One pot synthesis of NiMo-Al2O3 catalysts by solvent-free solid-state method for hydrodesulfurization

Article abstract of DOI:10.1039/c7ra11892a

A simple and solvent-free solid-state method was used to prepare NiMo-Al₂O₃ hydrodesulfurization (HDS) catalysts using Ni(NO₃)₂·6H₂O, (NH₄)₆Mo₇O₂₄·4H₂O, and AlCl₃·6H₂O as the solid raw materials and polyethylene glycol (PEG) as an additive. The effects of PEG addition on the precursor thermal decomposition, catalyst properties and dibenzothiophene (DBT) HDS activity were investigated. The as-prepared catalysts were characterized by nitrogen adsorption-desorption measurements, powder X-ray diffraction (XRD), thermogravimetric analysis/differential scanning calorimetry (TGA/DSC), H₂ temperature-programmed reduction (H₂-TPR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). The results showed that an increase in PEG addition dramatically increases specific surface area and pore volume of the catalyst, and improves Mo sulfidability and active MoS₂ dispersion by blocking the aggregation of metals, and consequently increases the number of HDS active sites. However, excess PEG leads to the decrease in specific surface area and pore volume attributed to the metal sintering caused by the strong heat release during thermal decomposition. As a result, dibenzothiophene HDS activity enhanced with increasing PEG addition and peaked at NiMoAl-15 (15% weight ratio of PEG to alumina), which exhibited a significantly higher activity as compared to the NiMo/Al₂O₃ catalyst prepared by wetness co-impregnation.

Full text of DOI:10.1039/c7ra11892a

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One pot synthesis of NiMo–Al2O3 catalysts by
solvent-free solid-state method for
hydrodesulfurization†
Cite this: RSC Adv., 2017, 7, 54468
*
Xiaodong Yi,
Weikun Lai
Dongyun Guo, Pengyun Li, Xinyi Lian, Yingrui Xu, Yunyun Dong,
and Weiping Fang
A simple and solvent-free solid-state method was used to prepare NiMo–Al2O3 hydrodesulfurization (HDS)
catalysts using Ni(NO3)2$6H2O, (NH4)6Mo7O24$4H2O, and AlCl3$6H2O as the solid raw materials and
polyethylene glycol (PEG) as an additive. The effects of PEG addition on the precursor thermal
decomposition, catalyst properties and dibenzothiophene (DBT) HDS activity were investigated. The as-
prepared catalysts were characterized by nitrogen adsorption–desorption measurements, powder X-ray
diffraction (XRD), thermogravimetric analysis/differential scanning calorimetry (TGA/DSC), H2
temperature-programmed reduction (H2-TPR), X-ray photoelectron spectroscopy (XPS), scanning
electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). The results
showed that an increase in PEG addition dramatically increases specific surface area and pore volume of
the catalyst, and improves Mo sulfidability and active MoS2 dispersion by blocking the aggregation of
metals, and consequently increases the number of HDS active sites. However, excess PEG leads to the
decrease in specific surface area and pore volume attributed to the metal sintering caused by the strong
heat release during thermal decomposition. As a result, dibenzothiophene HDS activity enhanced with
increasing PEG addition and peaked at NiMoAl-15 (15% weight ratio of PEG to alumina), which exhibited
a significantly higher activity as compared to the NiMo/Al2O3 catalyst prepared by wetness co-
impregnation.
Received 29th October 2017
Accepted 24th November 2017
DOI: 10.1039/c7ra11892a
rsc.li/rsc-advances
Many approaches have been employed to improve metal
dispersion of the catalysts for high HDS activity. The commonly
1. Introduction
used HDS catalyst preparation methods include simultaneous
or sequential impregnation, hydrothermal deposition and sol–
gel synthesis.13–15 For these methods, a complicated and time-
consuming process is always required and active metal
components on the support surface tend to agglomerate into
large crystallites. Therefore, a novel and efficient solid-state
reaction for preparing metal complexes, metal oxides or
suldes with uniform sizes and shapes has been developed.
