G Model
CATTOD-9723; No. of Pages7
ARTICLE IN PRESS
A.E. Cruz Pérez et al. / Catalysis Today xxx (2015) xxx–xxx
Table 1
2
the high MgO surface area allows a reaction with water leading
to Mg(OH)2 formation [14]. In order to solve this problem Zdrazil
et al. [15] propose the use of organic solvents such as dimethyl-
sulphoxide and methanol. Apparently, the textural properties of
Nomenclature of prepared catalysts.
Catalyst
Mol% TiO2
Impregnation method
Treatment
NiW/MT10-AD
NiW/MT25-AD
NiW/MT50-AD
NiW/MT10-NAD
NiW/MT25-NAD
NiW/MT50-NAD
NiW/MT10-AC
NiW/MT25-AC
NiW/MT50-AC
NiW/MT10-NAC
NiW/MT25-NAC
NiW/MT50-NAC
10
25
50
10
25
50
10
25
50
10
25
50
Aqueous
Dried
MgO are stable with these solvents. However, (NH ) Mo7O and
Aqueous
Dried
4
6
24
Ni(NO ) solubility in those solvents is very low, which complicates
Aqueous
Dried
3
2
Non-aqueous
Non-aqueous
Non-aqueous
Aqueous
Aqueous
Aqueous
Non-aqueous
Non-aqueous
Non-aqueous
Dried
Dried
Dried
Calcined
Calcined
Calcined
Calcined
Calcined
Calcined
the conventional impregnation process. On the other hand, a num-
ber of papers report on the role of additives in the sintering of MgO
[
16–18]. Additions of tetravalent Si, Ti and Zr enhance sintering.
Thereby, some authors reported that the addition of TiO promotes
MgO densification and grain growth at relative low temperature
2
[
19]. Furthermore, the mixing of solid oxides produces systems that
have novel acid or basic properties. These properties can be mod-
ified varying their mixing concentration. In the MgO–SiO system,
2
which has a basic behavior, its basicity strongly depends on MgO
content [20,21] being maximal for 50 wt% MgO. TiO –MgO mixed
recorded in the wavelength range 200–900 nm using AvaSpec-2048
spectrophotometer equipped with a diffuse reflectance attach-
ment. Sulfided catalysts were characterized by high resolution
transmission electron microscopy (HRTEM). HRTEM studies were
performed using a JEOL-JEM-2010 microscope. The solids were
ultrasonically dispersed in 2-propanol and the suspension was
collected on carbon coated grids. Slab length and layer stacking
distributions of WS2 crystallites in each sample were stablished
from the measurement of at least 300 crystallites detected on sev-
eral TEM micrographs taken from different parts of the sample
dispersed on the microscope grid.
2
oxide shows a basic behavior [22]. The substitution of titanium
ions for magnesium ions in magnesia lattice. This deforms magne-
sia crystalline structure and produces unbalanced electron charges
[
22], thus it contributes to the basicity observed in these mate-
rials. In the present work, two series of NiW catalysts supported
on MgO–TiO were synthesized, they were impregnated by aque-
2
ous and non-aqueous methods. The aim of this work is to explore
the effect of the solvent used for impregnation on the stability of
the support MgO–TiO (x) during the preparation and activation of
2
NiW/MgO–TiO catalysts and consequently on the activity in HDS
2
of DBT.
2.4. Catalytic activities measurements
2
. Experimental
The HDS of DBT was performed in a 500 mL batch reactor, mag-
2
.1. Preparation of MgO–TiO supports
netically stirred (700 rpm) (Parr Instrument Co.). The conditions of
the tests were as follows: temperature of 593 K under a hydrogen
atmosphere of 5 MPa for 8 h, using 200 mg of presulfided cata-
2
MgO–TiO2 supports with 10, 25 and 50 mol% of TiO2 were
−
3
prepared by sol–gel method [23]. Magnesium ethoxide and tita-
nium isopropoxide (Aldrich-Chemical ≥75%) were dissolved into
methanol and 2-propanol respectively (1 g alcoxide/100 mL alco-
hol). Then, both solutions were mixed and stirred during 4 h at
lyst and 1.25 × 10 mol of DBT dissolved in 100 mL hexadecane
(Aldrich-Chemical). Before each reaction, the catalysts were acti-
vated by ex situ sulfidation in a U-shape glass flow reactor. First, the
sample was flushed in a nitrogen flow and gradually increased the
temperature up to 423 K. After reached this temperature the flow
was switched to the sulfidation mixture (H2/H2S 15 vol% H2S) with
3
43 K. The gel was obtained by hydrolysis with dropwise addition
of deionized water. Then, the gel was dried at 393 K for 12 h and
then calcined at 823 K for 6 h. Nomenclature for supports: MT10
for 10 mol% TiO ; MT25 for 25 mol% TiO and MT50 for 50 mol%
−
1
a flow of 40 mL min and then increased the temperature up to
673 K in ca. 2.5 h. The sulfidation continued under these conditions
for 2 h. Then the sample was cooled down to room temperature,
changing the sulfidation mixture to nitrogen when the temperature
decreased to 423 K. The sulfided sample was carefully transferred to
the reactor in an argon atmosphere with the aim to avoid contact
with air. Then the reactor was flushed with nitrogen and heated
under stirring to reach the reaction temperature, hydrogen was
then introduced (PTot = 5 MPa). The reaction time was counted from
this moment. The total pressure was controlled constantly during
the course of reaction by adding hydrogen to compensate for its
consumption. Samples were periodically collected and analyzed
quantitatively by gas chromatography. The catalytic activity was
expressed by the initial reaction rate (mol DBT transformed per
second and per gram of catalyst).
2
2
TiO2.
2.2. Synthesis of NiW/MgO–TiO catalysts
2
In order to obtain catalyst with 26 wt.% WO3 and 4 wt.%
NiO, the support (100–150 mesh) was co-impregnated, using
the incipient wetness method, with a solution of ammonium
metatungstate (NH ) W12O39·H O (Aldrich ≥65%) and nickel
4
6
2
nitrate Ni(NO ) ·6H O (Aldrich ≥70%) using H O as solvent for
3
2
2
2
aqueous impregnation and methanol for non-aqueous impregna-
tion. After 12 h, the obtained solids were dried at 393 K for 10 h and
calcined at 673 K for 4 h. The solids were characterized and eval-
uated, before and after the calcination process. Nomenclature for
catalysts is shown in Table 1.
3. Results and discussion
2.3. Characterization of the solids
3.1. Supports and catalysts characterization
Surface area, pore volume, and pore size distribution of the sup-
ports and catalysts were obtained from N2 adsorption/desorption
isotherms using the conventional BET and BJH methods. The sam-
ples were outgassed at 573 K under vacuum. Afterwards, the N2
adsorption measurements were carried out at 73 K. XRD patterns
Textural properties for the supports and catalysts were
determined by N2 physisorption. Fig.
1
shows nitrogen
adsorption–desorption isotherms of the supports synthesized.
According to IUPAC classification, the MT10 and MT25 supports
work [21]. For the three mixed oxides the pore size distribution was
◦
◦
diffractometer using CuK␣ radiation (ꢁ = 1.5406 A˚ ) and a goniome-
◦
−1
ter speed of 1 (2ꢂ) min . UV–vis spectra of the samples were
Please cite this article in press as: A.E. Cruz Pérez, et al., NiW/MgO–TiO catalysts for dibenzothiophene hydrodesulfurization: Effect of
2