Deep desulfurization of fuels based on an oxidation/extraction process with acidic deep eutectic solvents

Article abstract of DOI:10.1039/c5gc00709g

The presence of organic sulfides in fuels has become a significant contributing factor to the formation of haze weather. The desulfurization of fuels has become a frontier scientific topic demanding urgent solutions. Research shows that acidic deep eutectic solvents (DESs) can provide a new route for the deep oxidation/extraction desulfurization of fuels because of their cheap and easily obtained raw materials, higher desulfurization efficiencies, environmentally friendly properties and simple synthetic procedures. Through smart design, a series of acidic DESs with different acidities were synthesized. These acidic DESs were applied for the successful deep oxidation/extraction desulfurization of model and real fuels. Results show that the acidity of deep eutectic solvents is the main factor that determines the oxidation/extraction process. The desulfurization capability has a positive correlation with the acidity of DESs, that is, the stronger a DES's acidity, the higher the desulfurization efficiencies achieved. Under optimal conditions, the desulfurization efficiencies of DESs choline chloride/p-toluenesulfonic acid (ChCl/p-TsOH) and tetrabutylammonium chloride/p-toluenesulfonic acid (TBAC/p-TsOH) can reach up to 99.99%. For the real fuel, the desulfurization efficiency can reach up to 97.25% and 95.90%, for (ChCl/p-TsOH) and (TBAC/p-TsOH) respectively. Finally, the final product was characterized and the oxidation/extraction mechanism was also investigated and the possible desulfurization route was proposed. The above study will provide new technology for the desulfurization of fuels.

Full text of DOI:10.1039/c5gc00709g

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Green Chemistry
Deep desulfurization of fuels based on oxidation/extraction process with
acidic deep eutectic solvents
Jingmei Yin,a Jinping Wang,a Zhuo Li,*b Dan Li,a Guang Yang,a Yingna Cui, a Ailing Wang,a Changping
Li,*a
5
Abstract: Organic sulfide in fuels has become an important reason for the formation of haze weather.
The desulfurization of fuels has become a frontier scientific topic demanding prompt solutions. Research
shows that the emergence of acidic deep eutectic solvents (DESs) has provided a new route for the deep
oxidation/extraction desulfurization of fuels because of its cheap and easily obtained raw materials,
10 higher desulfurization efficiencies, environmental friendly property and simple synthetic process.
Through the smart design, series of acidic DESs with different acidity were synthesized. These acidic
DESs were applied for the deep oxidation/extraction desulfurization of modeling and real fuels
successfully. Results show that the acidity of deep eutectic solvent is the main factor that determines the
oxidation/extraction process. The desulfurization capability has positive correlation with DESs’ acidity,
15 that is, the stronger of DESs’ acidity, the higher of desulfurization efficiencies were achieved. In optimal
conditions, the desulfurization efficiencies of DESs choline chloride/p-toluenesulfonic acid (ChCl/p-
TsOH) and tetrabutylammonium chloride/p-toluenesulfonic acid (TBAC/p-TsOH) can reach up to
99.99%. For the real fuel, the desulfurization efficiency can reach up to 97.25% and 95.90%, respectively.
Finally, the final product was characterized and oxidation/extraction mechanism was also investigated and
20 the possible desulfurization route was proposed. The above study will provide new technology for
desulfurization process of fuels.
60 Its physical and chemical properties are similar to ionic liquids
(ILs). So it is also known as the ionic liquids analogues. Its
excellent physical and chemical properties have gained
widespread attention. Deep eutectic solvents refers to the melting
1. Introduction
In recent years, production of clean fuels has become an
25 important research topic around the world. SOX, which are
generated by the burning of organic sulfide in fuels, have become
one of the most important reasons for the serious environmental
pollution and the formation of haze weather. Stringent
requirements have been made on sulfur content in fuels by most
30 countries. In order to prevent the pollution from vehicle emission,
the decrease of sulfur content from the source has become the
main trend. The new technologies were suggested to be adopted
substance formed
by
two
or
three
65 cheap, green components connecting with each other through
hydrogen bonding[6,7]. It has good chemical stability and thermal
stability. At the same time, the hydrogen bond donor (HBD)
and acceptor (HBA) can be chosen according to demands.
