Characterization and catalytic performance of Co-Mo-W sulfide catalysts supported on SBA-15 and SBA-16 mechanically mixed

Article abstract of DOI:10.1016/j.cattod.2015.07.049

A group of hydrodesulfurization (HDS) catalysts, based on transition metal sulfides (Co-Mo-W) and supported on mechanically-mixed mesoporous silicas (SBA-15 and SBA-16), have been synthesized and characterized by physicochemical methods (DRS-UV-vis, Micro Raman spectroscopy, XRD, SEM, HRTEM, EDS and catalytic activity). It has been demonstrated that the use of a mixture of silicas with two different porous structures has an advantage with respect to the use of the SBA-15 and SBA-16 separately for the preparation of supported catalysts for hydrodesulfurization reactions of refractory sulfur compounds, such as dibenzothiophene. This is because the presence of different porous structures has a positive effect over the diffusion processes of the precursors of the active phases on the support, and the whole result is a higher catalytic activity for the HDS reactions of dibenzothiophene, even more than that of the commercial catalyst used as a comparative model; this activity is also related with the stage of the synthesis process in which the mixture is done.

Full text of DOI:10.1016/j.cattod.2015.07.049

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Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Characterization and catalytic performance of Co-Mo-W sulfide
catalysts supported on SBA-15 and SBA-16 mechanically mixed
a
b
a
a
c
M.A. López-Mendoza , R. Nava , C. Peza-Ledesma , B. Millán-Malo , R. Huirache-Acu n˜ a ,
a
a,∗
P. Skewes , E.M. Rivera-Mu n˜ oz
Centro de Física Aplicada y Tecnología Avanzada, Universidad Nacional Autónoma de México, Departamento de Nanotecnología, A.P. 1-1010 Querétaro,
Qro. C.P. 76000, Mexico
Facultad de Ingeniería, Universidad Autónoma de Querétaro, Centro Universitario, Cerro de las Campanas, 76010, Querétaro, Mexico
Facultad de Ingeniería Química, Universidad Michoacana de San Nicolás de Hidalgo, Ciudad Universitaria, 58060, Morelia, Michoacán, Mexico
a
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
A group of hydrodesulfurization (HDS) catalysts, based on transition metal sulfides (Co-Mo-W) and sup-
ported on mechanically-mixed mesoporous silicas (SBA-15 and SBA-16), have been synthesized and
characterized by physicochemical methods (DRS-UV–vis, Micro Raman spectroscopy, XRD, SEM, HRTEM,
EDS and catalytic activity). It has been demonstrated that the use of a mixture of silicas with two different
porous structures has an advantage with respect to the use of the SBA-15 and SBA-16 separately for the
preparation of supported catalysts for hydrodesulfurization reactions of refractory sulfur compounds,
such as dibenzothiophene. This is because the presence of different porous structures has a positive
effect over the diffusion processes of the precursors of the active phases on the support, and the whole
result is a higher catalytic activity for the HDS reactions of dibenzothiophene, even more than that of
the commercial catalyst used as a comparative model; this activity is also related with the stage of the
synthesis process in which the mixture is done.
Received 26 March 2015
Received in revised form 23 July 2015
Accepted 28 July 2015
Available online xxx
Keywords:
HDS catalysts
Mesoporous silicas
SBA-15
SBA-16
©
2015 Elsevier B.V. All rights reserved.
1
. Introduction
sulfur such as thiophenes and benzothiophenes which are difficult
to remove with conventional commercial catalysts.
Currently, oil and its derivatives, natural gas and coil are the
The most of the catalytic materials used in hydrotreatment pro-
cesses are based on transition metal sulfides, typically those of
molybdenum and tungsten, promoted by cobalt or nickel and sup-
ported in inert materials. Alumina is the most common support
material for HDS reactions for their mechanical properties, low cost
and easy regeneration; however, this material has several disad-
vantages, mainly related to the presence of undesired interactions
between transition metal and support which has promoted the
research of alternative materials such as titania and silica, which
latter have demonstrated alumina equally advantageous properties
in terms of physical and chemical properties. Numerous previous
studies have reported the convenience of using mesoporous sili-
cas with cubic (SBA-16) or hexagonal (SBA-15) mesostructure as a
support material for active species to be used in the hydrodesulfur-
ization reactions since they have shown outstanding performance
for the removal of sulfur atoms within the molecular structure of
thiophenes and benzothiophenes.
main sources of primary energy worldwide and this tendency will
not change in the next two decades according to recent studies in
energy topics.
Currently, oil and its derivatives are, along with natural gas and
coal, the main sources of primary energy worldwide and, accord-
ing to recent studies, this tendency will continue for the next two
decades [1]. It involves a great quantity of challenges related to the
separation of the fractions in the oil, specifically those associated
with the removal of undesired elements such as nitrogen, oxygen,
sulfur and metals, because of the environmental effects of those
compounds. Hydrodesulfurization (HDS) focuses in the removal of
sulfur-containing molecules in oil, typically by the use of catalytic
materials. In the recent years, the research in this topic has focused
its efforts in the development of new catalysts, which has to be
effective in the removal of molecules of complex compounds of
Different mesoporous silica materials have been studied as
supports for hydrodesulfurization catalysts, such as MCM, HMS
and SBA materials, because of their ordered porous structure and
high surface area. Particularly, many studies have been published
∗ Corresponding author. Tel.: +52 4422381158; fax: +52 4422381165.
E-mail addresses: erivera@unam.mx, emrivera@fata.unam.mx
E.M. Rivera-Mu n˜ oz).
(
http://dx.doi.org/10.1016/j.cattod.2015.07.049
920-5861/© 2015 Elsevier B.V. All rights reserved.
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regarding the use of SBA type materials as catalyst supports
for the HDS reactions. According to the studies by Nava and
co-workers [2–6], the SBA-15 and SBA-16 materials are suit-
able supports for depositing transition metal sulfides enabling
the synthesis of bimetallic and trimetallic catalysts, which have
demonstrated greater activity than commercial catalysts supported
on alumina. Similar results have been reported by Soni and co-
workers who have synthesized NiMo catalyst supported on SBA-15
and SBA-16 which have been tested in different hydrotreatment
reactions showing a good activity and obtaining a better perfor-
mance for those supported on SBA-16 [7,8]. Meanwhile, Klimova
and co-workers have used SBA-15 and SBA-16 silicas as cat-
alytic support for the system Ni-Mo-W under different conditions
obtaining improvement in catalytic activity for hydrodesulfur-
ization of dibenzotiophene-like molecules [9–11]. Adjaye and
co-workers have proven the effectiveness of SBA-15 as support
for HDS catalysts using system Ni-Mo and Fe-W obtaining cat-
alytic activities comparable to those of commercial NiMo-alumina
catalysts [12,13]. Rayo and co-workers have studied the effect of
the acid–base conditions of the impregnation media and the pres-
ence of dopant heteroatoms such as Al, Ti and Zr over Ni-Mo
sulfided catalysts supported in SBA-15 used in the HDS reaction
of thiophene, obtaining good catalytic performance, improved by
the presence of conditions or heteroatoms capable to promote
an increase the Brønsted acidity in the catalyst surface and avoid
the NiMoO4 formation, additionally they have reported that the
synthesis of transition metal sulfide precursors under acid con-
ditions favors the preservation of the porous structure of the
SBA-15 [14,15].
oven. Afterward, the obtained solid was vacuum filtered, washed
thoroughly with deionized water and dried first in air at room tem-
◦
◦
perature and then at 110 C for 18 h and finally calcined at 500 C for
◦
6 h in a static air muffle with a 2 C per minute temperature ramp,
in order to remove the organic template.
