Extraction process of sulfur compounds from fuels with protic ionic liquids

Article abstract of DOI:10.1039/c4ra16186f

Sulfur compounds in fuels are hazardous to the environment and present a great challenge. In this study, a series of protic ionic liquids (PILs) was designed, such as N,N-dimethyl-N-cyanoethyl ammonium butyrate ([DMAPN]⁺[Bu]^-), N,N-dimethyl-N-cyanoethyl ammonium propionate ([DMAPN]⁺[Pr]^-), N,N-dimethyl-N-cyanoethyl ammonium acetate ([DMAPN]⁺[Ac]^-), N,N-dimethyl-N-cyanoethyl ammonium formate ([DMAPN]⁺[Fo]^-), N,N-dimethyl-N-(2-(2-hydroxyethoxy)) ammonium propionate ([DMEE]⁺[Pr]^-), N,N-dimethyl-N-hydroxyethyl ammonium propionate ([DMEA]⁺[Pr]^-), N,N-dimethyl-N-hydroxyethyl ammonium acetate ([DMEA]⁺[Ac]^-), N,N-dimethyl-N-hydroxyethyl ammonium formate ([DMEA]⁺[Fo]^-), N,N-dimethyl-N-butyl ammonium acetate ([DMBA]⁺[Ac]^-), N-methyl-N-hydroxyethyl ammonium propionate ([MEA]⁺[Pr]^-), and N-methyl-N-hydroxyethyl ammonium acetate ([MEA]⁺[Ac]^-), and their extraction desulfurization capability was evaluated. Through selection, two PILs were chosen to optimize the desulfurization process. The results showed that benzothiophene (BT) and thiophene (T) can be removed using PILs effectively. Both cations and anions have an influence on the extraction efficiencies. However, cations have a greater influence on the desulfurization process. Under optimal conditions, the extraction efficiency can be higher than 80% for one cycle. After 6-8 cycles, the removal efficiency could reach up to 99.5%. It is worth noting that the PILs could be recycled via simple vacuum distillation. After 4 times of recycling, the desulfurization efficiency changed slightly. Finally, a mechanistic study shows that hydrogen bonding interactions between the sulfur atom and active hydrogen of the PILs account for the high desulfurization efficiency. This green process would provide a new route for the extraction desulfurization of fuels. This journal is

Full text of DOI:10.1039/c4ra16186f

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Extraction process of sulfur compounds from fuels
with protic ionic liquids†
Cite this: RSC Adv., 2015, 5, 15892
a
c
c
Zhuo Li,ab Jinggang Xu, Dan Li and Changping Li
*
*
Sulfur compounds in fuels are hazardous to the environment and present a great challenge. In this study, a
series of protic ionic liquids (PILs) was designed, such as N,N-dimethyl-N-cyanoethyl ammonium butyrate
([DMAPN]+[Bu]ꢀ), N,N-dimethyl-N-cyanoethyl ammonium propionate ([DMAPN]+[Pr]ꢀ), N,N-dimethyl-N-
cyanoethyl ammonium acetate ([DMAPN]+[Ac]ꢀ), N,N-dimethyl-N-cyanoethyl ammonium formate
([DMAPN]+[Fo]ꢀ), N,N-dimethyl-N-(2-(2-hydroxyethoxy)) ammonium propionate ([DMEE]+[Pr]ꢀ), N,N-
dimethyl-N-hydroxyethyl ammonium propionate ([DMEA]+[Pr]ꢀ), N,N-dimethyl-N-hydroxyethyl
ammonium acetate ([DMEA]+[Ac]ꢀ), N,N-dimethyl-N-hydroxyethyl ammonium formate ([DMEA]+[Fo]ꢀ),
N,N-dimethyl-N-butyl ammonium acetate ([DMBA]+[Ac]ꢀ), N-methyl-N-hydroxyethyl ammonium
propionate ([MEA]+[Pr]ꢀ), and N-methyl-N-hydroxyethyl ammonium acetate ([MEA]+[Ac]ꢀ), and their
extraction desulfurization capability was evaluated. Through selection, two PILs were chosen to optimize
the desulfurization process. The results showed that benzothiophene (BT) and thiophene (T) can be
removed using PILs effectively. Both cations and anions have an influence on the extraction efficiencies.
However, cations have a greater influence on the desulfurization process. Under optimal conditions, the
extraction efficiency can be higher than 80% for one cycle. After 6–8 cycles, the removal efficiency
could reach up to 99.5%. It is worth noting that the PILs could be recycled via simple vacuum distillation.
