SDBS-assisted hydrothermal synthesis of flower-like Ni-Mo-S catalysts and their enhanced hydrodeoxygenation activity

Article abstract of DOI:10.1039/c5ra20086e

Ni-Mo-S catalysts were prepared by sodium dodecyl benzene sulfonate (SDBS) assisted hydrothermal synthesis. The presence of SDBS increased the NiS₂ crystallite size, enlarged the interlayer distance of MoS₂ plane and formed loose flo

Full text of DOI:10.1039/c5ra20086e

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SDBS-assisted hydrothermal synthesis of flower-
like Ni–Mo–S catalysts and their enhanced
hydrodeoxygenation activity†
Cite this: RSC Adv., 2015, 5, 94040
Received 29th September 2015
Accepted 30th October 2015
Weiyan Wang,*ab Song Tan, Guohua Zhu, Kui Wu, Liang Tan, Yingze Li
and Yunquan Yang*
a
a
a
a
a
ab
DOI: 10.1039/c5ra20086e
www.rsc.org/advances
Ni–Mo–S catalysts were prepared by sodium dodecyl benzene catalysts from industrial commercialization. Mo based suldes,
sulfonate (SDBS) assisted hydrothermal synthesis. The presence of a kind of commercial catalysts for hydrodesulfurization reac-
2
SDBS increased the NiS
distance of MoS
which contributed to the enhanced HDO activity. Compared with Ni– played an important role in the catalytic activity and product
2
crystallite size, enlarged the interlayer tions, had been extensively applied into the HDO reactions, but
2
plane and formed loose flower-like architecture, their morphologies depended on the synthesis methods and
23
Mo–S synthesized in the absence of SDBS, p-cresol conversion, distribution. Customarily, traditional suldes were prepared
methylcyclohexane selectivity and deoxygenation degree was by sulfurization with toxic gas H S, and their activities was
2
24
increased by 24%, 25.1% and 26.3%, respectively.
closely related to the sulfurization conditions. Moreover, this
process required high temperature to obtain the desired phases
and resulted in an ordered crystalline MoS with large particle
size. It had conrmed that amorphous structure supplied more
available unsaturated active sites for reactions and then
2
Due to the increasing energy consumption and environmental
pollution, bio-oil, derived from the fast pyrolysis of biomass,
has been considered as a sustainable, renewable and clean
replacement material for the production of fuel and chemicals
25,26
enhanced the catalytic activity.
For example, B. Yoosuk
27
et al. had prepared amorphous unsupported Ni–Mo bimetal
suldes with high surface area by one step hydrothermal
method and concluded their high HDO activities.
1,2
and consequently attracted much attention. Unfortunately,
this bio-oil has high oxygen content, leading to some delete-
rious properties, e.g., a low heating value, meaning that it could
not be utilized directly as a supplement for transportation fuel.
Aiming to decrease the oxygen content, catalytic hydro-
deoxygenation (HDO) is an interesting and effective technology
for the selective removal of oxygen atom from bio-oil in the
presence of hydrogen and with a moderate reaction tempera-
Adding surfactant as a so template during the synthesis of
Mo based suldes was a normal strategy to control their
28
29
morphologies. C. Li et al. had reported that Pluronic F-127
had a signicant effect on the formation of ower-like MoS
2
.
30
S. Miao et al. had found that tetraethylorthosilicate was crucial
to form exfoliated MoS with ultra-small sheets and high
specic surface area (101.6 m
2
3
ture. Since phenols were the major monomers for the lignin
2
ꢀ1
31
g
). Y. Tang et al. had
fraction of biomass and the scission of Caromatic–OH bond was
demonstrated that PVP-assisted synthesis was an efficient and
4
harder than other C–O bonds, they were usually used as model
scalable method for the preparation of MoS nanosheets. L. Ma
2
compounds to evaluate the HDO activity of the prepared
catalysts.
