Desulfurization of dibenzothiophene by chemical oxidation and solvent extraction with Me3NCH2C6H 5Cl·2ZnCl2 ionic liquid

Article abstract of DOI:10.1039/b815575e

Desulfurization of dibenzothiophene (DBT) by a combination of both chemical oxidation and solvent extraction was investigated. Me₃NCH ₂C₆H₅Cl·2ZnCl₂ ionic liquid was prepared from cheap starting materials and used as extractant for oxidative desulfurization of DBT in n-octane. DBT in oil phase was extracted into ionic liquid phase and then oxidized to its corresponding sulfone by H ₂O₂ and equal volume of acetic acid. The desulfurization yield of DBT in n-octane was 94% at 30 min under the conditions of H ₂O₂/DBT molar ratio at 6 and V (ionic liquid): V (n-octane) = 1: 5, which was remarkably higher than that by mere extraction with ionic liquid (28.9%). The Me₃NCH₂C₆H ₅Cl·2ZnCl₂ ionic liquid could be recycled six times without a significant decrease in activity. Kinetics of oxidative desulfurization of DBT by H₂O₂ and acetic acid was first-order with an apparent rate constant of 0.0842 min^-1 and half-time of 8.23 min.

Full text of DOI:10.1039/b815575e

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Desulfurization of dibenzothiophene by chemical oxidation and solvent
extraction with Me3NCH2C6H5Cl·2ZnCl2 ionic liquid
Fa-tang Li,*a Rui-hong Liu,a Jin-hua Wen,a Di-shun Zhao,b Zhi-min Suna and Ying Liua
Received 5th September 2008, Accepted 12th March 2009
First published as an Advance Article on the web 1st April 2009
DOI: 10.1039/b815575e
Desulfurization of dibenzothiophene (DBT) by a combination of both chemical oxidation and
solvent extraction was investigated. Me3NCH2C6H5Cl·2ZnCl2 ionic liquid was prepared from
cheap starting materials and used as extractant for oxidative desulfurization of DBT in n-octane.
DBT in oil phase was extracted into ionic liquid phase and then oxidized to its corresponding
sulfone by H2O2 and equal volume of acetic acid. The desulfurization yield of DBT in n-octane
was 94% at 30 min under the conditions of H2O2/DBT molar ratio at 6 and V (ionic liquid) : V
(n-octane) = 1 : 5, which was remarkably higher than that by mere extraction with ionic liquid
(28.9%). The Me3NCH2C6H5Cl·2ZnCl2 ionic liquid could be recycled six times without a
significant decrease in activity. Kinetics of oxidative desulfurization of DBT by H2O2 and acetic
acid was first-order with an apparent rate constant of 0.0842 min-1 and half-time of 8.23 min.
of oils by extraction with ILs has been reported from
Introduction
2001.16–25 Compared with VOCs or water, ionic liquids
Air pollution, caused by diesel exhaust gas (SOx), is one of the
most serious problems in the world, and much attention has
have much higher extraction abilities which lead to shorter
reaction time. Ionic liquids also have a much higher density,
been focused on deep desulfurization of light oils. To protect the
which makes it possible to readily recycle the ionic liquids
environment against contamination, the S-limit will be reduced
for multiple extractions without additional environmental
in USA to a maximum of 15 ppm by 2010 and to 10 ppm
concern. Many types of ILs, such as [BMIm]AlCl4,16,17
in Europe by 2009.1 Many other countries also have tightened
[EMIm]AlCl4,16 [EMIm]BF4,18 [BMIm]BF4,18,19 [BMIm]PF6,18,19
their sulfur content specifications.
[BMIm][OcSO4],20
[EMIm][EtSO4],20
[BMIm]Cu2Cl3,21
The removal of sulfur-containing compounds is carried
out industrially via hydrodesulfurization (HDS). However,
the HDS is limited in treating benzothiophenes (BTs) and
dibenzothiophenes (DBTs).2 Therefore, approaches based on
adsorption,3 extraction,4 oxidation5 and biodesulfurization6
have been explored. Among them, oxidative desulfurization
(ODS) is more attractive because BTs and DBTs can be oxidized
to their corresponding sulfoxides and sulfones easily, which
can be removed by extraction with water7 or water-soluble
polar solvents, such as dimethyl sulfoxide (DMSO),8 1-methyl-
2-pyrrolidinone (NMP) and dimethylformamide (DMF), etc.9
Although the ODS processes are effective, one of the prime
concerns is that the flammable and volatile organic compounds
(VOCs) or water, employed as extractants, are leading to further
environmental and safety concerns, such as wastewater emission
and fire hazards.
