HDS performance of NiMo catalysts supported on nanostructured materials containing titania

Article abstract of DOI:10.1016/j.cattod.2015.07.028

In the present work, NiMo catalysts supported on titania-containing materials of the SBA family (Ti-SBA-15 and Ti-SBA-16) and on titania nanotubes of different pore diameters were prepared with the aim of selecting the most promising catalysts for deep hydrodesulfurization (HDS) of gas oil. Supports and catalysts were characterized by nitrogen physisorption, XRD, temperature programmed reduction, ammonia TPD, UV-vis DRS and scanning electron microscopy (SEM-EDX). Sulfided catalysts were characterized by HRTEM and tested in the simultaneous HDS of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT). For comparison purposes, a conventional NiMo/γ-Al₂O₃ catalyst was used. In HDS of DBT, all catalysts supported on titania-containing nanostructured materials showed similar activity as the NiMo/γ-Al₂O₃ reference, but in HDS of 4,6-DMDBT their activities were about twice higher than that of NiMo/γ-Al₂O₃. Regarding selectivity, for DBT HDS, the presence of titania in the supports resulted in an increase in the proportion of CHB product, whereas for 4,6-DMDBT different behavior was observed. Thus, NiMo catalysts supported on Ti-SBA-15 and Ti-SBA-16 showed similar selectivity as the NiMo/γ-Al₂O₃. On the contrary, the catalysts supported on titania nanotubes allowed reaching high conversions of 4,6-DMDBT with a much lower proportion of hydrogenated products, which is an interesting result not observed by us previously.

Full text of DOI:10.1016/j.cattod.2015.07.028

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Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
HDS performance of NiMo catalysts supported on nanostructured
materials containing titania
Julio Cesar Morales-Ortun˜oa, Rodrigo Arturo Ortega-Domíngueza,
Patricia Hernández-Hipólitoa, Xim Bokhimib, Tatiana E. Klimovaa,∗
a Laboratorio de Nanocatálisis, Departamento de Ingeniería Química, Facultad de Química, Universidad Nacional Autónoma de México (UNAM),
Cd. Universitaria, Coyoacán, 04510, México D.F., Mexico
b Instituto de Física, Universidad Nacional Autónoma de México (UNAM), Apartado Postal 20-364, 01000, México D.F., Mexico
a r t i c l e i n f o
a b s t r a c t
Article history:
In the present work, NiMo catalysts supported on titania-containing materials of the SBA family (Ti-
SBA-15 and Ti-SBA-16) and on titania nanotubes of different pore diameters were prepared with the
aim of selecting the most promising catalysts for deep hydrodesulfurization (HDS) of gas oil. Sup-
ports and catalysts were characterized by nitrogen physisorption, XRD, temperature programmed
reduction, ammonia TPD, UV–vis DRS and scanning electron microscopy (SEM-EDX). Sulfided catalysts
were characterized by HRTEM and tested in the simultaneous HDS of dibenzothiophene (DBT) and
4,6-dimethyldibenzothiophene (4,6-DMDBT). For comparison purposes, a conventional NiMo/␥-Al2O3
catalyst was used. In HDS of DBT, all catalysts supported on titania-containing nanostructured materials
showed similar activity as the NiMo/␥-Al2O3 reference, but in HDS of 4,6-DMDBT their activities were
about twice higher than that of NiMo/␥-Al2O3. Regarding selectivity, for DBT HDS, the presence of titania
in the supports resulted in an increase in the proportion of CHB product, whereas for 4,6-DMDBT differ-
ent behavior was observed. Thus, NiMo catalysts supported on Ti-SBA-15 and Ti-SBA-16 showed similar
selectivity as the NiMo/␥-Al2O3. On the contrary, the catalysts supported on titania nanotubes allowed
reaching high conversions of 4,6-DMDBT with a much lower proportion of hydrogenated products, which
is an interesting result not observed by us previously.
Received 13 April 2015
Received in revised form 25 July 2015
Accepted 27 July 2015
Available online xxx
Keywords:
Deep hydrodesulfurization
NiMo catalysts
Titania nanotubes
Ti-SBA-15
Ti-SBA-16
Support effect
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
alyst. Traditionally, bimetallic catalysts based on Mo/W, promoted
by Ni/Co and supported on ␥-Al2O3 are used in the HDS pro-
In the last decade, it has become necessary to increase signif-
icantly the production of clean fossil fuels and at the same time,
to improve their quality in order to comply with new environ-
mental legislations worldwide. Decreasing the sulfur and nitrogen
content in petroleum derived fuels should improve air quality by
diminishing CO, CO2, NOx and SOx emissions in urban zones. In
general, hydrotreatment (HDT) processes are employed in the oil-
refining industry to remove heteroatoms (S, O and N), metals and
aromatics. Therefore, the development of novel more active and
effective HDT catalysts is currently an important task. Among HDT
processes, hydrodesulfurization (HDS) is commonly used to elimi-
nate sulfur from different petroleum fractions [1]. The HDS process
is commonly performed at high temperature and pressure, in a
hydrogen atmosphere and in the presence of a heterogeneous cat-
cess. These catalysts exhibit high activity in removing sulfur from
non-refractory thiophene, benzothiophene and dibenzothiophene
compounds, but their activity is relatively low in removing sulfur
from refractory dibenzothiophenes with alkyl groups in positions
4 and 6 of the molecule (positions close to the sulfur atom) [2,3].
This leads to the production of diesel fuel, which after conventional
hydrotreatment still contains a noticeable amount of unreacted
refractory dibenzothiophene compounds (about 500–1000 ppm of
S). Nowadays, the challenge is the development of a new gen-
eration of catalysts highly active for the removal of low reactive
dibenzothiophenes, which is the key for obtaining ultra-low sulfur
diesel (ULSD) containing less than 15 ppm of S [4]. Recently, a new
generation of commercial bulk hydroprocessing catalysts has been
developed for the production of ULSD, such as NEBULA (New Bulk
Activity) and STARS (Super Type II Active Reaction Sites) catalysts
[5–7]. These catalysts have high activity in HDS [8], but they are
expensive because of their high metal content and high consump-
tion of hydrogen [9–12]. From this point of view, supported HDS
∗
Corresponding author.
E-mail addresses: klimova@unam.mx, tklimova@gmail.com (T.E. Klimova).
http://dx.doi.org/10.1016/j.cattod.2015.07.028
0920-5861/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: J.C. Morales-Ortun˜o, et al., HDS performance of NiMo catalysts supported on nanostructured materials
containing titania, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.07.028

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catalysts with low metal content and better textural characteris-
tics can be an attractive option for the substitution of expensive
bulk HDS catalysts in industry.