Zhang et al. reported a low-temperature solid-state reaction for
the synthesis of ammonium aluminum carbonate hydroxide
(AACH), which used poly-glycol (PEG), NH4HCO3 and
Al(NO3)3$9H2O as precursors.16 The alumina obtained by the
calcination of AACH showed unique bimodal porous structure
and high content of surface acid. Without any solvents, Ren
et al. synthesized zeolites with MFI structure by mechanically
mixing of solid raw materials.17 Because of the simple, rapid,
solvent-free and energy-saving process, this method has
attracted extensive research interest.
Due to increasingly stringent environmental regulations for the
sulfur content in fuel oils,1 catalytic hydrodesulfurization (HDS)
is widely used to remove suldes in petroleum rening. In
typical commercial HDS catalysts, active components
MoS2(WS2) and Co(Ni) promoters are supported on g-alumina.
The HDS activity of the multicomponent catalysts depends on
the synthetic procedure and their compositions. In recent years,
different supporters such as SiO2, ZrO2, TiO2 and mixed oxides
have been widely studied for improving HDS performance.2–5
Another source of activity enhancement can be an alternative
active phase based on transition metals (e.g., Ru, Fe, Rh, Pt) or
carbides, phosphides.6–8 In addition, the addition of different
organic compounds such as ethylene diamine tetraacetic acid
(EDTA), citric acid, thioglycolic acid or polyethylene glycol also
has crucial inuence.9–12
National Engineering Laboratory for Green Chemical Productions of Alcohols, Ethers
and Esters, College of Chemistry and Chemical Engineering, Xiamen University,
Xiamen 361005, P. R. China. E-mail: xdyi@xmu.edu.cn
Recently, solid-state reaction has been applied to the
synthesis of supported catalysts with a high dispersion of active
metal, in which solid state reactions occur between hydrated
transition metal salts and organics. With this method, Cu/ZnO/
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra11892a
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ZrO2 catalysts were prepared for CO2 hydrogenation; the copper owing air atmosphere up to 800 ꢁC at a heating rate of
dispersion and catalytic activity of the catalysts are strongly 10 C minꢀ1 with TGA/DSC (SDT Q600) thermal analyzer.
inuenced by calcination temperature.18 Our group systemati-
Nitrogen adsorption–desorption measurements of the cata-
ꢁ
cally studied the properties of NiMo–Al2O3 catalysts prepared by lysts were performed on a Micromeritics Tristar 3000 adsorp-
a similar process using starch and urea as additive.19,20 The tion automatic instrument at liquid nitrogen temperature. The
ꢁ
results showed that an appropriate concentration of starch samples were outgassed at 300 C for 3 h in vacuum. Surface
improves the HDS activity, but an excess of starch addition areas were calculated by the Brunauer–Emmett–Teller (BET)
decreases the activity because of agglomeration of Mo particles. method, and the pore size distribution and total pore volume
Compared with conventional impregnation method, intro- were determined by the Barrett–Joyner–Halenda (BJH) method.
ducing active metal during the synthesis process of mesoporous
alumina possesses especial advantages, including high disper- out on an X'Pert Pro automatic powder diffractometer operated
sion of metallic composition and preferable anti-sintering.
at 40 kV and 30 mA using CuKa (l ¼ 0.15406 nm) mono-
As mentioned, such solid-state method has seldom been chromatized radiation in all cases.
Powder X-ray diffraction (XRD) characterization was carried
used to load active metal components and synthesis carrier in
Temperature-programmed reduction (TPR) experiments
a one-step process, particularly in using this method to were performed with a GC-TPR apparatus. The samples of as-
prepared supported HDS catalyst. Herein, we report the solvent- made NiMoAl-x (50 mg) were pretreated in an Ar stream at
ꢁ
free solid-state synthesis of NiMo–Al2O3 catalysts using PEG as 300 C for 1 h and then cooled to room temperature. Next, the
additive. The solid-state reaction is not only used for producing catalyst was heated to 900 ꢁC at a rate of 10 ꢁC minꢀ1 under a 30
alumina but also used for dispersing Ni–Mo active species onto ml minꢀ1 ow of an 5% H2/Ar mixture. H2 consumption was
the support simultaneously. A detailed study on the inuence of determined by a thermal conductivity detector (TCD).
