The use of inorganic Al-HMS as a support for NiMoW sulfide HDS catalysts

Article abstract of DOI:10.1016/j.ica.2021.120450

Inorganic hexagonal mesoporous silica (HMS) and aluminum modified HMS materials (Al-HMS) were prepared and used as supports of transition metal sulfide hydrodesulfurization (HDS) catalysts based on nickel, molybdenum, and tungsten as active phase. The samples were characterized with XRD, HRTEM, TPD, N₂ physisorption and UV–Vis. The catalytic activity of the trimetallic catalysts was performed in the HDS of dibenzothiophene (DBT). When Al was incorporated into the inorganic support, important changes and effects were observed on the physicochemical properties. On the other hand, the incorporation of Al into the HMS led to a decrease in the reaction rate (k) and a trend toward a direct path of desulfurization was observed for all materials.

Full text of DOI:10.1016/j.ica.2021.120450

Inorganica Chimica Acta 524 (2021) 120450
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Research paper
The use of inorganic Al-HMS as a support for NiMoW sulfide HDS catalysts
a
,
*
b
a
c
a
˜
´
˜
R. Huirache–Acuna , T.A. Zepeda , P.J. Vazquez , E.M. Rivera-Munoz , R. Maya-Yescas ,
d
b
˜
B. Pawelec , G. Alonso-Núnez
a
´
´
Facultad de Ingeniería Química, Universidad Michoacana de San Nicolas de Hidalgo, 58060 Morelia, Michoacan, Mexico
b Centro de Nanociencias y Nanotecnología – UNAM, 22800 Ensenada, B.C., Mexico
c
´
Centro de Física Aplicada y Tecnología Avanzada-UNAM, 76230 Juriquilla, Queretaro, Mexico
d
´
Instituto de Catalisis y Petroleoquímica, CSIC, c/Marie Curie 2, Cantoblanco, Madrid 28049, Spain
A R T I C L E I N F O
A B S T R A C T
Inorganic hexagonal mesoporous silica (HMS) and aluminum modified HMS materials (Al-HMS) were prepared
and used as supports of transition metal sulfide hydrodesulfurization (HDS) catalysts based on nickel, molyb-
denum, and tungsten as active phase. The samples were characterized with XRD, HRTEM, TPD, N2 physisorption
and UV–Vis. The catalytic activity of the trimetallic catalysts was performed in the HDS of dibenzothiophene
(DBT). When Al was incorporated into the inorganic support, important changes and effects were observed on the
physicochemical properties. On the other hand, the incorporation of Al into the HMS led to a decrease in the
reaction rate (k) and a trend toward a direct path of desulfurization was observed for all materials.
This paper is dedicated to our wonderful
colleague, Dr. Maurizio Peruzzini. Thank you
very much for your dedication, professionalism
and for motivating so many people in the sci-
entific field.
Keywords:
Inorganic
HMS
Al-HMS
Transition metal sulfide
NiMoW
Hydrodesulfurization
1. Introduction
sieves based on mesoporous silica as catalyst supports [6–8]. The design
procedure makes use of the high surface areas that exhibit this type of
Currently, research projects for the development of more effective
HDS catalysts have become an important topic of study for environ-
mental catalysts around the world [1–4]. Environmental regulations in
many countries require the production of transport fuels with lower
sulfur contents (less than 10 ppm) [1–4].
materials to achieve higher activity per unit weight. At the same time,
the uniform mesoporous structure facilitates the dissemination of the
polycyclic compounds of sulfur and the surface acidity contributes the
metals’ dispersion.
Pure or modified inorganic mesoporous silica-based materials with
heteroatoms such as Ti, Al, Zr or Nb [9–22], have attracted considerable
attention as possible supports for the preparation of hydroprocessing
catalysts. These features of HMS supports have enabled the preparation
of transition metal sulfided CoMo and NiMo bimetallic catalysts with
higher catalytic activity in comparison with a CoMo/Al2O3 commercial
catalyst in the HDS reaction of DBT [6,13,18], and even better than
other inorganic materials such as SBA-15 or MCM-41 [23]. Regarding
the active phase generally, bimetallic formulations have been used
[24–26]. However, in 2001, Soled et al. patented a novel unsupported
trimetallic catalyst called NEBULA (New Bulk Activity) based on tran-
sition metals, which presents the highest catalytic activity for HDS re-
Over the past 20 years has been a global trend to reduce levels of
sulfur compounds in fossil fuels because they are precursors of sulfur
oxide and in addition to poison the transition metals present in other
stages of refinement. Among the sulfur compounds in petroleum frac-
tions include thiols, sulfides, disulfides, thiophenes, benzothiophenes,
and dibenzothiophenes compounds. The ability to remove the sulfur
content in the organic molecules depends on the molecule complexity
(molecular size and structure of organo-soaked compounds) [5]. The
complexity of sulfur removal increases in the following order: thio-
phenes > benzothiophenes > dibenzothiophenes. In recent years, for
deep HDS processes, it is explored and investigated the design of new
catalysts based on the synthesis and application of inorganic molecular
˜
actions [27]. Subsequently, R. Huirache-Acuna and co-workers [28]
* Corresponding author.
