Synthesis of Textured Tungsten Disulfide Nanosheets and their Catalysis for Benzylamine Coupling Reaction

Article abstract of DOI:10.1002/cctc.201901576

Tungsten disulfide (WS₂) has a graphite-like structure with the properties of strippable and indirect/direct band gap conversion, which makes it have important applications in the fields of optoelectronics, catalysis and lubrication. Here, ultr

Full text of DOI:10.1002/cctc.201901576

Accepted Article
Title: Synthesis of textured tungsten disulfide nanosheets and their
catalysis for benzylamine coupling reaction
Authors: Hua Liang, Bingqian Zhang, and Ji-Ming Song
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Synthesis of textured tungsten disulfide nanosheets and their
catalysis for benzylamine coupling reaction
Hua Liang§, Bing-Qian Zhang§, Ji-Ming Song*
School of Chemistry & Chemical Engineering,
Anhui Province Key Laboratory of Chemistry for Inorganic/Organic Hybrid
Functionalized Materials,
Anhui University,
Hefei, Anhui 230601, P R China.
Tel:+86 0551 63861279 : Fax: +86 0551 63861279
E-mail: jiming@ahu.edu.cn
§These authors contributed equally to this article.
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Abstract: Tungsten disulfide (WS2) has a graphite-like structure with the properties
of strippable and indirect/direct band gap conversion, which makes it have important
applications in the fields of optoelectronics, catalysis and lubrication. Here, ultra-thin
WS2 nanosheets with texture on the surface were obtained by colloid synthesis
method, and its phase, morphology and valence state were characterized. TEM images
displayed that the obtained sample had flower-like morphology composed of “petal”
nanosheets with several stripes on the surface. Interestingly, the fabricated WS2
nanosheets exhibited excellent catalytic performance for the conversion of
benzylamine with the yield reaching 97.9% and excellent catalytic activity still being
retained after 5 cycles of catalysis, and the WS2 nanosheets also had good catalytic
activity for coupling reactions of benzylamine homologues to synthesize
corresponding imines. Besides, the coupling reaction could be done in natural light
without the need for additional light and oxygen atmosphere at certain temperature.
Further research showed that in our reaction system, the benzylamine oxidative
coupling reaction was a photothermal co-catalytic process with thermal catalysis
being dominant role and photocatalysis further improving the yield.
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Introduction
Transition metal chalcogenides include a big family with the characteristics of layered
structure and tunable band gaps, which exhibit good electric and optical properties.[1-3]
As a member of the big family, tungsten disulfide (WS2) nanosheet has recently
attracted widespread attention from scientists by virtue of its good photoluminescence
effect[4-6] and considerable absorption intensity in the visible range as well as excellent
[7]
mechanical properties and flexibility. There are many synthesis methods to be
[8]
[9]
developed to prepare WS2 nanosheets, including stripping, vapor deposition,
solvothermal[10] and colloidal method. [11]The colloidal synthesis has the advantages of
simple and fast synthesis method with large amount and controllable morphology of
products, which is favored. The geometric crystallization indicates that the monolayer
tungsten disulfide displays a sandwich-like structure composed of three layers of
atoms, where the sulfur atoms are located on the outer layer, and the tungsten atom is
located on the inner layer. Because of its unique crystal structure, it has excellent
optical and electrical properties. [5]So, the application of WS2 nanosheets in the field of
catalysis is worth exploring. [12-14]
The imine, as key intermediate in many reactions, has wide applications in the
fields of industry, agriculture and biological medicine. [15-16] It can not only be used to
synthesize corresponding nitrogen-containing organic compounds, drugs, biochemical
active substances and natural products, but also possess unique bacteriostatic,
bactericidal, antitumor, antiviral biological activities and good coordination chemical
properties. Imines could be obtained by several synthesis methods, such as
hydrogenation coupling reaction of nitro compounds, [17-18] oxidative condensation of
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amines with alcohols. [19-20] Oxidative Coupling Reaction of Benzylamine (OCRB) is
one of the effective ways to prepare corresponding imines. Mou prepared
N-benzylidene benzylamine via OCRB under visible-light irradiation using Au/TiO2
as catalyst.21 In addition, precious metals and their alloys, such as Pd3Pb, [22] Au, [23-24]
Au-Pd nanoparticles, [25] were also used as catalysts to prepare imines via OCRB.
