Sublimation-Induced Sulfur Vacancies in MoS2 Catalyst for One-Pot Synthesis of Secondary Amines

Article abstract of DOI:10.1021/acscatal.9b01429

MoS₂ catalysts with abundant S and Mo defects have been developed for the one-pot reductive amination of nitro compounds with aldehydes to synthesize secondary amines. The combination of multiple structural characterizations demonstrates that the density of S vacancies can be tuned by changing the thermal sublimation temperature. The experimental results and DFT calculations demonstrate that S vacancies on the surface of MoS₂ are the active sites for the hydrogenation of the intermediate imines to the final products secondary amines.

Full text of DOI:10.1021/acscatal.9b01429

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Article
Sublimation-induced sulfur vacancies in MoS2
catalyst for one-pot synthesis of secondary amines
Yunrui Zhang, Yongjun Gao, Siyu Yao, Siwei Li, Hiroyuki
Asakura, Kentaro Teramura, Hai-Jun Wang, and Ding Ma
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b01429 • Publication Date (Web): 19 Jul 2019
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Sublimation-Induced Sulfur Vacancies in MoS2
Catalyst for One-Pot Synthesis of Secondary
Amines
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,
Yunrui Zhang, † Yongjun Gao, *† Siyu Yao, *‡ Siwei Li, ‡ Hiroyuki Asakura, ‖ ǂ Kentaro
,
Teramura, ‖ ǂ Haijun Wang, † Ding Ma*‡
†
Key Laboratory of Analytical Science and Technology of Hebei Province, College of
Chemistry and Environmental Science, Hebei University, 071002 Baoding, PR China.
‡
Beijing National Laboratory for Molecular Sciences, College of Chemistry and
Molecular Engineering and College of Engineering, and BIC-ESAT, Peking University,
Beijing 100871, China.
‖
Department of Molecular Engineering, Graduate School of Engineering, Kyoto University,
Kyotodaigaku Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
ǂ Elements Strategy Initiative for Catalysts & Batteries (ESICB), Kyoto University, 1-30
Goryo-Ohara, Nishikyo-ku, Kyoto 615-8245, Japan
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KEYWORDS: reductive amination, sulfur vacancies, one-pot strategy, cascade reaction,
molybdenum sulfide
ABSTRACT: MoS catalysts with abundant S and Mo defects have been developed for
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the one-pot reductive amination of nitro compounds with aldehydes to synthesize
secondary amines. The combination of multiple structural characterizations demonstrates
the density of S vacancies can be tuned by changing the thermal sublimation
temperature. The experimental results and DFT calculations demonstrate that sulfur
vacancies on the surface of MoS are the active sites for the hydrogenation of the
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intermediate imines to final products secondary amine.
1. INTRODUCTION
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As the important building blocks for pharmaceutical, agricultural and other fine
chemicals, functionalized secondary amines are among the most important reactants and
intermediates in today’s synthetic chemistry.1-3 In general, the functionalized secondary
amines can be synthesized via N-alkylation of primary amines with corresponding
alcohols or the reductive amination of nitro compounds with aldehydes in the presence of
hydrogen.4-6 Based on the criterions of green and sustainable chemistry, it is highly
desirable to develop one-pot catalytic system for the reductive amination of nitro
compounds to fabricate secondary amines to avoid the tedious separation / purification
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steps and decrease the release of waste. According to the reaction mechanism, the
reductive amination of nitro compounds with aldehydes can be seen as a cascade
reaction of a serial of elementary steps including the reduction of nitro- group to primary
amine, the reversible formation of imine from the coupling of aldehydes with amine and
the reduction of imines into secondary amine. Therefore, the hydrogenation rate of the
catalysts is one of the key factors to enhance the efficiency of the cascade reaction.
In the previous studies, a number of precious metal-based catalysts have been used in
the one pot synthesis of secondary amines from nitro compounds and aldehydes. For
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instance, Gum-acacia-stabilized palladium nanoparticles has been reported to efficiently
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catalyze this reaction under mild conditions. Other Pd based catalysts, such as Pd-Ag
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alloy and AuPd@Fe O were also active for the conversion of nitroarenes.
