Hydrogenation of Functionalized Nitroarenes Catalyzed by Single-Phase Pyrite FeS2 Nanoparticles on N,S-Codoped Porous Carbon

Article abstract of DOI:10.1002/cssc.201901867

Catalytic hydrogenation of nitroarenes is an industrially very important and environmentally friendly process for the production of anilines; however, highly chemoselective reduction of nitroarenes decorated with one or more reducible groups in a nitroarene molecule remains a challenge. Herein, a novel hybrid non-noble iron-based nanocatalyst (named as FeS₂/NSC) was developed, which was prepared from biomass as C and N source together with inexpensive Fe(NO₃)₃ as Fe source through high-temperature pyrolysis in a straightforward and cost-effective procedure. Comprehensive characterization revealed that single-phase pyrite FeS₂ nanoparticles with precisely defined composition and uniform size were homogeneously dispersed on N,S-codoped porous carbon with large specific surface area, hierarchical porous channels, and high pore volume. The resultant catalyst FeS₂/NSC demonstrated good catalytic activity for hydrogenation of functionalized nitroarenes with good tolerance of various functional groups in water as a sustainable and green solvent. Compared with bulk pyrite FeS₂ and other non-noble metal-based heterogeneous catalysts reported in the literature, a remarkably enhanced activity was observed under mild reaction conditions. More importantly, FeS₂/NSC displayed exclusive chemoselectivity for the reduction of nitro groups for nitroarenes bearing varying readily reducible groups.

Full text of DOI:10.1002/cssc.201901867

Accepted Article
Title: Highly Active and Chemoselective Hydrogenation of
Functionalized Nitroarenes Catalyzed by Single Phase Pyrite
FeS2 Nanoparticles on N,S-Codoped Porous Carbon
Authors: Yong Yang, Yanan Duan, Xiaosu Dong, Tao Song, Zhaozhan
Wang, Jianliang Xiao, and Youzhu Yuan
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the VoR from the journal website shown below when it is published
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To be cited as: ChemSusChem 10.1002/cssc.201901867
Link to VoR: http://dx.doi.org/10.1002/cssc.201901867
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10.1002/cssc.201901867
ChemSusChem
Highly Active and Chemoselective Hydrogenation of Functionalized Nitroarenes
Catalyzed by Single Phase Pyrite FeS2 Nanoparticles on N,S-Codoped Porous Carbon
Yanan Duan,a Xiaosu Dong,a Tao Song,a Zhaozhan Wang,a Jianliang Xiao,b Youzhu Yuan,c
Yong Yanga*
aQingdao Institute of Bioenergy and Bioprocess Technology, Qingdao 266101, P R China
E-mail: yangyong@qibebt.ac.cn
bDepartment of Chemistry, Liverpool University, Liverpool L69 7ZD, UK
cState Key Laboratory of Physical Chemistry of Solid Surfaces, National Engineering
Laboratory for Green Chemical Production of Alcohols-Ethers-Esters, College of Chemistry
and Chemical Engineering, Xiamen University, Xiamen 361005, P R China
Keywords
Pyrite FeS2 nanoparticles, N,S-codoped porous carbon, hydrogenation, functionalized
nitroarenes, chemoselectivity
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Abstract
Catalytic hydrogenation of nitroarenes is a very industrially important and environmentally
friendly process for the production of anilines; however it still remains a challenge for highly
chemoselective reduction of nitroarenes decorated with one or more reducible groups in a
nitroarene molecule. Herein we developed a novel hybrid non-noble iron-based nanocatalyst
(named as FeS2/NSC), which was prepared using biomass as C and N source together with
inexpensive Fe(NO3)3 as Fe source via high temperature pyrolysis in a straightforward and
cost-effective procedure. Comprehensive characterizations including XRD, BET, Raman,
HR-TEM, XPS, 57Fe Mössbauer spectroscopy reveal that single phase pyrite FeS2
nanoparticles with precisely defined composition and uniform size were homogeneously
dispersed on N,S-codoped porous carbon with large specific surface area, hierarchical porous
channel and high pore volume. The resultant catalyst FeS2/NSC demonstrated superior
catalytic activity for hydrogenation of functionalized nitroarenes with good tolerance of
various functional groups using water as a sustainable and green solvent. Compared with bulk
pyrite FeS2 and other non-noble metal-based heterogeneous catalysts reported in the literature,
a remarkably enhanced activity was observed under mild reaction conditions. More
importantly, the catalyst FeS2/NSC displayed exclusive chemoselectivity to the reduction of
nitro group for nitroarenes bearing varying readily reducible groups. Control experiments and
DFT calculations reveal that, owing to the large electronegative N and S atoms incorporation
into the carbon, the FeS2/NSC not only had a preferential adsorption of nitro group but also
had a strong interaction between FeS2 nanoparticles and N,S-codoped porous carbon, which
substantially boosted the catalytic activity, chemoselectivity, and stability as well.