Compared with the ionic liquids, the raw materials of DESs are
70 cheap and easily obtained. Notably, the synthetic process is
simple, and the other organic solvents need not to be introduced,
which can be very environmentally friendly. Therefore, DESs are
considered to be an alternative to volatile organic solvents as
‘green’ solvent[8-10]. DESs has been widely concerned in
to reduce the sulfur content in fuels [1-3]
.
Presently, hydrodesulfurization process (HDS) is the “golden”
35 desulfurization method, which is being applied in most refineries.
However, for organic sulfide with larger steric, such as thiophene
and their derivative, HDS is not so efficient. It is difficult to
75 preparation
of
materials[11], catalytic reaction[12], electrical
and so
chemistry[13]
,
chemical separation process[14-19]
,
on. DESs can receive or give electron or proton to form
the hydrogen bonds; therefore, it has good capability to
achieve deep
desulfurization
of
fuels. Therefore, as
a
complementary approach, non-hydrodesulfurization (Non-HDS)
dissolve many
compounds. However,
compared with
40 methods were widely investigated, which has become
a
80 the widespread use of ILs in chemical separation process,
application of DESs has been just at its beginning stage, mainly
including extraction of glycerol[15] and phenol[16] from the
research focus[4]. Many Non-HDS methods are being
probed, such as biological desulfurization process, membrane
biodiesel, uptake of saponins[17] and phenolic metabolites[18]
,
technology, alkylation, adsorption, extraction and
oxidation/
extraction process[5]. Oxidation/extraction desulfurization
45 technology can be divided into two processes. First,
organic sulfide in fuels is oxidized to the corresponding sulfoxide
and sulfone, then extracted by efficient polar solvents. This
process can be carried out under normal temperature and pressure.
Furthermore, its investment is lower than HDS. So, it is
separation of the cellulose component[19] and extraction of
85 protein[20] desulfurization of fuels[21-23]. Until now, the
,
application of DESs as "three agents in one" for the removal
of organic sulfide with oxidation/extraction coupling process has
not been reported. Nevertheless, as analogs of DESs, ionic
liquids were widely probed in liquid-liquid extraction[24],organic
90 sulfide extraction or oxidation/extraction desulfurization process,
50 a very promising desulfurization
technology.
However, the
commonly used extractants are most organic ones, which are
volatile and toxic, serious environmental pollution will be
caused. Therefore, the development of green solvents can be great
significance to realize and simplify the desulfurization process.
55 This green solvents should act as catalyst, solvent and extractant
so that “one pot” deep desulfurization of fuels can be
mainly about the
desulfurization with imidazolium and pyridyl based ILs[25-
30], desulfurization using Lewis and Bronsted acid ILs[31-33]
extraction or
oxidation/extraction
,
extraction and catalytic oxidation desulfurization with ionic
95 liquids[34-36] and catalytic oxidation/extraction desulfurization
with functionalized ionic liquids [37-39]
.
achieved, which
industrialization of oxidation/extraction desulfurization process.
Deep eutectic solvents (DESs) are new type of solvents.
will
greatly
promote the
During the past few years, we have been committed to the
investigation of ILs and DESs applied in chemical separation
a

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process, especially extraction removal of organic sulfide from
the experimental errors were within 3%. As for the
fuels[21,40-42]. In our previous work, acidic functionalized ionic
liquids can be effectively applied to remove organic sulfide from
fuels[41]. It was also found that acidity is also important factors
affecting oxidation / extraction desulfurization process. To our
regret, the acidity of functionalized ionic liquids is still difficult
to achieve precise control. At the same time, the cost of raw
materials for synthesizing functionalized ILs is still high.
Moreover, the complex synthesis process and the introduction of
10 the organic solvent will influence the greenness of ILs. The above
reasons have become obstacles for application of functionalized
ILs in desulfurization process. To be noted, for real diesel fuels, it
is also not efficient. How to design and develop new solvents that
can conquer with the above problem efficiently has become an
15 urgent problem. Through our previous research, DESs was first
used as extractants for desulfurization of fuels[21]. Research
shows that the emergency of deep eutectic solvents (DESs) has
desulfurization of real fuels, similar process was adopted.
