable.
Au, Ag and Cu have been found to be selective for this reac-
tion, but these catalysts were not active enough even under
harsh reaction conditions.3 Therefore, it is highly desirable
2 Linggong Road, Dalian 116024, China. E-mail: zkzhao@dlut.edu.cn;
Fax: +86-411-84986354
†Electronic supplementary information (ESI) available: The XRD, XPS, TPR, GC
results for typical reaction mixture and GC for establishing degree of detectable
sensitivity, GC results for reaction mixtures by using nitrosobenzene and phenyl-
hydroxylamine as substrates, GC-MS, LC-MS and 1H-NMR analytic results. See
DOI: 10.1039/c3gc42049c
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but challenging to develop a highly-efficient and less-expensive
catalyst for chemoselective reduction of substituted nitroar-
enes to synthesize their corresponding arylamines.
Experimental section
General procedure for catalyst preparation
In recent years, transition-metal carbides, such as Pt-like The as-received activated carbon derived from coconut shells
metals have attracted much attention as promising materials (Aladdin Chemical Co., for catalyst support, China) was
for their outstanding bulk and surface physicochemical pro- ground and sieved into a powder with grain size less than
perties.4 Previous reports reveal that metal carbides have been 62 μm for preparation of the catalyst support. The surface
used as efficient catalysts and have been applied in decomp- modification process of activated carbon via H2O2 oxidation
osition reactions of hydrazine, ammonia and methanol, was performed as follows. A given amount of ground activated
Fischer–Tropsch reaction, hydrogenolysis, water gas shift and carbon particles was immersed into 35 wt% H2O2 aqueous
electrocatalysis.5 Also they had been tried as additives or sup- solution (10 ml g−1 AC) at 5–20 °C with continuous stirring for
ports for precious metals to catalyze the reduction of nitro- 6 h. The sample was filtered, washed with deionized water,
arenes. But the use of precious metals is indispensable.2a,6 The and subsequently dried at 105 °C overnight, and then the AC
unique properties allow metal carbides to have great potential supports were obtained. Afterwards, an aqueous solution con-
in hydrogen-transferring related reactions. The addition of taining M (M = Fe, Co, Ni, Cu or Cr) nitric acid compounds
transition metals has been found to enhance the formation of and ammonium molybdate with the desired M and Mo con-
metal carbides.7 Thus, it can be envisioned that the metal tents were impregnated onto the AC support using the incipi-
carbide catalyst modified by appropriate less-expensive tran- ent wetness impregnation (IWI) method as reported in the
sition metals like Fe, Co, Ni, Cu or Cr may exhibit excellent literature,8 followed by drying overnight and calcinating at
catalytic performance for chemoselective reduction of nitro-
a certain temperature in an N2 atmosphere to obtain
arenes. However, to the best of our knowledge, there have been MOy–MoOx/AC. Subsequently, a series of MOy–MoOx/AC were
no reports so far to utilize metal carbides as noble metal-free carburized in a CH4–H2 mixture as described previously,9 and
catalysts for the efficient transformation of substituted nitro- finally passivated in 1% O2/N2 when the temperature was
arenes to corresponding functionalized arylamines.
cooled down to room temperature. The resultant samples are
In this study, we describe the first report of the facile and the as-prepared M–Mo2C/AC catalysts.