[BMIm]DBP,22 [C8MIm]BF4,23 [C4MPy]SCN,24 [EMIm]FeCl4,25
have been employed in the extraction desulfurization of oils
because of their high affinity to sulfur-containing compounds.
The desulfurization yield of light oils, however, is relative low
by only extraction with ILs. To improve efficiency of sulfur
removal from light oils, a new effectual approach has been
explored, which is chemical oxidation in conjunction with
ILs extraction.26–30 For instance, Gao and coworkers26 used
[HMIm]BF4 as extractant and H2O2 as oxidant for sulfur
removal of DBT-containing model oil, which could reach 60–
93%. Li and coworkers27,28 developed an extraction and catalytic
oxidative desulfurization system composed of IL, H2O2 and
catalyst. [BMIm]BF4, [OMIm]BF4, [BMIm]PF6, [OMIm]PF6,
[BMIm]TA and [OMIm]TA doped with peroxotungsten and
peroxomolybdenum complexes or molybdic compounds were
selected for desulfurization as catalysts. The desulfurization
yield of DBT-containing model oil could reach above 98%. Wei
and coworkers29 found [BMIm]PF6 was a more effective solvent
than [BMIm]BF4 for providing an environment that results in a
higher rate of chemical oxidation. Zhao et al.30 studied the ODS
of DBT by using [HNMP]BF4 as extractant in the presence of
H2O2 and pointed out that [HNMP]BF4 was also a catalyst,
which could decompose H2O2 to form hydroxyl radicals that
were strong oxidizing agents for DBT.
Ionic liquids (ILs) have been extensively employed in gas
separation,10 liquid/liquid extraction,11 chemical synthesis,12
catalysis,13 etc.,14 because of their non-volatility, non-
flammability, easy to handle, recyclable, good solubility
characteristics, and high thermal stability.15 Desulfurization
aCollege of Science, Hebei University of Science and Technology,
Shijiazhuang, 050018, China. E-mail: lifatang@126.com;
Fax: +86-311-88632009; Tel: +86-311-81668536
However, the imidazolium- or pyrrolidonium-based ILs used
as extractants previously are relatively expensive and it is
impossible to use these ILs in industry. More recently, a series
bCollege of Chemical and Pharmaceutical Engineering, Hebei University
of Science and Technology, Shijiazhuang, 050018, China
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Fig. 1 Negative ion ESI-MS spectrum of BTMAC·2ZnCl2.
of inexpensive ILs have been prepared from cheap starting
materials, such as choline chloride·xZnCl2 and trimethylammo-
nium chloride·xAlCl3, which have been used in Fischer indole
reactions,31 protection of carbonyls32 and also desulfurization
of fuels.18 The Lewis acidic ILs containing metal halide anions,
CH3COOOH. The decrease of DBT concentration in ionic
liquid promotes the extraction process and the sulfur content in
the oil phase decreases continuously. On the basis of this and the
combined oxidative process developed by Wei and coworkers,29
the process of extraction and oxidation mechanism of DBT is
proposed, as shown in Scheme 1.
such as AlCl4,-19 FeCl4 and CuCl2,-21 have showed promising
-25
results on the removal of sulfur-containing compounds because
of their inherent low fluidity and the p-complexation of metal
anions with thiophenes. In this paper, Me3NCH2C6H5Cl·xZnCl2
(BTMAC·xZnCl2, x = 1~3), has been prepared from cheap
starting materials of Me3NCH2C6H5Cl and ZnCl2 and used as
extractant in ODS of DBT.
Results and discussion
Structures of anions in ZnCl2-based ionic liquid
To identify the Zn2+ species of BTMAC·xZnCl2 ionic liquid and
verify its stability to air, electrospray ionization tandem mass
(ESI-MS) spectral analysis was carried out. ESI-MS spectrum
of BTMAC·2ZnCl2 ionic liquid, as typical representative of
BTMAC·xZnCl2, is shown in Fig. 1. Intensive peaks are
Scheme 1 The process of extraction and oxidation mechanism of
DBT using H2O2 and AcOH as the oxidants in an oil–ionic liquid
system.