However, these reactions often require precious metals as catalysts with complex
synthesis steps and harsh reaction conditions, which limit the development of these
synthetic methods. So, researchers have been looking for cheap and efficient catalysts
for coupling reaction of benzylamine. [26-28] However, to our best knowledge, the
application of WS2 in catalytic coupling reaction of amine has been rarely studied and
its photothermal co-catalysis has not been reported.
In this work, we sucessfully synthesized the petal-like WS2 nanosheets with 2H
phase by oil phase method. The obtained WS2 nanosheets were used as catalyst for
coupling reaction of benzylamine, and we found that the conversion of benzylamine
to imine was more than 97% under the conditions of 60 ℃, air atmosphere, and
natural light. Different reaction conditions, including different solvents, temperature,
reaction time were explored. Interestingly, the oxidative coupling reaction of
benzylamine in our reaction system was a photothermal co-catalytic reaction, and
thermal catalysis played a leading role. The catalysts had good reaction activity and
cycle stability after undergoing five cycles. Furthermore, it also had excellent catalytic
activity for the benzylamine homologs.
Experimental section
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Chemicals and characterization (see the Supporting Information, SI)
Synthesis of WS2 nanosheets
Preparation of precursor solution: 0.5 mmol tungsten hexachloride (WCl6) was
dissolved in 2 ml oleic acid (OA) for 10 min by ultrasound until the color of the
solution turned brown; then 20 ml of oleylamine (OLA) was added with magnetic
stirring, and yellow solution was formed; 1 mmol sulfur powder was added to the
above yellow solution and the orange precursor solution was obtained under
magnetic stirring.
Next, 40 ml OLA was placed in a 100 ml three-neck flask and degassed at 65oC
for 1 hour to remove the low-boiling substance, then was heated up to 320℃ with
nitrogen condition; the above precursor solution was slowly injected into the
three-neck flask, and the mixed solution was kept at 320℃for one hour, then naturally
cooled to room temperature. The final product was obtained by centrifugation and
washing using a mixture of ethanol and chloroform for several times, then dried in a
vacuum oven for later characterization and testing.
Catalytic and cycle experiments of benzylamine coupling reaction
In the typical catalytic process, 0.5 mmol benzylamine as substrate, 30 mg WS2 as
catalyst and 0.5 ml acetonitrile as solvent were added in the Schlenk tube. The
reaction mixture was heated to 60℃ in an oil bath with magnetic stirring for 30 h.
After the reaction, the catalyst was separated by centrifugation, and the supernatant
was taken and determined by gas chromatography (GC) and gas
chromatography-mass spectrometry (GC-MS). The effect of different solvents, such
as n-hexane, ethanol and water, on the yield of the reaction was explored, and
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acetonitrile was replaced by certain solvent in catalytic process, while other
conditions remained unchanged. In addition, reaction temperature and catalyzed
coupling of benzylamine homologues experiments with single conditional change was
also investigated, and other experimental conditions were the same as typical
experimental conditions.
Recycle experiments of WS2 as catalyst
The recycle stability of benzylamine coupling catalyzed by WS2 nanosheets was
explored, and five cycle experiments were carried out. The catalyst was separated by
centrifugation after each reaction, and the supernatant was taken and determined by
GC, then trichloromethane was used as dispersing agent and ethanol was used as
precipitator for washing the catalyst for several times under the aid of centrifugation,
and the regenerated catalyst was dried in a vacuum drying box at 60 ℃ for the next
reaction.
Results and Discussion
Figure 1 (a) XRD pattern of the obtained samples, (b) The schematic diagram of WS2 crystal
structure
The phase analysis of the synthesized product was measured by XRD. As shown in
Figure 1a, the product was the WS2 of 2H phase. [11] There was a strong diffraction
peak around 34° corresponding to the (100) crystal plane of WS2, while other
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diffraction peaks at 42°, 48° and 58° arose from the (006), (105) and (110) crystal
planes, respectively. The apparent asymmetry of the diffraction peak corresponding to
the (100) in the XRD pattern might be caused by the superposition of the diffraction
peaks of the oxides on the surface of the sample, which could be learned from the
subsequent XPS characterization. Figure 1b was the schematic diagram of 2H-WS2
crystal structure, in which sandwich structure was composed of three atomic layers
from the diagram by hexagonal closest stacking. [29] The layered WS2 was formed by
the combination of single-layer structure with van der Waals forces.
Figure 2 (a), (b), (c) TEM images of the obtained samples and the illustration in Figure 2 (b, c)
were photographs of a lotus and single petal with veins, respectively, (d) HRTEM image of the
obtained sample, and the inset showed the SAED pattern.