chemoselective reduction of nitroarenes containing other sen-
sitive substituents to the corresponding functionalized aryl-
Characterization
amines by using the supported cobalt-assisted molybdenum XRD patterns were collected from 10 to 80° at a step width of
carbide on AC as an efficient precious metal-free catalyst 0.02° using Rigaku Automatic X-ray Diffractometer (D/Max
(Scheme 1). The synergistic effect between cobalt and Mo2C 2400) equipped with a CuKα source (λ = 1.5406 Å). H2-TPR
allows the catalyst to exhibit unexpected catalytic performance experiments were performed in an in-house constructed
in this transformation. 100% conversion of various substrates system equipped with a TCD (thermal conductivity detector) to
with 100% selectivity of the corresponding functionalized aryl- measure H2 consumption. A quartz tube was loaded with
amines has been achieved, which is comparable to the results 100 mg of catalyst pretreated by calcination in Ar at 300 °C for
for precious metal catalysts; in addition N2 is the only bypro- 30 min and then was cooled down to ambient temperature in
duct, suggesting that this work presents a highly-efficient and Ar. After that, it was reduced with a 10 vol% H2–Ar mixture
clean approach for functionalized arylamines production. This (30 ml min−1) by heating up to 800 °C at a ramp rate of 10 °C
work illustrates that metal carbides, once modified by an min−1. XPS measurements were performed with a V.G. Scienti-
appropriate additive, can be considered as an efficient and fic ESCALAB 250 spectrometer using monochromic Al Kα (hν =
low-cost alternative to precious metal catalysts for producing 1486.6 eV, 150 W). The binding energies of Mo 3d and Co 2p
valuable functionalized arylamines. Moreover, this finding were analyzed at 220–240 and 775–805 eV, respectively, using
also opens a horizon to extend the application of this promis- the Shirley baseline-correction method. The binding energy
ing class of less-expensive metal carbides to replace scarce was calibrated using the C 1s photoelectron peak at 284.6 eV
and highly expensive precious metals in the other organic as a reference. The leaching of the catalyst was investigated by
reactions.
measuring Co and Mo contents in reaction mixtures using the
ICP technique on the ICP spectrometer (Optima 2000DV,
Perkin Elmer).
Catalytic performance measurement
94 mg of catalyst prepared as in this work, 1 mmol nitro-
benzene (Tianjin Beilian Fine Chemicals Corp., Ltd), the desired
amount of hydrazine hydrate (80 wt%, Tianjin Bodi Chemicals
Corp., Ltd), and ethanol (Tianjin Fuyu Chemicals Corp., Ltd)
as solvent were added into a round-bottom flask, and the
mixture was refluxed under strong stirring for the desired reac-
tion time. In our experiments, all the reagents were analytical
Scheme 1 Chemoselective reduction of various substituted nitroarenes
to the corresponding functionalized arylamines over the developed sup-
ported metal carbide catalysts on AC.
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Table 1 Hydrogenation of nitrobenzene over various catalystsa
grade and used without further purification. The real hydra-
zine concentration calibrated in our lab is 79.2 wt%. Then the
reaction mixture was cooled down to room temperature after
the reaction finished. The catalyst was separated easily from
the reaction mixture by a filtration procedure, and then it was
washed with a certain amount of ethanol. The filtrate and
washing liquid were combined to obtain the mixture for analy-
sis. The mixture was quantitatively analyzed by gas chromato-
graphy (GC) and liquid chromatography (LC). The molecular
structures of the obtained arylamines were confirmed by Mass
Spectrometry (MS) and 1H NMR (ESI†). The conversion was cal-
culated by the weight percent of the consumed aromatic nitro
compounds in the total amount added; the selectivity to aryl-
amines was calculated by the 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 exper-
iment. The separated and washed catalyst was dried and
reused for recyclability measurement without extra catalyst
being added. In order to probe the perspective of the de-
veloped catalyst to practical application in industrial pro-
duction, the experiment was scaled-up by a factor 10 and was
performed on the basis of above procedure; the isolated yield
was calculated as the percentage of the separated aniline
through an evaporation method to the theoretical value based
on the amount of nitrobenzene.
Entry
Catalyst
Reaction time (h)
Sel./yieldb (%)
1
2
3
4
5
6
7
8
25% W2C/AC
25% Mo2C/AC
1
1
1
1
2
1
1
1
2
1
1
1
1
2
2
100/29.9
100/40.6
100/73.9
100/82.4
100/92.0
100/77.7
100/73.9
100/84.4
100/100
—/99c
2.5% Fe 25% Mo2C/AC
2.5% Ni 25% Mo2C/AC
2.5% Ni 25% Mo2C/AC
2.5% Cu 25% Mo2C/AC
2.5% Cr 25% Mo2C/AC
2.5% Co 25% Mo2C/AC
2.5% Co 25% Mo2C/AC
Rh–Fe3O4
2.5% Co/AC
AC
None
2.5% Co 25% Mo2C/AC
2.5% Co 25% Mo2C/AC
9
10
11
12
13
14
15
100/21.4
100/24.7
0
100/100d
100/100d
a Reaction conditions: 94 mg catalyst, 1 mmol nitrobenzene, 2 equiv.
