Oxidative desulfurization of fuel oil by pyridinium-based ionic liquids

Article abstract of DOI:10.3390/molecules14114351

In this work, an N-butyl-pyridinium-based ionic liquid [BPy]BF₄ was prepared. The effect of extraction desulfurization on model oil with thiophene and dibenzothiophene (DBT) was investigated. Ionic liquids and hydrogen peroxide (30%) were tested in extraction-oxidation desulfurization of model oil. The results show that the ionic liquid [BPy]BF₄ has a better desulfurization effect. The best technological conditions are: V(IL)/V(Oil)/V(H₂O₂) = 1:1:0.4, temperature 55 °C, the time 30 min. The ratio of desulfurization to thiophene and DBT reached 78.5% and 84.3% respectively, which is much higher than extraction desulfurization with simple ionic liquids. Under these conditions, the effect of desulfurization on gasoline was also investigated. The used ionic liquids can be recycled up to four times after regeneration.

Full text of DOI:10.3390/molecules14114351

Molecules 2009, 14, 4351-4357; doi:10.3390/molecules14114351
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Oxidative Desulfurization of Fuel Oil by Pyridinium-Based
Ionic Liquids
1
,2,
1
3
Dishun Zhao *, Yanan Wang and Erhong Duan
1
School of Chemical Engineering, Tianjin University, Tianjin, 300072, China;
E-Mail: mmsafin@hotmail.com
2
3
School of Chemistry and Pharmaceutical Engineering, Hebei University of Science and
Technology, Shi jiazhuang 050018, China
School of Environmental Science and Engineering, Hebei University of Science and Technology,
Shijiazhuang 050018, China; E-Mail: deh@tju.edu.cn
*
Author to whom correspondence should be addressed; E-Mail: dishunzhao@yahoo.com.cn.
Received: 9 September 2009; in revised form: 8 October 2009 / Accepted: 19 October 2009 /
Published: 28 October 2009
Abstract: In this work, an N-butyl-pyridinium-based ionic liquid [BPy]BF
The effect of extraction desulfurization on model oil with thiophene and dibenzothiophene
DBT) was investigated. Ionic liquids and hydrogen peroxide (30%) were tested in
extraction-oxidation desulfurization of model oil. The results show that the ionic liquid
BPy]BF has a better desulfurization effect. The best technological conditions
are: V(IL)/V(Oil) /V(H ) = 1:1:0.4, temperature 55 °C, the time 30 min. The ratio of
4
was prepared.
(
[
4
2
O
2
desulfurization to thiophene and DBT reached 78.5% and 84.3% respectively, which is
much higher than extraction desulfurization with simple ionic liquids. Under these
conditions, the effect of desulfurization on gasoline was also investigated. The used ionic
liquids can be recycled up to four times after regeneration.
Keywords: ionic liquids; extraction-oxidation; desulfurization; pyridinium-based
1
. Introduction
The dramatic environmental impact of sulfur oxides contained in engine exhaust emissions has been
widely recognized in recent years. As a result specifications regarding sulfur content in fuels are

Molecules 2009, 14
4352
becoming more and more stringent [1], and desulfurization of gasoline has attracted wide attention. In
European countries, the sulfur component in gasoline was restricted to a maximum 50 μg/mL in 2005,
and gasoline will be sulfur-free in 2011 [2]. Research on deep desulfurization of gasoline is of great
importance in both academic and industrial fields.
A traditional desulfurization method is catalytic hydrodesulfurization (HDS) using catalysts such as
CoMo or NiMo[3], which requires both high temperature and high pressure. Also HDS is only
effective on aliphatic sulfur structures such as thiols, thioethers and disulfides, etc. Sulfur-containing
aromatic compounds (including thiophene and its derivatives) are barely removed by this process. Due
to this reason, alternative desulfurization technologies have been developed, including adsorption [4],
extraction [5], and selective oxidation [6]. Extractive desulfurization is believed to be one of the best
methods because the process operation is easy. However, using volatile organic compounds (VOCs) as
extractant leads to more environmental and safety concerns.
Ionic liquids (ILs) have been employed extensively in Green Chemistry. ILs have the characteristics
of nonvolatility, nonflammablity and high thermal stability. As a result, ILs are considered as green
solvents and are employed in processes of catalysis, chemical synthesis and separations. The use of ILs
as extractants to remove aromatic sulfur compounds in extractive desulfurization has been reported in
recent research studies [7-14]. Bösmann [7] first described that imidazolium-based ILs can be utilized
as extractant. Most of the literatures have described with ILs as simple extractant; the results indicate
that ionic liquids can efficaciously extract the sulfur content from the oil phase. Wen-Hen Lo and co-
workers [8] combined the methods of oxidation and extraction, using, H O - acetic acid as the
2
2
4 6
oxidant, and [BMIM]BF and [BMIM]PF as the extractant; this process increases the desulfurization
rate by about an order of magnitude relative to that of merely extracting with RTILs.
According to the reports [14], the presence of acid could increase the desulfurization, but when it
comes to industrial applications, the equipment would be corroded by acid. In this work, we use a type
4
of pyridinium-based IL [BPy]BF as extractant, which is combined with hydrogen peroxide as oxidant
for the first time. The desulfurization of thiophene and dibenzothiophene (DBT) in model oil as well as
fluidized catalytic cracking (FCC) gasoline are studied.
2
. Results and Discussion
Both the cation of IL and thiophene (as well as its derivatives) have aromatic rings. When the IL
cation, which had an aromatic ring structure with a large π bond, interacted with thiophene sulfur
compounds, a complex was produced and the thiophene compounds were extracted to the IL phase due
to the easily polarized sulfur molecule [15].
2
.1. Effect of extractive desulfurization for model oil
4
The extractive desulfurization results of IL[BPy]BF are shown in Figure 1. When IL was used as
extractant for thiophene and DBT containing model oil, the extraction equilibrium will reached after
about 15 min. the ratio of sulfur removal reached 44.5% and 47.2%, respectively, with the reaction
conditions set at 30 °C for 15 min. The IL has better extracting ability to remove DBT than thiophene
from the model oil, the reason may be that DBT has more aromatic rings on the thiophene branch, so
the interaction effect between them was enhanced.