65 2.3 Analytical methods
The sulfur content of fuels was measured by Gas Chromatograph
(GC). The chromatogram conditions are listed as follows:
chromatogram column: HP-5; injection volume: 1 μL; carrier gas
(N2): 210 mL/min; H2: 40 mL /min; air: 350 mL/min; flux: 1.6
70 mL/min, constant flow mode; inlet temperature: 300 °C; detector
temperature: 300 °C; column temperature: heating from 120 °C to
150 °C with 15 °C/min increase, 150 °C to 160 °C with 2 °C/min
increase, 160 °C to 250 °C with 25°C/min increase. The
concentration of BT was measured with internal standard method,
75 using hexadecane as the internal standard solvent. The
concentration of hexadecane was 2000 ppm. The analysis of
standard curve is shown in S.Fig.9. The correlation coefficient
achieves 0.99997. The sulfur content of real fuels was determined
by micro-coulometry. EPR signal of radicals trapped by DMPO
5
provided
a new route for the deep oxidation/extraction
80 were recorded at ambient temperature on
a JES FA200
desulfurization of fuels because of its cheap and easily obtained
20 raw materials, higher desulfurization efficiencies, environmental
friendly property and simple synthetic process. As the follow-up
work, acidic DESs were designed, synthesized and their
oxidation/extraction desulfurization capability were also
investigated. To optimize the oxidation/extraction desulfurization
25 process, the main factors that affect the desulfurization of fuels
were studied systematically. With the above study, the theoretical
foundation of desulfurization process for the smart design of
DESs and the adjustment of desulfurization process was built up.
This will provide a new theoretical and technological support for
30 deep desulfurization of fuels.
spectrometer. After bubbling O2 for 10 min, the samples were
introduced into the quartz cup inside the microwave cavity. The
setting for the EPR spectrometer were listed as follows: center
field, 336.50 mT; sweep width, 5 mT; microwave frequency, 9.5
85 GHz; modulation frequency, 100 kHz; power, 0.998 mW.
3. Results and discussions
3.1 Effect of different types of DESs
Our previous research showed that the acidity of functionalized
ILs has a significant influence on the oxidation/extraction
90 desulfurization process, while the acidity of functionalized ionic
liquids is difficult to control precisely[42]. However, the acidity
control of deep eutectic solvent is relatively simple, which can be
realized through the proper choosing of hydrogen bond donor
(HBD). This provides a good choice in solvent selection for
95 oxidation/extraction desulfurization. Therefore, in accordance
with the acidity of HBD, series of deep eutectic solvents were
designed and synthesized. Based on the above principle, p-
toluenesulfonic acid (p-TsOH), 5-sulfosalicylic acid (SSA), para
aminosalicylic acid (PAS) were chosen as HBD, and choline
2. Experimental
2.1 Synthesis of acidic DESs
Quaternary ammonium salts were chosen as HBA and some
typical acids with different acidity were chosen as HBD.
35 Synthesis process was carried out in a round-bottomed flask. Raw
materials were purified before use. Then HBA and HBD were
mixed at a certain molar ratio (mostly 1:1 or 1:2 in this research).
After two compositions become liquid with heating, the system
was stirred vigorously with a magnetic stirrer at a certain
40 temperature, ranging from 60 ℃ to 120 ℃. The reaction process
would be about 3～5 hours. Synthesis process of ChCl/ p-TsOH
was taken as an example to illustrate the synthesis of DESs. In a
100 ml round bottom flask, equipped with a magnetic stirrer,
the molar ratio 1:2 of choline chloride and p-toluene sulfonic acid
45 were mixed. After those two compositions become liquid with
heating, the system was stirred at 80℃ for 4 hours. Then ChCl/p-
TsOH can be got (S.Fig.1a). Finally, the purity of DESs was
confirmed by using 1H NMR listed in supporting information
S.Fig.2. The characterization of the other DESs were listed as
50 S.Fig.3~S.Fig.8).
100 chloride(ChCl),
tetrabutylammonium
chloride
(TBAC),
tetrabutylammonium bromide (TBAB), tetraethylammonium
chloride (TEAC) and tetraethylammonium bromide (TEAB) as a
hydrogen bond acceptor (HBA). In addition, their oxidation/
extraction desulfurization capability is probed, which was shown
105 in Fig.1. The results show that the hydrogen bond donor and
acceptor affect the oxidation/extraction process greatly. HBD has
greater influence on the desulfurization process than HBA.
Furthermore, the acidity of deep eutectic solvent is the main
factor that determines the oxidation/extraction process. Under the
110 same conditions, the desulfurization capability has positive
correlation with DESs’ acidity, that is, the stronger of DESs’
acidity, the higher of its desulfurization efficiencies. The molar
ratio of HBA to HBD influences the desulfurization greatly.
