The synergic effects at the molecular level in CoS2 for selective hydrogenation of nitroarenes

Article abstract of DOI:10.1039/c7gc03122j

Catalytic functionalized aniline formation from nitroarenes is a core technology in the synthesis of pharmaceuticals, agrochemicals, and fine chemicals. However, control of chemoselectivity still poses particular challenges with aromatic nitro substrates bearing one or more reducible groups. Here, we report the low-cost synthesis of a porous carbon supported CoS₂ catalyst (CoS₂/PC) and successfully apply the catalyst in the chemoselective hydrogenation of nitroarenes. For hydrogenation of 3-nitrostyrene, the catalyst furnishes a superior selectivity of 99% towards 3-aminostyrene at a conversion of >99%. Density functional theory calculations together with X-ray absorption fine structure spectroscopy reveal that terdentate and tetrahedral coordinated Co atoms in CoS₂ (labeled as Co₃ and Co₄) are possible active sites. The face to face located Co₃ and Co₄ sites make the reaction rather local, and Co₃ and Co₄ sites are occupied by substrates and H₂, respectively, which is beneficial to the superior activity and selectivity. The Co₃-Co₄ "synergic active site pair" in CoS₂ makes the investigation of the synergic effects at the molecular level a reality on heterogeneous catalysts.

Full text of DOI:10.1039/c7gc03122j

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The synergic effects at molecular level in CoS2 for selective
hydrogenation of nitroarenes
Received 00th January 20xx,
Accepted 00th January 20xx
Zhongzhe Wei,a, ‡ Shanjun Mao,a, ‡ Fanfei Sun,b Jing Wang,a Bingbao Mei,b Yiqing Chen,a Haoran Li,a
Yong Wang*,a
DOI: 10.1039/x0xx00000x
Catalytic functionalized aniline formation from nitroarenes is a core technology in the synthesis of pharmaceuticals,
agrochemicals, and fine chemicals. However, control of chemoselectivity has still posed particular challenges when
aromatic nitro substrates bearing one or more reducible groups. Here, we report the low-cost synthesis of porous carbon
supported CoS2 catalyst (CoS2/PC) and successfully apply the catalyst in the chemoselective hydrogenation of nitroarenes.
For hydrogenation of 3-nitrostyrene, the catalyst furnishes a superior selectivity of 99% towards 3-aminostyrene at
conversion >99%. Density functional theory calculations together X-ray absorption fine structure spectroscopy reveal that
terdentate and tetrahedral coordinated Co atoms in CoS2 (labeled as Co3 and Co4) are possibe active sites. The face to face
located Co3 and Co4 sites make the reaction rather local, and Co3 and Co4 sites are occupied by substrates and H2,
respectively, which is beneficial to the superior activity and selectivity. The Co3-Co4 “synergic active site pair” in CoS2
makes the investigation of the synergistic effect at molecular level a reality on heterogeneous catalysts.
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techniques have been used to control the selectivity by the use
of suitable additives3, 8-11 or modification of active metals12-14
.
Introduction
Although these approaches may improve the selectivity in
specific cases, it is necessary to develop more sustainable
process to reproducibly control the selectivity of the catalysts.
Impressively, supported Au15-17 and Ag18, 19 nanoparticles (NPs)
emerging as state-of-the-art catalysts achieved high
chemoselectivity (90%–99%) of nitroarenes, including 3-
nitrostyrene, which presented a new strategy. By tuning the
metal-support interaction, nonselective hydrogenation metals
such as Pt, Ru, and Pd can also be transformed into selective
catalysts.20-22 Nevertheless, the risk in supply and volatile price
of noble metal catalysts severely limited their
industrial applications. Development of efficient non-noble
metal catalysts to replace precious metal catalysts
has practical significance because of their abundance and low
price.23-26 To date, great progress has been made in developing
earth-abundant metals-based heterogeneous catalysts for
Catalysis is the core field of nanoscience and nanotechnology.1
Selectivity is a major issue in heterogeneous catalysis. High
selectivity means less waste, simpler and cheaper separation
units, which is of growing importance to sustainability in
modern society. Furthermore, achieving high selectivity along
with high conversion has emerged as the prime concern in
designing catalysts.