Molecules 2009, 14
Figure 1. Effect of desulfurization to thiophene and DBT in model fuel with IL.
4353
52
48
44
40
36
32
28
24
(
2)
(
1)
3
6
9
12
15
t/min
Extraction conditions: 30 °C; V (IL): V (Model fuel) = 1:1; (1) Thiophene (2) DBT
2
.2. Effects of oxidative desulfurization for model oil
Because hydrogen peroxide can be dissolved in the IL, upon addition of hydrogen peroxide as an
oxidant into IL[BPy]BF , the H was decomposed to hydroxyl radical in the system, and then was
4
2 2
O
further decomposed to hydroxyl radical (HOO-). This oxidizes the thiophene sulfides to the
corresponding sulfone, and as the polarity of the sulfone was larger, it was remained in the IL phase.
The process of this reaction is shown in Figure 2.
2 2
Figure 2. Process of extraction-oxidation of thiophene and DBT in model oil by IL- H O system.
S
S
Oil phase
IL phase
H O
H O
2 2
2
2
S
S
[
BPy]BF4
[BPy]BF
4
O
S
S
O
O
O
2
2 2
.2.1. Effect of the amount of H O on desulfurization of model oil
The influence of the amount of H
desulfurization was not changed after about 30 min. It is obvious that the desulfurization of model oil
is increased after adding H to IL, with the presence of H , the desulfurization were up from
4.5% to 69.8% (thiophene) and from 47.2% to 74.0% (DBT). The optimal ratio by volume of H to
IL is 0.4:1. No change in the desulfurization was visible when the ratio was greater. This means that
when the ratio is 0.4:1, free radicals decomposed by H were sufficient to oxidize thiophene and
DBT to sulfone, so we chose the ratio of 0.4:1 in the subsequent research.
2 2
O (30%) on desulfurization is shown in Figure 3. The rate of
2
O
2
2 2
O
4
2 2
O
2
O
2

Molecules 2009, 14
Figure 3. Effect of amount of H O on desulfurization in model oil.
4354
2
2
80
76
72
68
64
60
56
52
48
44
40
(
2)
(1)
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
ratio of H O to IL
2
2
Reaction conditions: temperature = 30 °C; V (IL) / V (Model oil) = 1; time = 30 min; (1)
Thiophene (2) DBT.
2
.2.2. Effect of temperature on desulfurization of model oil
The effect of reaction temperature (from 30 °C to 65 °C) on the desulfurization of model oil is
shown in Figure 4. The reaction rate increased obviously when the temperature increased from 30 °C
to 55 °C, and when the temperature increased to 55 °C, the desulfurization reached the maximum
7
8.5% and 84.3% values, respectively. When the temperature was higher than 55 °C, the
desulfurization decreased appreciably. The reason is that the H is decomposed to H O at a high
2
O
2
2
temperature. The temperature of 55 °C was the optimal one in this study.
Figure 4. Effect of reaction temperature on desulfurization in model oil.
86
84
82
80
78
76
74
72
70
(2)
(1)
25
30
35
40
45
50
55
60
65
70
Degree Centigrade
2 2
Reaction conditions: V (IL) / V (Model oil) / V (H O ) = 1:1:0.4, time = 30 min; (1) Thiophene (2) DBT.