Higher amount of acids is beneficial the desulfurization process.
115 With the same molar ratio, the sequence of desulfurization
efficiency is listed as follows: p-toluenesulfonic acid (p-TsOH) >
5-sulfosalicylic acid (SSA) > 4-aminosalicylic acid (PAS). The
desulfurization efficiencies of DESs ChCl/p-TsOH and TBAC/p-
TsOH (molar ratio of HBA:HBD is 1:2) can reach up to 99.99%.
120 Deep desulfurization of fuels was realized with ‘one pot’ process.
2.2 Oxidation/extraction process
Benzothiophene (BT) was chosen as typical sulfur content and n-
octane as typical fuels. The sulfur content of the model oil was
adjusted to 1600 ppm. Desulufurization process was carried out
55 in self-made equipment. DES, H2O2 and Oil were mixed by
certain molar ratio and then added into pear-shaped bottle
subsequently (S.Fig.1b and S.Fig.1e).The mixture was stirred for
60 min at 25 °C in a water bath except for otherwise defined.
Finally, the oxidation/extraction equilibrium reached (S.Fig.1c
60 and S.Fig.1f). The ratio of reactants was adjusted according to the
different experimental conditions. All the experiments in this
study were done in duplicates to determine reproducibility, and

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DOI: 10.1039/C5GC00709G
Page 3 of 8
Green Chemistry
of ChCl/p-TsOH is more suitable for the oxidation/extraction of
sulfur content.
100
80
60
40
20
0
100
ChCl/p-TsOH
TBAC/p-TsOH
80
60
40
20
0
ChCl/PAS(1:2)
TEAC/SSA(1:2)
ChCl/p-TsOH(1:1)
ChCl/SSA(1:2)
TBAB/p-TsOH(1:2)
TEAB/p-TsOH(1:2)
TEAC/p-TsOH(1:2)
TBAC/p-TsOH(1:2)
ChCl/p-TsOH(1:2)
9
1
1
2
3
4
5
6
7
8
5
5
2
05 0.
1
025
0.
0
0.
3
5
0.
4
0.
2
6
25
0.
0.
067
0.
0
0031
.
0
0
.
0.
0
Types of DES
m(H O ):m(Oil)
2
2
40
Fig.1 Effect of different types of DESs
Fig.3 Effect of the amount of oxidant
3.2 Effect of the oxidant
3.3 Optimization of the oxidation/extraction process
As it is known that many DESs also have extraction
desulfurization capability [21]. In optimal conditions, its extraction
efficiency can reach 71.06%. So, the necessity of introducing
oxidant should also be investigated for the acidic DESs based
desulfurization process. From Fig.2, if oxidant is not introduced,
the desulfurization efficiency can be only 25.17% and 37.79% for
10 ChCl/p-TsOH and TBAC/p-TsOH, respectively. However, after
the introduction of H2O2, the desulfurization efficiency can reach
99.99%. Therefore, oxidant can be very significance to achieve
deep desulfurization process.
Many factors affect the desulfurization process, such as the
amount of DESs, extraction time, system temperature, stirring
45 speed. So the above parameters were investigated to optimize the
desulfurization process, which can be of great importance for its
application in real fuels system.
5
As mentioned above, DESs are cheaper. However, less amounts
of DESs needed and higher oxidation/extraction efficiencies
50 achieved is always our goals. Effect of the amount of DESs and
extraction time were investigated in detail shown in Fig.4. It can
be seen that with the increase of the amount of DESs, the
desulfurization efficiency also increase. When the amount of
DESs to fuels comes to 0.5:1, the desulfurization efficiency can
55 achieve 99.99% in 60 min. While for 0.4:1 or 0.2:1, the
equilibrium time would be longer. The reason is that larger
consumption of DESs will lead to its higher DESs concentration.
The electron density of sulfur atoms on benzothiophene is easy
to be polarized because of the interactions between DESs and BT.
60 Furthermore, more proportion of DESs will also provide more
active acidity spot, which enhances the oxidation ability of H2O2.
