Deep oxidative desulfurization of fuels in the presence of Br?nsted acidic polyoxometalate-based ionic liquids

Article abstract of DOI:10.3390/molecules25030536

Polyoxometalate-based ionic liquid hybrid materials with a pyridinium cation, containing Br?nsted acid sites, were synthesized and used as catalysts for the oxidation of model and real diesel fuels. Keggin-type polyoxometalates with the formulae [PMo12O40]^3?, [PVMo11O40]^4?, [PV2Mo10O40]^4?, [PW12O40]³? were used as anions. It was shown that increasing the acid site strength leads to an increase of dibenzothiophene conversion to the corresponding sulfone. The best results were obtained in the presence of a catalyst, containing a nicotinic acid derivative as cation and phosphomolybdate as anion. The main factors affecting the process consisting of catalyst dosage, temperature, reaction time, oxidant dosage were investigated in detail. Under optimal conditions full oxidation of dibenzothiophene and more than a 90% desulfurization degree of real diesel fuel (initial sulfur content of 2050 ppm) were obtained (the oxidation conditions: NK-1 catalyst, molar ratio H2O2:S 10:1, molar ratio S:Mo 8:1, 1 mL MeCN, 70 °C, 1 h). The synthesized catalysts could be used five times with a slight decrease in activity.

Full text of DOI:10.3390/molecules25030536

Article
Deep Oxidative Desulfurization of Fuels in the
Presence of Brönsted Acidic Polyoxometalate-Based
Ionic Liquids
1
1
1
1
1,2
Argam Akopyan , Ekaterina Eseva , Polina Polikarpova , Anastasia Kedalo , Anna Vutolkina
1,2,
and Aleksandr Glotov
*
1
Department of Petroleum Chemistry and Organic Catalysis, Moscow State University, 119991 Moscow,
Russia; arvchem@yandex.ru (A.A.); esevakatya@mail.ru (E.E.); polikarpova-polina@rambler.ru (P.P.);
nastya.kedalo@mail.ru (A.K.); annavutolkina@mail.ru (A.V.)
Department of Physical and Colloid Chemistry, Gubkin Russian State University of Oil and Gas, 119991
Moscow, Russia
2
* Correspondence: glotov.a@gubkin.ru; Tel.: +7(499)507-90-32
Academic editor: Joaquín García Álvarez
Received: 30 December 2019; Accepted: 24 January 2020; Published: 26 January 2020
Abstract: Polyoxometalate-based ionic liquid hybrid materials with a pyridinium cation,
containing Brönsted acid sites, were synthesized and used as catalysts for the oxidation of model
3−
4−
and real diesel fuels. Keggin-type polyoxometalates with the formulae [PMo12
O
40] , [PVMo11
O
40] ,
4−
3−
[PV
2Mo10
O40] , [PW12
O
40] were used as anions. It was shown that increasing the acid site strength
leads to an increase of dibenzothiophene conversion to the corresponding sulfone. The best results
were obtained in the presence of a catalyst, containing a nicotinic acid derivative as cation and
phosphomolybdate as anion. The main factors affecting the process consisting of catalyst dosage,
temperature, reaction time, oxidant dosage were investigated in detail. Under optimal conditions
full oxidation of dibenzothiophene and more than a 90% desulfurization degree of real diesel fuel
(initial sulfur content of 2050 ppm) were obtained (the oxidation conditions: NK-1 catalyst, molar
ratio H
2
O2:S 10:1, molar ratio S:Mo 8:1, 1 mL MeCN, 70 °C, 1 h). The synthesized catalysts could be
used five times with a slight decrease in activity.
Keywords: oxidative desulfurization; Keggin-type polyoxometalate; ionic liquids; Brönsted acidic
1. Introduction
In recent years, the produced hydrocarbon feedstock has contained an increasing number of
organosulfur compounds, which adversely affect the refining processes and the quality of the
resulting fuels [1]. Sulfur oxides generated during the combustion of fuels have a negative impact
both on the environment and human health. In this regard, in many countries stringent
environmental requirements, limiting the sulfur content in light fuels to 10 ppm, have been
introduced [2]. Since the increase in sulfur content of oils leads to an increase in the cost of the
hydrodesulfurization, the interest of researchers has been increasingly attracted by non-hydrogen
desulfurization methods [3–5]. Oxidative desulfurization is one of the most promising ones among
the non-hydrogen desulfurization methods (extraction, adsorption, biodesulfurization) [1,6,7]. The
method of oxidative desulfurization is based on the oxidation of sulfur-containing moieties and
subsequent removal thereof from petroleum products [8,9].
The main advantages of oxidative desulfurization are lower temperatures and pressures, as
well as the absence of hydrogen [10,11]. Another advantage of oxidative desulfurization is the ability
Molecules 2020, 25, 536; doi:10.3390/molecules25030536
www.mdpi.com/journal/molecules