A very similar procedure was used to synthesize the SBA-16
support where Pluronic® F-127 (EO106PO70EO106, Sigma–Aldrich)
triblock copolymer was used as the structure directing and com-
pletely dissolved in the corresponding amount of HCl 2 M under
stirring. After total dissolution, TEOS was added and allowed to
◦
react at 28 C for 20 h. Then, the reaction mixture was transferred
◦
into polypropylene bottles and heated at 80 C for 48 h in oven. The
solid residue was vacuum filtered, washed thoroughly with deion-
◦
ized water and dried at room temperature and then at 110 C for
◦
18 h. Finally, the sample was calcined at 500 C in a static air muffle
with a 2 C per minute temperature ramp, for 6 h for the removal
◦
of the copolymer.
The catalytic supports based on mesoporous silicas were
obtained by the mechanical mixing of equal molar quantities of
SBA-15 and SBA-16 materials. For the preparation of the mix-
tures pure silicas were meshed in Tyler Starndard Meshes No. 120
and 150 in order to obtain product with particle sizes between
106 and 125 ␮m; meshed silicas were blended by hand during
15 min in order to achieve a complete incorporation of the two
silicas. Supports were mixed in three different ways: a blend
was made prior to the impregnation of the active phase precur-
sors (MMIC – Mechanical Mixture + Impregnation + Calcination),
another one after the impregnation of SBA-15 and SBA-16 sepa-
rately but before calcination (IMMC – Impregnation + Mechanical
Mixture + Calcination) and finally, after separate impregnation and
calcination of SBA-15 and SBA-16 supports (ICMM – Impregna-
tion + Calcination + Mechanical Mixture).
Additional studies have focused on the surface modifica-
tion/functionalization of the SBA materials by incorporation of
other transition metals such as titania, zirconia or alumina, observ-
ing an improvement in the dispersion and reducibility of the
active species, which results in a better catalytic performance
[
2,5,16–18]. Wang and co-workers have demonstrated the impor-
2.2. Catalysts preparation
tance of porous morphology in catalysts supported in mesoporous
silica SBA-15 by the synthesis of materials with different struc-
ture showing different catalytic performance as consequence of
differences in the diffusion proceses in the porous structure
Oxide-state catalysts were prepared by the simultaneous
impregnation via incipient wetness method. Each support was
loaded with fixed equal amounts of molybdenum (5.75 wt%
as MoO3), tungsten (10.92 wt% as WO3) and cobalt (3.05 wt%
as CoO). In a typical synthesis, 1 g of support was impreg-
nated with 1 mL of impregnation aqueous solution containing
ammonium heptamolybdate tetrahydrate ((NH4)6Mo7O24-4H2O,
assay: 81–83%, Sigma–Aldrich), ammonium metatungstate hydrate
((NH4)6H2W12O40-xH2O, assay: 99%, Sigma–Aldrich) and cobalt
nitrate hexahydrate (Co(NO3)2-6H2O, assay: 98%, Sigma–Aldrich).
The concentrations of each transition-metal precursor were cal-
culated to achieve a Mo(W)/(Mo + W) atomic ratio of 0.5 and a
Co/(Mo + W) atomic ratio of 0.43. The impregnated supports were
[
19].
Despite all these studies, to date there have been no reported
research on the combination of supports, with different porosities,
for transition metal sulfides, HDS catalysts.
In this work, a group of HDS catalysts, based on transition metal
sulfides (Co-Mo-W), supported in mixtures of mesoporous silicas
(
SBA-15 and SBA-16) have been synthesized and characterized by
different physicochemical techniques in order to determine their
catalytic properties and performance.
◦
dried first at room temperature for 5 h and then at 85 C for 16 h.
◦
2
. Materials and methods
Finally, they were calcined at 500 C for 4 h in a static air muf-
◦
fle with a 2 C per minute temperature ramp. It is important
2
.1. Support preparation
to mention that the aqueous solution containing the precur-
sors of the three metals is stable during preparation of the
catalysts.
The siliceous SBA-15 and SBA-16 mesoporous materials were
synthesized according to the procedure described previously by
Fresh sulfided catalysts were prepared by sulfidation of the
oxide-state catalysts. The sulfidation reaction was carried out in a
U-shaped glass tubular reactor; oxide-state catalysts were charged
Zhao et al. [20] and Flodström and Alfredsson [21]. For the
®
synthesis of SBA-15 material, Pluronic P-123 (EO20PO70EO20
,
◦
◦
Sigma–Aldrich) triblock copolymer was used as the structure-
in the reactor and heated until 400 C in N2 at 2 C per minute; when
reaction temperature was raised, sulfidation was carried out using
®
directing agent. In a typical synthesis, the Pluronic P-123 was
3
completely dissolved in a solution of water and HCl 4 M under
stirring. After that, the required amount of tetraethyl orthosili-
cate (TEOS, 98%, Sigma–Aldrich) was added to the solution, the
a stream of 15 (v/v)% of H2S in H2 with a flux of 0.71 m per minute
◦
per catalyst gram, at controlled constant temperature (400 C) for
4 h. Once the sulfidation reaction was carried out, sulfided cata-
lysts were cooled to room temperature in a nitrogen stream and
charged directly to the HDS reactor in inert atmosphere in order to
avoid oxidation of the transition metal sulfides.
◦
sol–gel reaction was carried out with stirring at 35 C for 24 h. The
reaction mixture was subsequently transferred into polypropylene
◦
bottles and heated at 80 C for 24 h in a controlled-temperature
Please cite this article in press as: M.A. López-Mendoza, et al., Characterization and catalytic performance of Co-Mo-W sulfide catalysts
supported on SBA-15 and SBA-16 mechanically mixed, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.07.049

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2.8. DRS-UV–vis spectroscopy
3
2.3. Characterization techniques
2
.3.1. Scanning electron microscopy
Morphological properties of supports and catalysts were studied
Diffuse reflectance spectroscopy in the UV–visible range was
aimed to determine the coordination environment of transition
metals in oxide-state catalysts, spectra were recorded in the range
of 200–800 nm at room temperature using a Varian Cary 5000
UV–vis spectrometer equipped with an integration sphere. Spec-
tra were determined using an internal MgO reference material
and using the SBA-15 as blank in order to avoid the appearance
of the electronic transitions corresponding to the siliceous mate-
rial. Before the analysis, samples were meshed in order to obtain
particles with size in the range of the 100–125 ␮m.
by scanning electron microscopy. Images for supports and oxide-
state catalysts were obtained using secondary electrons in a JEOL
JSM-6060 LV microscope operated at 20 kV under high vacuum con-
ditions. Samples were ground into a fine powder and deposited
over copper stubs and covered with a thin gold film with the aid
of a metal evaporator EMS 550 Sputter Coater. Sulfided catalysts
were characterized with a JEOL JSM-7600F microscope operated at
1
kV and under high vacuum and by the use of secondary electrons
for the collection of images; powdered samples were treated in an
inert atmosphere and deposited on copper stubs and analyzed in
an uncoated way.
2.9. Micro Raman spectroscopy
The characterization and aggregation of transition metal oxides
2
.4. Chemical analysis
in catalysts was studied by Micro-Raman spectroscopy, spectra
were recorded at room temperature in a Dilor Labram II Micro-
Raman system equipped with an Ar laser emitting at 488 nm and
The chemical analysis of the oxide-state and fresh sulfided
catalysts (wt%) was determined by energy-dispersive X-ray spec-
troscopy (EDS); oxide-state catalysts were analyzed with a Oxford
Inca X–Sight system taking into account the average of five mea-
surements in different points of the samples. Elemental distribution
maps and composition of fresh sulfided catalysts were recorded in
a similar system making a complete scanning of the samples.
−1
operated at 30 mW, with a resolution of ± 1 cm , and a holographic
notch filter from Kaiser Optical Systems, Inc. (model Super Notch-
Plus).
