Synthesis of highly ordered TiO2-Al2O3 and catalytic performance of its supported NiMo for HDS of 4, 6-dimethyldibenzothiophene

Article abstract of DOI:10.1016/j.cattod.2020.03.003

The highly ordered mesoporous TiO₂-Al₂O₃ composite oxides were prepared via a facile evaporation-induced self-assembly (EISA) method. All of those synthesized materials were characterized by means of XRD, XRF, N₂ adsorption, FTIR, Py-FTIR and TEM. The results show that the hexagonally ordered mesoporous TiO₂-Al₂O₃ oxides with high specific surface area, high thermal stability and narrow pore size distributions were successfully synthesized by EISA. The corresponding NiMo catalysts supported on TiO₂-Al₂O₃ oxides were further characterized by H₂-TPR, HRTEM and XPS. Their hydrodesulfurization catalytic performances were tested in a fixed-bed reactor, using 4,6-dimethyldibenzothiophene as the probe. The analyzing results exhibit that the incorporation of TiO₂ could effective weaken the support-metal interactions, thus enhancing the reduction degree of active metals and forming more “Type II” Ni-Mo-S active phases that would be beneficial for the HDS of highly refractory organosulfur compound. The catalytic results reveal that the 4,6-DMDBT conversion over NiMo/TA-n catalysts gradually increased as the Ti/Al molar ratio increases, and reaches a maximum values at NiMo/TA-0.4. The NiMo/TA-0.4 exhibited the highest hydrodesulfurization catalytic performance of 4,6-DMDBT due to the synergistic effect of suitable textural properties, high thermal stability and moderate metal-support interactions.

Full text of DOI:10.1016/j.cattod.2020.03.003

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Catalysis Today
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Synthesis of highly ordered TiO2-Al2O3 and catalytic performance of its
supported NiMo for HDS of 4, 6-dimethyldibenzothiophene
Pengfei Zhanga, Fujun Mua, Yasong Zhoua,*, Yuchi Longa, Qiang Weia, Xiaodong Liua,
Qingxiang Youa, Yacheng Shana, Wuwen Zhoub
a China University of Petroleum, State Key Laboratory of Heavy Oil Processing, Beijing 102249, PR China
b School of Chemistry and Chemical Engineering, Xi’an University of Science and Technology, Xi’an 710054, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
The highly ordered mesoporous TiO2-Al2O3 composite oxides were prepared via a facile evaporation-induced
self-assembly (EISA) method. All of those synthesized materials were characterized by means of XRD, XRF, N2
adsorption, FTIR, Py-FTIR and TEM. The results show that the hexagonally ordered mesoporous TiO2-Al2O3
oxides with high specific surface area, high thermal stability and narrow pore size distributions were successfully
synthesized by EISA. The corresponding NiMo catalysts supported on TiO2-Al2O3 oxides were further char-
acterized by H2-TPR, HRTEM and XPS. Their hydrodesulfurization catalytic performances were tested in a fixed-
bed reactor, using 4,6-dimethyldibenzothiophene as the probe. The analyzing results exhibit that the in-
corporation of TiO2 could effective weaken the support-metal interactions, thus enhancing the reduction degree
of active metals and forming more “Type II” Ni-Mo-S active phases that would be beneficial for the HDS of highly
refractory organosulfur compound. The catalytic results reveal that the 4,6-DMDBT conversion over NiMo/TA-n
catalysts gradually increased as the Ti/Al molar ratio increases, and reaches a maximum values at NiMo/TA-0.4.
The NiMo/TA-0.4 exhibited the highest hydrodesulfurization catalytic performance of 4,6-DMDBT due to the
synergistic effect of suitable textural properties, high thermal stability and moderate metal-support interactions.
TiO2-Al2O3
Evaporation-induced self-assembly
Hydrodesulfurization
4,6-Dimethyldibenzothiophene
1. Introduction
significant improvement for HDS activity, the promising industrial ap-
plications were limited by its high cost and low stabilities. As a result of
The petroleum fractions contain many kinds of sulfides, including
mercaptans, thioethers, thiophenes, dibenzothiophenes and their alkyl
derivatives [1,2]. Hydrodesulfurization (HDS) [3] is an effective way to
remove most of the sulfides from the petroleum fractions. However, due
to more and more stringent environmental regulations (strictly limited
sulfur content < 10 ppm), academia and industry need to find an ef-
fective way to remove organosulfur compound such as dibenzothio-
phenes and their alkyl-substituted dibenzothiophenes [4–8], which are
highly refractory for the S-removal over traditional Ni(Co)MoS2(WS2)
catalysts supported on Al2O3. Increasing the operating temperature and
pressure of hydrodesulfurization process would result in additional
economic burden. Therefore, the development of novel HDS catalysts
with the excellent catalytic performance for the refractory organosulfur
compound is the most effective strategy to produce engine fuel that
acquires the requirement of ultra-low sulfur diesel (ULSD) [9].
high surface area, high thermal-stability and low prices, γ-alumina is
widely employed as support of HDS catalysts. Unfortunately, the strong
interactions between γ-alumina and active metals via Mo-O-Al linkages
limit the sulfidation of active metals and result in the formation of vast
“Type I” NiMoS active phases [11], which exhibit a relatively low HDS
catalytic performance. In order to overcome this disadvantage, binary
composite supports, such as TiO2-Al2O3 [12–17], MgO-Al2O3 [18–20],
ZrO2-Al2O3 [21] and SiO2-Al2O3 [22] have been considered and de-
veloped as the support of HDS catalysts. Among them, TiO2-Al2O3
composite oxides [12–17] had caused a significant amount of attention.
