Ultra-deep oxidative desulfurization of fuel with H2O2 catalyzed by molybdenum oxide supported on alumina modified by Ca2+

Article abstract of DOI:10.1039/c7ra08900g

A highly active catalyst of molybdenum oxide supported on mesoporous alumina modified by Ca²⁺ was synthesized by an in situ method and applied in the catalytic oxidative desulfurization (CODS) system. This catalyst was characterized by FT-IR, XRD, BET and XPS. The influences of m(catalyst)/m(model oil), V(H₂O₂)/V(model oil), reaction temperature and reaction time on oxidative desulfurization of 4,6-dimethyldibenzothiophene (4,6-DMDBT), dibenzothiophene (DBT) and benzothiophene (BT) were investigated. This catalyst has high desulfurization activity in the removal of organic sulfides under mild conditions. The catalytic oxidation reactivity of sulfur-containing compounds is in the order of DBT > 4,6-DMDBT > BT. The kinetic studies reveal that the oxidative desulfurization of the organic sulfides can present a pseudo first-order kinetic process, and the apparent activation energies of 4,6-DMDBT, DBT and BT are 34.67 kJ mol^-1, 33.01 kJ mol^-1 and 40.16 kJ mol^-1, respectively. The recycling experiments indicate that 4,6-DMDBT, DBT and BT removal can still reach 90.0%, 89.9% and 87.3% after eight cycles.

Full text of DOI:10.1039/c7ra08900g

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Ultra-deep oxidative desulfurization of fuel with
H2O2 catalyzed by molybdenum oxide supported
on alumina modified by Ca2+†
Cite this: RSC Adv., 2017, 7, 48208
*
Wei Jin,
Yongsheng Tian,
Guanghui Wang, Danlin Zeng,
Qian Xu
and Jiawei Cui
A highly active catalyst of molybdenum oxide supported on mesoporous alumina modified by Ca2+ was
synthesized by an in situ method and applied in the catalytic oxidative desulfurization (CODS) system.
This catalyst was characterized by FT-IR, XRD, BET and XPS. The influences of m(catalyst)/m(model oil),
V(H2O2)/V(model oil), reaction temperature and reaction time on oxidative desulfurization of 4,6-
dimethyldibenzothiophene (4,6-DMDBT), dibenzothiophene (DBT) and benzothiophene (BT) were
investigated. This catalyst has high desulfurization activity in the removal of organic sulfides under mild
conditions. The catalytic oxidation reactivity of sulfur-containing compounds is in the order of DBT >
4,6-DMDBT > BT. The kinetic studies reveal that the oxidative desulfurization of the organic sulfides can
present a pseudo first-order kinetic process, and the apparent activation energies of 4,6-DMDBT, DBT
and BT are 34.67 kJ molꢀ1, 33.01 kJ molꢀ1 and 40.16 kJ molꢀ1, respectively. The recycling experiments
indicate that 4,6-DMDBT, DBT and BT removal can still reach 90.0%, 89.9% and 87.3% after eight cycles.
Received 11th August 2017
Accepted 9th October 2017
DOI: 10.1039/c7ra08900g
rsc.li/rsc-advances
refractory sulfur compounds such as 4,6-dimethyldibenzothio-
phene (4,6-DMDBT), dibenzothiophene (DBT) and benzothio-
phene (BT).6–9 In order to solve these problems in
1. Introduction
Nowadays, manufacturing of ultra-low sulfur fuels has caused
worldwide research general concern.1,2 Sulfur oxides and sulfate
particulates are produced in the combustion process of the
sulfur organic compounds in fuel oil, which comes amid fears,
creating environmental pollution such as acid rain.3 Explosion
engines are also etched because of these sulfur organic
compounds in fuel oil.4 Furthermore, these hazardous
substances produced from the sulfur organic compounds in
fuel oil also play an important role in contaminating the cata-
lysts for tail gas treatment in catalytic converters.5 Thus, the oil
renery industry is facing more stringent environmental regu-
lations and laws all over the world. In order to meet the
requirements of environmental protection, various of desul-
furization methods were developed.
Conventionally, hydrodesulfurization (HDS) is the most
important technology for removal of the sulfur compounds
from fuel oil in the oil renery industry. However, HDS process
usually needs harsh reaction conditions including high
temperatures, high pressures and huge amounts of hydrogen.