32
et al. had prepared few-layer and edge-rich MoS nanospheres
2
by adding SDBS. However, although previous studies had
Up to now, the catalysts for the HDO of phenols included
shown the high activity of Ni–Mo bimetal sulde in the HDO of
5
–7
8–10
amorphous metal borides,
metal catalyst,
noble metal
27,33
phenols,
these catalysts were not prepared by SDBS-assisted
11–14
15–17
18
19
catalysts,
transition metal phosphide,
carbide, oxide
hydrothermal synthesis before. Therefore, in this communica-
tion, the effects of SDBS addition amount on the structures and
HDO activities of Ni–Mo–S catalysts were studied.
20–22
and sulde catalysts.
However, the low thermal stability,
high cost or low HDO activity of prevented some of these
Fig. 1 showed the XRD patterns of Ni–Mo bimetal sulde
catalysts prepared by adding different amount of SDBS. Without
a
School of Chemical Engineering, Xiangtan University, Xiangtan City, Hunan 411105,
ꢁ
P R China. E-mail: wangweiyan@xtu.edu.cn; yangyunquan@xtu.edu.cn
b
adding SDBS, the peak at 2q ¼ 14 in the XRD pattern of Ni–Mo–
34,35
National
&
Local United Engineering Research Center for Chemical Process S was attributed to the typical (002) plane of MoS
,
while the
2
(ref. 36)
2
Simulation and Intensication, Xiangtan University, Xiangtan 411105, P. R. China
ꢁ
ꢁ
ꢁ
ꢁ
peaks at 2q ¼ 32 , 36 , 54 and 57 to cubic phase NiS
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c5ra20086e
ꢁ
37,38
and 2q ¼ 45 to the characteristic of amorphous structure.
1
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Fig.
1 XRD patterns of the as-prepared Ni–Mo bimetal sulfide
catalysts.
3
5,39
Compared with crystalline MoS
2
in the previous studies,
all
ꢁ
ꢁ
ꢁ
peaks at 2q ¼ 33 , 39 and 59 became much weaker, indicating
that Ni–Mo–S had a poorly crystallization and MoS and NiS
2
2
were uniformly dispersed in the resultant catalysts. At the
ꢁ
presence of SDBS, some new peaks were observed at 2q ¼ 27 ,
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
36
3
2 , 36 , 39 , 45 and 53 , attributing to cubic NiS
2
,
which
became obvious and sharp, especially for Ni–Mo–S-0.3. These
suggested that the addition of SDBS had a great effect on NiS
crystallite size in Ni–Mo–S catalysts.
2
Fig. 2 SEM and TEM images of Ni–Mo bimetal sulfide catalysts.
2
According to the XRD results, the NiS crystallite size was
40
calculated by the Scherrer formula: D ¼ Kl/(b cos q). As shown sphere morphology vanished, but sphere-like particles with
in Table 1, the average crystallite size of NiS in Ni–Mo–S, Ni– a diameter of about 0.6 mm appeared. Ni–Mo–S-0.3 presented
2
Mo–S-0.1, Ni–Mo–S-0.2, Ni–Mo–S-0.3 and Ni–Mo–S-0.4 was 11.8 a loose ower-like architecture, which was resulted from the
nm, 27.5 nm, 31.5 nm, 41.2 nm and 39.7 nm, respectively, random self-assembly of nano-sheets. According to the possible
indicating that adding SDBS during the synthesis process formation mechanism for Mo based suldes in the previous
42,43
increased the NiS
2
crystallite size. It had reported that Ni–Mo–S literatures,
the hollow sphere morphology was elucidated as
phase formed and possibly presented as nano-size particles in following: (1) nucleation and growth into primary nano-
the catalysts when they were prepared by one step hydrothermal particles, (2) orientated growth into nano-sheets and (3)
41
method, leading to no appearance in the XRD pattern.