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to reduce the price of catalyst, some non-precious cobalt-7, 11-12, and iron -based
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catalysts have been reported recently to be able to catalyze the reductive amination
reactions with organic compounds (formic acid for example) and molecular hydrogen as
the reducing reagents. However, due to the poor activity of the 3d metals, the optimal
reaction conditions of these systems are generally higher than 150 °C. Constructing an
inexpensive catalyst with good low temperature activity and selectivity is still a big
challenge for the cascade reductive amination reactions.
Molybdenum sulfide, one of the most representative layered compounds with planar Mo
atom layer sandwiching between two sulfur ion layers. Due to the lack of accessible Mo
sites in bulk MoS structure, the molybdenum sulfide is generally unreactive. Only at the
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structural defects could the MoS be able to activate small molecules, such as H .
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Hence, manufacturing high density of defects in MoS is the key measure to improve the
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catalytic activity of MoS in hydrogenation reactions. Although MoS has been applied to
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catalyze the hydrogenation of nitro compounds with hydrazine hydrate or hydrogen as
reducer,17-20 no research concerning the one pot reductive amination of nitro compounds
with aldehydes has been reported.
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Herein, we report that by utilizing the thermal sublimation method at different
temperature it is possible to tune the density of S vacancies on the MoS catalyst
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synthesized via hydrothermal method. Under the optimal condition, the thermal-treated
MoS catalysts exhibit over 92% yield to target secondary amine products at only 120 °C
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in the one-pot reductive amination of nitrobenzene with benzaldehyde. The correlation of
the catalytic performances with structural characterizations and DFT calculations
demonstrates that higher density of sulfur vacancies in the MoS catalysts, the higher
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catalytic activity to hydrogenate intermediate imine to final secondary amine can be
achieved. The MoS catalyst has also exhibit excellent substrate adaptability in the
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synthesis of secondary amines.
2. EXPERIMENTAL SECTION
2.1. Chemicals. Ammonium molybdate (AR. 99 %), carbon disulfide (GC. 99 %), ethyl
acetate (AR. 99.5 %) and ethanol (AR. 99.8 %) are produced by Tianjin Kermel Chemical
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Reagent Co., Ltd. Nitrobenzene (AR. 99 %) is produced by Tianjin Fuchen Chemical
Reagent Factory. Benzaldehyde (AR. 99 %) was purchased from Tianjin Guangfu Fine
Chemical Research Institute. Other nitro-compounds and dodecane (AR. 99 %) was
purchased from Aladdin Industrial Corporation. All chemicals were used without further
treatment.
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.2. Synthesis of the CAT：The hydrothermal process for synthesizing MoS is based
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on previous report.21 Typically, (NH ) Mo O ·4H O (0.9 g) was dissolved in deionized
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water (20 mL) in a small beaker (50 mL) and transferred into a 100 mL autoclave
(stainless steel). Then carbon disulphide (10 mL) was introduced into the solution. The
autoclave was sealed and heated at 400 °C for 4 hours in muffle furnace. The resulting
product was filtered, washed several times with deionized water, and finally freeze-dried
for 12 hours. The product was named as CAT. CAT (0.5 g) was then further calcined at
o
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50 C for 1 hour in nitrogen atmosphere (40 mL/ min) in a tubular furnace and the sample
was named as CAT-450. If CAT-450 is exposed to air for few days, the catalytic activity
will decrease dramatically. The deactivated CAT-450 is named as CAT-450-D. Sodium
molybdate was used instead of ammonium molybdate to synthesize the references. The
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corresponding references were named as CAT-Na and CAT-Na-450. The commercial
MoS is named as MoS -com.