Functionalized amines, produced from aromatic nitro
compounds by reduction, are important platform chemicals in
the production of multifunctional molecules, such as
3
pharmaceuticals, dyes and agrochemicals.2, Compared with
the use of the stoichiometric reducing agents such as sodium
hydrosulfide, Fe and Zn, development of heterogeneous
catalysts is the primary trend due to their environmentally
friendliness. Selective hydrogenation of nitroarenes using H2 is
preferred in industry because it is cheap, readily available and
forms water as the reaction byproduct.4 Many amines
substituted with diverse functional groups, including nitrile,
aldehyde, keto, amide and halide can be successfully obtained
by well-defined catalysts.5-7 However, the nitrobenzene
bearing a carbon-carbon double bond has still posed particular
challenges as both groups are easily hydrogenated. A series of
chemoselective hydrogenation of nitroarenes, such as Fe27, 28
,
Co29-33 and Ni20, 34-36. In terms of 3-nitrostyrene, these catalysts
afforded the desired 3-aminostyrene in 82%-97% selectivity
and the conversion should be precisely controlled. Once
prolong the reaction time to further improve the conversion,
byproduct 3-vinylaniline will produce. Although the selectivity
is comparable to that obtained by noble metal catalysts, there
is still room for improvement. Identifying and developing
advanced base-metal catalysts with superior activity and
selectivity is an ongoing challenge.
a. Advanced Materials and Catalysis Group, ZJU-NHU United R&D Center,
Department of Chemistry, Zhejiang University, Hangzhou 310028, P. R. China.
b. Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics,
Chinese Academy of Science, Shanghai 201204, P. R. China.
Here we report a sort of catalysts with regular isolated
metal active sites, earth-abundant transition metal pyrites
(MS2, M=Fe, Co, and Ni), for heterogeneous catalytic
hydrogenation systems with molecular hydrogen as the
‡ These authors have contributed equally.
Electronic Supplementary Information (ESI) available: Detailed analysis of the
catalysts, the analysis of “synergic active site pair”, DFT calculations of reaction
mechanism, the catalytic activity of the catalyst. See DOI: 10.1039/x0xx00000x
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reducing agent considering their metallic resembling electronic Table 1. Catalytic hydrogenation of 3-Nitrostyrene a
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structures.37, 38 Gratifyingly, CoS2 NPs on porous carbon (PC)
hybrids (CoS2/PC) synthesized in this work exhibit an
extraordinarily high selectivity of 99% even if we prolong the
reaction time after the fully conversion of nitro-groups.
Further, combining the experimental and theoretical analysis,
we conclude that the higher intrinsic activity and the
preferential adsorption of nitro group guarantee the perfect
DOI: 10.1039/C7GC03122J
Entry
Catalyst
Fe2O3/PC
Co3O4/PC
NiO/PC
t (h)
8
Conv. (%)b Yield (%)b
1
2
3
4
5
6
7
-
-
8
-
-
8
-
-
FeS2/PC
FeS2/PC
CoS2/PC
NiS2/PC
CoS2/AC
8
76
>99
>99
51
26
76
99
99
50
96
selectivity. The reaction mechanism follows
a
direct
hydrogenation route. These new scenario can be attributed to
the synergic effect between the terdentate and tetrahedral
coordinated Co atoms in CoS2, which can be revealed by
density functional theory (DFT) calculations and X-ray
absorption fine structure spectroscopy (XAFS).
12
8
12
8
8
a
Reaction conditions: 0.5 mmol 3-nitrostyrene, 4 mol% M, 3 mL
o
CH3OH, 110 C, 3.0 MPa H2. b Determined by GC (internal standard:
n-dodecane).