HDS is high efficiency in removing of thiols, suldes, disuldes,
and some thiophenic compounds, but it is hard to remove the
hydrodesulfurization process, some other supplementary
desulfurization methods were researched extensively, such as
oxidative desulfurization (ODS),10 adsorption desulfurization,11
alkylation desulfurization,12 biodesulfurization13 and so on.
ODS is considered to be a promising method for ultra-deep
desulfurization under mild conditions because it is efficient
and economical.14 In ODS, non-polar organic suldes are
oxidized by oxidizing agent to their homologous sulfones under
catalysts and then remove it by rectication, extraction,
adsorption or other separation operation.15
In recent years, various oxidizing agents such as oxygen,16
ozone,17 hydrogen peroxide,18 cumene hydroperoxide (CHP),19
tertiary butyl hydroperoxide (t-BuOOH)20 have been used for the
oxidative desulfurization. However, hydrogen peroxide is more
popular than the other oxidants due to the environmental
friendliness, safety, high oxidation capacity and high selectivity.
A number of homogenous and heterogeneous catalysts are
developed in diverse ODS systems in order to increase the
oxidation activity of the oxidizing agent, for example, sodium
bicarbonate,21 organic acids,22 polyoxometalates,23 molecular
sieve,24 molybdenum oxide25 and so on. Among these, molyb-
denum oxide have been extensively researched and employed
because of its good performance in the CODS system. Such as,
Jia and coworkers26 reported an efficient oxidative desulfuriza-
tion with MoO3 as the active phase and H2O2 as oxidant under
Hubei Key Laboratory of Coal Conversion and New Carbon Material, School of
Chemistry and Chemical Engineering, Wuhan University of Science and Technology,
Wuhan 430081, Hubei, China. E-mail: wangguanghui@wust.edu.cn
† Electronic supplementary information (ESI) available: Additional experimental
details. See DOI: 10.1039/c7ra08900g
´
´
mild and solvent free conditions. Garcıa-Gutierrez and
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coworkers1 reported that Mo/gAl2O3 as the catalyst and H2O2 as water) : V(ethanol) : V(ammonia) ¼ 20 : 10 : 1) in a 100 ml three-
oxidant in the CODS. The sulfur level of diesel fuel can be neck ask. Aer that, AlCl3, ammonium molybdate, and calcium
ꢁ
reduced from about 320 to less than 10 ppmw at 60 C under nitrate were dropwise added in turn every 10 minutes and stirred
atmospheric pressure. Meanwhile, the catalyst support also at 1000 rpm. Next the mixture was continuously stirred for 1 h
shows great effects on catalytic oxidative desulfurization. before aging for 4 h at room temperature, subsequently separated
Silica,27 titanium dioxide,28 active carbon29 and alumina by ltration, thoroughly washed with deionized water and dried in
(Al2O3)30 as the catalyst supports have been widely investigated. a vacuum oven at 105 ꢁC. Finally, the dried samples were calcined
ꢁ
From the perspective of application, Al2O3 has received much at 540 C for 12 h and labeled as Ca/MoO3/Al2O3 (n(Mo)/n(Al) ¼
attention in both academia and industry community because of 16.7%, n(Ca)/n(Mo) ¼ 6.1%). The control group catalyst was
its good mechanical properties, adjustable pore structure and prepared without calcium nitrate according to the above method
high resistance to attrition. Such as, Wang et al.31 used t-BuOOH and labeled as MoO3/Al2O3.
as oxidant in DBT ODS in a ow-type xed-bed reactor. They
found that Al2O3 as catalyst supports showed higher catalytic
activity than those supported on TiO2 and SiO2. Al2O3 is also
easy to synthesize and a cheap material compared to other
supports. The study found that the introduction of different
kinds of promoters can adjust surface acidity and pore structure
in a wide range and increase the activity and selectivity of the
catalysts. Lanthanides and transition metal elements are the
common promoters of catalysts in the oxidation desulfurization
system. There are many reports about them. For instance,
Zhang and coworkers32 synthesized the catalyst MoO3/SiO2 with
the modication of La3+ and applied it in the catalytic oxidative
desulfurization system. Yan and coworkers33 reported that
mesoporous HPW/SiO2 modied with Ag species was used as
the catalyst for the oxidative desulfurization of both model and
real diesel oils. However, there are few studies about alkali-earth
metal elements as the promoters of catalysts in the oxidation
desulfurization system.
