Effect of the amount of citric acid used in the preparation of NiMo/SBA-15 catalysts on their performance in HDS of dibenzothiophene-type compounds

Article abstract of DOI:10.1016/j.cattod.2013.06.002

In the present work, NiMo catalysts supported on SBA-15 were prepared with the addition of different amounts of citric acid (CA) in the impregnation solutions. The aim of this study was to inquire into the effect of the amount of citric acid on the activity and selectivity of the NiMo/SBA-15 catalysts in deep hydrodesulfurization (HDS). Catalysts were prepared by coimpregnation of Ni and Mo species from acidic aqueous solutions containing citric acid without further adjusting the solution's pH. The amount of citric acid used in the catalyst preparation was varied from CA:Mo molar ratio 0.5 to 2.0. In addition, a reference NiMo/SBA-15 catalyst was prepared without citric acid. After the impregnation, catalysts were dried (100 C, 6 h) and calcined (500 C, 4 h). The prepared catalysts were characterized by nitrogen physisorption, small-angle and powder X-ray diffraction (XRD), UV-vis diffuse reflectance spectroscopy (DRS), temperature-programmed reduction (TPR), high resolution transmission electron microscopy (HRTEM) and tested in simultaneous HDS of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) in a batch reactor at 300 C for 8 h. XRD, DRS and TPR characterizations showed that Ni and Mo oxide species were well dispersed in all catalysts prepared with CA. In contrast, a NiMoO₄ crystalline phase was detected by XRD in the reference NiMo/SBA-15 catalyst prepared without citric acid. Addition of citric acid to the impregnation solutions used for the catalyst preparation also resulted in an increase in the degree of sulfidation and in the dispersion of catalytically active MoS ₂ phase (elemental analysis, HRTEM). In accordance with this, HDS activity of the NiMo catalysts prepared with the addition of citric acid resulted to be significantly higher than that of the reference NiMo/SBA-15 sample for both sulfur-containing compounds tested (DBT and 4,6-DMDBT). It was found that the optimum amount of citric acid, which allows achieving the highest catalytic activity, corresponds to CA:Mo molar ratio equal to 1. Further increase in the amount of citric acid resulted in a slight decrease in the HDS activity. Regarding selectivity, addition of small amounts of CA, in general, resulted in an increase of the hydrogenation ability of the NiMo/SBA-15 catalysts. However, some differences in the selectivity of the catalysts were observed with different amounts of citric acid used.

Full text of DOI:10.1016/j.cattod.2013.06.002

Catalysis Today 220–222 (2014) 78–88
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Effect of the amount of citric acid used in the preparation of
NiMo/SBA-15 catalysts on their performance in HDS of
dibenzothiophene-type compounds
∗
Miguel Ángel Calderón-Magdaleno, Juan Arturo Mendoza-Nieto, Tatiana E. Klimova
Departamento de Ingeniería Química, Facultad de Química, Universidad Nacional Autónoma de México (UNAM), Cd. Universitaria, Coyoacán, México, D.F.
4510, Mexico
0
a r t i c l e i n f o
a b s t r a c t
Article history:
In the present work, NiMo catalysts supported on SBA-15 were prepared with the addition of different
amounts of citric acid (CA) in the impregnation solutions. The aim of this study was to inquire into the
effect of the amount of citric acid on the activity and selectivity of the NiMo/SBA-15 catalysts in deep
hydrodesulfurization (HDS). Catalysts were prepared by coimpregnation of Ni and Mo species from acidic
aqueous solutions containing citric acid without further adjusting the solution’s pH. The amount of citric
acid used in the catalyst preparation was varied from CA:Mo molar ratio 0.5 to 2.0. In addition, a refer-
ence NiMo/SBA-15 catalyst was prepared without citric acid. After the impregnation, catalysts were dried
Received 31 March 2013
Received in revised form 31 May 2013
Accepted 5 June 2013
Available online 9 July 2013
Keywords:
Deep hydrodesulfurization
NiMo catalysts
Citric acid
SBA-15
Dibenzothiophene
◦ ◦
100 C, 6 h) and calcined (500 C, 4 h). The prepared catalysts were characterized by nitrogen physisorp-
(
tion, small-angle and powder X-ray diffraction (XRD), UV–vis diffuse reflectance spectroscopy (DRS),
temperature-programmed reduction (TPR), high resolution transmission electron microscopy (HRTEM)
and tested in simultaneous HDS of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-
◦
4
,6-Dimethyldibenzothiophene
DMDBT) in a batch reactor at 300 C for 8 h. XRD, DRS and TPR characterizations showed that Ni and Mo
oxide species were well dispersed in all catalysts prepared with CA. In contrast, a NiMoO4 crystalline
phase was detected by XRD in the reference NiMo/SBA-15 catalyst prepared without citric acid. Addition
of citric acid to the impregnation solutions used for the catalyst preparation also resulted in an increase
in the degree of sulfidation and in the dispersion of catalytically active MoS2 phase (elemental analysis,
HRTEM). In accordance with this, HDS activity of the NiMo catalysts prepared with the addition of citric
acid resulted to be significantly higher than that of the reference NiMo/SBA-15 sample for both sulfur-
containing compounds tested (DBT and 4,6-DMDBT). It was found that the optimum amount of citric
acid, which allows achieving the highest catalytic activity, corresponds to CA:Mo molar ratio equal to 1.
Further increase in the amount of citric acid resulted in a slight decrease in the HDS activity. Regarding
selectivity, addition of small amounts of CA, in general, resulted in an increase of the hydrogenation
ability of the NiMo/SBA-15 catalysts. However, some differences in the selectivity of the catalysts were
observed with different amounts of citric acid used.
©
2013 Elsevier B.V. All rights reserved.
1
. Introduction
of a heterogeneous catalyst. In order to attend the demand of
production of lower contaminating diesel fuel, many efforts are
aimed to develop novel hydrotreating catalysts, highly active and
selective especially for HDS of the refractory polyaromatic sulfur
compounds.
Nowadays, the demand for high quality petroleum-derived
transportation fuels continues growing. Low and ultra-low sulfur
content fuels are particularly desired in order to solve environmen-
tal problems induced by SOx emission and to improve air quality
Among different approaches used in the last decade to improve
traditional HDS catalysts based on Mo(W) sulfide promoted by
[
1,2]. The main industrial process to accomplish this purpose is
hydrodesulfurization (HDS), in which S-containing molecules react
with hydrogen at high temperature and pressure in the presence
Ni(Co) and supported on ␥-Al O3 [3,4], one of the most promis-
2
ing is related to the use of novel mesostructured materials, such
as MCM-41, SBA-15 and KIT-6 as supports. These materials have
different advantages in comparison with alumina, namely, they
possess appropriate physicochemical properties, large surface area,
tunable pore size, good thermal and mechanical resistance and
∗ Corresponding author. Tel.: +52 55 5622 5371; fax: +52 55 5622 5371.
E-mail addresses: klimova@unam.mx, tklimova@gmail.com (T.E. Klimova).
0
920-5861/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cattod.2013.06.002

M.Á. Calderón-Magdaleno et al. / Catalysis Today 220–222 (2014) 78–88
79
the ability to disperse well NiMoS and CoMoS active phases lead-
ing to attractive HDS catalysts [5–7]. In our group, good results
were obtained with NiMo and NiW catalysts supported on SBA-15,
especially when this silica material was modified by the incorpora-
tion of different heteroatoms (Ti, Zr, Al) [8–13]. It was shown that
heteroatoms incorporated on the SBA-15 surface serve as anchor-
ing sites for the deposited Mo and W active species, significantly
improving by this means catalysts’ performance compared to pure
silica SBA-15-supported analogs [11]. When SBA-15 silica with no
incorporated heteroatoms is used as a support for the HDS cata-
lysts, other approaches can be used to modify and improve catalytic
activity. This can be achieved, for example, by the correct selection
of metal precursors and solvents, or by the addition of different
additives and organic ligands. Thus, the use of Keggin-type het-
eropolyacids (H PMo12O40 and H PW12O40) as distinct precursors
with the addition of a constant amount of citric acid to the impreg-
nation solution, but varying the pH value and conditions of the
subsequent thermal treatment, in simultaneous HDS of DBT and
4,6-DMDBT. It was found that the catalyst prepared from acidic
impregnation solution (pH 1) containing citric acid and calcined at
◦
500 C for 4 h before sulfidation showed the best catalytic activity
in HDS of both model compounds tested. This exceptional behav-
ior was attributed to a high hydrogenation ability of this catalyst,
which is an important feature for the HDS of DBT-type compounds
with alkyl-substituents in positions 4 and 6 of the molecule, reac-
ting preferentially through the hydrogenation pathway of HDS [30].