N2H4·H2O, calibration concentration of N2H4·H2O is 79.2%. b Detected
by GC-MS and 1H-NMR. c From reference. d The nitrosobenzene and
phenylhydroxylamine being used as substrates, respectively.
detected, corresponding to the solvent (ethanol) and aniline,
and neither nitroso nor phenylhydroxylamine exists in the
reaction mixture. Even if the GC is magnified to the same mul-
tiple as that in Fig. S4b,† there is still no peak corresponding
to the nitroso or phenylhydroxylamine. Therefore, we can
safely say that our analysis method is sensitive enough to
exclude the existence of the carcinogenic nitroso and phenyl-
hydroxylamine intermediates.
Results and discussion
From Table 1, we can see that the as-prepared carbides,
especially Mo2C/AC modified with diverse transition metals, to
be efficient catalysts for the chemoselective reduction of nitro-
benzene with 100% of selectivity. No transformation in the
blank experiment and poor reaction results over AC supports
confirm the essential catalytic role of Mo2C/AC in this trans-
formation. However, the catalytic activity is depressed if the
supported carbide is solely employed, which is consistent with
the reported results.6 According to the previous reports, Fe,
Co, Ni are active catalysts for the hydrogenation of nitroben-
zene, although the selectivity is not satisfactory,7a–g as well as
demonstrating that Fe, Co and Ni promote the formation of
molybdenum carbides,5a,7h which suggest the possibility of a
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
found that the addition of a small amount of non-precious
transition metals, especially cobalt can dramatically improve
the conversion from 40.6% to 84.4% (100% selectivity), which
is comparable with those over the reported noble-metal cata-
lysts.2a Through prolonging the reaction time up to 2 h, 100%
conversion and 100% selectivity can be achieved, suggesting
great potential in chemoselective reduction of functionalized
nitroarenes. By using Ni as a promoting agent, only 92.0%
yield can be obtained, although it has a comparable yield for
1 h to Co as promoting agent. As a result, Co is considered to
be the superior promoting agent for this reduction reaction.
The effect of loading of cobalt was optimized and the results
The supported metal carbides (Mo2C/AC and W2C/AC) and the
modified Mo2C (M–Mo2C/AC) catalysts on AC were prepared by
the reported carbothermal reduction method.5a,b From
Fig. S1,† the shift of the XRD peaks towards Mo2C phases from
M–Mo2C/AC catalysts shows the successful introduction of
transition metals into the Mo2C matrix. The nitrobenzene
hydrogenation was used as a probe of the reaction to evaluate
the catalytic performance of the obtained catalysts. The selecti-
vity and yield were calculated based on GC analytical results.
Table 1 illustrates the catalytic properties of various catalysts.
As we all know, the possible reduction intermediates such as
nitroso or phenylhydroxylamine in the reaction mixture may
be highly carcinogenic, and therefore to establish a highly sen-
sitive analysis method for investigating the possible existence
of trace nitroso or phenylhydroxylamine is a very important
issue. Firstly we investigated the degree of sensitivity of our
analytical methods. We prepared an ethanol solution (due to
ethanol being used as solvent in this work) with a total concen-
tration of 10 ppm of possible reduction products including
4 ppm nitroso, 2.2 ppm phenylhydroxylamine and 3.8 ppm
aniline as a sample. From the GC trace (Fig. S4b†), the three
definitely detectable peaks at 4.38, 5.02 and 12.00 min corres-
ponding to nitroso, aniline and phenylhydroxylamine, respect-
ively, are observed, which shows that our analytical method
possesses a sufficiently high degree of sensitivity for the detec-
tion of carcinogenic nitroso or phenylhydroxylamine as well as
the desired product. From Fig. S4a,† only two GC peaks can be
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are presented in Table S1 (ESI†). Results show that the amount visible peak corresponding to Co can be detected, which is
of added cobalt has a visible effect on yield but no effect on ascribed to the fine dispersion of Co or the insertion into the
selectivity, and the 2.5% of optimal loading of additive is β-Mo2C matrix (the cobalt is definitely introduced into the
essential for achieving high yield. To elucidate the promoting sample, confirmed by Co 2p XPS data in Fig. S2, ESI†).