oriented self-assembly into hollow sphere. Compared with the
However, in this study, the Ni/Mo molar ratio was only 0.3, but different morphologies in Fig. 2, the third step that nano-sheets
there displayed obvious NiS2 phase in Fig. 1. Moreover, the self-assembly was the crucial step for the nal morphology.
crystallite size of NiS was changed with the SDBS addition Because SDBS had an aromatic headgroup and a hydrophobic
2
amount. These revealed that no or very small proportion of Ni– tail, it could insert in the interspace between nano-sheets and
Mo–S phase formed in the resultant Ni–Mo bimetal sulde effectively inhibit the aggregation of nano-sheets. Conse-
catalysts.
quently, the hollow sphere was broken, and then the nano-
The morphologies of Ni–Mo bimetal sulde catalysts were sheets self-assembled to form ower-like morphology. In addi-
observed using SEM and TEM, as shown in Fig. 2. Ni–Mo–S tion, it had presumed that SDBS could produce an interaction
2
ꢀ
showed a hollow sphere morphology with a diameter of about with MoS4 anions due to the chemical coupling effect origi-
.5 mm and wall thickness of 0.4 mm. This morphology might be nating from the highly delocalized p electrons of benzene ring
1
32
formed from the self-assembly of sheet-like subunits in ordered and the external electrons of the sulfur atoms. This interaction
orientation. Aer adding SDBS, e.g., Ni–Mo–S-0.2, the hollow could stabilize the nano-sheets and then decreased the thick-
ness. As shown in Fig. 2, when the SDBS amount increased from
0
.2 g to 0.4 g, the nano-sheet thickness gradually reduced. Their
surface areas were in line with the SEM results. As listed in
Table 1, because of the ll of NiS in MoS pores, all the surface
Table 1 Physical properties of Ni–Mo bimetal sulfide catalysts
2
2
Surface area
2
NiS crystallite
size (nm)
2
ꢀ1
areas were very low. Ni–Mo–S, Ni–Mo–S-0.1, Ni–Mo–S-0.2, Ni–
Catalyst
(m g
)
2
Mo–S-0.3 and Ni–Mo–S-0.4 presented a surface area of 7.9 m
Ni–Mo–S
7.9
7.2
5.6
7.5
9.0
11.7
27.5
31.5
41.2
39.7
ꢀ1
2
ꢀ1
2
ꢀ1
2
ꢀ1
2
ꢀ1
g
, 7.2 m
g
, 5.6 m
g
, 7.5 m
g
and 9.0 m g ,
Ni–Mo–S-0.1
Ni–Mo–S-0.2
Ni–Mo–S-0.3
Ni–Mo–S-0.4
respectively. The surface area was decreased aer the change of
hollow sphere into the solid sphere, but which was increased
when the nano-sheet thickness became thinner.
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To further investigate the effect of SDBS on the microstruc-
ture of Ni–Mo bimetal sulde catalysts, the prepared samples
were characterized by HRTEM, as shown in Fig. 3. All the images
presented two kinds of parallel fringes: one group with an
interlayer distance of 0.65–0.72 nm and the other one with an
interplanar spacing of 0.27 nm, characterizing the (002) plane of
46,47
MoS
2 2
(ref. 44 and 45) and (200) plane of cubic NiS ,
respectively. These also suggested that Ni and Mo were existed
in the separated sulde phases in Ni–Mo bimetal sulde cata-
lysts. Interestingly, with the increment of SDBS amount, the
interlayer distance for (002) plane of MoS was enlarged, which
2
was caused by the aromaticity of the headgroup in SDBS. At the
presence of SDBS, it acted as an exfoliator and inserted into the
interlayer of nano-sheets to disrupt the van der Waals interac-
48,49
tion between MoS
2
layers.
This enlarged interlayer distance
was expected to be favorable for the co-planar absorption of
phenols via the aromatic ring and exhibited high hydrogenation
selectivity.