Figure 2 showed the morphology and structure of the obtained WS2 at different
magnification. Figure 2a and Figure 2b revealed that the WS2 displayed a flower-like
morphology, which was similar with a lotus (inset in Figure 2b). From the local
magnification diagram of Figure 2c, we could find that the single “lotus” was
composed of many petals with several stripes on the surface of the petal (inset in
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10.1002/cctc.201901576
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Figure 2c). HRTEM images showed the lattice-stripe distance was 0.270 nm
corresponding to the (100) crystal plane of WS2, and the SAED pattern displayed two
distinct diffraction rings correspond to (100), (110) crystal planes in Figure 2d,
respectively.
Figure 3 displayed the XPS spectra of 2H-WS2. It could be observed from Figure
3a that the two characteristic peaks near 31.8 eV and 34.0 eV were attributed to
W4f7/2 and W4f5/2, respectively; the two peaks at 33.3 eV and 35.3 eV were derived
from W 4f7/2 and W4f5/2 of W-O bond due to surface oxidation of the sample; [30-31] the
wide peak at 37.6 eV was belong to XPS spectrum of W5p. Figure 3b showed that the
XPS spectra of S 2p3/2 and S 2p1/2, and the two characteristic peaks were located at
161.5 eV and 162.8 eV, [32-33] respectively.
a
b
W 4f
S 2p
7/2
5/2
3/2
1/2
W 5p
42
40
38
36
34
32
30
168
166
164
162
160
Binding energy (eV)
Binding energy (eV)
Figure 3 XPS spectra of the WS2: (a) W 4f and (b) S 2p
The UV-Vis, Fluorescence, and Raman spectra of the obtained WS2 nanosheets
were shown in the Figure 4. There was an obvious broad absorption band in the range
of 200-800 nm, indicating the WS2 had good light absorption capacity, which would
be beneficial to photocatalysis (Figure 4a). According to Tauc
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2
'
formula
, the extrapolated band gap of WS2 was about 1.5 eV
(hv)  C (hv  Eg )
(Figure 4b), which was consistent with the band gap of the single-layer WS2
[34]
nanosheets reported in the literature.
Figure 4c showed photoluminescence
spectrum of WS2 nanosheets, and the maximum emission wavelength was 405 nm
using 260 nm as the excitation light with broadening peak shape. Figure 4d displayed
that Raman spectra of the product, and two peaks at 350, 417 cm-1 could be assigned
to E2g mode and A1g mode of 2H-WS2 nanosheets, respectively; this result indicated
that the synthesized WS2 nanosheet had a structure with individual and few layers. [35]
1.1
a
b
1.0
0.9
0.8
0.7
0.6
200
300
400
500
600
700
800
1
2
3
4
5
6
7
Wavelength (nm)
hν (eV)
E2g
c
d
A1g
340
360
380
400
420
440
150
200
250
300
350
400
450
500
Raman Shift (cm-1)
Wavelength(nm)
Figure 4 Several spectra of WS2: (a) UV–Vis, (b) Tauc plot of UV-Vis, (c) Fluorescence, (d)
Raman.
Then, the catalytic performance of the obtained WS2 nanosheets was examined by
using benzylamine being oxidized to imine as the probe reaction. The steps of the
catalytic coupling reaction were referred to in the experimental section. Some reaction
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conditions, such as reaction temperature, the effect of catalyst and solvents, have been
explored (See SI, Figure S1, Table S1). We found that if there were no catalysts (WS2
nanosheets) being added into the reaction system, the other experimental conditions
were consistent with typical synthesis conditions and no coupling product
N-benzylidene benzylamine was detected, indicating that WS2 nanosheets played a
crucial role in the coupling reaction of benzylamine. From the results of different
temperatures, we found that the optimum temperature was 60 ℃. Then, the catalytic
reactions in different solvents such as acetonitrile, n-hexane, cyclohexane, ethanol and
aqueous were carried out (Table 1 Entry 1-5). The results could be seen from Table 1,
when acetonitrile was used as the solvent, the yield, conversion, and selectivity
reached as high as 97.9%, 98.6% and 99.3%, respectively, after 30 hours of reaction.
This yield was higher than the reported value (94%) in the literature with catalytic
conditions of oxygen atmosphere and 5 W LED light source for 30 h. [36] When
n-hexane or cyclohexane was used as solvents, the yields were 76.1% and 70.0% in
turn, as well as favorable conversion and selectivity (Table 1 Entry 2-3).