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to remove organosulfur compounds not destroyed in the process of hydrodesulfurization [8].
Hydrogen peroxide is the most common oxidant, because only water is formed as a byproduct [12].
Transition metal salts, in particular containing molybdenum, copper, vanadium, and tungsten,
have become the most common catalysts for oxidation [13–17]. Such catalysts have become
widespread due to their ability to form peroxocomplexes in the presence of hydrogen peroxide
[7,18,19]. Interest in oxometallic catalysts is also growing [15]. An important advantage of such
catalysts is that they can be used for a minimum of five cycles [16]. The main problem associated
with the use of catalysts based on transition metal salts is the presence of two reaction phases: the
organic phase with organosulfur compounds, and the polar phase, where oxidant is present. To
eliminate phase transfer constraints, interfacial carriers are used, which contribute to a faster and
more efficient reaction [20]. Ionic liquids have become widespread as interfacial carriers in the
oxidation of sulfur compounds [1,21–25]. For example, the authors of [24] managed to increase the
conversion of dibenzothiophene (DBT) from 10% to 100% in 1 h by adding an ionic liquid (IL) to the
catalytic mixture. In another article [26], it was shown that more than 90% of DBT can be removed by
adding to hydrogen peroxide [(C2H4OH)N(CH3)3]FeCl4 (ChFeCl4) and an ionic liquid
(1-butyl-3-methylimidazole
tetrafluoroborate
[BMIM]BF4
or 1-n-octyl-3-methylimidazole
tetrafluoroborate [OMIM]BF4) as a catalyst. This catalytic system containing the IL and the catalyst
can be easily separated and used up to five times without loss of activity. Addition of the ionic liquid
+
−
Bmim BF4 to the commercial catalyst H3PW12O40·14H2O allows complete conversion of DBT and
4,6- dimethyldibenzothiophene in 3 h of reaction at 70 °C [27].
As far as is known from the literature, the oxidation of sulfur compounds proceeds easily in
acidic media [28–30]. Brønsted acids (sulfuric acid, phosphoric acid, formic acid, acetic acid, etc.) are
often used as catalysts for oxidation of sulfur compounds [28,31]. Catalytic systems based on ionic
liquids containing Brønsted acidity are also known [32,33]. The disadvantage of such catalytic
systems is their lower activity compared to catalysts based on molybdenum and tungsten [1,34,35].
In comparison with the above mentioned catalysts. the synthesized system has the following
advantages: content of ionic liquid in the catalytic system (0.5 mL of ionic liquid in ref. [24], less than
100 mg in our case) and reaction time (3 h in ref. [27], 1 h in this paper).
The ionic liquids used in this work as catalysts consist of cation-containing Brønsted acidity and
an anion containing a transition metal. The Brønsted acid sites are presented by carboxylic groups,
bonded with a pyridinium cation and the transition metal containing anion is presented by the
phosphoromolybdate anion. The advantage of such an approach is the possibility to provide a single
catalyst having the high catalytic activity of transition metals and acids as well as the functions of an
interfacial carrier.
2. Results and Discussion
2.1. Characterization of the Catalysts
The elemental CHN composition of ILs with Bronsted acidity was determined. The results
obtained show similar values of experimentally detected and calculated values (weight percentage)
which proves the composition of the synthesized catalysts and the proposed molar ratios of
corresponding cation and anion. The measurement error did not exceed 1%.
Calc. for Py-1: C, 20.84; H, 1.99; N, 3.47. Found: C, 20.87; H, 1.98; N, 3.53.
Calc. for Py-2: C, 23.02; H, 2.4; N, 3.36. Found: C, 22.95; H, 2.35; N, 3.38.
Calc. for Py-3: C, 28.76; H, 3.49; N, 3.05. Found: C, 27.96; H, 3.27; N, 3.0.
Calc. for NK-1: C, 26.97; H, 3.15; N, 3.15. Found: C, 26.69; H, 3.09; N, 3.17.
Calc. for NK-2: C, 21.78; H, 2.61; N, 2.54. Found: C, 20.97; H, 2.13; N, 2.27.
Calc. for NK-3: C, 10.84; H, 1.49; N, 1.26. Found: C, 10.97; H, 1.63; N, 1.57.
Calc. for NK-4: C, 38.78; H, 5.39; N, 2.51. Found: C, 39.94; H, 5.99; N, 2.37.
1
13
H, C NMR, and ESI-MS spectra are shown as follows (Supporting information Figures S1–15):