PEG addition on the catalytic behavior of NiMo–Al2O3 catalyst
XPS spectra were obtained in MultiLab 2000 spectrometer
for dibenzothiophene (DBT) hydrodesulfurization has been using Mg Ka radiation under vacuum (<10ꢀ9 mbar). The oxidic
performed.
catalysts were pressed into thin sheets, sulfurized in a 15% H2S/
H2 stream at 400 C for 2 h, and then cooled to room temper-
ꢁ
ature. Before measurement, the sulded catalysts were kept
under inert atmosphere without contact air.
2. Experimental section
2.1. Synthesis samples
Scanning electron microscopy (SEM) images were obtained on
a eld emission scanning electron microscopy LEO-1530 device,
which was manufactured by Leo Company in Germany. Emission
voltage was 15 eV, and accelerating voltage was 20 kV.
High-resolution transmission electron microscopy (HRTEM)
images of the samples were obtained using a Philips Analytical
FEI Tecnai 30 electron microscope operating at 300 kV. Freshly
sulded catalyst was ultra-sonically dispersed in ethanol. The
suspension was then collected on a car bon-coated copper grid.
Ni(NO3)2$6H2O,
(NH4)6Mo7O24$4H2O,
AlCl3$6H2O
and
alkapolpeg-6000 (PEG-6000, molecular weight: 6000) were
purchased from Sinopharm Chemical Reagent (China).
Commercial alumina (g-Al2O3, SSA 260 m2 gꢀ1) was obtained
from Fushun Research Institute of Petroleum and Petrochem-
ical, SINOPEC, Fushun, China. All of the above chemicals were
used as purchased without any further purication.
A series of NiMo–Al2O3 catalysts with different PEG addition
were prepared by solid-state synthesis. In a typical preparation,
the solid raw materials Ni(NO3)2$6H2O, (NH4)6Mo7O24$4H2O,
AlCl3$6H2O and PEG-6000 (molecular weight: 6000) were mixed
and grinded at ambient temperature for 10 min, then calcined
at 500 ꢁC at a heating rate of 10 ꢁC minꢀ1 for 4 h in muffle
furnace. The samples were denoted as NiMoAl-x (x ¼ 0, 2, 6, 10,
15 and 20), where x represents the weight ratio of PEG to
alumina. In the nal catalyst, the content of MoO3 and NiO are
12 wt% and 4 wt%, respectively.
2.3. Catalytic tests
Dibenzothiophene (DBT) HDS reaction was measured in a high-
pressure xed-bed continuous-ow stainless steel reactor with
internal diameter 6.0 mm. Before the reaction, the catalyst was
sulfurized in situ at 400 ꢁC for 2 h in a stream of 15 vol% H2S/H2
under atmospheric pressure. The reaction protocol is as follows:
ꢁ
300 C with a total pressure of 2.0 MPa, a liquid hourly space
velocity (LHSV) of 12 hꢀ1, and a H2/hydrocarbon volumetric
ratio of 600, using 2.0 wt% DBT in decalin as a model
compound. The liquid product was collected every hour aer
a steady state had been reached and analyzed by a gas chro-
matograph equipped with a FID detector and a HP-5 capillary
column. The GC spectra and product distribution of DBT HDS
were shown in S-Fig. 1.†
For comparison, commercial alumina (g-Al2O3, SSA 260 m2
gꢀ1) supported NiMo catalyst have been prepared by co-
impregnation of aqueous solutions of ammonium heptamo-
lybdate and nickel nitrate. The catalyst precursor was dried at
110 ꢁC for 12 h, followed by calcining at 500 ꢁC for 5 h, and the
nal catalysts were denoted as NiMo/Al2O3. The content of
MoO3 and NiO in the nal catalyst was the same as the catalyst
prepared by solid-state synthesis method.
