Journal of Environmental Protection, 2011, 2, 796-802
doi:10.4236/jep.2011.26091 Published Online August 2011 (http://www.SciRP.org/journal/jep)
Ionic Liquid Assisted Extraction of Nitrogen and
Sulphur-Containing Air Pollutants from Model Oil
and Regeneration of the Spent Ionic Liquid
Ikenna Anugwom1, Päivi Mäki-Arvela1, Tapio Salmi1, Jyri-Pekka Mikkola1,2
1Industrial Chemistry and Reaction Engineering, Process Chemistry Centre, Åbo Akademi University, Turku, Finland; 2Technical
Chemistry, Department of Chemistry, Chemical-Biological Center, Umeå University, Umea, Sweden.
Received April 21st, 2011; revised June 9th, 2011; accepted July 16th, 2011.
ABSTRACT
Removal of air pollutants, such as nitrogen and sulphur containing compounds from a model oil (dodecane) was stud-
ied. An ionic liquid (1-ethyl-3-methylimidazolium chloride [C2 mim] [Cl]) was used as an extractant. Liquid-liquid ex-
traction by using 1-ethyl-3-methylimidazolium chloride [C2 mim] [Cl] was found to be a very promising method for the
removal of N- and S-compounds. This was evaluated by using a model oil (dodecane) with indole as a neu tral nitrogen
compound and pyrid ine as a basic nitrogen compound. Dibenzothiop hene (DBT) was used as a sulphur compound. An
extraction capacity o f up to 90 wt% was achieved for the model oil containing pyridine, while only 76 wt% o f indole in
the oil was extracted. The extraction capacity of a model sulphur compound DBT was found to be up to 99 wt%. Re-
generation of the sp ent ionic liquid was carried out with toluene back-extraction. A 1:1 toluene-to-IL wt ratio was per-
formed at room temperature. It was observed that, for the spent ionic liquid containing DBT as a model compound
more than 85 wt% (corresponding 3852 mg/kg) could be removed from the oil. After the second regeneration cycle, 86
wt% of the DBT was recovered from the ionic liquid to toluene. In the case of indole as the nitrogen containing species,
more than 99 wt%, (corresponding to 2993 mg/kg) of the original indo le was tran sferred fro m the model oil to the ionic
liquid. After the first-regeneration cycle of the spent ionic liquid, 54 wt% of the indole-in -IL was tr ansferre d to tolu ene.
Thus, both extractions of nitrogen and sulphur model compounds were successfully carried out from model oil and the
back-extraction of these compounds from the ionic liquids to toluene demonstrated the proved the concept of the regen-
eration point of view.
Keywords: Ionic Liquids, Extraction, Air Pollutants, Nitrogen Compound, Sulfur Com pounds
1. Introduction
Developments in the field of separation of pollutants,
such as nitrogen and sulphur-containing compounds in
the refinery industry have been experiencing a rapid
growth due to more and stricter environmental protection
regulations on the release of these pollutants from trans-
portation fuels. Ionic liquids on the other hand are poten-
tial alternative solvents for the separation of aromatic
hydrocarbons (benzene, toluene, ethyl benzene) from C4
to C10 aliphatic hydrocarbon mixtures [1].
The prospect of ionic liquids as alternative solvents to
substitute less desirable organic media is under intensive
research. The low vapor pressure and ability to adjust the
physical and chemical properties of ionic liquids by se-
lection of the cations and anions, makes them attractive
for a wide range of applications. For example, ionic liq-
uid have already illustrated their potential as alternative
solvents and reaction media [2-5]. Furthermore, ionic
liquids have been applied in solvent extraction processes
and in gas separations [6,7].
Nitrogen compounds are harmful impurities in petro-
leum products. In liquid fuels, nitrogen compounds act as
precursors for nitrogen oxides (NOX), which are envi-
ronmental pollutants. Nitrogen compounds are known to
be poisons for acidic and metallic refinery catalysts and,
thus, they cause problems in the petroleum hydrotreat-
ment. Nitrogen compounds have also been considered to
be responsible for the color and gum formation as well as
for formation of deposits. Nitrogen compounds in petro-
leum products, when combusted, have carcinogenic and
mutagenic properties [8]. Furthermore, the adsorption of

Ionic Liquid Assisted Extraction of Nitrogen and Sulphur-Containing Air Pollutants from 797
Model Oil and Regeneration of the Spent Ionic Liquid
nitrogen-compounds on catalysts is higher compared to
that of sulphur-compounds, even though their reactivity
is lower. Thus, as the concentrations of the refractory
sulphur compounds in the oil have been lowered via
normal hydrodesulphurization (HDS) process, the nitro-
gen-compounds present in the oil could inhibit the HDS
process through their competitive adsorption on the cata-
lyst surface [1,9]. The removal of such HDS catalyst in-
hibitors could promote the improvement of the HDS
conditions and thus, contribute in reducing the difficul-
ties of meeting the strict emission specifications in the
transportation fuels.