Results and discussion
Fabrication and Characterization of Catalysts. PC was synthesized
by a novel “leavening” strategy through simple pyrolysis of α-
cellulose, (NH4)2C2O4 and NaHCO3.39 The metal pyrites supported on (Table 1, entries 1-3). However, once transforming TMOs into MS2,
PC (denoted as MS2/PC) were fabricated by the “oxide-to-sulfide” things turn around. All the tested MS2 displayed perfect selectivity
method (Figs S1-9). The high-angle annular dark field-scanning with CoS2/PC performing the highest activity (Table 1, entries 4-7),
transmission electron microscopy (HAADF-STEM) image showed furnishing 99% yield of 3-vinylaniline exclusively. The TOF (defined
that the lattice spacing of 0.160 and 0.320 nm were assigned to the as moles of 3-nitrostyrene converted per mol of exposed CoS2 per
(222) and (111) facets of CoS2 (Fig. 1). The Z-contrast intensity hour) was estimated to be 112 h-1. As far as we know, this is among
distribution captured in the selected area in Fig. 1, marked with a the highest values ever reported on transition metal based
21, 40
yellow rectangle reveals Co-S-Co periodic pattern. STEM and catalysts.18,
Obvious particle aggregation occurred in FeS2/PC
elemental mapping analysis of CoS2/PC revealed that C, Co and S and NiS2/PC, which may cause the relatively poor catalytic activity
species were uniformly distributed in the entire skeletal framework (Fig. S10). Besides, the reaction conditions were systematically
(Fig. S6). Powder X-ray diffraction (XRD) pattern of CoS2/PC (Fig. 3c) optimized by variation of the pressure, temperature, and solvent
validated the formation of CoS2 pyrite (JCPDS 41-1471) with high (Table S2). To elaborate the role of support, we designed the
crystalline. XPS investigations further verified the formation of CoS2 catalyst of CoS2 NPs loaded on activated carbon (CoS2/AC). The
(Fig. S7). The textural properties of CoS2/PC were measured by N2 catalytic performance of CoS2/AC was tested under the same
sorption isothermal analysis. CoS2/PC exhibited high specific surface reaction conditions with CoS2/PC. 25% yield of 3-vinylaniline was
area (424 m2g-1) and a wide range of pore size distribution (1-100 achieved, much lower than CoS2/PC (Table 1, entry 8). The different
nm) (Fig. S11).
support greatly influenced the catalytic activity. First, the doped
Activity Test. Here hydrogenation of 3-nitrostyrene is used as the nitrogen atoms in PC could provide more surface nucleation sites,
probe reaction to explore the performance of MS2/PC. Methanol
was selected as solvent and the optimized reaction conditions were
o
110 C and 3 MPa H2. Transition metal oxides (TMOs) (Fe2O3, Co3O4
and NiO) supported on PC were all totally inactive for the reaction
Fig. 2. Evolution of reactant and product concentrations with
CoS2/PC. Reaction conditions: 3-nitrostyrene (0.5 mmol), CoS2/PC
(3.7 mol% Co), CH3OH (5 mL), 3 MPa of H2, 110 °C.
Fig. 1. HAADF-STEM image of CoS2/PC.
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DOI: 10.1039/C7GC03122J
Fig 3. Analysis of possible active sites in CoS2/PC. (a) Crystal structures of CoS2 and the possible active sites. Color code: magenta is cobalt
and yellow is sulfur. (b) Projected DOS and spin density (the numbers in the right) of the d orbitals for the possible active sites. (c) XRD
patterns of CoS2/PC catalyst before and after reaction. (d) Typical fitting curves of the EXAFS signal in R-space for samples.
leading to a relatively stable and uniform dispersion of CoS2. Second, when the scale of NPs shrinks to single-atoms. Molecular
the hierarchically porous structure greatly inhibited the limitation of adsorption of H2 occurs since no additional metal atoms are
44
mass transfer of the reactant to the active sites and promoted the available for H binding.43, Here both molecular and dissociated
free diffusion of molecules, thus accelerating the reaction.
adsorption of H2 are considered. Results showed that the true
The extraordinarily high selectivity of the CoS2/PC catalyst was stable and active surface is possibly (110) with Co3 and Co4, which
further evidenced by the evolution of the reactant and product serve as the active sites (Figs S13 and S14). DFT calculations
distributions with reaction time in Fig. 2. The curve showed that 3- indicated that during the reaction, the bridge S in Co4 can be
nitrostyrene can be smoothly transformed into 3-vinylaniline in 7.5 removed by H2, forming Co3 without suffering from phase
h without forming any other side or intermediate products. The transformation, which can be deduced from XRD patterns (Fig. 3c).
selectivity still remained as high as 97%, even if prolonging the The generation of Co3 site can be further revealed by XAFS and
reaction time to 23 h. This distinguishes the CoS2/PC catalyst from exteneded XAFS (EXAFS) spectroscopy, which is a powerful tool to
the noble metal catalysts, which will hydrogenate C=C groups with a ascertain quantitative structural parameters for the atoms
considerable reaction rate, resulting in undesired selectivity at full surrounding of the central Co atoms.
conversion.
Table 2. Structural parameters of different samples obtained from
the XAFS fitting.