the structure-directing agent and tetraethyl orthosilicate (TEOS,
Aldrich, 99.999%) as the silica source. Pluronic P123 (4 g) was dis-
solved in water (30 g) and 2 M HCl (120 g) solution at 35 ◦C. Then
TEOS (8.5 g) was added into the solution. The mixture was stirred
at 35 ◦C for 20 h and then aged at 80 ◦C for 48 h without stirring. The
solid product was recovered by filtration, washed with deionized
water and air-dried at room temperature. Calcination was carried
It is well-known that the catalytic performance of HDS cata-
lysts can be modified by changing the nature of the active phase or
of the support. Among many different materials tested as supports
for HDS catalysts, titania has attracted special attention due to high
intrinsic HDS activity demonstrated by Mo catalysts supported on
this oxide [13,14]. This behavior of TiO2-supported Mo catalysts
was attributed to the good dispersion of Mo oxide species and their
easy reduction and sulfidation. In order to take advantage of the
attractive properties of titania and increase its specific surface area,
different combinations of TiO2 with other oxide supports or TiO2-
containing mixed oxides were assayed as supports giving promis-
ing results. Among many different titania-containing supports, new
nanostructured materials have attracted special attention in the
last decade. This is due to their unique physical and chemical prop-
erties, novel morphology and attractive texture. Thus, in our group,
the well-ordered mesoporous molecular sieves SBA-15 and SBA-16
modified with titania were synthesized and tested as supports for
NiMo catalysts, giving good results, especially for the elimination of
low reactive aromatic sulfur compounds, such as alkyl-substituted
dibenzothiophenes [15,16]. In addition, novel titania-based nano-
scale materials (nanocrystals, nanoparticles, nanotubes, nanowires
and nanofibers) have appeared recently. Some of them have already
been tested as supports for HDS catalysts. For example, CoMo
catalysts supported on high surface area (more than 300 m2/g)
nano-structured titania powder showed activity twice higher in
dibenzothiophene HDS than a commercial CoMo/alumina catalyst
[17]. CoMo catalysts supported on titania nanotubes also showed
high activity in HDS of dibenzotiophene [18,19] and selective
hydrodesulfurization of thiophene [20]. High HDS activity observed
in the above works was attributed to high dispersion of catalyt-
ically active MoS2 particles and high sulfidability of Mo species
[17–19]. Regarding NiW catalysts, it was also found that the cat-
alysts supported on TiO2 nanotubes resulted in two times higher
thiophene conversion in comparison with the alumina and titania
supported ones [21]. The above results show that nanotubular tita-
nia is a promising support for hydrotreating catalysts. However, up
to now, the catalytic behavior of the HDS catalysts supported on
titania nanotubes has not been studied in the elimination of sulfur
from refractory alkyl-substituted dibenzothiophenes.
◦
¯
out in static air at 550 C for 6 h. SBA-16 silica with cubic Im3m struc-
ture was synthesized according to the literature [24,25] using the
triblock copolymer Pluronic F127 (EO106PO70EO106, BASF) as the
structure-directing agent and tetraethyl orthosilicate as the silica
source. The same amounts of reactants were used as for the syn-
thesis of the SBA-15 material. The mixture was stirred at 35 ◦C for
20 h and then aged at 100 ◦C for 72 h without stirring. The solid
product was recovered by filtration, washed, dried and calcined as
described for SBA-15. Synthesized SBA-15 and SBA-16 materials
were modified with titania by chemical grafting using titanium(IV)
isopropoxide (Ti(i-PrO)4, Aldrich, 97%) as the titania source and dry
ethanol as the solvent. In the grafting procedure, calcined SBA-15
or SBA-16 material was slurried in ethanol solution containing Ti(i-
PrO)4 for 6 h. To eliminate the excess Ti(i-PrO)4, the filtered material
was washed three times with dry ethanol. After drying at room
temperature Ti-SBA-15 and Ti-SBA-16 materials were calcined in
static air at 550 ◦C for 5 h. Elemental analysis of titania-containing
SBA-15 and SBA-16 materials (SEM-EDS) showed that they con-
tained 16 and 14 wt.% of TiO2, respectively. Titania nanotubes (NT)
were synthesized by alkaline hydrothermal treatment (10 M NaOH,
140 ◦C, 20 h) of commercial TiO2 followed by ion exchange with
0.1 M HCl solution and drying (120 ◦C, 12 h) [26,27]. Two NT sup-
ports were synthesized using two different anatase precursors:
low surface area TiO2 (7.6 m2/g, Aldrich) and anatase nanopowder
(89 m2/g, Aldrich). Nanotubular materials with internal pore diam-
eter of 3 nm and 5 nm, respectively, were obtained. These supports
were labeled as NT(3) and NT(5).
NiMo catalysts supported on Ti-SBA-15, Ti-SBA-16, NT(3), NT(5)
and ␥-Al2O3 were prepared by incipient wetness co-impregnation
of aqueous solutions of nickel nitrate, Ni(NO3)2·6H2O (Aldrich), and
ammonium heptamolybdate, (NH4)6Mo7O24·4H2O (Merck). After
co-impregnation, all catalysts were dried first at room temperature,
then at 100 ◦C for 24 h, and calcined at 500 ◦C for 4 h (Ti-SBA and
alumina-supported samples) and at 350 ◦C for 2 h (NT-supported
catalysts) in air atmosphere. The nominal composition of the cat-
alysts was 12 wt.% of MoO3 (8 × 10−4 mol/g) and 3 wt.% of NiO
(4 × 10−4 mol/g), which corresponds to the Ni:Mo molar ratio = 1:2.
Prepared catalysts will be denoted as NiMo/corresponding
support.
In the present work, our interest was to inquire into the catalytic
behavior of the NiMo catalysts supported on titania nanotubes
in deep hydrodesulfurization (HDS) and to compare their activity
and selectivity with those of Ti-SBA-15 and Ti-SBA-16 supported
analogs. For this purpose, NiMo catalysts supported on titania-
containing materials of the SBA family (Ti-SBA-15 and Ti-SBA-16)
and on titania nanotubes of different pore diameters were prepared
and characterized. Their catalytic activity was tested in simulta-
neous HDS of two model compounds: dibenzothiophene (DBT) and
4,6-dimethyldibenzothiophene (4,6-DMDBT). These compounds
were selected as representative molecules of the gas oil fraction,
which have different reactivity toward the direct desulfurization
and hydrogenation routes of HDS.
2.2. Support and catalyst characterization
Supports and prepared NiMo catalysts were characterized by N2
physisorption, X-ray diffraction (XRD), UV–vis diffuse reflectance
spectroscopy (DRS), NH3 temperature-programmed desorption
(TPD), temperature-programmed reduction (TPR), scanning elec-
tron microscopy (SEM-EDX) and high-resolution transmission elec-
tron microscopy (HRTEM). N2 adsorption/desorption isotherms
were measured with a Micromeritics ASAP 2020 automatic ana-
lyzer at liquid N2 temperature. Prior to the experiments, the
samples were degassed (p < 10−1 Pa) at 270 ◦C for 6 h. Specific sur-
face areas were calculated by the BET method (SBET), the total
pore volume (Vp) was determined by nitrogen adsorption at a rel-
ative pressure of 0.98 and pore size distributions were obtained
from the adsorption isotherms by the BJH method. The mesopore
diameter (Dp) corresponds to the maximum of the pore size distri-
2. Experimental
2.1. Support and catalyst preparation
Gamma alumina support (␥-Al2O3) was synthesized by calcina-
tion of Boehmite Catapal B at 700 ◦C for 4 h. Mesoporous SBA-15
silica support with hexagonal p6mm structure was prepared
according to a well-known procedure [22,23] using the triblock
copolymer Pluronic P123 (Mav = 5800, EO20PO70EO20, Aldrich) as
bution. The micropore area (S ) was estimated using the correlation
␮
of t-Harkins & Jura (t-plot method). XRD patterns were recorded
in the 3◦ ≤ 2ꢀ ≤ 90◦ range on a Siemens D5000 diffractometer,
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3
˚
used for kDBT and kDMDBT calculations are shown in Fig. 11. Obtained
using CuK␣ radiation (ꢁ = 1.5406 A) and a goniometer speed of
1◦(2ꢀ) min−1. Small-angle XRD (2ꢀ = 0.5–10◦) was performed on a
Bruker D8 Advance diffractometer using small divergence and scat-
tering slits of 0.05◦. UV–vis electronic spectra of the samples were
recorded in the wavelength range 200–800 nm using a Varian Cary
100 Conc spectrophotometer equipped with a diffuse reflectance
attachment. Polytetrafluoroethylene was used as reference. NH3
TPD and TPR experiments were carried out in a Micromeritics
AutoChem II 2920 automatic analyzer equipped with a TC detec-
tor. Before ammonia adsorption, the samples were pretreated
in situ at 500 ◦C for 30 min in a helium flow in order to remove
water and other contaminants. The samples were then cooled to
120 ◦C and contacted with a He/NH3 mixture (90/10 mol/mol and
20 mL/min) for 30 min. The desorption step was performed in a
He stream (50 mL/min) with a heating rate of 10 ◦C/min. After
reaching 500 ◦C, the sample was kept at this temperature until
the trace reached the base line. Before TPR experiments, the sam-
ples were pretreated in situ at 500 ◦C for 2 h under air flow and
cooled in an Ar stream. The reduction step was performed under
a stream of an Ar/H2 mixture (90/10 mol/mol and 50 mL/min),
with a heating rate of 10 ◦C/min up to 1000 ◦C. Chemical compo-
sition of the synthesized materials was determined by SEM-EDX
using a JEOL 5900 LV microscope with OXFORD ISIS equipment.