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1. Introduction
Functionalized anilines are industrially important intermediates for a variety of speciality and
fine chemicals as well as key building blocks for high performance rubbers and polymer. [1] In
particular, structurally complex anilines decorated with diverse functional groups have
widespread applications in life science. To date, more than 4,000,000 tons of aniline and its
derivatives were produced per year over the worldwide markets.[2] However, it still remains a
great challenge for chemoselective hydrogenation of the nitro group when the nitroarene
molecule bears with readily reducible functional groups, such as C=C, CC, C=O, and CN.
Specifically, precious metal-based catalysts generally give superior activity yet with low
selectivity to the desired amines. Since the breakthrough of gold-based catalysts for selective
hydrogenation of 3-nitrostyrene was reported by Corma and co-workers in 2006, [3] significant
progress toward improved selectivity has been achieved. Various metal-based catalysts (e.g.,
platinum, [4] silver, [5] palladium, [6] rhodium[7] and gold[8]) have been developed through
varying preparation strategies, such as adding surface modifiers, tailoring the metal particle
sizes, alloying with second component metals, controlling metal-support interaction, or
fabricating the unique core-shell or well-confined nanostructured catalysts. Nevertheless, the
high cost and scarcity of these precious metals accompanied with complicated and tedious
preparation procedures for these catalysts significantly hamper their practical potential,
especially for large-scale industrial applications. Therefore, searching for more earth-
abundant alternatives with high catalytic efficiency and excellent chemoselectivity is highly
desirable. Besides, a facile and environmentally friendly process to prepare such non-noble
metal-based catalyst with high stability is also of great importance from both economic and
sustainable viewpoints.
The sustainable, copious supply of non-noble metal salts, especially iron, couples with their
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environmentally benign nature and low toxicity make them appealing catalysts. In this context,
Beller and co-workers have made significant achievements and developed nanosized iron or
cobalt-based metal/metal oxide catalysts for highly selective hydrogenation of functionalized
nitroarenes. [9] However, in their cases, relatively demanding reaction conditions, for example,
long reaction time (>12 h), high temperature (>120 oC), high H2 pressure (50 bar), and toxic
and flammable organic solvents are usually required to obtain satisfactory reaction efficiency,
although the catalysts feature good tolerance of functional groups with excellent selectivity. In
addition to the relatively harsh reaction conditions, structurally sophisticated and expensive
organic ligands are prerequisite for fabricating those well-defined iron or cobalt-based
catalysts with strong ligand-dependent catalytic performance. Such limitations spurred us to
develop a novel inexpensive non-noble metal catalyst with facile preparation procedure and
can allow for a general reduction of all kinds of nitroarenes with superior catalytic efficiency
and outstanding selectivity using water as a green, non-toxic and non-flammable solvent
under mild reaction conditions.