Non-catalytic processes for the selective removal of
N-compounds have been studied, whereby basic N-
compounds, such as pyridine and quinoline, have been
removed by using ion-exchange resins [9]. Additionally,
liquid–liquid extraction of nitrogen and sulphur com-
pounds by using volatile carboxylic acids [10] and ionic
liquids as extracting solvents have been demonstrated
[1,2].
Liquid-liquid extraction has been extensively used in
industrial in the separation and purification of aromatic
hydrocarbon, because of severe operation conditions and
straightforward processes [11]. The use of conventional
polar organic compounds such as glycol or sulfolane has
been extensively applied for commercial extraction of
aromatic hydrocarbons from aromatic-aliphatic mixtures
[12]. However, due to the high volatilities of these chemi-
cals, there are usually losses of extractants by evapora-
tion. These organic compounds are generally toxic and
flammable [12]. Liquid-liquid extraction has some gen-
eral drawbacks as conventional polar organic compounds
are used. It is of particular importance to develop novel
extractants with high distribution coefficients, high se-
lectivities and low solvent losses.
Ionic liquids are organic salts composed of organic
cations and inorganic anions, and their melting points are
under or at ambient temperatures. Ionic liquids are inter-
esting due to their tuneable properties, making them suit-
able as reaction media and extraction solvents among
other applications [13]. It should be noticed that ionic
liquids have negligible vapour pressures and, thus, the
regeneration of spent ionic liquids with minor solvent
losses might be possible. Ionic liquids have high chemi-
cal and thermal stabilities and a wide liquidus range [1,9,
12,13].
The aim of this work was to investigate the liquid-liquid
extraction of a model oil, dodecane, with an ionic liquid,
1-ethyl-3-methylimidazolium chloride ([C2 mim] [Cl]]).
The model oil contained pyridine and indole as basic and
neutral nitrogen compounds, respectively, whereas diben-
zothiophene was used as sulphur compound (Figure 1).
N+
N
Cl-
(a)
N
(b)
N
H
(c)
S
(d)
Figure 1. Extraction of dodecane with (a) an ionic liquid [C2
mim] [Cl]. Following compounds were used as model N and
S containing compounds: (b) pyridine, (c) indole, (d) diben-
zothiophene.
2. Experimental Section
2.1. Sample Preparation
The model oil used was either pure dodecane or 1:1 do-
decane- toluene mixture (99.5 wt%, J.T: Baker) (98 wt%,
Fluka), while indole (99 wt%, Aldrich) and pyridine (99
wt%, Acros) were used as nitrogen-containing com-
pounds, respectively. Dibenzothiophene (DBT) (99 wt%,
Sigma-Aldrich) served as a sulphur containing compound.
The model oil contained 3000 ppm of N-compound,
whereas the concentration of the sulphur compound was
4545 ppm. The toluene (99.5 wt%, J.T: Baker) was used
as the back-extraction liquid for ionic liquid recovery. The
ionic liquid was a commercially available 1-ethyl-3-
methyl-imidazolium chloride [C2 mim] [Cl] (95 wt%,
Merck, water content was 1.4 wt%) and it was used as
such. The samples were prepared by dissolving the re-
quired amounts of the N- or S-containing compounds into
20 ml of dodecane. The effect of the aromatics was tested
by dissolving the required amount of N- or S-compound
into a mixture of dodecane and toluene (1:1 wt-ratio, 20
ml).