Because TiO2-Al2O3 composite oxides could not only accurately reg-
ulate the support-metal interactions, but also overcome the defect of the
low surface area and the poor stability of titania.
Synthesis TiO2-Al2O3 composite oxides with different physico-che-
mical properties via various methods, including impregnation [14], co-
precipitation [15], sol-gel [16] and chemical evaporation deposition
[17] has been reported. For instance, V. Santes [14] used three different
strategies (impregnation, co-precipitation and sol-gel) to prepare TiO2-
In the last decade, many attempts had been proposed to develop the
novel HDS catalysts, such as using of noble metals instead of the
transition metal sulfides [8,10]. Although these noble metals showed a
⁎ Corresponding author.
E-mail address: zhouyasong2011@163.com (Y. Zhou).
https://doi.org/10.1016/j.cattod.2020.03.003
Received 10 April 2019; Received in revised form 7 December 2019; Accepted 2 March 2020
0920-5861/©2020ElsevierB.V.Allrightsreserved.
Pleasecitethisarticleas:PengfeiZhang,etal.,CatalysisToday,https://doi.org/10.1016/j.cattod.2020.03.003

P. Zhang, et al.
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Al2O3 oxides. All these three methods successfully could synthesize
TiO2-Al2O3 oxides with high surface area but while the pore volume
and average pore diameter were relatively low. A. Duan [16] prepared
a series of TiO2-Al2O3 composite oxides via sol-gel, using Tetra-n-butyl-
titanate and pseudoboehmite/AlCl3 as precursor. They found catalytic
behavior of NiW/TiO2-Al2O3 catalysts was superior to the reference
NiW/Al2O3 catalyst in HDS of diesel oil, especially for the TiO2-Al2O3
with 15 wt% TiO2 content that could produce the product with sulfur
content close to zero. Furthermore, S. M. Morris [23] has also suc-
cessfully synthesized mesoporous TiO2-Al2O3 by evaporation-induced
self-assembly strategy [24]. Pluronic P123 tri-block copolymer was
used as structure-directing agent and anhydrous ethanol was used as
solvent with appropriate amounts of nitric acid to adjust pH values.
To the best of our knowledge, there are no reports on systematically
studying the performance of TiO2-Al2O3 oxides prepared by evapora-
tion-induced self-assembly method as the support of HDS catalyst. The
aim of this study is to synthesize and characterize TiO2-Al2O3 composite
oxides prepared by EISA method and investigate its role as a carrier on
the ultra-deep HDS. In order to reveal the physicochemical properties of
the synthesized TiO2-Al2O3 oxides, the obtained samples were char-
acterized by XRD, XRF, N2 adsorption, FTIR, Py-FTIR and TEM.
Meanwhile, the corresponding NiMo catalysts were also characterized
by H2-TPR, HRTEM and XPS, and tested in a fixed-bed reactor by using
4,6-dimethyldibenzothiophene (4,6-DMDBT) as a probe.
The N2 adsorption-desorption analysis of TiO2-Al2O3 oxides and the
corresponding catalysts were performed on a Micromeritics ASAP 2010
equipment at ― 196 °C. The surface areas were calculated by the
Brunauer-Emmett-Teuer (BET) equation. The pore volumes and pore
sizes were determined by Barrett-Joyner-Halenda (BJH) method from
the nitrogen adsorption branch of isotherms. To determine the chemical
composition of the synthesized samples, X-ray fluorescence (XRF)
analysis was performed on a PANalytical Axios instrument with the
condition of a vacuum atmosphere. The X-ray diffraction (XRD) mea-
surements of the TiO2-Al2O3 samples were carried out on a PANALY-
RICALL advanced power diffractometer instrument with Cu Kα radia-
tion (40 kV, 40 mA). H2 temperature-programmed reduction
characterization was performed on aTP-5000 multifunction adsorption
equipment. Before the measurement, the catalysts need to be pre-
treated at 200 °C for 60 min using a high-purity Ar. The following step is
that the feed gas was switched to the reduction gas, containing 10 vol.
% H2 in Ar, at the flow rate of 30 ml/min. At the same time, the tem-
perature of the furnace was increased from 100 °C to 1000 °C with a
ramp rate of 10 °C/min. High-resolution transmission electron micro-
scopy (HRTEM) analysis was performed on a Philips Tecnai G2 F20
equipment at 200 kV. Before the experiment, the catalysts must be
sulfide using a fixed bed reactor at 320 °C and 4 MPa for 5 h, using a
cyclohexane solution containing 2 vol. % CS2. After sulfidation, the
catalysts were immediately removed from the reactor and preserved in
cyclohexane. Then, the pre-treated catalysts were dispersed on a
carbon-coated copper grid. Meanwhile, the grid was quickly transferred
into TEM chamber. The average lengths and stacking number of MoS2
crystallites were calculated by statistical analyses based on more than
400 slabs from 20 TEM images of each catalyst. The average length (L)
and average stacking number (N) are calculated using Eq (1) and Eq (2)
[7,27,28]:
2. Experimental
2.1. Preparation of TiO2-Al2O3
The ordered mesoporous TiO2-Al2O3 oxides were prepared ac-
cording to reported procedure [23–26]: 2.00 g Pluronic P123 was firstly
dissolved in 20.0 ml anhydrous ethanol in beaker A. The beaker A was
covered with polyethylene film and stirred at room temperature for 4 h.