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1
1-(carboxymethyl) pyridinium bromide (3): Yield 80%. Н NMR spectrum (DMSO, 400 MHz): δ = 5.72 (s,
13
2Н), 8.20–8.24 (t, 2Н), 8.66–8.7 (t, 1Н), 9.18–9.2 (d, 2Н). С NMR δ = 167.6, 146.4, 146.2, 127.7, 60.6
+
ppm. ESI-MS: positive ion m/z = 138 [C5H5NCH2COOH] . pKa = 1.9.
1
1-(2-carboxyethyl) pyridinium bromide (4): Yield 88%. Н NMR spectrum (DMSO, 400 MHz): δ = 3.07–
13
3.11 (t, 2Н), 4.78–4.82 (t, 2Н), 8.13–8.15 (t, 2Н), 8.57–8.60 (t, 1Н), 9.13–9.15 (d, 2Н). С NMR δ = 171.5,
+
145.8, 145.4, 127.8, 127.1, 56.5, 34.5 ppm. ESI-MS: positive ion m/z = 152 [C5H5N(CH2)2COOH] . pKa =
4.2.
1
1-(5-carboxypentyl) pyridinium bromide (5): Yield 86%. Н NMR spectrum (DMSO, 400 MHz): δ = 1.27
(m, 2H), 1.5–1.54 (qt, 2H), 1.91 (qt, 2H), 2.19–2.22 (t, 2H), 4.57–4.61 (t, 2H), 8.14–8.17 (t, 2H), 8.58–8.6
13
(t, 1H), 9.08–9.10 (d, 2H). С NMR δ = 179.1, 146.2, 144.9, 128.9, 62.5, 34.3, 31.1, 25.5, 24.4 ppm.
+
ESI-MS: positive ion m/z = 194 [C5H5N(CH2)5COOH] . pKa = 4.4.
1
1-butyl-3-carboxypyridinium bromide (6): Yield 68%. Н NMR spectrum (DMSO, 400 MHz): δ = 0.88–
0.92 (t, 3H), 1.3 (m, 2H), 1.89 (m, 2H), 4.67–4.71 (t, 2H), 8.24–8.27 (t, 1H), 8.93–8.95 (d, 1H), 9.26–9.28
13
(d, 1H), 9.58 (s, 1H). С NMR δ = 165.1, 149.3, 148.4, 146.7, 128.2, 126.7, 63.6, 30.3, 19.5, 12.9 ppm.
+
ESI-MS: positive ion m/z = 180 [C5H4N(C4H9)COOH] . pKa = 0.2.
1
1-dodecyl-3-carboxypyridinium bromide (7): Yield 48%. Н NMR spectrum (DMSO, 400 MHz): δ = 0.82–
0.84 (t, 3H), 1.22 (m, 14H), 1.39 (m, 2H), 1.73 (qt, 2H), 1.9 (m, 2H), 4.36–4.39 (t, 2H), 4.67–4.70 (t, 2H),
13
8.27 (t, 1H), 8.96–8.98 (d, 1H), 9.28 (d, 1H), 9.59 (s, 1H) С NMR δ = 164.8, 149.3, 148.3, 146.5, 128.1,
126.6, 64.1, 31.98, 31.6, 30.4, 30.23, 29.58, 29.54, 29.36, 29.27, 28.03, 22.72, 14.10 ppm. ESI-MS: positive
+
ion m/z = 292 [C5H4N(C12H25)COOH] .
Elemental analysis of the metal content in the obtained catalysts with Bronsted acidity was
carried out. The results are presented in Table 1. The metal amounts found agree with the
theoretically calculated values.
Table 1. The content of molybdenum and phosphorus in ionic liquids according to elemental analysis
data.
Calculated Values, % Mass.
Found Amount, % Mass.
Ionic Liquid
Designation
Mo
P
Mo
37.44 ±
0.31
P
(NKBu)7PMo12O42
(NKBu)5H2PMo12O42
(NKBu)2H5PMo12O42
(NKC12)7PMo12O42
(PyAc)7PMo12O42
NK – 1
NK – 2
NK – 3
NK – 4
Py – 1
36.98
41.81
52.01
29.55
40.84
0.99
1.12
1.40
0.81
1.10
1.29 ± 0.1
1.45 ± 0.11
1.54 ± 0.13
1.24 ± 0.08
1.27 ± 0.11
41.48 ± 0.3
51.04 ±
0.27
27.61 ± 0.3
40.64 ±
0.31
37.57 ±
0.31
37.40 ±
0.31
(PyPr)7PMo12O42
Py – 2
Py – 3
39.47
35.85
1.06
0.96
1.20 ± 0.09
1.08 ± 0.1
(PyHex)7PMo12O42
2.2. Spectral Characterizations
The intermolecular electronic interaction between heteropolyacid anions and IL-cations are
7–
illustrated by IR spectra in Figures 1 and 2. The anion [PMo12O42] shows four characteristic peaks at
−1
1058, 952, 875, and 798 cm , corresponding to the P–O, M = Oa, M–Ob–Mo, and Mo–Oc–Mo bonds
(where Oa is the terminal oxygen, Ob—bridging oxygen between corner-sharing octahedral,
Oc— bridging oxygen between edge-sharing octahedral) for the catalysts based on nicotinic acid
–1
(Figure 1) and characteristic peaks at 1060, 956, 877, and 787 cm for the catalysts based on pyridine
(Figure 2). The presence of four characteristic absorption peaks above indicates that the obtained

