Facile synthesis of hydrophobic MoS2 and its activity and stability in the hydrodeoxygenation reaction

Article abstract of DOI:10.1039/c8nj05980b

In this study, MoS₂ with different hydrophobicity was prepared by adding silicomolybdic acid and their catalytic activity and stability were tested by using the HDO of p-cresol as a probe. The addition of silicomolybdic acid enhanced the hydrophobicity and surface area, and changed the morphology and S/Mo atomic ratio. The air-water contact angle reached up to 132.3°, suggesting a high hydrophobicity. During the HDO reaction, with increasing hydrophobicity of MoS₂, p-cresol conversion was slightly decreased but the product distribution was almost unchanged, which was attributed to the fact that the hydrophobicity weakened the adsorption of reactant molecules due to the hydrophilic phenolic hydroxyl group. Most importantly, the hydrophobicity prevented water from contacting the MoS₂ surface and then protected its structure from sulfur loss, leading to a low decrement of conversion (0.4%) and low increment of toluene selectivity (5.3%) after three cycles of reactions at 300 °C. Hence, enhancing the hydrophobicity was a good strategy to improve the stability of sulphide catalysts for HDO reactions.

Full text of DOI:10.1039/c8nj05980b

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The role of atropisomers on the photo-reactivity and fatigue of
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Facile synthesis of hydrophobic MoS and its activity and stability
in the hydrodeoxygenation reaction
2
Received 00th January 20xx,
Accepted 00th January 20xx
Kui Wu , Chao Wang , Xiangxiang Chen , Weiyan Wang †, Yunquan Yanga, b
a
a
a
a, b
†
DOI: 10.1039/x0xx00000x
2
In this study, MoS with different hydrophobicity was prepared by adding silicomolybdic acid and their catalytic activity
and stability were tested by using the HDO of p-cresol as a probe. The addition of silicomolybdic acid enhanced the
hydrophobicity and surface area, and changed the morphology and S/Mo atom ratio. The air-water contact angle reached
www.rsc.org/
2
up to 132.3°, suggesting a high hydrophobicity. During the HDO reaction, with the hydrophobicity of MoS increasing, p-
cresol conversion was slightly decreased but the product distribution was almost unchanged, which was attributed to that
the hydrophobicity weakened the adsorption of reactant molecules due to the hydrophilic phenolic hydroxyl group. Most
2
importantly, the hydrophobicity prevented water from contacting MoS surface and then protected its structure from
sulfur loss, leading to a low decrement of conversion (0.4%) and low increment of toluene selectivity (5.3%) after three
cycle reactions at 300 °C. Hence, enhancing the hydrophobicity was a good strategy to improve the stability of sulphide
catalysts for HDO reactions.
INTRODUCTION
form of water with the presence of hydrogen. In this process,
the deoxygenation rate and efficiency mainly depends on the
adopted catalyst, and then much attention concentrates on
The increasing demand of energy and some environment
problems caused by the utilization of fossil fuel force us to
exploit a renewable and sustainable energy source. Until now,
several alternative energies such as nuclear, solar, wind, bio-
2
, 3, 5,
the development of novel catalysts with high HDO activity.
6
Mo based sulphides are good catalysts for
hydrodesulfurization (HDS) reactions, which were also widely
applied into the HDO reactions due to their similarity.
1
oil, and so on, have appeared. Taking the energy density,
7
-11
stability, transportation and existing infrastructure into
account, only bio-oil, derived from biomass such as cellulose,
hemicellulose and lignin via fast pyrolysis, has the same
elements as fossil fuel and attracts much attention. Moreover,
biomass produces zero net CO emission and it is highly
2
regenerable and abundant on the earth. Consequently, this
bio-oil is considered as the most potential candidate to
substitute fossil fuel. However, it contains many significant
oxygenated compounds, resulting in the high oxygen content
(even up to 35%-40%), low heat value and some other
disadvantages,3 which is unsuitable for direct use as a
transportation fuel (or as a fuel additive) and required to be
However, these sulphides are structure-sensitive catalysts and
their activities are closely related to the preparation
1
2-14
methods.
phenol on the amorphous MoS
hydrothermal method was almost 2.4-fold higher than that on
For example, the conversion in the HDO of
2
catalyst prepared by
1
5
commercial MoS
2
.