The catalytic activity of the prepared Ni–Mo bimetal sulde
catalysts were tested taking the HDO of p-cresol as a probe. The
concentration changes of reactant and products versus reaction
time on Ni–Mo–S and Ni–Mo–S-0.3 are shown in Fig. 4. Previ-
ously, two reaction routes had been proposed in the HDO of
Fig. 4 Concentration changes of reactant and products versus reac-
phenols based on the product distributions: (i) direct deoxy- tion time in the HDO of p-cresol on (a) Ni–Mo–S and (b) Ni–Mo–S-
genation (DDO) via C–O bond scission and (ii) hydrogenation– 0.3.
deoxygenation (HYD) via saturation of the aromatic ring
followed by the dehydration. Fig. 4(a) showed that the main
products in the HDO of p-cresol on Ni–Mo–S were toluene,
methylcyclohexane and 3-methylcyclohexene, and not any
oxygen-containing compound was detected during the whole
reaction, suggesting that the hydrogenation of the aromatic ring
was the limited step for HYD route. The increasing toluene and
methylcyclohexane concentrations with the reaction time indi-
cated that DDO and HYD routes were simultaneously occured,
where toluene was the product of DDO route while methyl-
cyclohexane and 3-methylcyclohexene produced from the HYD
route. According to these product concentrations, it could be
concluded that DDO route was superior to HYD route on Ni–
27,50,51
Mo–S catalyst. However, previous studies
had reported
that the addition of promoter Ni into MoS2 enhanced the
hydrogenation activity and made HYD to be the main HDO
route for phenols. This change on the HDO reaction route was
resulted from the different synthesis method of the catalyst. For
52
example, D. Wang et al. had reported that toluene selectivity
was higher than 90% on Ni–Mo–W sulde catalyst prepared by
mechanical activation method. In contrast, although the HDO
of p-cresol on Ni–Mo–S-0.3 presented the same products as that
on Ni–Mo–S, their contents varied greatly. The product
concentration at the same reaction time decreased in the order
of methylcyclohexane > toluene > 3-methylcyclohexene, indi-
cating that the main HDO route changed toward to HYD. These
demonstrated that adding SDBS into the synthesis of Ni–Mo
bimetal sulde enhanced the hydrogenation activity.
The comparison of Ni–Mo–S, Ni–Mo–S-0.1, Ni–Mo–S-0.2, Ni–
Mo–S-0.3 and Ni–Mo–S-0.4 on the HDO activity are shown in
53,54
Fig. 3 TEM images of Ni–Mo bimetal sulfide catalysts.
Table 2. According to the previous conclusions,
the
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Table 2 The HDO of p-cresol on Ni–Mo–S, Ni–Mo–S-0.1, Ni–Mo–S-0.2, Ni–Mo–S-0.3 and Ni–Mo–S-0.4 at 275 C for 6 h
RSC Advances
ꢁ
Catalyst
Ni–Mo–S
Ni–Mo–S-0.1
Ni–Mo–S-0.2
Ni–Mo–S-0.3
Ni–Mo–S-0.4
Conversion (mol%)
71.5
2.4
82.8
3.3
85.8
3.4
95.5
4.6
92.6
3.7
2
k ꢂ 10 , mL per s per g catalyst
Products selectivity (mol%)
Methylcyclohexane
30.8
10.4
58.8
1.47
68.6
33.7
9.3
57.0
1.48
80.8
49.1
4.8
46.1
1.59
84.2
56.0
5.5
38.5
1.65
94.9
59.3
6.7
34.0
1.69
87.3
3
-Methylcyclohexene
Toluene
H/C molar ratio
D. D. (wt%)
decomposition of p-cresol was modeled to be a pseudo-rst- degree on Ni–Mo–S-0.3 was increased by 24%, 25.1% and 26.3%
order reaction and the reaction rate constant k (mL per s per under the same reaction conditions, respectively. But further
g catalyst) was calculated according to the equation ln(1 ꢀ x) ¼ increasing SDBS amount would reduce the HDO activity, e.g.,
ꢁ
ꢀ
kCcatt. Aer reaction at 275 C for 6 h, p-cresol conversion on 87.3% deoxygenation degree on Ni–Mo–S-0.4. Hence, adding an
Ni–Mo–S was 71.5% with a selectivity of 30.8% methyl- appropriate SDBS in the synthesis of Ni–Mo bimetal sulde
cyclohexane and a deoxygenation degree of 68.6%. With the could maximize its HDO activity.