Recently, the cubic pyrite-type transition metal disulfides (MS2, M = Fe, Co, Ni) with very
diverse and intriguing properties have attracted considerable attention. Amongst the family of
transition metal pyrites, iron disulfide (FeS2), an inexpensive and earth-abundant mineral,
commonly known as fool’s gold, has demonstrated exceptionally high promise in numerous
cutting-edge energy technologies, such as solar cells, lithium/sodium-ion batteries,
electrocatalytic hydrogen and oxygen evolution, and oxygen reduction as well. [10] However,
to date, the investigation of FeS2 or other pyrites as catalyst for organic transformations is still
limited, and only sporadic studies are available.[11,12] For instance, Guo and Schaak
independently investigated bulk FeS2 or FeS2 nanoparticles as catalyst for the selective
hydrogenation of nitroarenes. [11] In both cases, nitroarenes bearing C=C and other functional
groups could be converted to the corresponding amines with full selectivity at complete
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conversion; however complete selectivity for 4-iodonitrobenzene, one of the most demanding
substrates, was obtained only at 75% conversion, indicating the possibility of dehalogenation
at higher conversion. Nonetheless, despite the easy availability, good biocompatibility and
low price of pyrite FeS2 with generally good selectivity for hydrogenation of nitroarenes, the
overall activities achieved so far are very poor and need to be further improved. Furthermore,
the presence of iron oxides on the surface of the as-prepared bulk FeS2 also raises a question
of which phase is actually the active catalyst, which needs to be elucidated by formation of a
single phase pyrite FeS2 in the catalyst. On the other side, the synthesis of single phase pyrite
FeS2 with precisely defined composition remains challenging. Specifically, demanding
preparation conditions are necessary due to unavoidable segregation of Fe and S species,
which generally causes changes of the variation of the stoichiometry, crystalline structure, and
material phase, thereby substantially influencing its photovoltaic efficiency and catalytic
performance..[13]
Our recent studies and other previous reports have demonstrated that the hybridization of the
non-noble metal catalysts, such as cobalt, nickel, and iron, with heteroatoms-doped porous
carbon can lead to improved catalytic activity and selectivity as well as stability in catalysis.[14]
The enhancement of catalytic performance was attributed to the synergistic effect of chemical
compositions (appropriate N, P, or S-doping) and unique porous structures (large specific
surface area, high pore volume, hierarchical porous channel). We envision that a boosted
reaction efficiency could be expected if nanosized hybrid pyrite catalyst, composing of FeS2
nanoparticles and heteroatom-doped porous carbon materials, was fabricated, yet such
investigation has not been explored before.
Herein, we report, for the first time, the fabrication of novel single phase pyrite FeS2
nanoparticles (NPs) supported on N,S-codoped hierarchical porous carbon (FeS2/NSC) as
highly active and stable catalyst for chemoselective hydrogenation of functionalized
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nitroarenes. The FeS2/NSC was prepared via a facile and three-step (hydro-)thermal process,
employing naturally renewable and easily available biomass as N and C sources together with
earth-abundant and low cost iron salt as starting materials, in an operationally simple,
environmentally benign, and cost-effective manner. The FeS2/NSC can selectively
hydrogenate a broad spectrum of functionalized nitroarenes into more versatile and valuable
anilines using water as the solvent under mild conditions (2.0 MPa H2, 120oC). Compared
with unsupported bulk FeS2, FeOx, and even other non-noble metal-based heterogeneous
catalysts reported in the literature, a remarkably enhanced catalytic activity with exclusive
selectivity was achieved successfully. Characterization studies reveal that uniform size and
single-phase FeS2 NPs with precisely defined composition were homogeneously dispersed on
N,S-codoped porous carbon, and the FeS2/NSC possesses large specific surface area,
hierarchical porous channel, high pore volume with strong interaction between FeS2 NPs and
support. DFT calculations disclose that FeS2/NSC has a preferential adsorption capacity for
the nitro group, which account for the enhanced catalytic activity and exclusive
chemoselectivity. Furthermore, the FeS2/NSC could be readily recovered for successive
reuses without an appreciable loss in catalytic performance, highlighting it practical potential.
2. Results and discussion
2.1. Preparation and Characterization of the FeS2/NSC Catalyst
The single phase pyrite FeS2 NPs supported on N,S-codoped hierarchical porous carbon
(denoted as FeS2/NSC) was synthesized in a sequential hydrothermal, pyrolysis, and
sulfurization proedure as shown in Scheme 1 (see details in the Supporting Information). First,
fresh bamboo shoots were cut into slices, dried and ground into powder followed by the
hydrothermal process to get a brown hydrochar solid. Second, the resultant solid was
homogeneously mixed with Fe(NO3)3 followed by pyrolysis under N2 atmosphere at 800 oC
for 2 h to get black powder, labeled as FeOx/NC. Subsequently, the FeS2/NSC was obtained
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by annealing the FeOx/NC with sublimed sulfur at 500 oC with a heating rate of 5 oC min-1 for
2 h in a N2 atmosphere. The iron content in FeS2/NSC was determined to be 6.8 wt% by the
coupled plasma optical emission spectrometry (ICP-OES), and N and S contents were
estimated to be 3.07 and 26.75 wt% by elemental analysis.
