Deep oxidative desulfurization of model oil catalyzed by magnetic MoO3/Fe3O4

Article abstract of DOI:10.1039/c5ra11028a

A magnetic MoO₃/Fe₃O₄ catalyst was prepared and used in the oxidative desulfurization (ODS) of dibenzothiophene (DBT), benzothiophene, and thiophene. The morphology and components of the magnetic composite material were characterized by X-ray diffraction, scanning electron microscopy and X-ray photoelectron spectroscopy techniques. Results indicated that MoO₃ as an active species was supported on Fe₃O₄. High calcination temperature leads to the formation of Fe₂(MoO₄)₃ compounds which showed lower activity than MoO₃. The highest conversion of 99.5% for ODS of DBT can be achieved on MoO₃/Fe₃O₄ calcinated at 400 °C. In addition, the amount of MoO₃/Fe₃O₄, TBHP/DBT molar ratio and reaction temperature played vital roles in desulfurization efficiency. This work showed the possibility for the utilization of low cost magnetic MoO₃/Fe₃O₄ composite in the oxidative desulfurization because it shows high activity and could be easily separated by applying an external magnetic field, achieving ten times recycling without significant decrease in activity.

Full text of DOI:10.1039/c5ra11028a

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DOI: 10.1039/C5RA11028A
Deep oxidative desulfurization of model oil catalyzed by
magnetic MoO3/Fe3O4
Qin Zhou1, Shurong Fu1, Min Zou1, Yiming He2, Ying Wu1*, Tinghua Wu1*
1 Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, P R
China
2 Department of Material Physics, Zhejiang Normal University, Jinhua, 321004, P R
China
Corresponding author, Tel: +86 0579 82282234; fax: +86 0579 82282234.
Eꢀmail: yingwu@zjnu.cn (Y. Wu), thwu@zjnu.cn (T. Wu).
1

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Abstract
The magnetic MoO3/Fe3O4 catalyst was prepared and used in the oxidative
desulfurization (ODS) of dibenzothiophene (DBT), benzothiophene, and thiophene. The
morphology and components of the magnetic composite material were characterized by
Xꢀray diffraction, scanning electron microscopy and Xꢀray photoelectron spectroscopy
techniques. Results indicated that MoO3 as an active species was supported on Fe3O4.
High calcination temperature leads to the formation of Fe2(MoO4)3 compounds which
showed lower activity than MoO3. The highest conversion of 99.5 % for ODS of DBT
can be achieved on MoO3/Fe3O4 calcinated at 400oC. In addition, the amount of
MoO3/Fe3O4, TBHP/DBT molar ratio and reaction temperature played vital roles in
desulfurization efficiency. This work showed the possibility for the utilization of low
cost magnetic MoO3/Fe3O4 composite in the oxidative desulfurization because it shows
high activity and could be easily separated by applying an external magnetic field,
achieving ten times recycling without significant decrease in activity.
Keywords: MoO3/Fe3O4, magnetic composite particles, tertꢀbutyl hydroperoxide,
oxidative desulfurization
2

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1. Introduction
The combustion of fuels results in sulfur oxide (SOx) production, which leads to
acid rain, haze, or many other environmental issues.1 Thus, numerous countries have
promulgated regulations to restrict the sulfur content in fuels.2 Dibenzothiophene (DBT),
benzothiophene (BT), and thiophene (Th), as well as their derivatives, are main
sulfurꢀcontaining compounds.3 These substances are difficult to remove by traditional
hydrodesulfurization technology. In recent years, oxidative desulfurization was
developed to overcome this disadvantage and found to have the high removal efficiency
with convenient and simple operating procedures.4,5
In general, the catalysts for oxidative desulfurization can be divided into
heteropolyacid6,7, organic acids8,9, ionic liquids,10,11 and nanometal oxides. The main
12
focus in this field is deep oxidative desulfurization by nanometal oxides, such as V2O5
ZrO213, TiO214, MnO215, MoO316, and WO317. Compared with the traditional catalyst,
nanocatalysts possess higher surface and more active sites. However, these
,
nanocatalysts cannot be separated simply or rapidly in their applications. In recent years,
magnetic materials have been widely used in various fields because of their high
catalytic reaction activity and easy separation. The magnetic ionic liquids18ꢀ20 and
magnetizable complex21,22 have been used as catalysts in oxidative desulfurization to
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enhance separation efficiency. As of this writing, no report has been published on the
application of magnetic composite oxides in the oxidative desulfurization reaction.