Unfortunately, unlike HDS, the water
2
produced during HDO reaction has a negative effect on the
1
6, 17
sulphide structure and then causes catalyst deactivation.
It has been well-documented that the hydrophobization of
catalyst could markedly improve its activity and/or stability in
1
8
the reactions where water co-existed in the system. Our
previous investigations also evidenced that the hydrophobicity
of catalyst was beneficial to protect the active phases and
promote their dispersion in the oil phase and then presented
upgraded by selectively removing oxygen from oxygenated
compounds.4
Hydrodeoxygenation (HDO) is an efficient technology to lower
the oxygen content, where the oxygen atom is removed in the
1
0, 19
high HDO activity and stability during the HDO reactions.
2
0, 21
Normal MoS
2
has no hydrophobicity,
but its hydro-
phobicity can be regulated by both the surface chemical
a. School of Chemical Engineering, Xiangtan University, Xiangtan, Hunan, 411105,
PR China
National & Local United Engineering Research Centre for Chemical Process
Simulation and Intensification, Xiangtan University, Xiangtan 411105, P. R. China
Corresponding Author, E–mail: wangweiyan@xtu.edu.cn (W. Wang), and
composition and surface roughness.
22-24
Based on these
mentioned above, in this study, we tried to add silicomolybdic
acid into the hydrothermal preparation of MoS and expected
b.
2
†
to obtain hydrophobic catalysts. Their HDO activity and
stability were tested by using p-cresol as a model compound.
yangyunquan@xtu.edu.cn (Y. Yang)
Electronic Supplementary Information (ESI) available: [Detailed characterization
procedures of XRD analysis, BET analysis, water contact angle analysis, XPS This work mainly concentrated on the effect of hydrophobicity
analysis, SEM analysis, HRTEM analysis and activity test for hydrophobic MoS2
on the activity, product distribution and catalyst stability.
catalysts. The definitions of conversion, selectivity and deoxygenation degree for
each experiment.]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 2018
New J. Chem., 2018, 00, 1-3 | 1
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in the HDO reactions by preventing water from contacting
EXPERIMENTAL
View Article Online
DOI: 10.1039/C8NJ05980B
catalyst surface.
All solvents and reagents were obtained from Sinopharm
Chemical Reagent Co., Ltd. in high purity (≥99%) and used
without further purification. Sodium molybdate (0.73 g),
thiourea (1.14 g) and silicomolybdic acid (0 g, 2 g, 4 g, 6 g and 8
g) were dissolved in 100 mL water, and hydrochloric acid was
added to adjust the pH value to 0.8, and then transferred to a
0
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00 mL Teflon-lined stainless-steel autoclave and treated at
40 C for 24 h. After reaction, the resulting black precipitate
o
was separated and washed with water and absolute ethanol,
and dried under vacuum at 60 C for 5 hours. The resultant
o
catalysts were denoted as Mo-S-X, where X represented the
weight of silicomolybdic acid. These catalysts were
characterized by X–ray Diffraction, Nitrogen Physisorption,
Raman Spectra, Scanning Electronic Microscopy, High
Resolution Transmission Electron Microscopy technologies and
Water Contact Angles. Their catalytic activities were tested in a
batch reactor using the HDO of p-cresol as a probe reaction.
The detailed characterization procedure, activity tests and the
definitions of conversion, selectivity and deoxygenation
degree for each experiment were shown in the Supporting
Information.