After that, 3.40 g of aluminum isopropoxide was dissolved in 20.0 ml
anhydrous ethanol in beaker B, containing 3.2 ml 67 % HNO3 and
1.4 ml distilled water. Once it dissolved, the solution in beaker B was
added dropwise into glass beaker A and stirred at room temperature for
2 h. After that, 0.95 g of titanium isopropoxide was dropwise added and
the solution was further stirred at room temperature for 2 h. Then the
obtained material were transferred to an oven with a temperature of
60 °C for 48 h. After ethanol was completely evaporated, the resulting
samples were calcined at 500 °C with a ramp rate of 1 °C/min. The
obtained TiO2-Al2O3 composite oxides with different titania content
were denoted as TA-n, where n is the molar ratio of Ti to Al. For ex-
ample, TA-0.2 denotes that the Ti/Al molar ratio equal 0.2.
x
i=1 xili
n
L =
i=1 xi
(1)
y
N =
yN
i
i
y
i=1
y
i
(2)
i=1
Where xi represents the number of slabs with length li, li represents the
length of the slab i. y represents the number of slabs in Ni layers, Ni
i
represents the number of layers in slab li.
Assuming that the MoS2 nanoparticles are perfect hexagons, the
dispersion of MoS2
(
fMo) could be calculated using Eq (3) [7,28,29]:
t
i=1 6(ni 1)
Moedge
Mototal
fMo
=
=
t
(3ni2 3ni + 1)
(3)
i=1
where Moedge represents the number of Mo atoms at the edge of MoS2
nanoparticles, Mototal represents the total number of Mo atoms,
t
re-
2.2. Preparation of NiMo/TA-n catalysts
presents the total number of slabs displayed in the TEM images and ni
represents the number of Mo atoms along a MoS2 slab edge, ni could be
obtained from its slab length by the following equation: (L = 3.2(2ni -
1)Å). X-ray photoelectron spectroscopy (XPS) analysis was carried out
on a PerkinElmer PHI-1600 ESCA spectrometer. The binding energies of
measured elements were calibrated by the binding energy of C 1s
(284.6 eV).
Prior to the impregnation, the TiO2-Al2O3 oxides need to be pressed
into a tablet shape and crushed into 20–40 mesh. The NiMo/TA-n
catalysts with the fixed 14 wt. % MoO3 and 3.5 wt. % NiO were pre-
pared by incipient wetness co-impregnation method, using an aqueous
solution containing the required amounts of nickel nitrate hexahydrate
(Aldrich) and ammonium heptamolybdate tetrahydrate (Aldrich). After
that, NiMo/TA-n catalysts were standing at room temperature over-
night, and then the obtained samples were transferred to an oven with a
temperature of 120 °C for 3 h. Finally, the obtained NiMo/TA-n cata-
lysts were calcined at 500 °C for 4 h under air atmosphere. In addition,
NiMo/TA-0.0 sample was used as the reference catalyst.
2.4. Catalytic activity evaluation
The HDS of 4,6-DMDBT were performed in a fixed bed reactor
(8 mm inner diameter and 300 mm length) loaded with 1.0 g catalysts
diluted by the same volume quartz particles. Prior to reaction, the in-
vestigated catalysts were first pre-sulfide at 320 °C and 4 MPa for 5 h
with a weight hourly space velocity (WHSV) of 10 h−1and an H2/oil
ratio of 100 (v/v), using a cyclohexane solution with 2.0 vol. % CS2
concentration. After that, the reactor temperature was adjusted to room
temperature, and then the feed was switched to the cyclohexane con-
taining 0.5 wt% 4,6-DMDBT by a SZB-2 double–piston pump. The
2.3. Materials characterization
The TiO2-Al2O3 oxides synthesized via EISA method are character-
ized by means of XRD, XRF, N2 adsorption, FTIR, Py-FTIR and TEM. The
NiMo/TA-n catalysts were further characterized by H2-TPR, HRTEM
and XPS.
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reactions are performed in the temperature rang 280−300 °C with
different WHSVs, while H2/oil ratio and pressure were fixed at 100 (v/
v) and 4 MPa, respectively. The liquid product was collected after a
stabilization period of 6 h and analyzed offline immediately, using an
Agilent 4890D gas chromatograph with a 60-m capillary Rtx-1 column
(0.25 mm, RESREK). In order to identify each of compounds in the li-
quid product, a Finnigan TRACE gas chromatography/mass spectro-
metry (GC/MS) system consisting of a TRACE Ultra gas chromato-
graphy capillary column and HP 5790 MS detector was used to analyze
the liquid product.
which are in good agreement with the published results [34–37]. On
the other hand, with the further increase of TiO2 contents, TA-0.6
shows new diffraction peaks at 2θ = 25.3°, 38.0°, 48.1°, 54.3°, 62.8°
and 75.5°, corresponding to the (1 0 1), (0 0 4), (2 0 0), (2 1 1), (2 0 4)
and (3 0 1) planes of anatase TiO2 (PDF#21-1272).
In order to investigate the relationship between thermal stability
and Ti/Al molar ratio of TiO2-Al2O3 oxides, the WXRD patterns of TA-
0.2 and TA-0.4 are presented in Fig. 2, calcined at 500 °C, 700 °C and
900 °C, respectively. As shown in Fig. 2(A), the feature of amorphous
for TA-0.2 can still remain even after calcined at 700 °C. And the
crystalline phases of anatase and alumina were detected until calcined
at 900 °C, suggesting that TA-0.2 possesses high thermal stability.
However, as for the TA-0.4 sample calcined at 700 °C, new diffraction
peaks at 2θ = 25.3°, 38.1°, 48.1° and 54.6° were observed, indicating
the crystalline phase of anatase TiO2 formed, and the corresponding
peaks changed sharply and intensely as the increase of calcining tem-
perature to 900 °C. This phenomenon indicates that the thermal stabi-
lity decreases as the Ti/Al molar ratio increases. Furthermore, com-
pared with TA-0.4, no rutile TiO2 crystalline phase on TA-0.2 composite
oxide was detected even after calcined at 900 °C. According to literature
[38,39], anatase and rutile are two primary existing forms of titania.