In this work, molybdenum oxide was loaded on the meso-
porous alumina by in situ synthesis method using the alkali-earth
metal Ca2+ as the promoter. The catalytic reaction conditions
m(catalyst)/m(model oil), V(H2O2)/V(model oil), reaction temper-
ature and reaction time, kinetic studies of the catalytic oxidation
and reuse of the catalyst were investigated in the oxidative
desulfurization of the model fuel oil. The results indicated that
the mesoporous alumina loaded MoO3 materials of Ca2+ modi-
ed exhibited a high catalytic activity in the oxidation desulfur-
ization system of 4,6-DMDBT, DBT and BT model fuel oil.
2.3. Characterization of the samples
FT-IR spectra (4000–500 cmꢀ1) were recorded at room temper-
ature on a Bruker VERTEX 70 FT-IR spectrometer using KBr in
the ratio of 1 : 200. The X-ray diffraction (XRD) patterns were
recorded on a Phillips Xpert Pro powder diffraction system
using Cu Ka radiation with a Ni lter over the range 10ꢁ # 2q #
90ꢁ. The pore structure of the catalysts were obtained from N2
adsorption isotherms which were obtained at ꢀ195.8 ꢁC in the
relative pressure range of 0.001–1 with a Quantachrome
instrument for Quadrasorb SI. The X-ray photoelectron spec-
trum (XPS) measurements were performed with a Multilab 2000
X-ray photoelectron spectrometer (VG, UK) (see ESI†).
2.4. Catalytic oxidative desulfurization
The model fuel was prepared by dissolving a certain amount of
4,6-DMDBT, DBT and BT in n-octane to get the solution with
a sulfur content of 400 mg gꢀ1 respectively. The catalytic oxida-
tion reaction was performed in a three necked glass ask with
a condenser, stirrer and thermometer. In these experiments,
10.0 ml of the model fuel oil and a certain amount of catalyst
were put into the reactor, which were placed in a water bath at
a constant temperature. When the reaction mixture was heated
to the set temperature, a certain amount of hydrogen peroxide
was added into the mixture, and the reaction was kept at the set
temperature under stirring at 1000 rpm for a certain time. Then
the mixture was rapidly cooled down to room temperature and
separated by centrifugation. The reaction products were
analyzed by GC-FID equipped with a HP-5 capillary column.
2. Experimental
2.1. Materials
3. Results and discussion
3.1. Oxidative desulfurization of the model fuel oil
All chemicals including ammonium molybdate, aluminium chlo-
ride, polyethylene oxide–polypropylene oxide–polyethylene oxide
(F127), methanol, ethanol, ammonia, calcium nitrate, 4,6-dime-
thyldibenzothiophene (4,6-DMDBT), dibenzothiophene (DBT),
benzothiophene (BT), n-octane, hydrogen peroxide (H2O2, 30%
wt), were purchased from Sinopharm Chemical Reagent Co., Ltd.
In order to verify the CODS activity, the catalytic oxidative
desulfurization of the 4,6-DMDBT, DBT and BT model fuel oil
was performed with the catalyst Ca/MoO3/Al2O3. The effect of
mass ratio of catalyst/model oil on 4,6-DMDBT, DBT and BT
removal rate of the 10 ml model oil was shown in Fig. 1. Clearly,
with the increase of catalyst amount, the oxidative removals of
4,6-DMDBT, DBT and BT increased gradually and then reached
2.2. Catalyst preparation
For the preparation of the catalysts, 3.17 g AlCl3, 0.67 g a plateau. When m(catalyst)/m(model oil) reached 0.56% for 4,6-
ammonium molybdate and 0.056 calcium nitrate were DMDBT or DBT (and 0.70% for BT), the removal rate of the
g
rstly dissolved respectively in 5.0 ml deionized water to spare. sulfur compound was close to 100.0%. The conversion of 4,6-
Then 0.38 g F127 was added into a 62.0 ml mixed solution DMDBT or DBT was higher than that of BT with the same
of deionized water, ethanol and ammonia (V(deionized amount of catalyst.