2
Fig. 2 SEM images of MoS prepared by adding different
amounts of silicomolybdic acid
RESULTS AND DISCUSSION
Characterization of MoS catalysts
2
Table 1 The structure properties of Mo-S-X catalysts
Surface
Pore
Pore
Contact
angle ( )
Catalyst
area
m /g)
volume diameter
I
002/I100
o
2
3
(
(cm /g)
0.15
(nm)
10.0
Mo-S-0
Mo-S-2
Mo-S-4
Mo-S-6
Mo-S-8
63.6
1.34
1.30
1.24
1.04
0.93
42.3
65.5
99.8
0.20
0.25
0.49
0.59
9.4
8.8
110.8
212.4
235.3
96.4
9.2
118.5
132.3
10.1
Fig. 1 Photographs of Mo-S-0 and Mo-S-8 distribution between
dodecane and water and water Contact angles of Mo-S-0, Mo-
S-4
Fig. 2 shows the scanning electron microscopy (SEM) images of
MoS catalysts prepared by adding different amounts of
silicomolybdic acid. Without adding silicomolybdic acid, MoS
and
Mo-S-8
2
2
The hydrophobicity of the prepared MoS
2
was first tested in
was built up with many curly and interlaced nanosheets with
thickness of several nanometers randomly. With the increase
of silicomolybdic acid amount, the self-assembled nanosheet
morphology gradually became indistinct. According to the
possible formation mechanism of the self-assembled
the mixture of dodecane and water. According to the density,
MoS had larger density than water and dodecane. If the
prepared MoS had weak hydrophobicity, black MoS would be
2
2
2
dispersed in water. As shown in Fig. 1, Mo-S-0 was dispersed in
water while Mo-S-8 was only dispersed in dodecane. Contact
angle meter was further used to measure and compare the
2
5, 26
2-
nanosheets proposed in the previous investigation,
ions reacted with sulfur ions to form MoS and the residual
ammonia prevented the stacking of MoS nanostructures, and
then MoS grew with two-dimensional direction to be
MoO
4
2
2
hydrophobicity. The air-water contact angle of MoS prepared
2
by normal hydrothermal method was only 42.3°, but which
was increased to 96.3° and 132.3° after adding 4 g and 8 g
silicomolybdic acid, respectively. These suggested that adding
2
2
7
nanosheet nanostructure. After adding silicomolybdic acid,
because of its acidity, the produced ammonia from the
decomposition of thiourea existed in the solution in the form
, which decreased the residual ammonia and promoted
the three-dimensional direction growth and then influenced
the morphology. As shown in the SEM image of Mo-S-8, the
nanosheet morphology almost disappeared. Consequently, it
could be concluded that the morphology was one of factors for
2
silicomolybdic acid during the preparation of MoS could
enhance its hydrophobicity and Mo-S-8 had a very high
hydrophobicity, which might be attributed to the co-existence
of Si and the modification of catalyst surface microstructure.
This was further analyzed by XPS and SEM characterizations.
This high hydrophobicity was expected to enhance its stability
+
of NH
4
2
8
2
| New J. Chem., 2018, 00, 1-6
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enhancing the hydrophobicity of MoS
hydrophobicity of the prepared MoS
disturbed and the particle aggregation was inhibited, measured S/Mo atom ratio was 1.82, which was very close to
producing smaller particles, which enlarged the surface area. that of Mo-S-8 (1.81). Hence, the decreasing S/Mo atomic ratio
As shown in Table 1, the surface area of MoS
the added amount of silicomolybdic acid. The surface area and the water solution from contacting with solid surface.
pore volume of Mo-S-8 were 235.3 m /g and 0.59 cm /g,
respectively, both of which were much larger than those of
Mo-S-0.
