Catalytic Activity of Polyfunctional Ionic Liquids in Oxidation of Model Sulfur Organic Compounds

Article abstract of DOI:10.1134/S1070427219040141

Ionic liquids having acid centers in the imidazolium cation and molybdenum, tungsten, and vanadium atoms in the anion were synthesized and their catalytic activity in the oxidation of organosulfur compounds was examined. The highest catalytic activity is exhibited by the ionic liquid containing a molybdenum atom. The infl uence exerted by the reaction duration and by the amount of the ionic liquid and oxidizing agent on the conversion of methyl phenyl sulfide. The conditions are found for reaching the 100% conversion of methyl phenyl sulfide under mild conditions in the presence of the catalysts, ionic liquids [ionic liquid: 3-(carboxymethyl)-1-methyl-1H-imidazol-3-ium molybdate with S: Mo molar ratio = 24 ∶ 1, 2 h, 40°C, H₂O₂: S molar ratio = 12 ∶ 1]. The efficiency of using polyfunctional catalysts based on ionic liquids containing a Br?nsted acid center in the cation for oxidation of organosulfur compounds as compared with application of a simple mixture of an ionic liquid and a transition metal salt.

Full text of DOI:10.1134/S1070427219040141

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2019, Vol. 92, No. 4, pp. 569−575. © Pleiades Publishing, Ltd., 2019.
Russian Text © The Author(s), 2019, published in Zhurnal Prikladnoi Khimii, 2019, Vol. 92, No. 4, pp. 531−537.
CATALYSIS
Catalytic Activity of Polyfunctional Ionic Liquids
in Oxidation of Model Sulfur Organic Compounds
a,
a
a
a
A. V. Akopyan *, E. A. Eseva , P. D. Polikarpova , T. M. Baigil’diev ,
a
a
I. A. Rodin , and A. V. Anisimov
a
Chemical Faculty, Lomonosov Moscow State University, Moscow, 119234 Russia
e-mail: arvchem@yandex.ru
*
Received October 8, 2018; revised December 14, 2018; accepted January 28, 2019
Abstract—Ionic liquids based on 1-methylimidazole were synthesized. The liquids contain Brønsted acid centers
in the cation and a transition metal atom in the anion. The polyfunctional ionic liquids synthesized in the study
are effective catalysts for the oxidative desulfurization process. The conditions are found for reaching the 100%
conversion of methyl phenyl sulfide under mild conditions in the presence of the catalysts, ionic liquids [ionic
liquid: 3-(carboxymethyl)-1-methyl-1H-imidazol-3-ium molybdate with S : Mo molar ratio = 24 : 1, 2 h, 40°C,
H O : S molar ratio = 12 : 1].
2
2
Keywords: ionic liquids, oxidative desulfurization, 1-methylimidazole, peroxo complexes, molybdenum, hy-
drogen peroxide.
DOI: 10.1134/S1070427219040141
Because of the introduction of severe ecological re-
The most widely used catalytic systems for oxidative
quirements to fuels, the demand increases for a search to desulfurization contain compounds of transition metals,
find new solutions for purification of oil fractions to remove such as molybdenum, tungsten, or vanadium, which can
sulfurous compounds [1, 2]. The most widely used desul- form peroxo complexes in the presence of hydrogen
furization method, hydrodesulfurization, requires a gross peroxide or alkyl peroxides [4–9]. However, application
capital and energy expenditure in its implementation, and of these systems is associated with phase constraints
the increase in the content of sulfur in hydrocarbon raw because sulfurous compounds are in the organic phase,
materials results in that the conditions of this process be- and the catalyst and oxidizing agent, in the aqueous phase.
This constraint can be diminished by using ionic liquids
come more severe, which requires that the already existing
catalysts should be improved and reactors be made larger
(IL), which can serves as interphase transfer agents in the
course of oxidation [10–12].
[3]. Because of the rising cost of the hydrodesulfurization
process, hydrogen-free desulfurization method find steadily
The application of Brønsted acids as catalysts
increasing application, the most promising of which is for oxidation of sulfurous compounds is extensively
the oxidative desulfurization [4, 5]. The method is based described in the literature (inorganic acids: sulfuric,
on the oxidation of organosulfur compounds, with their phosphoric; organic acids: formic, acetic, trifluoroacetic,
subsequent extraction with a polar solvent or adsorbent etc.) [13–15]. Examples are known of using ionic liquids
from the hydrocarbon medium [6]. One of the main ad- with Brønsted acid centers as catalysts for oxidation of
vantages of the oxidative desulfurization is constituted by sulfurous compounds [16–18]. However, systems of
the mild oxidation conditions (temperatures of up to 80°C this kind are inferior in efficiency to catalysts based on
and atmospheric pressure).Apromising oxidizing agent is salts of molybdenum or tungsten [19–21]. In the present
hydrogen peroxide, and it is an ecologically safe solvent study, we used polyfunctional ionic liquids composed
yielding water as product of its reduction.
of imidazolium cation with a carboxy group and anion
5
69

570
AKOPYAN et al.
containing molybdenum, tungsten, or vanadium. An
important advantage of this approach is that a catalyst
combines a high catalytic activity inherent in transition
metal compounds and acids with functions of an
interphase transfer.