˜
E-mail address: rafael_huirache@yahoo.it (R. Huirache–Acuna).
https://doi.org/10.1016/j.ica.2021.120450
Received 29 November 2020; Received in revised form 6 May 2021; Accepted 7 May 2021
Available online 9 May 2021
0020-1693/© 2021 Elsevier B.V. All rights reserved.

˜
R. Huirache–Acuna et al.
Inorganica Chimica Acta 524 (2021) 120450
reported the synthesis of a trimetallic-type catalyst mass (unsupported)
with high catalytic activity in the HDS of DBT. However mass type
catalysts have a higher cost compared to typical supported catalytic
materials. Therefore, it is very important to investigate the use of tri-
metallic transition metal sulfide as active phases of catalysts supported
on inorganic supports. Nowadays, to lowering the catalyst cost, one of
the procedures for deep desulfurization on which this research proposal
is based, is to increase the catalytic activity by obtaining more effective
hydrodesulfurization catalysts. For this reason, it is of great importance
the search of inorganic HMS and Al-HMS supports for the preparation of
novel trimetallic catalysts, which favors the formation of active sites for
the HDS reaction, in addition to reducing the catalyst cost.
(PSD) curves were calculated by applying the Barret-Joyner-Halenda
method (BJH) to the adsorption branches of the N2 isotherms. The
total pore volume (Vt) was obtained from the isotherms at P/P0 = 0.99.
The identification of the crystalline phases of the catalysts in the
form of oxides and sulfides were characterized by X-ray diffraction by
the powder method with a computerized Seifert 3000 diffractometer,
using Ni-filtered Cu Kα (λ = 0.1541 nm) radiation, a PW 2200 Bragg-
Brentano θ/2θ goniometer equipped with a bent graphite mono-
chromator and an automatic slit, and a step size of 0.01◦.
The acidity of the materials was determined by desorption of
ammonia at programmed temperature (TPD of NH3) carried out in a
Micromeritics TPR/TPD 2900 with a thermal conductivity detector
(TCD). Samples (0.050 g) were degassed in a He (99.996%) flow at 250◦
C for 1 h. Then, the sample was cooled to a temperature of 100◦ C and
saturated with a stream of NH3 (5% NH3/He) at a flow rate of 50 mL/
min for 1 h, followed by He flow for 15 min. Desorption was carried out
by heating the sample up to 700◦ C with a heating rate of 15 ◦C / min. To
determine the total acidity based on its NH3 desorption profile, the area
under the curve was integrated. Weak, medium and strong acidities
were defined as the area under the curve of the peaks in the temperature
ranges of 100–250 ◦C, 250–400 ◦C, and 400–600 ◦C, respectively.
The diffuse reflectance spectra were obtained using a UV–vis spec-
trophotometer Varian Cary by 5000. The acquisition range of 200–900
nm was at room temperature with a recording speed of 120 nm/min.
The study by transmission electron microscopy (TEM) was per-
formed to the sulfided catalysts. A JEOL 100 CXII microscope operating
at a 200 kV voltage and equipped with an X-ray signal INCA (Oxford
Instruments) was used. The sample was dispersed in ethanol with an
ultrasonic bath at room temperature. Finally, a drop of the suspension
was placed on a copper grid coated with carbon. Statistical evaluation of
about 50 particles was performed from various TEM images.