effect of the added cobalt on the catalytic performance of the Through carefully comparing the XRD patterns of Co-modified
supported Mo2C catalyst on AC for the chemoselective and unmodified Mo2C/AC catalysts, a slight shift to higher
reduction of nitroarenes, the two samples 25% Mo2C/AC and diffraction angle in comparison with unmodified sample can
2.5% Co/AC were also tested. In a comparison of the 25% be observed, implying the incorporation of Co into the β-Mo2C
Mo2C/AC and 2.5% Co/AC, the combined catalyst (2.5% Co– crystal phase. Thus it allows the existence of a strong inter-
25% Mo2C/AC) exhibits unexpectedly better catalytic activity, action between Co and Mo2C. The remarkable high intensity
implying the existence of the notable promoting effect of the of the diffraction peaks in Co–Mo2C/AC in comparison with
addition of Co. Various techniques like XRD, XPS and H2-TPR those obtained from Mo2C/AC (collected under the same con-
were employed to further probe the roles of adding Co in ditions) implies the promoting effect of Co on the formation
improving the catalytic performance.
of crystal Mo2C, consistent with the previous reports.5a,7h The
Fig. 1(A) presents the XRD patterns of as-prepared Mo2C/AC broadened XRD peaks show that the unmodified Mo2C/AC
catalysts with and without Co modification. The diffraction catalyst has smaller average crystalline size than Co–Mo2C/AC.
peaks at 34.6°, 38.1°, 39.6°, 52.4°, 61.8°, 69.8°, 74.7° and 75.7° Generally, the smaller size is favorable for enhancing reaction.
can be assigned to the (100), (002), (101), (102), (110), (103), However, the Co-modified Mo2C catalyst with larger average
(112) and (201) crystal faces of β-Mo2C with a hexagonal close- crystalline size exhibits the significantly superior catalytic per-
packed structure (PDF# 35-0787)5c,10 (Fig. 1A). Furthermore, no formance to the unmodified Mo2C catalyst, which shows the
excellent catalytic performance of Co–Mo2C/AC catalyst is a
result of the possible synergistic effect between Co and Mo2C,
but not from the average crystalline size. The Mo 3d XPS
spectra of the Co-modified and unmodified Mo2C/AC catalysts
are given in Fig. 1(B). The peaks at 229.5 and 232.6 eV assigned
to Mo4+ and Mo6+ in the carbides5b,7h,11 become weaker and
shift to lower binding energy as the cobalt is introduced,
further suggesting the existence of the interaction between
cobalt and Mo2C,5a,7h which is consistent with those reported
in references.5a,b,7h,11 From the H2-TPR profiles,12 the second
peak corresponding to MoO3 from Co-modified MoO3/AC
shows a remarkable shift to lower temperature in comparison
with that obtained from the unmodified one (Fig. S3 in ESI†),
suggesting the promoting effect of Co on the reducibility of
MoO3, which may be favorable for the formation of carbide via
a carbothermal reduction method.5a,7h The confirmed syner-
gistic interaction between Co and Mo2C significantly improves
the catalytic performance of Mo2C in chemoselective reduction
of nitro groups.
A plausible reaction mechanism is proposed (Scheme 2).
The tungsten and molybdenum carbides are highly active cata-
7j,k
lysts for the decomposition of N2H4
and active H and N
Fig. 1 (A) XRD patterns and (B) XPS spectra of 25% Mo2C/AC and 2.5%
Co 25% Mo2C/AC catalysts.