Analysis of Possible Active Sites. The possible catalytic surface or
active site is verified firstly by DFT calculations. Two most
stableplanes,41 namely, (100) and (110) are examined for CoS2. For
(100), each Co atom is penta coordinated by S atoms, with four S
atoms coordinated in one plane and the other one vertically below
(labeled as Co5, Fig. 3a). For (110), two kinds of planes with
different terminal S atoms are considered. One contains two kinds
of different four-coordinate Co atoms (labeled as Co4 and Co4, Fig.
S12b). The other one has no bridge S atoms, resulting in terdentate
and tetrahedral coordinated Co atoms (labeled as Co3 and Co4, Fig.
3a).
σ 2 (10- ∆E
0
Sample
CoS2/PC
Shell
N
R(Å)
Rrt (Å)
3Å)
6.5
4.4
6.2
(eV)
-0.4
1.6
Co-S
6.0
2.7
5.3
2.29
3.47
3.88
2.31
3.48
3.91
Co-S
Co-Co
-1.1
CoS2/PC
(used)
Co-S
4.5
2.28
2.31
5.9
1.9
Co-S
6.4
3.5
6.7
2.31
3.49
3.88
2.31
3.48
3.91
6.5
4.3
6.2
1.6
4.2
-1.9
CoS2/PC
(adsorption)
The premise for efficient hydrogenation with H2 lies in the
activation of H2. Generally, H2 dissociates into atomic H on
transition metal NPs directly.42 However, the situation changes
Co-S
Co-Co
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DOI: 10.1039/C7GC03122J
Fig. 4. DFT studies on the hydrogenation of 3-nitrostyrene on CoS2/PC. (a) Eads of the reactants, intermediates and product for the most
energy favorable cases at “synergic active site pair” on Co3 of CoS2 (110). Color code: grey is carbon; blue is nitrogen; red is oxygen and
white is hydrogen. (b) Possible reaction pathways on CoS2. (c) Projected DOS on the process of the first hydrogenation step in 3-
nitrostyrene with molecular adsorbed H2 and atomic H. IS, initial state; TS, transition state; FS, final state.
As shown in Table 2, for the fresh CoS2/PC catalyst, the reaction. Before the reaction, 3-nitrostyrene and H2 should readily
coordination numbers of Co-S in the first shell is 6. For the reused adsorb on the catalyst. Since 3-nitrostyrene binds stronger than H2
CoS2/PC catalyst, the coordination number of Co-S in the first shell (Fig. 4a), Co3 site in the “active site pair” is occupied by 3-
is 4.5. The significantly lower coordination number of Co-S in the nitrostyrene and Co4 site by H2 as seen in the IS state in Fig. 4b.
reused CoS2/PC catalyst suggests that part S atoms in the bulk CoS2 Then the hydrogenation reaction proceeds in the “synergic active
vanish during the reaction. Moreover, the coordination number of site pair”, during which the synergic effect occurs. It is noteworthy
4.5 demonstrates that more Co3 sites formed and the possible that this synergic effect makes the “active site pair” a very efficient
active sites are Co3 and Co4 sites, which is consistent with the DFT local catalytic unit since the global atomic H source supply and the
calculations. In the fitting curve of the EXAFS signal in R-space data migration of some of the intermediates are quite limited owing to
(Fig. 3d), it is found that the peak pattern of the reused CoS2/PC either the unfavorable diffusion barriers across the active sites (Fig.
catalyst is similar with the fresh catalyst, again manifesting that the S13c) or strong adsorption (Fig. 4a). In addition, the hydrogenation
crystal of CoS2 is preserved during the reaction.
activity of molecular adsorbed H2 is much higher than that of
Actually, H2 prefers molecular adsorption and Co3 site is much atomic H, which again indicates the low efficiency of hydrogenation
more active. The origin of the activity stems from the unsaturation through global atomic H source and the local hydrogenation
of the Co atoms, similar with that of TMOs.45 As shown in Fig. 3b, character. For instance, the energy barrier for the first
the density of states (DOS) for the d orbitals of lower coordinated hydrogenation step on 3-nitrostyrene requires only 0.32 eV with
Co atoms shows more significant single peaks caused by spindensity, molecular adsorbed H2 while nearly twice of the value (0.54 and
an indicator for unpaired electrons or orbitals. Only up to one 0.50 eV) is required with atomic H (Fig. 4b and Fig. S15a). The
orbital is accessible for Co5 with 0.6 e spin density, which certainly distinctive activities rise from different activation modes (Fig. 4c).
cannot activate H2 since two orbitals are required. However, for Co3 When hydrogenating with molecular adsorbed H2 , only ~1 eV has to
and Co4, the high spin density ensures enough orbitals for H2 be driven up to break the bonding σg state of H2 by injecting
adsorption. More important is that the face to face located Co3 and electrons from the d band of Co4 to the antibonding σu* state of
Co4 sites array parallel in line uniformly (Fig. S12c), forming Co3-Co4 H2.42 However, ~2 eV has to be overcome to break 1s bond of H
“active site pairs”, which makes it possible to investigate the when atomic H is used as H source.
synergic effect at the molecular level in hydrogenation reaction.