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2.3. Catalyst characterizations. Powder X-ray diffraction (XRD) patterns were recorded
on the Bruker D8 ADVANCE diffractometer with Cu Kα (λ=1.5418 Å) radiation at 40 kV
and 20 mA. The morphology characterizations of CAT and CAT-450 were researched on
transmission electronic microscopic (TEM, Tecnai G2 20). The X-ray photoelectron
spectroscopy (XPS) data were collected on a Thermo ESCALAB 250XI XPS
spectrometer. The Fourier Transform infrared spectroscopy (FTIR) analysis was carried
out on a Bruker ALPHA FTIR Spectrometer. Raman spectra were collected on Laser
Confocal Micro-Raman Spectroscope (LabRAM HR800, Horiba Jobin Yvon, France). The
samples were placed on a glass slide and measured directly with the laser wavelength
532 nm. The nitrogen adsorption-desorption isotherms are measured on a surface area
and pore size analyzer ( V-sorb-2800, Gold APP Instruments, Beijing ). The samples were
firstly pre-treated at 120 °C for 4 h in vacuum. The BET surface area is achieved
according to the adsorption data in the relative pressure ( P/P ) range of 0.05–0.35. The
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distribution of pore size and pore volume are based on BJH analysis of adsorption
isotherm data.
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Mo K-edge XAS measurements were conducted at the public beamline, BL01B1,
SPring-8 (Hyogo, Japan). The incident X-rays were monochromatized with a Si(311)
double crystal monochromator. Higher harmonic X-rays radiation were removed by
setting the glancing angle of the Rh-coated X-ray mirrors to 1.5 mrad. The incident (I0)
and transmitted (It) X-ray fluxes were measured using ion chambers filled with N2
(50%)/Ar (50%) and Ar (75%)/Kr (25%), respectively. The photon energy was calibrated
at the inflection point of the Mo K-edge X-ray ab-sorption near-edge structure (XANES)
spectrum of Mo metal foil to 20000 eV. Data were reduced using the Athena and Artemis
software ver. 0.9.25 included in the Demeter package.
2.4. Catalytic reaction. In general, nitroarene (0.5 mmol), aldehydes (0.7 mmol), catalyst
(10 mg), solvent (3 mL) were added into an autoclave (stainless steel 316, 20 mL)
sequentially. The reactor was purged with hydrogen twice and then charged with
hydrogen pressure to 2 MPa. The autoclave was heated in an oil bath at 120 ℃ for 5
hours. After reaction, the reaction mixture was cooled to room temperature and dodecane
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(
50 uL) was added into the system as internal standard. Then the mixture was diluted
with ethyl acetate (20 mL) and filtered into a sample vial for analysis on a gas
chromatography with a HP-5 column (Agilent GC 7820).
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2.5. Density functional theory (DFT) calculations. The DFT calculations are performed
using the Vienna Ab Initio Simulation Package (VASP). The projector-augmented wave
method was adopted to describe the interaction between the ionic cores and electrons.
The approach taken by VASP is based on the local density approximation (LDA). The
calculation precision is normal and a 1×1×1 Monkhorst-Pack K-point sampling was used
for all calculations. The value of plane wave cutoff is 400 eV. The van de Waals interacts
corrections were not taken into consideration. A supercell 7*7 MoS plane involving the
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consecutive S vacancies enlarged from 0 to 6 are used as model catalysts. The lattice
type is 3D triclinic, where a = b = 25.328 Å and c = 18.410 Å. The adsorption energy for
hydrogen or imine on MoS plane is calculated out basing on following equation, E =
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a
E(CAT+sub) – ECAT – E . In the formula, E
is the total energy of MoS catalyst
sub
(CAT+sub)
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adsorbing substrates, such as hydrogen or imine, ECAT is the total energy of MoS catalyst
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and Esub is the total energy of free substrate.
