Oxidation of dibenzothiophene with cumene hydroperoxide on MoO3/SiO2 modified with alkaline earth metals

Article abstract of DOI:10.1016/j.cattod.2009.04.026

Catalytic oxidation of dibenzothiophene (DBT) in decalin was performed using an oil-soluble oxidant, cumene hydroperoxide (CHP), over molybdenum oxide (MoO₃) supported on silica. The effects of MoO₃ loading, reaction time and the molar ratio of CHP/DBT were investigated. At a MoO₃ loading of 15 wt%, the conversion of DBT reached 82% at 70 °C, WHSV 30 h^-1, and O/S molar ratio 3. Alkaline earth metals, such as Ca, Ba, Sr and Mg were introduced on the surface of silica, prior to the impregnation of MoO₃. The results showed that the activity in the oxidation of DBT with CHP decreased in the order: MoO₃/Ca-SiO₂ > MoO₃/Ba-SiO₂ > MoO₃/SiO₂ > MoO₃/Sr-SiO₂ > MoO₃/Mg-SiO₂. The MoO₃/Ca-SiO₂ catalysts were characterized by XRD. The DBT conversions on MoO₃/Ca-SiO₂ catalysts with various Ca/Mo ratios were studied. When the Ca/Mo ratio was 0.05, the DBT conversion was the highest (95%) at 60 °C, WHSV 30 h^-1, and O/S molar ratio 3.0.

Full text of DOI:10.1016/j.cattod.2009.04.026

Catalysis Today 149 (2010) 122–126
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Oxidation of dibenzothiophene with cumene hydroperoxide on MoO /SiO
3
2
modified with alkaline earth metals
a
a,b,
a,b
a,b
a,b
Jie Chang , Anjie Wang *, Jing Liu , Xiang Li , Yongkang Hu
a
Department of Catalysis, State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, China
Liaoning Key Laboratory of Petrochemical Technology and Equipments, Dalian University of Technology, Dalian 116012, China
b
A R T I C L E I N F O
A B S T R A C T
Article history:
Catalytic oxidation of dibenzothiophene (DBT) in decalin was performed using an oil-soluble oxidant,
cumene hydroperoxide (CHP), over molybdenum oxide (MoO ) supported on silica. The effects of MoO
Available online 10 June 2009
3
3
loading, reaction time and the molar ratio of CHP/DBT were investigated. At a MoO
3
loading of 15 wt%,
À1
Keywords:
the conversion of DBT reached 82% at 70 8C, WHSV 30 h , and O/S molar ratio 3. Alkaline earth metals,
Oxidative desulfurization
Cumene hydroperoxide
Dibenzothiophene
such as Ca, Ba, Sr and Mg were introduced on the surface of silica, prior to the impregnation of MoO
results showed that the activity in the oxidation of DBT with CHP decreased in the order: MoO
SiO > MoO /Ba-SiO > MoO /SiO > MoO /Sr-SiO > MoO /Mg-SiO . The MoO /Ca-SiO catalysts were
characterized by XRD. The DBT conversions on MoO /Ca-SiO catalysts with various Ca/Mo ratios were
3
. The
3
/Ca-
2
3
2
3
2
3
2
3
2
3
2
MoO
3 2
/SiO
3
2
Calcium
studied. When the Ca/Mo ratio was 0.05, the DBT conversion was the highest (95%) at 60 8C, WHSV
À1
3
0 h , and O/S molar ratio 3.0.
ß 2009 Elsevier B.V. All rights reserved.
1
. Introduction
In recent years, much attention has been paid to deep
properties of the resulting sulfone are significantly different from
those of hydrocarbons in fuel. Therefore, they can be easily
removed by distillation, solvent extraction, and adsorption [1].
Quite a few types of oxidants have been reported, such as
hydrogen peroxide [8–15], nitrogen dioxide or nitric acid [16], air
desulfurization of diesel fuel because the exhaust gases containing
SOx are the major origin of air pollution and acid rain. In a refinery,
the removal of sulfur from petroleum distillates is mainly through
the so-called hydrotreating process, in which hydrodesulfurization
[17,18],
2
O /aldehyde [19,20], tert-butyl hydroperoxide [21],
cyclohexanone peroxide [22], and cumene peroxide [23]. Hydro-
gen peroxide is the most commonly used oxidant among these
(HDS) is involved. However, it is considerably difficult to achieve
ultra low sulfur levels by HDS, due to the significantly low
reactivity of sulfur containing polyaromatic compounds, such as
dibenzothiophene (DBT) and its derivatives [1]. Another dis-
advantage of HDS is the high consumption of hydrogen, which is
costly and sometimes not readily available.
2 2
oxidants. Nevertheless, the ODS by H O usually takes place in a
batch reactor, because at least two phases are involved in the
reaction. It is apparent a flow-type reaction system is preferable in
a modern refinery with a high throughput. Wang et al. [21] used
tert-butyl hydroperoxide (t-BuOOH) as oxidant in DBT ODS in a
An alternative or polishing process is therefore desirable.
Several non-HDS approaches to desulfurization have been
extensively investigated recently [2–6]. Among them, oxidative
desulfurization (ODS) is considered to be one of the promising new
methods for ultra-deep desulfurization of engine fuels. ODS
generally takes place under mild conditions (<100 8C and atmo-
spheric pressure). In an oxidation process, DBT, which is one of the
most refractory sulfur compounds in HDS, is readily oxidized to
yield the corresponding sulfone [7]. The chemical and physical
flow-type fixed-bed reactor. They found that Al
MoO showed higher catalytic activity than those supported on
TiO and SiO –Al . Nevertheless, our preliminary study showed
that Al -supported MoO was not stable in the oxidation.
In the present study, cumene hydroperoxide (CHP), an inter-
mediate in the simultaneous production of phenol and acetone, was
used as an oil-soluble oxidant. Alkaline earth metal modified Mo/
2 3
O -supported
3
2
2
2 3
O
2
O
3
3
SiO
. Experimental
.1. Catalysts preparation
2
was used as the catalyst in DBT ODS in a fixed-bed reactor.
2
2
*
Corresponding author at: Department of Catalysis, State Key Laboratory of Fine
Chemicals, Dalian University of Technology, 158 Zhongshan Road, Dalian 116012,
China. Fax: +86 411 39893693.
2
Aerosil silica (surface area 250 m /g) was used as the support.
Ammonium heptamolybdate, calcium nitrate, magnesium nitrate,
E-mail address: ajwang@dlut.edu.cn (A. Wang).
0920-5861/$ – see front matter ß 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2009.04.026