In the current paper, the magnetic MoO3/Fe3O4 catalyst was synthesized and used
for the oxidative desulfurization of DBT model oil with tertꢀbutyl hydroperoxide
(TBHP) as oxidant. The catalyst exhibited high catalytic efficiency in oxidative
desulfurization and could be quickly separated and recovered by an external magnetic
field. The effects of reaction time, temperature, O/S molar ratio and catalyst amount
were studied on the sulfur removal rate of model oil.
2. Experimental
2.1 Materials
FeCl3, C19H42BrN (CTAB), anhydrous sodium acetate, ethylene glycol (EG),
HNO3, (NH4)6Mo7O24·4H2O (AMM), and Nꢀoctane were purchased from the Shanghai
Chemical Factory. Tertꢀbutyl hydroperoxide (TBHP), tetradecane, dibenzothiophene
(DBT), benzothiophene (BT), and thiophene (Th) were purchased from the Aladdin
Industrial Corporation. All reagents were commercially available and used as received
without further purification. Deionized water was used throughout all experimental
procedures.
2.2 Preparation
The Fe3O4 microspheres were prepared by hydrothermal treatment. Anhydrous
FeCl3 (2.43 g) was dissolved in C2H6O2 (60 mL) to form a clear solution, in which
anhydrous NaAc (4.92 g) and CTAB (0.72 g) were subsequently added under stirring.
The obtained homogeneous mixed solution was transferred into Teflonꢀlined stainless
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steel autoclave for hydrothermal treatment at 200 C for 10 h. After the autoclave was
cooled down to room temperature, the precipitate was collected by magnetic separation
and washed several times with deionized water and anhydrous ethanol, then dried under
vacuum at 50 oC before characterization and application.
The magnetic MoO3/Fe3O4 particles were prepared by precipitation method.
(NH4)6Mo7O24·4H2O (0.3 g) was dispersed in deionized water (50 mL) to form a clear
solution, followed by the addition of Fe3O4 (0.2 g). The obtained homogeneous solution
by ultrasonication was transferred to a threeꢀnecked flask. Nitric acid was added into the
solution to adjust pH value equal to1.5, and then the mixture was heated in water bath at
o
70 C for 10 h under mechanical stirring. After cooling down to room temperature, the
precipitate was collected by magnetic separation and washed with deionized water for
o
several times, then dried in vacuum oven at 60 C for 8 h. Finally, the obtained sample
was annealed in air at 400 oC for 4 h.
2.4 Oxidative desulfurization
Model oils were prepared by dissolving DBT, BT, or Th in nꢀoctane separately,
with the initial sulfur content of 500 ppm (ꢁg/g). The catalyst, model oil (5 mL) and
TBHP were added into a 50 mL twoꢀnecked flask in experimental proportion. Then the
flask was heated in oil bath under stirring vigorously with a mechanical stirrer. After the
reaction, the catalyst was separated from the reaction system under external magnetic
field.