The elemental composition was examined by X-ray
fluorescence microanalysis (XFMA) with an ARL
Perform’X X-ray fluorescence wave spectrometer
(Thermo Fisher Scientific, New Wave).
Samples were prepared by the following procedure:
powdered samples were compacted into pellets on a boric
acid substrates and covered with a Lavsan film, which was
pressed with an annular casing against a cuvette. Liquid
samples placed in the cuvette and covered with a Lavsan
film, which was pressed with a casing.
EXPERIMENTAL
The ionic liquids were synthesized from chloroacetic
acid(97.6%, RusKhim), ethanol(96%), 1-methylimidazole
(99%, Lanster), sulfuric acid (95%, Komponent-Reaktiv),
The composition of the reaction products was
determined and the purity of the starting substances was
controlled by gas chromatography on a Kristall-2000M
chromatograph with flame-ionization detector, Zebron
column (L = 30 m, d = 0.32 mm), ZB-1 liquid phase,
temperature programmed within the range from 100 to
and hydrochloric acid (37%, Sigma-Tek).
The model mixture was composed of methyl phenyl
sulfide (MeSPh, 99%, Acros Organics) dissolved in
dodecane (99%, Sigma Aldrich). The initial content of
the total sulfur in the system was 1000 ppm. Mixtures
of dibenzyl sulfide (Bn S, 98%, Sigma Aldrich), ben-
2
2
50°C. The chromatograms were recorded and analyzed
zothiophene (BT, 98%), and dibenzothiophene (DBT,
with a computer and Khromatek Analitik 1.5 software.
The reaction mixture was analyzed before and after the
oxidation under the following conditions:
9
8%, Sigmal Adrich) were prepared in a similar way.
The oxidizing-catalytic mixtures were synthesized
from sodium molybdate (Na MoO ·2H O, 99%, Sigma
2
4
2
Aldrich), sodium tungstate (Na WO ·2H O, 99%, Sigma
– nitrogen as carrier-gas (p = 200 kPa), with the
volumetric flow rate of 30 mL min ;
2
4
2
–
1
Aldrich) sodium vanadate (NaVO , 99%, SigmaAldrich),
3
hydrogen peroxide (H O , 50%, Prime Chemicals Group),
2
2
– initial column temperature 100°C:
tert-butyl hydroperoxide (t-BuOOH, 70%,ABCR), and
-butyl-3-methylimidazolium tri(pentafluoroethyl) tri-
–
injector temperature 150°C;
1
1
– detector temperature 250°C;
fluorophospate (99%, Merck). H NMR spectra were re-
corded with a Varian=XR-400 spectrometer with working
frequency of 400 MHz. We used 2% solutions of samples
in deuterated water or deuterated dimethyl sulfoxide. The
chemical shifts are given in ppm on a scale relative to
hexamethyldisiloxane as internal standard.
–
1
– column heating rate 20 deg min .
Synthesis of ionic liquids. The ionic liquids (3)–(5)
used in the study as catalysts for oxidation of sulfurous
compounds were synthesized in accordance with
Scheme 1.
The structure of the compounds obtained was
Synthesis of 3-(2-ethoxy-2-oxoethyl)-1-methyl-1H-
imidazol-3-ium chloride (IL-1). The quaternization
rection was performed by the procedure described [22].
The product was analyzed by the NMR and ESI-MS
1
confirmed by H NMR spectroscopy, ESI-MS mass
spectrometry, and also by the method of elemental
analysis.
ESI-MS spectra were recorded with a Dionex Ultimate
1
methods. H NMR spectrum (DMSO, 400 MHz): δ =
3
000 liquid chromatograph with an AB Sciex Qtrap
1
5
.20–1.23 m (3H), 3.85–3.92 s (3H), 4.15–4.20 m (2H),
.38 s (2H), 7.82–7.84 m (2H), 9.41 s (1H), ESI-MS:
m/z = 169.
tandem quadrupole mass-spectrometric detector and
sample ionized via electrospraying (ESI) in the mode
with positive ions recorded. As a mobile phase served a
mixture of 0.1% formic acid in water and acetonitrile. A
sample was introduced with an autosampler via a zero-
volume connector directly into the mass spectrometer.