Scheme 1. Illustration of the procedure for preparation of the FeS2/NSC.
Powder X-ray diffraction (XRD) pattern of FeS2/NSC (Figure 1A) shows that all diffraction
peaks, exclusively indexed to the (111), (200), (210), (211), (220), (311), (222), (023), and
(321) facets, respectively, were clearly observed, which are in good line with a cubic FeS2
(JCPDS No. 42-1340; space group Pa-3, a = b = c = 5.4179 Å). No other peaks from
impurities such as marcasite FeS2, or hexagonal troilite FeS could be detected. The average
lattice constant of the pyrite FeS2 NPs was found to be 5.424 Å, in agreement with
stoichiometric pyrite. Besides, two characteristic diffraction peaks with less intensity at 26.2
and 44.0o corresponding to the (002) and (004) facets of graphitic carbon were also observed,
indicating the formation of graphitic carbon in the sample. Raman spectrum verifies the
formation of graphitic carbon in the catalyst FeS2/NSC, in which two conspicuous peaks at
1590 and 1350 cm-1 appeared ( Figure 1B). Of them, the peak at 1350 cm-1 is attributable to
the D band for the disordered carbon, edge defects, and other defects, while the peak at 1590
cm-1 corresponds to the G band for the vibrational mode in the sp2-bonded graphitic carbon.[15]
Moreover, three peaks located at 332, 377 and 430 cm-1 were also detected, which are the
characteristic active modes for bulk pyrite FeS2 corresponding to the S2 vibration (Eg), S-S in-
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phase stretch (Ag), and stretch (Tg) modes,[16] respectively. This observation further confirms
the formation of single phase pyrite in the sample.
Figure 1. (A) XRD pattern and (B) Raman spectroscopy of FeS2/NSC. (C, D) HR-TEM
images and (E) HAADF-STEM images and EDX mappings of C, O, N, S and Fe of
FeS2/NSC. (F) EDX analysis result for the FeS2/NSC.
Transmission electron microscope (TEM) imaging discloses that FeS2 NPs with rather
uniform sizes of 30 ± 10 nm are evenly distributed on the surface of carbon support (Figure
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1C). The high resolution TEM (HR-TEM) image presents lattice fringes with interplanar
spacings of 0.270 and 0.312 nm, corresponding to the (200) and (111) crystal planes of the
pyrite FeS2 (Figure 1D). The homogeneous distribution of FeS2 NPs and each component of
Fe, N, S, and C atoms over the entire sample are further supported by the high-angle annular
dark-field scanning transmission electron microscopy (HAADF-STEM) images and the
corresponding energy-dispersive X-ray (EDX) elemental mapping analysis (Figure 1E). The
atomic ratio of Fe and S is estimated to be 1:2.03 from EDX analysis, being consistent with
the stoichiometric pyrite FeS2 (Figure 1F). To better convince the formation of single phase
pyrite FeS2 in the FeS2/NSC, 57Fe Mössbauer spectroscopy was carried out (Figure 2E) and
showed the presence of only one paramagnetic doublet with chemical shift = 0.31 mm s-1 and
quadrupole splitting = 0.62 mm s-1, which is clear evidence for the formation of single phase
FeS2 and are well consistent for FeS2 with the pyrite structure. [17]
The surface elemental status of the FeS2/NSC was further characterized by X-ray
photoelectron spectroscopy (XPS). The S 2p3/2 and S 2p1/2 peaks (Figure 2C) observed at
162.7 and 163.8 eV, respectively, are consistent with the sulfur binding energy in bulk pyrite.