After 4 times of recycling, the desulfurization efficiency changed slightly. Finally, a mechanistic study
shows that hydrogen bonding interactions between the sulfur atom and active hydrogen of the PILs
account for the high desulfurization efficiency. This green process would provide a new route for the
extraction desulfurization of fuels.
Received 11th December 2014
Accepted 21st January 2015
DOI: 10.1039/c4ra16186f
www.rsc.org/advances
sulfur compounds can react with hydrogen and be converted
into hydrogen sulde and the corresponding hydrocarbons.
1. Introduction
Although HDS is efficient in removing mercaptans, suldes,
and sulfur from fuels, it is less effective for the removal of sulfur
containing compounds with a larger steric hindrance, such as
thiophene and its derivatives.6,7 The harsh operating conditions
and high costs of HDS hinder its application in the deep
removal of thiophene-based sulfur contents. Another disad-
vantage of HDS is the easy olen saturation and the notable
decrease in the octane number of gasoline aer treatment.8
Therefore, non-hydrodesulfurization technologies have attrac-
ted great attention. Some new desulfurization processes have
been developed in recent years, such as oxidative,9,10 adsorp-
tion,11,12 biological,13,14 extraction,15,16 and oxidative coupling
with extraction processes,17,18 etc.
The extraction desulfurization process is based on the prin-
ciple that organic suldes and hydrocarbons have different
solubilities in a solvent. However, traditional extractants applied
for the desulfurization process are volatile organic solvents,
which pollute the environment seriously because of their strongly
volatile and highly toxic properties. Therefore, the development
of new green and efficient extractants can be of great importance
to effectively remove sulfur compounds from fuels.
In recent years, with the increase of motor vehicles, automobile
exhausts and especially the sulfur content in fuels have become
a major source of air pollution. Acid rain resulting from auto-
mobile exhausts has become a more and more serious problem,
which not only causes damage to buildings and soils, but is also
hazardous to the environment.1,2 Therefore, most countries put
forward strict regulations on sulfur emission. According to the
Directive of the European Union, the total sulfur content in
gasoline or diesel fuels in Europe should not exceed 10 ppm
aer 2010.3 So, considerable attention has been given to the
deep desulfurization of fuels to eliminate their hazardous
inuence on the environment.4 Hydrodesulfurization (HDS) is a
conventional ‘gold’ method widely adopted for the removal of
sulfur contents in petroleum reneries.5 In the HDS process,
aCollege of Resources and Environment, Northeast Agricultural University, Harbin,
China. E-mail: jinggangxu@126.com
bSchool of Science, Dalian Ocean University, Dalian, China
cDepartment of Chemical Engineering, Dalian University, Dalian, China. E-mail:
changpingli@dicp.ac.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra16186f
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Ionic liquids (ILs) are usually composed completely of ions the reaction ask and stored under N2 until use for further
with melting points lower than 100 ꢁC.19,20 Compared with experiments. The synthesis process of [DMAPN]+[Pr]ꢀ was taken
traditional organic solvents, they have unique properties such as an example for describing the synthesis process. 3-Dime-
as high thermal and chemical stabilities, a negligible vapor thylaminopropionitrile (49.08 g, 0.5 mol) was placed in a
pressure, high ionic conductivity, non-ammability and a 250 mL round-bottomed ask equipped with a stirrer and
tunable solvation capacity.21–25 With the above special proper- cooled using an ice bath for 30 min. Propionic acid (37.04 g,
ties, the loss of ILs resulting from distillation and dissolution in 0.5 mol) was added dropwise using a dropping funnel. Aer the
other phases during the desulfurization process can be avoided, addition was completed, the mixture was stirred for 24 h. Then,
as ILs have a good extraction capability for aromatic sulfur [DMAPN]+[Pr]ꢀ was obtained. The structures of the PILs are
1
compounds, and do not dissolve in fuels. So, the extraction shown in Fig. 1. The purity of the ILs was conrmed using H
desulfurization based on ILs has become a hot topic in recent NMR spectroscopy, which is listed in Fig. S1 of the ESI.†
years.26–30 However, the high costs and high viscosities of [DMAPN]+[Pr]ꢀ: 1H NMR (CDCl3) d 11.55 (m, 1H); d 2.77 (t, 2H); d
commonly used imidazolium based ILs hinder their application 2.60 (dd, 2H); d 2.36 (s, 6H); d 2.31 (q, 2H); d 1.12 ppm (t, 3H).
in the desulfurization process.