2
.10. Catalytic performance measurements
HDS reaction of DBT was carried out in a Parr model 4848
2
.5. Nitrogen adsorption–desorption studies
high pressure batch reactor. In order to minimize internal diffu-
sion limitations, all catalysts were thoroughly ground in a mortar
to a fine powder and meshed for use materials with particle size
between 106 and 125 ␮m. For the experiment 0.5 g of sulfided
catalyst were introduced into the batch reactor containing a solu-
tion of DBT in hexadecane with a concentration of 12 mmol/L
The textural properties of the catalysts and supports were deter-
mined from the adsorption–desorption isotherms of nitrogen at
7
7 K, recorded with a Quantachrome Autosorb iQ2 instrument.
◦
Prior to the analysis, the samples were degassed at 200 C in
vacuum (0.2 atm) for 12 h. The volume of the adsorbed N2 was
normalized to the standard temperature and pressure. The specific
surface area of the samples was calculated by applying the BET
method taking into account the nitrogen adsorption data within
the 0.05–0.30 relative pressure range. The pore diameter distribu-
tion was calculated by the BJH method using the desorption branch
of the isotherms. The accumulated pore volume was obtained from
the adsorption–desorption isotherms for a relative pressure (P/Po)
of 0.99.
◦
at room temperature. The reactor was heated up to 320 C in
presence of inert atmosphere with N2 (at an inicial pressure
of 200 psi). Once reaction temperature was raised, nitrogen was
purged out and the reactor was pressurized with hydrogen to
8
00 psi, hydrodesulfurization reaction was carried out at con-
◦
stant temperature (320 C) with intense stirring (700 rpm). These
conditions allow to exclude external diffusion effects and mini-
mize internal diffusion limitations [2,4,24–26]. The reaction time
was averaged to 5 h from the incorporation of hydrogen to the
reaction system. The residual reactants and resulting products
concentrations were analyzed using gas chromatography by the
use of an Agilent Technologies 7990A chromatograph provided
with a 30-m packed column containing 3% OV-17 as separating
phase.
2.6. Small-angle X-ray diffraction
The ordered mesoporous structure of the supports was inves-
◦
tigated by small-angle X-ray diffraction in the 0.5–5 range for
2
ꢀ. Samples were analyzed on a Rigaku Ultima IV diffractometer,
Catalytic performance was measured taking into account two
parameters: the apparent rate constant (related with the rate of
reaction) and selectivity (related with the capability of the cata-
lyst to promote the formation of desiderable products). Apparent
rate constant is a good approximation to the reaction rate in the
sense that it involves the effect of the concentration of reactants
and products, the effect of temperature and the negative effect
using Ni-filtered Cu-K␣ radiation (ꢁ = 1.54 A˚ ), operated at 40 kV and
◦
3
0
0 mA and with a step size of 5 per minute and scanning every
.02 s, with rotation of the specimen at 30 revolutions per minute.
2.7. Wide-angle X-ray diffraction
The presence of crystalline phases of oxides and sulfides of tran-
of the presence of the H S in the batch reactor. Previous stud-
2
sition metals in the catalysts was determined by X-ray diffraction
in the 5–80 range for 2ꢀ; diffractograms for oxide-state catalysts
ies have shown that HDS reactions of simple sulfur compounds
follows a pseudo-first order kinetics [19,22–26], so the apparent
rate constants (kapp) were calculated by this model (Equiation (01),
where rHDS is the reaction rate for the global HDS reaction, kapp is
the apparent rate constant and [DBT] is the DBT concentration along
the time) and assuming pseudo-zero order respect to hydrogen,
under the consideration that an excess of hydrogen was employed,
thus, its partial pressure could be considered as constant during
the overall reaction time. Apparent reaction constants were calcu-
lated using an exponential fit of the DBT concentration data and a
linearization of this fitting according to Equation (02), where [DBT]0
◦
were recorded in a Rigaku Miniflex diffractometer, using Ni-filtered
Cu-K␣ radiation (ꢁ = 1.54 A˚ ), operated at 30 kV and 15 mA, with a
◦
step size of 2 per minute and sampling every 0.02 s.
Those for fresh sulfided catalysts were obtained in a Rigaku
Ultima IV diffractometer, working with Co-filtered Cu-K␣ radia-
◦
tion (ꢁ = 1.54 A˚ ), with a step size of 5 per minute and sampling
every 0.02 s and rotating the specimen at 30 revolutions per minute.
The determination of the crystalline phases and indexing was done
®
using the MDI -Jade V 5.0.37 software.
Please cite this article in press as: M.A. López-Mendoza, et al., Characterization and catalytic performance of Co-Mo-W sulfide catalysts
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represents the initial concentration of DBT and XDBT is the DBT
conversion at overall reaction time.
relative pressures 0.40–0.75, in which the adsorption and desorp-
tion branches of the isotherm are substantially parallel, which is
indicative of the presence of mesopores with regular shape, thus
agreeing with the presence of cylindrical mesoporous channels in
an hexagonal two-dimensional arrangement, typical of the SBA-15.
This type of isotherm has been widely reported in previous studies
of this material [4,20,21,30].
d[DBT]
dt
rHDS,app
=
= kapp[DBT]
(01)
(02)
[
DBT] (1 − XDBT) = kappt
0
HDS of DBT can occur by two parallel pathways: direct desul-
furization (DDS) and hydrogenation (HYD); the first leads to the
formation of biphenyl (BP) via hydrogenolysis, this is, the sul-
fur atom is removed from the molecule by the breaking of the
carbon–sulfur bonds, while the hydrogenation of one or two aro-
matic rings produces cyclohexylbenzene (CHB) and bicyclohexyl
Meanwhile, the SBA-16 shows a H2 Type hysteresis loop in
the range of relative pressures of 0.39–0.65 [28,29], in which the
desorption branch is wider and vertical than that of adsorption,
related with the presence of mesopores with non-uniform shape,
which is consistent with the fact that this material is character-
ized by the presence of mesopores in the form of interconnected
spheres, creating differences between the inlet and the pore diam-
eters; this isotherm has also been widely documented in previous
studies [4,20,21].
The mechanical mixture of mesoporous silicas (MMS) clearly
shows two hysteresis loops, as expected considering that it con-
sists of SBA-15 and SBA-16, the first one in the range of relative
pressures of 0.40–0.55, which can be classified as H2 type accord-
ing to the IUPAC rules [28,29], so that this first hysteresis loop is
mainly related to the SBA-16 present in the mixture; the second
hysteresis occurs in the range of relative pressures between 0.55
and 0.75, and clearly corresponds to an H1 type hysteresis loop
(
BCH), respectively. The ability of the catalyst to promote prefer-
ably one of the two routes is known as selectivity and in this study
was determined as the ratio between the generated quantities of
hydrogenation products over those of direct desulfurization at con-
version of 30% for dibenzothiophene according to the Equations
(
03)–(05), where the factors in brackets are the quantities of the
HDS products.
[
BP] × 100
DDS = [
(03)
(04)
(05)
CHB] + [BCH] + [BP]
{
[CHB] + [BCH]} × 100
HYD = [
CHB] + [BCH] + [BP]
[
28,29] associated with the SBA-15 existent in the sample.
Fig. 2 shows the pore diameter distributions for the supports, in
HYD
Selectivity = DDS
all cases one can see very narrow distributions in which the vari-
ation in the pore size does not exceed 20 A˚ (2 nm). In the case of
SBA-15, the distribution is centered at 58.9 A˚ (5.89 nm), while the
SBA-16 shows a narrower distribution centered at 34.9 A˚ (3.49 nm).