Although larger amount of DESs is beneficial for desulfurization
process including shorter equilibrium time and higher
desulfurization efficiency, its cost must be considered so that
65 applied in real fuels system. So the mass ratio of DESs to fuels
0.5:1 was chosen, with which the equilibrium time,
desulfurization efficiency and the cost is more preferable. To be
noted, until now the above results is one of the least amount of
solvents used that can get so much higher desulfurization
70 efficiency. Moreover, to guarantee a sufficient equilibrium, 60
min was chosen as reaction time in the following experiment.
System temperature is a useful criterion for evaluating the
possibility of oxidation/extraction desulfurization process applied
in real fuels. Effect of system temperature was investigated
75 systematically listed Fig.5. It is indicated that the higher of the
system temperature, the higher of desulfurization efficiency can
be achieved at the beginning stage. However, after 60 min the
desulfurization can reach 99.99% at all temperature, which mean
that temperature will not influence the desulfurization process
80 anymore. The result also shows that higher temperature will
result in shorter equilibrium time. The probable reason for this is
that higher temperature is more beneficial for the decomposing of
H2O2 at beginning. However, with the increase of reaction time,
adequate H2O2 decompose to achieve the same desulfurization
85 efficiencies. In this specific case, 25 ℃ was chosen as system
temperature throughout the investigation because 60 min is
preferable in this work. The above results would be significant for
its application in real fuels because this process can be operated
just in normal temperature.
100
80
60
40
20
0
l
i
l
i
O
O
2
H 2
O 2
H 2
O
+
+
+
il
O
+
+
l
i
H
-TsO
H
-TsO
O
+
H
H
p
/
p
/
l
C
A
B
-TsO
p
/
hC
-TsO
p
/
l
C
T
C
A
hC
B
C
T
15
Fig.2 Necessity of introducing oxidant
The amount of oxidant is an important factor that influences the
oxidation/extraction desulfurization process. And it is also an
important standard that can measure the suitability of the solvent.
The less of the amount of oxidant used and the higher
20 desulfurization efficiency can be obtained is an important basis
for the choice of the new solvents. With the above
reason, effect of amount of oxidant is investigated. Results show
that desulfurization efficiency increases gradually with the
increase of oxidant dosage (Fig.3). If DES ChCl/p-TsOH was
25 chosen, the desulfurization efficiency can reach 99.99% when
m (H2O2): m (Oil) is 0.05:1. To be noted that this result is out of
our expectation because so little oxidant is needed and so much
higher desulfurization was got. When the TBAC/p-TsOH
as extractant, the desulfurization efficiency achieves 99.99%
30 when m (H2O2): m (Oil) is 0.3:1. Further increase of the amount
of H2O2 will not lead to the increase of desulfurization efficiency.
Compared with the above two DESs, ChCl/p-TsOH
desulfurization system need less oxidant, so it will be more
significant for its future industrial application. It can be easily
35 seen that in ChCl/p-TsOH system, less amount of oxidant is
needed compared with TBAC/p-TsOH and acidic functionalized
ionic liquids [41]. The probable reason for this is that the structure

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DOI: 10.1039/C5GC00P70a9gGe 4 of 8
Stirring speed is another factor that influences the desulfurization 20 3.4 Recycling of used DESs
process. Fig.6 shows the effect of stirring speed on
desulfurization efficiency. As the speed increases, the
For industrialization of DESs, recycling and reuse can be
importance not only from economic aspect but from
desulfurization efficiencies are gradually increasing. To achieve
environmental aspect. ChCl/p-TsOH is taken as an example to
5
deep desulfurization efficiency more than 99.99%, ChCl/p-TsOH
investigate the possibility of recycling and reusing of DESs. It is
and TBAC/p-TsOH can reach their equilibrium in 50 min and 60 25 indicated that the recycling of the DES can be realized only
min at 800 rpm, respectively. The reason would be the higher of
the stirring speed will be beneficial for the mass transfer and
promotion of collision area, and then the reaction time was
10 shortened. In the following experiments, 800 rpm was chosen as
stirring speed.