2
. In addition, due to the also prepared hydrophobic MoS
2
with the same synthesis
View Article Online
, oriented growth was conditions for Mo-S-8 but doubled thDiOoIu: r1e0.a103a9m/Co8uNnJt0.59T8h0Be
2
2
-
2
increased with was caused by that the hydrophobicity prevented S ions in
2
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(
a)
Mo-S-0
Mo-S-4
Mo-S-8
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Fig. 3 TEM images of Mo-S-0, Mo-S-4 and Mo-S-8
2
The microstructure of MoS was further characterized by
transmission electron microscopy (TEM), as shown in Fig. 3. In
all TEM images, black thread-like fringes were clearly observed,
attributing to the MoS
about 0.65 nm, characterizing the (0 0 2) basal planes of
240
238
236
234
232
230
228
226
224
Electron Binding Energy (eV)
(b)
Mo-S-0
2
slabs. These fringes have a spacing of
2
9, 30
crystalline MoS
industrial MoS
2
.
Comparing with the TEM image of the
2
, these prepared MoS catalysts presented
2
Mo-S-4
short lengths of wrinkled layers with highly disordered and
minimal layer stacking, suggesting the nanometer particles or
poor crystallinity.31 After adding silicomolybdic acid, the slab
length of MoS
formation of smaller particles, which mainly resulted from that
the high hydrophobicity inhibited the growth of MoS
crystallized particles. Notably, in the TEM image of Mo-S-8,
some defects were observed in the black fringes. These
defects corresponded to the sulfur vacancies according to our
previous investigation, indicating that the ratio of S/Mo was
lower than theoretical value in MoS
confirmed by XPS characterization.
The X-ray photoemission spectroscopy (XPS) was used to
identify the chemical state of the prepared MoS , as displayed
in Fig. 4. Due to the addition of silicomolybdic acid, the
element Si was also analyzed. The XPS result showed that the
atom content of Si in Mo-S-8 was only 5.6%. The detailed
binding energy profiles for Mo and S in Mo-S-0, Mo-S-4 and
Mo-S-8 were similar. Three peaks appeared at 229.3, 232.5 eV
and 226.5 eV in the XPS spectra of Mo 3d level, attributed to
2
was further shortened, indicating the
Mo-S-8
159
2
1
66
165
164
163
162
161
160
Electron Binding Energy (eV)
1
7
Fig. 4 XPS spectra of (a) Mo 3d and (b) S 2p levels of Mo-S-0,
Mo-S-4 and Mo-S-8
2
, which was further
Fig. 5 shows the X-ray diffraction (XRD) patterns of the
2
catalysts by adding different silicomolybdic
acid. Four diffraction peaks appeared at 2θ=14°, 33°, 36° and
9°, corresponding to (002), (100), (103) and (110) planes of
respectively. Notably, the intensity of
002) peak was decreased and the peak intensity for other
planes almost unchanged with the increase of silicomolybdic
acid amount. The corresponding ratio of I002/I100 was
calculated according to the XRD data and summarized in Table
2
prepared MoS
5
34, 35
hexagonal MoS
2
,
(
4
+
2-
the doublet Mo 3d5/2 and Mo 3d3/2 in Mo ions and S for
3
2
MoS
62 eV, corresponding to the S 2p1/2 and S 2p3/2 of S ions,
respectively. These suggested that the prepared samples were
MoS and the addition of silicomolybdic acid did not change its
2
, respectively. Fig. 4(b) showed two peaks at 163.3 and
1
. Obviously, the ratio of I002/I100 in the XRD pattern of
2
-
33
1
decreased from 1.34 for Mo-S-0 to 0.93 for Mo-S-8, suggesting
that there had some defects on (002) plane. However, our
previous investigation found that the low ratio of I002/I100 was
2
sulfide structure. The S/Mo atomic ratio was estimated from
the integrated peak areas of XPS survey spectra, which was
17
attributed to the defects caused by the loss of sulfur. This
unsaturation of sulfur in the prepared MoS had also been
confirmed by the XPS analysis. Hence, the addition of
silicomolybdic acid into the hydrothermal preparation of MoS
2
1
.94, 1.88 and 1.81 for Mo-S-0, Mo-S-4 and Mo-S-8,
respectively. These suggested that S/Mo atom ratio in the
surface of MoS was decreased with the addition amount of
2
2
had no effect on the typical sulphide structure but changed the
S/Mo molar ratio.