2. Experimental
2.1. Synthesis of inorganic HMS and Al-HMS
Inorganic Hexagonal Mesoporous Silica materials (HMS) and Al-
HMS (Si/Al = 60, 40 and 20) were prepared by the method of sol–gel
[29]. A summary of this procedure is described below: first, a homo-
geneous solution of dodecylamine surfactant (DDA, 98% Aldrich,
4.7554 g or 5.9 mL), was dissolved in ethanol (58.4 mL, Sigma-Aldrich)
and the right quantity of deionized water was added under constant
stirring (H2O, 234.2 mL); then, mesitylene (MES, 97% Aldrich, 16.1 mL)
was added to the above solution with stirring at room temperature for
20 min. Subsequently, a solution of tetraethylorthosilicate (TEOS, 98%,
22.8 mL, Aldrich) was slowly added and allowed to perform the reaction
(hydrolysis and condensation) with stirring at room temperature for 20
h. Finally, the solid obtained was recovered by filtration, washed with
deionized water and dried at room temperature, followed by drying at
110 ◦C. The organic material is removed by calcination in air at 550 ◦C
for 4 h. The molar ratios of the reagents used are 1 mol TEOS: 0.25 mol
DDA: 10 mol Ethanol: 130 mol H2O: 1.125 mol MES. Mesoporous silica
containing aluminum (Al-HMS) was also synthesized using the same
procedure as for the material of HMS, but this time by dissolving the
appropriate amount of aluminum (AlIPO, 97%, Aldrich) in TEOS in
order to obtain samples with Si/Al molar ratios of 20, 40 and 60.
2.4. HDS performance (catalytic activity)
The HDS of DBT reaction was performed in a batch reactor (Parr
model 4848 high-pressure). First, sulfurization of the catalysts was
performed in a fixed bed reactor made of borosilicate glass with a gas
mixture of 15%H2S/85%H2 (Praxair) at a flow of 60 mL minꢀ 1, raising
the temperature at a rate of 10 ◦C to 400 ◦C minꢀ 1, where the samples
stayed for 2 h. Then, the samples were cooled down to room temperature
under a nitrogen flow and were placed in a beaker with hexadecane to
prevent oxidation of the activated catalyst. The transition metal sulfide
catalysts were then introduced into the reactor. The reactor was heated
at a rate of 5.5 ◦C minꢀ 1 up to 320 ◦C and pressurized by using hydrogen
to 5.4 MPa. Regarding the stirring speed, this remained intensive (700
rpm) to prevent external diffusion constraints. The reaction time com-
prises 270 min and sampling for chromatographic analysis was con-
ducted in the following periods: 0, 15, 30, 45, 60, 90, 120, 150, 210 and
270 min. Analyses of the samples were performed during the course of
each run to determine the conversion against time dependence. Each
reaction required 500 ppm of sulfur (about 0.226 g of DBT), 100 mL of
hexadecane as solvent and 0.255 g of each catalyst. The course of each
reaction was followed by taking liquid samples at the times aforemen-
tioned and analyzing then in a gas chromatograph Agilent Technologies
7890A GC System Model which features a 30 m HP5 column with
capillary length and an internal diameter of 0.32 mm, equipped with a
detector type Agilent 355 SCD. The products obtained after the con-
version of the DBT were biphenyl (BP) as direct desulfurization (DDS)
product, while bicyclohexyl (BCH), cyclohexylbenzene (CHB) and traces
of tetrahydrodibenzothiophene (THDBT) were the products resulting
from the hydrogenation pathway (HYD). Direct desulfurization (DDS)
and hydrogenation (HYD) selectivity were defined as:
2.2. Trimetallic transition metal catalysts synthesis
All NiMoW/HMS and NiMoW/Al-HMS catalysts were prepared by
simultaneous support impregnation with metal oxide precursors via
incipient wetness method. In a typical synthesis, precursors salts were
dissolved in deionized water: (NH4)6Mo7O24⋅6H2O (AHM, ammonium
heptamolybdate)), (NH4)6H2W12O40⋅xH2O (AMT, ammonium meta-
tungstate)), and Ni(NO3)2⋅6H2O (nickel nitrate hexahydrate) were
employed as the precursors of MoO3, WO3 and NiO, respectively. The
supports were loaded with fixed amounts of molybdenum (8.53 wt%),
tungsten (13.75 wt%) and nickel (3.81 wt%). The concentrations were
calculated to achieve a Mo/W atomic ratio of 0.5 and a Ni/Mo atomic
ratio of 0.3.
The solution of AHM was poured into the solution of AMT with
constant stirring, and then the solution of nickel nitrate hexahydrate was
added into the mixture. The solution was added dropwise using constant
stirring until the homogeneous humidification of the support was
completed. Then, it was immediately subjected to drying in a furnace at
85 ◦C for 16 h, and calcined at 500 ◦C for 4 h, in both treatments using a
temperature profile of 2 ◦C/min.
2.3. Materials characterization
Determination of the surface area (SBET) was performed by N2
Physisorption at ꢀ 196 ◦C using a Micromeritics TriStar 3000 apparatus,
having each sample a pretreatment to remove any organic component
and humidity (degassing) of 5 h at 270 ◦C in an inert atmosphere. In
order to avoid the tensile strength (TSE) artifact, pore size distribution
(DDS): (BP) × 100% /(CHB + BP + BCH + THDBT)
(HYD): (CHB + BCH + THDBT) × 100% / (CHB + BP + BCH +
THDBT)
HDS kinetic constants (k) were calculated assuming pseudo-first
2

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R. Huirache–Acuna et al.