[18] An intense peak at 164.9 eV corresponding to C-S-C bond was observed, indicating the
success of incorporation of S atom into carbon framework. [19] Besides, two peaks around
168.8 eV assignable to the S-O oxidized species of sulfate were also detected, mainly due to
the oxidation in the storage. [20] Note that, no signals for elemental S at 164.0 (2p3/2) and 165.2
eV (2p1/2) were observed, demonstrating that the catalyst is free of elemental S impurity. [21]
The Fe 2p spectrum (Figure 2D) shows a pair of spin-orbit split peaks at 707.4 (2p3/2) and
720.1 eV (2p1/2), respectively, which is consistent with the literature values for pyrite FeS2. [22]
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Figure 2. (A-D) XPS spectra of C 1s, N 1s, S 2p, Fe 2p for FeS2/NSC. (E) 57Fe Mössbauer
spectroscopy at 293 K and (F) N2 adsorption isotherm of FeS2/NSC.
Two less intensive shake-up peaks located at 711.6 and 725.6 eV were observed, assignable to
the partial Fe3+-S coordination bond and the surface oxidation states. [23] In addition, the signal
at approximate 709.8 eV was also detected, which can be assigned to Fe in Fe-Nx
configuration. [9a, 24]The high-resolution spectrum of C (Figure 2A) was fitted into five
subpeaks, i.e., C-C/C=C (284. 7 eV), C-N (285.4 eV), C-S (286.6 eV), C-O (287.3 eV), and
COOH (290.1),[25] verifying that the N and S atoms are incorporated into the porous carbon
framework. Five types of N (Figure 2B), including pyridinic N (398.5 eV, 15%), Fe-Nx (399.6
eV, 16%)[26], pyrrolic N (400.9 eV, 37%), graphitic N (402.0 eV, 27%), and oxidized N
(403.2 eV, 5%), could be distinguished from N 1s spectrum. Notably, the apperance of Fe-Nx
configuration in the sample from Fe 2p and N 1s XPS analysis implies the interaction of FeS2
NPs with N atoms in the carbon framework.
N2 adsorption/desorption measurements (Figure 2F) clearly demonstrates that the FeS2/NSC
obtained with this preparation strategy posseses hierarchically micro-, meso-, and macro-
pores with high specific surface area (734.5 m2 g-1) and large pore volume (0.408 cm3g-1).
Note that, the hierarchical porous structure with a high specific surface area is expected to not
only favor the reactant adsorption and rapid mass-transfer but also can provide good
accessibility to the active sites, thereby can boost the catalytic performance.
2.2 Catalytic Hydrogenation of Nitroarenes.
Catalytic hydrogenation of nitrobenzene (1a) is initially used as a model reaction to evaluate
the catalytic activity of FeS2/NSC. To pursue the reaction under milder, greener and more
sustainable conditions, the hydrogenation was first conducted at 120oC under a H2 pressure of
20 bar using water as solvent, and the presentative results are shown in Table 1. To our
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pleasant, full conversion of 1a to aniline (2a) was achieved within 5 h over the FeS2/NSC
(entry 1). A decrease of reaction temperature or pressure of H2 resulted in a great lower
reaction efficiency (entries 2-4). With this impressive result in mind, a series control
experiments were subsequently performed. When the NSC itself was used as the catalyst, only
trace amount of 2a (6%) was observed (entry 14), demonstrating the almost inert nature of
NSC for hydrogenaton of nitrobenzene under the reaction conditions. Bulk FeS2 as a catalyst
delivered only 26.7% of conversion 1a to 2a under the same reaction conditions (entry 10).
These two results indicate that the combination of FeS2 NPs and NSC is necessary to
synergistically facilitate the reaction to achieve high reactivity. To prove this conclusion, the
catalyst FeS2/SC without N-doping was prepared from commercially available activated
carbon and was employed as a catalyst for the model reaction. The catalyst FeS2/SC gave less
efficiency than that of FeS2/NSC under otherwise identical conditions (entry 5), while its
catalytic activity was still higher than that of bulk pyrite FeS2 (entry 10). It should be noted
that the catalyst FeS2/SC has a similar size of FeS2 NPs to that of FeS2/NSC, as shown in
Figure S2. In addition, control test of the reaction in the absence of catalyst under identical
conditions only showed a negligible activity (entry 13), highlighting the critical role of the
FeS2/NSC for the hydrogenation of nitroarenes. For comparison, other non-noble metal-based
heterogeneous catalysts, including Fe-phen/C-800[8a], FeOx/NC, CoS2/NSC, and NiS2/NSC
(entries 6-9), were also employed as the catalysts for the reaction, and none of them suppassed
the reaction efficiency of the FeS2/NSC. Particularly, the catalysts Fe-phen/C-800 and
FeOx/NC, which consist Fe2O3 NPs as major phases, showed rather lower catalytic activity
compared with FeS2/NSC under the present conditions (entry 9). Taking into account these
results, we can come to a conclusion that the large specific surface area, high pore volume,
and N, S atoms codoping of the carbon framework substantially contribuate to the enhanced
catalytic performance.