increment of SDBS, p-cresol conversion increased rst and then
For the HDO of p-cresol, the different absorption ways
56,57
decreased while methylcyclohexane selectivity increased grad- decided the reaction routes.
Vertical orientation absorption
ꢁ
ually. The corresponding k at 275 C increased with the order of facilitated the direct C–O bond scission to form toluene while
ꢀ
2
ꢀ2
Ni–Mo–S (2.4 ꢂ 10 ) < Ni–Mo–S-0.1 (3.3 ꢂ 10 ) < Ni–Mo–S-0.2 coplanar adsorption caused to the saturation of aromatic ring
ꢀ2
ꢀ2
(
3.4 ꢂ 10 ) < Ni–Mo–S-0.4 (3.7 ꢂ 10 ) < Ni–Mo–S-0.3 (4.6 ꢂ rst and then deoxygenated to yield methylcyclohexane as the
ꢀ2
1
0 ), showing the highest HDO activity of Ni–Mo–S-0.3. It had nal product. The XRD and TEM results indicated that no or
reported that the highest reaction rate constant k on Ni–Mo very little of Ni–Mo–S phase but seperated Ni and Mo suldes
ꢀ
2
bimetal suldes prepared without adding SDBS was 3.2 ꢂ 10
phases presented in the resultant catalysts, hence, the HDO
ꢁ
33
mL per s per g catalyst at 300 C, which was much lower than reaction mechanism could be explained by the control remote
that in this study because the k increased with the raising of model. Spillover hydrogen generated on the donor phase (NiS2)
5
5
reaction temperature. Compared with Ni–Mo–S, p-cresol and then migrated onto the acceptor phase (MoS ) for the HDO
2
58
conversion, methylcyclohexane selectivity and deoxygenation reaction aer p-cresol was absorbed on MoS2. The positive
effect of adding SDBS in the synthesis of Ni–Mo bimetal sulde
catalyst on the HDO activity might be related to following
factors. Firstly, as shown in Fig. 5(a), p-cresol conversion
increased with NiS
2
average crystallite size. Ni–Mo–S-0.3 had the
largest NiS crystallite size (41.2 nm) and exhibited the highest
2
conversion (95.5%). The larger the NiS crystallite size was, the
2
more hydrogen supplied on NiS , and the higher was the
2
conversion. Secondly, Fig. 3 and Table 2 displayed that the
interlayer distance for (002) plane of MoS and H/C molar ratio
2
increased with the SDBS amount. This suggested that higher
interlayer promoted the coplanar adsorption and then raised
the hydrogenation products selectivity. As shown in Fig. 5(b),
the sum selectivity of methylcyclohexane and methylcyclohex-
ene on Ni–Mo–S-0.4 was the largest. Last but not least, Ni–Mo–S-
0.3 presented a loose ower-like structure. Compared with the
hollow spheres that had most of the surface area inside and not
as accessible to p-cresol molecules, this special structure could
provide more void space for the transfer of substrate and
45
product molecules and more active sites for reactions, which
enhanced the HDO catalytic activity.
Conclusions
Ni–Mo–S catalysts were prepared by SDBS-assisted hydro-
thermal synthesis and the effect of SDBS addition amount
during the synthesis on their structures and HDO activities were
Fig. 5 The changes of (a) p-cresol conversion versus Ni
size and (b) the sum of selectivity of methylcyclohexane and methyl-
cyclohexene versus interlayer distance between MoS (002) plane.
2
2
S crystallite
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2
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ꢀ2
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