The reaction begins with NO2 being hydrogenated by molecular
Synergic Effect between Co3 and Co4. Since the activity of Co3 and adsorbed H2, forming NOOH (Fig. 4b), since the dissociation of NO2
Co4 is different, different roles they will play in the hydrogenation
to Co4 before hydrogenation requires a barrier as high as 0.65 eV
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(Fig. S15b). Then NOOH continually reacts to the left H atom, the barrier required to react forward (0.24 eV). To further support
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leaving chemisorbed H2O, and N-nitroso compound (NO) is formed. our hypothesis, hydrogenation of nitroDsoObI:e1n0z.1e0n3e9/C(7NGOC’0)31w22aJs
Further hydrogenation of NO proceeds after H2O was extracted by conducted (Fig. 5). Results showed that NO’ was mostly converted
methanol and the vacancy left (Co4) reoccupied by H2, resulting in into azoxybenzene (AZOXY’) in less than 10 min, implying that NO’
NOH. The sequential formation of N-hydroxylamine product (NHOH) can be firstly transformed into phenylhydroxylamine efficiently,
will encounter a relative bigger barrier (0.60 eV), which is which, in turn, react with NO’ to form AZOXY’ as Haber et al.
understandable since atomic H addition is involved. When it comes proposed.46 DFT studies also reveal the participation of NO in
to the desirable product 3-vinylaniline (NH2), two possible energy solution along the possible condensation route with or in absence
favorable pathways may contribute. The bond between N and O in of the catalyst (Fig. S16). That is, once NO existed in the solution,
NHOH has to break first overcoming 0.31 eV, forming aniline radical AZOXY can be detected. However, neither NO nor AZOXY was
(NH) before the two pathways start. In one way, NH is captured by detected in our catalytic system, which verified our hypothesis. In
the OH group left on Co3 through the OH…NH hydrogen bond. Then addition, the results summarized in Table S2 showed that the
atomic H is added to NH by hydrogen transferring through the OH selectivity towards 3-vinylaniline was perfect in different solvents
group, forming NH2 with a tiny barrier of 0.12 eV. Note that this without forming any other intermediates. Likely, the reaction
pathway involves the consumption of lower active atomic H, which performed in a non-polar solvent also underwent the direct toute.
is meaningful to release the Co4 active site for H2. Another way
A bird's-eye view of the energy profile illustrates that the rate-
occurs when NH diffuses to another Co3-Co4 “synergic active site determining step seems to be NO to NHOH. However, comparing
pair” where adsorbed NH is hydrogenated by H2 with a barrier the reaction rate of NO2 and NO, results suggest that NO reacts
slightly bigger than that of the former way (0.24 eV). However, this much faster than NO2, which seems in paradox with the theoretical
way is only favorable for H2 addition (0.62 eV is required for atomic implication. This inconsistence can be explained as the strong
H addition). After the yielding of NH2, the OH group can be reduced adsorption of NO on the catalyst. The slow reaction rate (highest
to H2O to recover the active site with a moderate barrier (Fig. S15c). barrier) of NO to NHOH results in the enrichment of NO or poison of
Analysis of the Reaction Mechanism. It has been generally the Co3 site which in turn greatly decreases the efficiency of the
accepted that the mechanistic scheme of catalytic reduction of “synergic active site pair”. Eventually, the conversion of NO2 is
nitroarenes follows either the direct route or the condensation slowed down. This can be easily confirmed by mixing NO’ and
route (Scheme S1).46-48 The whole calculated surface reaction nitrobenzene together as the reactants and measuring their
network indicates a direct route. As reported before, intermediates, reaction rates respectively. As shown in Fig. S17, NO’ reacts quickly
such as azoxybenzene and azobenzene, can be detected along the and reaches full conversion in 10 min, while no obvious conversion
condensation route.49 However, no other side products have been of nitrobenzene was observed before the full conversion of NO’,
detected before the full conversion of nitroarenes (Fig. 2), further which clearly indicates the conversion suppression of NO2 by
suggesting a direct route. Based on the fact that NO and NHOH can poisoning the active site through NO.