Inorganica Chimica Acta 524 (2021) 120450
order kinetics referred to initial DBT concentration.
k ⋅ t = ꢀ ln(1 ꢀ x)
(1)
where t is reaction time, and x is the conversion of DBT. Catalytic
tests were performed varying the particle size and stirring rate to found
an experimental error value between 3.5 and 5.4%.
HMS Si/Al = 20
3. Results and discussion
HMS Si/Al = 40
HMS Si/Al = 60
3.1. Characterization of oxide catalyst precursors
3.1.1. Textural properties of synthesized materials
The textural properties of the HMS-Al (40) pure support and calcined
NiMoW/HMS-Al(40) catalyst precursors were studied by N2 phys-
isorption at ꢀ 196 ◦C (Fig. 1).
Al-free
As expected, a large decrease in the amount of adsorbed nitrogen is
observed after the incorporation of a large amount of metal oxides. The
N2 isotherms of NiMoW/HMS-Al(40) catalyst have two capillary
condensation steps: (i) the first hysteresis loop for these materials starts
at a partial pressure of about 0.40, indicating the presence of framework
mesoporosity and (ii) the second hysteresis loop starting at a relative
pressure of about 0.88 might to be attributed to either to macropores or
to textural interparticle mesoporosity, which takes place when the
particle diameters are very small and closely packed [30].
0.0
0.2
0.4
0.6
0.8
1.0
P/P0
Fig. 2. N2 adsorption–desorption isotherms at ꢀ 196 ◦C for the oxide cata-
lyst precursors.
Table 1
Textural properties of the calcined catalysts and Al-
modified supports as determined by N2 adsorp-
tion–desorption isotherms at ꢀ 196 ◦C.
The N2 adsorption–desorption isotherms corresponding to transition
metal NiMoW catalysts supported on inorganic HMS and Al-modified
HMS material (varying the Si/Al = 20, 40, 60) are presented in Fig. 2.
It is observed that all materials exhibit type IV isotherms (IUPAC clas-
sification), which is associated with mesoporosity and show “bottle-
neck” hysteresis [31], mainly in catalysts with Si/Al = 60 ratio and for
pure HMS.
Catalyst
SBET (m2/g)
Al-free
544
517
451
483
HMS Si/Al = 60
HMS Si/Al = 40
HMS Si/Al = 20
The SBET values of the catalysts in their oxide state are shown in
Table 1. It was observed that when the aluminum concentration in-
creases (Si/Al = 20 > Si/Al = 40 > Si/Al = 60) it leads to a decrease in
the SBET values in all cases, probably due to a progressive blockage of the
support with a Si/Al = 40 ratio (451 m2/g). Table 1 shows the influence
of Al loading on the specific SBET values of the oxide catalyst precursors
suggesting a partial blockage of the pores by the oxide species formed by
the heteroatom [23].
́
supports pores by the metal particles formed during impregnation-
calcination process. However, this behavior was not lineal since the
support with a higher amount of aluminum (Si/Al = 20) shows a SBET
value of 483 m2/g, which is higher compared with that observed for the
On the other hand, NiMoW catalysts supported on Al free (HMS) and
HMS with a Si/Al = 60 ratio, show the highest SBET value among the
catalysts studied. The lowest SBET value was recorded for the catalysts
containing a moderate Al-loading (Si/Al = 40). This result suggests that
Ni–Mo-W species might be located not only inside the pores, but also an
important fraction was probably located at the entrance of the meso-
pores [23]. The specific surface area of the Si/Al = 20 sample is higher
than Si/Al = 40 sample. This effect can be related with a major segre-
gation of the species (see XRD section) blocking the pores of support.
The pore size distributions of the HMS-Al (40) pure support and
calcined NiMoW/HMS-Al(40) catalyst precursors calculated from the
adsorption branch of the N2 isotherm by using the Barret-Joyner-
Halenda model are shown in Fig. 3. As seen in this figure, both sam-
ples show a uniform, narrow pore size distribution centred at 3.2 nm.
The chemical and textural properties of the Al-HMS(40) and NiMoW/Al-
HMS (40) samples are given in Table 2.
900
800
700
0.88
600
500
HMS-Al(40)
0.4
400
A comparison of the specific surface area values of the pure support
and calcined catalyst indicates that SBET decreases upon the incorpora-
tion of metal oxides. Simultaneously, the total pore volume decreases
after metal oxides incorporation during catalyst preparation. The
possible explanation of this increase is that the introduction of metals to
the support blocked the small pores, so the decrease in surface area
occurs. The large loss of total pore volume after metal loadings suggest
the loss of microporosity is probably due pores blocking by guest metal
oxides. To confirm the location of guest phases, the normalized BET
surface area (NSBET) was calculated.
