Application of ionic liquids for extraction and synthesis of organosulfur compounds

Article abstract of DOI:10.1134/S1070363213110170

We consider the possible application of pyridinium ionic liquids to desulfurize hydrocarbon raw products and to transform the extracted toxic admixtures into promising sulfur compounds. Anodic oxidation of the extracted thiols as well as oxidation in the presence of electron mediators (N,N,N′,N′-tetramethyl-1,4-phenylenediamine, tri-p-bromophenylamine, and tri-p-tolylamine) leads to the corresponding disulfides.

Full text of DOI:10.1134/S1070363213110170

ISSN 1070-3632, Russian Journal of General Chemistry, 2013, Vol. 83, No. 11, pp. 2062–2065. © Pleiades Publishing, Ltd., 2013.
Original Russian Text © V.V. Okhlobystina, A.O. Okhlobystin, Yu.Yu. Koldaeva, N.O. Movchan, A.A. Litvin, N.T. Berberova, 2013, published in Zhurnal
Obshchei Khimii, 2013, Vol. 83, No. 11, pp. 1868–1872.
Application of Ionic Liquids for Extraction and Synthesis
of Organosulfur Compounds
a
a
a
V. V. Okhlobystina , A. O. Okhlobystin , Yu. Yu. Koldaeva ,
b
a
a
N. O. Movchan , A. A. Litvin , and N. T. Berberova
a
Astrakhan State Technical University, ul. Tatishcheva 16, Astrakhan, 414025 Russia
e-mail: ionradical@gmail.com
b
Southern Scientific Center, Russian Academy of Sciences, Rostov-on-Don, Russia
Received December 6, 2012
Abstract—We consider the possible application of pyridinium ionic liquids to desulfurize hydrocarbon raw
products and to transform the extracted toxic admixtures into promising sulfur compounds. Anodic oxidation of
the extracted thiols as well as oxidation in the presence of electron mediators (N,N,N',N'-tetramethyl-1,4-
phenylenediamine, tri-p-bromophenylamine, and tri-p-tolylamine) leads to the corresponding disulfides.
DOI: 10.1134/S1070363213110170
Due to approval of the new fuel standards, deve-
lopment of approaches to eliminate residual admixtures
of highly toxic thiols from hydrocarbon mixtures has
become an important issue. Ionic liquids have been
recognized as promising extractants of thiols from
nonpolar media [1].
thus can be applied in tiny quantity [5]. The interest to
electron mediators has emerged due to their ability to
significantly decrease the activation energy of certain
reactions [6, 7].
In this study, we considered the possible applica-
tion of ionic liquids for extraction of thiols and their
transformation into non-toxic disulfides via anodic
oxidation of cation-radicals, including the processes in
the presence of electron mediators. The following ionic
liquids were studied: 1-butylpyridinium tetrafluoro-
borate I prepared according to [8, 9] and 1-butyl-4-
methylpyridinium tetrafluoroborate II (Aldrich). Aromatic
amines were used as electron mediators: (tri-p-tolyl-
amine, tri-p-bromophenylamine, and N,N,N',N'-tetra-
methyl-1,4-phenylenediamine).
Ionic liquids have been discovered quite recently
[2, 3]. These compounds consisting of bulky organic
cation and inorganic or organic anion have revealed a
set of unique properties. The asymmetric structure and
spatial separation of the charges favor the formation of
ion-containing liquid phase. This phase is stable over
wide temperature range, showing low melting point
and vapor pressure in combination with electro-
chemical stability and electric conductance. Owning to
high polarity, some ionic liquids are not mixed with
hydrocarbons, rather giving two- or multiphase
systems, thus capable of extracting the polar admix-
tures from hydrocarbons [1, 2]. Ionic liquids are
considered “green” solvents, as they are not flam-
mable, non-toxic, non-explosive, and environmentally
friendly.
One of the most important characteristics of a
solvent for electrochemical study is its electrochemical
window, the difference between potentials of anodic
and cathode redox reaction of background electrolyte
[10]. We determined that the electrochemical window
of the ionic liquids I and II was of 5.1 V and 5.4 V,
respectively (Fig. 1).
Ionic liquids are promising for electrochemical
studies as well, due to high electric conductivity and
wide range of polarization potential [4].
The study of electrochemical properties of the
ferrocene/ferrocenium redox pair in ionic liquids I and
II revealed that the oxidation potentials were slightly
shifted as compared to standard potentials of the same
system in acetonitrile. In particular (Table 1), in the
Electron mediators are special catalytic systems,
they are not consumed in the chemical processes and
2
062