Scheme 2 Plausible reaction mechanism for the reduction of nitro-
arenes over Co–Mo2C/AC in the presence of hydrazine hydrate.
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Table 2 Summary on the chemoselective reduction of various nitro-
arenes over the as-developed Co–Mo2C/AC catalysta
N2H4·H2O Reaction Sel./yieldb
Entry Substrate
1
Product
(equiv.)
time (h)
(%)
2
2
100/100
2
3
4
5
6
2
2
6
6
6
2
2
1
1
1
100/100c
100/99.3d
100/100
100/100
100/100
Fig. 2 Recyclability test of the developed 2.5% Co 25% Mo2C/AC cata-
lyst for chemoselective reduction of nitrobenzene to yield aniline. Con-
ditions: 94 mg catalyst, 1 mmol nitrobenzene, 2 equiv. N2H4·H2O, at
refluxing temperature for 2 h of reaction time.
atoms can be formed. The addition of transition metals has
been found to enhance the formation of metal carbides, which
can efficiently catalyze many reactions by the synergistic effect
between the metal carbide and transition metals like Fe, Co
and Ni.7 The synergism can hopefully enhance the activation
of N2H4. As a result, the chemoselective reduction of nitro-
arenes can take place smoothly over the developed Co-promoted
Mo2C/AC catalyst. The active N atoms can be transformed into
N2 which is harmless for the environment while the active H
atoms (some of the active H is transformed into H2, and there-
fore excess N2H4 is required) as the source of the reducing
agent leads to the quick reduction of the nitro group to
produce a nitroso intermediate, with subsequent fast conver-
sion to phenylhydroxylamine, followed by final reduction in
the rate-determining step to the product aniline; a similar
mechanistic profile has been also proposed in the litera-
ture.7i,13 In order to confirm that the effective reducing agent
is the active H atom and not the formed H2 (H2 can be pro-
duced by decomposition of N2H4 7j), we performed the Co–
Mo2C/AC catalyzed reduction of nitrobenzene at 5 bar of hydro-
gen pressure, but no detectable aniline is formed. According
to the reported reference,2m only a 1% trace of aniline was
detected in the presence of N2H4 (without adding catalyst),
implying that N2H4 is not the reducing species. From above,
neither H2 nor N2H4 is an effective reducing species but it is
the active H atom formed on-line by the decomposition of
N2H4. The proposed reaction mechanism is further confirmed.
From references,13 the nitrosobenzene and phenylhydroxyl-
amine are the possible intermediates in the reduction process
of nitroarenes by using N2H4 as reducing agent. In order to
shed further light on the above mechanism, the control experi-
ments using nitrosobenzene and phenylhydroxylamine inter-
mediates 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 1 (entries
14 and 15). The reaction results demonstrate that the aniline is
7
8
6
7
1
3
100/100
100/100
9
1
1
1
1
1
3
100/100
100/100e
100/100
100/100
100/100
10
11
12
13
6
6
6
10
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take place if the reaction is scaled-up by factor 10, as well as
99.3% of isolated yield can be obtained, which is very close to
the GC yield. From above, the non-precious metal Co–Mo2C/
AC catalysed chemoselective reduction could be a practical
approach for the production of arylamines. Leaching of cata-
lyst can lead to the residual metal in the chemical product,
besides the deactivation of catalyst. Therefore, the investi-
gation on leaching of the catalyst is quite important for a prac-
tical catalyst. We carefully performed the leaching experiments
through ICP analysis, finding that only 0.1544 and 2.336 ppm
of Co and Mo, respectively can be detected. Therefore, the
leaching of Co and Mo is negligible.
To investigate the scope of the developed method in this
work, the reduction reactions of structurally diverse nitro com-
pounds were carried out, and reaction conditions were also
optimized (Table S2, ESI†). The optimized reaction results are
presented in Table 2.
Table 2 (Contd.)