-
observed at m/z 171, 309 and 442, which correspond to ZnCl3 ,
Extraction desulfurization performance of ILs for DBT in
n-octane
-
-
Zn2Cl5 and Zn3Cl7 , respectively, using the atomic weights of
Zn and Cl.
Wicelinski et al.33 prepared ionic liquids composed of
AlCl3 with 1-butylpyridinium chloride (BPC) or l-methyl-3-
ethylimidazolium chloride (MEIC) and found there were hy-
droxychloro and oxychloro anionic species (such as Al2Cl6OH-,
Al3Cl7O2H-) in the ionic liquids because of the strong Lewis
acidity of anions. It indicated that the AlCl3-based ionic
liquids are moisture sensitive. Whereas there were no obvious
oxygen-containing anionic species in Fig. 1, which showed that
BTMAC·2ZnCl2 was stable to moisture and air.
Table 1 shows the Nernst partition coefficients kN sulfur of DBT
with BTMAC·xZnCl2 ILs. For comparison, some results of kN
with other ILs from literature are also listed.
As can be seen by the Nernst partition coefficients kN
listed in Table 1, all of the BTMAC·xZnCl2 ionic liquids
show remarkable ability for sulfur removal. It is found that
sulfur removal of model oil can reach above 23.3% after
extraction with BTMAC·xZnCl2 ionic liquids, whereas the
sulfur removal of DBT with imidazolium-based ILs are all below
20%. The reason for the difference of extraction capabilities
is that the desulfurization mechanism with ZnCl2-based ionic
liquids is different from that with imidazolium-based ionic
liquids. Herna´ndez-Maldonado and Yang et al.37 found there
was p-complexation between DBT with Zn2+ when performing
desulfurization with Zn(II)–X and Y zeolite sorbents. The
mechanism for the extraction of sulfur-containing compounds
with imidazolium-based IL is likely relevant to the formation
of liquid clathrate due to the p–p interaction between the
unsaturated bonds of the S-compound and the imidazolium
ring of ILs.22,36 In reference 19, Zhang et al. compared the
absorption capacities of several ILs, including trimethylam-
monium chloride salt-aluminium trichloride (AlCl3-TMAC),
The process and mechanism of ODS/extraction by
BTMAC·2ZnCl2
There are two steps in the process of the oil–ionic liquid
phase system of oxidation of DBT, containing the extractive
equilibrium and the oxidative reaction.34 In the first step, DBT
is extracted from oil phase into ionic liquid until the extractive
equilibrium is stabled. The second step is oxidation of DBT. In
presence of H2O2, the acetic acid (AcOH) gives peracetic acid
(CH3COOOH), which is a strong oxidant and its oxidative
ability is stronger than that of H2O2.35 DBT in ionic liquid
phase is oxidized to its corresponding sulfone (DBTO2) by
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Table 1 Nernst partition coefficients kN for extraction of DBT with ILs
IL
kN [mg of S (L of IL)-1/mg of S (L of oil)-1]
IL
kN [mg of S (L of IL)-1/mg of S (L of oil)-1]
a
a
BTMAC·ZnCl2
1.52
1.82
2.03
1.89
BTMAC·3ZnCl2
[BMIm]BFb
1.68
0.96
0.79
1.02
a
a
BTMAC·1.5ZnCl2
a
BTMAC·2ZnCl2
[BMIm]TAb
[OMIm]TAb
BTMAC·2.5ZnCl2
a Reaction conditions: model oil = 10 mL, IL = 2 mL, the mixture was stirred at 30 ◦C for 10 min. b Results from Li and coworkers.27 Model oil
was prepared by dissolving DBT in n-octane to give a solution with S-content of 1000 ppm. Reaction conditions: model oil = 5 mL, IL = 1 mL, the
mixture was stirred at 30 ◦C for 15 min.