HRTEM studies were performed using a JEOL 2010 microscope
k (h−1) values were also expressed in L s
g
cat
−1 (Tables 6 and 7) in
−1
order to obtain values comparable with those published previously.
3. Results and discussion
3.1. Supports
Fig. 1 shows TEM micrographs of the supports synthesized in
the present work. It was observed that the Ti-SBA-15 support,
Fig. 1(a), was formed by grains of relatively uniform size (typically
around 300 nm diameter and 700–1000 nm long). A well-ordered
hexagonal array of mesopores can be seen for this sample. The Ti-
SBA-16 material, Fig. 1(b), was formed by aggregates of spheroidal
shape particles with some spaces between them. For the NT(3) and
NT(5) materials, nanotubular structures can be clearly observed in
Fig. 1(c) and (d). These open-ended structures with multilayered
walls had the external diameter around 8–10 nm and internal pore
diameter of around 3 and 5 nm, respectively.
Textural characteristics of the synthesized supports were
determined by nitrogen physisorption. Fig. 2(a) shows nitrogen
adsorption-desorption isotherms of the Ti-SBA-15 and Ti-SBA-16
supports. According to IUPAC classification, the Ti-SBA-15 sup-
port shows a type IV isotherm with a H1 hysteresis loop [22,24].
This hysteresis is characteristic of solids consisting of particles
crossed by nearly cylindrical channels [28]. The Ti-SBA-15 mate-
rial had a narrow monomodal pore size distribution centered on
˚
(resolving power 1.9 A). The solids were ultrasonically dispersed in
heptane and the suspension was collected on carbon-coated grids.
Slab length and layer stacking distributions of MoS2 crystallites in
each sample were established from the measurement of at least
300 crystallites detected on several HRTEM pictures taken from
different parts of the same sample dispersed on the microscope
grid.
˚
78 A, Fig. 3(a). The Ti-SBA-16 support also has a type IV isotherm,
but with a H2 hysteresis loop, which is typical of ink-bottle pores
of SBA-16 materials [15]. For the Ti-SBA-16 material, a bimodal
pore size distribution was observed, Fig. 3(a). The first peak cen-
2.3. Catalytic activity tests
˚
tered at 53 A can be ascribed to the internal ink-bottle pores of
˚
the material, whereas the second one (at about 220 A) can be
The HDS activity tests were performed in a batch reactor at
300 ◦C and 7.3 MPa total pressure for 8 h using n-hexadecane solu-
tion of DBT (1300 ppm of S) and 4,6-DMDBT (500 ppm of S). Prior to
the catalytic activity evaluation, the catalysts were sulfided ex situ
in a tubular reactor at 400 ◦C for 4 h in a stream of 15 vol.% of H2S
in H2 under atmospheric pressure. The sulfided catalysts (0.15 g)
were transferred in an inert atmosphere (Ar) to a batch reactor
(Parr) with 40 mL of n-hexadecane solution containing the model
compounds (DBT and 4,6-DMDBT). The course of the reaction was
followed by withdrawing aliquots each hour and analyzing them on
an Agilent 6890 gas chromatograph. To corroborate product iden-
tification, the product mixture was analyzed on an Agilent 7890A
GC system with 5975C MS detector.
attributed to the spaces between agglomerates of spheroidal par-
ticles forming SBA-16-type materials. Regarding two nanotubular
titania supports NT(3) and NT(5), both of them show the typical
type IV nitrogen adsorption isotherms, Fig. 2(b), with a noticeable
hysteresis loop, which confirms the presence of meso- and macro-
pores. The hysteresis profiles of NT materials can be interpreted
as a combination of H1 and H3 types. Type H1 is characteristic of
mesoporous materials having uniform nearly cylindrical channels;
whereas H3 hysteresis is frequently associated with solids consist-
ing of aggregates or agglomerates of particles forming slit shaped
pores of non-uniform size [28]. Both nanotubular materials have
bimodal pore size distributions with two principal signals, Fig. 3(b).
For the NT(3) sample the first maximum was observed at around
Kinetic studies were performed in the absence of the external
and internal mass transfer/diffusion limitations. Catalyst activity
was determined by measuring DBT and 4,6-DMDBT concentrations
at different reaction times. DBT and 4,6-DMDBT conversions (X)
were calculated as:
˚
˚
30 A, whereas the second broad peak had a maximum at 440 A.
The NT(5) material showed two broad peaks; the first one between
˚
˚
˚
30 and 80 A (the mean value around 50 A) and the second at 700 A.
Considering that nanotubular titania supports are formed by aggre-
gates or agglomerates of nanotubes, Fig. 1(c) and (d), the smaller
pores can be ascribed to the pores inside the nanotubes, whereas
larger pores can correspond to the voids in the aggregation of the
nanotubes.
Co − C
X =
(1)
Co
where Co is the initial DBT (or 4,6-DMDBT) concentration (mol L−1
in the reaction mixture and C is its concentration (mol L−1) at dif-
ferent reaction times (t, h). The rate constants (kDBT and kDMDBT
)
Textural characteristics of the synthesized supports (SBET, S , Vp
␮
and Dp) are shown in Table 1. In general, the TiO2-containing sup-
ports of the SBA family had better textural characteristics (higher
surface area and total pore volume) than the conventional ␥-
alumina. Contrary to all other supports, Ti-SBA-15 and Ti-SBA-16
showed the presence of a noticeable amount of micropores (98 and
62 m2/g micropore area, respectively) in addition to larger meso-
pores. Nanotubular titania supports showed the BET surface areas
slightly higher (238–258 m2/g) than that of the conventional ␥-
Al2O3 support (216 m2/g).
,
Tables 6 and 7) were determined considering that the DBT and
4,6-DMDBT HDS reactions are of the pseudo-first-order using the
following equation:
1
t
k = − × ln (1 − X)
(2)
where k is the pseudo-first-order reaction constant (h−1), X the DBT
(or 4,6-DMDBT) conversion and t the reaction time (h). Linear plots
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Fig. 2. Nitrogen adsorption–desorption isotherms of supports: (a) Ti-SBA-15 and
Ti-SBA-16; (b) NT(3) and NT(5).
Fig. 4 shows the small-angle XRD patterns of the synthesized
Ti-SBA-15 and Ti-SBA-16 samples. For the Ti-SBA-15 support with
pore structure ordered in two dimensions (2D), three well-resolved
peaks were observed: a very intense peak at about 1◦(2ꢂ) and two
less intense signals at 1.7◦(2ꢂ) and 1.9◦(2ꢂ). These peaks can be
assigned to (1 0 0), (1 1 0) and (2 0 0) reflections associated with
p6mm hexagonal symmetry of the SBA-15-type materials [22,23].
˚
All three reflections yield a unit cell parameter (ao) value of 105 A,
confirming that the measured structure is indeed of the p6mm sym-
metry. Regarding the Ti-SBA-16 material, it has a three dimensional
(3D) cubic pore arrangement. The XRD pattern of this material
shows a strong reflection at about 0.9◦(2ꢂ) and two small shoul-
Table 1
Textural characteristics of supports.
d
SBET (m2/g)a
S
(m2/g)b
␮
Vp (cm3/g)c
Dp (A)
˚
Sample
Ti-SBA-15
Ti-SBA-16
NT(3)
NT(5)
␥-Al2O3
636
521
238
258
216
98
62
–
–
–
0.83
1.43
0.79
0.49
0.52
78
53; 220
30; 440
50; 700
109
a
Specific surface area calculated by BET method.