N2H4·H2O Reaction Sel./yieldb
Entry Substrate
14
Product
(equiv.)
time (h)
(%)
8
2
100/100
15
16
14
6
3
1
100/100
100/100
From Table 2, the catalyst has excellent selectivity and yield
for the chemoselective reduction of various functionalized
nitroaromatics under the optimized reaction conditions (the
details can be seen in Table S2, ESI†). Interestingly, halogen
substituted nitrobenzenes were reduced to the corresponding
chloroanilines in high yields and without any dehalogenation
(Table 2, entries 4–6). The reducible functional groups in reac-
tion substrates like OH, CHO, COOC2H5 and COOH remained
totally unaffected under the reaction conditions (Table 2,
entries 7–11). Notably, the reduction reactions of nitroaniline
also occurred selectively to give the corresponding phenyl di-
amines with 100% selectivity (Table 2, entries 13 and 14). The
multiamino aromatic compounds, as key fragments in dyes,
can also be efficiently and selectively produced by the chemo-
selective reduction of corresponding nitroarenes using the
developed Co–Mo2C/AC catalysts (Table 2, entries 15 and 16).
The above results exhibit the developed Co–Mo2C/AC material
in this work is a promising catalyst for the clean and efficient
production of functionalized arylamines via a chemoselective
reduction reaction from their corresponding nitroarenes, the
vulnerable moieties in the various substrates were well toler-
ated during the reaction process in this approach.
In summary, we present an efficient and practical approach
for the chemoselective synthesis of various functionalized aryl-
amines from corresponding nitroarenes by Co modified metal
carbides whose catalytic properties are comparable to those of
precious metals, which is ascribed to the remarkably synergis-
tic effect between Co and Mo2C confirmed by XRD, XPS,
H2-TPR and a test reaction as well. The catalyst can be easily sep-
arated and reused eight times without any loss in its catalytic
performance. ICP analysis shows no obvious leaching of
the catalyst, suggesting a very low concentration of residual
transition metals in the product. In view of the importance of
functionalized arylamines in industrial manufacture, the stra-
tegy presented in this work, once fully developed, may open a
window for the production of valuable amine-containing
chemicals from nitro groups. In addition, the substitution of
precious metal catalysts with less-expensive transition metal
carbides in various transformation reactions will be of great
17
18
6
6
1
1
100/100
100/100
a Reaction conditions: 94 mg catalyst, 1 mmol substrate, 80% of N2H4
aqueous solution as reducing reagent. b Detected by GC-MS and
1H-NMR. c Scaled up by factor 10. d Isolated yield. e LC-MS was used (in
ESI).
the only product (Fig. S5–S7†) 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 confirm
the above proposed reaction route for the chemoselective
reduction of nitro group over the developed Co-promoted
Mo2C/AC catalyst in this work.
In industrialized production, the catalyst recycling is defi-
nitely important in the heterogeneous catalytic reactions.
Therefore, we investigated the recyclability of our developed
catalyst for chemoselective reduction reaction by using nitro-
benzene as a model substrate. The separated spent catalyst
was used only by washing with ethanol and drying before use,
with no extra fresh catalyst being added. Fig. 2 presents the
experimental results. From the reaction results, we can observe
that there is no loss in the yields of aromatic anilines over
eight recycling runs of the used catalyst, suggesting the poten-
tial catalyst for the chemoselective reduction of nitroarenes in
practical applications.
As we know, the scaled-up catalytic properties and isolated
yield are very important for a practical catalyst, and thereby
using nitrobenzene as a model substrate, the experiment
scaled-up by a factor 10 was performed, and the GC yield and
isolated yield are presented in Table 2 (entries 2 and 3). Reac-
tion results show that no difference in selectivity and GC yield
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This work is financially supported by the Joint Fund of
Coal, set up by National Natural Science Foundation of China
and Shenhua Co., Ltd (grant no. U1261104), the Chinese Min-
istry of Education via the Program for New Century Excellent
Talents in University (grant no. NCET-12-0079) and the Funda-
mental Research Funds for the Central Universities (grant no.
DUT12LK51).
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