APPLICATION OF IONIC LIQUIDS FOR EXTRACTION AND SYNTHESIS
acetonitrile solution {0.1 mol/L of [Bu N]ClO )}, the
2063
4
4
I, mA
anodic peak of ferrocene was of Еpa = 0.42 V, the
cathode peak potential Еpc = 0.37 V, their difference
being of ∆Е = 0.058 V, Ipc/Ipa = 1.00. In the ionic
liquids I and II, for ferrocene Е = 0.44 V, Е = 0.41 V,
pa
pc
∆
Е = 0.033 V, and I /I = 1.00. The results were in
pc pa
line with the published data that demonstrated the
close potentials of ferrocene redox pair oxidation in the
acetonitrile solution of [Bu N][PF ] and in a variety of
Е, V
4
6
ionic liquids [11–13]. Oxidation of ferrocene in
acetonitrile as well as in the ionic liquids was com-
pletely reversible, as confirmed by the values of ∆E
Fig. 1. Cyclic voltammogram of the (1) cathode branch
oxidation) and (2) anodic branch (reduction) of II (Pt-
anode, Ag/AgCl).
(
and I /I . However, in the cases of thiols the solvent
pc pa
effect on the potentials was more significant (see Table 1).
Rheological studies of ionic liquids demonstrated
that addition of water significantly changed the ionic
liquid viscosity and, thus, the sensitivity of electro-
chemical determination of various species in ionic
liquid media. Furthermore, Pt electrode was found
sensitive to traces of water in the ionic liquids; that
was revealed as a maximum around 1.30 V [14, 15].
The influence of water on oxidation of N,N,N',N'-
tetramethyl-1,4-phenylenediamine in 1-methyl-3-[2,6-
the diffusion of the species in the ionic liquid. At the
same time, the water impurity in the ionic liquid
complicated the electrochemical identification. In the
cyclic voltammogram of ionic liquid I with admixture
of water, the oxidation peak at 1.27 V was observed,
thus the identification of species at 1–2 V was poorer
than in the case of dry solvent.
Extraction properties of ionic liquids I and II were
studied using a model of heptane with admixture of
thiols (C H SH, C H SH, C H CH SH, and others).
(S)-dimethyloctene-2-yl]imidazolium tetrafluoroborate
was studied in [16]. It was found that the presence of
water (5%) in N,N,N',N'-tetramethyl-1,4-phenylenedi-
amine significantly increased the oxidation current.
The influence of water on the ionic liquids viscosity
was confirmed by determination of the diffusion coef-
ficients.
4
9
7
15
6
5
2
After addition of I or II to that model mixture, a
heterogeneous system was formed. After extraction
(
30 min, continuous stirring), the residual content of
the thiols was determined with X-ray fluorescence
sulfur analyzer (Fig. 2). It was to be seen that the
extraction of aromatic thiol was twice as efficient as of
the aliphatic thiol.
We found that in the anhydrous ionic liquid the
sensitivity of C H SH determination (Е = 1.8 V in the
4
9
ionic liquid) was decreased by 20 times. The effect
was ascribed to the diffusion limitations in the case of
viscous ionic liquid.
Further, the effect of temperature of the extraction
degree of thiols was studied. Increasing the extraction
temperature to 55°С led to more than 2-fold increase
of the thiols recovery, mainly due to decrease of the
ionic liquid viscosity and, consequently, diffusion
Thus, the effect of water on the ionic liquids
properties was ambiguous. Addition of water enhanced
–3
Table 1. Electrochemical parameters of thiols and disulfides (υ 200 mV/s, Ag/AgCl, c 5×10 mol/L, Pt)
Е
pa (RSH), V
Е
pa (RSSR), V
2 2 6 5
CH SSCH C H
Solvent/background
C
4
H
9
SH
C
6
H
5
CH
2
SH
C
4
H
9
SSC
4
H
9
C
6
H
5
CH
2
3
Cl
2
/0.1 mol/L [Bu
4
N]ClO
4
1.70
1.62
1.83
1.76
1.72
1.98
1.40
1.32
1.28
1.27
1.43
1.65
CH
CN/0.1 mol/L [Bu
4
N]ClO
4
I or II
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 83 No. 11 2013