In the same figure it highlights the fact that the mechanical mixture
of the two materials produces a bimodal pore diameter distribu-
tion with maxima at 35.8 A˚ (3.58 nm) and 56.4 A˚ (3.58 nm), which
For comparative purposes, in the reaction experiments were
used equal catalyst mass, with similar particle size (100–125 ␮m)
and with the same metal charge (added during catalysts synthesis);
in addition, the HDS performance of a commercial CoMo/Al O cat-
2
3
alyst was measured as comparative model (Mo = 14.2%; Co = 3.8%;
2
P = 0.83%; SBET = 223 m /g; average pore diameter = 6.6 nm).
correspond with the maxima of the individual distributions for
SBA-15 and SBA-16, moreover they preserve the same relative ratio
of height with respect to the pure silicas.
3
. Results and discussion
3.1. Nitrogen adsorption–desorption studies
The nitrogen adsorption–desorption isotherms and pore diam-
eter distributions for the oxide-state and sulfided catalysts (not
showed) have the same tendency and shape that those for the sup-
ports, being indicative that the porous morphology of the materials
is preserved through the synthesis process; only with low displace-
ments to lower relative pressures related with the decreasing of the
pore diameter associated with the deposition of the precursors and
active phases over the silicas surface onto the pores.
Fig. 1 shows the obtained nitrogen adsorption–desorption
isotherms at 77 K for the synthesized supports, it can be seen
that all the supports show nitrogen adsorption–desorption Type-
IV isotherms, according to the classification of the IUPAC [27–29],
which are characteristic of mesoporous materials, with a hystere-
sis loop due to capillary condensation and evaporation within the
mesopores. In the case of the SBA-15 material, it shows a type H1
hysteresis loop (according to IUPAC rules, [28,29]) in the range of
Fig. 1. Nitrogen adsorption–desorption isotherms for synthetized supports (MMS
Fig. 2. Pore diameter distribution for synthetized supports (MMS – mechanical
–
mechanical mixture of mesoporous silicas).
mixture of mesoporous silicas).
Please cite this article in press as: M.A. López-Mendoza, et al., Characterization and catalytic performance of Co-Mo-W sulfide catalysts
supported on SBA-15 and SBA-16 mechanically mixed, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.07.049

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Table 1
Textural properties for the synthesized catalysts.
2
3
Surface area (m /g)
Pore diameter (nm)
Pore volume (cm /g)
Support
Oxide state
Sulfided
Support
Oxide state
Sulfided
Support
Oxide state
Sulfided
S15ICS
S16ICS
MMICS
IMMCS
ICMMS
819
462
771
447
342
355
402
386
235
106
116
169
105
4.788
3.413
4.126
3.415
3.409
3.404
3.055
0.897
0.275
0.504
0.464
0.134
0.256
0.309
0.277
0.327
0.040
0.182
0.256
0.154
*
*
*
3.411
3.402
*
*
3.422
3.400
*
*
3.419
3.406
*
It refers an average pore diameter for the statistical distribution, it must be remained that the distribution is bimodal and has two media (S15ICS: sulfided catalyst supported
in SBA-15, S16ICS: sulfided catalyst supported in SBA-16, MMICS: sulfided catalyst supported on mixed silicas following the route mechanical mixture – impregnation –
calcination – sulfidation, IMMCS: sulfided catalyst supported on mixed silicas following the route impregnation – mechanical mixture – calcination – sulfidation, ICMMS:
sulfided catalyst supported on mixed silicas following the route impregnation – calcination – mechanical mixture – sulfidation).
The textural properties of the synthesized catalysts are sum-
marized in the Table 1, in the table the samples are referred to
the sulfided catalysts and the Support and Oxide-State columns
refer to the properties of the materials previous to sulfidation; it is
possible to observe that the pure SBA materials have large surface
areas suitable for their use as catalyst supports, while the samples
prepared by mechanical mixing of these materials provide interme-
diate values for the area, volume and pore diameter, as expected;
however, the effect on the values of these parameters appears not
to be linearly related to the amount of each material in the mixture
(1 1 0), (2 1 1) and (2 2 0) present in an Im3m cubic structure, which
have also already extensively characterized in the literature [20,31].
Meanwhile, the mixture of silicas made mechanically shows a
very strong peak corresponding to the reflection (1 0 0) of SBA-15,
a shoulder corresponding to the (1 1 0) reflection of the SBA-16,
which are accompanied by other peaks related with both meso-
porous phases, confirming the incorporation of both materials in
the mixture.
3.3. Wide-angle X-ray diffraction
(
50–50%). In the same Table it can be appreciated a drastic reduction
in surface area after impregnation and calcination steps (Oxide-
State column), between 50 and 60%, related with the deposition
and formation of the oxide precursors of the active phases; and after
sulfidation the reduction is almost 80–90% respect to the original
support surface area, which is indicative of the high dispersion of
the active phases on the support and the possible formation of clus-
ters of oxides of transition metals which occlude smaller pores; this
effect is greater in the catalysts supported in pure SBA-16 (S16ICS)
because of its smaller pores dimension.
Wide-angle diffractograms obtained for mesoporous silica sup-
ports show, in all the cases, a very broad peak centered around
◦
2
4 (in a 2ꢀ scale) corresponding to amorphous silica, which has
been widely reported in numerous previous studies [4,6,20], this
peak still appears in the oxide state and fresh sulfided catalysts
as showed in the Figs. 4 and 5. Fig. 4 shows diffractograms for
the oxide-state supported catalysts, in all of them the presence of
low intensity peaks at angles 2ꢀ of 19.5, 25, 28, 32.5, 38, 43, 28,
+
X
*+ +*
*
3
.2. Small-angle X-ray diffraction
+
O
O
+
X
*++
+
O
+O
+
* *
X-ray diffractograms obtained at small angles for the meso-
*
porous silica supports are shown in Fig. 3. The presence of defined
peaks in all samples is evidence of the existence of well-structured
mesoporous phases. It can be seen in the Fig. 3 that the SBA-15
shows peaks corresponding to the reflections (1 0 0), (1 1 0) and
ICMM
IMMC
X
(
2 0 0) of a 2D hexagonal array with space group P6mm, themselves
already have been widely reported for SBA-15 [20,30,31]. Mean-
while, SBA-16 also shows the peaks corresponding to the reflections
MMIC
S16IC
S15IC
2
θ [°]
5
10 15 20 25 30 35 40 45 50 55 60 65 70 75 80
Fig. 4. Wide-angle X-ray diffractograms for oxide-state catalysts and the main iden-
tified crystalline phases: o – MoO3, x – WO3, + – CoMo(W)O4, * – MoxW1−xO3, (S15IC:
oxide-state catalyst supported on SBA-15, S16IC: oxide-state catalyst supported on
SBA-16, MMIC: oxide-state catalyst supported on mixed silicas following the route
mechanical-mixture – impregnation – calcination, IMMC: oxide-state catalyst sup-
ported on mixed silicas following the route Impregnation – mechanical mixture –
calcination, ICMM: oxide-state catalyst supported on mixed silicas following the
route impregnation – calcination – mechanical mixture).
Fig. 3. Small-angle X-ray diffractograms for synthesized silica supports, for each of
them is showed an amplification of the difractogram in the range of 1–3 for 2ꢀ
values (MMS – mechanical mixture of mesoporous silicas).
◦
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◦
5
(
7 and 59.5 has been identified, corresponding to ␤-CoMo(W)O4
sizes for the Mo(W)S2 were determined as 28–31 A˚ for all the
samples with the PDLX software Version 2.3.1.0 by Rigaku , using
®
JCPDS-ICDD 21-0868, 15-0867), additionally with peaks corre-
sponding to the isolated species MoO (JCPDS-ICDD-76-1003) and
the Williamson–Hall method.