Amine based protic ionic liquids (PILs) are a new type of ILs,
2.2 Extraction process
which can be designed according to our own demand through
BT or T was dissolved in n-octane as a model oil. The sulfur
proton exchange between Brønsted acid and Brønsted base.31
content of the model oil was 1600 ppm. The extraction process
PILs not only have the excellent physical and chemical proper-
was performed in the self-made devices at 25 ꢁC. The reactants
ties of traditional ionic liquids, they also have their own unique
contained 1 g of IL and 1 g of model oil. The mixture was stirred
properties, such as cheap raw materials, a simple synthesis
at room temperature for 30 min except as otherwise dened.
process, wide range of viscosities (10–650 mPa s), and a simple
The ratio of reactants was adjusted according to the different
conditions. All experiments in this study were carried out in
recycling process with vacuum distillation. Most importantly,
PILs are mainly composed of non-toxic, biodegradable mate-
duplicate to determine its reproducibility, and the experimental
rials. Therefore, PILs exhibit special environmentally friendly
errors were within 3%.
and biocompatible properties compared with traditional
ILs.31–35 In our previous research, two PILs, namely
[DMEE]+[Pr]ꢀ and [DMAPN]+[Pr]ꢀ, were probed for removing
2.3 Analytical methods
DBT from fuels, and a higher desulfurization efficiency was
The sulfur content of the model oil was measured by the gas
chromatograph (GC). The chromatogram conditions are listed
achieved.36
With the above preliminary research, more systematic
as follows: chromatogram column, HP-5; injection volume,
research is performed for the removal of organic sulfur from
0.6 mL; carrier gas (N2), 210 mL minꢀ1; H2, 35 mL minꢀ1; air,
fuels. In this study, a series of amine based PILs was designed
350 mL minꢀ1; ux, 1.6 mL minꢀ1, at constant ow mode; inlet
and synthesized, such as [DMAPN]+[Bu]ꢀ, [DMAPN]+[Pr]ꢀ,
temperature, 300 ꢁC; detector temperature, 300 ꢁC; and column
[DMAPN]+[Ac]ꢀ, [DMAPN]+[Fo]ꢀ, [DMEE]+[Pr]ꢀ, [DMEA]+[Pr]ꢀ,
ꢁ
temperature (BT), heating from 120 ꢁC to 150 Cꢀw1 ith 15 ꢁC
[DMEA]+[Ac]ꢀ, [DMEA]+[Fo]ꢀ, [DMBA]+[Ac]ꢀ, [MEA]+[Pr]ꢀ, and
minꢀ1 increase, 150 ꢁC to 160 ꢁC with 2 ꢁC min increase,
[MEA]+[Ac]ꢀ, which contain cyano, hydroxyl and ether groups,
ꢀ1
ꢁ
ꢁ
ꢁ
ꢁ
160 C to 250 C with 25 C min increase; column tempera-
and their extraction capability for benzothiophene (BT) and
thiophene (T) in fuels was also probed. Some factors that affect
the desulfurization efficiency were investigated, such as the type
of PIL, extraction time, rotation speed, PIL dosage, initial sulfur
concentration, etc., to determine the optimal conditions for the
desulfurization process, and recycling of the PILs was realized
successfully. Finally, the extraction mechanisms were also
studied systematically using quantitative 1H NMR and FT-IR
spectroscopy.
ꢁ
ꢁ
ture (T), heating from 60 C, stay 2 min, to 100 C with 10 C
minꢀ1 increase, 100 ꢁC to 250 ꢁC with 25 ꢁC minꢀ1 increase. The
concentration of T and BT were measured with an internal
2. Experimental
2.1 Preparation of PILs
The synthesis process was carried out under an inert atmo-
sphere (N2). Raw materials were puried before use. The amine
was placed in a 250 mL round-bottomed ask equipped with a
stirrer and cooled using an ice bath for 30 min. Acid was added
dropwise using a dropping funnel. Care must be taken to keep
the reaction mixture cool because the reaction is very
exothermic. Aer the addition was completed, the mixture was
stirred for 24 h. Aer this period, the PILs were removed from Fig. 1 Structures of the PILs.
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standard method, using hexadecane as the internal standard
solvent. The concentration of hexadecane was 2000 ppm. The
analysis of the standard curve is shown in Fig. S2 and S3 of the
ESI.† The correlation coefficients achieved were 0.99992 and
0.99993.
3. Results and discussions
3.1 Selection of PILs
The designable nature of the structuresmakes a diversity of PILs
possible. So, the proper choice of PIL is very important for the
desulfurization process. The selection of PILs is shown in Fig. 2.
As derived from the results, the structures of the cations and
anions both inuence the desulfurization process. However, the
cations have a greater inuence on the desulfurization process.