washing by water. Since the sulfur oxides are insoluble and the
ChCl/p-TsOH is soluble in water, the used ChCl/p-TsOH is
extracted by washing with water. The sulfur oxides precipitated in
the form of a white solid and was filtered. Finally, the aqueous
30 ChCl/p-TsOH solution was dried using vacuum distillation. The
desulfurization efficiencies are very stable after four times of
recycling (Fig.7). The desulfurization can reach 98.87% after four
times recycling, and the loss of DESs each cycle is not more than
100
80
1
2%. Furthermore, from H NMR characterization, the structures
35 of DESs do not change (Fig.8). Pure ChCl/p-TsOH: 1HNMR (500
MHz, CD3CN) δ7.69(d, J=8.2Hz, 4H),7.66 (s,3H),
7.27(d,J=8.1Hz,4H),3.93(m,2H),3.38(t,J=5.0Hz,2H),3.09(s,9H),
2.39(s,6H). Recycled ChCl/p-TsOH: 1HNMR (500 MHz,
CD3CN) δ8.03 (s,3H), 7.69(d, J=8.0 Hz, 4H), 7.29(d,
40 J=7.9Hz,4H), 3.93(m,2H), 3.38 (t, J=5.0 Hz, 2H), 3.09 (s,9H),
2.40 (s,6H). Based on the above investigation, DESs was
recycled and reused successfully. TBAC/p-TsOH were also
recycled with the same procedure. The result of its 1H NMR
characterization was listed in S.Fig.10.
60
40
ChCl/p-TsOH:BT:H
O
TBAC/p-TsOH:BT:H O
2 2
2
2
0.5:1:0.05
0.4:1:0.05
0.2:1:0.05
0.5:1:0.3
0.4:1:0.3
0.2:1:0.3
20
0
20
40
60
80
100
120
Extraction time/min
Fig.4 Effect of the amount of DESs on extraction efficiency
ChCl/p-TsOH
（25℃,800rpm）
100
80
60
40
20
0
100
80
60
ChCl/p-TsOH:Oil:H O =0.5:1:0.05,800 rpm
2 2
25℃
40℃
60℃
40
20
0
TBAC/p-TsOH:Oil:H O =0.5:1:0.3,800 rpm
2
2
25℃
40℃
60℃
0
1
2
3
4
0
20
40
60
80
100
120
Recycling times
Extraction time/min
45
15
Fig.7 Extraction performance of the recycled DES
Fig.5 Effect of system temperature
100
80
60
ChCl/p-TsOH:BT:H
O =0.5:1:0.05, 25℃
2
2
400 rpm
800 rpm
1200 rpm
40
TBAC/p-TsOH:BT:H
O =0.5:1:0.3, 25℃
2
2
400 rpm
800 rpm
1200 rpm
20
0
20
40
60
80
100
120
Extraction time/min
Fig.8 Structures of pure and recycled ChCl/p-TsOH
(a) Pure ChCl/p-TsOH (b) Recycled ChCl/p-TsOH
Fig.6 Effect of stirring speed
50 3.5 Characterization of final product

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Green Chemistry
Form the recycling process, the white solid, namely sulfur oxide 65 compounds more acceptable for the oxygen atoms to be oxidized
was obtained. The revealing of its structure would be valuable for
the investigation of the oxidation/extraction mechanisms. To get
into the corresponding sulfoxide. The formation of sulfone will
increase the polarity of sulfide significantly and easy to dissolve
in polar DESs. Notably, DESs has three functions in this
desulfurization process: catalyst, solvent and extractant.
1
the right composition and structure of the white solid, H NMR
1
5
and GC-MS were used for the characterization. The result of H
NMR was listed in S.Fig.11, and summarized as follows: 1H
NMR (500 MHz, CD3CN) δ 7.75 (d, J = 7.5 Hz, 1H), 7.67 (t, J =
7.5 Hz, 1H), 7.61 (t, J = 7.5 Hz, 1H), 7.53 (d, J = 7.5 Hz, 1H),
7.43 (d, J = 6.9 Hz, 1H), 6.90 (d, J = 6.9 Hz, 1H). It can be seen
70 3.7 Desulfurization of real fuels
Through the above investigation, deep desulfurization of
modeling fuels can be achieved by using this specific
oxidation/extraction process. Its desulfurization efficiency can be
more than 99.99%. However, the composition of the real fuels
75 can be more complex than the modeling ones. Anyway, whether
the process can be applied in the real fuel system is the key that
determine the possibility of its future industrialization. Therefore,
oxidation/extraction desulfurization of real fuel was carried
10 that this solid is probably BTO2. To confirm the structure of the
oxidized products, the separated sulfur oxide was also
characterized by GC-MS listed in S.Fig.12. GC-MS (EI, 70ev),
m/z (%) 166 (43, M+), 137 (100), 118 (16), 109 (43), 89(15). It is
clearly shown that final oxidation products, benzothiophene
15 sulfone (BTO2) is obtained. Unfortunately, the intermediate
cannot be observed. Perhaps the benzothiophene sulfoxide (BTO)
also appeared as transition state during the oxidation/extraction
process, but it was not found in the final product.