Molecules 2009, 14
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2
.3. Oxidative desulfurization of gasoline with IL
The extraction system and extraction-oxidation system were applied to gasoline containing
7
80 μg/mL sulfur, which was provided by the Shijiazhuang Refinery. The reaction conditions for the
extraction desulfurization ares: mass ratio of IL to gasoline is 1:1, reaction for 30 min at 55 °C. The
reaction conditions of extraction-oxidation desulfurization are: mass ratio of IL to gasoline is 1:1, the
amount of H O to IL is 0.4:1, time and temperature are the same. The concentrations of sulfur in
2
2
gasoline before and after reaction are shown in Table 1, where it can be seen that the sulfur content of
gasoline can be reduced by 35.9% (extraction desulfurization) and 56.3% (extraction-oxidation
desulfurization). We found that the desulfurization was decreased compared to the data of
desulfurization on model oil. The reason is that gasoline contains many non-aromatic sulfur
components such as thiol sulfide which cannot be extracted by IL effectively. On the other hand, the
aromatic hydrocarbons and olefins in gasoline influence the sulfur extraction as well.
Table 1. Effect on desulfurization in diesel oil.
IL system
Extraction
Extraction-oxidation
Primal sulfur content Desulfurization
a
780μg/mL
780μg/mL
35.9%
56.3%
b
Reaction conditions: a: temperature = 55 °C; V (Model oil) / V (IL) = 1:1; time = 30 min; b:
temperature = 55 °C; V (Model oil) / V ( IL) / V(H O ) = 1:1:0.4; time = 30 min.
2
2
2
.4. Regeneration of used IL
The recycling of the xtraction-oxidation system for IL was investigated in the desulfurization of
model oil with thiophene and DBT. The used IL was distilled in an oil bath at 110 °C to remove H
2
O
2
entirely, and then the IL was regenerated by re-extraction in tetrachloromethane. The results of
desulfurization on model oil by regenerated IL are shown in Table 2, which indicates that the
regenerated IL could be recycled four times with negligible decrease in activity.
Table 2. The desulfurization of model oil by regenerated IL.
Regenerated time
Thiophene
78.5%
77.0%
76.4%
75.8%
74.3%
72.1%
DBT
Fresh IL
84.3%
83.2%
82.0%
81.1%
80.3%
77.5%
1
2
3
4
5
2 2
Reaction conditions: temperature = 55 °C; V (Model oil) / V (IL) / V (H O ) = 1:1:0.4; time = 30 min.
3
. Experimental
The IL [BPy]BF
0.2 mol), butyl bromide (0.2 mol) and cyclohexane (50 mL) were stirred in a round-bottom flask at
0 °C for 12 h. The white precipitate (N-butylpyridinium bromide) was filtrated off and evaporated in
4
was prepared using the methods described in the literature [16]. Briefly, pyridine
(
6

Molecules 2009, 14
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a vacuum drying oven. The IL was prepared by mixing N-butylpyridinium bromide (0.2 mol) and
sodium tetrafluoroborate (0.2 mol) with acetone (100 mL) in a round-bottom flask, and stirring the
mixture at room temperature for 12 h. The precipitate was filtered off, and then the solvent was
1
removed by rotary evaporation to leave the desired yellow IL. The IL was analyzed by H-NMR,
1
3
C-NMR, and Fourier Transform Infrared (FTIR) spectroscopy.
Model oil was prepared by dissolving thiophene (0.525 g) in n-octane (500 mL), and DBT (0.920 g)
in n-octane (500 mL), The sulfur contents of the model oils were 400 μg/mL and 320 μg/mL
respectively. The gasoline was provided by the Shijiazhuang Refinery of China, The sulfur content
was 780 μg/mL. Desulfurization experiments were carried out as follows: model oil (or gasoline)
(
10 mL)e and IL (10 mL) and a certain amount of hydrogen peroxide were added to a 50 mL flask, the
resulting mixture was stirred for 30 min at the desired reaction temperature. The concentration of
sulfur in model oil and FCC gasoline were detected by a microcoulometric detector (WK-2D) and a
gas chromatograph equipped with a flame photometric detector (GC-FPD).
4
. Conclusions
The ionic liquid [BPy]BF was found to be effective in the extraction of sulfur components in
4
model oils and gasoline at room temperature. The desulfurization process was achieved after H
2 2
O was
added to the extraction system; the oxidizability of H was utilized for oxidizing the sulfur
2
O
2
components to the corresponding sulfones which were extracted by the IL. The desulfurization
ofthiophene and DBT by the extraction-oxidation system can reach values of 78.5% and 84.3%,
respectively, which are much higher than those achieved by extractive desulfurization. Because of the
other complex components in gasoline, the desulfurization of that substance was lower than the
desulfurization of model oil. The used IL was regenerated through re-extraction in tetrachloro-
methane, and can be recycled four times without any obvious decrease in activity. This proves the
feasibility of desulfurization with a simple, mild and environmentally benign method in industry in the
future.
Acknowledgements
The work was financially supported by the National Natural Science Foundation of China
(
20576026).
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4
Sample Availability: Samples of the compounds IL([BPy]BF ) and gasoline are available from the
authors.
©
2009 by the authors; licensee Molecular Diversity Preservation International, Basel, Switzerland.
This article is an open-access article distributed under the terms and conditions of the Creative
Commons Attribution license (http://creativecommons.org/licenses/by/3.0/).