One-pot extractive and oxidative desulfurization of liquid fuels with molecular oxygen in ionic liquids

Article abstract of DOI:10.1039/c4ra09659b

Benzothiophene (BT), dibenzothiophene (DBT) and 4,6-dimethlydibenzothiophene (4,6-DMDBT) were extracted from an oil phase to ionic liquid phase, and then oxidized to the corresponding sulfone by the cheap catalyst, N-hydroxyphthalimide (NHPI), using molecular oxygen as the oxidant in one-pot. The system can be recycled 5 times without a significant decrease in desulfurization.

Full text of DOI:10.1039/c4ra09659b

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One-pot extractive and oxidative desulfurization of
liquid fuels with molecular oxygen in ionic liquids
Cite this: RSC Adv., 2014, 4, 59885
a
Jianlong Wang, Qingping Guo,b Changming Zhangac and Kaixi Lia
*
Received 2nd September 2014
Accepted 5th November 2014
DOI: 10.1039/c4ra09659b
www.rsc.org/advances
Benzothiophene (BT), dibenzothiophene (DBT) and 4,6-dimethlydi- organic acid, AcOH, as a catalyst in the presence of H2O2.4 The
benzothiophene (4,6-DMDBT) were extracted from an oil phase to Li group used the peroxotungsten complex, polymolybdates as
ionic liquid phase, and then oxidized to the corresponding sulfone by the catalyst, and ILs as the extractant to desulfurize organic
the cheap catalyst, N-hydroxyphthalimide (NHPI), using molecular sulfur of fuels in the presence of H2O2.5 The different acid ILs
oxygen as the oxidant in one-pot. The system can be recycled 5 times and ILs based on metal were also employed as extractant and
without a significant decrease in desulfurization.
catalyst at the same time for desulfurization of fuel.6 However,
the oxidant, H2O2, in the above-mentioned ODS system is
expensive.
Sulfur compounds in fuels are converted to SOx during
combustion, which results in acid rain and poisons catalytic
converters in cars. Primarily for environmental concerns,
governments worldwide have issued increasingly stringent
regulations to limit sulfur levels in fuels. In the petroleum
rening industry, the conventional method for reducing sulfur
is catalytic hydrodesulfurization (HDS), which is highly effective
for removing thiols, suldes, and disuldes. However, it is
difficult to remove thiophenes with steric hindrance on the
sulfur, such as DBT and it derivatives.1 Severe HDS conditions
are required in order to remove such compounds. These oper-
ating conditions result in large hydrogen consumption and a
signicant increase in the operating expenses. Therefore,
extensive research is carried out to propose alternative tech-
nologies to obtain low-sulfur fuels.2 Ionic liquids (ILs) as a type
of “green solvent” were widely used as extractant in lab for
extractive desulfurization of fuel.3 The efficiencies of sulfur
removal, various kinds of ILs used as extractants, however, are
rather low because of the similar polarity between the sulfur-
containing molecules and the fuels. To obtain deep desulfur-
ization, multistage extraction must be operated. In order to
obtain deep desulfurization, oxidative desulfurization was
combined with extraction using ILs as extractant or extractant/
catalyst. Lo et al. combined oxidative desulfurization (ODS) and
extraction with ILs, [BMIM]BF4 or [BMIM]PF6 as extractant and
The oxidation of the refractory sulfur-containing compounds
using molecular oxygen instead of hydrogen peroxide as
oxidizing agent under mild conditions has long been desired
due to it's the economical and environmental points of view.
However, the direct introduction of molecular oxygen to thio-
phenes is difficult, because the ground state of molecular
oxygen is the inert triplet. In order to use molecular oxygen as
oxidant for desulfurization, the aldehydes were used as co-
oxidants.7 The polyoxometalates, the metallophthalocyanines
and the lactones were also examined as catalyst for desulfur-
ization under different conditions.8
NHPI is a cheap, nontoxic catalyst easily prepared by the
reaction of phthalic anhydride and hydroxylamine. It acts as a
precursor of phthalimido-N-oxyl (PINO) radical, which is the
effective abstracting species in all of the free radical processes
mediated by this N-hydroxy derivative.9 In this work, we rstly
report the one-pot oxidative and extractive desulfurization of BT
and DBTs in fuel oil by NHPI as catalyst with molecular oxygen
as the oxidant, and using IL, 1-butyl-3-methylimidazolium tet-
rauoroborate ([Bmim]BF4) as extractant of low volatility.
The desulfurization of DBT was investigated with the
extraction combined with catalytic oxidation system using
molecular oxygen as oxidant.† Extractive and oxidative desul-
furization of model oil containing DBT was conducted in a 70
mL autoclave with 0.3 MPa molecular oxygen at 120 ꢀC. The oil
phase was examined at different reaction time with the sulfur
specic gas chromatography (GC-FPD). Fig. 1 shows GC-FPD
analyse before and aer the extractive and catalytic oxidation
of DBT. The peak area of DBT decreased with increasing reac-
tion time. The DBT presented in n-octane was not detected aer
aInstitute of Coal Chemistry, Chinese Academy of Science, Taiyuan, 030001, PR China.
