Bio-inspired noble metal-free reduction of nitroarenes using NiS2+x/g-C3N4

Article abstract of DOI:10.1039/c4ra10127h

We introduce the concept of bio-inspired catalytic hydrogenation of nitroarenes by mimicking the catalytic behavior of enzymes with NiS_2+x nanoparticles and polymeric melon (g-C₃N₄). The g-C₃N₄-supported NiS_2+x nanoparticles functioned as ligand-free and noble metal-free catalysts and offered high efficiency, comparable to noble metal-based catalysts, but at a much better selectivity.

Full text of DOI:10.1039/c4ra10127h

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Bio-inspired noble metal-free reduction of
nitroarenes using NiS2+x/g-C3N4†
Cite this: RSC Adv., 2014, 4, 60873
*
*
Ya-Nan Zhang, Xin-Hao Li, Yi-Yu Cai, Ling-Hong Gong, Kai-Xue Wang
and Jie-Sheng Chen
Received 10th September 2014
Accepted 31st October 2014
DOI: 10.1039/c4ra10127h
www.rsc.org/advances
We introduce the concept of bio-inspired catalytic hydrogenation of novel, inexpensive, active and selective catalysts. Cobalt oxide
nitroarenes by mimicking the catalytic behavior of enzymes with catalysts were recently introduced as an excellent heteroge-
NiS2+x nanoparticles and polymeric melon (g-C3N4). The g-C3N4- neous catalyst for the hydrogenation of nitroarenes into the
supported NiS2+x nanoparticles functioned as ligand-free and noble corresponding anilines, whilst specic ligands were still
metal-free catalysts and offered high efficiency, comparable to noble involved in the fabrication of the catalysts.11,12 For practical
metal-based catalysts, but at a much better selectivity.
applications, abundant, cheap and reusable catalysts are
appreciated without the involvement of noble metal or expen-
sive organic ligands.
Reduction/hydrogenation of nitroarenes to corresponding
substituted anilines is of great importance, due to the wide
applications of aniline as one of the most important interme-
diates in industry for fabricating polymers and ne chemicals.
Typically, the hydrogenation of nitroarenes is catalyzed by
either supported noble metal nanoparticles or metal complexes
with expensive ligands.1–5 The majority of those catalysts
investigated in the literature were involved with expensive noble
metal components or specic ligands, limiting their applica-
tions in large scale. Moreover, only limited samples of Au or Pd
nanocatalysts and Fe complexes can access the hydrogenation
of nitroarenes under ambient conditions.6–8 Recently, the M.
Beller group reported that a series of iron complexes exhibit
remarkable activities for catalytic or photocatalytic dehydroge-
nation of FA under mild conditions with very high hydrogen
evolution rates.9,10 However, practical utilization of those
homogeneous catalysts for releasing H2 from FA was obviously
prevented by the cost of expensive ligands and the difficulty in
reuse.
In nature, enzymes composed of amino acids and [Ni–Fe–S]
clusters (Scheme 1) can use binding and proximity effects to
achieve an astounding rate for specic reactions (e.g. hydroge-
nation of CO) with high selectivity under mild conditions.
Enzyme mimics have long been an important branch of bio-
inorganic chemistry, which is inspired by nature and aims to
follow the general principles of natural enzymes using alter-
native materials. Simply mimicking the binding and proximity
effects of enzymes, by using some inorganic clusters or
Heterogeneous catalysts are usually preferred for practical
applications because of their better stability, ease of separation
and recyclability as compared to homogeneous counterparts.
Despite numerous developments of heterogeneous catalysts for
the reduction of nitroarenes, there still exists a great interest in
School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University,
Shanghai 200240, People’s Republic of China. E-mail: xinhaoli@sjtu.edu.cn; k.
wang@sjtu.edu.cn
† Electronic supplementary information (ESI) available: Experimental details,
characterizations, XPS, XRD, FTIR and PL spectra, and measurements. See DOI:
10.1039/c4ra10127h
Scheme 1 Chemical structures of typical enzymes with [Ni–S] or [Ni–
N] centers and the proposed structure of NiS2+x/g-C3N4 nanohybrids
(only one unit cell of NiS2+x was depicted for simplification).