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Fig. 1 Effect of catalyst/model oil on 4,6-DMDBT, DBT and BT
removal. Reaction conditions for 4,6-DMDBT removal: V(H2O2)/
V(model oil) ¼ 0.20%, 55 ꢁC, 10 min; reaction conditions for DBT
removal: V(H2O2)/V(model oil) ¼ 0.20%, 55 ꢁC, 8 min; reaction
conditions for BT removal: V(H2O2)/V(model oil) ¼ 0.20%, 55 ꢁC,
16 min.
Fig. 2 Effect of H2O2/model oil on 4,6-DMDBT, DBT and BT removal.
Reaction conditions for 4,6-DMDBT removal: m(catalyst)/m(model oil)
¼ 0.56%, 55 ꢁC, 10 min; reaction conditions for DBT removal:
m(catalyst)/m(model oil) ¼ 0.56%, 55 ꢁC, 8 min; reaction conditions for
BT removal: m(catalyst)/m(model oil) ¼ 0.70%, 55 ꢁC, 16 min.
Fig. 3 Effect of reaction time on 4,6-DMDBT, DBT and BT removal.
Reaction conditions for 4,6-DMDBT removal: m(catalyst)/m(model oil)
¼ 0.56%, V(H2O2)/V(model oil) ¼ 0.20%; reaction conditions for DBT
removal: m(catalyst)/m(model oil) ¼ 0.56%, V(H2O2)/V(model oil) ¼
0.20%; reaction conditions for BT removal: m(catalyst)/m(model oil) ¼
0.70%, V(H2O2)/V(model oil) ¼ 0.20%.
The effect of volume ratio of H2O2/model oil on 4,6-DMDBT,
DBT and BT removal rate of the 10 ml model oil is shown in
Fig. 2. When V(H2O2)/V(model oil) reached 0.2% (n(O/S) ¼ 2.3),
the removal rate of 4,6-DMDBT, DBT and BT was close to
100.0%. However, when V(H2O2)/V(model oil) got up to more
than 0.2%, the removal rate of 4,6-DMDBT, DBT and BT
decreased signicantly. It is probably because excess H2O2
caused a side reaction catalyzed by Mo and produced excess
H2O. On the one hand, the side reaction occupied the active
center and increased the consumption of H2O2.34 On the other
hand, a percentage of the hydrophilic catalyst was transferred to
the water phase, thus reducing the catalyst dispersed in the oil
phase.34 This resulted the reduction of the desulfurization rate.
The effect of reaction temperature and reaction time on 4,6-
DMDBT, DBT and BT removal rate of the 10 ml model oil is
shown in Fig. 3. The experimental results show that both
enhancing reaction temperature and prolonging reaction time
are advantageous to the removal rate of model fuel oil. The
higher reaction temperature improved the performance of
catalyst greatly, because the higher temperature aggravated the
molecules thermal motion, meanwhile the formation quantity
of the metallic peroxide species which were produced by the
hydrogen peroxide and molybdenum oxide in the catalyst in
a unit time increased with increasing of reaction temperature.35
At 55 ꢁC, the oxidative removal rate of 4,6-DMDBT, DBT and BT
were close to 100.0% only in 10 min, 8 min and 16 min,
respectively. Meanwhile, under the same reaction conditions,
the oxidative removal rate of 4,6-DMDBT, DBT and BT catalyzed
with MoO3/Al2O3 were 64.4%, 68.3%, 53.9%, respectively. This
showed that the desulfurization rate of 4,6-DMDBT, DBT and
BT catalyzed with Ca/MoO3/Al2O3 was higher than with MoO3/
Al2O3 under the same conditions. The results indicated that
oxidation reactivities of these sulfur compounds decreased in
the order of DBT > 4,6-DMDBT > BT under the same reaction
conditions. This was because of the lower electron density of the
sulfur atom on BT (5.739). As calculated by Otsuki et al.36 The
electron densities on sulfur atoms are 5.760 for 4,6-DMDBT and
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Table 1 Reaction rate constant (k) and correlation coefficient (R2) for
oxidation reaction of 4,6-DMDBT, DBT and BT at different
temperatures
reported that the apparent activation energy of DBT in the
MoO3/Al2O3–CHP catalytic system was 67.5 kJ molꢀ1. The
experiment and comparison results indicate that the meso-
porous Ca/MoO3/Al2O3–H2O2 catalytic system in the present
work displays the higher catalytic activity.