In the present work, following up on our previous research, we
tried to inquire on the effect of the amount of citric acid used in the
preparation of the NiMo/SBA-15 catalysts from acidic impregnation
solutions (pH 1 or below) on their activity and selectivity in deep
hydrodesulfurization (HDS). We were interested in determining if
varying the amount of citric acid can bring further improvement in
activity and selectivity of these catalysts in deep HDS. For this aim,
NiMo catalysts supported on SBA-15 were prepared with different
amounts of citric acid, characterized and tested in simultaneous
HDS of two model compounds (DBT and 4,6-DMDBT). This is the
last work of this series dealing with the search for the optimum
experimental conditions (solution pH, amount of citric acid and
thermal treatment) for the preparation of active NiMo/SBA-15 cat-
alysts with desired selectivity for HDS of DBT-type compounds.
3
3
for NiMo and NiW catalysts supported on SBA-15 has also led
to good results [14]. Another option consists on the addition of
different types of organic acids to modify the characteristics of
HDS catalysts, among them nitriloacetic acid (NTA), ethylenedi-
aminetetraacetic acid (EDTA), cyclohexanediaminetetraacetic acid
(
CyDTA), citric acid (CA), etc. [15–21]. It is well-known that, under
the appropriate experimental conditions (solution pH, metal and
ligand concentrations, temperature, etc.), organic or inorganic acids
are able to interact with metal precursors and thus increase solu-
bility and stability of the metal species used for the preparation
of supported catalysts leading to a better catalytic performance.
Among different organic additives, citric acid attracted our atten-
tion, because it is cheap, easily available and easy to handle.
In works [22,23], citric acid was employed to synthesize
2. Experimental
2.1. Catalyst preparation
Co [Mo (C H5O7) O11] complex in aqueous solution at acidic pH
which then was used as a precursor of the CoMoS phase. CoMo/␥-
2
4
6
2
SBA-15 silica with hexagonal p6mm structure was prepared
according to a reported procedure [31,32] using the triblock
copolymer Pluronic P123 (Mav = 5800, EO20PO70EO20, Aldrich) as
the structure-directing agent and tetraethyl orthosilicate (TEOS,
Aldrich, 99.999%) as the silica source. Pluronic P123 copolymer (4 g)
Al O catalysts prepared from this complex showed improved
2
3
activity for deep HDS. Catalysts prepared with citric acid and sup-
ported on metal oxides other than Al O or alumina modified
2
3
with zeolites were found to be active in hydrodesulfurization of
refractory compounds such as dibenzothiophene (DBT) and 4,6-
dimethyldibenzothiophene (4,6-DMDBT). For example, attractive
results were obtained with the CoMoP/HY-Al O catalyst prepared
◦
was dissolved in water (30 g) and 2 M HCl (120 g) solution at 35 C.
Then TEOS (8.5 g) was added into the solution. The mixture was
◦
◦
stirred at 35 C for 20 h and then aged at 80 C for 48 h without stir-
ring. The solid product was recovered by filtration, washed with
deionized water and air-dried at room temperature. Calcination
2
3
with CA [24]. Addition of CA to this catalyst resulted in a con-
siderable increase in activity when commercial diesel was used.
This was ascribed to a better dispersion of the active phase and
the prevention of the sulfidation of Co at low temperatures. Citric
acid was also used in the preparation of CoMo/␥-Al O catalysts
◦
was carried out in static air at 550 C for 6 h.
NiMo catalysts supported on SBA-15 were prepared by a
standard incipient wetness co-impregnation technique reported
elsewhere [33]. The calcined support was co-impregnated
using aqueous solutions of ammonium heptamolybdate,
2
3
modified with boron [16]. Addition of citric acid increased the
dispersion of Co and Mo species, resulting in a larger amount of
Co–Mo–S phase after sulfidation without active-site blocking by
cobalt sulfide clusters [25]. NiMo catalysts supported on wide-pore
ZrO –TiO prepared at acidic pH with the addition of EDTA or CA,
(
(
NH ) Mo7O ·4H O (Merck, 99%), nickel nitrate, Ni(NO ) ·6H O
4
6
24
2
3
2
2
Baker), and citric acid, C H O7·H O (Merck, 99.5%). Amounts of
6
8
2
citric acid used corresponded to CA:Mo molar ratios between
.5 and 2.0. All impregnation solutions had acidic pH (pH 1 or
2
2
0
and sulfided without previous calcinations, showed high catalytic
activity in dibenzothiophene hydrodesulfurization [26]. For citric
acid, better activity results were obtained for the Ni:CA molar ratio
equal to 1:2, although no selectivity modifications were observed
for any molar ratio.
In our group, a series of NiMo catalysts supported on SBA-15
were prepared with citric acid and tested in hydrodesulfurization
of DBT [27,28]. In work [27], NiMo/SBA-15 catalysts were prepared
by varying the pH of the impregnation solution and conditions of
the thermal treatment after the impregnation step. We observed
that these two factors modified activity and selectivity of the cat-
alysts in HDS of DBT. A detailed kinetic study undertaken in this
work confirmed this result. In the following work [28], NiMo/SBA-
below) which was not adjusted to any specific value. In addition, a
reference NiMo/SBA-15 catalyst was prepared by co-impregnation
of nickel nitrate and ammonium heptamolybdate, without CA
addition. After co-impregnation, all catalysts were dried first at
◦
◦
room temperature, then at 100 C for 6 h, and calcined at 500 C for
h in static air atmosphere. The nominal composition of the cat-
4
alysts was 12 wt.% of MoO3 and 3 wt.% of NiO, which corresponds
to the Ni:Mo molar ratio = 1:2. As all catalysts were supported
on SBA-15, hereinafter, we will denote them just as NiMo and
NiMoCA(x), where x represents CA:Mo molar ratio used in the
catalyst preparation (x = 0.5, 1.0, 1.5, 2.0).
1
9
5 catalysts were prepared using basic impregnation solutions (pH
) containing different amounts of citric acid. These catalysts were
2.2. Support and catalyst characterization
sulfided without previous calcination and showed very high selec-
tivity for the direct desulfurization of DBT. In the more recent work
SBA-15 support and NiMo(CA)/SBA-15 catalysts were char-
acterized by N2 physisorption, small-angle and powder X-
ray diffraction (XRD), UV–vis diffuse reflectance spectroscopy
[
29], we compared behavior of the NiMo/SBA-15 catalysts prepared

8
0
M.Á. Calderón-Magdaleno et al. / Catalysis Today 220–222 (2014) 78–88
(
DRS), temperature-programmed reduction (TPR), and high
in the SBA-15 support and, probably, to some pore blockage by
the agglomerated Ni and Mo oxides species. Pore diameter also
decreased from 7.8 nm in the starting SBA-15 support to 7.4 nm
in the NiMo catalyst, which supports our supposition about the
agglomeration of Ni and Mo species inside the mesopore chan-
nels of the support. Regarding calcined NiMoCA(x) catalysts, their
textural properties varied with the amount of citric acid used in
the synthesis. Thus, the sample prepared with small amount of cit-
ric acid (x = 0.5) showed significantly better textural characteristics
(SBET and pore volume equal to 662 m /g and 0.88 cm /g, respec-
tively) than the reference NiMo catalyst prepared without CA. This
illustrates that the addition of a small amount of citric acid to the
impregnation solution resulted in about 10–13% increase in the
specific textural characteristics of the catalyst. The possible expla-
nation for this can be an increase in the dispersion of the deposited
Ni and Mo species in the NiMoCA(0.5) catalyst in comparison with
the analog prepared without CA. However, further increase in the
amount of citric acid used in the catalysts’ preparation resulted in a
slight decrease in the textural characteristics (Table 1). The catalyst
prepared with CA:Mo molar ratio equal to x = 2.0 showed textu-
ral characteristics similar to those of the reference NiMo sample.
This fact, possibly, is related to different degrees of agglomeration
of Ni and Mo oxide species, and subsequently in a change in the
dispersion of metal oxide particles over the SBA-15 support, or to
the fact that not all citric acid was eliminated from the NiMoCA(x)
catalysts upon calcinations, thus giving rise to an increase in the
amount of residual carbon with increasing the amount of CA used.