NiMoW/HMS-Al(40)
300
200
100
0
0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure P/P0
The normalized SBET (NSBET) was calculated from the equation:
NSBET = SBETof catalyst/[(1 ꢀ y) × SBETof support]
(2)
Fig. 1. Nitrogen adsorption–desorption isotherms of pure HMS-Al (40) support
and calcined NiMoW/HMS-Al(40) catalyst.
3

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R. Huirache–Acuna et al.
Inorganica Chimica Acta 524 (2021) 120450
4
3
2
1
3.2 nm
HMS-Al(40)
NiMoW/HMS-Al(40)
NiMoW/HMS-Al(60)
NiMoW/HMS-Al(40)
NiMoW/HMS-Al(20)
NiMoW/HMS
0
0
0
1
2
3
4
5
6
2
4
6
8
10
2 Theta (º)
Pore size distribution (nm)
Fig. 3. Pore size distribution of the pure HMS-Al(40) support and calcined
NiMoW/HMS-Al(40) catalyst as determined from adsorption branch of the N2
adsorption data using the BJH method.
Fig. 4. Low-angle XRD patterns of oxide precursors.
* MoNiO4 (00-008-0357)
+ Ni2Mo3O8 (00-037-0855)
Table 2
*
Chemicala and textural propertiesb of Al-HMS (40) and NiMoW/Al-HMS (40)
catalysts.
*
*
Support/
Catalyst
Ni
Mo
(wt.
%)
W
SBET
NSBET
Vtotal
d (nm)
(m2
(cm3
HMS Si/Al = 60
(wt.
%)
(wt.
%)
gꢀ 1
)
gꢀ 1
)
NiMoW/Al-
HMS(40)
Al-HMS
(40)
3.8
7.1
8.7
451
0.88
0.77
4.7
5.0
HMS Si/Al = 40
HMS Si/Al = 20
–
–
–
689
–
1.30
a
b
As determined by EDX/TEM analysis.
As determined from N2 adsorption–desorption isotherms at ꢀ 196 ◦C for
oxide precursors: SBET: BET surface area (m2/gsupport); Vtotal : total pore volume;
d: average pore diameter (nm) calculated from adsorption data by BJH method
where y is the nominal metals content (wt%).
The NSBET value close to 0.9 suggests the bimodal location of the
metal oxides: on the external surface and within the inner porous
structure (larger part).
Al-free
0
20
40
60
80
100
3.1.2. X-ray diffraction (XRD)
2 Theta (º)
Samples were analyzed by this characterization technique with the
objective of studying the phases present in the synthesized materials.
The low-angle X-ray patterns of the NiMoW/Al-HMS oxide precursors
are shown in Fig. 4. As seen in this figure, all materials shows an intense
single d100 reflection accompanied by more or less pronunciated diffuse
scaterring centered at 0.65◦, which indicates hexagonal symmetry.
Higher order Bragg reflections are not resolved in the patterns of this
material. However, it was demonstrated that similar “single-reflection”
materials can still exhibit local hexagonal symmetry, as observed by
selected area electron diffraction and TEM [32].
Fig. 5. X-ray diffractograms for the oxide catalysts precursors.
can be clearly seen in the angles of 28.7◦, 32.5◦, and 43.7◦ respectively,
regardless the type of support. In addition Ni2Mo3O8 phase (JCDPS 00-
037-0855) was detected. It is possible that another type of crystalline
phase was formed, however, if the crystal size of the present species is
less than 5 nm, it is not possible to detect them by means of this
technique.
3.1.3. UV–vis diffuse reflectance spectroscopy (DRS)
Fig. 5 shows the diffractograms of NiMoW catalysts supported on
pure and modified HMS varying the aluminum content. All catalysts
present a broad peak at approximately 2θ = 24◦ (diffraction angle),
which is typical of amorphous silica [1]. On the other hand, reflections
on the pattern belonging to the MoNiO4 phase (JCDPS 00–008-0357)
The coordination environment of Ni2+ and Mo6+ (W6+) ions in the
precursors was studied by UV–vis diffuse reflectance spectroscopy (DRS-
UV–vis). The UV–vis DRS spectra of the catalysts are shown in Fig. 6 as a
Kubelka-Munk-Schuster function. Independently of the incorporation of
4

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R. Huirache–Acuna et al.