E-mail: jianlong.wang@hotmail.com; Fax: +86-351-4250292
bCollege of Materials Science and Engineering, Taiyuan University of Technology,
Taiyuan, 030024, PR China
cTaiyuan University of Technology, Taiyuan, 030024, PR China
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reaction temperature, as shown in Fig. 2. Increasing reaction
temperature from 80 ꢀC to 140 ꢀC led to a noticeable increment
of removing DBT from oil phase. The desulfurization ratio of
DBT was up to 100% at 140 ꢀC in 3 h compared with 82.3% at 80
ꢀC. The DBT can be also removed completely at 120 ꢀC in 3 h. In
addition, the inuence of pressure on sulfur removal was
examined. The results are shown in Fig. 3. With increasing
pressure of oxygen from 0.17 MPa to 0.3 MPa, more DBT was
removed from oil phase. But the desulfurization ratio did not
change when the pressure exceeded 0.3 MPa in 3 h. The results
demonstrate that oxidation of DBT can be facilitated by dis-
solved oxygen in liquid phase, which is more concentrated at
higher pressure.
Fig.
1 GC-FPD chromatograms for the extractive and oxidative
desulfurization of DBT at different time (conditions: DBT model oil
(500 mg mLÀ1), 10 mL; [Bmin]BF4, 4 mL; catalyst NHPI, 10 mmg;
oxidant O2, 0.3 MPa; reaction temperature 120 ꢀC; reaction time 5 h).
The reactivity of different sulfur-containing compounds,
including BT, DBT and 4,6-DMDBT which the initial concen-
tration of sulfur in model oil was 500 mg mLÀ1 respectively, was
estimated for oxidation with this system at the same condition.
The removal of sulfur-containing compounds versus the reac-
tion times in the system is shown in Fig. 4. The desulfurization
ratio of sulfur-containing compounds decreases in the order
DBT > BT > 4,6-DMDBT. The sulfur removal order of four sulfur
compounds by extraction and catalytic oxidation process was
not consistent with the previous reports. As calculated by Shir-
aishi et al. and Otsuki et al., the electron densities on sulfur
atoms are 5.739 for BT, 5.758 for DBT and 5.760 for 4,6-
DMDBT.10 These calculated results indicate that these model
sulfur-containing compounds are oxidized easily with
increasing electron density on the sulfur atoms. The difference
of sulfur removal in this desulfurization system with previous
reports may be due to the extractive desulfurization system. For
sulfur compounds, DBT and BT, the extractive performance
grows with the increase of the aromatic p-electron density. But
for 4,6-DMDBT, the methyl substitution at the 4 and 6 positions
of DBT remarkably retards the extractive performance of
[Bmim]BF4, which leads to the less 4,6-DMDBT removal than
DBT and BT from oil phase.6
extraction and catalytic oxidation at 120 ꢀC in 5 h. The oil phase
did not contain the corresponding oxides, which indicated that
all of DBT had been removed from model oil to IL phase.
The different desulfurizations systems with and without O2
were employed to investigate whether the molecular oxygen is
oxidant. The results are listed in Table 1. The sulfur removal of
DBT was only 32.9% and 33.5% with [Bmin]BF4 or [Bmin]BF4/
NHPI in the presence of N2 (entry 1, 2), respectively. The
desulfurization depends on the extractive property of [Bmin]
BF4. But the sulfur removal of DBT increased sharply and
reached 100% with O2 at the same temperature and reaction
time (entry 3). The desulfurization ratio can be reached 51.6%
when air is used as oxidant at the same conditions. Compared
with molecular oxygen, air contains less molecular oxygen
(about 21%). The results clearly demonstrate that the molecular
oxygen is the oxidant and joins the oxidation of the sulfur
compounds. The catalyst is active for the oxidation of DBT to
corresponding oxides. Zhang et al. reported that no IL was
detected in the organic phase aer absorption measurement.3
The GC with nitrogen and phosphorus detector (NPD) showed
no nitrogen peak in model oil. It indicated that model oil did
not contain IL and catalyst. Few n-octane can be extracted into
IL because of the n-octane inserting into the dynamic molecular
structure of IL, but it can be recovered with distilling the used IL
aer desulfurization.3
The effect of temperature on the conversion efficiency of DBT
was investigated at 0.3 MPa of initial pressure. The removal of
sulfur-containing compound increased with enhancing
Table 1 Influence of different desulfurization systemsa
Sulfur removal
Entry
Desulfurization system
(%)
1
2
3
4
[Bmin]BF4 + O2
32.9
33.5
100
51.6
[Bmin]BF4 + NHPI + N2
[Bmin]BF4 + NHPI + O2
[Bmin]BF4 + NHPI + Air
a
Conditions: DBT model oil (500 mg mLÀ1), 10 mL; [Bmin]BF4, 4 mL;
catalyst NHPI, 10 mmg; N2, O2, Air, 0.3 MPa; reaction temperature 120
ꢀC; reaction time 5 h.