Elemental analysis performed to confirm this supposition showed
the presence of 0.3 wt.% of carbon in the calcined NiMoCA(2.0)
catalyst.
resolution transmission electron microscopy (HRTEM). Nitro-
gen adsorption–desorption isotherms were measured with a
Micromeritics ASAP 2020 automatic analyzer at liquid N2 tem-
perature. Prior to the experiments, the samples were degassed
− ◦
1
(
p < 10 Pa) at 270 C for 6 h. Specific surface areas (SBET) were
calculated by the BET method, the total pore volume (Vp) was
determined by nitrogen adsorption at a relative pressure of 0.98
and pore size distributions were obtained from the adsorption
isotherms by the BJH method. The mesopore diameter (DP) corre-
sponds to the maximum of the pore size distribution. The micropore
area (Sꢀ) was estimated using the correlation of t-Harkins &
2
3
◦
Jura (t-plot method). XRD patterns were recorded from 3 to
◦
8
0
(2ꢁ) on a Bruker D8 Advance diffractometer, using Cu K␣
◦
−1
radiation (ꢂ = 1.5406 A˚ ) and a goniometer speed of 1 (2ꢁ) min
.
◦
Small-angle XRD (2ꢁ = 0.5–10 ) was performed on a Bruker D8
Advance diffractometer using small divergence and scattering slits
of 0.05 . UV–vis electronic spectra of the samples were recorded
in the wavelength range of 200–800 nm using a Varian Cary
◦
1
00 Conc spectrophotometer equipped with a diffuse reflectance
attachment. Polytetrafluoroethylene was used as a reference. TPR
experiments were carried out in a Micromeritics AutoChem II 2920
automatic analyzer equipped with a TC detector. Before TPR experi-
ments, the samples were pretreated in situ at 400 C for 3 h under air
flow and cooled in an Ar stream. The reduction step was performed
◦
under a stream of a H /Ar mixture (10/90 mol/mol and 50 mL/min),
2
◦
◦
with a heating rate of 10 C/min up to 1000 C. Elemental analy-
sis of calcined and sulfided NiMo(CA) catalysts was performed on
an automatic Perkin-Elmer 2400 Series II CHNS/O analyzer with
TC detector. HRTEM studies of sulfided catalysts were performed
using a Jeol 2010 microscope (resolving power 1.9 A˚ ). The solids
The nitrogen adsorption–desorption isotherm of the SBA-15
support and NiMo(CA) catalysts are shown in Fig. 1(a). They all
correspond to a type IV isotherm characteristic of mesoporous
materials, with a H1 hysteresis loop characteristic of a well-formed
SBA-15 silica [31,32]. The shape of the isotherm and of the hystere-
sis loop did not suffer important changes after the incorporation of
NiMo(CA) species. Isotherms of all catalysts shown in Fig. 1(a), pre-
pared with or without citric acid, maintain the characteristic shape
of the SBA-15 support. This indicates that the characteristics of the
original pore structure of the SBA-15 support were conserved in
the catalysts. However, all NiMo(CA) catalysts showed a decrease
in the amount of adsorbed nitrogen, which is in line with textural
characterization results from Table 1. Pore size distributions of the
SBA-15 support and NiMo(CA) catalysts are shown in Fig. 1(b). All
they show a monomodal pore size distribution with a maximum
between 7 and 8 nm. Only a slight decrease in the pore diameter
and in the intensity of the peaks can be observed in this figure after
the incorporation of Ni and Mo species in the SBA-15 support.
Fig. 2 shows small-angle XRD patterns of the SBA-15 support
and NiMo(CA) catalysts. All of them exhibit diffraction patterns
characteristic of SBA-15 material with three well-resolved peaks
corresponding to (1 0 0), (1 1 0) and (2 0 0) reflections associated
with p6mm hexagonal symmetry of mesopores. The presence of
these three signals in the XRD patterns of the NiMo(CA) catalysts
points out that the long-range pore arrangement of the SBA-15 sup-
port was maintained in all catalysts after deposition of Ni and Mo
species and citric acid. Results from the calculation of the pore wall
thickness for the SBA-15 support and NiMo(CA) catalysts (Table 1,
last column) indicate a small increase in the thickness of the pore
walls of the starting SBA-15 support after the impregnation of Ni
and Mo species. This increase was slightly higher for the reference
NiMo catalysts prepared without CA (∼0.2 nm) than for the cat-
alysts of the NiMoCA(x) series (∼0.1 nm). This confirms that the
metal species mentioned above were deposited mostly inside the
SBA-15 mesopore channels and that their dispersion was better in
the catalysts prepared with the addition of citric acid.
were ultrasonically dispersed in n-heptane and the suspension was
collected on carbon coated grids. Slab length and layer stacking
distributions of MoS2 crystallites in each sample were established
from the measurement of at least 300 crystallites detected on sev-
eral HRTEM micrographs taken from different parts of the same
sample dispersed on the microscope grid.
2.3. Catalytic activity
The HDS activity tests were performed in a batch reactor at
◦
3
00 C and 7.3 MPa total pressure for 8 h with constant stirring.
Prior to the catalytic activity evaluation, the catalysts were sul-
◦
fided ex situ in a tubular reactor at 400 C for 4 h in a stream of
1
5 vol.% of H S in H under atmospheric pressure. The sulfided cat-
2 2
alysts (0.15 g) were transferred in an inert atmosphere (Ar) to a
batch reactor (Parr) with 40 mL of n-hexadecane solution contain-
ing both DBT (Aldrich, 1300 ppm of S) and 4,6-DMDBT (Aldrich,
5
00 ppm of S). The course of the reaction was followed by with-
drawing aliquots each hour and analyzing them in an Agilent 6890A
chromatograph. To corroborate product identification, the product
mixture was analyzed in a Hewlett Packard GC–MS instrument.
3
. Results
3.1. Characterization of support and catalysts in their oxide form
The textural properties of NiMo(CA) catalysts are given in
2
Table 1. The SBA-15 support had a BET surface area of 801 m /g,
2
3
micropore area of 127 m /g, total pore volume of 1.04 cm /g and
pore diameter of 7.8 nm. All NiMo(CA) catalysts showed lower
specific textural characteristics. Thus, the NiMo catalyst prepared
2
without citric acid had surface area of 598 m /g and pore volume
of 0.78 cm /g. This decrement can be related to an increase in the
3
sample’s density after the incorporation of Ni and Mo precursors

M.Á. Calderón-Magdaleno et al. / Catalysis Today 220–222 (2014) 78–88
81
Table 1
Textural and structural characteristics of SBA-15 support and NiMo(CA) catalysts.
SBETa (m /g)
2
Sꢀb (m /g)
2
Vpc (cm /g)
3
Dpd (nm)
a0e (nm)
ıf (nm)
Sample
SBA-15
NiMo
NiMoCA(0.5)
NiMoCA(1.0)
NiMoCA(1.5)
NiMoCA(2.0)
801
598
662
617
610
592
127
83
87
76
99
90
1.04
0.78
0.88
0.84
0.82
0.81
7.8
7.4
7.5
7.5
7.5
7.5
10.4
10.2
10.2
10.2
10.2
10.2
2.6
2.8
2.7
2.7
2.7
2.7
a
Specific surface area calculated by BET method.
b
c
d
e
f
Micropore area estimated using the correlation of t-Harkins & Jura (t-plot method).
Total pore volume determined at a relative pressure of 0.98.
Pore diameter corresponding to the maximum of the pore size distribution calculated from the adsorption isotherm by the BJH method.
√
a0, unit-cell parameter estimated from the position of the (1 0 0) diffraction line (a0 = d1 0 0 × 2/ 3).
Pore wall thickness (ı = a0 − Dp).
Fig. 3 shows powder XRD patterns of the SBA-15 support
dispersion of Ni and Mo oxide species deposited on the SBA-15
surface.
and NiMo(CA) catalysts. The diffraction pattern of the SBA-15
support has the shape characteristic for this amorphous sil-
ica material [14]. In the case of the reference NiMo catalyst
prepared without addition of citric acid, signals corresponding
to NiMoO4 crystalline phase (JCPDS 33-0948) were observed.