Micropore area estimated using the correlation of t-Harkins & Jura (t-plot
b
Fig. 1. TEM images of supports (a) Ti-SBA-15, (b) Ti-SBA-16, (c) NT(3) and (d) NT(5).
method).
c
Total pore volume.
d
Pore diameter corresponding to the maximum of the pore size distribution
calculated from the adsorption isotherm by the BJH method.
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5
3.5
(a)
3.0
Ti-SBA-15
Ti-SBA-15
Ti-SBA-16
2.5
Ti-SBA-16
*
2.0
1.5
1.0
0.5
*
*
*
*
*
γ-Al2O3
*
+
+
+
+
+
+
NT(3)
NT(5)
0.0
10
100
1000
10
20
30
40
50
60
70
80
Pore diameter (Å)
2θ (º)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Fig. 5. Powder XRD patterns of supports: Ti-SBA-15, Ti-SBA-16, ␥-Al2O3, NT(3) and
NT(5). *␥-Al2O3 (JCPDS card 29-63), +H2Ti3O7 (JCPDS card 41-192).
NT(3)
(b)
diffractograms of the Ti-containing SBA-15 and SBA-16 materials
do not show the presence of any crystalline phase. A broad signal
observed in these patterns between 15 and 35◦(2ꢂ) corresponds
to the amorphous silica of the SBA-15 and SBA-16 supports. On
the contrary, other three diffractograms show signals indicating
that these materials have a crystalline structure. As expected, the
alumina support had a ␥-Al2O3 structure (JCPDS card 29-63). Nan-
otubular titania materials NT(3) and NT(5) showed similar X-ray
diffraction patterns with three principal signals located at 11.2, 24.4
and 48.4◦(2ꢂ). These reflections are typical of monoclinic layered
trititanate phase (H2Ti3O7, JCPDS card 41-192), which forms the
curved nanotubular walls, and can be indexed to its (2 0 0), (1 1 0)
and (0 2 0) crystal planes, respectively. The signal at 11.2◦(2ꢂ) is
attributed to the interlayer distance of hydrogen trititanate which
NT(5)
10
100
1000
Pore diameter (Å)
˚
is equal to 8.0 A. The broadness of all observed diffraction signals of
Fig. 3. Pore size distributions of supports: (a) Ti-SBA-15 and Ti-SBA-16; (b) NT(3)
and NT(5).
NT(3) and NT(5) samples points out the poor crystalline nature of
the synthesized nanotubes, which is in line with previous reports
for similar materials [29,30].
ders at about 1.2◦(2ꢂ) and 1.5◦(2ꢂ). These signals correspond well
to the (1 1 0), (2 0 0) and (2 1 1) reflections of the cubic Im3m
structure of Ti-SBA-16 sample with a unit cell parameter (ao) of
144 A. Nanotubular titania materials NT(3) and NT(5) have a one
dimensional nanostructure (1D), and because of this they do not
present characteristic signals in the small-angle XRD region.
Powder XRD patterns of the synthesized supports and the
reference ␥-Al2O3 are shown in Fig. 5. It can be seen that the
Additional information about characteristics of titania species
in different supports synthesized in the present work was obtained
by UV–vis diffuse reflectance spectroscopy (Fig. 6). The DRS spec-
trum of the low-surface area titania used as precursor exhibits
broad absorption at ∼330 nm with the absorption edge at about
380 nm due to ligand-to-metal charge transfer (LMCT) between the
O2− ligand and the titanium(IV) ion. This position of the absorp-
tion edge indicates that the TiO2 precursor has a band-gap of ca.
¯
˚
120000
(100)
Ti-SBA-15
100000
(110)
20000
Ti-SBA-16
15000
80000
60000
40000
(200)
(211)
10000
5000
0
(110)
20000
(200)
0
1
2
3
4
1
2
3
4
2θ (º)
2θ (º)
Fig. 4. Small-angle XRD patterns of Ti-SBA-15 and Ti-SBA-16.
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Table 2
6
13
12
11
Quantitative analysis of ammonia TPD results obtained for supports.
Ti-SBA-16
Sample
Amount of acid sites (␮mol NH3/g)
NT(3)
10
9
8
7
6
5
4
3
2
1
0
Total
Weak
Medium
Strong
(120–200 ◦C)
(200–400 ◦C)
(400–500 ◦C)
SBA-15
Ti-SBA-15
Ti-SBA-16
NT(3)
NT(5)
␥-Al2O3
188
336
204
512
533
728
13
161
88
129
109
100
99
147
63
280
314
381
76
28
53
103
110
247
NT(5)
TiO2 precursor
synthesized supports revealed the differences in the type of titania
species in them; namely, in the NT(3) and NT(5) samples, nano-
sized titania species were found, whereas in the Ti-SBA-15 and
Ti-SBA-16 samples, only isolated Ti species were present.
Ti-SBA-15
200
300
400
500
Wavelength (nm)
Ammonia TPD characterization results are shown in Fig. 7 and
Table 2. Pure silica SBA-15 material used for the preparation of the
Ti-SBA-15 support has the lowest total amount of acid sites among
all samples. Titania grafting on this material resulted in an increase
in the total amount of acid sites, especially of the weak and medium
strength ones, and a slight decrease in the amount of strong acid
sites. Similar distribution of acid sites of different strength was
also observed for the Ti-SBA-16 support, presenting mostly weak
and medium acid sites. Alike acid characteristics of the above two
materials (Ti-SBA-15 and Ti-SBA-16) should be due to the presence
of similar titania species in them, namely isolated tetrahedral and
octahedral Ti oxide species detected by DRS. The principal differ-
ence between these materials is a larger total amount of acid sites
in the Ti-SBA-15 sample compared to the Ti-SBA-16 analog, which
can be ascribed to a larger amount of titania grafted in the first case
(16 wt.% of TiO2 for Ti-SBA-15 versus 14 wt.% for Ti-SBA-16). Nan-
otubular titania supports NT(3) and NT(5), showed a significantly
larger acidity than the SBA-based materials (Fig. 7, Table 2). Thus,
the total amount of acid sites in NT(3) and NT(5) was almost twice
higher than in the Ti-SBA samples. In addition, medium strength
sites were predominant in both cases in line with the presence of a
noticeable amount of weak and strong acid sites. These acid charac-
teristics can be due to the presence of nano-sized titania species in
the nanotubular supports. However, in general, acidity of the nan-
otubular titania supports was smaller than that of the reference
␥-alumina support, which showed the highest total amount of acid
sites as well as of medium and strong acid sites among all supports
tested.
Fig. 6. UV–vis DRS supports: Ti-SBA-15, Ti-SBA-16, NT(3), NT(5) and low surface
area TiO2 anatase precursor.
3.3 eV, which is well in line with previous literature reports for
the bulk TiO2 anatase phase [31]. Transformation of the precursor
TiO2 into nanotubular titania structures NT(3) and NT(5) resulted
in a blue shift of the absorption signals observed in the corre-
sponding UV–vis DRS spectra (Fig. 6). Thus, the absorption band
maximum was observed at about 300 nm for both nanotubular
materials, whereas the absorption edge was located at 365 and
360 nm for NT(3) and NT(5), respectively, indicating an increase
in the band-gap energy (Eg) of these materials to 3.4–3.5 eV. These
differences in the electronic properties of the TiO2 precursor and
nanotubular titania samples can be attributed to a transforma-
tion of the bulk TiO2 anatase into nano-sized titania structures
(titania nanotubes) [31,32]. In the DRS spectra of Ti-SBA-15 and
Ti-SBA-16 further blue shift can be observed; the absorption band
maxima are located at about 230 nm and the absorption edge at 320
and 300 nm, respectively. These Ti-containing samples of the SBA-
family prepared by chemical grafting of TiO2 species had still higher
Eg values (3.9 eV for Ti-SBA-15 and 4.1 eV for Ti-SBA-16). Previously,
the band at 230 nm was assigned to LMCT transitions involving iso-
lated Ti atoms in tetrahedral coordination [33]. In addition, in the
spectrum of the Ti-SBA-15 sample a shoulder can be observed at
about 250 nm, which indicates the presence of some isolated octa-
hedral Ti species [16,33]. Therefore, the DRS characterization of the
The above differences in the characteristics of the titania species
in different supports are expected to affect the supports’ interaction
with the deposited Ni and Mo species and their catalytic activity.