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Table 1. Hydrogenation of nitrobenzene over the FeS2/NSC catalyst.a
Entry
Catalyst
Temperature Pressure
Conversion
(%)b
>99
Selectivity
TOF
(h-1)c
4.4
(oC)
120
100
80
( bar)
20
20
20
10
20
20
20
20
20
20
20
20
20
20
20
(%)b
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
1
FeS2/NSC
FeS2/NSC
FeS2/NSC
FeS2/NSC
FeS2/SC
FeOx/NC
CoS2/NSC
NiS2/NSC
Fe-phen/C-800
FeS2
2
53.6
35.2
84.9
51.3
37.5
84.9
41.4
52.6
26.7
17.5
15.2
10.6
3.7
2.3
3
1.5
4
120
120
120
120
120
120
120
120
120
120
120
120
3.7
5
2.3
6
1.7
7
3.7
8
1.8
9
2.3
10
11
12
13d
14
15
1.2
Fe2O3
0.77
0.67
0.47
0.16
0.26
Fe3O4
Nano Fe powder
-
NSCe
6
aReaction conditions: nitrobenzene (0.5 mmol), catalyst ( 2.3 mol% of Fe), H2O (2 mL), H2
(20 bar), 120oC, 5 h. bDetermined by GC using dodecane as an internal standard. cTOF was
calculated by mole of nitrobenzene converted divided by mole of metal per hour. dIn the
absence of any catalyst. eNSC represents N,S-codoped carbon with same preparation
procedure without addition of iron salt.
After identifying the optimized reaction conditions, we further applied the FeS2/NSC for
hydrogenation of the most demanding 4-iodonitrobenzene (1x) and 3-nitrostyrene (1y) among
a variety of functionalized nitroarenes, due to their being considerably prone to C=C bond
reduction and dehalogenation. Delightfully, the FeS2/NSC not only showed high activity for
both reactants, but more importantly greater than 99% of the selectivity to their respective
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desired 4-iodoaniline (2x) and 3-aminostyrene (2y) was achieved at full conversion, without
observation of any reduction of C=C bond and dehalogenation even with prolonged reaction
times (Figure 3). These results are significantly different from those of platinum-group
catalysts exhibited, where low chemoselectivity to the desired amines was usually achieved
after full conversion of 1x and 1y. For example, the commercially available Pd/C gave
complete over-reduction product 3-ethylaniline (3y) after full conversion with slightly
prolonged times, although it exhibited rather higher reaction efficiency (Figure 3A). Notably,
in the case of hydrogenation of 3-nitrostyrene, 96% of isolated yield with a TOF as high as
8.4 h-1 was achieved, which is 42 and 120 times higher than those catalyzed by bulk pyrite
FeS2 reported by Guo and Schaak (Table S3), [10] respectively. To the best of our
knowledgement, this is the highest value among many state-of-the-art nonprecious metal-
based heterogeneous catalysts (Table S3). More importantly, the reaction conditions (20 bar
H2, 120 oC, 5 h, water as solvent) in this case are comparatively much milder and greener than
those previous reports, making the process more operationally practical and cost-effective.
