Highly selective reduction of nitroarenes to anilines catalyzed using MOF-derived hollow Co3S4 in water under ambient conditions

Article abstract of DOI:10.1016/j.catcom.2017.07.001

We developed a new strategy for the efficient reduction of nitroarenes using sodium sulfide as reducing agent with MOF-derived Co₃S₄ as catalyst in water and at room temperature. The introduction of sodium sulfite enhanced the reactant conversion and product selectivity, and the as-synthesized catalyst was used repeatedly five times and retained its activity and selectivity. A wide spectrum of reducible functional moieties kept unaffected under the reaction conditions, and isotope labeling experiment showed the hydrogen atom was derived from water.

Full text of DOI:10.1016/j.catcom.2017.07.001

Accepted Manuscript
Highly selective reduction of nitroarenes to anilines catalyzed
using MOF-derived hollow Co3S4 in water under ambient
conditions
Yong Xu, Xiao-Jun Lv, Yong Chen, Wen-Fu Fu
PII:
DOI:
Reference:
S1566-7367(17)30291-1
doi: 10.1016/j.catcom.2017.07.001
CATCOM 5120
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Catalysis Communications
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Revised date:
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2 March 2017
13 June 2017
4 July 2017
Please cite this article as: Yong Xu, Xiao-Jun Lv, Yong Chen, Wen-Fu Fu , Highly
selective reduction of nitroarenes to anilines catalyzed using MOF-derived hollow Co3S4
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as affiliation for all authors. Please check if appropriate. Catcom(2017), doi: 10.1016/
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ACCEPTED MANUSCRIPT
Highly selective reduction of nitroarenes to anilines catalyzed using
MOF-derived hollow Co3S4 in water under ambient conditions
Yong Xu,a Xiao-Jun Lv,a Yong Chen,a Wen-Fu Fu*a,b
aKey Laboratory of Photochemical Conversion and Optoelectronic Materials, CAS-HKU Joint
Laboratory on New Materials, Technical Institute of Physics and Chemistry, Chinese Academy of
Sciences, University of Chinese Academy of Sciences, Beijing 100049, P.R. China
E-mail: fuwf@mail.ipc.ac.cn
bCollege of Chemistry and Chemical Engineering, Yunnan Normal University, Kunming 650092,
P.R. China

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Abstract: We developed a new strategy for the efficient reduction of nitroarenes
using sodium sulfide as reducing agent with MOF-derived Co3S4 as catalyst in water
and at room temperature. The introduction of sodium sulfite enhanced the reactant
conversion and product selectivity, and the as-synthesized catalyst was used
repeatedly five times and retained its activity and selectivity. A wide spectrum of
reducible functional moieties kept unaffected under the reaction conditions, and
isotope labeling experiment showed the hydrogen atom was derived from water.
Keywords: Metal-organic frameworks; Hollow Co3S4; Nitro reduction;
Heterogeneous catalysis; Sodium sulfide

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1. Introduction
The development of a new kind of organic synthesis reaction is a big but
worthwhile challenge. As a class of significant industrial chemicals,
approximately 95% of nitroarenes are transformed into anilines, which are
important precursors for the synthesis of pharmaceuticals, pesticides, dyes,
rubber chemicals, and agricultural chemicals [1-4]. The pioneering work for the
reduction of nitroarenes generally involved catalytic hydrogenation of the nitro
group using precious metals such as Au, Pt, and Pd deposited on active carbon
or metal-organic frameworks (MOFs) [5-8]. In addition to hydrogen, other
reducing agents such as silane, ammonia borane, sodium borohydride, formic
acid and hydrazine have been used to reduce nitroarenes in combination with
metal-bearing catalysts [8-12]. Moreover, it is also a challenge to selectively
reduce only the nitro group at a fast reaction rate while keeping any other
reducible functional groups in the molecule unchanged. There have been a few
reports on the use of sodium sulfide as a reductant, which is known as the Zinin
Reaction. However, some byproducts such as aldehydes and condensation
products of amines are found in this reaction using the model substrate
p-methyl nitrobenzene (Fig. 1) [13-15].
(Embed Fig. 1 here)
Recently, noble-metal-free catalysts have caused extensive concerns for
researchers, taking into account the cost and resource storage [16-18].