Table 1. Oxidative coupling of benzylamine catalyzed by WS2 nanosheets.
Entry
WS2
acetonitrile
n-hexane
cyclohexane
ethanol
Conv. (%)
98.61
Sel. (%)
99.29
80.72
88.97
98.79
52.33
Yield (%)
97.91
1
2
3
4
5
94.27
76.09
78.69
70.01
41.46
40.96
water
65.69
34.38
Reaction conditions: 0.5 mmol of benzylamine, 30 mg catalyst, air, 60℃for 30 h, determined by GC.
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However, if the catalytic reactions were carried out in ethanol or water as
solvents, the yields were only 41.0% in ethanol and 34.4% in water, respectively
(Table 1 Entry 4-5). Therefore, like typical synthesis conditions, we chose the
obtained WS2 nanosheets as the catalyst and acetonitrile as a solvent to conduct
experiments at 60 oC under natural light. Natural light in the laboratory could meet the
experimental lighting requirements, without the use of sunlight or LED additional
light sources. Reaction kinetic experiments showed that the catalytic coupling
reaction of benzylamine in acetonitrile belonged to the first order reaction (Figure
S3).
In order to determine the role of light and heat in the catalytic process, we further
investigated the effects of temperature and light on the catalytic reaction. The reaction
in the dark chamber for 30 h was carried out and kept the other conditions unchanged;
the result displayed that the yield obtained was 93.2%, slightly lower than the yield
(97.9%) under typical catalytic condition. Therefore, the benzylamine coupling
reaction could be well carried out in the dark chamber for our catalytic reaction
system. To further prove the relationship between photocatalysis and thermocatalysis,
we carried out the experiments under natural light and dark at 20 ℃ and 60 ℃ for
12 h, respectively. The results showed that the yield of reaction under darkroom at
20℃ was 10.5%, while the yield in darkroom at 60℃ was 63.2%, which yield could
be further increased to 80.4% when natural light was introduced at the same time. As
a comparison, when there was only natural light without heating (room temperature),
the yield was only 12.9% as shown in Table 2. Therefore, we can conclude that the
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oxidative coupling reaction of benzylamine in our reaction system was a photothermal
co-catalytic reaction, and thermal catalysis played a leading role.
Table 2. Catalytic Performances of WS2 under different conditions
Entry
T (℃)/ Light
60 / natural light
60 / dark
Time (h)
30
Yield (%)
97.9
1
2
3
4
5
6
7
8
9
30
93.2
60 / natural light
60 / dark
12
80.4
12
63.2
20 / natural light
20 / dark
12
12.9
12
10.5
60 /blue light
60 /red light
12
77.8
12
72.1
60 /fluorescent lamp
12
68.8
In addition, the effect of different light sources, such as blue light, red light, and
fluorescent lamp, on the catalytic reaction were also investigated at 60 ℃ for 12 h.
As shown in Table 2, under the irradiation of blue and red light source, the yield of
the reaction at 60 ℃ for 12 h was 77.8% and 72.1%, respectively. The yield was
68.8% when irradiation under fluorescent lamp with 60 W in laboratory. Those results
showed that different light sources had certain effects on the catalytic yield with little
difference, which were consistent with the fact that the reaction was dominated by
thermal catalysis, and the catalyst WS2 had good absorption in a wide wavelength
range (Figure 4a).
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WS2
Figure 5 Schematic diagram of proposed mechanism for the oxidative coupling of benzylamine
catalyzed by WS2 nanosheets
e-
O2
O2
(a)
(b)
-e-
NH2
NH2
O2
+
NH
NH2
(c)
(d)
H2O2
+
NH
NH2
-NH3
N
+
The mechanism of the catalytic reaction process was illustrated in Figure 5 and
Formula (a-d). Firstly, under the irradiation of hot source and natural light,
electron-hole pairs were generated on the surface of the WS2 nanosheets, and large
quantities of electron were captured by the adsorbed oxygen to produce superoxide
radicals with very high reaction activity; meanwhile, benzylamine captured the
positive charge from the hole and formed benzylamine radicals (Formula a and b).
Secondly, freshly formed benzylamine radicals further reacted with superoxide
radicals to form hydrogen peroxide and corresponding benzimide (Formula c). Finally,
reaction between benzimide and benzylamine produced the final product by
deamination (Formula d). [37]
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Table 3. Oxidative coupling reaction of several benzylamine homologues catalyzed by WS2
nanosheets.