2.2. Extraction Procedure
The extraction experiments were performed in 10 ml vials,

Ionic Liquid Assisted Extraction of Nitrogen and Sulphur-Containing Air Pollutants from 798
Model Oil and Regeneration of the Spent Ionic Liquid
with the ionic liquid to model oil ratios of 1:1, 1:5 and
1:10. The ionic liquid-model oil mixture formed a
two-phase sample, in which the top phase was the model
oil and the lower phase was the ionic liquid. A PTFE
magnetic stirrer unit with an IKA magnetic stirrer
equipped with a thermocouple was used for the agitation
and temperature control. The extraction was carried out at
60˚C for 15 min. The phases were separated after the
experiment by decantation and the model oil was analyzed
by GC. The ionic liquid was regenerated by back–ex-
traction by using toluene, at room temperature. The ionic
liquid to toluene ratio was 1:1 (vol.) in the back extraction
experiment.
2.3. Analytical Procedure
The model oil was analyzed by gas chromatography
(Agilent technologies 6890N network GC System with an
on-column injector having a flame ionization detector
(FID). The column used was Agilent 122-10A6, DB-Petro,
(length 100 m, internal diameter 250 μm and film thick-
ness 0.50 μm). The following temperature programme
was used: 50˚C (3.8˚C/min) - 220˚C (5min).
3. Results and Discussions
3.1. Effect of an Aromatic Compound on the
Extraction Capacity of the Ionic Liquid
The effect of an aromatic compound present in the model
oil for the extraction capacity is shown in Figure 2. The
nitrogen content in the model oil was 3000 ppm. The
extraction capacity of the ionic liquid for the removal of
indole (N-compound 1) was reduced by 20% as the
amount of aromatics in the model oil was increased to 50
wt%. In the case of pyridine (N-compound 2), the pres-
ence of aromatics in the model oil reduced the extraction
capacity of the ionic liquid by 20% as well, compared to
the case in the absence of aromatics Figure 3. Therefore,
it is evident that the presence of aromatic compounds in
the model oil decreases the extraction capacity of the
ionic liquid. The presence of an aromatic compound in
the sample affected the extraction capacity of the ionic
liquid probably due to the fact that the ionic liquid exhib-
its a high affinity towards the aromatics. The origin of
this affinity most probably resides in the CH-π interac-
tions between the hydrogen in the ionic liquid ring and
the aromatics [13]. An increase in the aromatic content of
the model oil, therefore, reduces the extraction capacity
of the ionic liquid as observed by Xie et al. [1]. The high
extraction capacity of the ionic liquids for heterocyclic
aromatic compounds has been reported by several au-
thors [14-16]. It is suggested that the interactions of the
ionic liquids with the aromatic compounds are linked to
the electron pair-acceptor properties of the 1,3-dialky-
limidazolinium rings. It was assumed that in the presence
of a high concentration of aromatics, the extraction of
other compounds is compromised due to the saturation of
ionic liquids by aromatics. Thus, as the mass fraction of
toluene in the hydrocarbon mixture was increased, the
distribution coefficient for heterocyclic aromatic com-
pounds decreased linearly as has been reported Xie et al.
[1]. Analogously it was concluded that the ionic liquids
(IL) exhibited a better extraction capacity of the model
oil if no aromatic compounds were present.
3.2. Effect of the Ionic Liquid to Oil Ratio
The effect of the ionic liquid-to-oil ratio was investigated
in the extraction of N-compounds from the model oil.
The extraction capacity improved as the ratio of IL-to-oil
was increased, for indole Figure 4 and pyridine Figure 5,
respectively. The extraction capacity also improved with
an increase in the IL to oil ratio. A similar observation
was reported by Zhao et al. [13], in which 1-butyl-3-
methylimidazolium chloride with aluminium chloride
([C4mim] [Cl] / [AlCl3]) ([Cl]/ [AlCl3] (ratio 0.35/0.65))
was used as an ionic liquid and the model oil contained
500 ppm of DBT in n-dodecane. Four consecutive ex-
tractions were carried out at 60˚C and a fresh ionic liquid
was in each extraction. The model oil used by Eber et al.
[2] and Bösmann et al. [14] contained 500 ppm of DBT
in dodecane and the extraction was carried out at room
temperature [2,14]. The ionic liquid used by Eber et al.
[2] was 1-butyl-3-methyl-imidazolium octylsulfate ([C4
mim] [OcSO4]). Generally, they used a mixing contact
time 15 min between ionic liquid and oil [2,14]. The
model oil in the current study was a mixture containing
Figure 2. Extraction of the model oil containing indole as
the N-compound with IL [C2 mim] [Cl] at 60˚C.