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Especially, catalysts containing cobalt have attracted increasing attention as
alternatives to replace noble-metal-based catalysts, owing to the low cost and
remarkable catalytic activities for the reduction of water, dioxygen, carbon
dioxide, and hydrogenation [19-22]. In comparison, metal-organic frameworks
constructed from metal ions and polyfunctional organic ligands have become
attractive and important materials in many fields due to their excellent physical
and chemical properties such as low density, extra high specific surface area,
and well defined pore structure [20, 23-24]. However, the fabrication of hollow
nanomaterials with controlled morphology and composition is still a bottleneck.
Interestingly, zeolitic imidazolate framework-67 (ZIF-67), a cobalt-based MOF,
has been used as a self-templating precursor to prepare hollow Co3S4 through a
solvothermal reaction with thioacetamide (TAA) and a thermal annealing
process. The synthesized Co3S4 maintained a dodecahedral configuration and
the special properties of MOFs including high conductivity and thermal
stability over a broad pH range from 0 to 14 [25]. To the best of our knowledge,
we demonstrate for the first time that this material also displays excellent
activity for the catalytic reduction of nitroarenes to anilines without any
disproportionation or condensation products by using sodium sulfide as a
reductant in the presence of sodium sulfite.
2. Experimental
2.1. Preparation of hollow Co3S4
To prepare Co3S4, first, ZIF-67 was synthesized based on previous report with some

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modifications. Typically, Co(NO3)2·6H2O (1.74 g) was dissolved in 30 mL of
methanol. Another solution containing 3.94 g 2-methylimidazole was rapidly added to
the above solution with violently stirring for 1 min. Then the mixture was kept for 24
h at room temperature. Deep purple ZIF-67 products were obtained by centrifugation
o
and washed with water and ethanol three times, followed by vaccum drying at 60 C
overnight.
For the preparation of hollow Co3S4, 80 mg as-produced ZIF-67 was dispersed into
20 mLof ethanol accompanied with ultrasonic irradiation for 20 min. 0.12 g TAA was
dissolved in another 20 mL of ethanol, which was poured into ZIF-67 mixture. The
o
obtained mixture was transferred into 50 mL autoclave, heated and kept at 120 C for
6 h. The produced sulfide was centrifuged, washed with ethanol three times and dried
o
o
at 70 C in vaccum oven. The sample was heated at 350 C for 2 h in tube furnace
with a heating rate of 3.3 oC/min under Ar atmosphere.
2.2. Preparation of ordinary Co3S4 (o-Co3S4)
Co(NO3)2·6H2O (582 mg) and TAA (751.3 mg) were dissolved in 40 mL of ethanol.
The process followed is similar to that of hollow Co3S4
2.3. Catalytic reduction of nitroarenes
Typically, 5 mL Na2S/Na2SO3 aqueous solution containing 0.4 mmol substrate and
5 mg hollow Co3S4 was added into 15 mL cuvette sealed with a rubber septum. A
stream of Ar was bubbled into the reaction system for 10 min and then the reaction
solution was stirred with a magnetic stirrer at room temperature. In order to reduce
random error, the position of cuvettes was swapped every half an hour.