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nanoparticles as model materials, can make articial enzyme similar contrasts of both g-C3N4 and NiS2+x in the view range of
progress for catalysis or photo-catalysis.13,14
TEM, it is difficult to distinguish NiS2+x from g-C3N4 at a rela-
Although important reports have been made recently tively low resolution. A high resolution TEM (HRTEM) image
regarding the use of metal oxide and manganese sulde mate- (Fig. 1B), however, ambiguously displays a fringe with lattice
˚
rials as robust catalysts for photo-splitting and electro-splitting spacing of approximately 2.52 A, corresponding to the (210)
of water,15–17 few such materials exist for hydrogenation of plate of NiS2. The successful deposition of NiS2 can also be
organic compounds. Thus, we turned towards a noble metal- conrmed by XRD (See Fig. S2†). The main peaks in the X-ray
free heterogeneous catalyst based on transition metal sulde diffraction pattern can be indexed to NiS2 (JCPDS no. 11-0099).
nanoclusters and amine-group-rich polymeric carbon nitride (g-
The X-ray photoelectron spectroscopy (XPS) spectra of the
C3N4). The g-C3N4 materials have recently been described as an samples further demonstrated the contents and chemical status
excellent support to embed and activate metallic nanoparticles of the NiS2+x nanoclusters. As revealed by the XPS analysis result
for both photocatalysis and organic synthesis.18–25 More (Fig. S3†), the atomic number ratio of nickel and sulphur is
importantly, the unique heterocyclic macrocycle structure of g- 1 : 2.2, which is slightly lower than the theoretical value (1 : 2) of
C3N4 with an N–C–N-bonding pattern resembling porphyry in NiS2. Consequently, the typical sample was named as NiS2+x
/
Scheme 1 can principally act as in situ “solid ligands” to provide CN, due to the non-stoichiometric composition of our sample.
strong coordination to metal ions and thus can tune the elec- The high resolution Ni 2p XPS spectrum of the typical sample
tronic environment of the supported nanoclusters.26–29 We here (Fig. 1C) revealed that small parts of Ni2+ were oxidized into Ni3+
establish that highly selective reduction of nitroarenes to the during the process of depositing NiS2+x nanoclusters on the
corresponding anilines is efficiently catalysed by g-C3N4 sup- surface of the g-C3N4 support. The fact that the self-catalytic
ported NiS2+x nanoclusters (NiS2+x/CN) under mild conditions. method for depositing metal sulphides was conducted at 80
A commercially available reductant, NaBH4, was selected as a ꢀC rather suggests a photocatalytic oxidation of Ni2+ using g-
model hydrogen source, because NaBH4 has been widely used C3N4 in air. Such a photocatalytic oxidation reaction has been
with metallic nanoparticle-based catalysts for the reduction of well dened for g-C3N4 with a specic nanostructure for organic
nitroarenes at an ambient temperature.8,10,30,31 The selectivity of synthesis, where the formation of O2À ions was detected.35 The
the reduction of –NO2 over NiS2+x/g-C3N4 is very high, even in possibility of a photocatalytic oxidation reaction during the
the presence of other functional groups.
formation process of NiS2 nanoclusters was further conrmed
The NiS2+x/CN catalysts were prepared by a modied self- by the detailed S 2p XPS spectrum (Fig. 1D), with two strong
catalytic method previously described in our group (see the peaks at 168.56 eV and 169.6 eV attributed to sulfur oxoanions
ESI† for details).32,33 A typical TEM image revealed the stability (denoted as S–Ox in Fig. 1D).36 The typical peaks at 162.55 eV
of mesoporous g-C3N4 aer the deposition of the NiS2+x and 163.68 eV suggested the presence of S–S bonds of NiS2 in
components on the surface (Fig. 1A). Aer the deposition of the samples, again revealing the formation of NiS2 on the
NiS2+x, both the surface area and the pore size distribution of surface of the g-C3N4 support.37,38 All these observations
mesoporous g-C3N4 remained unchanged (Fig. S1†). Due to the revealed a functional group-rich surface of the NiS2 nano-
particles, well explaining the slightly lower Ni/S ratio. Obviously,
these groups will benet from the formation of a coupled
interface between g-C3N4 and the NiS2+x nanoclusters, and thus
shows a potential application of NiS2+x/CN as a catalyst for
organic synthesis.