3.2. Catalysts
Ti-SBA-15
Ti-SBA-16
Impregnation of Ni and Mo species on the Ti-containing sup-
ports, Ti-SBA-15, Ti-SBA-16, NT(3) and NT(5) did not produce
significant changes in the characteristic shape of the isotherms,
which confirms that the original pore structure of the supports
was preserved in the NiMo catalysts. As expected, for all cata-
lysts the amount of adsorbed nitrogen as well as specific textural
characteristics (SBET and Vp, Table 3) decreased after metal deposi-
tion. However, this decrease was much stronger for the catalysts
supported on Ti-SBA-15 and Ti-SBA-16 materials than for other
supports. Thus, surface area and pore volume values of the NiMo/Ti-
SBA-15 and NiMo/Ti-SBA-16 catalysts were about ∼30–40% smaller
than those of the corresponding supports (Tables 1 and 3). This
decrease is much larger than the increase in the sample’s den-
sity (about 18%) after the deposition of metal species. This can be
due to the deposition of the impregnated metal species inside the
γ-Al2O3
NT(5)
NT(3)
SBA-15
150
200
250
300
350
400
450
500
Temperature (ºC)
Fig. 7. Ammonia TPD profiles of supports (Ti-SBA-15, Ti-SBA-16, ␥-Al2O3, NT(5) and
NT(3)) and parent SBA-15 material.
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Table 3
427
Textural characteristics of NiMo catalysts.
d
SBET (m2/g)a
S
(m2/g)b
␮
Vp (cm3/g)c
Dp (A)
˚
Sample
NiMo/Ti-SBA-15
NiMo/Ti-SBA-15
NiMo/Ti-SBA-16
NiMo/NT(3)
NiMo/NT(5)
NiMo/␥-Al2O3
435
289
209
207
178
60
200
–
–
–
0.61
0.94
0.72
0.40
0.40
76
53; 210
30; 430
50; 640
100
435
NiMo/Ti-SBA-16
NiMo/NT(5)
417 466
721
a
Specific surface area calculated by BET method.
Micropore area estimated using the correlation of t-Harkins & Jura (t-plot
b
494
419
method).
772
c
Total pore volume.
NiMo/NT(3)
d
Pore diameter corresponding to the maximum of the pore size distribution
774
378
463
calculated from the adsorption isotherm by the BJH method.
NiMo/γ-Al2O3
200
400
600
Temperature (ºC)
800
1000
mesopores of the SBA-family supports, which is in line with the
incipient wetness impregnation method used for the catalysts’
preparation. In addition, some agglomeration of the deposited Ni
and Mo oxide species can also take place inside the pores lead-
ing to a blocking of mesopore channels. As a comparison, for
the NiMo catalysts supported on NT(3) and NT(5), a decrease in
the textural characteristics was just about 12–19% in reference to
the corresponding supports. Therefore, for the last two catalysts,
no agglomeration of the deposited Ni and Mo oxide species was
expected.
Small-angle XRD patterns of the NiMo catalysts supported on
Ti-SBA-15 and Ti-SBA-16 materials were similar to those shown
in Fig. 4 for the corresponding supports. The position of the reflec-
tions characteristic of Ti-SBA-15 and Ti-SBA-16 and their intensities
almost did not change after the impregnation of Ni and Mo, indi-
cating that the long-range periodicity order of the 2D and 3D
SBA-family supports was preserved in the prepared catalysts and
their unit-cell parameters did not suffer a significant change after
metal deposition.
Fig. 8. TPR profiles of the NiMo catalysts supported on different materials (solid
lines). TPR profiles of the corresponding supports are shown as dashed lines below
the reduction profiles of the catalysts.
Results from the TPR characterization of the synthesized NiMo
catalysts supported on Ti-SBA-15, Ti-SBA-16, NT(3), NT(5) and ␥-
alumina are shown in Fig. 8. TPR profiles of the corresponding
supports are also shown for comparison (dashed lines). It can be
seen that the obtained TPR profile (number of reduction peaks,
position and intensity) strongly depends on the support used.
Thus, for the reference NiMo/␥-Al2O3 catalyst, three reduction
peaks can be observed at 378, 463 and 774 ◦C. According to lit-
erature [34,35], hydrogen consumption in the low temperature
region (300–500 ◦C) can be attributed to the first step of reduc-
tion (from Mo6+ to Mo4+) of octahedral Mo oxide species, Mo(Oh),
of different degrees of agglomeration. An increase in the degree of
agglomeration of octahedral Mo oxide species is accompanied by
an increase in reduction temperature. In addition, the signal with
the maximum at 463 ◦C can be ascribed to the reduction of the
NiMoO4 species (main peak at 475 ◦C [36]). The formation of these
species can be expected in our catalysts, since they were prepared
by the co-impregnation of Ni and Mo precursor species, which
favor the formation of an Anderson-type heteropolymolybdate
Powder X-ray diffraction patterns of the NiMo catalysts pre-
pared in the present work were recorded in the 2ꢂ range from 5◦ to
80◦. No signals attributable to the presence of any new crystalline
phase (MoO3, NiO, NiMoO4, etc.) were detected in the XRD pat-
tern of any catalyst. Only reflections corresponding to the supports
(Fig. 5) were observed. This result points out to a good dispersion
of the deposited metal oxide species in all prepared catalysts.
Fig. 9. HRTEM of sulfided catalysts: (a) NiMo/Ti-SBA-15, (b) NiMo/Ti-SBA-16, (c) NiMo/NT(3) and (d) NiMo/NT(5).
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Table 4
< 2
Length (nm):
70
Reduction behavior of different NiMo catalysts.
2.1-4
4.1-6
6.1-8
> 8
(A)
b
Sample
H2 consumption (mmol/g)a
˛
R
60
200–600 ◦
C
600–1000 ◦
C
Total
50
40
30
20
10
0
NiMo/Ti-SBA-15
NiMo/Ti-SBA-16
NiMo/NT(3)
NiMo/NT(5)
NiMo/␥-Al2O3
1.51
1.48
1.66
1.78
0.98
0.80
0.97
1.11
0.96
1.29
2.31
2.45
2.77
2.74
2.27
0.80
0.84
0.96
0.94
0.78
a
Hydrogen consumption determined from TPR results: H2 consumptions due to
the supports’ reduction were subtracted from the H2 consumptions of the corre-
sponding catalysts.
b
˛R, degree of the reduction of Ni and Mo oxide species determined from total
H2 consumption of each sample and the theoretical value corresponding to their
complete reduction (2.90 mmol/g).