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. RESULTS AND DISCUSSION
3.1 Catalytic activity. The catalytic hydrogenation of nitrobenzene to aniline over
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hydrothermal synthesized CAT catalyst has been evaluated in the presence of 2 MPa H2
(Table S1). At 120 °C, 38 % yield of aniline can be achieved in one hour. Extending the
reaction time to 4 hours, the yield of aniline reached to 98 %. In comparison, the
commercial MoS shows no activity for the hydrogenation reaction. When appropriate
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amount of benzaldehyde is introduced into the reaction system, it is normal that the
commercial MoS is unable to catalyse the cascade secondary amine formation reaction.
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However, although nitrobenzene could be total hydrogenated within four hours over CAT,
the aniline was still the main product and the selectivity to N-benzyl aniline (desirable
product) and N-benzylideneaniline (intermediate) were only 12.4 % and 17.8 % (Table 1,
Entry 1) respectively. In addition, a small amount of 2-phenylquinoline and its
hydrogenated product 2-phenyl-1,2,3,4-tetrahydroquinoline were detected as side
products, which is due to the reaction of ethanol solvent with the reaction substrates and
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further hydrogenation. To produce more defects in the MoS , we tried to anneal the CAT
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catalyst at above 450 °C (The sublimation temperature of MoS ) and used the treated
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catalysts for the reaction. Under the same reaction conditions, CAT-450 presented 100
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conversion of nitrobenzene and 73.2 % selectivity to N-benzyl aniline (Table 1, Entry
2). Further extending reaction times to 5 h, the selectivity to N-benzyl aniline reached 85.2
%
(Table 1, Entry 3), suggesting the thermal treatment has positive influence on the yield
of secondary amine product. To further confirm the effectiveness of thermal sublimation,
we used sodium molybdate as the precursor and obtained another kind of CAT-Na using
the same hydrothermal synthesis procedure. The CAT-Na and CAT-Na-450 catalysts
exhibited similar trend in the reaction (Table 1, Entry 4 and 5), although with relatively
weaker activity. These results demonstrate that partial sublimation is an efficient method
to improve the catalytic activity of MoS . If the cascade reaction starts with equal molar
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nitrobenzene and benzaldehyde, the selectivity to N-benzyl aniline was only 52.1% and
a large amount of aniline was left because a part of benzaldehyde reacted with ethanol
to generate by-products, such as benzaldehyde diethyl acetal (Table 1, Entry 6).
Therefore, excess benzaldehyde or nitrobenzene is essential to ensure the maximum
yield of target product. We further optimized the thermal sublimation condition (Table S1)
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2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
with the hydrogenation of nitrobenzene as a probe reaction. The CAT catalysts treated at
400~550 °C exhibit almost identical promotion effect for the hydrogenation of
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
nitrobenzene. However, when the treatment was done over 600 °C, the catalysts lost the
activity dramatically. The optimization of the reaction temperature and solvent (Table 1,
Entry 7-11) points out that lower reaction temperature would decrease yield of target
products and ethanol is best solvent for the cascade reaction. The CAT-450 catalyst can
be recycled easily and only a little compromise of catalytic performance is observed after
th
the 4 run (Figure 1a). However, If CAT-450 was exposed to air for a few days, the
catalyst (denoted as CAT-450-D) would deactivate and lose the hydrogenation ability
(Table 1, Entry 12). As control experiments, commercial MoS donated as MoS -com has
2
2
no catalytic activity for this reaction, which is same with the blank reaction (Table 1, Entry
13, 14). Compare to with other catalytic system, such as cobalt catalyst and Pd-based
catalysts, this MoS -based catalytic system is low-cost and efficient (Table S2). More
2
importantly, when the scale of the reaction was enlarged to ten times of the optimized
conditions, that is, 5 mmol of nitrobenzene and 7 mmol benzaldehyde as substrates, 83.7
% yield of N-benzyl aniline can be achieved with 80 mg CAT-450 under 2 MPa H at 120
2
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Page 13 of 38
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ºC in 19 hours (Table 1, Entry 15). This result points out that the catalytic system has
potential to be applied in practical scenarios.