silicomolybdic acid, which seemed to be caused by the
increased amount of Mo in the system. To confirm this, we
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Table 2 The conversion and toluene selectivity in the HDO of p-
View Article Online
DOI: 10.1039/C8NJ05980B
cresol on Mo-S-X after different run numbers at 300 °C for 5 h
Toluene selectivity
Conversion (%)
Decrement
%)
Increment
(%)
(%)
Catalysts
(
Mo-S-0
Mo-S-2
1st 2nd 3rd
1st
2nd 3rd
Mo-S-0 100 100 90.8
Mo-S-2 99.3 97.4 91.1
Mo-S-4 99.0 95.2 93.3
Mo-S-6 96.2 93.8 93.2
Mo-S-8 91.0 91.0 90.6
9.2
8.3
6.2
3.1
0.4
85.8 89.5 93.1
85.8 92.2 92.5
86.0 91.7 92.6
86.1 89.9 91.3
86.2 87.8 90.8
8.5
7.8
7.7
6.0
5.3
0
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0
Mo-S-4
Mo-S-6
Mo-S-8
1
0
20 30 40 50 60 70 80 90
To further study the effect of hydrophobicity on the stability,
the reusability of all catalysts were tested in the HDO of p-
cresol, as shown in Table 2. After three cycle reactions, the
conversion and toluene selectivity on Mo-S-0 was 90.8% and
85.8%, respectively. In comparison with the first run, the
conversion was decreased by 9.2% while the toluene
selectivity increased by 7.3%, which resulted from the negative
effect of the produced water during the HDO reaction on the
2
Theta (degree)
Fig. 5 XRD patterns of MoS
different
2
catalysts prepared by adding
amounts
of
silicomolybdic acid
HDO activity
Conversion
3
Methylcyclohexane
-Methylcyclohexene Toluene
1
00
80
60
1
7
sulfide structure. These obviously presented the low stability
of Mo-S-0 and a considerable deactivation. Surprisingly, this
deactivation was greatly inhibited by enhancing the
hydrophobicity of MoS
both the decrement of conversion and the increment of
toluene selectivity were decreased, especially for MoS with a
2
. With the hydrophobicity increasing,
4
0
0
0
2
high hydrophobicity. For example, the conversion on Mo-S-8
was 90.6% after three cycles, which was only decreased by 0.4%
compared with the first run. However, the modification on the
sulfide structure during the HDO reaction was hard to
completely eliminate, leading to a small increment on toluene
selectivity.
2
Mo-S-0 Mo-S-2 Mo-S-4 Mo-S-6 Mo-S-8
Catalyst
Fig. 6 Conversion and product selectivity in the HDO of p-cresol
on Mo-S-X catalysts after reaction at 300 °C for 5 h
Table 3 The change of peak intensity and S/Mo atom ratio
after HDO reaction
The prepared MoS
2
was applied into the HDO of p-cresol to
I002/I100 ratio
S/Mo atom ratio
Decrement
%)
Decrement
(%)
Catalyst
study the effect of hydrophobicity on the catalytic activity and
reaction route. The activity test results are shown in Fig. 6.
After reaction at 300 °C for 5 h, the conversion and toluene
selectivity on Mo-S-0 was 100% and 85.8%, respectively.