3. Result and discussion
2.2. Characterization
3.1. Thermal analysis
Thermogravimetric analysis/differential scanning calorimetry Thermal oxidative decomposition of the various precursors,
(TGA/DSC) of the precursor before calcination was conducted in which determines the structure and property of the resultant
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catalyst, is signicantly affected by the PEG addition. The thermal
analysis results for the NiMoAl-x precursors with different PEG
contents are reported in Fig. 1. As seen from TG curves in Fig. 1A,
the weight loss for NiMoAl-x at about 100 ꢁC corresponds to the
removal of moisture. The major weight loss for NiMoAl-x between
150 ꢁC and 350 ꢁC is attributed to a series of overlapping oxidative
decomposition reactions, including the process of dehydration–
decomposition and dehydroxylation of PEG and aluminum
chloride and the subsequent total transformation of nickel
nitrate and ammonium heptamolybdate.21,22 The presence of
PEG exhibits a new weight loss at 300 ꢁC, while it becomes greater
with the increase of PEG content. The total weight losses coin-
cided with the expected values, indicating that both the PEG and
the precursors are completely decomposed.
3.2. Textural properties and crystalline phase
Nitrogen adsorption–desorption isotherms of the NiMoAl-x
catalysts are shown in Fig. 2A. All the samples exhibit type IV
isotherms with H1 hysteresis loops, which indicate the feature
of mesoporous structure. The area of the hysteresis loops and
quantity of N2 adsorbed for the NiMoAl-x samples increase
signicantly with increasing PEG addition, which indicates that
the porous system develops as PEG content increases. Texture
parameters calculated by BET and BJH methods (Table 1) show
that the BET surface area and pore volume increase obviously
with increasing PEG addition. However, when the PEG addition
is sufficiently high, comparing to the NiMoAl-15 sample, the
BET surface area and pore volume of NiMoAl-20 decrease. On
the one hand, swelling is not completely free, but is constrained
by steric effects; on the other hand, the considerable heat
released via roasting excessive amount of PEG results to the
agglomeration of nickel–molybdenum particles. Pore size
distribution in Fig. 2B shows that the most probable pore size
improves with increasing PEG content, yielding a wide pore size
distribution. Meanwhile, a series of Al2O3-x (where x represents
the weight ratio of PEG to alumina, and x ¼ 0, 10, 15, 20) were
synthesized via solvent-free solid-state route with different PEG
addition (Table S2 and S-Fig. 2†). All these results indicate that
PEG plays a key role in directing framework of the as-prepared
catalyst.
As shown in Fig. 1B, the DSC curves indicate several endo-
thermic processes at temperature lower than 250 ꢁC and an
exothermic reaction at higher temperature. A weak endo-
thermic band occurs at 60 ꢁC for PEG-containing samples
corresponds to the melting of the extended chains, the other
endothermic peak locates in 100–250 ꢁC is caused by the partial
thermal decomposition of the precursors. Whereas, a strong
exothermic peak at about 300 ꢁC is assigned to the combustion
decomposition of PEG, in this process, most of aluminium
chloride is decomposed into oxide. With the increasing PEG
ꢁ
addition, the exothermic peak area and peak height at 300 C
increase proportionally. Otherwise, most of the chloride ion is
removed as gaseous HCl or NH4Cl during the calcining process,
but there is still a trace residual on the oxide catalysts. Inter-
estingly, we have not detected the presence of Cl on the sulde
catalysts, which might generate HCl during the suldation
process (Table S1†).
X-ray diffraction patterns of the oxide (A) and sulde (B)
NiMoAl-x catalysts are shown in S-Fig. 3.† For the oxide cata-
lysts, a weak peak corresponding to g-Al2O3 can be observed,
which indicates the formation of poorly crystalline. Typical
features of molybdenum and nickel oxides are absent, which
Fig. 1 TG curves (A) and DSC curves (B) of the NiMoAl-x precursors
measured by TGA/DSC: (a) NiMoAl-0, (b) NiMoAl-2, (c) NiMoAl-6, (d) Fig. 2 Nitrogen adsorption–desorption isotherms (A) and pore size
NiMoAl-10, (e) NiMoAl-15, and (f) NiMoAl-20.
distributions (B) of the NiMoAl-x catalysts.