Similar results have already been reported previously for the
catalysts prepared by co-impregnation of ammonium hepta-
molybdate and nickel nitrate on SBA-15 and Al-SBA-15 supports
In order to obtain more information regarding the dispersion of
Ni and Mo oxide species in the catalysts prepared with citric acid,
UV–vis diffuse reflectance spectroscopy (DRS) and temperature-
programmed reduction (TPR) were used. UV–vis DR spectra for
NiMo(CA) catalysts supported on SBA-15 are shown in Fig. 4. In
these spectra, absorption bands observed in the 200–400 nm region
correspond to ligand-to-metal charge transfer (LMCT) O2 → Mo
−
6+
.
[
(
34]. It was shown that an Anderson-type heteropolymolybdate
As it was described previously, the exact positions of these bands
depend on the coordination around Mo(VI) cations, which can be
octahedral (Oh) or tetrahedral (Td), and the size of the agglom-
erated Mo oxide species [35]. In the spectrum of the reference
NiMo catalyst prepared without CA (Fig. 4, curve (a)), signals corre-
sponding to different types of Mo species can be clearly observed.
Thus, the absorption band with the maximum at 250 nm evidences
the presence of tetrahedral Mo species, whereas the absorption
at 280–330 nm is characteristic for agglomerated octahedrally-
coordinated polymolybdate species. The latter ones showed a UV
absorption edge energy (Eg) of 3.3 eV (Table 2), which is close to
the previously reported values for ammonium heptamolybdate and
NH ) [Ni(OH) Mo O ]·4H O was rapidly formed in the impreg-
4
4
6
6
18
2
nation solution, and within a few minutes crystallites of this phase
were precipitated. Low solubility of this ammonium salt led to the
precipitation of the crystallites of this compound in and outside the
porosity of the support. Calcination of the catalyst resulted in the
elimination of NH4+ cations and formation of MoO3 and NiMoO4.
The above explains why the NiMoO crystalline phase was detected
4
in the reference sample prepared without citric acid. Regarding the
catalysts prepared with citric acid, in all cases no signals of any
crystalline phase were detected. It is worth to mention, that results
obtained by XRD for the reference NiMo sample are consistent with
results obtained for this catalyst by nitrogen physisorption, where
a decrease in the SBET value and pore volume after Ni and Mo depo-
sition makes us suppose the possibility of some pore blocking by
the agglomerated Ni and Mo oxide species. As expected, addition
of citric acid to the impregnation solutions increased solubility
and stability of Ni and Mo precursors used, avoiding the forma-
tion of an Anderson-type heteropolymolybdate and increasing the
small MoO clusters [35]. This result is well in line with powder XRD
3
results for this catalyst. Thus, the signal corresponding to tetrahe-
dral Mo species can be associated with the formation of a NiMoO4
crystalline phase evidenced by XRD, in which Mo(VI) cations are
coordinated tetrahedrally. At the same time, small MoO3 clusters,
the formation of which can be expected as a result of the decompo-
sition of an Anderson-type precipitate upon calcination and which
Fig. 1. Nitrogen adsorption–desorption isotherms (a) and pore size distributions (b) of SBA-15 support, NiMo and NiMoCA(x) catalysts.

8
2
M.Á. Calderón-Magdaleno et al. / Catalysis Today 220–222 (2014) 78–88
Fig. 4. UV–vis diffuse reflectance spectra of the catalysts: (a) NiMo, (b) NiMoCA(0.5),
c) NiMoCA(1.0), (d) NiMoCA(1.5) and (e) NiMoCA(2.0).
(
were not detected by XRD because of their small size, can be
responsible for the absorption band corresponding to octahedrally
coordinated Mo oxide species. The signals which can be observed
in the spectra of the NiMo catalysts prepared with the addition
of citric acid (curves (b)–(e), Fig. 4) can be ascribed to tetrahedral
or well-dispersed octahedral Mo species. Eg values determined for
the catalysts of the NiMoCA(x) series varied between 3.8 and 4.0 eV
Fig. 2. Small-angle XRD patterns of SBA-15 support, NiMo and NiMoCA(x) catalysts.
(Table 2), which points out to a better dispersion of octahedral Mo
species in the catalysts prepared with citric acid in comparison with
the reference NiMo sample. An interesting behavior was observed
between Eg values and the amount of citric acid used in the cat-
alyst preparation; namely, catalysts prepared with small amounts
of citric acid (x = 0.5 and 1.0) had higher Eg values (4.0 eV) than
the samples prepared with larger amounts of CA (x = 1.5 and 2.0).
It seems that when a small amount of citric acid is added to the
impregnation solution containing both Ni and Mo precursor salts,
dispersion of octahedral Mo species increases. However, when the
amount of citric acid increases, there is a contrary effect consisting
in an increase in the size of polymolybdate species.
TPR characterization results for NiMo(CA) catalysts supported
on SBA-15 are shown in Fig. 5. It can be appreciated that the NiMo
catalyst prepared without citric acid shows a complex TPR pro-
◦
file. A low-temperature reduction peak with a maximum at 386 C
◦
has two shoulders at about 360 and 432 C. The presence of three
overlapped peaks in the 300–500 C region points out to the coex-
Fig. 3. Powder XRD patterns of SBA-15 support, NiMo and NiMoCA(x) catalysts.
Signals of NiMoO4 crystalline phase, JCPDS card 33-0948.
◦
*
istence in this sample of octahedral Mo species of different degrees
of agglomeration [36–38]. These low temperature reduction peaks
can be attributed to the first step of reduction from Mo to Mo
Table 2
6+
4+
Absorption edge energy (Eg) and reduction behavior of NiMo(CA) catalysts.
of octahedral Mo species deposited on the SBA-15 surface. The sig-
Ega (eV)
H2 consumptionb (mmol/g)
˛Rc
nal at 610 C can be attributed to the second step of reduction of
◦
Sample
octahedral Mo species (from Mo4+ to Mo ) and to the first step
0
◦
◦
C
200–650
C
650–1000
Total
of reduction of small MoO3 clusters. Finally, low-intensity hydro-
NiMo
3.3
4.0
4.0
3.9
3.8
1.38
1.55
1.67
1.51
1.31
0.48
1.00
0.83
0.78
0.79
1.86
2.55
2.50
2.29
2.10
0.64
0.88
0.86
0.79
0.73
◦
gen consumption in the 700–1000 C region corresponds to the
NiMoCA(0.5)
NiMoCA(1.0)
NiMoCA(1.5)
NiMoCA(2.0)
reduction of tetrahedral Mo species detected by XRD as a NiMoO4
crystalline phase. TPR profiles obtained for the NiMo catalysts pre-
pared with the addition of citric acid were significantly different
from the above-described one of the reference NiMo/SBA-15, espe-
a
b
c
Eg values were determined from UV–vis DRS spectra of the catalysts [35].
Hydrogen consumption determined from TPR results.
◦
cially in the low temperature region (below 650 C). Thus, only one
˛R, degree of the reduction of Ni and Mo oxide species determined from total
◦
well-defined peak was observed between 300 and 450 C for all
H2 consumption of each sample and the theoretical value corresponding to their
complete reduction (2.90 mmol/g).
NiMoCA(x) catalysts. The exact position of the maximum of this
peak changed with the amount of citric acid used in the synthesis.

M.Á. Calderón-Magdaleno et al. / Catalysis Today 220–222 (2014) 78–88
83
Table 3
Results from the characterization of the sulfided NiMo(CA) catalysts: average length
(
L) and average stacking degree (N) of MoS2 crystallites, estimated fMo fractiona and
sulfur contents.
Catalyst
HRTEM
Chemical analysis
L (nm)
N
fMo
S (wt.%)
˛Sb
NiMo
7.1
5.2
4.3
4.5
5.0
4.1
2.4
2.2
2.1
2.7
0.17
0.23
0.28
0.26
0.24
5.59
6.71
6.48
6.41
6.26
0.84
1.01
0.98
0.97
0.95
NiMoCA(0.5)
NiMoCA(1.0)
NiMoCA(1.5)
NiMoCA(2.0)
a
fMo, estimated fraction of Mo atoms on the edge surface of MoS2 crystallites [12].
Degree of the sulfidation of Ni and Mo species determined from elemental anal-
b
ysis results and the theoretical value corresponding to their complete sulfidation
6.62 wt.% S).