react to form AZOXY readily without the catalyst,49 it is reasonable The Original of the High Selectivity of CoS2/PC. To reveal the
to deduce that as long as one of the intermediates strongly adsorb reason of the superior selectivity of CoS2/PC, we conducted the
on the surface of the catalyst and react forward before desorption, hydrogenation of nitrobenzene and styrene respectively. Results
the condensation route can be eliminated. The remarkable Eads of (Table S3) showed that the nitro group had a much higher reaction
NO (-1.04 eV) in Fig. 4a clearly shows that it prefers reacting rate than vinyl group, suggesting CoS2/PC was intrinsically more
forward rather than desorbing into solution when comparing with active for nitro group hydrogenation. What’s more, when the
mixture of nitrobenzene and styrene as reactants, hydrogenation of
styrene was completely inhibited, while the hydrogenation of
nitrobenzene was unaffected (Fig. S18). Note that hydrogenation of
the nitro group and vinyl group requires pre-adsorption.
Preferential adsorption must be considered in that hydrogenation
of the vinyl group cannot be completely inhibited if both the nitro
group and the vinyl group adsorb on the active site.50 Then the
adsorption of nitro group and vinyl group of 3-nitrostyrene on the
catalyst was calculated by DFT. The much bigger Eads of the nitro
group (-0.80 eV) than that of the vinyl group (-0.36 eV) suggests the
preferential adsorption of the nitro group, in consistent with the
suppressed hydrogenation phenomenon for the vinyl group. The
strong adsorption of nitro group on CoS2/PC can be furthur
corroborated by XAFS. As listed in Table 2, the coordination number
of Co-S in the first shell of the fresh catalyst after adsorbed with
nitrobenzene is 6.4, higher than the fresh catalyst, indicating that
the nitro group can adsorb on the Co atoms in CoS2. Therefore, one
can conclude that the extremely superior selectivity observed on
CoS2/PC to the nitro group are not only contributed to the higher
Fig. 5. Distribution of products during the hydrogenation of
nitrosobenzene. Reaction conditions: substrate (0.5 mmol), CoS2/PC
(3.7 mol% Co), 3 MPa of H2, 110 °C.
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Table 3. Chemoselective hydrogenation of various substituted
intrinsic activity but also, especially, accounted for the preferential
adsorption.
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nitroarenesa
DOI: 10.1039/C7GC03122J
Reusability of the Catalyst. The separation and recycling of catalyst
is a key parameter for heterogeneous catalysis. The catalytic activity
of CoS2/PC was tested in successive runs and compiled the results in
Fig. S19. Unfortunately, a progressive loss of activity occurred but
maintenance of perfect chemoselectivity through all cycles. The
activity can improve through extending the reaction time in the
recycles. When the activity of the reused catalyst was evaluated
after 16 hours, both the conversion and selectivity remained
constant through all cycles. TEM image showed that no obvious
aggregation of CoS2 NPs occurred for the reused catalyst (Fig. S20).
The XPS spectra suggested that CoS2 remained unchanged after the
reuse (Fig. S21). Metal leaching shown in Table S4 may be the main
reason for the decreased activity of CoS2/PC during the recycle
reactions. In a separate test, 49% yield of 3-aminostyrene was
achieved within 2.5 h at 110 °C and 3 MPa. Then the liquid phase
was collected by centrifugation. The yield of 3-aminostyrene
remained almost unchanged after a further 2.5 h reaction,
suggesting that the reaction indeed underwent heterogeneous
process.
Hydrogenation of Substituted Nitroarenes. We examined the
general scope of catalytic hydrogenation of various substituted
nitroarenes employing the CoS2/PC catalyst. As shown in Table 3,
various substrates were reduced cleanly with CoS2/PC, affording
high conversion and excellent selectivity. Successful catalytic
hydrogenation was conducted when methyl, methylthio, hydroxyl
and phenyl groups were substituted on the nitrobenzene,
furnishing the corresponding amines in good-to-excellent yields
(Table 3, entries 1-5). Notably, halogen-substituted amines have
been fruitfully achieved without any dehalogenation (Table 3,
entries 6-8). Also noteworthy is the catalyst also works well for
heteroaromatic nitroarenes (Table 3, entry 9). Similarly, CoS2/PC
showed good tolerance to a wide scope of reducible groups, such as
keto, nitrile, aldehyde, amide and alkene, delivering the desired
products in almost full yields (Table 3, entries 10-14). These results
underscore again the unprecedented selectivity of cobalt disulfide
catalyst in the hydrogenation of aromatic compounds with
reducible groups.