[BMIm]PF6, [BMIm]BF4 and [EMIm]BF4 for model organosul-
fur compounds and found that the absorption capacity of AlCl3-
TMAC was higher than that of others. So the p-complexation
between lone pair electrons on organosulfur compounds and
the unoccupied orbital on transition metal ions was higher
than the p–p interaction between the unsaturated bonds of
the S-compound and the imidazolium ring of IL. Compared to
ionic liquids, the organic solvents show lower kN. For example,
Shiraishi et al.8 reported that acetonitrile was considered to
be the most suitable solvent for the extraction of the sulfur-
containing compounds. As a result, the kN was 1.32 and 0.19
when acetonitrile wa◦s employed as extractant to extract DBT
from n-octane at 60 C38 and commercial light oil at ambient
temperature,8 respectively.
The viscosities of the ionic liquids appear to be governed
essentially by H-bonding.40 When the molar ratio of ZnCl2 to
BTMAC increased from 1 to 1.5 or 2, the H-bonding between H+
on the cation and Cl- on the anion weakened and the viscosity
decreased because of the existence of the large anion41 of ZnCl3 ,
-
-
Zn2Cl5- and Zn3Cl7 . However, when the molar ratio of ZnCl2 to
BTMAC exceeded 1.5 or 2, some ZnCl2 could not fully react with
BTMAC and the ionic liquid was not really pure, so the viscosity
increased. It can be seen that the extraction performance of ionic
liquids depends on both Lewis acidity and viscosity. That is, the
extraction ability increases with the increase of Lewis acidity and
the decrease of viscosity. By combining the influence of the two
factors on the extraction ability of ionic liquid, BTMAC·2ZnCl2
is the ideal ionic liquid.
The high sulfur removal ability of BTMAC·xZnCl2 ionic
liquids make them good extraction solvents for desulfurization.
Although the Nernst partition coefficients are lower than
that of DBT in n-dodecane with [BMIm]Cl/AlCl3 (4.09),16
BTMAC·xZnCl2 are also competitive ILs with wide developing
prospect because they are cheap and insensitive to moisture and
air. It also can be seen that the highest Nernst partition coef-
ficient was 2.03 when BTMAC·2ZnCl2 was used as extractant.
That is, BTMAC·2ZnCl2 is the ideal ionic liquid.
Effect of H2O2/DBT molar ratio and IL/oil volume ratio on
sulfur removal of DBT
To investigate the effect of the amount of oxidizing agent
and extractant on the sulfur removal properties, the oxida-
tion/extraction of DBT in n-octane under different H2O2/DBT
(O/S) molar ratios and IL/oil volume ratios is carried out at
◦
30 C. The results are shown in Fig. 3 when BTMAC·2ZnCl2,
H2O2 and an equal volume of AcOH are employed as extractant
and oxidants in the system, respectively. As can be seen, the
sulfur removal of model gasoline increases with the increase of
IL/oil volume ratio. When the IL/oil volume ratio is low (below
1 : 5), the extraction effect of IL for model gasoline is bad. As the
IL/oil volume ratio goes above 1 : 5, i.e. 1 : 1, the sulfur removal
of model gasoline increases slightly and much IL is wasted. So
the most appropriate IL/oil volume ratio is 1 : 5.
Relative Lewis acidity and viscosities of ionic liquids
The Lewis acidic strength of [HSO3–(CH2)3–NEt3]Cl–ZnCl2
ionic liquids could be adjusted by varying the molar fraction of
ZnCl2 reported by previous literature.39 Here the relative acidity
of BTMAC·xZnCl2 ionic liquids is compared by FT-IR to
investigate the reason why the molar ratio of ZnCl2 to BTMAC
affects the extraction performance. As shown in Fig. 2, pure
acetonitrile shows two characteristic bands around 2292 and
2253 cm-1, which are assigned to CN stretching vibrations. When
acetonitrile is mixed with ionic liquids, a higher wavenumber
around 2313 cm-1 appears. The band at around 2313 cm-1 is
indicative of the existence of Lewis acidic sites. It is known that
the higher intensity of characteristic band means the stronger
Lewis acidity of the corresponding ionic liquid, so the Lewis
acidity strength of the ionic liquids increased with the increase
of the fraction of ZnCl2. Zhang et al.19 reported that the acidity
change did not have a significant effect on the sulfur removal
efficiency with ionic liquids and our extraction desulfurization
results are in agreement with them. That is, Lewis acidity is not
the only factor affecting the sulfur removal.