2.5 Characterization
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Xꢀray diffraction (XRD) patterns of the catalysts were obtained on a Philips Xꢀ ray
diffractometer (PW 3040/60) using Cu Kα target at 40 kV and 40 mA. Scanning
electron microscopy (SEM) was performed with a Hitachi Sꢀ4800 scanning electron
microanalyzer with an accelerating voltage of 15 kV. The characterizations of surface
information and valence state of the catalysts were carried out with Xꢀray photoelectron
spectroscopy (XPS), using an ESCA System Xꢀray photoelectron spectrometer with Al
Kα Xꢀray as the excitation source. The FTꢀIR spectra of DBT oxidation products were
recorded on a NEXUS 670 FTꢀIR spectrometer with KBr pellets prepared by manual
grinding. The product was analyzed by gas chromatographyꢀflame ionization detector
(GCꢀFID) (Shanghai Tian Mei Scientific Instrument Co., Ltd., GCꢀ7890) equipped with
a TMꢀ5 capillary column (30 m × 0.32 mm i.d. × 0.5 ꢁm) for the detection of DBT, BT
Co ꢀCt
X =
×100%
and Th. The removal of the sulfides was calculated as follows:
, where
Co
C0 is the initial concentration of sulfur in the model fuel, and Ct is the concentration of
sulfur in the oil phase after the reaction began for a period of time.
3 Results and discussion
3.1 Characterizations of catalysts
XRD patterns of the catalysts are shown in Fig. 1. For the Fe3O4 sample, the main
peaks at 30.10°, 35.45°, 43.12°, 53.48°, 57.01°, and 62.58° corresponded to the (220),
(311), (400), (422), (511) and (440) planes. For MoO3/Fe3O4, the characteristic
diffraction peaks of orthorhombic MoO3 also appeared at 2θ = 12.77°, 23.31°, 25.70°,
27.33°, and 38.98°, aside from the characteristic diffraction peaks of Fe3O4.
The influence of the calcinations temperature on the catalyst was also analyzed by
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XRD characterization. The characteristic diffraction peaks of MoO3 weakened as the
calcination temperature increased. Only the diffraction peaks of Fe3O4 and MoO3 were
o
detected for the MoO3/Fe3O4 catalysts calcined at 400 C. When the calcination
temperature was raised to 500 oC and 600 oC, new diffraction peaks appeared at 15.27°,
20.40°, 21.73°, 22.88°, 25.61°, 27.46°, 30.12°, 34.06°, 35.77°, and 40.89°, which were
attributed to the characteristic diffraction peaks of Fe2(MoO4)3 (JCPDS No.
00ꢀ035ꢀ0183). Moreover, the peak intensity increased when the calcination temperature
was raised, and the Fe3O4 and MoO3 diffraction peaks couldnot be detected. These
results showed that the catalysts were converted to Fe2(MoO4)3 compound with
increasing calcination temperature. The optimal calcination temperature of catalyst was
o
400 C because the performance of MoO3 was much higher than that of Fe2(MoO4)3 in
the model oil desulfurization reaction.
Fig. 2 displays the scanning electron microscopy images of Fe3O4 and
MoO3/Fe3O4. The uniform Fe3O4 spherical structure was composed of small
selfꢀassembled Fe3O4 particles (Fig.2A and 2B). In comparison, the MoO3/Fe3O4
catalyst in Fig. 2C and 2D presented a smoother surface, with a diameter of 600 and
700 nm. The result was due to the fact that the Fe3O4 particles were covered by small
MoO3 particles.
o
The surface chemical states of MoO3/Fe3O4 calcined at 400 C were studied by
Xꢀray photoelectron spectroscopy (XPS). In Fig. 3A, a typical full XPS spectrum of
MoO3/Fe3O4 spheres showed photoelectron lines at binding energies of about 232, 711,
530 and 285 eV, corresponded to Mo3d, Fe2p, O1s, and C1s, respectively. In the Mo3d
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spectrum (Fig. 3B), the binding energies of Mo3d3/2 and Mo3d5/2 were about 232.7 eV
and 235.9 eV, respectively. The values were in accord with the MoO3 binding energy.
The result further proved the presence of MoO3 in the MoO3/Fe3O4 catalysts. The
Fe2p3/2 and Fe2p1/2 peaks were located at 711.2 and 725.2 eV, respectively, consistent
with the binding energy of Fe3O4. Fig. 3C shows the spectrum of O1s, and the peak at
530.4 eV may be attributed to anionic oxygen in Fe3O4 and MoO3.