Entry
Reactants
Products
Yield (%)
97.91
NH
2
N
1
2
3
4
5
NH
2
N
92.80
NH
2
N
N
N
93.97
NH
2
94.93
Cl
Cl
Cl
NH
2
96.19
F
F
F
Reaction conditions: 0.5 mmol benzylamine or its homologue, 30 mg WS2, Air, 60℃, 30 h,
determined by GC-MS.
To demonstrate the formation of superoxide radicals during the reaction, we used
p-benzoquinone as superoxide radical scavenger to verify. At the beginning of the
reaction, an appropriate amount of p-benzoquinone was added to the reaction system,
and other conditions remained unchanged. The catalytic results showed that the yield
was only 0.94% (Table S2, 2a), indicating that most of the superoxide radicals
generated during the reaction were captured by p-benzoquinone, which greatly
reduced the reaction rate. Besides, a DPD/ POD method was used to detect the
content of hydrogen peroxide in the reaction process. The results revealed that two
peaks of the hydrogen peroxide signal appeared near 510 nm and 551 nm (Figure S2),
indicating that the superoxide free radicals reacted with benzylamine radicals to
produce hydrogen peroxide (Formula c). [38]
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Based on those results, oxidative coupling of a series of benzylamine
homologues using WS2 as catalyst was tested. The results could be seen from Table 3
that WS2 nanosheets had same great catalytic performance for the coupling reaction of
a variety of benzylamine homologues, and all yields were higher than 92% (Table 3
Entry 2-5). Furthermore, we could see that those yields were slightly lower than that
of benzylamine (Table 3 Entry 1), which might be related to the type and position of
substituent group on the benzene ring. The methyl group belongs to the electron donor
group, when it is in the ortho-position and para-position of the benzylamino group,
the electron cloud density of the benzene ring increases, thereby increasing the p-π
conjugate effect and reducing the reaction activity of the benzylamine homologue.
While the halogen atom is an electron-absorbing group, which can reduce the density
of electron cloud, thereby reducing the p-π conjugate effect with the amine group and
increasing the amino group reaction activity, and the greater the halogen polarity, the
more obvious the effect (Table 3 Entry 2-5). [39]
Figure 6 The recyclability of WS2 nanosheets as catalyst.
The cyclic stability of the WS2 nanosheets as catalyst for benzylamine coupling was
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investigated. The results of five cycle experiments were shown in Figure 6. High
catalytic performance was maintained for every reaction cycle, and after five cycles,
the conversion and yield of the coupling reaction still remained at 94.0% and 92.3%,
indicating WS2 nanosheets having high cyclic stability.
Conclusion
Ultra-thin WS2 nanosheets with 2H-phase were successfully synthesized by oil phase
method, and assembled the products with flower-like morphology. TEM and HRTEM
images revealed that there were many stripes on the surface of nanosheets, similar to
texture of petal. The obtained WS2 nanosheets had a strong absorption in the range of
200-800 nm, and exhibited strong fluorescence near 405 nm under excitation of 260
nm. Importantly, the WS2 nanosheets possessed high photothermal co-catalytic
activity for oxidative coupling of benzylamine and benzylamine homologues under
natural light and air atmosphere at 60℃. Under typical catalytic conditions, the yield,
conversion and selectivity of the coupling reactions reached as high as 97.91%,
98.61% and 99.29%, respectively. Besides, the WS2 nanosheets had good catalytic
stability and high catalytic activity still maintained after 5 cycles. The results show
that the synthesized WS2 nanosheets have a good catalytic application in the
benzylamine coupling for imine, and are expected to replace the precious metal
catalyst for the synthesis of imine in the future.
Acknowledgements
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The authors are thankful for the support rendered by the National Science Foundation
of China (NSFC) (Grants 21641007), and Major Project of Education Department of
Anhui Province (KJ2016SD63). We also thank the Key Laboratory of Environment
Friendly Polymer Materials of Anhui Province.
Keywords: Tungsten disulfide nanosheets; Benzylamine; Photothermal co-catalysis;
Coupling reaction; Imine
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Entry for the Table of Contents
Flower-like WS2 composed of nanosheets like petals with several stripes on the
surface were synthesized. As-synthesized WS2 nanosheets showed excellent catalytic
properties for oxidation coupling reaction of benzylamine under mild conditions
without additional light and oxygen atmosphere. Further research showed that the
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10.1002/cctc.201901576
ChemCatChem
benzylamine oxidative coupling reaction was a photothermal co-catalytic process with
thermal catalysis being dominant role and photocatalysis further improving the yield.
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