J. Chang et al. / Catalysis Today 149 (2010) 122–126
123
Scheme 1. Oxidation of DBT by CHP.
strontium nitrate and barium nitrate were all of CR grade. SiO
2
-
detected, and no DBT sulfoxide was found. Both feed and
product were analyzed on an Agilent-6890+ gas chromatograph
equipped with an FID using an HP-5 capillary column (5% phenyl
supported MoO catalysts with various Mo contents were prepared
3
by an incipient wetness impregnation method. An amount of 1 g
silica was impregnated with 3 ml aqueous solution of ammonium
heptamolybdate with required concentration, and then kept for 8 h
at room temperature. The obtained slurry was dried overnight at
methyl polysiloxane, 30.0 m  320
m
m  0.25
mm).
2.3. Characterization
1
00 8C, and the resulting solid was calcined in air at 550 8C for 5 h.
MoO loading level was varied from 5 to 35 wt%.
The preparation of Ca promoted MoO /SiO (MoO
3
X-ray powder diffraction (XRD) patterns of catalysts were
collected on a D/max-2400 diffractometer using Cu-Ka radiation.
3
2
3
/Ca-SiO
2
) is
illustrated as an example. An amount of 1 g silica was impregnated
in 3 ml aqueous solution of calcium nitrate solution (4–8 g/l) for
3. Results and discussion
8
h at room temperature, followed by drying overnight at 100 8C
and calcination at 550 8C for 5 h. The resulting materials was
impregnated with 3 ml aqueous solution of ammonium hepta-
molybdate with required concentration, and then kept for 8 h at
room temperature. The obtained material was dried overnight at
The oxidation of DBT by CHP is represented in Scheme 1. The
product from the oxidation of DBT was DBT sulfone, whereas CHP
was transformed to 2-phenyl-2-propanol after donating the
oxygen atom. These compounds were confirmed by GC–MS. It
was found that no oxidation of DBT was observed in the absence of
the catalyst.
1
00 8C, followed by calcination at 550 8C for 5 h. In all alkaline
earth metal promoted catalysts, the MoO loading was 15 wt%, and
3
the Ca/Mo molar ratio was varied from 0.05 to 0.2. The preparation
of other alkaline earth metal modified catalysts was prepared in a
similar procedure, and molar ratio of alkaline earth metal to Mo
was 0.1.
3.1. MoO /SiO
2
3
Fig. 1 shows DBT conversion in the oxidation over Mo/SiO2
catalysts at various MoO
À1
3
loadings at 70 8C, WHSV 30 h and O/S
2.2. DBT oxidation
molar ratio 3.0. With increasing MoO loading level, the conversion
3
of DBT increased initially and then decreased. A maximum
Both decalin and cumene hydroperoxide (CR grade) were used
without purification.
The oxidation of DBT was performed in a fixed-bed glass
reactor (8 mm i.d.). Typically, the reactor was loaded with 0.2 g
catalyst particles (the catalyst bed height was about 2 cm). The
feed (1000 ppm DBT in decalin) was fed into the reactor at the
conversion of DBT (82%) was observed at a MoO3 loading of
15 wt%. The highest activity may be attributed to the monolayer
dispersion of MoO on SiO at 15 wt%. The decrease in the activity
3
2
of the catalyst at higher MoO3 loading may be attributed to the
aggregation of MoO particles on SiO [24]. Poor dispersion reduces
3
2
the coordination of Mo–O bond to SiO , leading to a lowered
2
reaction temperature by
a
syringe pump. The oxidation
polarity of Mo–O bond and the decreased ODS activity.
conditions were as follows: atmospheric pressure; WHSV,
Fig. 2 shows DBT conversion as a function of temperature over
MoO /SiO catalysts with various MoO3 loadings. Maximum DBT
À1
3
0 h ; O/S molar ratio, 3; reaction temperature, 40–80 8C.
3
2
The liquid products were collected every 60 min. The reaction
was carried out for 4 h. In our study, only DBT sulfone was
conversions were observed at 70 8C for all loading levels. The
increase of DBT conversion with increasing reaction temperature is
Fig. 1. Variation of DBT conversion with MoO
SiO catalysts.
3
loading in the oxidation over MoO
3
/
Fig. 2. Oxidation activities of DBT as a function of temperature over MoO
catalysts at various MoO loading.
3 2
/SiO
2
3