Table 1 shows XPS results of MoO3/Fe3O4 calcined at different temperatures. The
binding energy of Fe2p3/2 and O1s increased with increasing calcination temperature,
implying that Fe element mainly existed in Fe3O4 form for MoO3/Fe3O4 calcined at 400
oC. When the calcination temperature was raised to 500 oC, the formation of
Fe2(MoO4)3 resulted in the increase in the binding energy of Fe2p3/2. In addition, the
o
Mo/Fe ratio for MoO3/Fe3O4 calcined at 400 C was larger than that for MoO3/Fe3O4
o
calcined at 500 C. These results showed that higher calcination temperature led to
Fe2(MoO4)3 formation on the surface.
The magnetic properties of the Fe3O4 and MoO3/Fe3O4 catalysts were studied using
a magnetometer at room temperature. Fig. 4 shows the magnetization of Fe3O4 and
MoO3/Fe3O4 at 300 K by cycling the field between ꢀ70 kOe and 70 kOe. The Fe3O4 and
MoO3/Fe3O4 catalysts presented a typical hysteresis loop phenomenon, and their
coercivities were 12.41 and 10.05 Oe, respectively. This finding revealed the weak
ferromagnetism behavior for Fe3O4 and MoO3/Fe3O4. The saturation magnetizations
were 76.40 and 27.91 emg/g, respectively. The decrease in saturation magnetization of
MoO3/Fe3O4 could be attributed to the addition of molybdenum oxide. The inset in Fig.
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4 shows the magnetic separation renderings. The brown MoO3/Fe3O4 catalyst was
uniformly dispersed in an aqueous solution and the solution quickly changed to be clear
by an external magnet. The MoO3/Fe3O4 catalyst could be easily separated from the
solution. Thus, this catalyst has great potential as a desulfurization catalyst carrier.
3.2 Catalytic Activity of MoO3/Fe3O4
The sulfurꢀremoval performance of catalysts is displayed in Table 2. Fe3O4 showed
negligible performance, with sulfur removal rate of only 2%. By contrast, MoO3
o
presented a high sulfur removal rate of 97.6%. MoO3/Fe3O4 calcined at 400 C
exhibited the best sulfur removal of 99.5%. When the calcination temperature was
raised to 500 oC and 600 oC, the sulfur removal rates of the MoO3/Fe3O4 catalysts were
40.9% and 37.9%, respectively. As shown in Table 2, Fe2(MoO4)3 compounds were
generated with the increase in calcination temperature, and the catalytic activity of this
compounds was lower than MoO3. Therefore, the sulfur removal rate of the
MoO3/Fe3O4 catalysts calcined under high temperature was reduced. The results
suggested that MoO3 was the active component for the catalytic oxidation of DBT in the
MoO3/Fe3O4 catalysts system. Meanwhile, Fe3O4 mainly played the role of a magnetic
mediator, which resulted in the separation of the MoO3/Fe3O4 catalyst from the reaction
system under an extra magnetic field.
3.3 Influence of O/S Molar Ratios on Sulfur Removal
The amount of oxidant is one of the most important factors affecting DBT removal
in the oxidative desulfurization system. According to the chemical reaction equation of
DBT oxidation into dibenzothiophene sulfone, oxidation of 1 mol DBT requires 2 mol
TBHP. In fact, however, more oxidants were required. Fig. 5 shows that the effects of
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oxidative desulfurization were dependent on the amount of oxidant for DBT
desulfurization with different O/S molar ratios. The oxidative desulfurization rate of
DBT maintained an upward trend with O/S molar ratios from 1:1 to 6:1, which may be
attributed to increasing the concentration of the oxidizing agent TBHP greatly will
promote the chances of sulfur compounds contact with the oxidant molecules and thus
improve the desulfurization rate gradually. However, when the oxidizing agent TBHP
was gradually depleted, the desulfurization rate remained almost unchanged 3 h later.
Therefore, the most appropriate ratio of O/S was 4:1. Under this condition the
desulfurization rate of 98.7% was achieved after reaction for 3 h.