(a)
(b)
(c)
(d)
(e)
80
70
60
50
40
30
20
10
0
(NH4)4[Ni(OH)6Mo6O18]·4H2O in the impregnation solution [37],
which precipitates on the support surface and upon calcination
yields the NiMoO4 phase. Hydrogen consumption at a higher
temperature (500–700 ◦C) can be ascribed to the second step of
reduction of Mo(Oh) species (from Mo4+ to Mo0) and to the first step
of reduction of small MoO3 clusters. Finally, the signal observed in
the 700–1000 ◦C region corresponds to the first step of reduction of
tetrahedral Mo oxide species, Mo(Td), being in strong interaction
with the support. Therefore, the TPR results show that the NiMo/␥-
Al2O3 catalyst has both types of Mo oxide species, octahedral and
tetrahedral, with a considerable contribution of the Mo(Td) species
reducible at a high temperature. The TPR profiles of the NiMo cata-
lysts supported on nanotubular titania materials, NT(3) and NT(5)
(Fig. 8), also exhibit three main reduction peaks: the first intense
signal with the maximum at 417–419 ◦C, the second similar inten-
sity peak at a higher temperature (466–494 ◦C) and the last signal
in the temperature interval between 650 and 900 ◦C. The TPR pro-
files of the NiMo/Ti-SBA-15 and NiMo/Ti-SBA-16 catalysts (Fig. 8)
presented the principal reduction signal at about 430 ◦C with a
shoulder at about 500 ◦C and a low-intensity hydrogen consump-
tion at a 600–1000 ◦C interval. The TPR profiles of the catalysts
supported on Ti-containing materials of the SBA-family showed
clearly that octahedral Mo oxide species are predominant in these
two samples. It seems that the type of Mo species in NiMo cata-
lysts is related to the characteristics of the titania species in the
supports. Isolated, low acidity Ti moieties (Ti-SBA-15 and Ti-SBA-
16 supports) result in dispersed octahedral Mo species, while an
increase in the supports’ acidity (NT(3) and NT(5) supports) leads
to an increase in the proportion of tetrahedral ones.
In addition, hydrogen consumption and degree of reduction (˛R)
were determined for different NiMo catalysts (Table 4). In order to
quantify the amount of metal species that can be reduced at low
and high temperatures, hydrogen consumption in two tempera-
ture intervals (200–600 ◦C and 600–1000 ◦C) was also determined
(Table 4). The NiMo catalysts supported on Ti-SBA-15, Ti-SBA-
16 and ␥-Al2O3 showed similar degrees of reduction (˛R ∼ 0.8).
However, the catalysts supported on SBA-type materials showed
higher hydrogen consumption at a lower-temperature interval
(200–600 ◦C) than the alumina-supported NiMo sample. This indi-
cates that Ni and Mo oxide species can be reduced more easily when
supported on SBA-type materials than on ␥-alumina. Regarding the
NiMo catalysts supported on nanotubular titania materials, NT(3)
and NT(5), they showed a higher degree of reduction (˛R 0.94–0.96)
than the other three catalysts, as well as higher hydrogen consump-
tion at a low-temperature interval.
Number of layers:
1
2
3
4
5
6
(B)
(a)
(b)
(c)
(d)
(e)
Fig. 10. Length (A) and stacking (B) distributions of MoS2 crystallites in sulfided
catalysts: (a) NiMo/␥-Al2O3, (b) NiMo/NT(3); (c) NiMo/NT(5), (d) NiMo/Ti-SBA-15
and (e) NiMo/Ti-SBA-16.
interplanar distances were observed on micrographs of all sulfided
catalysts. In general, MoS2 particles observed in the Ti-SBA-15 and
Ti-SBA-16-supported catalysts had similar morphology (images (a)
and (b), Fig. 9). They were formed by 1–4 layers with lengths
between 2 and 8 nm. Regarding the NiMo catalysts supported on
two nanotubular titania materials, they showed mainly the pres-
ence of the active phase particles of just 1 or 2 layers with less than
6 nm length (images (c) and (d), Fig. 9). In this case, we also did
not observe a noticeable difference in the characteristics of the sul-
fided MoS2 particles deposited on NT materials of different internal
pore diameter. The above results indicate that the characteristics of
the deposited MoS2 crystallites were influenced principally by the
chemical composition of the supports (TiO2-SiO2 for Ti-SBA-15 and
Ti-SBA-16, or H2Ti3O7 for NT(3) and NT(5)), whereas the shape and
size of the pores and other textural characteristics of the supports
did not have a strong influence on the stacking degree and length
of the MoS2 particles. Fig. 10 shows the stacking and length distri-
butions of MoS2 crystallites in the sulfided catalysts supported on
Ti-SBA-15, Ti-SBA-16, NT(3) and NT(5), where the differences in the
morphology of the MoS2 phase can be observed in a more detailed
way. The information regarding the reference NiMo/␥-Al2O3 cat-
alysts was also included for comparison purposes. The reference
catalyst showed broad length and stacking distributions. At the
same time, in the Ti-SBA-15 and Ti-SBA-16-supported samples,
MoS2 particles of 2 layers and 2–4 nm in length were predominant.
For the NiMo catalysts supported on nanotubular titania materi-
als, the majority of the MoS2 particles had just 1 layer and 2–4 nm
in length. It seems that a stronger metal-support interaction on
NT supports resulted in less stacked MoS2 particles than on Ti-SBA
materials. Therefore, the use of Ti-containing supports allowed us
HRTEM characterization of sulfided NiMo catalysts was per-
formed in order to obtain information about the dispersion of
catalytically active MoS2 species. Fig. 9 shows HRTEM micrographs
of the NiMo catalysts supported on different titania-containing
materials. The typical fringes of MoS2 crystallites with 0.61 nm
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Table 5
tor of the amount of sulfided Mo species located on the catalytically
active surface of MoS2 particles [40]. The results from Table 5 show
that the fMo values found for all NiMo catalysts supported on SBA-
type materials and titania nanotubes were similar (∼0.3) and twice
higher than that of the reference NiMo/␥-Al2O3 sample (fMo = 0.15).
Therefore, in spite of the difference in the average stacking degree
(N, Table 5) of the SBA- and NT-supported catalysts, all four cata-
lysts supported on Ti-containing materials possess almost the same
amount of catalytically active sites, available for interaction with
reactant molecules.
Results from the characterization of the sulfided NiMo catalysts by HRTEM: average
length (L) and average stacking degree (N) of MoS2 crystallites and estimated fMo
fraction.a
˚
Catalyst
L (A)
N
fMo
NiMo/Ti-SBA-15
NiMo/Ti-SBA-16
NiMo/NT(3)
NiMo/NT(5)
NiMo/␥-Al2O3
36
36
35
35
80
1.9
2.1
1.3
1.2
2.4
0.32
0.32
0.33
0.33
0.15
a
fMo, estimated fraction of Mo atoms on the edge surface of MoS2 crystallites
[37,38].
3.3. Catalytic behavior in hydrodesulfurization
to improve the dispersion of the active phase in the sulfided cata-
lysts. The above observations were confirmed by the calculation of
the average length (L) and number of layers (N) of MoS2 crystallites
in different catalysts (Table 5). These average characteristics were
used to estimate the fraction of Mo atoms (fMo) located on the edge
surface of MoS2 crystallites. The fMo fraction was calculated using
equations reported in [38,39], assuming that the MoS2 crystallites
are perfect hexagons. This fraction can be considered as an indica-
In the present study, the catalytic activity of the NiMo cata-
lysts was tested in simultaneous hydrodesulfurization (HDS) of
dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-
DMDBT). These two sulfur-containing molecules were selected
as representatives of the refractory sulfur compounds contained
in the diesel fraction of petroleum [41]. Tables 6 and 7 show
DBT and 4,6-DMDBT conversions obtained with the synthesized
NiMo catalysts at different reaction times. Results obtained with
Table 6
Activity and selectivity of different NiMo catalysts in hydrodesulfurization of DBT.
b
−1
Catalyst
DBT conversion (%)a
kDBT (h−1
)
kDBT × 105 (L s−1 gcat
)
CHB/BPc
2 h
4 h
6 h
8 h
NiMo/Ti-SBA-15
NiMo/Ti-SBA-16
NiMo/NT(3)
NiMo/NT(5)
NiMo/␥-Al2O3
46
36
40
33
36
72
65
68
60
66
86
85
90
85
83
94
95
96
94
93
0.29
0.24
0.28
0.22
0.25
2.15
1.79
2.07
1.61
1.85
1.22
1.52
1.14
1.21
0.39
CHB, cyclohexylbenzene; BP, biphenyl.
a
DBT conversion at different reaction times.
kDBT, overall pseudo-first order rate constants determined from linear plots shown in Fig. 11(a).