And anatase TiO2 could transform into rutile TiO2 in the temperature
range of 600 °C–1000 °C [39]. Hua Song [38] found that the thermal
stability of TiO2-Al2O3 composite oxides increase as the Al2O3 content
increases, because alumina can delay the transformation of anatase
TiO2 to rutile TiO2. This is the reason why no rutile TiO2 crystalline
phase on TA-0.2 sample was detected.
The specific reaction rate constant (kHDS) is calculated according to
the pseudo-first-order reaction equation [7,30]:
F
kHDS
=
ln(1 x)
m
(3)
Where kHDS is the reaction rate constant of HDS for 4,6-DMDBT (mol
g−1 h−1), F is the reactant feed rate (mol h−1), m is the weight of the
x
catalyst (g) and is the conversion of 4,6-DMDBT.
The selectivity (HYD/DDS) is calculated using Eq. (4) [31]:
HYD/ DDS= ( DMCHB+ DMBCH+ DMTHDBT+ DMHHDBT)/DMBP
(4)
Where DMCHB is 3,3′-dimethylcyclohexylbenzene, DMBCH is 3,3′-di-
methylbicyclohexyl, DMBP is 3,3′-dimethylbiphenyl, DMTHDBT is di-
methyltetrahydrodibenzothiophene, DMHHDBT is dimethylhexahy-
drodibenzothiophene and no cracking products were detected in the
liquid products.
The turnover frequency (TOF) is calculated using Eq. (5) [32]:
TOF= kHDS/(NMo × fMo × 3600)
(5)
3.2. XRF analysis
Where kHDS is the reaction rate constant of 4,6-DMDBT HDS (mol
g−1 h−1), NMo is the amount of Mo species in catalysts (mol g−1), fMo is
the dispersion of MoS2 active phase.
The XRF characterization has been widely used to investigate the
chemical composition of material. The compositions of the above-
mentioned TiO2-Al2O3 oxides were investigated by XRF, and the cor-
responding results were exhibited in Table 1. The data of TiO2-Al2O3
oxides provided by XRF are in line with the theoretical calculations.
This suggested that the evaporation-induced self-assembly method
[23–26,33] can successfully synthesize TiO2-Al2O3 oxides with the de-
sired amount of components.
3. Results and discussion
3.1. XRD analysis
Fig. 1(A) exhibits the small-angle X-ray diffraction (SXRD) patterns
of the calcined TiO2-Al2O3 oxides. A strong peak centered at around
0.8° and a relatively weak peak centered at around 1.5° were observed,
which could be ascribed to the (1 0 0) and (1 1 0) reflections of hex-
agonally ordered mesoporous TiO2-Al2O3 oxides based on the further
analysis of TEM characterization [23–26,33].
3.3. BET analysis
The N2 adsorption-desorption isotherms and pore size distributions
of the calcined TiO2-Al2O3 oxides are exhibited in Fig. 3, and the cor-
responding data of the specific textural properties (BET surface area,
pore volume and average pore diameter) are collected in Table 1. As
shown in Fig. 3(A), all of those TiO2-Al2O3 oxides exhibit the similar
Type IV nitrogen adsorption-desorption isotherm with H1 shaped hys-
teresis hoop classified by IUPAC [25,26], which is the typical character
The wide-angle X-ray diffraction patterns (WXRD) of the calcined
TiO2-Al2O3 oxides with various Ti/Al molar ratios are also presented in
Fig. 1(B). WXRD patterns of TA-0.0, TA-0.2 and TA-0.4 exhibit no ap-
parent diffraction peaks between 20° and 90° (2θ), confirming that the
above-synthesized mesoporous TiO2-Al2O3 oxides are amorphous,
Fig. 1. Small-angle X-ray diffraction (A) and Wide-angle X-ray diffraction (B) patterns of the TiO2-Al2O3 oxides: (a) TA-0.0, (b) TA-0.2, (c) TA-0.4, (d) TA-0.6.
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Fig. 2. Wide-angle X-ray diffraction patterns of the TA-0.2 (A) and TA-0.4 (B): calcined at 500 °C, 700 °C and 900 °C, respectively.
conclude that most of active metals were located inside the mesoporous
of TiO2-Al2O3 oxides, which is in good agreement with published re-
sults [15,16].
Table 1
Textural Properties of the TiO2-Al2O3 Oxides Calcined at 500 °C.
Samples
Textural properties
Isotherm
Chemical composition
N2 adsorption-desorption isothermals and pore size distributions of
TA-0.2 and TA-0.4 samples are present in Figs. 1 and Figs. 2 (in the
Supporting Information), calcined at 500 °C, 700 °C and 900 °C, re-
spectively. Those samples calcined at 700 °C still possess IV type iso-
thermals with H1 shaped hysteresis loops, which confirm the high
thermal stability of TiO2-Al2O3 oxides. Compared to TA-0.4 sample, the
TA-0.2 still exhibits IV type isothermals with H1 shaped hysteresis
loops even after calcined at 900 °C, implying that the thermal stability
of TiO2-Al2O3 samples decreases as the increasing of Ti/Al molar ratio.
Besides, as for TA-0.2 sample, the calcined temperature does not change
the PSD curves. However, an enlargement phenomenon in the PSD
curves for the TA-0.4 sample calcined at 900 °C is visible, which may be
ascribed to the framework consolidation.