3.4 Influence of the Amount of Catalyst on Sulfur Removal
Fig. 6 presents the sulfurꢀremoval experimental results at varying usage amounts of
the MoO3/Fe3O4 catalyst under fixed conditions, as noted in the legend. The amount of
catalyst significantly influenced the sulfurꢀremoval performance. When the amount of
catalyst increased from 3 mg to 8 mg, the desulfurization rate increased rapidly with the
reaction time. The most suitable amount of catalyst for our experiment was 6 mg. The
desulfurization rate of 97.6 % was obtained after reaction for 3 h.
3.5 Influence of Reaction Temperature on Sulfur Removal
The influence of temperature on the sulfurꢀremoval kinetics was studied in the
o
temperature range of 60oC to 110 C, and the results are presented in Fig. 7. The
reaction temperature greatly enhanced the sulfur removal rate. The sulfur removal of
o
DBT was visibly advanced after reaction for 1 h. When the temperature was 90 C, the
sulfur removal of DBT reached 83.5% in 2 h and sulfur was nearly removed after 2.5 h,
o
whereas the desulfurization efficiency appeared to be much lower at 60 C. However, a
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higher temperature at 110 C resulted in the thermal decomposition of TBHP. Thus, for
the present experiment and from a practical perspective, 90 oC and 3 h were selected as
the appropriate temperature and reaction time, respectively.
3.6 Oxidation of Different Sulfur-Compounds
As mentioned above, the model oil of DBT could be effectively removed by
MoO3/Fe3O4. To examine the desulfurization efficiency of the MoO3/Fe3O4 catalyst for
other organic sulfides, the sulfurꢀremoval performance of BT and Th was also studied
and compared with the model oil of DBT under similar reaction conditions. The results
are shown in Fig. 8. The reactivity of the three organic sulfides oxidized by TBHP
followed the order of DBT > BT > Th, which was dependent on the electron density of
the sulfur atom of different organic sulfides. In general, lower electron density on sulfur
atom resulted in lower activity of the sulfur removal rate. For DBT, BT, and Th, the
electron densities of the sulfur atom are 5.785, 5.739 and 5.696, respectively. Therefore,
it is highly efficient for the removal of DBT which is a key organic sulfide of diesel oils.
By contrast, the lower electron density of sulfur atom on BT and Th directly resulted in
low catalytic activity over the MoO3/Fe3O4 catalyst.
3.7 Influence of Recycle Times on Sulfur Removal
In the oxidative desulfurization system, the MoO3/Fe3O4 catalyst with magnetic
behavior was convenient and conducive to be separated under an external magnet. The
magnetic catalysts were reused for oxidative desulfurization and the Sꢀremoval rates are
presented in Fig. 9. After 10 cycles, no significant change in the desulfurization rate was
observed, demonstrating the high stability of the magnetic MoO3/Fe3O4 catalyst.
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The structure of the obtained white precipitate was detected by IR and the result is
presented in Fig.10. Compared with DBT, the product presented the absorption bands
for sulfone at 1288 and 1166 cmꢀ1, and the absorption band for sulfoxide at 1047 cmꢀ1,
suggesting that DBT was oxidized to DBTO and DBTO2 by using the MoO3/Fe3O4
catalysts.
4. Conclusions
In summary, the magnetic MoO3/Fe3O4 catalyst was synthesized and applied in the
oxidative sulfur removal of DBT. This study showed that the catalyst can be easily
separated from the reaction system by adding a magnet and recycled. After 10 cycles, no
obvious decrease was observed for oxidative sulfur removal using the MoO3/Fe3O4
catalysts. In addition, the amount of catalyst used was relatively small, and the
conversion of 99.5% was achieved under optimal conditions.
ACKNOWLEDGEMENTS
This work was supported by grants from the Nature Science Foundation of China (No.
21373188) and Nature Science Foundation of Zhejiang Province (LY12B03001).