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Table 1 Summary of the several typical properties of the NiMoAl-x catalysts
NiMoAl-0
NiMoAl-2
NiMoAl-6
NiMoAl-10
NiMoAl-15
NiMoAl-20
NiMo/Al2O3 (imp)
SBET (m2 gꢀ1 a
)
72
306
0.52
5.3
3.9
2.6
384
0.84
8.6
3.7
4.4
11.0
0.54
86.9
3.5
407
0.96
10.0
3.9
420
0.88
9.9
4.1
5.4
13.2
0.57
89.8
2.9
243
0.66
8.4
4.0
2.5
250
0.57
8.1
4.5
3.4
12.3
0.43
81.2
4.4
Vp (cm3 gꢀ1 b
)
0.15
4.0
3.4
4.3
8.6
0.56
60.7
4.2
2.8
Dm (nm)c
Mo/Al (ꢃ102)d
Ni/Al (ꢃ102)d
S/Al (ꢃ102)d
Ni/(Ni + Mo)d
Mosul. (%)d
L (nm)e
5.0
8.4
12.3
0.56
89.0
3.0
9.1
0.40
75.9
3.6
0.38
89.5
3.7
Ne
2.5
2.4
2.2
2.1
1.9
3.4
a
BET surface area. b Pore volume. c Most probable pore diameter. d Atomic ratio, obtained by XPS. e Determined by statistical analysis of HRTEM
images.
can be due to the high dispersion of nickel and molybdenum that a fraction of Mo oxide species possibly incorporated into
species. For the sulde catalysts, the weak and broad XRD peaks alumina bulk phase, which are difficult to be reduced.
at 14.4ꢁ, 33.5ꢁ, and 58.6ꢁ (JCPDS card 37-1492) can be identied,
XPS spectra of the sulded NiMoAl-x catalysts with different
which corresponding to (002), (101) and (110) crystal planes of PEG addition were collected and decomposed. Mo 3d spectra of
MoS2, respectively. The broad diffraction peaks indicate that no the catalysts are shown in S-Fig. 4,† and the related parameters
aggregated particles were formed aer sulfurization and that are listed in Table 2. For Mo 3d spectra, 3 different contributions
there was a high dispersion of active components over the Al2O3 corresponding to MoS2, MoOxSy, and Mo oxide species were
support surface.
determined according to binding energy values reported in
previous works.24 The three Mo 3d doublets are as follows: (1) the
binding energy at approximately 228.7 eV (Mo4+ 3d5/2) associated
with MoS2 species, (2) the binding energy at 230.6 ꢂ 0.2 eV (Mo5+
3d5/2) correlated with a Mo oxy-sulde species, and (3) the
binding energy at 232.6 ꢂ 0.2 eV (Mo6+ 3d5/2) related to Mo oxide
species strongly bonded to alumina or enwrapped by alumina
that were not completely sulded. Furthermore, a broad peak at
approximately 226.0 eV was assigned to the S 2s line and sub-
tracted from the total spectrum of Mo 3d. For the NiMoAl-
0 catalyst, a high percentage of MoOx (Mo6+) was measured,
and the MoS2 species represented only about 60.7%. However,
the atomic percentage of Mo4+ in the sulde phase increases
signicantly (from 60.7% to 89.5%) with the increase in the PEG
content, demonstrating that the addition of PEG considerably
improves the suldation and reducibility of Mo species.