The extraction sequence for the cations is listed as follows:
[DMAPN]+[Pr]ꢀ > [DMEE]+[Pr]ꢀ > [DMEA]+[Pr]ꢀ > [MEA]+[Pr]ꢀ,
Fig. 3 Effect of extraction time on the extraction efficiency.
that the rotation speed has no inuence on the extraction
desulfurization efficiency. The main reason for this is that the
low viscosity of the two PILs is benecial for the mass transfer
during the extraction process. As the mass transfer can be very
important for the energy consumption, it will promote the
application of ILs. In this research, 800 rpm was selected for the
following experiments.
During the extraction process, the concentration gradient is
the driving force for the extraction. Therefore, the ionic liquid
dosage is an important factor. The results are shown in Fig. 5.
As derived from the result, a larger ionic liquid dosage would be
favorable for the desulfurization process. When the phase ratio
of [DMAPN]+[Pr]ꢀ to the fuel reaches 4 : 1, extraction efficien-
cies of 88.5% for BT and 84.7% for T can be achieved. When the
phase ratio of [DMEA]+[Pr]ꢀ to the fuel reaches 4 : 1, extraction
efficiencies of 81.9% for BT and 77.4% for T can be achieved.
The extraction efficiency is much higher than that of traditional
ionic liquids. Considering the economic efficiency, the mass
ratio of PILs to fuels was chosen as 1 : 1 for the following
experiments.
while that for the anions is: [DMAPN]+[Bu]ꢀ > [DMAPN]+[Pr]ꢀ
>
[DMAPN]+[Ac]ꢀ > [DMAPN]+[Fo]ꢀ. Among all PILs, [DMAPN]+[Bu]ꢀ
has the highest extraction desulfurization efficiency, and the
extraction efficiency can reach up to 62.5% for BT and 59.9% for
T. Taking all factors into account, [DMAPN]+[Pr]ꢀ and
[DMEA]+[Pr]ꢀ were chosen as representative PILs to optimize
the extraction desulfurization process.
3.2 Optimization of the extraction parameters
To optimize the desulfurization process, some factors inu-
encing the extraction, such as effect of extraction time, rotation
speed, phase ratio and initial concentration, were investigated.
Fig. 3 shows the effect of extraction time on the extraction
efficiency. The sulfur content in fuels was reduced tremen-
dously with the increase in time at the very beginning. However,
aer 10 min, the extraction equilibrium could be reached. The
lower viscosity of the PILs probably accounts for this fast
equilibrium. The shorter extraction equilibrium time is very
benecial for future industrial applications. However, 30 min
was chosen as the extraction time in the following study to
guarantee a sufficient equilibrium.
The initial concentration of the sulfur compounds is also a
factor that inuences the partition coefficients of sulfur. The
result is shown in Fig. 6. It can be seen that the partition coef-
cients increase and then decrease with the increase in initial
The effect of rotation speed was another factor to affect the
extraction process, which is shown in Fig. 4. The result indicates
Fig. 2 Effect of PIL types on the extraction efficiency.
15894 | RSC Adv., 2015, 5, 15892–15897
Fig. 4 Effect of rotation speed on the extraction efficiency.
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Fig. 5 Effect of mass ratio on the extraction efficiency.
Fig.
7
Effect of multiple extraction cycles on desulfurization
efficiency.
sulfur concentration, and the extraction efficiencies of
[DMAPN]+[Pr]ꢀ are higher than those of [DMEA]+[Pr]ꢀ. All
factors taken into account, 1600 ppm was chosen as the initial
concentration of sulfur in the fuels.
environmental aspects. In our specic case, the used PILs,
namely [DMAPN]+[Pr]ꢀ and [DMEA]+[Pr]ꢀ, both have lower
boiling points. This results in the development of a new recy-
cling method by vacuum distillation. Then, the recycled PILs
were used in a further extraction process, as shown in Fig. 9. It
can be found that the extraction efficiencies are very stable aer
four times of recycling. So, this result is very signicant for
future industrial applications and the facile recycling of PILs.
Furthermore, the structures of the PILs do not change, as shown
in Fig. 10. As can be seen from the above study, the PILs can be
recycled and reused easily and efficiently.
3.3 Multiple extraction cycles
Efficient desulfurization was achieved within one cycle;
however, deep desulfurization is always our goal. The sulfur
content in fuels should be less than 10 ppm to meet the Euro V
standards. To achieve deep desulfurization, multiple extraction
cycles were conducted and investigated, which is shown in
Fig. 7. Aer 6–8 extraction cycles, deep desulfurization was
realized with an extraction efficiency of more than 99.5%. The
sulfur content of fuels can be reduced below 7.8 ppm, which
cannot be achieved solely by the extraction process with tradi-
tional ILs.