(
to MoS2 crystallites with 0.61 nm interplanar distances and some
features of the SBA-15 mesopore structure were observed in micro-
graphs of the sulfided catalysts. Reference NiMo catalyst prepared
without citric acid showed MoS2 crystallites with lengths between
2
and 10 nm and with variable stacking degrees (the number of
stacking layers changed between 2 and 8). Addition of citric acid
during catalyst preparation, in general, resulted in a better disper-
sion of MoS2 particles. As it can be seen in Fig. 6 (b), NiMoCA(1.0)
catalyst, prepared from the impregnation solution containing cit-
ric acid at CA:Mo molar ratio equal to 1.0, had smaller (2–6 nm)
and less stacked (1–3 layers) MoS2 particles. However, when the
amount of citric acid was increased to x = 2.0, some more stacked
MoS2 particles were detected in addition to the well-dispersed
Mo active phase (micrograph (d), Fig. 6). This qualitative observa-
tion was confirmed by calculation of length and stacking degree
distributions for NiMo(CA) catalysts (Fig. 7), and by the deter-
mination on their basis of average morphology (average length
and number of layers) of MoS2 crystallites in different catalysts
Fig. 5. TPR profiles of NiMo and NiMoCA(x) catalysts supported on SBA-15.
Thus, the addition of a small amount of citric acid to the impregna-
tion solution (x = 0.5) resulted in a decrease in the temperature of
◦
the maximum to 369 C. However, further increase in the amount
of citric acid was accompanied by a slight increase in the posi-
tion of the maximum, which was observed at 373, 378 and 385 C
◦
for NiMoCA(1.0), NiMoCA(1.5) and NiMoCA(2.0) catalysts, respec-
tively. In addition, the first reduction peak became more intense
and symmetric than in the TPR profile of the reference NiMo
sample, which points out to a more homogeneous distribution of
octahedral Mo species in the samples prepared with citric acid.
Results from the quantification of the amount of hydrogen con-
(Table 3). Fig. 7 clearly shows a decrease in MoS2 slabs length
and stacking for all catalysts prepared with CA. NiMoCA(1.0) cata-
lyst showed MoS2 particles of the shortest length (4.3 nm) among
all the prepared samples, formed in average by 2.2 layered MoS2
slabs. The above HRTEM observations seem to be in line with
previously described results of characterization of the catalysts
in their oxide form. Thus, it was described above that the addi-
tion of small amounts of citric acid (x = 0.5 and 1.0) improves
dispersion of Ni and Mo precursor species in calcined NiMoCA(x)
catalysts, and in accordance with this, the same NiMoCA cata-
lysts in their sulfided state also showed a better dispersion on the
SBA-15 surface than the reference samples prepared without CA
◦
sumed during the reduction of the catalysts in low (200–650 C)
◦
and high-temperature (650–1000 C) regions, as well as the total
hydrogen consumption are shown in Table 2. It can be seen that
the reduction of the samples prepared with citric acid proceeds
with higher total consumption of hydrogen than of the reference
NiMo catalyst. Degrees of reduction (˛R) of NiMoCA(x) samples
were in the range of 0.73–0.88, which indicates that the majority
of Ni and Mo oxide species were reduced in the TPR experiments.
For comparison, NiMo catalyst showed ˛R value equal to 0.64. The
above results show that the addition of small amounts of citric
acid (x = 0.5 and 1.0) during catalyst preparation not only results
in a decrease in the temperature of reduction of Ni and Mo oxide
species (Fig. 5), but also makes their reduction more complete. This
fact can be explained by a better dispersion of Ni and Mo oxide
species in the catalysts prepared with citric acid and an increase
in the proportion of Mo6+ ions in octahedral coordination, which
can be reduced much easier than the corresponding tetrahedral
ones.
(Table 3).
Results from the HRTEM characterization of sulfided NiMo(CA)
catalysts were used to determine the average fraction of Mo
atoms on the edge surface of MoS2 crystallites (fMo, Table 3),
which can be considered an indicator of the amount of sulfided
Mo species located on the catalytically active surface of MoS2
particles [39]. The fMo fraction was calculated using equations
reported in [12], assuming that the MoS2 crystallites are per-
fect hexagons. The fMo values reported for NiMo(CA) catalysts
in Table 3 show a noticeable increase in the proportion of Mo
species on the catalytically active surface of the sulfided catalysts
prepared using citric acid. The highest value of the fMo fraction
(0.28) was found for the NiMoCA(1.0) catalyst. In addition, sulfur
3.2. Characterization of sulfided catalysts
content was determined if the sulfided NiMo(CA) catalysts by ele-
mental analysis in order to determine their degree of sulfidation
Sulfided NiMo(CA)/SBA-15 catalysts were characterized by
(˛ , Table 3). The reference NiMo catalyst prepared without cit-
S
HRTEM in order to obtain more information about the dispersion
of MoS2 crystallites in the catalysts prepared with the addition
of different amounts of citric acid. Representative micrographs
of selected catalysts are shown in Fig. 6. The typical fringes due
ric acid was sulfided in 84%, whereas all catalysts prepared with
citric acid showed higher amounts of sulfur and degrees of sul-
fidation corresponding to almost completely sulfided Ni and Mo
species.

8
4
M.Á. Calderón-Magdaleno et al. / Catalysis Today 220–222 (2014) 78–88
Fig. 6. HRTEM micrographs of sulfided catalysts supported on SBA-15: (a) NiMo, (b) NiMoCA(1.0), (c) NiMoCA(1.5) and (d) NiMoCA(2.0).
3
.3. Catalytic activity
the direct desulfurization pathway (DDS), sulfur is eliminated
through hydrogenolysis of C S bonds, yielding biphenyl-type com-
pounds. In the second route, called the hydrogenation pathway
(HYD), desulfurization occurs after hydrogenation of one of the
aromatic rings of dibenzothiophene molecules with the forma-
tion of tetrahydro- and hexahydrogenated intermediates, which
after desulfuration resulted in the products with one aromatic
and one hydrogenated rings (cyclohexylbenzene or methylcyclo-
hexyltoluene). Nowadays, it is well known that unsubstituted DBT
can react through any one of the reaction routes (the DDS or the
HYD) depending on the catalyst used, whereas 4,6-DMDBT, one of
the most refractory sulfur compounds in diesel fuels, has a clear
Catalytic behavior of the synthesized and characterized
NiMo(CA) catalysts supported on SBA-15 was evaluated in simul-
taneous hydrodesulfurization (HDS) of two model compounds,
dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-
DMDBT). These S-containing aromatic molecules were selected
because they are representative for diesel fuel and have quite
different reactivity toward two possible routes of HDS. It is well
known that HDS of dibenzothiophene-type compounds can occur
by the two parallel reaction routes. Fig. 8 shows the reaction
scheme of HDS of DBT-type molecules. In the first route, called
Fig. 7. Length and stacking distributions of MoS2 crystallites in sulfided catalysts: (a) NiMo, (b) NiMoCA(0.5), (c) NiMoCA(1.0), (d) NiMoCA(1.5) and (e) NiMoCA(2.0).

M.Á. Calderón-Magdaleno et al. / Catalysis Today 220–222 (2014) 78–88
85
HYD route
Table 6
Conversions of 4,6-DMDBT obtained over NiMo(CA) catalysts at different reaction
times and overall pseudo-first order rate constant (kDMDBT).
+
H2
+ H
2
4
−1
−1
)
Catalyst
4,6-DMDBT conversion (%)
kDMDBT × 10 (s gcat
S
S
S
R
R
R
R
R
2 h
4 h
6 h
8 h
R
NiMo
13
23
25
21
18
30
47
54
45
41
43
74
80
67
60
56
89
92
84
75
1.6
2.8
3.4
2.6
2.3
+
H2
-H 2
S
+ H2
-H 2S
NiMoCA(0.5)
NiMoCA(1.0)
NiMoCA(1.5)
NiMoCA(2.0)
DDS route
+
H2
R
R
R
R
R
Table 5 shows the reaction product distributions obtained
with different NiMo(CA) catalysts at 50% of DBT conversion. As it
was expected, biphenyl (BP) and cyclohexylbenzene (CHB) were
the principal reaction products accompanied by small amounts
of tetrahydrodibenzothiophene (4HDBT) and dicyclohexyl (DCH).
However, the proportion of CHB and BP in the reaction products
changed depending on the catalyst. Thus, the reference NiMo cat-
alyst prepared without citric acid, NiMoCA(1.5) and NiMoCA(2.0)
showed similar selectivities toward both routes of HDS, the DDS
and the HYD, leading to the formation of comparable amounts of
BP and CHB (Table 5). At the same time, two other catalysts pre-
pared with smaller amounts of citric acid (x = 0.5 and 1.0), showed
much higher hydrogenation ability and larger amounts of cyclo-
hexylbenzene in comparison with biphenyl. The highest CHB/BP
ratio was obtained with the NiMoCA(1.0) sample.