Inorganica Chimica Acta 524 (2021) 120450
The strength for the acid sites can be determined by using the tem-
perature at which the ammonia molecule desorbs. Based on the
desorption temperature, the acid sites were classified as weak
(100–250 ◦C), medium (250–450 ◦C) or strong (T > 450 ◦C). All cata-
lysts show weak, medium and strong acid sites. The first two peaks are
attributed to weak Brønsted acid sites (Tmax ≈ 200 ◦C) and moderate
(Tmax ≈ 300 ◦C), respectively. The concentration of acid sites expressed
as mmol of desorbed NH3 per gram of support is reported in Table 3.
As shown in this table, the total acidity shows the following trend:
HMS Si/Al = 20 > HMS Si/Al = 60 > HMS Al- free > HMS Si/Al = 40.
The total acidity could be related to the catalytic activity shown by the
system investigated in this work. It was observed that the materials with
lower acidity present higher overall rate values in the HDS reaction of
DBT. Similarly, as the total acidity increased, there was a decrease in the
catalytic activity (such as in the catalysts with Si/Al ratio of 20 and 60).
It would be expected that as the aluminum ratio increases in the HMS
structure, the acidity will increase; however, there is apparently no
tendency for the acid sites with the Si/Al ratio of each material. This may
be due to the modification of the structure of the support when
impregnated with the active phase. It is important to mention that the
active phase has an irregular dispersion on the surface of the support as
shown in the TEM micrographs and this also causes molybdenum sulfide
to modify the total acidity depending on the arrangement and stacking
of the Mo(W)S2 layers. As previously reported [36], a largest amount of
Brønsted acid sites are formed by substitution of Si4+ ions by Al3+ ions
during direct synthesis of the SBA-16-Al support. In this sense, one can
expect that this behavior could be followed by our Al-HMS modified
samples. In addition, A. Olivas and T. Zepeda [37], found that the
dispersion of the active species (sulfide species) was related to the
acidity of the support, this result can be asociated with the different
values of length and stacking as reported in TEM section.
Al-free
HMS Si/Al = 60
HMS Si/Al = 40
HMS Si/Al = 20
400
500
600
700
800
900
Wavelength (nm)
Al-free
HMS Si/Al = 60
HMS Si/Al = 40
HMS Si/Al = 20
200
250
300
350
400
450
Wavelength (nm)
Fig. 6. DR UV–Vis spectra of the oxide catalysts precursors.
aluminum in the catalysts, they present two bands near 240 nm and 320
nm. The first involves the presence of ligand–metal charge transfer,
which involves isolated transition metal sites attached to tungsten in
tetrahedral coordination [33]. The shift in the bands mentioned could be
attributed to the influence of tungsten on the electronic properties of this
type of catalysts. A decrease of this type of species is observed in the case
of the NiMoW catalysts supported on the HMS material with Si/Al ratio
of 40. The tetrahedral species are more difficult to reduce and therefore
to sulfide. The second band is assigned in literature to the transition
ligand–metal charge O2-Mo6+ (W6+) from molybdenum and tungsten
ions in octahedral coordination [34]. Additionally, DR UV–vis spectra
exhibit bands in the 600–900 nm region. A “shoulder” is observed at
650 nm which could be related to Ni2+ ions coordinated in tetrahedral
form. On the other hand, the band at 750 nm has been related to octa-
hedral type Ni2+ ions [35].
3.2. Characterization of catalysts in their sulfided state
3.2.1. Transmission electron microscopy (TEM)
Catalysts were studied by transmission electron microscopy since the
transition metal sulfides are the active phases for the HDS reaction.
Regarding the morphology of the support, in the images of TEM, we can
observe some distorted areas, collapsed and pores. It is well known that
the phases of Mo(W)S2 adopt a structure in the form of layers with Mo
atoms located in a trigonal prismatic coordination. All catalysts show
the typical fringes of MoS2 crystals. The location of the Mo(W)S2 sheets
in the HMS and Al-HMS carriers is shown in Fig. 8a–d.
As reported previously by us [28], by using Nuclear magnetic reso-
nance, 27Al NMR spectra for NiMoW/Al-HMS indicate that a larger
amount of Al is incorporated into the framework of the HMS than in
extraframework positions.
3.1.4. Desorption with ammonia at programmed temperature (TPD-NH3)
The effect of modification of inorganic HMS support with Al (Si/Al =
20, 40 and 60) on the acidity was determined by TPD of NH3 technique.
Fig. 7, compares the TPD profiles of NH3 of the precursors.