3.5 Extraction mechanisms
Although the extraction mechanism is always challengeable, it
is very important for understanding the extraction process. So,
quantitative 1H NMR and FT-IR spectroscopy were used for
mechanistic studies, and the spectra are shown in Fig. 11 and
12. In this part, [DMAPN]+[Pr]ꢀ was taken as an example to
systematically probe the extraction mechanisms. Fig. 11 shows
the 1H NMR results of different molar ratios between
[DMAPN]+[Pr]ꢀ and thiophene (T). It can be seen that hydrogen
bonding interactions exist between the active hydrogen of the
PIL and the sulfur atom of T. These hydrogen bonding
3.4 Repeated use and recycling of used PILs
To see if the PILs have become saturated, repeated use of the
used PILs for the extraction desulfurization was investigated. As
shown in Fig. 8, the PILs became saturated and their extraction
capability was lost aer ve repeated uses. So, the recycling and
reuse of PILs can be of great necessity from both economic and
Fig. 6 Effect of initial concentration of sulfur on the extraction
efficiency.
Fig. 8 Effect of repeated use of PILs.
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Fig. 12 FT-IR of different molar ratios (T/[DMAPN]+[Pr]ꢀ). (a) 1 : 0 (b)
1 : 1 (c) 0.5 : 1 (d) 0.1 : 1 (e) 0 : 1.
Fig. 9 Extraction performance of recycled PILs.
its corresponding hydrogen nucleus electron density. The
vibrating magnetic eld strength decreases and the chemical
shi values increase. With the increase in T, the phase ratio of
PIL/T from 1 : 0.1 to 1 : 1 increases, and the hydrogen atom
signals such as a-H (7.2981 ppm) and b-H (7.0920 ppm) both
obviously move to a lower eld, namely to 7.3606, 7.3472, and
7.3286 ppm for a-H and to 7.1284, 7.1230, and 7.1080 ppm for
b-H. At the same time, with increasing thiophene ratio, the
active hydrogen of [DMAPN]+[Pr]ꢀ also shis to the higher eld.
As can be seen from the above analysis, the driving force of the
extraction process is probably the hydrogen bonding
interactions.
To conrm the hydrogen bonding interaction mechanism,
characterization with FT-IR spectroscopy was also carried out,
which is shown in Fig. 12. The hydrogen bonds affect the C–S–C
stretching vibration of thiophene to a certain extent.37 So, the
C–S–C stretching vibration is taken as an example and was
observed at wavenumbers between 1000 and 1100 cmꢀ1. As seen
from Fig. 12, the C–S–C vibration of pure thiophene is observed at
1079 cmꢀ1, which shis to 1076 cmꢀ1 for T : [DMAPN]+[Pr]ꢀ ¼ 1 : 1
and 0.5 : 1 and to 1075 cmꢀ1 for 0.1 : 1. This phenomenon results
from the formation of hydrogen bonds between [DMAPN]+[Pr]ꢀ
and thiophene. Therefore, it can be concluded that the
hydrogen bond formation between the active hydrogen of the
PIL and the sulfur atom of thiophene accounts for the higher
extraction efficiency.
Fig. 10 1H NMR spectra of pure and recycled PILs. (a) Pure
[DMAPN]+[Pr]ꢀ. (b) Recycled [DMAPN]+[Pr]ꢀ.(c) Pure [DMEA]+[Pr]ꢀ. (d)
Recycled [DMEA]+[Pr]ꢀ.
For the 1H NMR results of different molar ratios between
[DMAPN]+[Pr]ꢀ and BT, we refer to Fig. S4 in the ESI.† The same
1
trends were also obtained in the H NMR spectra. So, we can
draw the conclusion that hydrogen bonding is the main driving
force for this specic extraction desulfurization process.
4. Conclusions
Fig. 11 1H NMR of different molar ratios ([DMAPN]+[Pr]ꢀ/T). (a) 1 : 0 (b)
A series of protic ionic liquids was designed and synthesized.
Their extraction desulfurization capability was estimated.
Through selection, two typical PILs were chosen as typical
examples to optimize the extraction desulfurization process.
The results show that BT and T can be effectively removed using
PILs. Both cations and anions have an inuence on the
extraction. However, the cations have a greater inuence on the
1 : 0.1 (c) 1 : 0.5 (d) 1 : 1 (e) 0 : 1.
interactions will result in the reinforcement of the sulfur atom’s
electron withdrawing ability for the a-H and b-H in the ring of T,
which will lead to a decrease in the T ring electron density and
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desulfurization process. The sequence is: [DMAPN]+[Pr]ꢀ
>
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