Molecules 2020, 25, 536
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heteropolyacid’ anion based on ILs keeps the Keggin-type structure [36]. There are characteristic
−1
absorption peaks of the carboxyl group of these ILs at 1725 and 1697 cm respectively. Bands at
−1
2800–3000 cm are the stretching vibrations of the saturated C–H bond in NK-4 (Figure 1, (d)). These
results indicate the formation of the catalyst containing IL, cation, and heteropolyacid anion.
798
NK.ESP
d
c
b
a
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1)
Figure 1. IR spectra of catalysts: (a) NK-1, (b) NK-2, (c) NK-3, (d) NK-4.
787
PY.ESP
g
f
e
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1)
Figure 2. IR spectra of catalysts: (e) Py-1, (f) Py-2, (g) Py-3.
2.3. Catalytic Tests
In order to evaluate the activity of catalysts, the oxidation of model mixtures containing
dibenzothiophene was studied under the same conditions (Figure 3).

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Figure 3. Conversion of dibenzothiophene over various catalysts. Reaction conditions: initial sulfur
content 500 ppm, 5 mL of model mixture, molar ratio H :S = 4:1, molar ratio sulfur: molybdenum
8:1, 1 mL MeCN, 50 °C, 1 h. Conversion of dibenzothiophene without catalyst is 7%.
2
O
2
Among the phosphoromolybdenum-containing ionic liquids, NK-1 and NK-4 appear to be
more active catalysts (Figure 3). The results obtained show that the conversion of dibenzothiophene
increases with the acidity of the cation (NK-1 > Py-1 > Py-2 > Py-3). Increasing the extent of proton
substitution also leads to higher DBT conversion (NK-1 > NK-2 > NK-3). This fact can be connected
to the catalytic activity of the carboxylic group by formation of the corresponding peracids due to
the multifunctional nature of the synthesized catalysts. The multifunctional nature of the catalyst is
explained by the fact that the catalyst used combines several functions such as:
- the formation of peroxocomplexes by anions, which are intended for the oxidation of
organosulfur compounds;
- the formation of the corresponding peracid, also capable of oxidizing organosulfur
compounds;
- Bronsted acid site for activation of sulfur atoms;
- a cation playing the role of an interphase carrier due to the ability to control hydrophobic
properties by varying the length of the alkyl chain.
Studies of the effect of the catalyst amount show, that a higher content of NK-1 leads to an
increase of DBT conversion achieving 98% at molar ratio S/Mo = 3:1 (Figure 4). The results obtained
for NK-4 show that the catalyst works even at a very small molar ratio S/Mo = 100:1. This fact may be
explained by the good surface activity of NK-4, that is why all the added amount of the catalyst is
concentrated on the interface. The increase in the NK-4 amount by 20 times did not lead to
significant rise in DBT conversion, which may be explained by a higher decomposition of hydrogen
peroxide.
Figure 4. Effect of amounts NK-1 and NK-4 on conversion of dibenzothiophene (DBT). Reaction
conditions: initial sulfur content 500 ppm, 5 mL of model mixtures, molar ratio H
2
O2:S = 4:1, 1 mL
MeCN, 50 °C, 1 h.