Type
a
SBET
,
Vp,
Dav
,
Al2O3
wt%
,
TiO2a, wt%
m2·g−1
cm−3·g-1
Nm
TA-0.0
TA-0.2
TA-0.4
TA-0.6
348
381
385
362
0.74
0.75
0.73
0.60
8.6
7.7
7.6
6.6
Ⅳ H1
Ⅳ H1
Ⅳ H1
Ⅳ H1
–
–
86.7
78.0
68.3
13.3
22.0
31.7
aDetermined by XRFanalysis.
of the ordered mesoporous structure. Moreover, due to the presence of
the capillary condensation in the mesoporous channels [26,33], TiO2-
Al2O3 oxides possess very steep hysteresis hoop, indicating that the
above-synthesized materials have excellent uniform mesoporous
structure. We could observe from Fig. 3(B) that all of those TiO2-Al2O3
oxides show a relatively narrow pore size distribution (6−13 nm) and
centered at around 9.0 nm. With the increase of Ti/Al molar ratio, the
BET surface area of TiO2-Al2O3 oxides shows an increment and it
reaches the maximum value at TA-0.4 sample. Then the BET surface
area shows a progressive decrease. The textural properties of the cor-
responding NiMo/TA-n catalysts are summarized in Table S1 (in the
supporting information). And these NiMo/TA-n catalysts were mea-
sured in the oxide form. Compared with the TiO2-Al2O3 oxides, we can
clearly see that all of the textural properties (surface area, pore volume
and the pore diameter) exhibit a marked decline after the active metals
loading. If outside surface of TiO2-Al2O3 materials was the position of
most active metals, the change of textural properties (surface area, pore
volume and pore diameter) would be incongruent. Therefore, we could
3.4. FTIR spectra analysis
Fig. 4 displays the FTIR of TiO2-Al2O3 oxides with various Ti/Al
molar ratios. The intense band centered at 3450 cm−1 and the band
around 1627 cm−1 can be assigned to stretching and bending modes of
hydroxyl group of composite oxides and adsorbed water. The FTIR
spectra of pure alumina shows
a broad band in the region of
500−750 cm−1, which can be attributed to the stretching vibration of
AlO6 octahedral. And the shoulder at 900 cm-1 is assigned to four-co-
ordinated aluminum vibration, which are the active sites for metal-
alumina interaction. With the increase of Ti/Al molar ratio, the in-
tensity of this band decreases, indicating the number of four-co-
ordinated aluminum atoms on the surface of composite oxides are re-
duced after the incorporation of TiO2. And thus, the corresponding
Fig. 3. N2 adsorption-desorption isotherms (A) and Pore size distributions (B) of TiO2-Al2O3 oxides: (a) TA-0.0, (b) TA-0.2, (c) TA-0.4, (d) TA-0.6.
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Table 2
Acid Properties of TiO2-Al2O3 Oxides.
Samples Weak (mmol g−1
)
Medium and Strong (mmol g−1
)
Total
BAS
LAS
BAS
LAS
BAS LAS
TA-0.0
TA-0.2
TA-0.4
TA-0.6
0
0
0
0
0.052
0.064
0.082
0.083
0
0
0
0
0.022
0.020
0.022
0.033
0
0
0
0
0.074
0.084
0.104
0.116
It can be apparently observed that TiO2-Al2O3 oxides have highly or-
dered mesoporous structure with the hexagonal arrangement pores
along the [0 0 1] direction from Fig. 6 (b, d). The ordered alignment of
cylindrical pores along the [1 1 0] direction could be also observed
from Fig. 6 (a, c), which is in good agreement with the result provided
by N2 adsorption based on the presence of H1 hysteresis hoop. This
highly ordered mesoporous structure would facilitate the adsorption
and diffusion of large molecule, such as 4,6-DMDBT. [40–43].
Fig. 4. FTIR spectra of TiO2-Al2O3 oxides with various Ti/Al molar ratios.
TEM images show that the uniform porous channel of TA-0.0, TA-
0.2, TA-0.4, and TA-0.6 are approximately 10.7 nm, 9.6 nm, 9.4 nm,
and 8.1 nm, respectively. Comparing the diameter data provided by N2
adsorption analysis, we could obtain that the thickness of TA-0.0, TA-
0.2, TA-0.4, and TA-0.6 are above 2.1 nm, 1.9 nm, 1.8 nm and 1.5 nm,
respectively. With the increase of Ti/Al molar ratio, the thickness of
TiO2-Al2O3 oxide gradually decreases. This is the reason why the
thermal stability of the above-mentioned composite oxides decreases as
the increasing Ti/Al molar ratio, which is in accordance with the result
from XRD and N2 adsorption analysis. Moreover, the elemental map-
ping of Al and Ti of TA-0.4 sample is presented in Fig. 7. The elemental
mapping displays that the Al and Ti species were homogeneously dis-
tributed across the whole TiO2-Al2O3 composite oxides.
metal-support interactions are weakened. The spectrum of pure titania
exhibits an intensive band centered around 500 cm-1 in the low spectra
region (400−1000 cm−1), which can be attributed to the -Ti-O-Ti-
bridge vibration. Due to the rearrangement of Al3+ in the particles,
with the decreasing of Ti/Al molar ratio, this band shifted and the in-
tensity decreased, implying the formation of -Ti-O-Al- bridges in TiO2-
Al2O3 oxides.
3.5. Py- FTIR analysis
The surface acid properties of TiO2-Al2O3 oxides are investigated by
pyridine-adsorbed FTIR at different temperature. Fig. 5 displays the
result of Py-FTIR spectra and the details summarize in Table 2. Ac-
cording to literature, the vibrational peak at around 1540 cm−3 could
be attributed to pyridinium ions adsorbed on Brønsted acid sites (BAS)
and the vibrational peak at around 1450 cm−3 is representative of
Lewis acid sites (LAS). The results show that Brønsted acid sites are not
observed over TiO2-Al2O3 oxides synthesized by EISA method, which is
in accordance with samples prepared by impregnation, co-precipitation
and sol-gel methods [14–16]. The amount of weak LAS increases with
increasing the Ti/Al molar ratio in TiO2-Al2O3 samples. It is interesting
to note that the amount of medium and strong LAS remains stable at
low amount of Ti, and then increases sharply.