Aromatic amines are key intermediates in the synthesis of
ne chemicals, biologically active compounds and more. As a
typical aromatic amine, aniline, with a worldwide production
exceeding four million tons per year, was obtained mainly
(ꢁ97%) from hydrogenation of nitrobenzene. The excess
amount of H2 gas (>100 times in moles) was usually used for the
catalytic transformation of nitrobenzene to aniline at high
ꢀ
pressure (greater than 5 atm) and high temperature (>200 C),
and the process is thus of “high-energy consumption” and
unsafe in terms of H2 handling. Thus, chemoselective hydro-
genation of nitro compounds to aniline and the corresponding
substituted derivates in a sustainable manner is of great
importance for both industrial applications and fundamental
researches. We tested the possibility of hydrogenation of
nitrobenzene to aniline for the NiS2+x/CN samples at room
temperature by using water as the green solvent and NaBH4 as
the only hydrogen source.
Fig. 1 TEM (A) and HRTEM (B) images of NiS2+x/CN. XPS spectra of Ni
2p (C) and S 2p (D). Two shake-up satellites of Ni2+ (denoted as “Sat.”)34
No conversion was observed without a catalyst (entry 1, Table
were also observed in the Ni 2p3/2 XPS spectrum.
1) or NaBH4 (entry 2, Table 1). Bare carbon nitride offered no
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Table 1 Hydrogenation of nitrobenzenea
Although full conversions (entry 8–10, Table 1) were also ach-
ieved using all these noble metal catalysts, only Pd/CN gave a
selectivity of nearly 100% to aniline. Due to the same efficiency
of NiS2+x/CN and Pd/CN in the reduction of nitrobenzene for
aniline, we further studied the reduction of halogenated nitro-
benzene using the two catalysts. As shown in Table 2, the
reduction of 4-chloronitrobenzene was found to have a yield of
96% for 4-chloroaniline and 4% for aniline using Pd/CN.
Signicant dehalogenation occurred in the reduction of 4-
bromine nitrobenzene using Pd/CN with only a selectivity of 5%
to the target product. Moreover, 4-iodonitrobenzene was
completely reduced to aniline using Pd/CN. Such a dehaloge-
nation reaction was normal for noble metal-based nano-
catalysts, suggesting their limitation in the hydrogenation of
halogenated compounds. Surprisingly, NiS2+x/CN could
completely transfer halogenated nitrobenzene into substituted
halogenated aniline without obvious dehalogenation under
standard conditions. The cheap and mild NiS2+x/CN catalyst has
obvious advantages for practical applications in industry.
Aer knowing the superior catalytic activity of NiS2+x/CN for
the hydrogenation of nitrobenzene and halogenated nitroben-
zene, we further tested its compatibility with various
substituted nitroarenes. The methyl substituted substrates are
completely reduced to the corresponding anilines in an excel-
lent yield of 99% (entries 1–3, Table 3). The reaction of 2,6-
dimethlnitrobenzene could also reach an excellent yield of the
target product of up to 99% (entry 7, Table 3). Typical haloge-
nated nitrobenzenes were efficiently reduced with both good
conversion and selectivity (entries 4–6, Table 3). Our protocol
also worked well for the reduction of other typical complex nitro
compounds, such as 1-nitro-naphthalene and 6-nitroquinoline
(entry 8, 9, Table 3). The activity of the NiS2+x/CN catalyst will
not be affected by steric hindrance and conjugate phenomenon.
All these results suggest that the noble metal-free NiS2+x/CN
catalyst can be used for the reduction of nitroarenes with an
excellent conversion and selectivity in a mild process.