3.3. Reducibility and surface composition
H2-TPR characterization of the as-prepared oxide NiMoAl-x
catalysts is shown in Fig. 3. For all the catalysts, three main H2-
consumption peaks could be clearly identied. On the basis of
the previous reports, the rst peak between 400 ꢁC and 500 ꢁC,
which slightly shied to lower temperature with increasing PEG
addition, could be associated to the reduction (Mo6+ / Mo4+) of
highly dispersed octahedral Mo species.23 The second signal,
identied in the temperature range of 500–700 ꢁC, could be
ascribed to oxidic Ni2+ species in weak interaction with the
alumina. The high-temperature H2-consuming peak between
ꢁ
750 ꢁC and 900 C is related to the second step of reduction of
octahedral Mo species and the rst step reduction of tetrahedral
coordinated monomeric Mo species, which strongly bonded to
the alumina support or enwrapped by alumina. It can be seen
Ni 2p XPS spectra of the sulded NiMoAl-x catalysts are shown
in S-Fig. 5.† Ni 2p3/2 envelope was decomposed by considering
three major contributions. The binding energy at 853.5–854 eV
corresponds to NiMoS phase. The peak at approximately 856.1 eV
corresponds to Ni oxide species, including NiAl2O4 and NiOx that
Table 2 XPS parameters of the different contributions of Mo 3d ob-
tained for the sulfide NiMoAl-x catalysts
Mo4+
Mo5+
Mo6+
Catalyst
BE (eV) % atom BE (eV) % atom BE (eV) % atom
NiMoAl-0
NiMoAl-2
NiMoAl-6
228.8
228.5
229.0
60.7
75.9
86.9
89.0
230.7
230.5
230.6
230.6
16.2
8.0
4.6
2.5
232.7
232.5
232.8
232.6
23.1
16.1
8.5
NiMoAl-10 228.8
8.5
Fig. 3 H2-TPR patterns of the oxide NiMoAl-x catalysts.
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were strongly bonded to alumina support or enwrapped by least 20 images of 500–600 slabs taken from the different parts
alumina. Ni may also exist as NiSx (Ni3S2, Ni9S8, and NiS), which of each catalyst. The average layer number (N) and the average
is located at 853.0 ꢂ 0.1 eV.25 The relative quantities of Ni species slab length (L) have been calculated according to the rst
(NiMoS, NiSx, and Ni oxide) are reported in Table 3. The moment of the distribution:26
,
proportion of NiMoS phase, which is assumed to be the active
x
x
X
X
phase for catalytic HDS, increases with PEG addition. A higher
proportion of the NiMoS phase is obtained for the NiMoAl-15
sample, in which 76.3% of Ni atoms are engaged as compared
to 65.1% in NiMoAl-0. The proportion of Ni oxide phase
decreases with the increase of PEG amount, which can be
attributed to the reduction of difficult-to-reduce species due to
the blocking of PEG. These results suggested the presence of PEG
suppresses the incorporation of Mo species into alumina bulk
phase.
L ¼
xili
xi
(1)
(2)
i¼1
i¼1
,
y
y
X
X
N ¼
yiNi
yi
i¼1
i¼1
where li is the length of slab i, Ni is the layer number of slab i, xi
is the number of slabs with length li, and yi is the number of
slabs with Ni layers.
The statistical results of the length and stacking layer
number distributions of the MoS2 slabs over the sulded
NiMoAl-x catalysts are showed in Fig. 4. Over 45% of the
observed MoS2 crystals are longer than 4 nm on the NiMoAl-
0 catalyst, while the percentage decreases with increasing PEG
content, as a result, the average slab length of MoS2 gradually
reduces. However, when the amount of PEG increases to 20
(NiMoAl-20), more MoS2 crystals longer than 4 nm are detected.
The NiMoAl-15 catalyst shows an average MoS2 slabs length of
2.9 nm, which is much shorter than that of NiMo/Al2O3 catalyst
prepared by impregnation method (L, Table 1). Particularly, the
NiMoAl-15 catalyst exhibits the most centralized length distri-
bution, more than 70% of the observed MoS2 crystals length is
between 2 nm and 4 nm. The stacking layer number distribu-
tions show that the proportion of multilayered structures (>4)
increases with increasing PEG addition, which results an
increase in the average stacking layer number (N, Table 1).