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Figure Captions
Fig. 1 XRD patterns of magnetic Fe3O4 and MoO3/Fe3O4 with different calcination
temperatures
Fig. 2 SEM images of magnetic Fe3O4 (A,B) and MoO3/Fe3O4 (C,D)
o
Fig. 3 XPS spectra of magnetic MoO3/Fe3O4 calcined at 400 C: (A) full spectrum; (B)
Mo3d spectrum; (C) Fe2p spectrum; (D) O1s spectrum.
Fig. 4 Magnetization curves of Fe3O4 (A) and MoO3/Fe3O4 (B) and the photographs of
MoO3/Fe3O4 separated from the solution by applying an added magnet (inset)
Fig. 5 Influence of the mole ratio of O/S on sulfur removal
Fig. 6 Influence of the catalyst amount on sulfur removal
Fig. 7 Influence of reaction temperature on sulfur removal
Fig. 8 Sulfur removal of different sulfur compounds
Fig. 9 Influence of the recycle times on sulfur removal
Fig. 10 IR spectra of DBT (A) and its oxidation products (B)
Table 1 XPS results of the catalysts
Table 2 Oxidative desulfurization performance of the catalysts
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Fig. 1 XRD patterns of magnetic Fe3O4, MoO3 and MoO3/Fe3O4 with
different calcination temperatures
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Fig. 2 SEM images of Fe3O4 (A,B) and MoO3/Fe3O4 (C,D)
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Fig. 3 XPS spectra of MoO3/Fe3O4 calcined at 400 oC: (A) full spectrum;
(B) Mo3d spectrum; (C) Fe2p spectrum; (D) O1s spectrum.
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Fig. 4 Magnetization curves of Fe3O4 (A) and MoO3/Fe3O4 (B) and the
photographs of MoO3/Fe3O4 separated from the solution by applying an
added magnet (inset)
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Fig. 5 Influence of the mole ratio of O/S on sulfur removal
Reaction Conditions: T = 90 oC, V(DBT oil) = 5 mL, m(catalyst) = 6 mg.
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Fig. 6 Influence of the catalyst amount on sulfur removal
Reaction Conditions: T = 90 oC, V(DBT oil) = 5 mL, n(O):n(S) = 4:1.)
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Fig. 7 Influence of reaction temperature on sulfur removal
Reaction Conditions: V(DBT oil) = 5 mL, m(catalyst) = 6 mg, n(O):n(S) =
4:1.
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DOI: 10.1039/C5RA11028A
Fig. 8 Sulfur removal of different sulfur compounds
Reaction Conditions: V(DBT oil) = 5 mL, m(catalyst) = 6 mg, n(O):n(S) =
4:1.
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Fig. 9 Influence of the recycle times on sulfur removal
Reaction Conditions: T = 90 oC, m(catalyst) = 6 mg, n(O):n(S) = 4:1.
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Fig. 10 IR spectra of DBT (A) and its oxidation products (B)
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Table 1 XPS results of the MoO3/Fe3O4 catalysts
Binding Energy（eV）
Catalyst
Mo/Fe atomic ratio
Fe 2p3/2
711.2
Mo 3d5/2
232.7
O 1s
530.4
530.6
MoO3/Fe3O4（400 oC）
MoO3/Fe3O4（500 oC）
2.89
1.25
711.4
232.8
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Table 2 Oxidative desulfurization performance of the catalysts
Catalyst
Fe3O4
S-removel Rate
2 %
MoO3
97.6 %
MoO3/Fe3O4（400 oC）
MoO3/Fe3O4（500 oC）
MoO3/Fe3O4（600 oC）
99.5 %
40.9 %
37.9 %
（Reaction Conditions: V(DBT oil) = 5 mL, mcatalyst= 6 mg, n(O):n(S) = 4:1,
T = 90 oC.）
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Graphical Abstract
The magnetic MoO3/Fe3O4 catalyst showed good catalytic performance and stability
for oxidation desulfurization of dibenzothiophene, and it could be easily separated by
applying an external magnetic field.