3
WO3 (JCPDS-ICDD-852460), which occurrence has already been
previously reported in many studies of hydrodesulfurization cata-
lysts containing cobalt, molybdenum and tungsten and which have
been documented as precursor phases of active sites for cataly-
sis [2–5,32]. Very low intensity peaks (not shown in the figure)
has been recognized relative to the presence of oxide species of
3
.4. Scanning electron microscopy
Morphology of the synthesized materials has been studied by
scanning electron microscopy. SBA-15 and catalysts supported
over it show the typical morphology associated with this mate-
rial consisting of particles in the form of rollers or bars of about
molybdenum and tungsten with formula MoxW1 O (JCPDS-ICDD
− x
3
2
8-0668) as well as polymolybdates and polytungstates corre-
1
[
␮m aggregated in long tubular clusters of several microns long
20,31,33]. SBA-16 and the catalyst supported over it present typ-
ical morphology [21,34] comprising spherical particles of about
sponding to compounds containing large number of atoms of
molybdenum and tungsten, in which the metal/oxygen ratio is
between 2.75 and 2.9, as in the case of compounds such as Mo O11
,
4
0
5
.5 ␮m aggregated in clusters with dimension between 10 and
0 ␮m; that morphology is retained during all the synthesis pro-
Mo17O47, Mo O26, W24O68, W17O47, W20O58, W19O55, which have
9
coordination structures similar to those of Mo(W)O . The low
3
cess and in the case of SBA-16-supported catalysts it can be seen
the presence of sintered particles generated during the calcination
of the material in the different steps of the catalysts preparation.
The corresponding images for the sulfided catalyst can be seen in
the Fig. 6A and B.
intensity of peaks in all samples is indicative that most species
are widely dispersed on the surface of the supports and the pres-
ence of not well-defined peaks is due to supported species that are
amorphous or have crystallite sizes below the detection limit of the
technique (<4 nm).
Meanwhile, the catalysts supported on mixtures of silicas
made mechanically show the same morphology of the pure sup-
ports, therein it can be seen that the incorporation of the two
phases is only physical and the subsequent steps of the prepa-
ration do not cause significant changes in the morphology and
average size of the materials particles. Small differences in the
dispersion and incorporation of the SBA-15 and SBA-16 in the
mixtures derived from the different sequences used for make the
mixture can be seen, IMMCS sample (Impregnated + Mechanically
Mixed + Calcined + Sulfided, Fig. 6D) has showed a greater disper-
sion and incorporation of the support phases, resulting in a better
preservation of the morphological properties of the initial mate-
rials and a minor sintering effect of the SBA-16 particles. MMICS
sample (Mechanically Mixed + Impregnated + Calcined + Sulfided,
Fig. 6C) showed little differences in morphology compared with
the catalysts supported in the pure silica materials. ICMMS catalyst
Fresh sulfided catalysts diffractograms are shown in Fig. 5;
in all of them characteristic peaks corresponding to disulfides
of molybdenum and tungsten (MoS , JCPDS-ICDD 75-1539; WS ,
2
2
JCPDS-ICDD 08-0237) have been identified, those with hexagonal
structure which have been extensively documented as catalytic
species in hydrodesulfurization [2–5,32]; it shows differences in
the strength of the signals due to the degree of dispersion of the
sulfides in the supports surface. The presence of broad peaks in
all samples is indicative of the presence of crystalline domains
of different sizes supported on mesoporous silicas. Additionally,
characteristic peaks have been recognized corresponding to cobalt
sulfided species in the form of Co9S8 (JCPDS-ICDD 02-1459) and
Co1.62Mo6S8 (JCPDS-ICDD 30-0450), which has also been docu-
mented as active species in the study of hydrodesulfurization
catalysts containing molybdenum and cobalt [32], these peaks have
low intensity and they suggest the formation of sulfided molyb-
denum and tungsten species doped with cobalt. The crystallite
(Impregnated + Calcined + Mechanically Mixed + Sulfided, Fig. 6E)
has shown the presence of greater quantity of SBA-16 sintered
particles than the other samples and a greater size of the sintered
domains, between 10 and 100 ␮m.
*
+
o
ox
+
+
+
+
o o
x
x
o
o
3.5. Chemical analysis
x
*
Chemical analysis of the catalysts was made by energy dis-
persive spectroscopy (EDS). Spectra for oxide-state catalyst (not
presented) have shown the presence of characteristic signals cor-
responding to the electronic transitions of the expected elements
in the samples (O, Si, Mo, Co and W); the absence of the signal
corresponding to carbon is indicative of complete removal of the
polymeric surfactant used as directing agent for the synthesis of
mesostructured materials. Fresh sulfided catalysts spectra show
the same signals, in addition to those corresponding to sulfur elec-
tronic transitions.
By this technique, semiquantitative elemental composition of
the samples can be determined which is reported in Table 2. It can
be noted that the values determined experimentally are close to
those established theoretically (from the calculated atomic ratios
proposed in the synthesis), which is evidence of the homogeneity
in the distribution of the metals transition in the samples, and the
effectiveness of the impregnation method used for the synthesis of
catalysts with homogeneous distribution of the active species. In
this table it can be also observed that the chemical composition is
similar for all the catalysts and the differences between samples are
into the range of the experimental error of the analytical technique
so it can be assumed a similar metal charge in all the catalysts.
ICMMS
IMMCS
MMICS
S16ICS
S15ICS
2θ
[°]
1
0
20
30
40
50
60
70
80
Fig. 5. Wide-angle X-ray diffractograms for fresh sulfided catalysts and the crys-
talline phases identified o – MoS2, x – WS2, + – Co9 S8, * – Co1.62Mo6S8, (S15ICS:
sulfided catalyst supported on SBA-15, S16ICS: sulfided catalyst supported on
SBA-16, MMICS: sulfided catalyst supported on mixed silicas following the route
mechanical-mixture – impregnation – calcination – sulfidation, IMMCS: sulfided
catalyst supported on mixed silicas following the route impregnation – mechanical
mixture – calcination – sulfidation, ICMMS: sulfided catalyst supported on mixed
silicas following the route impregnation – calcination – mechanical mixture – sul-
fidation).
Please cite this article in press as: M.A. López-Mendoza, et al., Characterization and catalytic performance of Co-Mo-W sulfide catalysts
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Fig. 6. Scanning electron microscopy images obtained for the fresh sulfided catalysts (A-S15ICS: sulfided catalyst supported on SBA-15, B-S16ICS: sulfided catalyst supported
on SBA-16, C-MMICS: sulfided catalyst supported on mixed silicas following the route mechanical-mixture – impregnation – calcination – sulfidation, D-IMMCS: sulfided
catalyst supported on mixed silicas following the route impregnation – mechanical mixture – calcination – sulfidation, E-ICMMS: sulfided catalyst supported on mixed silicas
following the route impregnation – calcination – mechanical mixture – sulfidation), in all the cases it can be seen preservation of the morphological properties along the
synthesis process and sintering effects in the SBA-16 domains.
Fig. 7 reproduces the elemental mapping images obtained
for the most active sulfided catalyst (IMMCS: Impreg-
nated + Mechanically Mixed + Calcined + Sulfided), similar results
have been obtained for all the samples. It can be observed a
homogeneous distribution of the transition metals and sulfur over
the sample surface, which is indicative of a high dispersion of
active phases on the surface of the catalysts. In such maps, also
is evident the similarity of the distribution obtained for molyb-
denum and sulfur, suggesting that used sulfurization process for
the catalysts synthesis allows the complete transformation of the
oxides deposited on the support and a high dispersion of disulfide
species.