Molecules 2020, 25, 536
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Effect of hydrogen peroxide amount was studied for catalysts NK-1 and NK-4 at temperature
50 °C (Figure 5).
Figure 5. Effect of the amount of hydrogen peroxide on conversion of the DBT. Reaction condition:
total sulfur 500 ppm, 5 mL of model mixtures, molar ratio S:Mo 8:1, 1 mL MeCN, 50 °C, 1 h.
For NK-1 catalyst an increase in hydrogen peroxide amount from molar ratio H
2
O2:S 4:1 to 10:1
did not lead to a significant rise in conversion of DBT which indicates the limitations connected with
phase transfer restrictions, but for catalyst NK-4 having less transfer limitations ,an increase of
hydrogen peroxide dosage allowed better results to be obtained, which also indicates the possible
decomposition of oxidant in the presence of NK-4.
Variation of the amount of acetonitrile shows that NK-1 is sensitive to the presence of
acetonitrile which plays the role of extractant and solvent both for catalyst and oxidant. It should be
noted that NK-4, having better lipophilic properties, is less sensitive to the presence of solvent
(Figure 6).
Figure 6. Effect of the acetonitrile amount on the conversion of the DBT. Reaction conditions: initial
sulfur content 500 ppm, 5 mL of model mixture, molar ratio H
2
O2:S 4:1, molar ratio S:Mo 8:1, 50 °C, 1
h.
The temperature dependence of DBT conversion was studied for both NK-1 and NK-4 catalysts
under two types of hydrogen peroxide dosage (molar ratio oxidant:sulfur 4:1 and 10:1) to
understand the thermal decomposition of hydrogen peroxide (Figure 7).

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(a)
(b)
Figure 7. Effect of temperature on the conversion of the DBT in the presence of NK-1 catalyst (a) and
NK-4 catalyst (b). Reaction conditions: initial sulfur content 500 ppm, 5 mL of model mixture, molar
ratio S:Mo 8:1, molar ratio H
2
O :S 4:1, 1 mL MeCN, 1 h.
2
The results obtained show that complete oxidation of DBT can be achieved by using NK-1
catalyst at 70 °C and oxidant:sulfur 10:1 molar ratio. While using four times excess of oxidant for
NK-1 and NK-4, increasing the temperature from 50 to 70 °C leads to a decrease in DBT conversion
which probably indicates the decomposition of hydrogen peroxide at higher temperature. At the
same time, increasing the oxidant amount allows a high conversion of the substrate to be obtained at
70 °C (100% for NK-1 catalyst and 93% for NK-4). The time dependence of DBT conversion obtained
is shown in Figure 8.
(a)
(b)
Figure 8. Effect of the oxidation duration on the conversion of the DBT in the presence of NK-1
catalyst (a) and NK-4 catalyst (b). Reaction conditions: initial sulfur content 500 ppm, 5 mL of model
mixture, NK-1 as catalyst, molar ratio H
2
O :S 4:1 (10:1), molar ratio S:Mo 8:1, 1 mL MeCN.
2
The results obtained confirm that the catalyst plays a significant role in the hydrogen peroxide
decomposition at temperature 70 °C. Thus, for both catalysts the corresponding kinetic curves reach
a plateau after 10 min of the reaction and additional dosage of oxidant allows continuation of the
oxidation reaction to achieve more than 90% conversion of DBT. It should be noted that
decomposition of hydrogen peroxide has higher rates for the NK-4 catalyst, which does not allow

Molecules 2020, 25, 536
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achievement of 100% conversion even after addition of an extra amount of oxidant. Also, for catalyst
NK-4 the kinetic curve reaches a plateau even at temperature 50 °C while NK-1 works after 6 h of the
reaction at the same temperature. Thus, despite the catalyst NK-4 reaching the plateau faster than
NK-1 due to better hydrophobic properties, for complete oxidation of DBT it is preferable to use
NK-1.
The results of the study of oxidation for different classes of organosulfur compounds are shown
in Table 2.
Table 2. Oxidation of different classes of organosulfur compounds. Reaction conditions: total sulfur
500 ppm, 5 mL of model mixtures, NK-1 as catalyst, molar ratio H
2
O2:S = 4:1, molar ratio S:Mo 8:1, 1
mL MeCN, 1 h.
Substrate
methylphenylsulfide
dibenzylsulfide
Conversion, %
100
100
51
benzothiophene
5-methylbenzothiophene
Dibenzothiophene
4-methyldibenzothiophene
4,6-dimethyldibenzothiophene
25
71
96
64
The results obtained show that the oxidation activity of substrate decreases in the order sulfides
4-methyldibenzothiophene dibenzothiophene 4,6-dimethyldibenzothiophene
>
>
>
>
benzothiophene > 5-methylbenzothiophene which correlates with literature data [10,37].
The possibility of catalyst reuse by recycling was studied for NK-1 (Figure 9).
Figure 9. Effect of recycling NK-1 on the conversion of DBT. Reaction conditions: initial sulfur
content 500 ppm, 5 mL of model mixture, molar ratio H
2
O2:S 10:1, molar ratio S:Mo 8:1, 1 mL MeCN,
70 °C, 1 h.
Upon completion of reaction the catalyst was separated by centrifuging and used in the
oxidation of a new portion of model mixture. The results obtained shows that the catalyst maintains
its activity even after 5 cycles.
Oxidation of model sulfides shows that the NK-1 catalyst is most effective in this reaction.
Activity of this catalyst was investigated in oxidative desulfurization of real diesel fraction (initial
sulfur content of 2050 ppm). After oxidation, the reaction mixture was passed through silica gel to
remove oxidized sulfur compounds. The oxidation of the diesel fraction was carried out under the
most effective conditions selected for the oxidation of model sulfide mixtures: molar ratio H
2
O2:S,
10:1, molar ratio S:Mo 8:1, 1 mL MeCN, 70 °C, 1 h. As a result of oxidation, the degree of
desulfurization of the diesel fraction was 92%. The degree of desulfurization during oxidation
without a catalyst was 15%. Based on the obtained data, it can be concluded that the synthesized
catalysts are effective catalysts for oxidative desulfurization.
3. Experimental