3.7. H2-TPR measurement
The H2-TPR measurement was performed in order to characterize
the reducibility of metal actives supported on TiO2-Al2O3 oxides, and
reveal the variation of metal-support interactions. The H2-TPR profiles
of NiMo/TA-n catalysts are presented in Fig. 8. The reduction profile of
NiMo/TA-0.0 exhibits two main H2 consumption centered at around
525 °C and 800 °C, respectively. The reduction of molybdenum species
is a complex process. According to literature [31,44–47]: the reduction
peak centered at 525 °C could be attributed to the first step reduction of
polymeric octahedral Mo species (from Mo6+ to Mo4+), the high-
temperature reduction peak centered at 800 °C could be assigned to the
second reduction of the polymeric octahedral Mo species (from Mo4+ to
Mo°) and bulk crystalline MoO3 [47]. Although the reduction step of
nickel species (Ni2+ to Ni°) is taking place in the range between
400 °C–500 °C, the reduction peak of nickel species probably overlaps
3.6. TEM measurement
Transmission Electron Microscopy (TEM) measurement can de-
termine the detailed structures of supports distinctly, and the re-
presentative TEM images of TiO2-Al2O3 oxides were presented in Fig. 6.
Fig. 5. Py-FTIR spectrum of TiO2-Al2O3 oxides: (A) after pyridine desorbed at 473 K, (B) after pyridine desorbed at 623 K.
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Fig. 6. TEM images of the TiO2-Al2O3 oxides: (a, b) TA-0.2 and (c, d) TA-0.4.
3.8. HRTEM
with the MoO3 reduction peak [45]. That could be explained by that,
compared with nickel species, the amount of molybdenum species is
predominant. Compared with the reference NiMo/TA-0.0 catalyst, the
reduction peak centered at 525 °C shifted to lower temperature re-
markably as the Ti/Al molar ratio increased. Meanwhile, the low-
temperature reduction peak area of NiMo/TA-0.2 and NiMo/TA-0.4
slightly increases, implying that the amount of octahedral Mo species
increased in those two catalysts. However, as for the NiMo/TA-0.6, the
high-temperature reduction peak became more intense and the corre-
sponding peak area increased, indicating that more isolated tetrahedral
Mo active species [48] generated due to the excessive weakened metal-
support interactions. The H2-TPR results reveal that the incorporation
of TiO2 could effectively regulate the metal-support interactions and the
detailed TPR situations strongly depend on the compositions of TiO2-
Al2O3 oxides. Furthermore, the moderate metal-support interaction will
promote the formation of octahedral Mo species, thus producing more
“Type II” Ni-Mo-S active phase [49,50].
HRTEM measurement had been widely used in the study of the
morphology and dispersion of active phase. According to literature
[33,51–54], the morphological characteristic of MoS2 active phase
strongly depends on the nature of supports. The representative HRTEM
micrographs of sulfided NiMo/TA-n are displayed in Fig. 9. It can be
observed that the characteristic (0 0 2) basal plane with 0.61 nm inter-
planar distance [33,55,56] is presented in the sulfided catalysts. Be-
sides, the distribution of stacking number and lengths of MoS2 crys-
tallites is presented in Fig. 3 and Fig. 4 (in the Supporting Information),
respectively. As seen in Fig. 3 and Fig. 4, the incorporation of TiO2 can
significantly change the crystallite size distributions of NiMo/TA-n
catalysts. Fig. 3 shows that with the increase of Ti/Al molar ratio, the
portion of stacking number in the range of 1∼2 gradually decreases,
while stacking number in 3∼4 gradually increases. The slab lengths of
MoS2 nanoparticles over NiMo/TA-0.4 and NiMo/TA-0.6 catalysts are
Fig. 7. The elemental mapping of Al and Ti for the TA-0.4 sample.
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moderate stacking degree of MoS2 active phase, which would improve
utilization of active sites and diminish the planar adsorption resistance
of reactants, principally for 4,6-DMDBT, thus improving the hydro-
genation ability [54,57].
3.9. XPS characterization
The X-ray photoelectron spectroscopy (XPS) analysis could not only
provide the specific information of surface elements concentration, but
also quantitatively determine the sulfurization state of active metals.
The sulfided NiMo/TA-n catalysts are characterized by XPS and the
corresponding information is summarized in Table 4.
Fig. 10 displays the Mo 3d XPS spectra and the decomposition de-
tails are collected in Table 4. According to the literature [7,33,51,58],
the decomposition of Mo 3d could easily be completed based on the
binding energies of the different covalent states: (1) binding energies at
228.9
0.1 eV and 231.7
0.1 eV are ascribed to the Mo 3d5/2 and
Fig. 8. H2-TPR of the NiMo/TA-n catalysts: (a) NiMo/TA-0.0, (b) NiMo/TA-0.2,
Mo 3d3/2 of Mo4+ species, respectively; (2) binding energies at
(c) NiMo/TA-0.4 and (d) NiMo/TA-0.6.
