6
0
A. López-Benítez et al. / Journal of Catalysis 344 (2016) 59–76
(
1) the use of new active phases based on transition metals (for
characterized at different steps of the preparation: after impregna-
tion and drying, after calcination, and finally, in their sulfide state.
The resulting NiMo catalysts were then evaluated in hydrodesulfu-
rization, using dibenzothiophene as a representative sulfur-
containing screening molecule.
example, Ru, Fe, Rh, Pt) [14], carbides [15], or the quite active phos-
phides [16–23], (2) the use of new methods for preparing sup-
ported MoS
the use of new supports to replace the traditional
such as SiO , Zr-TiO , C, MCM-41, or SBA-16, or (4) the modification
of the -Al support by additives or other oxides (e.g.,TiO
Al SiO –Al F–Al Ga–Al B–Al
25–31].
The ability to form the NiMoS phase also depends on the type of
2
and WS
2
catalysts promoted by Ni or Co [1,24], (3)
c
-Al support
2 3
O
2
2
c
O
2 3
2
–
2
O
3
,
2 2
–Al O
3
,
P
2
O
5
2
O
3
,
2
O
3
,
2
O
3
,
2 3
O )
2
. Experimental
[
2
.1. Synthesis of the Al O support covered with manganese
2 3
supported NiMo polyoxometalate [32]. Depending on the polyox-
ometalate and its interaction with the support, different NiMoS
active species can be formed. Also, nonactive species can be
obtained, such as Ni or Mo oxisulfides or nonpromoted Ni or Mo
sulfides.
The reactants used for the catalyst synthesis were as follows:
aluminum isopropoxide Al(OC (Sigma–Aldrich, 98+%), 1-
propanol (Sigma Aldrich >99.8%), manganese(II) acetate tetrahy-
drate Mn(CH COO) O (Sigma Aldrich, 99+%), nickel nitrate
ꢀ4H
hexahydrate Ni(NO O (J.T. Baker, ACS Reagent), ammonium
ꢀ6H
heptamolybdate tetrahydrate (NH Mo O (Fluka, >99%),
24ꢀ4H
ammonium hydroxide NH OH (J.T. Baker, ACS reagent), and deion-
ized water (18.2 M cm).
The Al solid was prepared by the sol–gel method [8]. For
this purpose, 40.85 g of aluminum isopropoxide was dissolved in
80 mL of 1-propanol. The gel was then obtained by hydrolysis
3 7 3
H )
3
2
2
The objective of the present study is to evaluate the influence
3
)
2
2
2 3
of adding manganese to Al O on dibenzothiophene hydrodesul-
4
)
6
7
O
2
furization activity of NiMo catalysts. Manganese oxide is used
as a catalyst for industrial oxidation processes such as oxidation
of CO [33], methanol [34], ethylene [35], or nitric oxide [36]
and for combustion reactions [37]. This is due to the ability of
Mn to present various oxidation states (II, III, IV, and VII) [38].
Particularly, manganese oxides are used to control the environ-
mental pollution caused by volatile organic compounds (VOCs)
4
X
2 3
O
3
with dropwise addition of deionized water (40 mL). The gel
was dried at 100 °C for 12 h and calcined at 600 °C (5 °C/min)
for 4 h.
[
39]. In a study of postmetallocene catalysts, Fujisawa et al. [40]
mentioned the importance of the oxidation state of transition
metals in the catalytic activity. Catalysts based on Mn (II) present
higher catalytic activity than catalysts based on Mn (III) and Mn
Addition of manganese on the alumina surface was performed
using the incipient wetness impregnation method. Al
2
O
3
with
with
1
mol.% Mn as MnO was prepared by impregnating 1.0 g Al O
2 3
(
IV). On the other hand, Ban et al. [41] showed that complexes
II
a solution of 0.0243 g of manganese acetate dissolved in 1.8 mL of
-propanol. The solid was then dried at 120 °C for 12 h and cal-
cined at 550 °C (5 °C/min) for 4 h. This support is named RMn1.
In the case of 5 mol.% of Mn as MnO, 1.0 g of Al was impreg-
2
based on Mn (II) as [Mn (Cp) ] can catalyze ethylene polymeriza-
1
tion. Therefore, the oxidation state of manganese undoubtedly
plays a major role in explaining catalytic behavior in many
applications [42].