+
H2
R = H (DBT); CH (4,6-DMDBT)
3
R
Fig. 8. Reaction network for hydrodesulfurization of dibenzothiophene-type com-
pounds.
Table 4
Conversions of DBT obtained over NiMo(CA) catalysts at different reaction times and
overall pseudo-first order rate constant (kDBT).
4
−1
−1
)
Catalyst
DBT conversion (%)
kDBT × 10 (s gcat
2
h
4 h
6 h
8 h
Table 6 shows the results obtained in hydrodesulfurization of
4,6-DMDBT over prepared NiMo and NiMoCA(x) catalysts. It can
be seen in Table 6 that conversions obtained for 4,6-DMDBT were
lower than those of DBT (Table 4), which is due to the fact that
NiMo
21
36
40
33
27
42
66
69
63
61
58
86
87
85
81
68
95
96
94
92
2.4
4.8
5.3
4.4
4.1
NiMoCA(0.5)
NiMoCA(1.0)
NiMoCA(1.5)
NiMoCA(2.0)
4
,6-DMDBT has lower reactivity in comparison with unsubstituted
DBT. In addition, a significant difference in the catalytic activity of
the tested samples in HDS of 4,6-DMDBT was observed. However,
trends similar to those obtained with DBT were found. Namely,
the NiMo catalyst prepared without citric acid showed the lowest
activity among all catalysts tested (56% of 4,6-DMDBT conversion
at 8 h reaction time). Catalysts prepared with small amounts of
citric acid, NiMoCA(0.5) and NiMoCA(1.0) showed good perfor-
mance in the hydrodesulfurization of 4,6-DMDBT (89 and 92% of
conversion at 8 h, respectively). Further increase in the amount of
citric acid to x = 1.5 and 2.0 resulted in a decrease in the catalytic
activity: 84 and 75% of 4.6-DMDBT conversion were obtained with
NiMoCA(1.5) and NiMoCA(2.0) catalysts, respectively (Table 6).
The above results, as well as the pseudo-first order rate constants
shown in Table 6, confirmed again that better catalytic performance
was obtained with the catalysts prepared with small amounts of
citric acid (x = 0.5 and 1.0). The most active NiMoCA(1.0) catalyst
showed 4,6-DMDBT conversions and a kDMDBT value almost twice
higher that those of the reference NiMo catalyst. A comparison of
the catalytic activity results from Tables 4 and 6 points out that the
NiMoCA(1.0) catalyst resulted to be the most active for HDS of both
DBT and 4,6-DMDBT, which indicates that the amount of citric acid
used in the preparation of this catalyst was the optimum. This cat-
alyst seems to be a promising one for deep HDS of diesel, since it
preference for the HYD route, which is attributed, in general, to the
steric hindrance because of the presence of two alkyl substituents
in positions 4 and 6 of this molecule [40,30].
The DBT conversions obtained over NiMo(CA) catalysts at 2, 4,
6
and 8 h reaction time are shown in Table 4. The lowest conver-
sion (68% at 8 h reaction time) was obtained with the NiMo catalyst
prepared without citric acid. All NiMoCA(x) catalysts prepared with
citric acid showed higher catalytic activity than the reference NiMo
sample; especially, the catalyst prepared from impregnation solu-
tions with CA:Mo molar ratio x = 1. This can be clearly observed from
DBT conversions obtained at different reaction times and values of
overall pseudo-first order rate constants shown in Table 4. Thus,
9
6% of DBT conversion was obtained with this catalyst at 8 h, and
the corresponding rate constant was more than twice higher than
that of the reference NiMo sample. Further increase in the amount
of citric acid used to x = 1.5 and 2.0 resulted in a slight decrease
in the catalytic activity. The above results show, in general, that
the addition of citric acid in the impregnation solutions used in
the preparation of NiMo HDS catalysts had a beneficial effect on
their activity. However, the best results were obtained with the
NiMoCA(1.0) catalyst having equimolar CA:Mo ratio.
Table 5
Composition of products obtained in hydrodesulfurization of DBT over NiMo(CA)/SBA-15 catalysts (at 50% of DBT conversion).
Catalyst
Producta (%)
BP
Product ratios
BP: 4HDBT
4HDBT
CHB
DCH
4HDBT: CHB
CHB: BP
NiMo
47.3
39.5
37.3
43.6
49.6
4.5
4.5
4.0
5.5
5.2
45.2
52.6
55.4
47.3
42.5
3.0
3.4
3.3
3.6
2.7
10.51
8.78
9.33
7.93
9.54
0.099
0.086
0.072
0.116
0.122
0.96
1.33
1.49
1.08
0.86
NiMoCA(0.5)
NiMoCA(1.0)
NiMoCA(1.5)
NiMoCA(2.0)
a
BP, biphenyl; 4HDBT, tetrahydrodibenzothiophene; CHB, cyclohexylbenzene; DCH, dicyclohexyl.

8
6
M.Á. Calderón-Magdaleno et al. / Catalysis Today 220–222 (2014) 78–88
Table 7
Composition of products obtained in hydrodesulfurization of 4,6-DMDBT over NiMo(CA) catalysts (at 50% of 4,6-DMDBT conversion).
Catalyst
Producta (%)
DMBP
Product ratios
4H
6H
MCHT
DMBCH
(4H + 6H): MCHT
MCHT: DMBP
NiMo
6.7
6.7
6.6
5.9
5.6
17.0
16.3
12.5
13.6
11.5
8.5
4.6
3.8
8.6
7.0
53.1
57.8
60.7
56.3
60.1
14.7
14.6
16.4
15.6
15.8
0.48
0.36
0.27
0.39
0.31
7.9
8.5
9.2
9.5
10.7
NiMoCA(0.5)
NiMoCA(1.0)
NiMoCA(1.5)
NiMoCA(2.0)
a
4
H, tetrahydro-4,6-dimethyldibenzothiophene (4H-4,6-DMDBT); 6H, hexahydro-4,6-dimethyldibenzothiophene (6H-4,6-DMDBT); DMBP, dimethylbiphenyl; MCHT,
methylcyclohexyltoluene; DMBCH, dimethylbicyclohexyl.
is able to eliminate sulfur from different types of sulfur-containing
aromatic molecules contained in this fraction of petroleum.
Product distributions obtained for different catalysts at 50%
of 4,6-DMDBT conversion are shown in Table 7. The preferen-
tial pathway of HDS, as expected, was found to be the HYD
route. Methylcyclohexyltoluene (MCHT) was the main desulfur-
ized product obtained with all catalysts tested. Other products
of the HYD route: pre-hydrogenated S-containing intermediates
citric acid (Table 1 and Fig. 1). However, further characterization
of the catalysts by diffuse reflectance spectroscopy and tempera-
ture programmed reduction showed that the effect of citric acid
was not linear with its amount used in the catalyst preparation.
Thus, addition of a small amount of citric acid to the impregna-
tion solution (x = 0.5 and 1.0) resulted in a noticeable increase in
the dispersion of Ni and Mo oxide species, which was evidenced
by an increase in the absorption edge energy (Eg, Table 2) and a
decrease in the temperature of reduction of octahedral Mo species
(Fig. 5). When the amount of citric acid used in the preparation was
increased to x = 1.5 and 2.0, this tendency changed. DRS and TPR
results showed an increase in the agglomeration of the deposited
Mo oxide species with an increase in the amount of citric acid used.
Therefore, results from the characterization of the catalysts in their
oxide form showed that a better dispersion of metal oxide pre-
cursors in calcined NiMoCA catalysts supported on SBA-15 was
reached when the amount of citric acid was small (x = 0.5–1.0).
Results from the characterization of the sulfided catalysts were
well in line with the above observations for calcined catalysts. Once
again, an increase was found in the dispersion of the sulfided Mo
species for the catalysts prepared with a small amount of citric acid,
reaching maximum for the NiMoCA(1.0) sample, and a decrease in
(
tetrahydro-4,6-dimethyldibenzothiophene, 4H-4,6-DMDBT, and
hexahydro-4,6-dimethyldibenzothiophene, 6H-4,6-DMDBT) and
dimethylbicyclohexyl (DMBCH) were also obtained in smaller pro-
portions. Dimethylbiphenyl (DMBP), the desulfurized product of
the DDS route, was formed in a much smaller amount (5.6–6.7%) for
all catalysts tested (Table 7). The highest proportion of DMBP (6.7%)
was obtained with the reference NiMo and NiMoCA(0.5) catalysts.