Figure 3. Comparison of 3-nitrostyrene hydrogenation over commercial Pd/C (A) and
FeS2/NSC (B)
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Subsequently, we further explored the generality of FeS2/NSC for other functionalized
nitroarenes. Overall, the FeS2/NSC showed outstanding tolerance to a broad spectrum of
substituted nitroarenes, as demonstrated in Table 2. Various functional groups are compatible
with the present reaction conditions, especially for those bearing aldehyde (1s), ketone (1u),
ester (1v), amide (1t), nitrile (1w), and halides, e.g., (1h-j, 1x). In all cases listed in Table 2,
superior activity with exclusive selectivity to the corresponding anilines was observed without
detection of the
Table 2. Substrate scopea
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aReaction conditions: nitroarene (0.5 mmol), FeS2/NSC ( 2.3 mol% of Fe), H2O (2 mL), H2
(20 bar), 120oC, 5 h. Isolated yield are given.
reduction of these reducible groups or dehalogenation. Furthermore, this protocol is also
applicable for heteroatom-containing nitros, such as 3-nitropyridine (1q) and 5-nitroquinoline
(1r), affording their respective anilines in high yields. In addition, marketed nitro-substituted
drugs, for example, Nimodipine (1ac), Nilutamide (1ad), and Niclosamide (1ae), were
hydrogenated to their corresponding amines in high isolated yields with excellent selectivity.
Durability and recyclability of a catalyst are important features for the advancement of
sustainable practical processes. Upon completion of the hydrogenation of 3-nitrostyrene, the
catalyst FeS2/NSC was recollected by centrifugation, washed and dried for subsequent cycles.
As shown in Figure S3, the FeS2/NSC is highly stable and can be conveniently recycled up to
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eight times to chemoselectively produce 3-aminostyrene without any loss of the selectivity,
although the activity had a slight decrease. No significant difference was observed for the
catalyst FeS2/NSC before and after the reaction by Raman and elemental analysis, clearly
indicating the strong stability (Figure S4 and Table S4).
2.3. Mechanism of Catalytic Behavior of FeS2/NSC.
Scheme 2. Comparison of selected supported iron catalysts for hydrogenation of styrene.
To unveil the underlying factors that provide the excellent activity and superior selectivity in
the hydrogenation of nitroarenes, some control experiments were carried out. First, we
performed the hydrogenation of styrene over the FeS2/NSC under the optimized conditions.
As shown in Scheme 2, the catalysts FeS2/NSC, FeOx/NC, FeS2/SC all showed a negligible
activity with almost complete recovery of styrene. However, high activity was achieved for
the hydrogenation of nitrobenzene as shown in Table 1. Furthermore, when a mixture of (1:1)
of styrene and nitrobenzene was subjected to the optimized conditions, only
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Figure 4. ATR-IR spectra for nitrobenzene (A) and styrene (B) adsorption behavior over the
FeS2/NSC
aniline was observed in quantitative yield with the C=C bond in styrene untouched. These
results clearly demonstrate that the FeS2/NSC was intrinsically active for the hydrogenation of
the nitro group, most likely due to the preferential adsorption of the nitro versus the olefinic
group. Second, we performed ATR-IR spectroscopy to study the adsorption behavior of the
FeS2/NSC for both nitro and olefinic group (Figure 4). Two obvious peaks around 1346 and
1524 cm-1are presented after adsorption of nitrobenzene on FeS2/NSC, which can be assigned
to the asymmetrical stretch of -NO2 group [8a,b]. In contrast, no characteristic peaks assignable
to the C=C group were observed after adsorption of styrene on FeS2/NSC. Such IR results
demonstrate that FeS2/NSC has a preferential adsorption of the nitro rather than the olefinic
group, which is line with the results of hydrogenation of nitrobenzene and styrene under the
same reaction conditions. It has been well-documented that heteroatoms (e.g., N, S, or P)
doped on carbon could effectively tune electron redistribution at the interface of heteroatoms-
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doped carbon and metal immobilized or encapsulated on carbon, and in turn could enrich the
positive charges on the side of metal via the Mott-Schottky effect.[27] As a consequence, the
large electronegative N and S atoms co-incorporated in the carbon makes single phase FeS2
nanoparticles more positively charged, which is more favorable for the adsorption of the nitro
group owing to its strong electron-withdrawing nature. In addition, H2-TPR experiments for
the catalyst FeS2/NSC suggest that FeS2 NPs has a relatively stronger interaction with N,S-
codoped carbon (Figure 4), which was also evidenced by the observation of Fe-Nx
configuration in Fe 2p and N 1s XPS.
Figure 4. H2-TPR profile for FeS2/SC and FeS2/NSC.