In the presence of H2O2 and AcOH, DBT was oxidized
chemically to DBTO2 etc. and the remaining DBT in n-octane
phase was further extracted into the IL phase. As shown in
Fig. 3, the sulfur removal of DBT in oil phase continued
to increase at different IL/oil volume ratios when the O/S
molar ratio was increased from 2 : 1 to 10 : 1. According to
the stoichiometric reaction, 2 mol of hydrogen peroxide are
consumed for oxidation of 1 mol of DBT to DBTO2. With the
increase of H2O2, the oxidant had more opportunity to react
with DBT and the desulfurization yield increased. It can be seen
that when V(IL)/V(oil) = 1 : 5, the sulfur removal of DBT
increased from 75.9% at a molar ratio of O/S = 2 to 94% at a
molar ratio of O/S = 6. When the O/S molar ratio reached 8, 10
and 12, the sulfur removal of DBT increased slowly to 98, 98.5
and 99.1%, respectively. The results indicated that the optimal
O/S molar ratio was 6 because less H2O2 was used, it is not
economical to use excess amounts of oxidant.
The viscosities of liquids always have a greater effect on
their extraction ability. The viscosities of BTMAC·xZnCl2 ionic
liquids are shown in Table 2.
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Fig. 2 FT-IR spectra of mixtures of acetonitrile and ionic liquids. (a) Pure acetonitrile, (b) molar ratio of ZnCl2 to BTMAC = 1.0, (c) molar ratio
of ZnCl2 to BTMAC = 1.5, (d) molar ratio of ZnCl2 to BTMAC = 2.0, (e) molar ratio of ZnCl2 to BTMAC = 2.5 and (f) molar ratio of ZnCl2 to
BTMAC = 3.0.
Table 2 Dynamic viscosities at 30 ◦C (Pa s) of the BTMAC·xZnCl2 ionic liquids
BTMAC·ZnCl2
BTMAC·1.5ZnCl2
BTMAC·2ZnCl2
BTMAC·2.5ZnCl2
BTMAC·3ZnCl2
3.548
3.067
3.112
3.358
3.649
Kinetics of oxidation of DBT in n-octane by H2O2 and AcOH
constant (min-1). Half-life (t1/2 (min)) is calculated using eqn (2),
which is derived from eqn (1) by replacing Ct with C0/2
To study kinetics of oxidation of DBT, samples (10 mL) were
taken out from n-octane phase and measured with a micro
coulometer to determine sulfur content every 5 min in the
reaction course. The results are shown in Fig. 4.
The oxidation of DBT in tetradecane has been studied by
Wei and coworkers29, who found that the rate of DBT oxidation
was first-order when excess H2O2 and AcOH were employed as
oxidants and [BMIm]BF4 or [BMIm]PF6 was used as extractant.
In this experiment, when the oxidation of DBT in n-octane by
H2O2 and AcOH is assumed to be the first-order reaction, the
sulfur content of model gasoline will meet eqn (1):7
t1/2 = 0.693/k
(2)
Fig. 5 illustrates the time-course variation in ln(C0/Ct) which
was obtained from data shown in Fig. 4. k and t1/2 could be
calculated. k and t1/2 were 0.0842 min-1 and 8.23 min, respec-
tively. Here, the results of the experiments strongly supported
the first-order reaction of oxidation kinetics of DBT.
Regeneration/recycling of ionic liquid
The recycling of the BTMAC·2ZnCl2 was investigated in ODS
of DBT in n-octane. After the reaction, the ionic liquid phase
was recycled and residual oxidizing agents (H2O2 and AcOH)
were evaporated from the ionic liquid. Then the system was
charged again with fresh H2O2 and AcOH for the next run.
ln(C0/Ct) = kt
(1)
Where C0 and Ct are the sulfur content of model gasoline at
time zero and time t (s), respectively, and k is the first-order rate
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Table 3 Recycle of BTMAC·2ZnCl2 in desulfurization of model oila
Cycle
Sulfur removal (%)
Cycle
Sulfur removal (%)
1
2
3
4
94
5
6
7
8
93.8
93.0
92.1
91.2
93.7
93.2
92.5
a Reaction onditions: T = 30 ◦C; t = 30 min; model oil = 10 mL, IL =
2 mL; molar ratio of O/S = 6.
decreased slightly with the increase of the cycle. The reason was
that DBTO2 in the ionic liquid was more and more and the
extraction performance of ionic liquid decreased.