Synthesis of 3-carboxymethyl)-1-methyl-1H-
imidazol-3-ium chloride (IL-2). The product of the
quaternization reaction was subjected to hydrolysis in
the presence of an excess amount of a 37% aqueous
solution of hydrochloric acid [22]. The product was
analyzed by the NMR and ESI-MS methods. H NMR
spectrum (DMSO, 400 MHz): δ = 3.90 s (3H), 5.16 s
(2H), 7.74–7.76 d (2H), 9.26 s (1H). ESI-MS: m/z = 141.
–
1
The flow rate of the mobile phase was 0.3 mL min .
The mass-spectrometric detection was performed in the
scanning mode within the m/z range 50–143 Da. The
samples were prepared in deionized water, with the mass
fraction of the substance being 2%.
1
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CATALYTIC ACTIVITY OF POLYFUNCTIONAL IONIC LIQUIDS
571
Scheme 1. Synthesis of ionic liquids containing Brønsted acid centers in the cation and a transition metal atom in the cation.
Exchange reactions of 3-carboxymethyl)-1-methyl-
H-imidazol-3-ium chloride (IL-2) with transition
2000 ppm of total sulfur, was added 0.02–0.08 mL of a
catalytic oxidizing mixture containing hydrogen peroxide
or tert-butyl hydroperoxide and ionic liquid (IL-2–IL-5) at
a metal : hydrogen peroxide molar ratio of 1 : 600 to 1 : 48.
The reaction was performed under permanent agitation
at 20–80°C in the course of 0.5–6 h. The composition
of the reaction mixture was analyzed, after the reaction
was complete, by gas chromatography. In each case, the
corresponding sulfones are oxidation products.
1
metal salts to give IL-3–IL-5. IL-2 (0.5 g) was dissolved
in 5 mL of distilled water, the resulting solution was
added dropwise to 10 mL of an aqueous solution of
sodium molybdate (the amount of sodium molybdate was
calculated to obtain IL-2 : sodium molybdate molar ratio
of 2 : 1). The resulting white precipitate was separated,
washed with water until sodium chloride was fully
removed, which was verified by a qualitative reaction
for chloride ions with the use of silver nitrate, and dried
at room temperature for 24 h. The anion exchange with
sodium tungstate and vanadate was performed in a similar
way (in the case of sodium vanadate, the IL-2 : sodium
vanadate molar ratio was 1 : 1). The thus synthesized
compounds were analyzed by the XFMA (see the table)
and ESI-MS methods. According to ESI-MS, m/z = 141.
RESULTS AND DISCUSSION
To compare under identical conditions the efficiencies
of the catalysts obtained, we performed reactions in
the presence of a catalytic system containing sodium
molybdate and a model ionic liquid, tri(pentafluoroethyl)
Preparation of a catalytic oxidizing mixture. To
.2–0.8 mL of a 50% solution and hydrogen peroxide
or tert-butyl hydroperoxide was added under permanent
agitation 0.015–0.035 G of an ionic liquid (IL-2–IL-5).
The agitation was continued for 5 min till the ionic liquid
was fully dissolved.
Content of metals in the ionic liquids synthesized in the study
according to elemental analysis data
0
Amount of a metal
Ionic
Metal
liquid
calculated
actual
IL-3
IL-4
IL-5
Molybdenum
Tungsten
21.4
34.1
13.0
20.5
34.7
12.6
Oxidation of model mixtures of organosulfur
compounds. The reaction of oxidation of model mixtures
Scheme 2) was performed as follows: to 5 mL of a
solution of an organosulfur compounds, which contains
(
Vanadium
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AKOPYAN et al.
Scheme 2. Reactions of oxidation of sulfur-containing compounds.
trifluorophosphate of 1-butyl-3-methylimidazolium (IL)
and ionic liquids obtained in the study with various anions
presence of hydrogen peroxide or alkyl peroxides. The
data obtained suggest that the catalysts synthesized in the
study are more efficient as compared with pure sodium
molybdate or a mixture of sodium molybdate with an
ionic liquid (IL). This indicates that the cationic part
containing a carboxy group is involved in the oxidation
process.
(IL-2–IL-5, Scheme 1). A mixture of sodium molybdate
and a model ionic liquid was used to evaluate the effect
of Brønsted acid centers contained in the catalysts
synthesized in the study (IL-2–IL-5) on the conversion
of methyl phenyl sulfide (MeSPh).