Ionic Liquid Assisted Extraction of Nitrogen and Sulphur-Containing Air Pollutants from 799
Model Oil and Regeneration of the Spent Ionic Liquid
Figure 3. Extrac tion of the model oil containing pyridine as
the N-compound with IL [C2 mim] [Cl] at 60˚C.
Figure 4. Extraction of the model oil containing indole as
the N-compound with IL ([C2 mim] [Cl]) at 60˚C.
Figure 5. Extrac tion of the model oil containing pyridine as
the N-compound with IL ([C2 mim] [Cl]) at 60˚C.
3000 ppm of the nitrogen compound (indole or pyridine)
in dodecane. Different ionic liquid to oil ratios were se-
lected 1:1, 1:5, 1:10 (wt-ratio) and the extraction condi-
tions were allowed to be the same in all cases: tempera-
ture was 60˚C while 15 min mixing contact time between
the oil and IL was used (Figures 4 and 5).
The extraction of the model oil containing indole as the
nitrogen compound was performed at 60˚C and the IL-to
oil ratio (wt) was 1:5. In equilibrium, 70% of the nitrogen
compound was removed (Figure 4). In the case of a 1:1
IL-to-oil ratio 76% of indole was removed in equilibrium
at 60˚C. This is an indication that a higher IL-to-oil ratio
does not improve the extraction effectiveness prominently.
Moreover, thinking about potential large-scale process,
1:5 IL to oil ratio is already rather high.
The extraction capacity of pyridine was strongly af-
fected, when a larger amount of the ionic liquid was used.
When the model oil containing pyridine as a nitrogen
compound was extracted (1:5 IL-to-Oil wt-ratio) at 60˚C,
70% of the nitrogen compound was removed at equilib-
rium. In the case that the IL to oil ratio (wt-ratio) was
increased from 1:5 to 1:1(wt-ratio), 90% of pyridine was
removed from the model oil at equilibrium. It is evident
that higher IL to oil ratio did not affect too much the ex-
traction capacity and, therefore, a 1:5-ratio seems to be
more appropriate however, already this ratio would result
in big liquid streams to be pumped around in large scale
applications.
The results from the present work cannot be directly
compared to the results presented in literature [1,2,13,14].
Several factors have been reported to improve the extrac-
tion capacity of ionic liquids, such as lowering the vis-
cosity of the ionic liquid [17] as well as selection of ionic
liquid with a longer alkyl chain in the cation part of the
ionic liquid [17]. For instance, 1-butyl-3-methylimida-
zolium chloride [1] has a longer alkyl length compared to
the present study, in which 1-ethyl-3-methylimidazolium
chloride was used as an extractant. The reason why we
wanted to study extraction with a cation containing a
shorter alkyl chain was simple; the goal was to investi-
gate, if an ionic liquid with more favorable toxicity pro-
file could be used, since butyl-imidazolium derivatives
are rather toxic. The lengths of the alkyl substitutes, on
both the cation and the anions of the IL, influence the
extraction efficiency of the ionic liquid [17]. One of the
biggest limitations for the use of ionic liquids lies in the
fact that they often have quite high viscosities. In this
work, 1-ethyl-3-methylimidazolium chloride was used.
However, 1-butyl-3-methylimidazolium chlorides, 1-butyl-
3-methylimidazolium chloride, 1-butyl-3-methylimida-
zolium trifluoromethylsufonate that were used by the
other authors [1,2,14]; all have viscosities within the

Ionic Liquid Assisted Extraction of Nitrogen and Sulphur-Containing Air Pollutants from 800
Model Oil and Regeneration of the Spent Ionic Liquid
same range [17], typically 14 - 48 cP. A lower viscosity
of an ionic liquid leads to more effective mass transfer,
thus, enhancing the extraction rate.
3.3. Extraction of S-Compounds from the Model
Oil Using Ionic Liquid
The extraction of dibenzothiophene as the sulphur com-
pound in the model oil was carried out with [C2 mim] [Cl]
as the ionic liquid Figure 6. The extraction was carried
out for 15 min at 60˚C, and the IL to oil ratio-wt was 1:1.
The results demonstrated that the concentration of the
S-compound in the model oil was reduced by 99% after a
completed extraction. This result demonstrated that the
extraction of dibenzothiophene from the model oil, with
1-ethyl-3-methylimidazolium chloride ([C2 mim] [Cl])
was successful.