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3.1. Materials
The ionic liquids and the model mixtures were synthesized from ethyl bromoacetate (99%,
Sigma Aldrich, Steinheim, Germany), bromopropionic acid (97%, Sigma Aldrich, Steinheim,
Germany), bromhexanoic acid (97%, Sigma Aldrich, Steinheim, Germany), bromobutane (99%,
Sigma Aldrich, Steinheim, Germany), 1-bromododecane (98%, Acros Organics, Steinheim,
Germany), pyridine (99,95%, Komponent-Reaktiv, Moscow, Russia), nicotinic acid (99%, Chemical
Line, St.Petersburg, Russia), hydrochloric acid (37%, Sigma-Tek, Moscow, Russia), acetonitrile
(99,85%, Komponent-Reaktiv, Moscow, Russia), dibenzothiophene (DBT, 98%, Sigma Aldrich,
Steinheim, Germany), benzothiophene (98%, Sigma Aldrich, Steinheim, Germany), methyl phenyl
sulfide (99%, Acros Organics, Geel, Belgium), dibenzyl sulfide (95%, Sigma Aldrich, Steinheim,
Germany),
4-methyldibenzothiophene
5- methylbenzothiophene
(96%,
(97%,
Sigma
Sigma
Aldrich,
Steinheim,
Steinheim,
Germany),
Germany),
Aldrich,
4,6-dimethyldibenzothiophene (95%, Sigma Aldrich, Steinheim, Germany), dodecan (99%, Sigma
Aldrich, Steinheim, Germany), ethanol (96%, Moscow, Russia), sodium phosphoromolybdate
(Na7PMo12O42, 98%, Sigma Aldrich, Steinheim, Germany), phosphoromolybdic acid
(H7PMo12O42*×H20, 99%, Sigma Aldrich, Steinheim, Germany), sodium hydroxide (KhimMed,
Moscow, Russia), hydrogen peroxide (H2O2, 50%, Prime Chemicals Group, Moscow, Russia).
3.2. Synthesis of Ionic Liquids
Synthesis of ionic liquids (ILs) having the cation-containing Brønsted acidity and the
heteropolyacid’s anion used as catalysts for oxidation of sulfur compounds are presented in
Schemes 1 and 2.
Br(CH
) COOEt
n
Na PMo
O
HCl (37%)
min
2
7
12 42
PMo
7
O
30
Δ,
12 42
+
-
-
+
+
Br
N
Br
N
N
N
(CH ) COOH
(CH ) COOEt
2 n
(CH ) COOH
2 n
2
n
(3) n=1
(4) n=2
(1) n=1
(2) n=2
Py - 1 (n=1)
Py - 2 (n=2)
Py - 3 (n=5)
Na PMo
O
H
Br(CH ) COO
7
12 42
2
n
-
+
Br
(CH ) COOH
N
2
n
(5) n=5
Scheme 1. The synthesis of ionic liquids (ILs) based on pyridine.
Stage 1. The quaternization reactions of the ILs (1)–(2) were performed according to the
literature procedure [38]. Equimolar amounts of pyridine, 1 mol and bromocarboxylic ethyl ester, 1
mol (or the corresponding ester) were placed in a flask equipped with a water-cooled condenser and
magnetic stirrer. The mixture was stirred for 3–6 h at room temperature. After that, the mixture was
heated in an oil bath to 80 °C. The product was isolated, washed with benzene (3 × 30 mL). Then, the
resulting product was dissolved in methanol, followed by extraction with petroleum ether (3 × 30
mL) and vacuum drying at 30 °C for 24 h. Synthesis of product (5) was similar to the procedure [39].
The reaction mixture was refluxed in a flask for 9 h, then cooled down to 20 °C and washed similarly
to (1)–(2).
Stage 2. Hydrolysis of products (1)–(2) was carried out by refluxing for 30 min using a double
excess of the concentrated hydrochloric acid. The solvent was removed in vacuo. The resulting solid
(3)–(4) was washed with acetone (3 × 30 mL), diethyl ether (3 × 30 mL), and dried in vacuo for 24 h to
constant weight.
Stage 3. The catalysts Py-1, Py-2, Py-3 were prepared according to the procedure described
previously [40]. The reaction of the anion exchange with sodium phosphoromolybdate was
performed using molar ratio 7:1 of IL (3) and Na7PMo12O42. A solution of IL (7 mmol) in deionized