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Dichloromethane was used as extraction agent and extracted 3 times in a row with
p-xylene as internal standard. Products were characterized by NMR analysis (ESI).
3. Results and discussion
The as-synthesized hollow Co3S4 was characterized using X-ray powder
diffraction (XRD), scanning electron microscopy (SEM), transmission electron
microscopy (TEM), selected area electron diffraction (SAED), elemental
mapping, and energy dispersive X-ray (EDX) spectroscopy (Fig. 2 and S1). The
XRD pattern corresponds to cubic Co3S4 (JCPDS No. 47˗1738). SEM and TEM
images provide details of the exterior and interior of the hollow structure,
which demonstrates a sharp contrast of the MOF’s morphology between the
center and the edge. The lattice spaces of 0.285, 0.236, and 0.168 nm are
ascribed to the (311), (400), and (440) planes. EDX analysis (Fig. S1(I)) reveals
an approximate Co/S ratio of 3:4 based on the calculated peak areas, confirming
the successful formation of the desired catalyst. The Si peak come from the
silicon pellet. EDX mapping further reveals that all elements are uniformly
distributed in the Co3S4. As shown in Fig. S2, X-ray photoelectron survey
spectra of Co3S4 shows low-energy peaks with binding energies of 778.4 and
780.0 eV, corresponding to Co 2p3/2 state, while high-energy peaks at 793.2 eV,
796.8 eV were assigned to Co 2p1/2 states. The first doublet at 778.4 eV, 793.2
eV and the second at 780.0 eV, 796.8 eV were assigned to Co2+ and Co3+,
respectively. A broaden peak (801.8 eV) at higher binding-energy can be
attributed to satellite signal.

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(Embed Fig. 2 here)
Initially, the reduction reaction was performed using p-methyl nitrobenzene
as the model substrate with sodium sulfide as the reductant and water as the
hydrogen source at room temperature (Table 1). After stirring for 10 h with the
hollow Co3S4 catalyst, the conversion and selectivity were 65.4 % and 73.5 %
in the absence of Na2SO3, respectively. However, both the conversion and
selectivity almost reached 100 % after stirring for 6 h in the presence of sodium
sulfite. The results demonstrated that present catalytic system could be used to
reduce the nitro group to an amino group without heating. Interestingly, the
nitro group could not be reduced using only sodium sulfite as the reducing
agent (Table 1, entry 3). Control experiments showed that the reduction
reaction could not be conducted in the absence of Co3S4 or sodium sulfide.
Control experiments showed that other sacrificial agents, including formic acid,
triethanolamine (TEOA), and isopropanol, could not be used for reduction of
the nitro group. In contrast, the conversion was only 12.1 % after stirring for 6
h when ordinary Co3S4 (o-Co3S4) was also used as catalyst (Table 1, entry 6, 7).
(Embed Table 1 here)
The results showed that sodium sulfite had a significant influence on both the
conversion and selectivity of the reduction reaction. To investigate the effect of
the sodium sulfide concentration on the reaction rate and to optimize the
catalytic reaction conditions, a series of experiments were performed by
varying the ratio of sodium sulfide and sodium sulfite while maintaining the

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concentration of sodium sulfite. As shown in Fig. 3, after stirring for 3 h, a
selectivity of up to 100 % for the reduction of p-methyl nitrobenzene to aniline
was achieved when the concentration of sodium sulfide was relatively small. As
the sodium sulfide content increased, the selectivity gradually declined whereas
the conversion increased. This indicated the acceleration of the reaction as a
result of increasing sodium sulfide concentration, but the catalytic activity at
higher sulfide concentrations slightly decreased. This can be related to the
effects of sulfur anion oxidated to elemental sulfur on reaction rate in the
process of substrate conversion, which is loaded on the surface of dodecahedral
Co3S4, decreasing the active sites of catalyst. Generally, the reduction of
nitrobenzene to aniline processes three steps, i. e. from nitrobenzene to
nitrosobenzene and to N-phenylhydroxylamine intermediates, then to formation
of product. Furthermore, cobalt has a high affinity to the nitro group compared
with the nitroso only having one oxygen atom, the generated nitrosobenzene
easily detach from the catalyst [26-28]. Higher sulfide concentration will lead
to accumulation of nitrosobenzene in system and subsequent formation of a
side product through a condensation reaction of it and N-phenylhydroxylamine
[29]. The optimal Na2S/Na2SO3 molar ratio was determined to be 5:7.
(Embed Fig. 3 here)
In comparison, no product was obtained if 1 atm of H2 was used as the
reductant, which suggested that activated hydrogen species could be involved
in the reduction. Based on previous studies, traditional metal-based catalysts