Associated with the contact angle listed in Table 1, it could be
found that the hydrophobicity had a slight effect on the
conversion but no effect on the product distribution. The
conversion on Mo-S-8 was still up to 91%, and the selectivity of
toluene, methylcyclohexane, 3-methylcyclohexene was 86.0%,
(
Before
After
Before
After
reaction reaction
reaction reaction
Mo-S-0
Mo-S-2
Mo-S-4
Mo-S-6
Mo-S-8
1.34
1.30
1.27
1.07
0.94
1.08
1.07
1.07
1.04
0.93
19.4
17.7
15.7
2.8
1.94
/
1.82
/
6.2
/
1.88
/
1.85
/
1.6
/
1.0
1.81
1.80
0.6
1
0.7% and 3.3%, respectively. The enhanced hydrophobicity of
To confirm the effect of hydrophobicity on the chemical
stability of MoS , XRD and XPS was employed to characterize
MoS prevented reactant molecules from contacting with
2
2
catalyst surface to some extent in the HDO of p-cresol because
of the hydrophility of phenolic hydroxyl group, and then
weakened the adsorption of p-cresol on catalyst surface and
lowered the conversion. However, Mo-S-8 had a large surface
area (235.3 m /g), which would expose more active sites and
facilitated the adsorption of p-cresol. Therefore, the
the spent catalysts. The XRD patterns and XPS spectra are
shown in the Supporting Information (seen in Fig. S1 and S2).
The corresponding results are summarized in Table 3. After
three cycle reactions, the decrement of I002/I100 and S/Mo
atom ratio for Mo-S-0 catalyst was 19.4% and 6.2%,
respectively, exhibiting a low stability. However, for spent Mo-
S-8 catalyst, the I002/I100 was only decreased by 1.0% while the
S/Mo atom ratio was almost unchanged in comparison with
2
conversion was decreased slightly.
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fresh Mo-S-8 catalyst. These suggested that the 3. M. Saidi, F. Samimi, D. Karimipourfard, T. NimmVaienwwArutidcliepOonnlinge,
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14,
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B
hydrophobicity had a great effect on the prevention of sulfur
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7
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operation temperature promoted sulfur loss. However, in this
study, the HDO reaction was carried out at 300 °C, but Mo-S-8
catalyst presented almost no sulfur loss. If the reaction
temperature is decreased, these hydrophobic MoS
would show higher stability.
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catalysts
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Conclusions
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During the hydrothermal preparation of MoS , the addition of
silicomolybdic acid enhanced the hydrophobicity and changed
the morphology. When the added weight of silicomolybdic
acid was increased to 8 g, the air-water contact angle of MoS
was increased from 42.3° to 132.3° while the surface area was
enlarged to 235.3 m /g, but the self-assembled nanosheet
morphology gradually became indistinct and the I002/I100 ratio
was decreased to 0.94, suggesting a high hydrophobicity and
low S/Mo atom ratio. In the HDO of p-cresol, the conversion 12. G. Liu, H. Ma, I. Teixeira, Z. Sun, Q. Xia, X. Hong and S. C. E.
was slightly decreased but the product distribution was almost
unchanged with the hydrophobicity. Most importantly,
because the hydrophobicity prevented water from contacting
catalyst surface, both the decrement on the conversion and
the increment on toluene selectivity were observably
decreased in the reusability tests. After the HDO reaction, the
002/I100 ratio and S/Mo atom ratio in Mo-S-8 changed very
little, demonstrating that the hydrophobicity was a significant
factor for the stability of MoS .
2
2
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The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript. The authors declare no competing financial
interest.
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Acknowledgements
This research was supported by the National Natural Science
Foundation of China (No. 21776236 and 21676225), Natural
Science Foundation of Hunan Province (2018JJ2384),
Collaborative Innovation Centre of New Chemical Technologies 25. T. Yang, Y. Chen, B. Qu, L. Mei, D. Lei, H. Zhang, Q. Li and T.
for Environmental Benignity and Efficient Resource Utilization,
Wang, Electrochim. Acta 2014, 115, 165-169.
Engineering Research Centre of Chemical Process Simulation 26. H. Liu, X. Su, C. Duan, X. Dong and Z. Zhu, Mater. Lett. , 2014,
and Optimization of Ministry of Education, and Hunan Key
Laboratory of Environment-Friendly Chemical Process
Integrated Technology.
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Chem. Commun. , 2013, 49, 9591-9593.
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