Manganese oxide is known to be unstable. However, man-
ganese can be stabilized in NiMnMo clusters [43], or when sup-
ported on Al O , ZrO , TiO , or SiO . MnO /Al O catalysts are
2 3 2 2 2 x 2 3
used, for example, in ozone decomposition [44], for the reduction
of benzaldehyde to ethanol [45], for the selective reduction of
2 3
O
nated with a solution of 0.1215 g of manganese acetate dissolved
in 2.5 mL of 1-propanol. The solid was then dried and calcined
using conditions similar to those described before for RMn1. This
support is called RMn5.
NO
x
[46], and for the catalytic oxidation of toluene [38]. Neverthe-
or ZrO gener-
less, manganese oxide catalysts supported on Al
2
O
3
2
2.2. Synthesis of the NiMo/RMn1 and NiMo/RMn5 catalysts
ally have low surface areas and can form spinels, as suggested by
Polato et al. [47].
NiMo/RMn1 and NiMo/RMn5 catalysts were prepared by co-
impregnation of nickel nitrate and ammonium heptamolybdate
on the RMn1 and RMn5 supports using the incipient wetness
Pecoraro et al. [48] and more recently Chianelli et al. [49]
reported the dibenzothiophene hydrodesulfurization activity of a
series of transition metal sulfide catalysts according to their posi-
tions in the periodic table. Manganese sulfide catalysts have the
lowest activity of the first series of transition metal sulfides. In
another study, Villaroel et al. [50] showed for different series of
3
method. Final loadings of 14 wt.% of MoO and 3.1 wt.% of NiO
were obtained, corresponding to a Ni/(Ni + Mo) molar ratio of 0.3.
Two different pHs (7 or 9) were used during the impregnation
of the Ni and Mo precursor solutions. For the impregnation solu-
tion at pH 7, nickel nitrate hexahydrate (0.1412 g) and ammonium
heptamolybdate tetrahydrate (0.1997 g) were dissolved in deion-
ized water (1.5 mL) before impregnation onto 1.0 g of support.
The same conditions were used for the impregnation at pH 9,
promoters of MoS
promotes the least molybdenum sulfide in dibenzothiophene
DBT) HDS. They concluded that manganese is not a good pro-
2 2 3
/c-Al O that manganese is the element that
(
moter. However, recently, Ho [43] reported that supported
NiMnMo catalysts show high activity in hydrodesulfurization of
sulfur compounds such as alkyldibenzothiophenes. This suggests
that Mn is poorly active as the MnS phase and does not act as a
4
except that the pH was adjusted with NH OH solution (0.5 M).
Similar protocols using the same amounts of Ni or Mo precursors
were used for preparing Ni-only or Mo-only catalysts supported
on RMn1 or RMn5. After the impregnation step, catalysts were first
dried at 120 °C for 12 h before being calcined at 400 °C (5 °C/min)
promoter atom of MoS
manganese seems able to increase the activity of already
Ni-promoted MoS /Al catalysts.
2
for hydrodesulfurization. However,
2
2
O
3
for 4 h. Calcined catalysts were then ground and the 80–125 lm
Manganese has not yet been studied as a support in the
hydrodesulfurization of dibenzothiophene. However, different oxi-
dation states of manganese make it possible to obtain a reducing or
oxidizing support that would influence the deposition of Ni and Mo
species. In order to study the role of manganese, we synthesized
particle size range was selected. NiMo catalysts were called
NiMo/RMn1 pH 7, NiMo/RMn1 pH 9, NiMo/RMn5 pH 7, and
NiMo/RMn5 pH 9, respectively, and suffixes -D and -C were used
to indicate dried and calcined samples, respectively.
The reference catalyst for activity measurements was a Ni–Mo/
two different Al
2
O
3
supports covered with 1 and 5 mol.% of MnO,
c
-Al
2 3 3
O catalyst containing 3 wt.% NiO and 14 wt.% MoO . The solid
2
ꢁ1
respectively. Supports have been impregnated with Ni/Mo precur-
sor solutions at pH 7 or 9. The NiMo polyoxometalates were then
presents a BET surface area of 250 m g and a pore volume of
0.48 cm g
3
ꢁ1
.