The integrated areas for Al 2p, Mo 3d, Ni 2p and S 2p peaks of
the sulded catalysts were used to calculate the atomic concen-
tration ratios of Mo, Ni and S over Al. As shown in Table 1, the
surface atomic ratio of Mo/Al increases with increasing PEG
addition, which should be attributed to the transfer of Mo atoms
from the bulk phase to the surface phase. However, Ni/(Ni + Mo)
and Ni/Al ratios both reached a maximum value at a PEG/
alumina weight ratio of about 15, in line with surface atomic
ratio of S/Al. The Mo suldation degree (Mosul.) of NiMoAl-x
catalysts was correlated with the fraction of Mo4+ determined by
the decomposition of Mo 3d XPS spectra. The Mo suldation
degree increases signicantly with PEG addition, indicating that
the addition of PEG considerably improve the suldability of Mo.
3.4. Morphology and dispersion of MoS2 clusters
The external morphologies of the NiMoAl-x and NiMo/Al2O3 are
presented in S-Fig. 6.† NiMoAl-0 with large particle size is
mainly formed by the accumulation of akes, and has a smooth
surface, whereas NiMoAl-x (x ¼ 6, 10, 15, 20) catalysts seem to
be formed by the agglomeration of the uniform spherical
particles with the size of 10 nm.
TEM characterization of the sulded NiMoAl-x and NiMo/
Al2O3 catalysts was performed for the determination of the
MoS2 dispersion and crystallites. Representative HRTEM
micrographs of the catalysts are shown in S-Fig. 7.† Typical
˚
MoS2 fringes with 6.2 A interplanar distance are observed in all
the micrographs of the sulded catalysts. The slab lengths and
stacking layer numbers of MoS2 slabs on the sulded NiMoAl-x
catalysts were obtained through statistical analyses based on at
Table 3 XPS parameters of the different contributions of Ni 2p ob-
tained for the sulfide NiMoAl-x catalysts
NiMoS
NiSx
NiOx
Catalyst
BE (eV) % atom BE (eV) % atom BE (eV) % atom
NiMoAl-0
NiMoAl-2
NiMoAl-6
NiMoAl-10 853.9
NiMoAl-15 853.9
NiMoAl-20 854.0
853.6
853.5
853.7
65.1
68.0
71.8
75.2
76.3
75.2
853.1
853.0
853.0
853.2
853.0
853.1
2.7
3.4
1.8
0.8
1.8
1.2
855.9
855.9
856.2
856.3
856.0
856.3
32.1
28.6
26.4
23.8
21.9
23.6
Fig. 4 Slab lengths (A) and stacking layer number (B) distributions of
MoS2 crystallites on the sulfide NiMoAl-x and NiMo/Al2O3 catalysts.
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Additionally, the stacking layer number distribution of the
NiMoAl-15 catalyst is the most homogenous, with over 70% of
MoS2 slabs having 2 to 3 layers. Compared with NiMo/Al2O3, all
NiMoAl-x catalysts show lower average stacking layer (1.9–2.8),
which suggests a better MoS2 dispersion. The addition of PEG
can block the aggregation of metals and support, and improve
Mo dispersion.27,28 Thus, more active MoS2 can be generated.
The average fraction of Mo atoms on the edge surface of the
MoS2 crystals, denoted as fMo, is a dispersion indicator of the
active surface of the crystal. The fMo value is determined from
eqn (3), by assuming that the MoS2 crystals are perfect hexa-
gons.29 In eqn (3), the numerator is the number of atoms in the
active surface (edges) and the denominator is the total number
of Mo atoms in the crystal; t is the total number of slabs shown
in the HRTEM images, and ni is the number of Mo atoms in one
Fig. 5 Schematic illustration of the solid-state synthesis of meso-
porous
NiMoS–Al2O3
catalysts
for
dibenzothiophene
hydrodesulfurization.
catalyst under the same conditions. It is noted that the CHB
selectivity over the NiMoAl-15 catalyst is higher than that over
NiMoAl-0 and NiMoAl-20 catalysts. As a result, the NiMoAl-15
catalyst shows a large HYD/DDS ratio, and the total HDS activ-
ities of DBT over the NiMoAl-x catalysts have the same changing
tendency as HYD/DDS ratio, which can be attributed to more
accessible NiMoS edge sites. However, the selectivity of BP
produced via the DDS pathway is much higher than that of CHB
and DCH formed through the HYD route, which was similar to
the ndings of previous studies.30 This feature should be due to
the low hydrogen pressure and high liquid hourly space velocity
(LHSV).