1
24
J. Chang et al. / Catalysis Today 149 (2010) 122–126
3 2
Fig. 3. Oxidation activities of DBT over MoO /SiO catalyst at various O/S ratios.
due to the enhanced reaction kinetics. Because all three catalysts
showed decreased ODS activity at temperatures over 70 8C, the
decrease in DBT conversion might be related with the increased
decomposition of CHP at high temperatures.
3 2
Fig. 5. XRD patterns of MoO /SiO modified by various alkaline earth metals.
The effect of O/S molar ratio on DBT conversion was
À1
investigated at 70 8C and WHSV 30 h
. Stoichiometrically,
2
mol of CHP is required for converting 1 mol of DBT to its
MoO
obtained at 60 8C for MoO
at 70 8C for MoO /SiO , MoO
and Ba significantly improved the performance of MoO
catalyst in the oxidation.
3
/Mg-SiO
2
.
Moreover, the highest DBT conversion was
/Ca-SiO and MoO /Ba-SiO , whereas
/Sr-SiO , and MoO /Mg-SiO . Both Ca
/SiO
2
sulfone. As shown in Fig. 3, DBT conversion increased with the
increase of O/S molar ratio until 3.0. This ratio is higher than the
stoichiometric O/S molar ratio. The difference may be attributed to
the thermal decomposition of CHP. When the O/S molar ratio was
over 3.0, DBT conversion was leveled off, probably due to the
limitation of residence time in the fixed-bed.
3
2
3
2
3
2
3
2
3
2
3
According to Wachs and coworkers [26], molybdenum oxide
supported on silica was a typical example of poorly dispersed
3
catalyst, because molybdenum oxide, such as MoO , and silica
3
.2. MoO
3
/SiO
2
modified by alkaline earth metals
are considered to have an acidic character, thus weak interaction
between molybdenum oxide and silica is expected. Since
alkaline earth metals possess strong basic characters, it is
Malka and Tatibou e¨ t [25] reported that Ca was a promoter of
MoO /SiO catalyst in the oxidation reaction of methanol. In the
present study, several alkaline earth metals were used to modify
MoO /SiO catalysts in DBT oxidation. Fig. 4 shows that the activity
of catalysts for oxidation of DBT with CHP decreased in the order:
MoO /Ca-SiO > MoO /Ba-SiO > MoO /SiO > MoO /Sr-SiO
3
2
3
therefore possible to improve the dispersion of MoO on silica
by the introduction of alkaline earth metals. On the other hand,
the presence of alkaline earth metals may lead to the formation
of molybdates, which are not active phases in DBT oxidation.
3
2
3
2
3
2
3
2
3
2
>
3 2
The XRD patterns of the modified MoO /SiO (Fig. 5) indicate the
appearance of various molybdates. It is reported that Ca can act
as germination sites towards molybdenum oxide [25], which
might be responsible for the improved dispersion of Mo oxides
3 2
and enhanced oxidation performance of MoO /Ca-SiO . BaO
react readily with CHP to form complex [27], suppressing CHP
decomposition and leading to an improved performance of
MoO
3
/Ba-SiO
2
. Among the alkaline earth metals, Ca modification
2
/SiO .
most significantly improved the performance of MoO
3
Therefore, the modification of Ca was investigated in more
detail.
Fig. 6 shows the oxidation activity of DBT in presence of
3 2
MoO /Ca-SiO catalysts at various Ca/Mo ratios. The highest DBT
conversion (95%) was observed at Ca/Mo ratio of 0.05. At higher
Ca/Mo ratio, such as 0.1, DBT conversion was decreased slightly.
3
Crystalline molybdenum oxide (MoO ) is the dominant species
at higher loadings and stable against dehydration [28]. The
polarity of Mo–O bond is the main factor that affects the activity
in oxidation. Increasing the polarity of Mo–O bond leads to an
increased electrophilicity of peroxy oxygen. Ca cations at low
Ca/Mo ratio may increase the polarity of Mo–O bond and thus
the activity of CHP on MoO
molar ratio, however, calcium molybdate (CaMoO
3
/Ca-SiO
2
catalyst. At high Ca/Mo
may
Fig. 4. Catalytic performance of MoO
3 2
/SiO modified by various alkaline earth
metals in DBT oxidation as a function of temperatures.
)
4