(Impregnated + Mechanically Mixed + Calcined + Sulfided) catalyst
is showed, in which the contrast is associated with the atomic
number of the atoms in the sample, such that the more brilliant
zones indicate the presence of elements with higher atomic num-
ber. Thus, the support mesoporous structure is observed as well as
the distribution of the active phases onto the pore channels forming
clusters of nanoparticles, observed as brilliant dots in the image. In
other words, this Z-contrast image reveals that the active phases
Mo(W)S are well dispersed over the support, both on the surface
2
as well as inside the pores. This is consistent with EDS and textural
results, and explains the reduction of pore diameter and surface
area reported in nitrogen adsorption–desorption studies.
Fig. 9A shows an HRTEM micrograph of fresh sulfided IMMCS
catalyst. The presence of two crystalline structures is observed,
corresponding to MoS2 and Co9S8 as identified in Fig. 9A and B,
respectively, which are segregated from a continuous crystalline
phase suggesting the presence of Mo(W)S2 phases doped with
cobalt which have been documented as active sites responsibles
of catalysis for hydrodesulfurization [35,36,63]. An interplanar
3
.6. High-resolution transmission electron microscopy
High-resolution transmission electron microscopy (HRTEM)
micrographs of supports are showed in Fig. 8. Concerning the sup-
port morphology, HRTEM images confirmed the regular porous
arrangement of the SBA-15- and SBA-16-supported catalysts.
Fig. 8A shows mesoporous silicas mechanically mixed before
impregnation. It can be observed the presence of the typical chan-
nels of the SBA-15 hexagonal structure as well as a zone with the
characteristic cubic Im3m structure of the SBA-16 along the [1 0 0]
direction. In Fig. 8B a Z-contrast image of fresh sulfided IMMCS
distance d0 0 4 of 3.13
A˚ , corresponding to (0 0 4) planes of molyb-
denum disulfide structure, was determined in Fig. 9B. Even if MoS2
fringes appear regularly stacked, their organization is relatively
disordered. Fig. 9C was obtained by performing a Fast Fourier
Transform (FFT) to the square marked in Fig. 9A. Bragg diffraction
Table 2
Elemental composition of the oxide-state catalysts obtained by EDS.
Sample*
O
Si
Co
Mo
W
Co/(Mo + W)
Mo/(Mo + W)
W/(Mo + W)
Expected
S15IC
S16IC
MMIC
IMMC
ICMM
67.22
71.13
69.77
70.07
70.76
70.77
28.81
24.54
26.32
25.22
24.88
25.21
1.19
1.12
1.23
1.14
1.11
1.12
1.39
2.07
1.44
1.66
1.58
1.38
1.39
1.16
1.26
1.92
1.74
1.54
0.43
0.35
0.46
0.32
0.33
0.38
0.5
0.5
0.64
0.53
0.46
0.48
0.47
0.36
0.47
0.54
0.52
0.53
*
S15IC: oxide-state catalyst supported on SBA-15, S16IC: oxide-state catalyst supported on SBA-16, MMIC: oxide-state catalyst supported on mixed silicas following the
route mechanical-mixture – impregnation – calcination, IMMC: oxide-state catalyst supported on mixed silicas following the route impregnation – mechanical mixture –
calcination, ICMM: oxide-state catalyst supported on mixed silicas following the route impregnation – calcination – mechanical mixture.
Please cite this article in press as: M.A. López-Mendoza, et al., Characterization and catalytic performance of Co-Mo-W sulfide catalysts
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Fig. 7. Elemental distribution obtained by EDS for the IMMCS catalyst (Impregnated + Mechanically Mixed + Calcined + Sulfided).
reflections, corresponding to (1 1 1), (2 0 0) and (2 2 0) planes of
3.7. DRS-UV–vis spectroscopy
Co9S8 structure were identified. These results are in agreement
with those obtained in the XRD analysis, and the presence of these
two phases is important for the catalytic activity [35,36], as will be
discussed later.
The diffuse reflectance spectra in the UV–visible range for the
oxide state catalysts are presented in Fig. 10 accompanied with the
corresponding deconvolution peaks obtained with a mathematical
Fig. 8. HRTEM micrographs of mesoporous silicas mechanically mixed before impregnation (A) and Z-contrast image of fresh sulfided IMMCS catalyst (Impregnated –
mechanically mixed – calcined – sulfided) (B).
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Fig. 9. HRTEM image of IMMCS catalyst [Impregnated + Mechanically Mixed + Calcined + Sulfided] (A), zoom on typical MoS2 slabs (B) and FFT of the square marked indicating
the Bragg reflections of (1 1 1), (2 0 0) and (2 2 0) of planes of Co9S8 (C).
software using Gaussian functions. In all the spectra a very intense
band between 200 and 350 nm has been identified which deconvo-
lution denotes the presence of three closer bands: a band between
as Co SiO , this suggests the presence of small amounts of these
2 4
species supported on the catalysts.
2
20 and 240 nm corresponding to the ligant-metal charge trans-
3.8. Micro Raman spectroscopy
6+
6+
fer related to the presence of Mo and W ions in tetrahedral
coordination like in WO42 and MoO4
presence on silica supported catalysts has already been reported
4–6,37]; a second band at 290–300 nm has been attributed by
−
2−
isolated species, whose
Micro Raman spectra obtained for the oxide-state catalysts are
shown in Fig. 11; it can be appreciated that all of them show a
−
1
[
very intense band in the range of 900–1000 cm , whose posi-
tion and intensity is indicative of the presence of several species
of tungsten and molybdenum with different symmetries. Decon-
volution of this band was done using a mathematical software and
based on Gaussian functions (not shown), in the deconvolution a
Jeziorowski et al. [38,39] to ligant-metal charge transfer transitions
in Mo–O–Mo bonds present in octahedral polymolybdates, while
that at 320–340 nm has been reported as the corresponding to the
2−
6+
6+
ligant-metal charge transfer transition from the O to Mo (W
)
−
1
in octahedral coordination compounds such as polymolybdates and
polytungstates [4–6,40,41]; both types of compounds were iden-
tified in the analysis of wide-X-ray diffraction, so these results are
consistent with the fact that the structure of the species supported
on environmental conditions is governed by the acid–base inter-
actions between transition metals and the support, so that the
silica having an acidic surface tends to form species which are
strong peak at 940–960 cm can be seen and it is attributed to
the symmetric stretching vibration of the terminal bond Mo(W) O
in various types of polymolybdates and polytungstates with octa-
hedral coordination of the metal, whose intensity is enriched
by the contribution of the Si–O stretching of the silanol groups
present in silica which appears in the same range. Additionally,
−
1
it is present a very low intense band at 980–985 cm
low intensity band near to 860 cm
and a
4−
6−
−1
stable under acidic conditions, such as Mo7O26
32,38,42–44].
The presence of broad bands indicates that the metal oxides
and Mo O
which have been reported
8
24
[
as corresponding to the O–Mo–Mo–O stretching vibrations of
distorted polymolybdates [41,48,49]. Polytungstates presence in
samples is confirmed by the appearance of a low intensity bands
are present in aggregates of different sizes. In all samples a
similar amount of octahedral molybdenum and tungsten species
is observed, except for the IMMC (Impregnated + Mechanically
Mixed + Calcined) which shows a higher band corresponding to
these species, similar results were obtained for the content of
species in tetrahedral coordination, which shows that in this cata-
lyst metal-support interactions are more significant.
−
1
−1
at 510 cm
and 200–300 cm corresponding to stretching and
angular deformation of the W–O–W bonds, respectively [40,50,51].