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water was added dropwise to an aqueous solution of sodium phosphoromolybdate (1 mmol) under
vigorous stirring at room temperature. The precipitate formed was separated, washed with
deionized water and ethyl alcohol, and dried in vacuo for 24 h.
COOH
PMo12O42
+
N
COOH
BrCH (CH ) CH
3
COOH
CH (CH ) CH
3
2
2 n
7
o
T=100 C
NK - 1 (n=2)
+
2
2 n
-
24 - 48 h.
NK - 4 (n=10)
N+
Br
N
COOH
CH2(CH2)nCH3
(6) n=2
H7-mPMo12O42
+
(7) n=10
N
CH CH CH CH
3
2
2
2
m
NK - 2 (m=5)
NK - 3 (m=2)
Scheme 2. Synthesis of ILs based on nicotinic acid.
Stage 1. The quaternization of IL (6)–(7) was carried out similarly to the methods described in
the literature [41,42]. Nicotinic acid (1 mol) and acetonitrile (50 mol) were placed in an autoclave
(sealed tube). Then, alkylbromide (3 mol) was added to the resulting suspension. To obtain products
(6) and (7), the reaction mixture was stirred at 100 °C for 24 h and 48 h respectively. After cooling
down to 0 °C for removal of unreacted nicotinic acid, the solution was decanted and dried at 40 °C in
vacuo to constant weight. The resulting precipitate (6) was dissolved in deionized water, washed
with hexane (3 × 15 mL) and diethyl ether (3 × 15 mL) and dried in vacuo for 24 h. The resulting solid
(7) was dissolved in benzene at 60 °C, filtered off from the precipitate and evaporated using a rotary
evaporator at 40 °C. Then product (7) was washed with petroleum ether (5 × 30 mL) and dried in
vacuo at 50 °C for 24 h.
Stage 2. The catalysts NK-1, NK-2, NK-3 and NK-4 were obtained by the reaction of the anion
exchange of IL (6)–(7) with Na7PMo12O42.
The sodium salt of phosphoromolybdic acid was prepared as follows. A saturated sodium
hydroxide solution was added to a solution of phosphoromolybdenum acid using alkali/acid molar
ratios of 7:1 for NK-1 and NK-4, 2:1 for NK-3, and 5:1 for NK-2. Separately, the solutions of IL (6) in
deionized water and (7) in ethanol were prepared. For NK-4, sodium phosphoromolybdate was
prepared in ethanol. The IL was added dropwise to the sodium salt and stirred at room temperature
for 24 h. The precipitate was separated, washed with ethanol and dried in vacuo for 24 h.
3.3. Catalyst Characterization
Infrared spectra (IR) of catalysts were recorded on a Nicolet (Thermo Scientific, Waltham,
−1
Massachusetts, USA) IR-2000 instrument in the range of 500–4000 cm by the method of multiple
o
violation of total internal reflection using a Multi-reflection HATR attachment containing a 45 ZnSe
1
13
crystal for various wavelength ranges with a resolution of 4 nm. NMR spectra of H and C were
measured on a Varian-XR-400 spectrometer (Bruker Avance 400, Watertown, MA, USA) with an
operating frequency of 400 MHz. Electrospray ionization mass spectrum (ESI-MS) were recorded on
a Dionex Ultimate 3000 (Dionex corporation, Sunnyvale, CA, USA and Berlin, Germany)
spectrometer liquid chromatography equipped with an AB Sciex 3200 Qtrap (AB Sciex, Redwood,
CA, USA) tandem quadrupole mass-spectrometric detector for the electrospray ionization (ESI) in
positive ion recording mode. Elemental analysis of metals in catalysts was performed on an ARL
PerformX X-ray fluorescence wave spectrometer (Thermo Fisher Scientific, Waltham, Massachusetts,
US). The elemental analysis of carbon, hydrogen, and nitrogen was carried out on a CHNS
instrument Thermo Flash 2000 (Thermo Fisher Scientific, GB). The calculation of pKa was carried out
according to formula (1), by measuring the pH values of the ionic liquids aqueous solutions at