Fig. 2 The effect of temperature on the DBT removal (conditions: DBT
model oil (500 mg mLÀ1), 10 mL; [Bmin]BF4, 4 mL; catalyst NHPI, 10
mmg; oxidant O2, 0.3 MPa).
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Fig. 3 The effect of O2 pressure on the DBT removal (conditions: DBT
model oil (500 mg mLÀ1), 10 mL; [Bmin]BF4, 4 mL; catalyst NHPI, 10
mmg; reaction temperature 120 ꢀC; reaction time 3 h).
Fig. 5 The proposed process and mechanism of extraction and
catalytic oxidation of DBT in IL–NHPI–O2 system.
Some research had been reported that the different catalyst,
such as heteropoly acid H5PV2Mo10O40/SiO2, poly-
oxomolybdates [(C18H37)2N(CH3)2]3Co(OH)6Mo6O18$3H2O, [C8-
H17N(CH3)3]3H3V10O28,
[C18H37N(CH3)3]7[PW10Ti2O38(O2)2],
[(C18H37)2NCH3]5[IMo6O24], and iron phthalocyanines or iron
porphyrin were used to desulfurize fuel oil with O2 as oxidant at
normal pressure or compression.8,12 The DBT can be removed at
normal pressure when heteropoly acid or polyoxomolybdates
were used as catalyst under 120 ꢀC or lower temperature.8 In
Fig. 4 Effect of different sulfur species on sulfur removal (conditions:
sulfur model oil (500 mg mLÀ1), 10 mL; [Bmin]BF4, 4 mL; catalyst NHPI,
10 mmg; oxidant O2, 0.3 MPa; reaction temperature 120 ꢀC).
The proposed process and mechanism whereby DBT is
extracted from oil phase and oxidized in the IL phase is as shown
in Fig. 5. In a combination of extraction and oxidation, DBT was
oxidized in the IL phase as it was extracted from the oil phase, so a
continuous decrease in the concentration of DBT in n-octane was
observed for each solvent during the oxidation process. In this
reaction system, the possible catalytic mechanism was due to free
radical oxidation. The generation of PINO from NHPI under
aerobic conditions was conrmed by Einhorn et al.11 This obser-
vation suggested that the hydrogen atom anchored to the N-
hydroxy moiety in NHPI is easily abstracted by molecular oxygen
to form PINO and hydroperoxyl radical, which is strong oxidizing
agents. The DBT was rstly oxidized to DBTO, and then DBTO2 by
the hydroperoxyl radical. The sulfur-containing compound in the
IL was oxidized to its corresponding sulfone by the hydroperoxyl
radical. Because of the high polarity of IL, sulfone does not exist in
the oil phase (Fig. 1). Aer reextraction, the DBT sulfone in IL can
be detected with GC-MS (Agilent 7890A-5975C) as shown in Fig. 6.
Fig. 6 The GC-MS spectrogram of the oxidized productions of DBT.
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conducted in 70 mL autoclave equipped with a magnetic stirrer, a pressure control
valve and two thermocouples inside and outside of reactor. In a typical run, the
model oil (DBT, 10 mL, 500 mg mLÀ1), the IL (4 mL) and catalyst (NHPI, 10 mg)
were added to the autoclave. The reactor was closed and pressurized to 0.3 MPa by
a ow rate of O2 (1 L minÀ1) and then released the gas to substitute the O2 for N2 in
air. Aer 3 times, the autoclave was heated to 120 ꢀC with stirring and kept 3 h.
The autoclave was cooled to room temperature in a water bath aer the reaction.
The O2 was depressurized slowly to atmospheric pressure and the reactor was
opened. The upper oil phase was withdrawn and analyzed for sulfur content using
the microcoulometric detector. The sulfur-containing compounds were analyzed
by gas chromatography coupled with ame photometric detection.
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catalyst, NHPI can oxidize BT and DBTs present in n-octane to
the corresponding sulfones using molecular oxygen as the
oxidant. The IL can be used as extractant. The corresponding
sulfones exist in IL phase. The oxidation and extraction can
proceed simultaneously. These results demonstrate that the
molecular oxygen can be employed as oxidant for oxidative
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compounds (BT, 4,6-DMDBT) in n-octane giving a corresponding sulfur content
500 mg mLÀ1. All the oxidative and extractive desulfurization experiments were
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