230.5
0.1 eV and 233.6
0.1 eV are assigned to the Mo 3d5/2 and
Mo 3d3/2 of Mo5+ species, respectively; (3) binding energies at
almost 50 % at about 4∼6 nm, while the corresponding values for
NiMo/TA-0.0 and NiMo/TA-0.2 are only around 34 % and 41 %, re-
spectively. Additionally, based on the statistical analysis by counting
more than 400 active MoS2 slabs over 20 different micrographs, the
232.7
0.1 eV and 236.0
0.1 eV are attributed to the Mo 3d5/2 and
Mo 3d3/2 of Mo6+ species, respectively. In general, the total Mo 3d
spectra needs to subtract the S 2 s centered at 226.0 eV. The reduction
degree of Mo species (Mosulfidation) was calculated using the equation:
Mo4+/(Mo4++ Mo5+ +Mo6+) [7,33,47,55]. From Table 4, it can be
clearly noted that the NiMo/TA-n catalysts showed a remarkable higher
value of Mosulfidation than the reference NiMo/TA-0.0 catalysts, which is
consistent with the H2-TPR analysis. This is possible because partly
hydroxyl groups on the alumina surface were substituted by inert hy-
droxyl groups on the TiO2-Al2O3 surface [41], thus inhibiting formation
of the strong Al-O-Mo linkages. And thus, the octahedral Mo6+ species
could be easily formed and reduced, allowing a better sulfidation of Mo
and favoring to the formation of a well sulfide particles.
average length L, average stacking number
N
and the dispersion fMo of
MoS2 slabs are calculated according to the Eq. (1), Eq. (2) and Eq. (3),
and the corresponding results are presented in Table 3. The average
stacking number of NiMo/TA-n catalysts increase from approximately
1.7 to approximately 3.5 as the Ti/Al molar ratio increased. Simulta-
neously, the average slab lengths of the MoS2 crystallites decrease from
approximately 4.3 nm to 3.6 nm, while the fMo of NiMo/TA-n catalysts
decrease from 0.26 to 0.20. The dispersion of MoS2 crystals decreased
after incorporation of TiO2, which is in accordance with results re-
ported by J. A. de los Reyes [53]. This indicates that the incorporation
of TiO2 could weaken the metal-support interaction. Furthermore, the
weakened metal-support interactions would be beneficial to yield the
The Ni 2p decomposition spectra of sulfided NiMo/TA-n catalysts
are shown in Fig. 11. The decomposition is based on that [7,33,51,59]:
the bonding energies at 853.5
0.1 eV, 855.5
0.1 eV and
Fig. 9. HRTEM images of the sulfided NiMo catalysts: (a) NiMo/TA-0.0, (b) NiMo/TA-0.2, (c) NiMo/TA-0.4 and (d) NiMo/TA-0.6.
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Table 3
species shows a steady improvement from 17 % (the reference catalyst)
Average length, Layer number and fMo of MoS2 slabs of sulfided catalysts.
to 30 % (NiMo/TA-0.6).
Catalysts
NiMo/TA-
NiMo/TA-
NiMo/TA-
NiMo/TA-
0.0
0.2
0.4
0.6
3.10. Catalytic performances
Average length (nm)
4.3
4.0
3.8
3.6
Average layer number 1.7
2.4
2.8
3.5
In the present work, catalytic activity of NiMo/TA-n catalysts were
assessed by HDS of 4,6-DMDBT. According to literature
[7,38,51,54,61], HDS of 4, 6-DMDBT takes place through two parallel
routes over conventional HDS catalysts: the direct desulfurization route
(DDS) and the hydrogenation route (HYD). In the DDS route, the sulfur
atom of 4,6-DMDBT could be removed directly by C–S bond cleavage to
produce 3,3′-dimethylbiphenyl (3,3′-DMBP). In the HYD route, 4,6-
DMDBT is firstly hydrogenated to produce the intermediate products,
including 4,6-dimethyltetrahydrodibenzothiophene (4,6-DMTHDBT),
4,6-dimethylhexahydrodibenzothiophene (4,6-DMHHDBT) and 4,6-di-
methylperhydrodibenzothiophene (4,6-DM-PHDBT). And these inter-
mediate products could further yield 3,3′-DMCHB and DMBCH via C–S
bond cleavage. The corresponding reaction network is presented in
Scheme 1. Compared to dibenzothiophenes (DBT), the HYD route is the
predominant reaction pathway in 4,6-DMDBT HDS reaction due to the
presence of two methyl groups located at the 4 and 6 positions [54].
The reaction results at 290 °C with different WHSVs are presented in
Fig. 12. It can be clearly observed that, compared to the reference
catalyst, the conversion of 4,6-DMDBT gradually increases as the Ti/Al
molar ratio increases. The 4,6-DMDBT conversion reaches the max-
imum value at NiMo/TA-0.4 catalyst, which is in good agreement with
the reaction results at other reaction temperatures from Figs. 5 (in the
Supporting Information). These results indicate the compositions of
TiO2-Al2O3 oxides play an important role in the catalytic activity of the
corresponding NiMo/TA-n catalysts.
fMo
0.26
0.24
0.23
0.20
Table 4
Mo 3d and Ni 2p XPS analysis data of sulfided NiMo/TA-n catalysts.
Catalyst
Mo4+
%
,
Mo5+
%
,
Mo6+
%
,
NiSx, NiMoS, Ni2+
,
Niasulfidation
%
,
%
%
%
NiMo/TA-0.0 62
NiMo/TA-0.2 74
NiMo/TA-0.4 82
NiMo/TA-0.6 79
19
16
14
6
19
10
5
17
21
25
30
53
64
69
64
30
15
6
70
85
94
94
15
6
857.3
0.1 eV are ascribed to NiSx, NiMoS and NiOx species, respec-
tively. The sulfidation degree of Ni species (Nisulfidation) is calculated by
the equation: (NiSx + NiMoS)/ (NiSx + NiMoS + NiOx) [59], and the
result is also collected in Table 4. The result shows that the in-
corporation of TiO2 leads to an improvement of Nisulfidation. Specifically,
the proportion of NiMoS species increased from 53 % over the reference
catalyst to 69 % over NiMo/TA-0.4. However, as for NiMo/TA-0.6
catalyst, the proportion of NiMoS phase (64 %) is smaller than NiMo/
TA-0.4 (69 %). This phenomenon could be explained by the fact that
the formation of big MoO3 cluster makes it more difficult for Ni to lo-
cate at the edge of MoS2 crystalline due to the excessive weakening
metal-support interactions [60]. In addition, the proportion of NiSx
In order to investigate the 4,6-DMDBT HDS selectivity over NiMo/
TA-n catalysts, the product distributions determined at around
(50
0.3)% of 4,6-DMDBT conversion is summarized in Table 5. Only
Fig. 10. Mo 3d XPS decomposition spectra of the sulfided catalysts: (a) NiMo/TA-0.0, (b) NiMo/TA-0.2, (c) NiMo/TA-0.4 and (d) NiMo/TA-0.6.