1
1
1
1
1
1
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1
1
1
2
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2
2
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2
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2
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5
6
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8
9
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1
2
3
4
5
6
7
8
9
0
To understand the reaction mechanism, the product distribution at different reaction
time was monitored (Figure 1b). It was found that the conversion of nitrobenzene
increased steadily along with the reaction time, while the selectivity of the target product
N-benzylaniline started increasing rapidly after 2 h reaction time. The selectivity of other
N containing molecules showed a trend of first increase then decrease with the increasing
reaction time, which is a typical characteristic of reaction intermediate. Based on the
reaction mechanism, the reaction between primary amine with aldehyde is a reversible
reaction. While the hydrogenation of the imine and the decomposition of imine are
competitive reactions. The kinetic behaviour of the reactants and intermediates also
indicates that the formation of imine and N=C bond hydrogenation are slower than the
nitro group hydrogenation. The imine hydrogenation to secondary amine is the rate-
limiting step of the cascade reaction. To confirm this conclusion, control experiments
starting with intermediate imine have been done. With CAT-450 as catalyst, 0.5 mmol N-
benzylideneaniline can be smoothly converted into N-benzyl aniline under 2 MPa H at
2
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1
1
1
1
1
1
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1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
1
20 ºC only in 0.5 hour. However, CAT cannot catalyze this reaction under same reaction
conditions and no N-benzyl aniline was detected. Only a small quantity of decomposed
products such as aniline, benzaldehyde and benzaldehyde diethyl acetal can be detected.
Therefore, enhancing the hydrogenation rate of the imine hydrogenation will significantly
promote the yield of the desired product.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Table 1. The catalytic performance of MoS samples in the one-pot reductive amination
2
of nitrobenzene with benzaldehyde.a)
Conv Sel. (%)
t/
.
Entry Catalyst
Solvent
T/℃
e
h
a
b
c
d
(%)
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Page 15 of 38
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1
2.
17.
8
62.
9
0
1
CAT
ethanol
ethanol
ethanol
ethanol
ethanol
ethanol
ethanol
ethanol
120
120
120
120
120
120
100
110
120
120
120
120
4
4
5
5
5
5
5
5
5
5
5
5
100
100
100
0.4
6.8
7.0
0
4
73.
2
11.
4
1.
6
2
CAT-450
CAT-450
6.8
0.7
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
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3
3
3
3
3
4
4
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4
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4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
85.
8.
1
3
6.0
2
16.
7
70.
2
4.
1
4
CAT-Na
CAT-Na-
3.8
5.9
5.1
3.0
2.8
2.5
7.4
0
44.
5
44.
0
2.
6
5
67.9
100
100
100
98.8
100
62.1
6.3
450
52.
1
43.
4
0.
8
6b)
7
CAT-450
CAT-450
CAT-450
CAT-450
CAT-450
CAT-450
1.3
36.
21.
3
38.
2
1.
1
9
74.
4
12.
1
2.
1
8
3.9
tetrahydrofur
an
32.
8
37.
4
29.
8
0
0
0
9
13.
35.
1
51.
4
10
acetonitrile
0.4
0
1
74.
22.
8
11
H
2
O
2.3
5.5
9
a)
14.
1
35.
0
41.
2
4.
1
12
CAT-450-D ethanol
1
3
4
MoS
-
2
-com
ethanol
ethanol
120
120
5
5
0
0
0
0
0
0
0
0
0
0
0
0
1
83.
8.
4
1
5c)
CAT
ethanol
120
19 100
1.2
0
6.6
7
Reaction conditions: nitrobenzene 0.5mmol, benzaldehyde 0.7mmol, catalyst 10 mg,
solvent 3mL, H 2 MPa, conversion and selectivity were determined by GC, b)
2
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nitrobenzene 0.5 mmol, benzaldehyde 0.5mmol, the other conditions are same to the
ones in a. c) nitrobenzene 5 mmol, benzaldehyde 7 mmol, catalyst 80 mg, the other
conditions are same to the ones in a.
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9
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1
2
3
4
5
6
7
8
9
0
Figure 1. a) Recycling experiment of CAT-450, b) products distribution along with reaction
time.