Further increase in the amount of citric acid used in the catalysts’
preparation resulted in a progressive decrease in the proportion of
DMBP in the reaction products and, consequently, in an increase
in the MCHT/DMBP ratio. The most active NiMoCA(1.0) catalyst
showed an intermediate value of MCHT/DMBP ratio between the
tested samples, but it also had a smallest ratio of the amount of
pre-hydrogenated intermediates (4H + 6H) to the corresponding
desulfurized product (MCHT), Table 7. It seems that the high cat-
alytic activity of this catalyst is due to the optimum ratio between
the catalytically active sites responsible for the hydrogenation of
the starting 4,6-DMDBT molecule and the ones responsible for their
further desulfurization.
the dispersion of MoS particles in the samples prepared with larger
2
amounts of CA. This can be clearly seen in the values of the fMo
fraction reported in Table 3. Catalytic activity tests also revealed
an increase in the HDS activity of the catalysts of the NiMoCA(x)
series to x = 1.0, and a further decrease for x values of 1.5 and 2.0.
Interestingly, similar trends in the activity of the catalysts with the
change in the amount of citric acid were found for both model com-
pounds used, DBT and 4,6-DMDBT, in spite of some differences in
the reactivity of these molecules and their preferences for particu-
lar reaction pathways of HDS.
The above results allow us to assume that the mechanism of
action of citric acid is somewhat different when it is added in small
or large amounts to the solutions used for the catalyst preparation.
It is well documented in the literature [22–26] that the addition
of citric acid improves activity of HDS catalysts. In general, it was
ascribed to a stabilization of the impregnation solutions and a better
dispersion of the active phase in presence of citric acid. In appro-
priate conditions citric acid can act as a chelating additive able to
form complexes with Ni(II) or Co(II) and prevent the sulfidation
of these promoters at low temperatures, leading to an increase
in the amount of Ni(Co)–Mo–S phase after sulfidation [16,25]. In
4
. Discussion
In the present work, NiMo catalysts supported on high sur-
face area SBA-15 silica were prepared by co-impregnation of Ni
and Mo precursor salts from aqueous solutions containing differ-
ent amounts of citric acid. All impregnation solutions containing
citric acid had acidic pH values (pH below 1). In addition, a ref-
erence NiMo/SBA-15 catalyst was prepared without citric acid.
After the impregnation, all catalysts were dried and calcined in air
atmosphere. The aim of this study was to inquire into the effect
of the amount of citric acid on the activity and selectivity of the
NiMo/SBA-15 catalysts in the simultaneous HDS of two model com-
pounds, DBT and 4,6-DMDBT. We tried to follow the relationship
between the characteristics of the Ni and Mo oxide species present
in the prepared catalysts after drying and calcination, their char-
acteristics after sulfidation and the catalytic performance of the
prepared formulations in HDS.
Characterization of the reference NiMo and NiMoCA(x) cata-
lysts by nitrogen physisorption and powder XRD showed that, in
general, dispersion of the deposited Ni and Mo oxide species was
better in all catalysts prepared with citric acid than in the refer-
ence NiMo sample. When citric acid was used, no formation of a
crystalline NiMoO4 phase (detected in the reference NiMo cata-
lyst) was observed (Fig. 3). In addition, NiMoCA(x) catalysts had
better textural characteristics than the sample prepared without
addition, a bimetallic Co [Mo (C H5O7) O11] complex was synthe-
2
4
6
2
sized in aqueous impregnation solution at acidic pH, which was
employed for the preparation of active HDS catalysts supported on
alumina [22,23]. However, formation of the complexes as described
above strongly depends on the solution pH, and, as it was shown in
our previous work [29], upon experimental conditions used by us
(Ni and Mo concentrations and pH below 1), no complexes can be
formed between citric acid and Ni(II) or Mo(VI) precursors. In some
works [28,29,41], it was observed that citric acid can act as a source
of a small amount of carbonaceous residues in HDS catalysts which

M.Á. Calderón-Magdaleno et al. / Catalysis Today 220–222 (2014) 78–88
87
can affect morphology of the MoS2 or WS2 slabs. However, in the
5. Conclusion
present work NiMoCA catalysts were calcined in air atmosphere
prior to sulfidation, and only small amount of carbon was found in
them by elemental analysis (for example, 0.3 wt.% of carbon in the
NiMoCA(2.0) sample). So, it seems that the presence of such a small
amount of carbon in these catalysts cannot explain the changes in
their catalytic behavior. In our opinion, when a small amount of cit-
ric acid is used in the NiMoCA catalyst preparation, the role of citric
acid can consist in the stabilization of the impregnation solution,
separation of Ni and Mo species in the solution, which avoids their
interaction leading to the formation of a NiMoO4 crystalline phase
in the calcined catalysts, and probably in an increase in the solution
viscosity. According to previous report [42], homogeneous distri-
butions and high dispersions of the supported metal oxide species
can be obtained from aqueous solutions containing citrate, acetate,
EDTA, etc. Upon solvent evaporation when drying the catalysts, a
steep increase in viscosity is observed, which inhibits redistribu-
tion of the impregnated solution upon drying of the support body.
In addition, a gel-like phase can be formed that favors high disper-
sion of the active phase even after full drying and calcination. In
order to confirm this supposition, we determined viscosity of the
solutions used for the preparation of our catalysts. It was found that
the viscosity of the aqueous solution containing only nickel nitrate
In the present work, NiMo catalysts supported on SBA-15 were
prepared by coimpregnation of Ni and Mo species from acidic
aqueous solutions (pH below 1) containing different amounts of
citric acid (CA). The aim of this study was to inquire on the
effect of the amount of citric acid on the activity and selectivity
of the NiMo/SBA-15 catalysts in simultaneous hydrodesulfuriza-
tion of dibenzothiophene and 4,6-dimethyldibenzothiophene. The
amount of citric acid used in the catalyst preparation was var-
ied from CA:Mo molar ratio 0.5 to 2.0. In addition, a reference
NiMo/SBA-15 catalyst was prepared without citric acid.
XRD, DRS and TPR characterizations showed that Ni and Mo
oxide species were well dispersed in all catalysts prepared with cit-
ric acid. No presence of a NiMoO crystalline phase, detected in the
4
reference NiMo/SBA-15 catalysts prepared without citric acid, was
found for the NiMoCA(x) catalysts. However, the maximum disper-
sion of deposited metal species was achieved for the intermediate
amounts of citric acid used (x = 0.5 and 1.0). Further increase in the
amount of citric acid to x = 1.5 and 2.0 resulted in some agglomer-
ation of octahedral Mo oxide species (DRS, TPR). Similar changes
in the dispersion of the sulfided MoS2 active phase were observed
by HRTEM. The NiMoCA(1.0) catalyst showed the best dispersion
of MoS2 particles and the highest fraction of Mo atoms located on
the catalytically active surface (fMo fraction, Table 3). In line with
results from the catalyst characterization, HDS activity of the NiMo
catalysts prepared with the addition of citric acid resulted to be sig-
nificantly higher than that of the reference NiMo/SBA-15 sample for
both DBT and 4,6-DMDBT. It was found that the optimum amount
of citric acid, which allows achieving the highest catalytic activity,
corresponds to CA:Mo molar ratio equal to 1. Further increase in
the amount of citric acid resulted in a slight decrease in the HDS
activity. We think that this behavior is due to two opposite effects
of citric acid. On the one hand, addition of citric acid to the impreg-
nation solution increases its stability and viscosity, especially upon
drying of the catalyst, leading to an increase in the dispersion of Mo
species on the catalyst surface. On the other hand, when the amount
of citric acid used in the catalyst preparation is large enough (x = 1.5
and 2.0), an excess of citric acid can react with the support surface
groups decreasing the amount available for the interaction with Mo
species from the solution and leading to the agglomeration of Mo
species upon calcination.
6
2
and ammonium heptamolybdate was equal to 0.715 × 10 m /s.
When citric acid was added to this solution, its viscosity increased
almost linearly with the amount of citric acid used, reaching a value
6
2
of 0.905 × 10 m /s for CA:Mo molar ratio x = 2.0. Further increase
in the viscosity can be expected upon drying of the impregnated
catalysts. Formation of a gel-like phase was observed by us upon
drying of the catalyst prepared with the largest amount of citric
acid (x = 2.0).