2
064
OKHLOBYSTINA et al.
Table 2. Electrochemical parameters of the organic mediators and the induced decrease of oxidation potential of C
4 9
H SH
–
3
a
6
and C H
5
CH
2
SH (υ 200 mV/s, Ag/AgCl, c 5×10 mol/L, Pt)
Е
pa, V
∆Е*, V
Mediator
CH
2
Cl
Bu
2
/0.1 mol/L
N]ClO
II
C
4
H
9
SH
6 5 2
C H CH SH
[
4
4
Tri-p-tolylamine
0.84
1.20
0.90
1.25
0.97
0.62
1.02
1.08
0.73
1.13
Tri-p-bromophenylamine
N,N,N',N'-Tetramethyl-1,4-phenylenediamine
0.20; 0.80
0.25; 0.85
a
Е
pa, oxidation potentials; ∆Е* = E
R
– E
M
; E
M
, mediator oxidation potential; E
R
, thiol oxidation potential.
enhancement. Above 55°С, thiols recovery from the
model mixture decreased.
tively. The introduction of additional amount of
the reference samples of the assigned substances
increased the peaks height, thus confirming the assignment
Electrolysis of C H SH in ionic liquids I and II was
4
9
(Fig. 3).
studied at the potential of 1.80 V. Analysis of the
electrolysis products revealed two oxidation peaks at
The mechanism of thiol oxidation in ionic liquid
the cyclic voltammogram, with Е' of 1.28 V and 1.87 V,
assigned to dibutyldisulfide and butanthiol-1, respec-
was similar to that in the classical solvents (acetonitrile
or methylene chloride).
pa
_
_
[BuPy]BF4
_2
e, [BuPy]BF4
[BuPy]BF4
RSH +
RSH
RS + RS
RS SR
H+
Thus, electrochemical oxidation of thiols allowed
their conversion into non-toxic disulfides under
environmentally friendly conditions.
Electrochemical oxidation of thiols C H SH, C H SH,
4 9 7 15
C H CH SH in the ionic liquid was performed at the
6
5
2
oxidation potential of the corresponding mediator
Table 2). Slight deviation of the mediator oxidation
potential in the ionic liquid as compared to that in
methylene chloride was observed.
(
In the next part of the study we investigated the
possibility to decrease the energy consumed for the
thiols oxidation by using electron mediators.
I, mA
benzyl thiol
butanethiol
Е, V
Before extraction
After
extraction
Fig. 3. Cyclic voltammogram of the products of
electrolysis of α-butanethiol in (1) II and (2) with standard
samples (Pt-anode, Ag/AgCl).
Fig. 2. Change of sulfur content in the model hydrocarbon
mixtures containing benzyl thiol and butanethiol.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 83 No. 11 2013

APPLICATION OF IONIC LIQUIDS FOR EXTRACTION AND SYNTHESIS
2065
From Tables 1 and 2 it is to be seen that the oxida-
tion potential of the electron mediators was significantly
less than that of thiols (C H SH, C H СН SH), there-
(0.20 mol) in 100 mL of acetone, and stirred at room
temperature during 12 h. The precipitate was filtered
off, the solvent was then evaporated.
4
9
6
5
2
fore, the electrochemical process overvoltage was
decreased by ∆Е*.
ACKNOWLEDGMENTS
The work was financially supported by Russian
Foundation for Basic Research (projects 12-03-
Electrolysis of C H SH in the presence of N,N,N',N'-
4
9
tetramethyl-1,4-phenylenediamine at constant potential
of 0.90 V in the ionic liquid II led to decrease of the
thiol content due to its dimerization into H C SSC H ,
3
1381mol_a and 12-03-00513-a), President of Russian
Federation (grant MK-923.2012.3) and in the frame of
Federal Target Program “Scientific and Pedagogical
Personnel of Innovative Russia” for 2009–2013
9
4
4
9
as confirmed by the oxidation current in the registered
cyclic voltammograms.
(
contract 16.740.11.0594).
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