Time Yield
Entry
Substrate
Product
(h)
3
(%)b
>99
1
2
3
4
5
6
7
8
9
12
6
95
>99
99
6
NH2
4
99
4
>99
>99
99
6
6
13
>99
Conclusions
10
8.5
96
We reported the straightforward, low-cost synthesis of CoS2
on porous carbon to study the synergic effects between active
sites for heterogeneous catalysis. The Co3-Co4 “synergic active
site pair” in CoS2/PC endows the catalyst with high activity and
superior selectivity in the hydrogenation of nitroarenes. The
concrete synergic interaction, which the face to face located
Co3 and Co4 sites are occupied by 3-nitrostyrene and H2,
respectively, has been illustrated in the discussion of the
reaction mechanism in detail from both theoretical
calculations and experiments. The molecular adsorbed H2
serves as more active hydrogen source in the reaction than
atomic H. The priority for nitro group adsorption guarantees
the exclusive hydrogenation of nitro group and the single
atomic adsorption greatly inhibit the formation of coupling
products. The strong intermediates binding, especially for
nitrosobenzene, is the key factor that makes the reaction
11
12
13
>99
>99
9.5
13
8
98
6 | J. Name., 2012, 00, 1-3
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contents of the liquid were analyzed on GC (ShimaVdiezwu,ArGticCle-2O0n1lin4e)
equipped with a Rtx-1071 column with n-DdOecI:a1n0e.10a3s9/tCh7eGCin0t3e1r2n2aJl
standard and the products were identified by GC-MS (Agilent
Technologies, GC 6890N, MS 5970). For recycling study, the
hydrogenation reaction was conducted with 2 mmol nitrobenzene
maintaining the same reaction conditions as mentioned above. The
catalyst was recovered by centrifugation, washed three times with
methanol, and direct used for the next run.
14
8
99
a
Reaction conditions: 0.5 mmol substarte, 3.7 mol% Co, 3 mL
o
CH3OH, 110 C, 3.0 MPa H2. b Determined by GC (internal standard:
n-dodecane).
The dispersion (D), i.e., the fraction of exposed CoS2 in the
catalyst, can be roughly calculated by the following equation:
proceeding along the direct route. This work paves the way to
explore synergic interaction between the active sites for
heterogeneous catalysis.
D (CoS2 dispersion) = 1.12/[d (mean diameter of CoS2 NPs, nm)
TOF data was defined as follows:
]
TOF = [moles of reactants reacted] / [(moles of metal used)*D
*(reaction time)]
TOF data was calculated at low conversion data (11% conversion
in 0.5 h).
Experimental
Computational Setup. Calculations were performed by using
periodic, spin-polarized DFT as implemented in Vienna ab initio
program package (VASP), 52, 53 using the projector augmented wave
.
A plane wave basis set with an energy cutoff of 400 eV was used
Materials. 3-Nitrostyrene was obtained commercially from Energy
Chemical (Shanghai, China). All other nitro compounds, cellulose,
(NH4)2C2O4, NaHCO3, Co(NO3)2·6H2O, Fe(NO3)3·9H2O, Ni(NO3)2·6H2O,
and S powder were purchased from Aladdin (Shanghai, China).
Unless otherwise stated, all solvents and chemicals used were of
commercially available analytical grade and used without further
treatment. All gases (H2, N2) used for catalyst preparation and
reaction process were ultrahigh purity (99.999%).
(PAW) method proposed by Blöchl 54 and implemented by Kresse 55
and exchange-correlation functional approximation was treated in
56
RPBE functional.