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such as Pd loaded on solid support transfer hydrogen to generate atomic
hydrogen on the surface of the catalysts and reduce organic compounds by
using sacrificial agents [11, 30]. Analogously, metallic Co3S4 displays the
function, transferring hydrogen from water to form H-Co3S4 species on its
surface, which is the key for the promotion of the whole reaction. In accordance
with our conjecture, an investigation was performed to verify the existence of
this species in the absence of nitroarenes. As expected, a small amount of H2
gas was detected using gas chromatography in a system containing an aqueous
solution of Na2S, Na2SO3, and Co3S4 after stirring for 4 days. Isotope labelling
experiment was also conducted using deuteroxide instead of water and the
result gained revealed that N-D bond formed in the reduction (Fig. S3).
The nitro group could coordinate to the cobalt center of the catalyst surface
during the nitroarene reduction process, as reported previously [28]. Before the
evolution of dihydrogen, the hydride generated on the surface of Co3S4 could
transfer hydrogen of water to the coordinated nitroarenes. In 1898, Haber first
put forward the well-known reduction pathways, that is, a stepwise pattern via
nitroso and hydroxylamine intermediates, which may condense to coupling
compounds such as azoxybenzene and azobenzene [31]. However, the reaction
procedure for the reduction of an azoxy to an azo and to aniline group was less
efficient. In the eyes of the mechanism, the coordinated nitro group was
hydrogenated to nitroso and hydroxylamine groups, followed by their coupling
products (Fig. S4) [29]. The results were further confirmed by the decreasing

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selectivity as the proportion of sodium sulfide was increased (Fig. 3). The
selectivity increased from 88.1% (Na2S/Na2SO3 molar ratio of 5:7, stirring for 3
h) to 99.8% (Table 1, entry 2) by prolonging the reaction time. To gain further
insight into the reaction pathways, the reduction of the condensation product,
azobenzene was used as substrate, was worked out, keeping the experimental
conditions identical to those for the reduction of nitroarenes. The obtained
results demonstrated that hydrazobenzene was the only product after stirring for
2 days (ESI). However, the details of the mechanism are still unclear, and more
studies are needed to gain a better understanding of the mechanism.
The catalytic system was generally applicable for the reduction of other
nitroarenes. Dehalogenation of sensitive halogen-containing nitroarenes was
expected to occur, as previously reported [32]. However, halogenated
nitroarenes were reduced to afford the corresponding anilines with excellent
selectivity and suppression of dehalogenation in our system (Table 2, entry 9,
10). From a synthetic point of view, it is challenging to selectively reduce
nitroarenes functionalized with other reducible moieties to valuable anilines at a
fast reaction rate. In this catalytic system, selective hydrogenation of the nitro
group was performed in the presence of vulnerable functional groups such as
aldehyde, ketone, cyano, amides and esters (Table 2, entry 1, 2, 11-13). The
activity of 4-nitrobenzonitrile was the lowest compared with other substrates.
Furthermore, the conversion and selectivity of nitroarenes containing
electron-withdrawing substituents was lower than those containing

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electron-donating
groups.
This
indicates that
substrates
bearing
electron-donating groups may be more difficult to detach from the surface of
the catalyst compared with those containing electron-withdrawing groups.
Taking steric hindrance effect into consideration, bulky substituents impede
intimate contact between the substrate molecules and the catalyst, resulting in
poor catalytic activity in the catalytic reduction reaction (Table 2, entry 5, 8).
(Embed Table 2 here)
The reusability of hollow Co3S4 catalyst was examined by reducing p-methyl
nitrobenzene to the corresponding p-methyl aminobenzene without any
regeneration, which is very critical for industrial applications. As shown in Fig.
4, the catalyst was collected by centrifugation and repeated washing with
o
deionized water and ethanol, and subsequently dried at 70 C in an oven. The
activity of the catalyst was retained after five successive runs without any
obvious decrease in conversion, selectivity and yield. XRD pattern after 5
cycles maintains the crystal phase of Co3S4 (Fig. S5) but some hollow
dodecahedrons are falling apart (Fig. S6).
(Embed Fig. 4 here)
4. Conclution
In summary, MOF-derived hollow Co3S4 was used as an excellent catalyst in
an aqueous solution of sodium sulfide with sodium sulfite for the reduction of
nitroarenes to anilines without the need for intensive heating. The introduction
of sodium sulfite to the catalytic system was essential for obtaining high