It can be seen in Fig. 1 that the most efficient catalyst
is the ionic liquid synthesized in the present study with
a molybdenum atom in its anion (IL3). Ionic liquids
with molybdenum and tungsten exhibit a high activity
because these metals for active peroxo complexes in the
Thus, the results obtained are indicative of the
efficiency of using polyfunctional ionic liquids containing
both an acidic center and a transition metal ion. This
approach makes it possible to improve the results
as compared with using ionic liquids containing no
Brønsted acid centers. It is important to note here that
an IL containing an acid functional group, but having no
metal, exhibits a catalytic activity due to the formation
of a peroxy acid under oxidation conditions, which is
characteristic of a peroxide oxidation of sulfur-containing
compounds in the presence [1].
The effect of the oxidation temperature on the
conversion of methyl phenyl sulfide was examined in the
presence of IL-3 containing a molybdenum atom in the
anion (Fig. 2).As oxidizing agents were chosen hydrogen
peroxide and tert-butyl hydroperoxide with a molar ratio
between oxidant and total sulfur of 6 : 1. It can be seen
in Fig. 2 that hydrogen peroxide is a more effective
oxidizing agent, compared with tert-butyl hydroperoxide.
The optimal oxidation temperature is 60°C for hydrogen
peroxide and 80°C for tert-butyl hydroperoxide.
Catalyst
The influence exerted by the oxidation duration on
the conversion of methyl phenyl sulfide was examined in
the presence of hydrogen peroxide and IL-3 ionic liquid
Fig. 1. Effect of the catalyst on the conversion of methyl
phenyl sulfide [H O : S = 6 : 1 (mol/mol), S : metal = 24 :
(mol/mol), 2 h, 40°C]. IL = 1-butyl-3-methylimidazolium
2
2
(Fig. 3) at 40°C. Under these conditions, full oxidation
1
tri(pentafluoroethyl)trifluorophosphate.
is reached in 8 h.
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CATALYTIC ACTIVITY OF POLYFUNCTIONAL IONIC LIQUIDS
573
1
00 : 1
50 : 1
S : Mo (mol/mol)
24 : 1
12 : 1
Temperature, °C
Fig. 2. Effect of temperature on the conversion of methyl phenyl
sulfide [oxidant : total sulfur = 6 : 1 (mol/mol), IL-3 as catalyst,
S : Mo = 24 : 1 (mol/mol), 2 h, 40°C].
Fig. 4. Effect of the amount of catalyst on the conversion of
methyl phenyl sulfide [H O : S = 6 : 1 (mol/mol), IL-3 as
catalyst, 2 h, 40°C].
2
2
1
: 2
1 : 4
1 : 6
1 : 12
Reaction time, h
S : H O (mol/mol)
2
2
Fig. 3. Effect of the oxidation duration on the conversion of
methyl phenyl sulfide [H O : S = 6 : 1 (mol/mol), IL-3 as
catalyst, S : Mo = 24 : 1 (mol/mol), 40°C].
Fig. 5. Effect of the amount of hydrogen peroxide on the
conversion of methyl phenyl sulfide [IL-3 as catalyst, S : Mo =
24 : 1 (mol/mol), 2 h, 40°C].
2
2
Varying the sulfur : molybdenum ratio (and, as a
consequence, the sulfur : catalyst ratio) demonstrated
respect to the sulfide in the range from 2 to 12 (Fig. 5)
demonstrated that a 12-fold excess of hydrogen peroxide
makes it possible to fully oxidize methyl phenyl sulfide
to the corresponding sulfone.
(Fig. 4) that raising the amount of added catalyst leads to
a higher conversion of methyl phenyl sulfide. A sulfur :
molybdenum molar ratio smaller than 12 : 1 cannot be
reached because of the limited solubility of the catalyst
in a hydrogen peroxide solution.
We carried out a comparative study of the oxidation
of sulfur-containing compounds in the presence of IL-3
(Fig. 6). It follows from these data that methyl phenyl
A study of the influence exerted by the amount of
hydrogen peroxide on the conversion of methyl phenyl
sulfide at a molar excess of hydrogen peroxide with
sulfide and dibenzyl sulfide are nearly fully oxidized,
dibenzothiophene is 30% oxidized, and benzothiophene is
worst of all in oxidation, which correlates with published
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AKOPYAN et al.
FUNDING
The study was supported by the Russian Science
Foundation (project no. 18-79-00116).
CONFLICT OF INTERESTS
The authors maintain that there is no conflict of
interests to be disclosed in the present study.
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