3.4. Regeneration of Spent Ionic Liquid
The regeneration of the spent ionic liquid was carried out
after the extraction of the N-compound from the model
oil. After the extraction of the model oil, the nitrogen
content of the model oil was reduced from the original
level of 3000 mg/kg to 6.7 mg/kg (very close to 100%).
Thus, the ionic liquid contained after the extraction 2993
mg/kg nitrogen in the form of indole Figure 7. The con-
centration of the N-compound in the ionic liquid after the
regeneration cycle one with toluene was found to be
1600 mg/kg and, thus, the nitrogen content was reduced
by 46%. After the second regeneration cycle with toluene,
the indole content in the IL was reduced to 1300 mg/kg,
corresponding to 54% of the original amount Figure 7.
The regeneration of the spent ionic liquid, it was ob-
served that, for the spent ionic liquid containing DBT as a
model sulphur compound contained 4532 mg/kgIL DBT
after the extraction. After the first regeneration cycle
using toluene as extractant in back-extraction in a 1:1
wt-ratio toluene-to-IL, over 85 wt% (corresponding 3852
mg/kg) of DBT could be removed from the IL by this
method. After the second regeneration cycle, 86 wt% of
the DBT was recovered from the ionic liquid to toluene
Figure 8.
4. Conclusions
In the present work, extraction of sulphur-and nitro-
gen-containing air pollutants from model oil, dodecane
or dodecane-toluene (1:1 wt ratio) with ionic liquids was
demonstrated. The effect of the oil-to-ionic liquid ratio
and the effect of the aromatic content were studied. Fur-
thermore, the spent ionic liquids were regenerated by
back-extraction.
The effect of the presence of aromatic compounds in
the model oil was investigated during the extraction with
Figure 6. The extraction of S-compound (DBT) from the
model oil. IL [C2 mim] [Cl] at 60˚C in IL to oil ratio1:1(wt).
Figure 7. The concentration of the N-compound (indole) in
spent IL. Toluene was used for IL regeneration at 25˚C.
Figure 8. The concentration of S-compounds (DBT) in IL.
Toluene was used for IL regeneration at 25˚C.
ionic liquid. The model oil contained 3000 ppm nitrogen
and 4545 ppm sulphur compounds, respectively. The

Ionic Liquid Assisted Extraction of Nitrogen and Sulphur-Containing Air Pollutants from 801
Model Oil and Regeneration of the Spent Ionic Liquid
results from the extraction of N-compounds and S-com-
pounds in the model oil with the ionic liquids at 60˚C
both in the absence and in the presence of aromatic
compounds, revealed that the nitrogen content starting
from indole was reduced by 76% from the model oil in
the absence of aromatic compound, whereas in the pres-
ence of an aromatic compound, the nitrogen concentra-
tion was reduced only by 50%. Analogously, as pyridine
was the nitrogen compound, the content of the nitrogen
compound was reduced by 70% from the model oil when
aromatic-free oil, and in the presence of aromatic com-
pounds only 55% reduction of the nitrogen content was
achieved.
The effect of the ionic liquid-to-oil ratio was investi-
gated during the extraction of the N-and S-compounds
from the model oil. The model oil samples were prepared
so that they contained equal concentrations of the nitro-
gen and sulphur compounds as being present in a real
sample heavy fuel oil (from refinery). The N- and
S-compounds were extracted with the ionic liquid at
60˚C. The nitrogen concentration using indole as a model
compound was reduced by 90% when the ratio of ionic
liquid-to-oil was 1:1 and by 65% when the ionic liq-
uid-to-oil ratio was 1:5. The reduction of the pyridine
content was 60% when the ionic liquid-to-oil ratio was
1:10.
Extraction of model oil with an ionic liquid revealed
that this method is suitable for removal of sulphur and
nitrogen compounds for fuel applications.
The results from the regeneration and reuse of the
spent ionic liquid revealed that the concentration of the
N-compound and S-compound decreased by 54% and by
86%, respectively, showing that continuous operation
with extraction and regeneration would be desired.
5. Acknowledgements
This work is part of the activities at the Åbo Akademi
University Process Chemistry Centre within the Finnish
Centre of Excellence Programme (2000-2011) appointed
by the Academy of Finland. The work was carried out by
financial support from Neste Oil Oyj.
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