Ratio of the principal reaction products at 50% of DBT conversion.
b
c
Table 7
Activity and selectivity of different NiMo catalysts in hydrodesulfurization of 4,6-DMDBT.
b
−1
Catalyst
4,6-DMDBT conversion (%)a
kDMDBT (h−1
)
kDMDBT × 105 (L s−1 gcat
)
MCHT/DMBPc
2 h
4 h
6 h
8 h
NiMo/Ti-SBA-15
NiMo/Ti-SBA-16
NiMo/NT(3)
NiMo/NT(5)
NiMo/␥-Al2O3
33
26
27
24
13
56
51
53
44
26
74
75
80
73
40
88
90
94
89
53
0.18
0.17
0.16
0.14
0.07
1.33
1.22
1.16
1.04
0.54
8.67
8.26
2.97
3.63
9.46
MCHT, methylcyclohexyltoluene; DMBP, dimethylbiphenyl.
a
4,6-DMDBT conversion at different reaction times.
kDMDBT, overall pseudo-first order rate constants determined from linear plots shown in Fig. 11(b).
Ratio of the principal reaction products at 50% of 4,6-DMDBT conversion.
b
c
1.4
1.2
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
NiMo/Al2O3
NiMo/TiSBA-15
NiMo/NT(5)
NiMo/Al2O3
NiMo/TiSBA-15
NiMo/NT(5)
(a)
(b)
NiMo/TiSBA-16
NiMo/NT(3)
NiMo/TiSBA-16
NiMo/NT(3)
0.8
0.6
0.4
0.2
0
0
1
2
3
4
5
0
1
2
3
4
5
Reaction time (h)
Reaction time (h)
Fig. 11. Pseudo-first-order kinetics linear plots for (a) DBT HDS and (b) 4,6-DMDBT HDS.
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the reference NiMo/␥-Al2O3 catalyst are also included. In gen-
BP
60
eral, DBT conversions reached with all catalysts (Table 6) were
higher than those of 4,6-DMDBT (Table 7). This is in line with
the well-known fact that dibenzothiophene molecules with alkyl
groups in positions 4 and 6 are less reactive and more difficult to
eliminate by HDS than unsubstituted DBT [42,43]. The reference
NiMo/␥-Al2O3 catalyst resulted in 93% of DBT conversion and 53%
of 4,6-DMDBT at 8 h reaction time. This low catalytic activity for the
hydrodesulfurization of 4,6-dimethyldibenzothiophene is a partic-
ular characteristic of the alumina-supported NiMo catalysts. It can
be noted in Table 6 that the NiMo/Ti-SBA-15 and NiMo/NT(3) cat-
alysts showed slightly higher catalytic activity in DBT HDS than
the ␥-alumina-supported reference and two other catalysts tested.
The pseudo-first-order rate constants calculated for HDS of DBT
with two more active catalysts were also slightly higher (12–15%)
than that of the NiMo/␥-Al2O3 (Fig. 11(a), Table 6). The above
activity results show that change in the support nature does not
have strong implications on the activity of the NiMo catalysts in
the hydrodesulfurization of DBT. In contrast, in the HDS of 4,6-
DMDBT, all NiMo catalysts supported on Ti-containing materials
showed noticeably improved catalytic activity than the reference
NiMo/␥-Al2O3 sample (Fig. 11(b), Table 7). In this case, 4,6-DMDBT
conversions of 88–94% were reached at 8 h. The pseudo-first-order
rate constants calculated for HDS of 4,6-DMDBT with the NiMo
catalysts supported on Ti-containing SBA materials were slightly
higher than those calculated for the catalysts supported on NT(3)
and NT(5). Nevertheless, all these constants were significantly
higher (1.8–2.5 times) than that of the NiMo/␥-Al2O3 (Table 7).
In addition, the pseudo-first-order rate constants determined in
the present work for simultaneous HDS of DBT and 4,6-DMDBT
are similar to those reported by us previously for HDS of DBT
(a)
50
40
30
20
10
CHB
BCH
THDBT
40
DBT conversion (%)
0
0
20
60
80
100
70
60
50
40
30
20
10
0
CHB
(b)
BP
in the presence of the NiMo catalyst supported on low-sodium
titania nanotubes (1.85 × 10−5 L s
g
cat
−1, [46−])1and for HDS of 4,6-
−1
DMDBT with NiMo/Ti-SBA-15 (∼1 × 10−5 L s gcat−1, [16]). That
means that the catalytic behavior of these catalysts is almost not
affected by the presence of different types of DBT molecules in the
reaction mixture. The NiMo/Ti-SBA-15 sample showed the highest
catalytic activity (highest values of kDBT and kDMDBT, Tables 6 and 7)
among all NiMo catalysts tested. Probably, this can be attributed to
the fact that this catalyst has the highest amount of dispersed octa-
hedral Mo oxide species easy to be reduced at low temperature
(TPR, Fig. 8) leading to the formation of well-sulfided and dispersed
MoS2 active species (Table 5). It seems that the amount of Ti species
loaded to this support and their characteristics (isolated Ti species)
provide an optimal metal-support interaction with the deposited
Ni and Mo oxide species. It is worth mentioning that all NiMo cat-
alysts prepared in the present work using Ti-containing materials
of different types resulted in high conversions of both DBT and 4,6-
DMDBT molecules, in contrast to the NiMo/␥-Al2O3 reference. Thus,
DBT conversions of 94–96% and 4,6-DMDBT conversions of 88–94%
were obtained with these catalysts at 8 h reaction time. From the
above results it can be clearly observed that (i) the NiMo catalysts
supported on Ti-containing materials (Ti-SBA-15, Ti-SBA-16, NT(3)
and NT(5)) were significantly more active for HDS of 4,6-DMDBT
than the alumina-supported reference, and (ii) the NiMo/Ti-SBA-
15 catalyst showed the highest catalytic activity for HDS of both
S-containing molecules tested.
BCH
THDBT
0
20
40
60
80
100
DBT conversion (%)
80
(c)
CHB
70
60
50
40
30
20
10
0
BP
BCH
THDBT
0
20
40
60
80
100
DBT conversion (%)
It is well-known that the hydrodesulfurization of DBT-
type molecules occurs through two parallel reaction routes,
namely, (i) direct desulfurization (DDS) yielding the corresponding
biphenyl-type products, and (ii) hydrogenation with subsequent
desulfurization (HYD), leading first to the pre-hydrogenated
intermediates (tetrahydro- and hexahydrodibenzothiophenes) and
then to cyclohexylbenzene-type compounds [44,45]. The main
reaction product obtained in the HDS of dibenzothiophene
through the DDS route is biphenyl (BP), whereas through the
Fig. 12. Product compositions obtained in HDS of DBT over NiMo catalysts: (a)
NiMo/␥-Al2O3, (b) NiMo/Ti-SBA-15 and (c) NiMo/NT(3). Detected products: BP,
biphenyl; CHB, cyclohexylbenzene; BCH, bicyclohexyl; THDBT, tetrahydrodiben-
zothiophene.
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11
70
40
(b)
MCHT
(a)
MCHT
35
60
30
25
20
15
10
5
50
40
30
20
DMBCH
DMBP
DMBCH
HH
DMBP
HH
10
TH
TH
0
0
0
20
40
60
80
100
0
10
20
30
40
50
DBT conversion (%)
DBT conversion (%)
60
50
40
30
20
10
0
MCHT
(c)
DMBCH
TH
DMBP
HH
0
20
40
60
80
100
DBT conversion (%)
Fig. 13. Product compositions obtained in HDS of 4,6-DMDBT over NiMo catalysts: (a) NiMo/␥-Al2O3, (b) NiMo/Ti-SBA-15 and (c) NiMo/NT(3). Detected products: DMBP,
dimethylbiphenyl; MCHT, methylcyclohexyltoluene; DMBCH, dimethylbicyclohexyl; TH, tetrahydrodimethyldibenzothiophene; HH, hexahydrodimethyldibenzothiophene.