To get deeper insight into such unique catalytic behaviors of FeS2/NSC, periodic density
functional theory (DFT) calculations were performed. Pyrite FeS2 (200) facet and positively
charged FeS2 (200) facet were adopted as models for representing FeS2 and FeS2/NSC. It is
well-known that the activation of molecular H2 and adsorption modes of 3-nitrostyrene or
other functionalized nitroarenes on the catalysts have profound influence on the activity and
selectivity for hydrogenation of nitroarenes. As shown in Figure 3, the product distribution of
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10.1002/cssc.201901867
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hydrogenation of 3-nitrostyrene as a function of reaction times reveals that the reaction
proceeded in a direct way without detection of any intermediate. As such, we firstly
calculated the dissociation energy for molecular hydrogen on both bulk FeS2 (200) and
positively charged FeS2 (200). In this case, only a slight difference (ca. 0.02 eV) between two
models with a dissociation energy of 0.30 and 0.28 eV, respectively, was observed (Figure
S5). Such an observation indicates that the formation of a single phase pyrite FeS2 NPs on
N,S-codoped carbon didn’t cause a great effect for the activation of H2 on the surface of
catalyst. Thus, the contribution from H2 activation for the highly boosted reaction efficiency
could be ruled out. Subsequently, DFT calculations for the adsorption energy of 3-
nitrostyrene on both models were carried out to study the interaction between nitro or olefinic
group and catalysts. As shown in Figure 5, -0.33 versus -0.50 eV of adsorption energies were
observed for the adsorption through the C=C and –NO2 groups with FeS2 (200), respectively,
strongly suggesting a more preferential adsorption through the nitro group. This phenomena
was further reinforced for the adsorption on positively charged FeS2 (200). In this case, the
calculated adsorption energy of -1.09 eV for the adsorption through nitro group was obtained,
which is 2 times higher than that on the FeS2 (200) surface. Meanwhile, a much lower
adsorption energy of -0.21 eV for the adsorption through C=C group was obtained, indicating
a less stable adsorption on the surface. Consequently, DFT calculations clearly disclose that
the positively charged FeS2 NPs greatly enhance adsorption of the nitro group. This is also in
good consistent with the results of ATR-IR and hydrogenation reaction. Taking into account
computational and control experiment results together, the unique features of preferential
adsorption of nitro group together with large specific surface area, hierarchical pores, high
pore volume and strong metal-support interaction shown by the FeS2/NSC account for the
enhanced activity and exclusive selectivity for the hydrogenation of functionalized nitroarenes.
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10.1002/cssc.201901867
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Figure 5. Adsorption energy of 3-nitrostyrene on pyrite FeS2 (200) and positively charged
FeS2 (200) surface. Color index: Fe, gray; S, yellow; C, dark gray; N, blue; O, red; H, white.
3. Conclusions
In summary, a single phase pyrite FeS2 nanoparticles supported on N,S-codoped hierarchical
porous carbon was fabricated in a straightforward and cost-effective process. FeS2
nanoparticles with uniform size were homogeneously dispersed on N,S-codoped porous
carbon derived from biomass with high specific surface area, hierarchical pores and large pore
volume. The resultant FeS2/NSC exhibited outstanding catalytic activity and exclusive
selectivity for catalytic hydrogenation of various functionalized nitroarenes with a broad
spectrum of functional groups. Significantly, a remarkable enhancement in catalytic activity
was achieved compared with the bulk FeS2 or other non-noble metal-based catalysts under
milder reaction conditions in water. DFT calculations disclose that FeS2/NSC have a
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10.1002/cssc.201901867
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preferential adsorption of the nitro group, which accounts for the highly boosted activity and
superior selectivity. In addition, the FeS2/NSC demonstrated a high stability and could be
recycled for reuse up to 8 times without an appreciable loss in catalytic performance. This
work represents an important advance forward for boosting the catalytic performance by
combination of a non-noble metal with heteroatoms-doped porous carbon materials.
Acknowledgements
The authors would like to acknowledge the financial support from the Key Technology R&D
Program of Shandong Province (2019GGX102075), DICP & QIBEBT (Grant No. DICP &
QIBEBT UN201704), and the Dalian National Laboratory for Clean Energy (DNL), Chinese
Academy of Sciences. Y.Y also thanks the Royal Society (UK) for a Newton Advanced
Fellowship (NAF-R2-180695).
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