Conclusion
BTMAC·2ZnCl2 ionic liquid is an excellent extraction solvent
because it is not only stable to moisture and air, but also
exhibits remarkable extraction desulfurization for DBT in n-
octane, which may be attributed to the p-complexing interaction
of DBT and Zn(II). In the presence of H2O2 and AcOH, DBT
is extracted from oil phase and oxidized to its corresponding
sulfone. The desulfurization yield of DBT can reach 94% at
30 min and 99% at 50 min. The BTMAC·2ZnCl2 ionic liquid
can be recycled six times without a significant decrease in
activity. The kinetics of oxidative desulfurization of DBT by
H2O2 and AcOH is first-order with an apparent rate constant
of 0.0842 min-1 and half-time of 8.23 min. All the experiments
were conducted at room temperature and it was easy to handle.
The study on extraction/oxidation desulfurization of DBT can
guide the desulfurization of light oils.
Fig. 3 Effect of H2O2/DBT molar ratio and IL/oil volume ratio on
sulfur removal of DBT. Reaction conditions: model oil = 10 mL, t =
30 min, 30 ◦C.
Experimental
Fig. 4 Time-course variation of sulfur removal of DBT in n-octane.
Reaction conditions: T = 30 ◦C; model oil = 10 mL, IL = 2 mL; molar
ratio of O/S = 6.
Preparation of ionic liquids and detection of the structure of
anions in ionic liquid
Anhydrous ZnCl2 and Me3NCH2C6H5Cl were used as received.
Me3NCH2C6H5Cl (0.1 mol) was mixed with ZnCl2 (0.1 xmol,
x = 1–3) and heated to ca. 120 ◦C in air with stirring until a
clear colorless liquid was obtained.
Structure of anions in ionic liquid was detected via ESI-MS,
which had been confirmed by Ko et al.25 to be an ideal analytical
tool in characterizing the structure of negative ions in IL. ESI-
MS spectrum of BTMAC·2ZnCl2 was collected on Shimadzu
2010EV LC-MS.
Evaluation of the Lewis acidity and measurement of viscosity of
ionic liquids
Fig. 5 Time-course variation of ln(C0/Ct). Reaction conditions: T =
30 ◦C; model oil = 10 mL, IL = 2 mL; molar ratio of O/S = 6.
FT-IR spectra were obtained on Shimadzu IRPrestige 21
spectrometer. Acetonitrile was used as a base probe molecule
to determine the acidity of ionic liquids. The detail process
was similar to that outlined in the previous literature.42 The
measurement of viscosity was carried out using an NDJ-8S
rotational viscometer.
After four times, DBTO2 was precipitated in the ionic liquid
phase and reclaimed from the ionic liquid by centrifugation. The
data shown in Table 3 indicated that the BTMAC·2ZnCl2 ionic
liquid could be recycled six times without a significant decrease
in activity. When six cycles had finished, the desulfurization yield
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model oil, the sulfur content of which was 1000 mg mL-1. IL
(1–10 mL) was then added and mixed with the oil. The sulfur
content of oil was detected using a micro coulometer with the
process of extraction. After 10 min, the sulfur content of oil did
not change which meant that the extraction equilibrium of DBT
in oil and ionic liquid phase was reached. 30 wt% H2O2 and
an equal volume of AcOH were added to the mixture, which
was then stirred vigorously. The upper phase (model oil) was
periodically withdrawn and analyzed for sulfur content using a
micro coulometer.
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At the end of each run, the ionic liquid phase was separated by
decantation from the oil phase. The oxidizing agents were then
evaporated from the ionic liquid phase at 100 ◦C for 3 h by rotary
evaporation. The DBTO2 was not removed from the ionic liquid.
The fresh H2O2, AcOH and model oil were introduced for the
next reaction under the same conditions as described above. By
this procedure, the DBTO2 was accumulated in the ionic liquid
successively. The solubility of DBTO2 in BTMAC·2ZnCl2 ionic
liquid is about 1805 ppm calculated by S-content and DBTO2 is
insoluble in n-octane. After four times, there was yellow solid,
i.e. DBTO2 in the ionic liquid phase. Then DBTO2 was reclaimed
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We are grateful for the financial support from the National
Natural Science Foundation of China (under No. 20806021 and
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