Entry
Catalyst
Conv. [%]
Sel. [%]
1b
2c
3
—
—
—
—
—
—
—
NiS2+x/CN
CN
4
5
6
NiS2/N-LC
NiS2/C
NiS2
48
99
42
79d
77d
53d
>99
>99
56d
54d
7
8
9
10
NiS2+x/CN
Pd/CN
Au/CN
Pt/CN
>99
>99
>99
>99
a
Reaction conditions: 0.5 mmol nitrobenzene, 50 mL of water, 10 mg of
b
catalyst, 0.5 mmol NaBH4, room temperature, 1 h. Without catalyst.
c
Without NaBH4. d The byproduct is nitrosobenzene.
conversion at all (entry3, Table 1). Unsupported NiS2 nano-
particles and carbon black (C, surface area: 62.5 m2 gÀ1) or
nitrogen doped layered carbon (N-LC, surface area: 190 m2
gÀ1 39
) supported NiS2+x nanoparticles could only offer moderate
conversions and selectivity (entry 4–6, Table 1). Under xed
conditions, NiS2+x/CN (9 wt%) offered a conversion of nearly
100% with a selectivity to aniline of up to 99% (entry 7, Table 1).
Note that the content of NiS2+x was calculated from the amounts
of precursors used in the synthetic process for a better clari-
cation, whilst the real content was further conrmed by XPS
(Fig. S3†) analysis. Obviously, carbon nitride has played an
important role in increasing both the activity and selectivity of
the NiS2 nanoparticles (Scheme S1†).
To determine the effect of the content of the NiS2+x nano-
particles in the catalyst on the catalytic performance, the NiS2+x
/
CN catalyst with weight percentages of the NiS2+x species from 1
to 15% were prepared under the same conditions and used for
the hydrogenation of nitrobenzene. The corresponding
conversions are summarized in Table S1.† When the weight
Table 2 Hydrogenation of halogenated nitrobenzene using Pd/CN
and NiS2+x/CNa
percentage of the NiS2 nanoparticles was only 1% (1 wt%-NiS2+x
/
CN), the catalyst gave a minor conversion (8%) and less selec-
tivity to aniline. The conversion and selectivity have been both
gradually improved to >99% as the weight percent of NiS2 was
increased to 9%. When more NiS2+x species (>12 wt%) were
introduced, the selectivity decreased via 95% to 86%. This is to
say that too much NiS2 nanoparticles deposited on the surface
of the g-C3N4 support disturbed the synergetic effect between
the two components. The synthetic temperature was optimized
to be 80 ꢀC (Table S2†). As the best catalyst here, NiS2+x/CN,
Pd/CN
NiS2+x/CN
Conv. [%]
Substrate
Conv. [%]
Sel. [%]
96b
Sel. [%]
>99
>99
>99
>99
>99
5b
>99
ꢀ
obtained at 80 C with 9 wt% of the NiS2+x nanoparticles, was
selected for all the following hydrogenation reactions of
nitrobenzene.
>99
0b
>99
>99
Some noble metal-based control catalysts (with a weight
percentage of 9 wt%) were tested under standard conditions.
a
Standard conditions described in Table 1. b The byproduct is aniline.
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Table 3 Hydrogenation of nitro compounds using NiS2+x/CNa
Conclusions
Entry
Substrate
Conv. [%]
Sel. [%]
In summary, we developed an efficient g-C3N4 supported NiS2+x
nanoparticle (NiS2+x/CN) catalyst system for the hydrogenation
of nitroarenes under mild conditions. Our noble metal-free
NiS2+x/CN catalyst offered excellent catalytic activity, which
was comparable to that of Pd nanocatalysts and much better
than those of Au and Pt nanocatalysts. More importantly, the
chemoselective hydrogenation of halogenated nitrobenzene to
corresponding anilines using the NiS2+x/CN catalyst is rather
speaking for the advantages of the NiS2+x/CN catalyst as a mild
catalyst against noble metal counterparts. The strategy reported
here may supply new possibilities for developing sustainable
nanocatalysts for practical applications.
1
99
>99
2
99
>99
3
4
>99
>99
>99
99
Acknowledgements
5
6
>99
>99
>99
98
This work was supported by the National Basic Research
Program of China (2013CB934102 and 2011CB808703) and the
National Natural Science Foundation of China (21331004 and
21301116).
Notes and references
7
8
>99
99
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