Dioxo compounds of molybdenum and tungsten are presented as
isolated species with tetrahedral coordination which is evidenced
−
1
by the appearance of an intense band at 970–975 cm
corre-
sponding to the asymmetric stretching of the O Mo(W) O bonds
and a contribution to the band at 990–995 related to the sym-
metric stretching of O Mo(W) O bonds [40,50]. The presence of
Concerning the band at 500–520 nm, it has been previously
2+
reported as corresponding to charge transitions in Co complexes
with octahedral coordination [45], and the band observed between
−
1
an intense band between 935 and 945 cm may be ascribed as
the W O symmetrical stretching related with tungsten in tetrahe-
5
65 and 580 nm has been attributed in previous studies to the d–d
dral position such as in WO42 ion, which is presented in isolated
−
electronic transitions (4T2g to 4A2g and 4T2g to 4T1g) in octahedral
high spin cobalt complexes present in the ␤-CoMoO , in which the
species of CoWO . The presence of this compound in the cata-
4
4
cobalt is interacting with molybdenum and whose presence already
been identified by X-ray diffraction; this octahedral ion has been
observed to be easily sulfided to generate active species for catalytic
hydrodesulfurization reactions [4–6,46]; however, in this range the
occurrence of charge transfer transitions for Co species in tetra-
hedral coordination also has been reported [47], that is indicative
of the presence of cobalt ions interacting directly with the support
lysts is confirmed by the appearance of a low intensity band at
−
1
730–740 cm
attributed to the O–W–O asymmetrical vibration
[37,40]. The displacement of this band is indicative of distor-
tion in the tetrahedral structure of the complex. The presence of
molybdenum species isolated in tetrahedral coordination, as ␤-
2+
CoMoO , is assumed by the presence in all samples of the bands at
4
−
1
890–900 cm corresponding to the Mo–Co–O stretching vibration,
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0
Fig. 10. Diffuse reflectance UV–vis spectra for the oxide-state catalysts and their deconvolution in Gaussian functions (S15IC: oxide-state catalyst supported on SBA-15, S16IC:
oxide-state catalyst supported on SBA-16, MMIC: oxide-state catalyst supported on mixed silicas following the route mechanical-mixture – impregnation – calcination, IMMC:
oxide-state catalyst supported on mixed silicas following the route impregnation – mechanical mixture – calcination, ICMM: oxide-state catalyst supported on mixed silicas
following the route impregnation – calcination – mechanical mixture).
−
1
−1
−1
at 830–840 cm associated with Mo–O asymmetric stretching and
observed in all spectra at 715 cm and 435 cm associated with
−
1
−1
at 317 cm related to the O–Mo–O angular deformation [52].
WO . Most of the samples exhibit bands at 990, 970 and 910 cm
which appear as masked signals on the main band, additionally
to low intensity bands at 635, 252 and 220 cm , which have
been previously reported as corresponding to silico-molybdic anion
,
3
−
1
The presence of bands at 990–995 and 815–820 cm , in addi-
tion to low intensity bands at 708, 666, 417, 377, 338, 290, 248, 217,
−
1
−
98 and 160 cm are signals of the presence of MoO supported on
3
1
1
4−
silica as has been reported previously [41]; low intensity bands are
[SiMo12O40
]
[53]. It can be seen a very low intensity band around
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Fig. 11. Micro Raman spectra for the oxide-state catalysts functions (S15IC: oxide-
state catalyst supported on SBA-15, S16IC: oxide-state catalyst supported on
SBA-16, MMIC: oxide-state catalyst supported on mixed silicas following the route
mechanical-mixture – impregnation – calcination, IMMC: oxide-state catalyst sup-
ported on mixed silicas following the route impregnation – mechanical mixture –
calcination, ICMM: oxide-state catalyst supported on mixed silicas following the
route impregnation – calcination – mechanical mixture).
Fig. 12. Conversion profile for the hydrodesulfurization reaction of DBT for the syn-
thetized catalysts and the commercial one used as comparative model (S15ICS:
sulfided catalyst supported on SBA-15, S16ICS: sulfided catalyst supported on
SBA-16, MMICS: sulfided catalyst supported on mixed silicas following the route
mechanical-mixture – impregnation – calcination – sulfidation, IMMCS: sulfided
catalyst supported on mixed silicas following the route impregnation – mechanical
mixture – calcination – sulfidation, ICMMS: sulfided catalyst supported on mixed
silicas following the route impregnation – calcination – mechanical mixture – sul-
fidation).
−
1
1
020 cm associated with the stretching vibration of the Mo O
bond in mono-oxided molybdenum species attached directly to the
support [41].
according to the pseudo-first order model proposed. The graph
shows that before 120 min of reaction, the conversion profile has a
linear behavior for all the catalysts as expected from the pseudo-
first order model proposed suggesting that in this step the rate is
controlled only for the kinetics of the reaction [19,22–25,55], after
that time they become important the diffusive and mass trans-
fer effects in the reaction system and the presence of collateral
reversible reactions. In this figure it can be seen that two of the
catalysts supported on silicas mechanical mixtures, the IMMCS and
MMICS, show a greater degree of conversion of dibenzothiophene
throughout the reaction time over those supported on pure SBA-
Table
3 shows the values calculated for the ratio of
metal–oxygen terminal bonds [Mo(W) O] and metal–oxygen bulk
bonds [Mo(W)–O–Mo(W)], obtained from the deconvolution peak
−
1
−1
areas of the bands at 950–960 cm and 980–985 cm ; the value
of this ratio is very important in catalytic hydrodesulfurization
reactions since previous studies have shown the influence of the
presence of terminal metal–oxygen bonds in the formation of cat-
alytic sites [54]. In the table it can be seen that the values of this ratio
are higher for catalysts supported on two of the mechanical mixture
of silicas, coinciding with the results for catalytic activity (mea-
sured as apparent reaction rate constant value and discussed in the
next section), showing that there is a close correlation between the
presence of terminal bonds and catalytic activity.
1
5 and SBA-16 and even the commercial catalyst used as reference,
while the catalyst ICMMS has a lower catalytic behavior.
The values obtained for the apparent rate constant for the tested
catalysts are shown in Table 4, these values were obtained from
the time and conversion data using a linear regression in which the
3.9. Catalytic performance
2
regression coefficient (R ) took values between 0.982 and 0.999
The catalytic activity of the synthesized materials in the hydro-
for the different samples and the standard error for the values of
apparent rate constants were between 3 and 5%. The fact that an
equal mass of catalyst was used for every experiment and that
all of them have similar metal charge (as determined by the EDS
results) allows to take this parameter as comparative element for
the catalytic activity of the samples; it can be seen that two of
the catalysts supported on mixtures of mesoporous silica (IMMCS
and MMICS) exhibit the highest activity in the series of synthe-
sized catalysts, in which the difference between the values is higher
that the experimental error (5%), indicating that the use of mix-
tures of mesoporous silica as support material is advantageous for
the preparation of catalysts based on metal transition sulfides for
hydrodesulfurization, in addition to the fact that the sequence of
preparation (mixing impregnation-calcination) has an effect on the
catalytic properties of the synthesized material.
The observed behavior is related to a higher density of sulfides
of molybdenum and tungsten formed on the surface of the cata-
lyst in the more active catalysts, as established by the results of
X-ray diffraction, in which corresponding peaks are more intense
for samples with higher catalytic activity and this is confirmed
with the diffuse reflectance spectroscopy results, in which the
more active catalysts contain a greater quantity of tetrahedral and
desulfurization reaction (HDS) of dibenzothiophene (DBT) was
measured using the apparent rate constant calculated by the
method described in the Experimental section. In Fig. 12 the diben-
zothiophene conversion profiles are shown, as was experimentally
determined; it can be seen that all catalysts follow an exponen-
tial behavior, as is expected for this type of chemical reactions
Table 3
Relation between terminal and bulk metal–oxygen bonds for the oxide-state
catalysts.