HYD route it is cyclohexylbenzene (CHB). For the HDS of
4,6-dimethyldibenzothiophene, the corresponding products are
dimethylbiphenyl (DMBP) and methylcyclohexyltoluene (MCHT).
Tables 6 and 7 show the ratios of the main desulfurized products
(CHB/BP and MCHT/DMBP) obtained with different catalysts in HDS
of DBT and 4,6-DMDBT, respectively. These ratios were determined
at 50% of DBT or 4,6-DMDBT conversion and can be used as a mea-
sure of the catalyst’s selectivity. The NiMo catalyst supported on
alumina showed low cyclohexylbenzene to biphenyl ratio (0.39,
Table 6). The main desulfurized product obtained with this cata-
lyst in the HDS of DBT was biphenyl (BP), which points out that the
DDS pathway of the reaction was the predominant reaction route.
This conclusion can be confirmed by the selectivity curve shown
in Fig. 12(a), for this catalyst, where it can be seen that BP is the
main product for all DBT conversions shown. For the NiMo cata-
lysts supported on different Ti-containing materials, CHB/BP ratios
from 1.14 to 1.52 were obtained at 50% of DBT conversion (Table 6),
indicating that all these catalysts have higher hydrogenation abil-
ity than that of the NiMo/␥-Al2O3 reference. At DBT conversions
higher than 45–50%, CHB became the main product obtained with
these catalysts (Fig. 12(b) and (c)).
Regarding the product distributions obtained for the
hydrodesulfurization of 4,6-DMDBT (Fig. 13), methylcyclohexyl-
toluene (MCHT) was the principal desulfurized product obtained
with all NiMo catalysts tested. This is in line with the well-known
preference of this molecule for the HYD pathway of HDS, which, in
general, is attributed to the presence of alkyl groups in positions
4 and 6 of the molecule strongly blocking the hydrogenolysis
(DDS) pathway [43]. In this case, the hydrogenation ability of the
catalysts (MCHT/DMBP ratio, Table 7) increases in the following
order:
NiMo/NT(3) < NiMo/NT(5) < NiMo/Ti-SBA-16 < NiMo/Ti-
SBA-15 < NiMo/␥-Al2O3. In general, a strong difference (of about
3 times) was observed in the MCHT/DMBP ratios obtained with
different catalysts. Thus, the amount of MCHT obtained with
the NiMo/NT(3) and NiMo/NT(5) catalysts at 50% of 4,6-DMDBT
conversion was 3–4 times larger than the amount of DMBP.
Under the same conditions, Ti-SBA-15 and Ti-SBA-16-supported
catalysts, as well as the NiMo/␥-Al2O3 reference, resulted in the
formation of a much larger proportion of MCHT product and only
a small amount of DMBP (MCHT/DMBP ratios ∼ 8.3–9.5, Table 7).
No clear relationship was observed between the hydrogenation
ability of the catalysts (MCHT/DMPB ratio) and their activity
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in HDS of 4,6-DMDBT. Thus, the NiMo/NT(3) and NiMo/NT(5)
catalysts showed high activity in line with a significantly lower
MCHT/DMBP ratio than all other catalysts. This is an interesting
result, since the majority of the catalysts active for the HDS of
4,6-DMDBT have high hydrogenation ability, which is normally
related to morphological characteristics, principally stacking
degree, of the MoS2 active phase. Thus, in general, it is consid-
ered that HYD rates increase with an increasing stacking degree
attributed to a less hampered planar adsorption geometry of
reactants on multilayered MoS2 crystallites [40]. In our previous
works, it was found that high activity for the HDS of 4,6-DMDBT
is normally obtained with the catalysts having well dispersed
short (3–4 nm) MoS2 particles formed by 2–3 layers [15,16,39].
On the other hand, a decrease in the stacking degree of the MoS2
clusters in general resulted in an increase in the selectivity of the
catalysts toward the DDS route of HDS, and diminishing of the
overall catalytic activity in HDS of refractory alkyl-substituted
dibenzothiophenes, such as 4,6-DMDBT. In the present work,
we found for the first time that the NiMo/NT(3) and NiMo/NT(5)
catalysts are able to desulfurize efficiently 4,6-DMDBT, not only
through the HYD pathway, but also through the DDS route of
the reaction. We think that this unusual catalytic behavior of the
NiMo/NT catalysts is due to high dispersion of the MoS2 active
phase and the particular morphology of the used nanotubular
titania support. It’s possible that the use of this material and its
surface curvature helps to diminish steric hindrance problems,
thus allowing the plug-in adsorption of 4,6-DMDBT molecules on
the hydrogenolysis active sites of low-stacked MoS2 active phase.
It should be mentioned that recently, similar results have been
obtained in ultra-deep hydrodesulfurization with the NiW catalyst
supported on 1D ␥-Al2O3 nanorods [47]. This catalyst showed
higher catalytic activity for HDS of both DBT and 4,6-DMDBT than
the reference NiWP/␥-Al2O3 commercial catalyst and unexpect-
edly high hydrogenolysis ability in HDS of 4,6-DMDBT (the DDS
rate with this catalyst was twice larger than the obtained with
the NiWP/␥-Al2O3 reference). It was also observed that the NiW
catalyst supported on 1D ␥-Al2O3 nanorods was formed by short
(∼1.9 nm) and low-stacked (1–2 layers) sulfide species which were
able to desulfurize 4,6-DMDBT through both the HYD and the
DDS routes of the reaction. Based on theoretical calculations, the
authors supposed that this behavior is due to the fact that Ni–W–S
sites on small slabs could act as hydrogenolysis sites rather than
as hydrogenation sites.
particles was different on the Ti-SBA supports and on titania nano-
tubes. In the first case, MoS2 crystallites were 3–4 nm in length and
formed by 2 layers. On the NT supports, MoS2 particles of simi-
lar length were observed, but formed principally by just 1 layer. In
both cases, the fraction of Mo atoms located on the edge surface
of MoS2 crystallites was the same (fMo ∼ 0.3). All NiMo catalysts
prepared using Ti-containing supports of different types showed
high catalytic activity for the simultaneous HDS of both DBT and
4,6-DMDBT molecules. In the HDS of 4,6-DMDBT, their activities
were about twice higher than that of the NiMo/␥-Al2O3 reference.
Regarding selectivity, for DBT HDS, the presence of titania in the
supports resulted in an increase in the proportion of CHB product,
whereas for 4,6-DMDBT, a different behavior was observed. NiMo
catalysts supported on Ti-SBA-15 and Ti-SBA-16 showed similar
selectivity as the NiMo/␥-Al2O3. These three catalysts desulfurized
4,6-DMDBT principally through the HYD pathway of the reac-
tion. On the contrary, the catalysts supported on titania nanotubes
allowed reaching high conversions of 4,6-DMDBT with a much
lower ratio of MCHT/DMBP products. This indicates that these cat-
alysts were able to desulfurize 4,6-DMDBT not only through the
HYD route, but also via the DDS pathway. This is an interesting
result which we relate to high dispersion of the MoS2 active phase
and the particular morphology of the nanotubular titania support.
Acknowledgments
Financial support by Consejo Nacional de Ciencia y Tecnología
(CONACYT, México) (Project 220175 and Ph.D. scholarship to J.C.M.-
O.) and DGAPA-UNAM (PAPIIT project IN-113715) is gratefully
acknowledged. The authors thank I. Puente Lee, C. Salcedo Luna, M.
Aguilar Franco and A. Morales Espino, for technical assistance with
electron microscopy (HRTEM, SEM-EDX), powder and small-angle
XRD characterizations.
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