A p (5×5) surpercell containing a 9-layer slab
with 162 atoms for (100) and 10-layer slab with 132 atoms for (110)
were modeled. Thus only gamma point was used for the Brillouin
zone sampling for structure optimization. However, a k-point of 3×
3 × 1 was performed for electronic structure calculation.. The
Synthesis of Porous Carbon (PC). 2 g cellulose, 6 g (NH4)2C2O4 and 6
g NaHCO3 were mixed homogeneously. Then the solid was
transferred into a crucible, directly heated to 800 °C (10 °C min−1),
and kept for 1 h by flowing N2 of 400 mL min−1. When it was cooled periodic condition is employed along the x and y direction. The
to room temperature, the black power was dissolved in 500 mL vacuum space along the z direction was set to be 13 Å. The upper
water and stirred at room temperature for several hours. Then the three or four layer atoms for (100) and (110) respectively in the cell
solution was filtered and washed with deionized water until the pH are allowed to relax during the structure optimization and the
of the filtrate reached 7. Finally, the residue was dried at 70 °C in an bottom six layer atoms are fixed. The relaxation is stopped when
oven overnight.
the force residue on the atom is smaller than 0.02 eV/ Å. The
Synthesis of MOx/PC Hybrids (M = Co, Fe or Ni). The preparation of transition states are calculated by using the climbing image nudged
MOx/PC hybrids was based on simple ethanol assisted impregnation elastic band (CI-NEB) method. 57
method. Typically, 100 mg PC was dispersed in 15 mL ethanol and
stirred for several hours. Separately, a certain amount of metal salt chemisorption or dissociation are defined respectively as: Eads/dis
The adsorption and dissociation energy for molecule
=
(0.125 g Co(NO3)2·9H2O, or 0.145 g Fe(NO3)3·9H2O, or 0.124 g Etot – Eslab – Emol, where Etot is the total energy after a molecule
Ni(NO3)2·6H2O) was added in the above PC-ethanol solution with adsorption or dissociation the on catalysts; Eslab is the energy of the
subsequent ultrasonic treatment at room temperature for 0.5 h. clean catalyst alone; Emol is the energy of the molecule in the gas
Next, the mixture was stirred, dried at the room temperature. phase. The energy barrier is defined as: Ea = ETS-EIS, where ETS is the
Finally, the solid was calcined at 350°C (10 °C min−1) for 3 h in a N2 total energy at the transition state and EIS the initial state.
flow of 400 mL min−1.
Synthesis of MS2/PC Hybrids (M = Co, Fe or Ni). The as-prepared
MOx on porous carbon were thermally sulfidized to MS2 using a
Conflicts of interest
51
modified previously described procedure.
Briefly, 50 mg of
There are no conflicts to declare.
MOx/PC and 2 g of S powder (Aladdin, 99.99%) were physically
mixed and grinded finely. Then the resulting solids were placed into
a crucible and heated up to 500 oC in 25 min, and held for 1 h in N2
flow. After cooling down to room temperature, the samples were
rinsed with CS2 (Sinopharm, 99%) to remove residual S and dried in
vacuum drying oven.
Acknowledgements
Financial support from the National Natural Science
Foundation of China (21376208, 21622308 and 91534114), the
National Key R&D Program of China (2016YFA0202900), the
Key Program Supported by the Natural Science Foundation of
Zhejiang Province, China(LZ18B060002), the Fundamental
Research Funds for the Central Universities, and the
computing time supported by Special Program for Applied
Research on Super Computation of the NSFC-Guangdong Joint
Fund (the second phase) are greatly appreciated.
Catalytic Applications in the Hydrogenation Reaction. The
hydrogenation of nitroarenes was studied in a 50 mL stainless steel
high-pressure batch reactor. Typically, 3-nitrostyrene (0.5 mmol),
the catalyst CoS2/PC (10 mg) and CH3OH (3 mL) were loaded into
the autoclave. First, the reactor was purged three times by pure H2
to remove air. After that, the reactor was charged with 3 MPa H2
o
and the reaction mixture was stirred at 110 C. After the reaction,
the reactor was placed into a water bath and cooled to room
temperature. The remaining H2 was carefully vented and the
reaction mixture was centrifuged to obtain the reaction liquid. The
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Journal Name
26. R. V. Jagadeesh, G. Wienhofer, F. A. WesterhauVsi,ewA.ArEti.clSeuOrnkliunes,
M. M. Pohl, H. Junge, K. Junge aDnOdI: 1M0..10B39e/lCle7rG,C0C3h1e2m2J.
Commun., 2011, 47, 10972-10974.
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Graphical Abstract
The face to face located terdentate and tetrahedral coordinated Co atoms in CoS2/PC
form “synergic active site pair”, showing superior activity and selectivity in the
hydrogenation of nitroarenes.