In Fig. 8(a–d) are compared their magnification Mo(W)S2 sheets. For
all catalysts, the arrangement of W(Mo)S2 layers parallel to the substrate
indicates the basal plane attachment. No edge-plane attachment was
observed because such species easily escape detection. The NiMoW/Al-
HMS (40) sample show slightly higher W(Mo)S2 slabs density than their
counterparts. In addition, there is a slight increase on the stacking
density of Mo(W)S2 for the catalysts supported on pure HMS and HMS
with Si/Al ratio of 40, than their counterparts HMS samples with Si/Al
ratio of 60 and 20. A higher density of Mo(W)S2 in the catalysts could
affect the catalytic activity in the HDS of DBT. The distribution of the
stacking length and number of stripes is shown in Table 4, respectively.
Data showed larger particles that are formed on the HMS catalysts with
Si/Al ratio of 60 (Fig. 8d). The interplanar distance of the (002) planes of
Table 3
Acidities of the oxide precursors (from TPD-NH3).
Catalyst
Amount of acid sites (mmol NH3/g)
Weak
Medium
Strong
Total
Al-free
1.2
0.0
0.3
0.3
1.1
3.5
1.8
3.4
1.0
0.5
0.8
0.7
3.3
4.0
2.9
4.4
HMS Si/Al = 60
HMS Si/Al = 40
HMS Si/Al = 20
Fig. 7. TPD-NH3 profiles for the oxide precursors.
5

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R. Huirache–Acuna et al.
Inorganica Chimica Acta 524 (2021) 120450
Fig. 8. TEM images of the fresh sulfided catalysts.
Table 4
Summary of the MoS2 crystal sizes determined by HRTEM.
Catalyst
Layer length (nm) (±1.8)
Stacking layers (±1)
Al-free
6.4
7.1
6.4
4.7
2.2
2.4
2.5
1.3
HMS Si/Al = 60
HMS Si/Al = 40
HMS Si/Al = 20
all catalysts was very similar (0.67 nm) and slightly higher than those of
the pure MoS2 (0.616 nm) and WS2 phase (0.618 nm). This could be
indicative that Al atoms were located along the (002) direction into the
crystalline structure of Mo(W)S2 or may be caused by the stress released
by curved fringes, the dislocation of the coplanarity of the MoS2 planes
by organic molecules or due to the synergistic effect resulting from the
intercalation of Ni atoms into the interlayer space of MoS2 crystallites
[38–40]. More information on the morphology of phases formed can be
revealed by statistical evaluation of about 50 particles from various TEM
images.
3.2.2. X-ray diffraction (XRD)
Fig. 9. X-ray diffractograms of the NiMoW sulfided catalysts.
Fig. 9a–d shows the X-ray diffractograms of sulfided catalysts sup-
ported on inorganic pure HMS and modified with aluminum. In these
patterns the typical planes (002), (100), (103), (105) and (110) are
observed with signals at 14, 33, 40, 50 and 58◦ 2θ (2θ = x “diffraction
angle”), respectively corresponding to both poorly crystalline 2H-MoS2
and WS2 structures (JCPDS-ICDD 8–237, 37–1492). When aluminum is
incorporated on the support, the intensity of the peak (002) is increased,
mainly for the catalyst supported on HMS with Si/Al ratio of 40, which is
related to a higher stacking of the Mo(W)S2 in the direction of the axis
“c”.
of the supports.
3.3. Evaluation of catalytic properties (activity and selectivity) in the
dibenzothiophene (DBT) hydrodesulfurization reaction (HDS)
The overall reaction rate and selectivity data at 30% DBT conversion
are shown in Tables 5 and 6, respectively.
In according to the catalytic activity, the evaluated materials showed
a good performance and a high activity in the HDS reaction of DBT, since
the DBT conversion values were 97–99%. The following trend was
observed with respect to the overall reaction rate in the HDS of DBT over
the trimetallic samples: Al-free > HMS Si/Al = 40 > HMS Si/Al = 60 >
HMS Si/Al = 20. It was observed that in the trimetallic catalysts, the
In addition, it is observed the presence of a wide band approximately
in 2θ = 24◦ characteristic of amorphous silica (SiO2). The presence of
nickel species is not clearly visible. The low intensity of these peaks
indicates that the species supported may be well dispersed on the surface
6

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R. Huirache–Acuna et al.
Inorganica Chimica Acta 524 (2021) 120450
sites leading to formation of S-saturated sites. Besides, some enhance-
ment of the catalyst HYD function may occur but the DDS reaction route
remains as the main reaction route.
Table 5
Conversion results and overall rates of the catalysts used in the HDS of DBT.
Reaction conditions: T = 320 ◦C; P = 5.4 MPa; Reaction time = 4.5 h.