We further expand the reaction substrates to a serial of functional groups modified
nitroarenes and benzaldehyde derivatives (Table 2). Both the electron-donating and
electron-withdrawing substituents are compatible with the process, resulting in a better
yield (72.0-93%) of the secondary amines. It is regular that the nitroarenes containing an
electron-donating group has a higher yield than that of an electron-withdrawing group.
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Table 2. Catalytic performance of CAT-450 in the one-pot reductive amination of
nitroarenes with aldehydes.
1
1
1
1
1
1
1
1
1
1
2
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2
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3
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9
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3
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8
9
0
3.2 Characterizations. Detailed structural characterizations have been performed to
elucidate the role of thermal treated MoS in the one pot cascade reductive animation
2
reactions, especially in the hydrogenation of imine intermediate. From the HRTEM
images (Figure 2a, 2b), it could be observed that the CAT and CAT-450 catalysts are
layered structures with a generally 3~8 layers stacked together. The edge defects (in
circle) and surface defects (in rectangle) are frequently observed compared to
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commercial MoS (Figure S1c). The TEM image of CAT shows that CAT is composed of
2
MoS sheets (Figure 2a inset). SEM images of CAT and CAT-450 also present flower-
2
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like MoS sheets (Figure 2c,2d). Compared with the commercial ones (Figure S1), these
2
relatively thin layered structure makes the CAT and CAT-450 expose more active surface.
The X-ray powder diffraction (XRD) profiles of the CAT and CAT-450 catalysts (Figure
2
e) suggested both of the catalysts maintain the crystal structures of the 2H-MoS2.23
Compared with the commercial MoS , the slight shift of the diffraction peak of (002) facet
2
in the CAT and CAT-450 samples suggests the increasing of d-spacing of between
adjacent MoS layers in the synthesized MoS . Meanwhile, it is clear that the intensity
2
2
ratio of (100) planes (I(100)) and (002) planes (I(002)) of the CAT and CAT-450 samples
are much larger than that of the commercial molybdenum disulfide, indicating the average
number of the stacked MoS layers in the hydrothermal synthesized MoS is significantly
2
2
24
reduced . The correlation of the XRD pattern with the reaction performance indicates the
edge rather than the plane of the MoS layers is active for the hydrogenation reaction.
2
However, the detailed crystal structures of the CAT and CAT-450 catalysts are almost
identical. To understand the reason for the significantly different reactivity of these two
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catalysts, the XAFS method which is more sensitive to the local environment was applied
to investigate the thermal sublimation induced structural changes.
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1
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9
0
As shown in Figure 2f, compared with MoS -com, the X-ray absorption of CAT at about
2
20010 eV is lower, which maybe results from the defects or sulfur vacancies in CAT. After
o
CAT was annealed at 450 C in nitrogen atmosphere, the absorption further decreased.
Fourier transform of EXAFS spectra of the three samples show two peaks at 1.9 Å and
2.8 Å respectively (Figure 2g), which correspond to the Mo-S and Mo-Mo (in the same
25
layer) coordination shells, respectively. Detailed fitting results of the Mo K edge EXAFS
spectra (Table 3), revealed clearly that the coordination numbers of Mo-S (C.N.Mo-S=5.5)
and Mo-Mo (C.N.Mo-S=4.7) shells are both smaller than the theoretical values,
demonstrating that there are both sulfur and molybdenum vacancies in a single MoS2
layers. In the thermal treated CAT-450 the C.N.Mo-S and C.N.Mo-Mo value further decreases
to only 5.1 and 4.3, confirming the further increase in the density of S and Mo vacancies
after the sublimation. The analysis of the sublimation substances (Figure S2) also
confirms the loss of S and Mo atoms after the thermal treatment. These structural
characterization results suggested that it is the variation of the S and Mo vacancies in the
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6
MoS single layer rather than the crystal structures of the MoS that is related with the
2
2
hydrogenation activities.