J. Chang et al. / Catalysis Today 149 (2010) 122–126
125
Fig. 6. DBT conversion over MoO
3
/Ca-SiO
2
with Ca/Mo atomic ratio.
Fig. 8. Comparison in oxidation reactivity of DBT and 4,6-DMDBT over MoO
SiO at various temperatures.
3
/Ca-
2
4. Conclusions
CHP is a suitable oil-soluble oxidant, which can be used to
oxidize DBT over silica supported MoO -based catalysts in a fixed-
3
bed reactor. When MoO
reached 82% at 70 8C, WHSV of 30 h and O/S molar ratio of 3.
3
loading was 15 wt%, DBT conversion
À1
Modification of MoO
positive or negative effects on the performance in DBT oxidation.
The activities in DBT oxidation decreased in the order: MoO /Ca-
/Mg-
. The introduction of Ca cation not only increased the
3 2
/SiO by alkaline earth metals could have
3
SiO
SiO
2
3 2 3 2 3 2 3
> MoO /Ba-SiO > MoO /SiO > MoO /Sr-SiO > MoO
2
conversion of DBT, but also decreased the optimal reaction
temperature. When the Ca/Mo ratio was 0.05, the DBT conversion
could reach 95%. 4,6-DMDBT showed lower oxidation activity than
DBT, probably due to the steric hindrance of methyl groups in the
molecule of 4,6-DMDBT.
Acknowledgements
This work is supported by the Natural Science Foundation of
China (Grant Nos. 20503003, 20333030, 20773020).
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