Catalyst
DBT Conversion
%
Overall reaction rate * 108
molDBT⋅(gCAT⋅s)ꢀ 1
In the case of Al-free sample, the formation of “brim sites” is highly
possible because this sample showed the formation of more “curved” Mo
(W)S2 phases. In this sense, Berhault and co-workers [46] observed that
the bending of the MoS2 slabs modifies the HDS properties of the sul-
fided CoMo/Al2O3 catalysts by creating active sites on curved layers of
the MoS2 phase. Moreover, the layers of MoS2 phase formed by sulfi-
dation of the irregular molybdenum oxide species might have a higher
amount of coordinatively unsaturated sites (CUS) than those formed
from regular MoO3 phase. In addition, we found that the catalytic per-
formance might be related to the dispersion and density of Mo(W)S2 in
the catalysts as observed in TEM.
Al-free
99
99
98
97
71.6
57.5
69.0
46.8
HMS Si/Al = 60
HMS Si/Al = 40
HMS Si/Al = 20
Table 6
Selectivity data at 30% DBT conversion for sulfided NiMoW/HMS and HMS-Al
catalysts.
Selectivity at 30% conversion
Finally, with respect to the selectivity, a trend toward the direct
desulfurization (DDS) path was observed: HMS Si/Al = 20 > Al-free >
HMS Si/Al = 60 > HMS Si/Al = 40. This behavior could be related to the
structure and morphology of the synthesized materials, since a very
similar tendency is observed in the stacking of the Mo(W)S2 measured
by TEM.
Catalyst
HYD
DDS
HYD/DDS
Al-free
26%
30%
34%
23%
74%
70%
66%
77%
0.35
0.42
0.51
0.29
HMS Si/Al = 60
HMS Si/Al = 40
HMS Si/Al = 20
4. Conclusions
incorporation of aluminum into the HMS framework decreases the cat-
alytic activity. On the other hand, the addition of a moderate amount of
aluminum (HMS Si/Al = 40 sample) produces a good performance in the
HDS of the DBT reaction, compared to that shown by the HMS Si/Al =
60 and HMS Si/Al = 20 catalysts. A tendency between activity and
acidity was observed in this work, since a decrease in the catalytic ac-
tivity might be related with an increase of the total surface acidity.
Materials with lower acidity present higher overall rate values in the
HDS reaction of DBT. Likewise, as the total acidity increased, there was a
decrease in the catalytic activity (HMS Si/Al = 20 and 60). However, the
correlation of the catalyst activity with the acidity of oxide precursors is
only a rough approximation and more investigation should be made to
elucidate this since the HDS of DBT occurs mainly via DDS route which
take place on the Ni-promoted metal sulfides by extracting an S-atom by
hydrogenolysis [41]. In such a case, the catalyst acidity may help only to
The incorporation of aluminum into the inorganic HMS framework
causes important changes in the structural, textural and physicochem-
ical properties of the trimetallic NiMoW catalysts, synthesized by an
incipient impregnation method. Regarding the structure, the presence of
poorly crystalline phases of Mo(W)S2 was observed, probably due to the
type of precursor used in addition to the synthesis method, nonetheless
an effect of aluminum incorporation is observed on the stacking of the
Mo(W)S2 layers. The catalytic activity could be related to the total
acidity quantified by TPD of NH3 for the system investigated in this
work. This is because there is a tendency between acidity and the overall
rate in the HDS reaction of DBT. Materials with lower acidity have
higher overall rate values in the HDS reaction of DBT. On the other hand,
when the total acidity was high, there was a decrease in the catalytic
activity. In addition, we found that the catalytic performance might be
related to the dispersion and density of Mo(W)S2 in the catalysts.
–
C
S bond cleavage by cracking. On the other hand, considering the
work reported by A.P. Glotov et al. [42] and our catalytic activity re-
sults, one may expect that the interaction between the active species and
the support surface should be increased when Al is incorporated in a
higher amount, decreasing the reducibility and the catalytic activity,
showing an optimum for NiMoW/Al-HMS (40) sample. In previous
works, we found that addition of Al, Ti or Zr leads to a noticeable
improvement during the performance of bimetallic catalysts (NiMo,
CoMo, NiW). However, in this work we study a ternary system (NiMoW).
The incorporation of Al increased the interaction between active species
and the surface, decreasing the reducibility of the species. On the other
hand, in a previous report [1] it was found that in a CoMoW trimetallic
system, the species of tunsgten can decorate the surface of molybdenum
species resulting in a minor reducibility and catalytic activity.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgements
This work was supported by CONACYT (CB 182191, SENER 117373
and 152012) and CIC 2020-2021 UMSNH projects (Mexico).
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