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Fig. 11. Ni 2p XPS decomposition spectra of the sulfided catalysts: (a) NiMo/TA-0.0,(b) NiMo/TA-0.2, (c) NiMo/TA-0.4 and (d) NiMo/TA-0.6.
Fig. 12. 4,6-DMDBT conversion obtained with the NiMo/TA-n catalysts at
different reaction times.
0.6 (7.0) < NiMo/TA-0.4 (7.6). To further compare the catalytic abil-
ities of NiMo/TA-n catalysts, Table 5 exhibits the reaction rate con-
stants (kHDS) and turnover frequencies (TOFs). According to the calcu-
lated values, the catalytic performance of the NiMo/TA-n catalysts
increases in the following order: NiMo/TA-0.0 (4.9 × 10−4 mol g-1 h-1,
1.9 h-1) < NiMo/TA-0.6 (5.4 × 10−4 mol g-1 h-1, 2.8 h-1) < NiMo/TA-
0.2 (5.8 × 10−4 mol g-1 h-1, 2.5 h-1) < NiMo/TA-0.4 (6.5 × 10−4 mol
g-1 h-1, 3.0 h-1), which is in good agreement with the trend of HYD/DDS.
The NiMo/TA-0.4 exhibits a superior HDS ability among all the sam-
ples.
Scheme 1. Reaction Network for Hydrofesulfurization of 4,6-DMDBT.
five products were detected via chromatographic analyses for all cata-
lysts, which are in accordance with the pioneer literature [38]. The
products of HYD pathway include 3,3′-DM-TH-DBT, 3,3′-DM−HH-DBT,
3,3′-DMCHB and 3,3′-DMBCH [38,47,58]. 3,3′-DMBP is assigned to the
product of the DDS route [38,51,62]. The selectivity (HYD/DDS) of
NiMo/TA-n catalysts is also calculated, and HYD/DDS increases as the
following order: NiMo/TA-0.0 (4.5) < NiMo/TA-0.2 (5.7) < NiMo/TA-
The catalytic performance could be attributed to that, although the
dispersion of the MoS2 active phases slightly decreases as the Ti/Al
molar ratio increased, the remarkable improvement of reduction degree
for active metals determined by H2-TPR and XPS characterizations
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Table 5
Product Distribution in the 4,6-DMDBT HDS over NiMo/TA-n catalysts.
Catalyst
Product Distributions (%)a
kHDS
TOFb (h−1
)
HYD
DDS
(10−4 mol g-1 h-1
)
DMDBT
DMBP
THDMDBT
DMCHB
DMBCH
NiMo/TA-0.0
NiMo/TA-0.2
NiMo/TA-0.4
NiMo/TA-0.6
50.2
49.9
50.0
50.1
9.0
7.4
5.8
6.2
0.7
0.6
0.3
0.5
33.3
33.1
34.5
33.2
6.8
4.5
5.7
7.6
7.0
4.9
5.8
6.5
5.4
1.9
2.5
3.0
2.8
9.0
9.4
10.0
a
b
Products compositions were detected at around 50 % 4,6-DMDBT conversion.
Calculated at 15 % of 4,6-DMDBT conversion.
yielded more Ni-Mo-S active phase, especially “Type II” Ni-Mo-S active
phase, which can effective enhance the hydrogenation ability of NiMo/
TA-n catalysts [41,47]. Therefore, the increasing stacking number of
MoS2 crystalline can effective diminish the steric hindrance and en-
hance the ability of the planar adsorption of 4,6-DMDBT and other
hydrogenated intermediates [50,59]. On the other hand, TiO2-Al2O3
oxides prepared by EISA method possess high surface area and pore
volume, which can provide adequate active sites for the 4,6-DMDBT
reaction. The regular mesoporous structure could also facilitate the
diffusion of reactants and hydrogenated intermediates [38,63]. This
may be the reason why the catalytic performance is in line with the
order of specific surface area of TiO2-Al2O3 oxides. Therefore, com-
bined with the moderate metal-support interactions and an optimal
textural properties, NiMo/TA-0.4 exhibits the highest catalytic ability
in HDS of 4,6-DMDBT.
CRediT authorship contribution statement
Pengfei Zhang: Writing - review & editing. Fujun Mu: Data cura-
tion. Yasong Zhou: Supervision. Yuchi Long: Investigation. Qiang
Wei:
. Xiaodong Liu: Software. Qingxiang You: Methodology.
Yacheng Shan: Methodology. Wuwen Zhou: Validation.
Acknowledgement
This work was supported by China National Petroleum Corporation
(CNPC) under grant No. 2016B-1902 and 2016E-0703.
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.cattod.2020.03.003.
4. Conclusions
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In the present work, the highly ordered TiO2-Al2O3 oxides with
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synthesis methods of TiO2-Al2O3 oxides, EISA method, which does not
requires the strict control of preparation conditions, provides an alter-
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