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conversion and selectivity. High chemoselectivity was achieved during the
reduction of nitroarene substrates containing other reducible functional groups.
Isotope labeling experiment showed the hydrogen atom was derived from water.
Co3S4 could be recycled five times and maintained high catalytic activity. This
strategy represents a new train of thought for the reduction of nitro compounds.
Acknowledgements
This work was financially supported by the National Key Basic Research
Program of China (973 Program 2013CB834804) and the Ministry of Science
and Technology (2012DFH40090) and the Strategic Priority Research Program
of the Chinese Academy of Sciences (XDB17030200). We thank the National
Natural Science Foundation of China (21267025, 21471155, 21367026) for
financial support.
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Graphical Abstract
MOF-derived hollow Co3S4 introduced reduction of nitroarenes to anilines
efficiently using sodium sulfide as reductant with sodium sulfite in water.

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Fig. 1. Schematic illustration of the strategies for the reduction of nitroarenes.
Fig. 2. (a) SEM and (b) TEM images of Co3S4.
Fig. 3. Changes of conversion, selectivity and yield depending on the concentration
ratio of sodium sulfide and sodium sulfite at room temperature. Reaction conditions:
0.4 mmol p-methyl nitrobenzene, 5 mg Co3S4 catalyst, 5 mL of H2O, fixed
concentration of Na2SO3 (0.35 M), stirring for 3 h.

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Entry
Reductant Conv. (%) Sel. (%) Yield (%) TOF (h-1)
1a
2b
3a
4c
5d
6e
7f
Na2S
65.4
100
0
73.5
99.8
—
47.4
99.8
0
1.60
4.07
0
Na2S/Na2SO3
Na2SO3
—
0
—
0
0
Na2S/Na2SO3
Na2S/Na2SO3
Na2S/Na2SO3
0
—
0
0
5.0
12.1
99.6
99.3
5.0
12.0
0.41
0.49
Table 1. Catalytic reduction of p-methyl nitrobenzene with hollow Co3S4 and sodium
sulfide at room temperature. Reaction conditions: 0.4 mmol p-methyl nitrobenzene, 5
a
mg Co3S4 catalyst, 5 mL H2O, Na2S (0.25 M), Na2SO3 (0.35 M), stirring for 10 h;
c
d
bStirring for 6 h; Without sodium sulfide and sodium sulfite; In the absence of
f
hollow Co3S4; eo-Co3S4 was used as catalyst, stirring for 3 h and 6 h.

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Table 2. Reduction of substituted nitroarenes to produce anilines in our catalytic
a
system. Reaction conditions: 0.2 mmol substrate, 5 mg Co3S4 catalyst, 5 mL of H2O,
Na2S (0.25 M), Na2SO3 (0.35 M), stirring at room temperature.

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Fig. 4. Reutilization of Co3S4 catalyst for nitroarenes reduction. Reaction conditions:
0.4 mmol p-methyl nitrobenzene, 5 mg Co3S4 catalyst, 5 mL solvent, Na2S (0.25 M),
Na2SO3 (0.35 M), stirring 5 h, at room temperature.

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Highlights
 MOF as a precursor to prepare hollow Co3S4 with dodecahedral configuration
 Co3S4 introduced reduction of nitroarenes to anilines keeping reducible groups
 MOF-derived Co3S4 having highly catalytic efficiency
 Sodium sulfide used as reductant without requiring heating in aqueous solution