On the other hand, when a large amount of citric acid was used
in the preparation of our catalysts (x = 1.5 and 2.0), a decrease in the
dispersion of oxide and sulfided Mo species was observed. We think
that this contrary effect of citric acid can be related with the interac-
tion of an excess of citric acid with the SBA-15 support. Recently, it
has been reported for the case of alumina-supported NiW catalysts
[
41], that citric acid preferentially reacts with basic and neutral
OH groups of alumina surface so as to moderate the interaction
between tungsten species and the support. We suppose that sim-
ilar interaction of citric acid with the SBA-15 surface groups can
result in a decrease of a number of anchoring points for deposited
Mo species, which can suffer agglomeration and loss of dispersion
during the calcination of the catalysts prepared with large amount
of citric acid.
Acknowledgments
Finally, it should be mentioned that the activity and selectivity
trends obtained in the present work for the NiMoCA(x) cata-
lysts prepared from acidic impregnation solutions with different
amounts of citric acid can be compared with those found previ-
ously for similar catalysts, but prepared from basic impregnation
solutions (pH 9) and sulfided without previous calcination [28]. In
both cases, catalysts prepared with a small amount of citric acid
showed higher catalytic activity than those prepared with a large
amount of CA. However, the effect on selectivity the amount of
citric acid used had, was significantly different depending on the
pH of the impregnation solution and the thermal treatment (dry-
ing or drying-calcination) of the catalysts before sulfidation step.
Thus in the work [28], a progressive increase in the selectivity of
the NiMo/SBA-15 catalysts toward the direct desulfurization path-
way of DBT HDS was observed with increasing the amount of CA
used. On the contrary, in the present work all the catalysts pre-
pared with CA showed high hydrogenation ability for both DBT
and 4,6-DMDBT model compounds used. The best activity results
were obtained with the NiMoCA(1.0) catalyst indicating that the
amount of CA used in the preparation of this catalyst was the
optimum.
Financial support from Consejo Nacional de Ciencia y Tec-
nología (CONACYT, Mexico) (project 100945 and Ph.D. scholarship
to J.A.M.-N.) is gratefully acknowledged. The authors thank A.
Morales Espino, C. Salcedo Luna and I. Puente Lee for technical
assistance with small-angle XRD, powder XRD and HRTEM char-
acterizations, respectively.
References
[
1] http://www.dieselnet.com/standards/fuels.html
[2] A. Stanislaus, A. Marafi, M.S. Rana, Catalysis Today 153 (2010) 1.
[
[
3] T.A. Pecoraro, R.R. Chianelli, Journal of Catalysis 67 (1981) 430.
4] H. Topsøe, B.S. Clausen, F.E. Massot, Hydrotreating Catalysis, Springer-Verlag,
Germany, 1996.
[5] U.T. Turaga, C. Song, Catalysis Today 86 (2003) 129.
[
[
6] K. Soni, K.C. Mouli, A.K. Dalai, J. Adjaye, Catalysis Letters 136 (2010) 116.
7] A. Wang, Y. Wang, T. Kabe, Y. Chen, A. Ishihara, W. Qiany, Journal of Catalysis
1
99 (2001) 19.
[8] T. Klimova, J. Reyes, O. Gutiérrez, L. Lizama, Applied Catalysis A: General 335
2008) 159.
9] O.Y. Gutiérrez, G.A. Fuentes, C. Salcedo, T. Klimova, Catalysis Today 116 (2006)
85.
(
[
4
[10] O.Y. Gutiérrez, D. Valencia, G.A. Fuentes, T. Klimova, Journal of Catalysis 249
(2007) 140.

8
8
M.Á. Calderón-Magdaleno et al. / Catalysis Today 220–222 (2014) 78–88
[
[
[
[
[
11] L. Lizama, T. Klimova, Journal of Materials Science 44 (2009) 6617.
12] O.Y. Gutiérrez, T. Klimova, Journal of Catalysis 281 (2011) 50.
13] D. Valencia, T. Klimova, Catalysis Today 166 (2011) 91.
14] L. Lizama, T. Klimova, Applied Catalysis B: Environmental 82 (2008) 139.
15] S. Badoga, K.C. Mouli, K.K. Soni, A.K. Dalai, J. Adjaye, Applied Catalysis B: Envi-
ronmental 125 (2012) 67.
16] N. Rinaldi, Usman, K. Al-Dalama, T. Kubota, Y. Okamoto, Applied Catalysis A:
General 360 (2009) 130.
17] G. Kishan, L. Coulier, V.H.J. de Beer, J.A.R. van Veen, J.W. Niemantsverdriet,
Journal of Catalysis 196 (2000) 180.
[27] D. Valencia, T. Klimova, Catalysis Communications 21 (2012) 77.
[28] D. Valencia, T. Klimova, Applied Catalysis B: Environmental 129 (2013)
137.
[29] T.E. Klimova, D. Valencia, J.A. Mendoza-Nieto, P. Hernández-Hipólito, Journal
of Catalysis 304 (2013) 29.
[30] C. Song, X. Ma, Applied Catalysis B: Environmental 41 (2003) 207.
[31] D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka, G.D. Stucky,
Science 279 (1998) 548.
[32] D. Zhao, Q. Huo, J. Feng, B.F. Chmelka, G.D. Stucky, Journal of the American
Chemical Society 120 (1998) 6024.
[
[
[
[
18] K. Al-Dalama, A. Stanislaus, Energy and Fuels 20 (2006) 1777.
19] M.S. Rana, J. Ramírez, A. Gutiérrez-Alejandre, J. Ancheyta, L. Cede n˜ o, S.K. Maity,
Journal of Catalysis 246 (2007) 100.
20] M.A. Lélias, E. Le Guludec, L. Mariey, J. van Gestel, A. Travert, L. Oliviero, F.
Maugé, Catalysis Today 150 (2010) 179.
21] M.A. Lélias, P.J. Kooyman, L. Mariey, L. Oliviero, A. Travert, J. van Gestel, J.A.R.
van Veen, F. Maugé, Journal of Catalysis 267 (2009) 14.
22] O.V. Klimov, A.V. Pashigreva, M.A. Fedotov, D.I. Kochubey, Y.A. Chesalov, G.A.
Bukhtiyarova, A.S. Noskov, Journal of Molecular Catalysis A: Chemical 322
[33] D. Valencia, I. García-Cruz, T. Klimova, Studies in Surface Science and Catalysis
175 (2010) 529.
[34] A. Sampieri, S. Pronier, S. Brunet, X. Carrier, C. Louis, J. Blanchard, K. Fajerwerg,
M. Breysse, Microporous and Mesoporous Materials 130 (2010) 130.
[35] R.S. Weber, Journal of Catalysis 151 (1995) 470.
[36] R. López Cordero, A. López Agudo, Applied Catalysis A: General 175 (2000) 23.
[37] R. López Cordero, F.J. Gil Llambias, A. López Agudo, Applied Catalysis 74 (1991)
125.
[
[
[
[38] R. Thomas, E.M. van Oers, V.H.J. de Beer, J.A. Moulijn, Journal of Catalysis 84
(1983) 275.
(
2010) 80.
[
23] A.V. Pashigreva, G.A. Bukhtiyarova, O.V. Klimov, Y.A. Chesalov, G.S. Litvak, A.S.
[39] E.J.M. Hensen, P.J. Kooyman, Y. van der Meer, A.M. van der Kraan, V.H.J. de Beer,
J.A.R. van Veen, R.A. van Santen, Journal of Catalysis 199 (2001) 224.
[40] D.D. Whitehurst, T. Isoda, I. Mochida, Advances in Catalysis 42 (1998) 345.
[41] H. Li, M. Li, Y. Chu, F. Liu, H. Nie, Applied Catalysis A: General 403 (2011)
75.
Noskov, Catalysis Today 149 (2010) 19.
[
[
24] T. Fujikawa, Topics in Catalysis 52 (2009) 872.
25] N. Rinaldi, T. Kubota, Y. Okamoto, Industrial and Engineering Chemistry
Research 48 (2009) 10414.
[
26] J. Escobar, M.C. Barrera, J.A. de los Reyes, J.A. Toledo, V. Santes, J.A. Colín, Journal
[42] A. Jos van Dillen, R.J.A.M. Terörde, D.J. Lensveld, J.W. Geus, K.P. de Jong, Journal
of Catalysis 216 (2003) 257.
of Molecular Catalysis A: Chemical 287 (2008) 33.