out. In optimal conditions, the desulfurization
efficiency of
80 ChCl/p-TsOH and TBAC/p-TsOH can reach up to 97.25% and
95.90% with the initial sulfur content 1600 ppm, which mean that
the deep desulfurization process can also be realized. However,
the oxidation/extraction desulfurization process based on
acidic functionalized ionic liquid is only efficient for modeling
3.6 Oxidation/extraction mechanisms
20 Oxidation/extraction mechanism is valuable for the understanding
of the desulfurization process. It is also significant for the smart 85 fuels, for real fuel system, it is not workable[38]. This would
design of DESs extractants. As a new type of solvent, acidic
DESs were first applied in oxidation/extraction desulfurization
process acting as ‘three agents in one’. So its mechanism is also
25 especially valuable. Combined with the spectrum characterization
and factor study, the proposed extraction mechanisms for the
given two processes were sketched in Fig.11. To confirm the
proposed oxidatioin/extraction desulfurization process, EPR spin-
trap technique (with DMPO) was used to probe the active oxygen
30 species generated by the desulfurization process. EPR signals was
listed in Fig.9~Fig.10. The process was described in detail as
follows. Interestingly, with the same oxidant H2O2, different
DESs used in this process can produce different active oxygen
species, which means that the reactions obey different
35 oxidation/extraction routes. Through EPR characterization, for
TBAC/p-TsOH, the characteristic peak can be observed can be
assigned to DMPO-.OH, that is to say, H2O2 decomposes to
hydroxyl radicals (·OH) in the desulfurization process listed in
Fig.9 [43-45]. However, for the ChCl/p-TsOH, the EPR signal can
probably open a new route for the deep desulfurization of real
fuels.
318.1
318.2
318.3
318.4
·-
40 be assigned to DMPO-O2 . In this process, H2O2 is converted
Magnetic field/mT
·-
into O2 radical species shown in Fig.10, which shows that the
·-
·-
typical signals of O2 trapped by DMPO ( DMPO-O2 ), and
Fig.9 EPR spectra of DMPO-·OH adducts in TBAC/p-
·-
characteristic peaks of the DMPO-O2 adducts are observed on
90
TsOH/H2O2 system
ChCl/p-TsOH systems[43-46]. This difference probably results from
45 the functional group of the hydrogen bond acceptor (HBA),
namely alcoholic hydroxyl group in ChCl is the main reason for
the production of O2·- for ChCl/p-TsOH. This phenomenon can
be of great importance for the revelation of the desulfurization
mechanisms. To be noted, the active oxygen species provided by
50 discomposed H2O2 could not oxidize BT at the interface of model
oil and DESs, which can be proved by the longer equilibrium
time. So BT was extracted by DESs phase at the first stage. Then
it was oxidized by the active species. Due to the polarity of BTO2
is larger than BT, it has more tendency to dissolve in DESs. To be
55 saturated, DESs will extract another part of BT, which was then
oxidized too. The above specific process repeated continuously
until reaching the reaction equilibrium. Because the dissolution
process of BT in DESs takes longer time, the extraction
equilibrium would be 60 min. It is much different from traditional
60 extraction desulfurization process with shorter equilibrium time.
From microstructure aspects, the S heteroatom in BT occurs sp2
hybrid, which will result in the existence of two single pair
electrons, one in 3pz hybrids orbital and the other in 3pz. At the
same time, 3d space orbit on the sulfur atom makes the sulfur
317.9 318.0 318.1 318.2 318.3 318.4 318.5 318.6
Magnetic field/mT
Fig.10 EPR spectra of DMPO-O2·- adducts in ChCl/p-TsOH
/H2O2 system

View Article Online
Green Chemistry
DOI: 10.1039/C5GC00P70a9gGe 6 of 8
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We would like to thank the National Natural Science Foundation
30 of China (Grant NO.21176033, 21306016) for financial support
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Green Chemistry
Page 8 of 8
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DOI: 10.1039/C5GC00709G
A table of contents entry
Series of acidic deep eutectic solvents were designed and synthesized，which could be used for
the oxidation/extraction desulfurization of fuels successfully.