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9
0
1
2
3
4
5
6
7
8
9
0
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9
0
Table 3. Structural parameters obtained from the Mo K-edge EXAFS spectra of MoS2
catalysts.
Sample
Shell
Bond length(Å) Coordination Number σ2
E0 shift (eV)
2
Mo-S
2.41
6 (S =0.96)
0.003
0.003
0.003
0.004
0.003
0.004
0
MoS -com.
3.1
2
2
Mo-Mo 3.17
6 (S =0.95)
0
Mo-S
2.41+/-0.01
5.5+/- 0.4
4.7+/-0.9
5.1+/-0.4
4.3+/-0.6
CAT
2.9
2.8
Mo-Mo 3.17+/-0.01
Mo-S
2.41+/-0.01
CAT-450
Mo-Mo 3.17+/-0.01
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Figure 2. a) TEM and HRTEM(inset) images of CAT, edge defects in cycles and surface
defects in rectangle; b) HRTEM image of CAT-450, edge defects in cycles and surface defects
in rectangles; c) SEM image of CAT; d) SEM image of CAT-450; e) XRD patterns of MoS2-
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9
0
com, CAT, CAT-450, CAT-450-D and CAT-800; f) Raman spectra of MoS -com, CAT,
2
CAT-450, CAT-450-D and CAT-800; g) XANES spectra of MoS -com, CAT and CAT-450,
2
inset: magnification of curves in circle; h) Fourier transform of EXAFS spectra of MoS2-
com, CAT and CAT-450.
Raman spectroscopy is a powerful non-destructive manner to study crystal structure,
layer thickness and defects of MoS . As shown in Figure 2h, two Raman-active peaks at
2
-1
-1
1
about 380 cm and 407 cm can be observed, representing the in-plane E and out-of-
2g
26
1
plane A vibration of hexagonal MoS , respectively. The relatively lager E peak width
1g
2
2g
and weaker intensity of CAT compared with CAT-450 and CAT-450-D suggest that the
crystal structure of CAT is not perfect and can be improved by annealing manner in
27
1
nitrogen atmosphere at 450 ºC. The lower intensity of in-plane E peaks also reveal
2g
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that the three samples compose of thin MoS sheets with basal-edge-rich property and
2
defects.28
1
1
1
1
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1
1
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9
0
1
2
3
4
5
6
7
8
9
0
Figure 3. a) Nitrogen-adsorption isotherms of MoS -com, CAT, CAT-450 and CAT-450-
2
D; b) the pore-size distribution of MoS -com, CAT, CAT-450 and CAT-450-D based on
2
BJH analysis of adsorption isotherm.
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Nitrogen adsorption–desorption experiments illustrate that CAT possesses larger
2
2
surface area than commercial MoS (217.3 m /g vs. 4.4 m /g). Moreover, CAT has a kind
2
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3
of mesoporous structure with 2.5 nm of pore size and 0.93 cm /g of pore volume. CAT-
4
50 exhibits larger BET surface area, wider pore diameter and higher total pore volume
than CAT, suggesting that thermal anneal procedure creates more pores or enlarge pore
size. Compared with CAT-450, the BET surface area and pore volume of CAT-450-D
decrease dramatically, resulting to the deactivation of CAT-450-D.
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Figure 4. a) Full XPS spectra of CAT, CAT-450 and CAT-450-D; b) S 2p XPS spectra of
CAT, CAT-450 and CAT-450-D; c) Mo 3d spectra of CAT, CAT-450 and CAT-450-D; d)
O1s spectra of CAT, CAT-450 and CAT-450-D (O/Mo represents the ratio of atom number
(At %) according to XPS analysis ).
XPS spectra of CAT before and after thermal treatment are displayed in Figure 4. It is
obviously observed that the content of oxygen in CAT-450 is extremely lower than CAT
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(
Figure 4a and d, the O/Mo ratio: 0.18 of CAT-450 vs. 0.48 of CAT), suggesting that
thermal treatment in nitrogen atmosphere effectively remove the oxygen from the MoS2
catalyst surface. In the deactivated CAT-450D catalysts, the O/Mo ratio is 0.40. The
detailed XPS spectra at S 2p and Mo 3d regions reveal that the oxygen species could be
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both SO or Mo(IV/VI)O .