Catalyst*
Mo(W) O/O-Mo(W)-O ratio
S15IC
S16IC
MMIC
IMMC
ICMM
1.13
2.77
4.37
6.66
0.79
*
(
S15IC: oxide-state catalyst supported on SBA-15, S16IC: oxide-state catalyst
supported on SBA-16, MMIC: oxide-state catalyst supported on mixed silicas follow-
ing the route mechanical-mixture – impregnation – calcination, IMMC: oxide-state
catalyst supported on mixed silicas following the route impregnation – mechan-
ical mixture – calcination, ICMM: oxide-state catalyst supported on mixed silicas
following the route impregnation – calcination – mechanical mixture).
Please cite this article in press as: M.A. López-Mendoza, et al., Characterization and catalytic performance of Co-Mo-W sulfide catalysts
supported on SBA-15 and SBA-16 mechanically mixed, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.07.049

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1
2
Table 4
Catalytic apparent constant and selectivity values (determined at 30% conversion) for the synthesized catalysts.
Catalyst*
kapp [mol/gcat s] (± 5% std error)
BF [mol]**
CHB [mol]**
BCH [mol]**
DSD [mol]**
HYD [mol]**
HYD/DDS Ratio (Selectivity)
−07
−07
−07
−07
−07
−7
S15ICS
S16ICS
MMICS
IMMCS
ICMMS
COMMERCIAL
6.7 × 10
8.1 × 10
8.2 × 10
9.2 × 10
3.4 × 10
4.7 × 10
109.79
114.66
110.70
117.80
117.93
138.00
14.54
9.83
13.34
13.25
11.54
4.00
0.00
6.21
0.96
0.00
3.32
48.00
109.79
114.66
110.70
117.80
117.93
138.00
14.54
16.03
14.30
13.25
14.86
52.00
0.13
0.14
0.13
0.11
0.13
0.38
*
(
S15ICS: sulfided catalyst supported on SBA-15, S16ICS: sulfided catalyst supported on SBA-16, MMICS: sulfided catalyst supported on mixed silicas following the route
mechanical-mixture – impregnation – calcination - sulfidation, IMMCS: sulfided catalyst supported on mixed silicas following the route impregnation – mechanical mixture
calcination – sulfidation, ICMMS: sulfided catalyst supported on mixed silicas following the route impregnation – calcination – mechanical mixture – sulfidation).
–
*
*
(
BF: biphenyl, CHB: cyclohexylbenzene, BCH: bicyclohexyl, DDS: direct desulfuration products, HYD: hydrogenation products).
octahedral molybdenum and tungsten species (derived from the
higher area for both deconvoluted spectral bands), being known
that an increase in octahedral species of these metals improves cat-
alytic activity. It has also found in all samples the presence of oxide
precursors with cobalt ions with octahedral coordination, which
are beneficial in the formation of active sites for catalysis of reac-
tions of hydrodesulfurization [54,56–62]. Additionally, the results
of Micro Raman spectroscopy indicates the presence of molybde-
num and tungsten species in octahedral coordination in all samples,
showing a greater intensity of the bands related with this species
the samples prepared by mechanical mixing of pure silicas, which
is related with the fact that these catalysts contain the highest
number of Mo(W) O terminal bonds, which has been reported as
responsible of the catalytic activity in the hydrodesulfurization of
dibenzothiophene [54].
On the other hand, it is known that the sulfidation of
CoMo(W)O4 results in the segregation of crystals of Co9S8 and
curved structures of Mo(W)S2 doped with cobalt, so that the cat-
alytic activity is the result of a “joint effect” derived from the
presence of metal centers with octahedral and tetrahedral coor-
dination, associated with particular arrangements of the active
constituents [35,36,63], which are very important for adsorption,
decomposition and chemical reaction of dibenzothiophene in sur-
faces of the catalysts [57,59,61–63], so that the most active catalysts
are those that show higher concentration of both phases and were
identified in the X-ray diffraction, HRTEM and spectroscopic anal-
yses.
Fig. 13. Comparison between the ratio of metal–oxygen terminal bonds and cat-
alytic performance. (S15ICS: sulfided catalyst supported on SBA-15, S16ICS: sulfided
catalyst supported on SBA-16, MMICS: sulfided catalyst supported on mixed silicas
following the route mechanical-mixture – impregnation – calcination – sulfida-
tion, IMMCS: sulfided catalyst supported on mixed silicas following the route
impregnation – mechanical mixture – calcination – sulfidation, ICMMS: sulfided
catalyst supported on mixed silicas following the route impregnation – Calcination
–
mechanical mixture – sulfidation).
The other important parameter studied for the catalytic perfor-
mance was the selectivity; HDS of DBT can occur by two parallel
pathways: direct desulfurization (DDS) and hydrogenation (HYD);
the first leads to the formation of biphenyl (BP) via hydrogenolysis,
while the hydrogenation of one or two aromatic rings produces
cyclohexylbenzene (CHB) and bicyclohexyl (BCH), respectively.
Selectivity results for synthesized catalysts are shown in Table 4.
It can be seen that all synthesized catalysts give low selectivity
values, as have seen in previous studies of catalysts supported
in mesoporous silicas [2–6,24,35,56,63], which implies that are
highly selective to the direct desulfurization route, unlike the com-
mercial catalyst which promotes equality both routes. Particularly,
the catalysts supported on mesoporous silica mechanical mixtures
give lower values with respect to those supported on pure SBA-
(
DA), followed by a direct desulfurization (DDS), which has been
previously reported [64]. Although differences in HYD/DDS ratio
values between the synthesized catalysts are small, they are above
the experimental error in measured concentrations (2–3%) and it
allows to assume that the difference between the samples is signif-
icant.
Fig. 13 shows a comparison between the ratio of metal–oxygen
terminal bonds and catalytic performance. It can be observed that
with increasing concentration of Mo(W) O terminal bonds, the cat-
alytic activity increases, which can be explained because there is a
better sulfidation of the oxides and an increase in edge sites.
1
5 and SBA-16, indicating that the use of mixtures with different
4. Conclusions
mesostructure as support tends to increase the selectivity of the
catalyst towards direct desulfurization route. This depletion in the
hydrogenation route of DBT is related to a confinement of the sul-
fided active species inside the mesopores of SBA materials which
hinders the adsorption of the DBT in an adequate position lead-
ing to an enhancement of the DDS selectivity [2,63]. Moreover, one
of the possible routes to remove sulfur atoms from more complex
molecules, such as 4,6,DMDBT, requires catalysts displaying selec-
tivity towards hydrogenation, however, can also be the case that the
route of elimination of sulfur can occur through one dealkylation
The use of mesoporous silicas mechanically mixed as support
for catalytic materials show advantages over the use of such mate-
rials separately in the preparation of trimetallic catalysts based
on transition metal sulphides. This improvement is attributed to
the presence of different mesoporous structures, which has a pos-
itive effect over the diffusion processes of the precursors of the
active phases on the support, which promotes a higher formation
of desirable precursor phases, as well as due to the fact that meso-
porous silicas have suitable pore dimensions for the diffusion of
Please cite this article in press as: M.A. López-Mendoza, et al., Characterization and catalytic performance of Co-Mo-W sulfide catalysts
supported on SBA-15 and SBA-16 mechanically mixed, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.07.049

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M.A. López-Mendoza et al. / Catalysis Today xxx (2015) xxx–xxx
13
the reactants and the products which inhibits the external mass
transfer limitations; the whole result is a higher catalytic activity
for the HDS reactions of dibenzotiophene, even more than that of
the commercial catalyst used as comparative model. It has seen
that the improvement in activity is also related with the stage of
the synthesis process in which the mixture is done, this suggest
that the higher activity result from different levels of electronic and
chemical interaction between precursors and the support material.
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Please cite this article in press as: M.A. López-Mendoza, et al., Characterization and catalytic performance of Co-Mo-W sulfide catalysts
supported on SBA-15 and SBA-16 mechanically mixed, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.07.049