Sample
t (ꢁC)
k (minꢀ1
)
R2
DBT
DBT
DBT
BT
BT
BT
4,6-DMDBT
4,6-DMDBT
4,6-DMDBT
45
50
55
45
50
55
45
50
55
0.201
0.247
0.294
0.119
0.154
0.190
0.189
0.235
0.283
0.981
0.985
0.992
0.989
0.993
0.989
0.977
0.981
0.987
3.3. Analysis of oxidation product
For analysis the oxidation product, GC-MS was used to research
the 4,6-DMDBT, DBT and BT oxidation reaction process cata-
lyzed by Ca/MoO3/Al2O3. Aer the oxidation reaction, the oil
phase and the solid catalyst were separated by centrifugation.
Aer that the catalyst was extracted by methanol. In the end, the
methanol phase were analyzed by GC-MS. Fig. 5 shows that MS
spectra of oxidized product of 4,6-DMDBT, DBT and BT. The
peak observed in the MS spectra at m/z ¼ 244.2, 216.0 and 166.1
were belonged to the molecular ion peak of 4,6-dimethyldi-
benzothiophene sulfone (4,6-DMDBTO2), dibenzothiophene
sulfone (DBTO2) and benzothiophene sulfone (BTO2). By
comparison with standard NIST data, 4,6-DMDBT, DBT and BT
5.758 for DBT. Therefore, it was clearly demonstrated that 4,6-
DMDBT and DBT can be oxidized more easily than that of BT.
The electron density of the sulfur atom on 4,6-DMDBT is higher
than that of DBT, however, 4,6-DMDBT has two methyl groups,
as well as the higher steric hindrance. As a result, the reactivity
of DBT was higher than that of 4,6-DMDBT. When catalyzed by
Ca/MoO3/Al2O3, the oxidative removal rate of DBT was close to
100.0% under 55 ꢁC aer 8 min. Chang and coworkers37 re-
ported that oxidation of dibenzothiophene with CHP on MoO3/
SiO2 modied with alkaline earth metals. The DBT conversions
ꢁ
on MoO3/Ca–SiO2 catalysts was the highest (95%) at 60 C, 4 h
and O/S molar ratio 3.0. It shows that the catalyst Ca/MoO3/
Al2O3 prepared in this research has higher activity than other
catalysts in ODS.
3.2. Apparent activation energy and reaction rate constant
The reaction rates determined at different temperatures were
summarized in Table 1. The apparent activation energies for the
CODS reactions (Fig. 4) were calculated by the Arrhenius
equation. The results indicated that the CODS reactions tted
pseudo rst order reaction kinetics. As can be seen from Fig. 4,
the activation energies of 4,6-DMDBT, DBT and BT oxidation
reactions were 34.67 kJ molꢀ1
,
33.01 kJ molꢀ1 and
40.16 kJ molꢀ1, respectively. The activation energies of DBT and
4,6-DMDBT are lower than that of BT, which proves the easier
oxidation of DBT and 4,6-DMDBT. M. A. Safa and coworkers4
Fig. 4 Arrhenius activation energies for 4,6-DMDBT, DBT and BT
oxidation.
Fig. 5 MS spectra of oxidized product of 4,6-DMDBT, DBT and BT.
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catalyst. The recycling experiments of Ca/MoO3/Al2O3 indicated
that 4,6-DMDBT, DBT and BT removal still reached 90.0%,
89.9% and 87.3% aer 8 cycles.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This work was nancially supported by the National Natural
Fig. 6 Effect of reuse times on the oxidative desulfurization rate of
Science Foundation (No. 21473126).
4,6-DMDBT, DBT and BT.
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experimental result indicated that the oxidation product of 4,6-
DMDBT, DBT and BT were 4,6-DMDBTO2, DBTO2 and BTO2,
which were adsorbed by the solid catalyst because of its highly
polar nature.
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