As these oxygen species mainly appears in the relatively
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low activity catalysts such as CAT and CAT-450-D, it infers that O possibly occupies the
defects of the MoS which tend to reduce the hydrogenation activity of the catalysts. The
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atomic ratio of S to Mo in CAT, CAT-450 and CAT-450-D are determined to be 1.81, 1.75
and 1.76, confirming the treatment at 450 ºC did create more sulfur vacancies. To further
confirm the relationship between sulfur vacancies and catalytic performance of MoS2
catalysts, a series of MoS catalysts with different atomic ratio of S to Mo were prepared
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by processing pre-synthesized MoS in nitrogen atmosphere at 450 ºC for 30s, 120 s,
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80s and 420s, respectively. It is obviously observed that the atomic ratio of S to Mo in
CAT decreased from 2.00 to 1.76 (S/Mo of CAT-450 is 1.75) when the annealing time
was extended. Correspondingly, the catalytic performance of these MoS catalysts
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became better along with the decrease of the atomic ratio of S to Mo. Furthermore, if the
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sulfur vacancies of CAT-450 were remedied by treating the CAT-450 in gaseous sulfur,
the catalytic activity for the one-pot cascade reaction decreased significantly.
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In summary, the combination of multiple characterization methods demonstrates the
excellent catalytic performances of the CAT-450 catalyst is from the high density of
surface defects in the MoS planes and the less occupancies of surface oxygen species
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at these defect sites.
3.3 DFT calculations. In order to understand the function of the surface defects in the
MoS on the reactivity of the one-pot cascade reductive amination of nitroarenes with
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aldehydes, DFT calculations were performed to evaluate the variation of surface S
vacancies on the adsorption of the key reactants and intermediates of the reaction (Figure
5). Based on the calculation results, the adsorption energy of two H* decreases from -0.5
eV to -1.3 eV when the S vacancies enlarged from 0 to 6 on a 7*7 MoS plane, indicating
2
the dissociated H* adsorbs more stable at the consecutive S vacancies (Figure 5, Figure
S3). Meanwhile, the consecutive S vacancies also benefit the adsorption of the imine
intermediate (Figure 5, Figure S4). This phenomenon could explain the positive effect of
the S vacancies on the reaction. Meanwhile, the DFT calculation also reveals that the
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affinity of the nitrobenzene with the S vacancies is much larger than that of the
benzaldehyde (Figure S5). As a result, the nitrobenzene reduction will be faster than
benzaldehyde.
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Figure 5. Adsorption energies of hydrogen on MoS with different S vacancies (black) and
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adsorption energies of intermediate imine on defective MoS containing dissociated H*
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on consecutive S vacancies.
4
. CONCLUSIONS
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One-pot cascade nitro-reduction and reductive amination for synthesizing secondary
amines is achieved by using defective MoS with sulfur vacancies. Thermal treatment of
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MoS in nitrogen atmosphere is a key and effective strategy to improve the catalytic
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activity of MoS due to the sublimation of MoS and sulfur. Characterizations and control
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experiments demonstrate that sulfur vacancies can be created because the sublimation
of sulfur is easier than MoS . DFT calculations also confirm that larger sulfur vacancies
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area on MoS can effectively adsorb and activate hydrogen, nitro compounds and
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intermediate imines so that MoS with more sulfur vacancies promotes the one-pot
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cascade reaction conduct smoothly. However, slow oxidation in air can deactivate the
active MoS catalyst. It is unprecedented that one-pot reductive amination of nitroarenes
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with aldehydes for synthesizing secondary amines is achieved by using defective MoS2
as effective catalyst.
ASSOCIATED CONTENT
Supporting Information. The Supporting Information is available free of charge on the
ACS Publications website at DOI:
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