Molecules 2020, 25, 536
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certain concentrations using an electronic pH meter. The concentration of an aqueous solution of
ionic liquid was determined by acid–base titration. The pKa values were determined only for water
soluble ILs.
pH = ½*(pKa + pC)
(1)
3.4. Catalytic Test
Solutions of methylphenylsulfide, dibenzylsulfide, benzothiophene, 5-methylbenzothiophene,
dibenzothiophene (DBT), 4-methyldibenzothiophene, and 4,6-dimethyldibenzothiophene in
dodecane were used as model oils, with an initial sulfur content 500 ppm. Also model mixtures of
DBT with a total sulfur content of 1000 ppm, 2000 ppm were prepared. The oxidative desulfurization
catalytic tests were conducted in a reactor equipped with a stirrer, thermostatic water bath, and
condenser. The catalyst, H2O2, and acetonitrile were added into the reactor, and then 5 mL of a model
mixture was injected. The resulting mixture was stirred for 10–360 min at 50 to 70 °C, and after
reaction, the upper oil layer was decanted at room temperature and analyzed by GC-FID with an
internal standard (Kristall 2000 M equipped with a capillary column (ZB-1 liquid phase, 30 m × 0.32
mm). The solution of catalyst in acetonitrile (lower phase) was collected and used in the recycling
experiments.
Oxidation of a real diesel fraction (initial sulfur content—2050 ppm) was performed by the
following procedure: NK-1 catalyst (molar ratio Mo:S = 8:1), 1 mL of acetonitrile and 29.5 μL of
hydrogen peroxide were added to 10 mL of fuel. The reaction was carried out for 1 h at 70°C. After
oxidation, the reaction mixture was passed through silica gel to remove the oxidized products. The
total sulfur content in the hydrocarbon fractions was determined using a sulfur analyzer ASE-2
(analyzer of sulfur energy dispersion) according to the ASTM D4294-10 standard (Standard Test
Method for Sulfur in Petroleum and Petroleum Products by Energy Dispersive Xray Fluorescence
Spectrometry) [43]. The method is based on X-ray fluorescence energy–dispersive spectrometry to
determine the sulfur mass fraction in diesel and unleaded gasoline in the range from 7 to 50 000 ppm
with a relative error of 3%.
Each experiment was made repeatedly to obtain a minimum of three convergent results, which
differ from the average value less than 5%. The average values were reported in Figures 3−9 and in
Table 2. The measurement error is less than 5%.
4. Conclusions
In this work, seven polyoxometallate based-ILs catalysts, including those with Bronsted acid
sites presented by carboxylic groups, were synthesized and used as catalysts for the oxidation of
organosulfur substrates. It was found that the activity was higher for catalysts containing Bronsted
acid sites in the cation. At the same time, it was shown that the catalytic activity depended on the
strength of the corresponding acid and was highest for NK-1, containing the carboxylic group
directly connected with the pyridinium cation. The influence of reaction conditions (reaction time,
oxidant and catalyst amount) on the conversion of DBT was investigated. Thus the increase of NK-1
catalyst content leads to a significant increase of DBT conversion and achieves 98% at molar ratio
S:Mo 3:1. The optimal molar ratio range of hydrogen peroxide to total sulfur content is H2O2:S from
4:1 to 10:1 and depends on reaction temperature. The higher reaction temperatures need higher
oxidant dosage for effective oxidation due to the increase of the hydrogen peroxide decomposition
rate. Also, it was shown that NK-1 catalyst is sensitive to the presence of acetonitrile which plays the
role of extractant and solvent for catalyst, oxidant, and the DBT oxidation products. The conditions
for complete oxidation of DBT were found as follows: NK-1 catalyst, molar ratio H2O2:S 10:1, molar
ratio S:Mo 8:1, 1 mL MeCN, 70 °C, 1 h. It was shown that the catalyst synthesized can operate for five
cycles without any decrease in its activity.
Supplementary Materials: Supplementary are available online.

Molecules 2020, 25, 536
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Author Contributions: Conceptualization, A.A. methodology, A.A. and E.E..; software, P.P.; validation, A.A.
and A.G.; formal analysis, E.E. and A.K.; investigation, E.E. and A.K., resources, A.K.; data curation, P.P. and
A.V.; writing—original draft preparation, E.E. and P.P.; writing—review and editing, A.A. and A.G.;
visualization, A.A. and E.E.; supervision, A.A.; project administration, A.A.; funding acquisition, A.A.
Funding: This work was financially supported by the Russian Science Foundation (project no. 18-79-00116).
Acknowledgments: This work was financially supported by the Russian Science Foundation (project no.
18-79-00116). The authors thank Valentine Stytsenko from the Gubkin Russian State University of Oil and Gas
for his input in this work.
Conflicts of Interest: The authors declare no conflict of interest.
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