edge, determined from the slab length L as shown in eqn (4)
,
t
t
X
X
ꢀ
ꢁ
fMO
¼
ð6ni ꢀ 6Þ
3ni2 ꢀ 3ni þ 1
(3)
i¼1
i¼1
ni ¼ L/6.4 + 0.5
(4)
The fMo values in Table 1 show a noticeably higher MoS2
dispersion on the sulded NiMoAl-x catalysts prepared using
PEG, as compared to the NiMo/Al2O3 catalyst. The MoS2
dispersion over the NiMoAl-x catalysts increases from 0.23 to
0.34 with increasing PEG content.
3.6. Effect of PEG on catalyst structure and HDS activity
3.5. HDS performance of the NiMoAl-x catalysts
As shown in Fig. 5, the solid-state method using PEG as additive
permit to synthesize highly active Al2O3 supported NiMo HDS
catalysts. The addition of PEG results in large surface area and
pore volume, which allow the catalyst to expose more active
sites. Indeed, the addition of PEG improves the reducibility and
suldability of Mo, as proved by TPR and XPS results, which
suggests a weak metal–support interaction. However, a larger
Mo dispersion can be observed from the HRTEM images with
PEG addition, which can be attributed to suppression of active
metal aggregation by PEG blocking.27,28 As previous studies
described, this positive effect may be caused by the fact that
there is a coordination/interaction between numerous hydroxyl
(OH) in PEG and active metals on the alumina support. During
the drying step, higher dispersion of Ni and Mo precursors was
maintained on the alumina surface even aer water removal,
since PEG has higher boiling point than water. These results are
the major factors for the signicant improvement in the HDS
activity of dibenzothiophene.
The HDS reaction results of dibenzothiophene (DBT) for the
NiMoAl-x catalysts are reported in Table 4. The conversion of
DBT occurs through two parallel routes: the direct desulfur-
ization pathway (DDS) yields mainly biphenyl (BP), whereas the
hydrogenation route (HYD) generates cyclohexylbenzene (CHB)
and dicyclohexyl (DCH). The specic rate of DBT HDS increased
with increasing PEG addition and peaked at NiMoAl-15,
decreasing thereaer. Compared to NiMo/Al2O3 catalyst, the
DBT HDS conversion was 10 percent higher for NiMoAl-15
Table 4 Initial activities and product distributions for dibenzothio-
phene HDS over the NiMoAl-x and NiMo/Al2O3 catalystsa
Product selectivityc
(%)
Catalyst
Rb (10ꢀ6 mol gꢀ1 sꢀ1
)
BP
CHB DCH HYD/DDS
NiMoAl-0
NiMoAl-10
NiMoAl-15
NiMoAl-20
0.47
1.33
1.41
1.09
92.3 5.4
90.5 7.8
89.8 8.5
91.9 6.1
90.9 7.3
2.3
1.7
1.7
2.0
1.8
0.083
0.105
0.114
0.088
0.100
4. Conclusions
In the present work, we develop a simple and solvent-free solid-
state route for synthesizing Al2O3 supported NiMo catalysts with
different PEG addition, which also can be used to synthesis
NiMo/Al2O3 1.25
a
Reaction conditions: temperature, 300 ꢁC; H2 pressure, 2.0 MPa;
LHSV, 12 hꢀ1; H2/liquid (v/v), 600; feed, 2 wt% DBT in decalin.
b
Initial specic rate (moles of DBT transformed per second and per Al2O3 supported other metal catalysts. An appropriate amount
c
gram of catalyst). DDS, direct desulfurization pathway; HYD,
of addition of polyethylene glycol (PEG) can dramatically
increases specic surface area and pore volume of the catalyst,
hydrogenation pathway; BP, biphenyl; CHB, cyclohexylbenzene; DCH,
dicyclohexyl.
and improves Mo suldability and active MoS2 dispersion by
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Acknowledgements
This work is supported by the National Natural Science Foun-
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54474 | RSC Adv., 2017, 7, 54468–54474
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