Heterogeneous Catalysts SILP with Phosphotungstic Acid for Oxidative Desulfurization: Effect of Ionic Liquid

Article abstract of DOI:10.1134/S0023158420050018

Abstract: SILP (supporting ionic liquid phase) catalysts for peroxide thiophene oxidation were obtained on silica gel, they included imidazole cations with sulfonic acid group and phosphotungstate anions. The composition of the active phase and the surface structure of the catalysts are characterized by a set of modern physicochemical methods, including mass spectrometry using the SALDI technique. It was found that the organic cation structure affects the stability of heteropolyanions, decomposition processes in the SILP and the catalytic properties of the samples. SILP catalysts are effective in desulfurization of the diesel fraction and provide fuel with an S content of less than 10 ppm.

Full text of DOI:10.1134/S0023158420050018

ISSN 0023-1584, Kinetics and Catalysis, 2020, Vol. 61, No. 5, pp. 775–785. © Pleiades Publishing, Ltd., 2020.
Russian Text © The Author(s), 2020, published in Kinetika i Kataliz, 2020, Vol. 61, No. 5, pp. 688–699.
Heterogeneous Catalysts SILP with Phosphotungstic Acid
for Oxidative Desulfurization: Effect of Ionic Liquid
A. A. Bryzhina, A. K. Buryakb, M. G. Gantmanc, V. M. Zelikmana,
M. I. Shilinaa, and I. G. Tarkhanovaa, *
aDepartment of Chemistry, Moscow State University, Moscow, 119991 Russia
bFrumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences,
Moscow, 119071 Russia
cHelmholtz Institut Erlangen-Nürnberg for Renewable Energy, Erlangen, 91058 Germany
*e-mail: itar_msu@mail.ru
Received November 7, 2019; revised January 28, 2020; accepted March 6, 2020
Abstract—SILP (supporting ionic liquid phase) catalysts for peroxide thiophene oxidation were obtained on
silica gel, they included imidazole cations with sulfonic acid group and phosphotungstate anions. The com-
position of the active phase and the surface structure of the catalysts are characterized by a set of modern
physicochemical methods, including mass spectrometry using the SALDI technique. It was found that the
organic cation structure affects the stability of heteropolyanions, decomposition processes in the SILP and
the catalytic properties of the samples. SILP catalysts are effective in desulfurization of the diesel fraction and
provide fuel with an S content of less than 10 ppm.
Keywords: thiophene oxidation, hydrogen peroxide, phosphotungstic heteropolyacid, SILP catalysts, diesel
desulfurization
DOI: 10.1134/S0023158420050018
INTRODUCTION
fuel purification from sulfur-containing compounds,
i.e., hydrodesulfurization (HDS), requires high tem-
perature, pressure, and high hydrogen consumption
[6]. The HDS process is effective for removal of thiols,
sulfides, and thiophene, but to remove heterocyclic
compounds, for example, dibenzothiophene and its
alkyl derivatives, it is necessary to use more severe
conditions for its implementation, which often leads to
an increase in the cost of the final product [7]. There-
fore, adsorption [8, 9], extraction [10, 11], and oxida-
tive desulfurization (ODS) [2, 4] are used in addition
to hydrodesulfurization. Among the listed methods,
ODS is most promising due to mild conditions and the
possibility of implementation on the traditional oil
refining equipment [12].
In the process of oxidative desulfurization, polar
sulfones and sulfoxides are usually formed, which are
then removed by adsorption or extraction methods [3,
13]. The catalyst of the process is a key component of
any oxidizing system. As a rule, compositions contain-
ing oxides in individual and mixed forms, as well as
salts of transition metals are used [2, 12, 14, 15]. Par-
ticular attention is paid to heterogeneous catalysts
based on HPAs and polyoxometallate compounds
deposited on porous materials with a large specific
surface area, which have a high catalytic activity in the
oxidation of organosulfur compounds [2, 4, 16]. How-
ever, the low stability of anions due to the strong inter-
Heteropolyacids (HPAs) and polyoxometallate
compounds based on them are widely used in combi-
nation with ionic liquids (ILs) to create catalytically
active hybrid materials. Due to their high acidity, oxi-
dation potential, and compatibility with green oxi-
dants, such as oxygen and hydrogen peroxide, these
kinds of compositions are effectively used in a number
of oxidative processes [1], including the desulfuriza-
tion of petroleum [2–4].
Desulfurization is an important process in the oil
refining industry, which is aimed at obtaining fuel that
meets modern environmental standards (total sulfur
content of less than 10 ppm) [5]. The problem of
petroleum processing is directly related to the removal
of heteroatomic compounds that have a negative
impact on the environment, corrode equipment, and
etch expensive catalysts. The traditional method of
Abbreviations: HPA, heteropolyacid; IL, ionic liquid; ODS, oxi-
dative desulfurization; ZI, Zwitterionic compound; PTA, phos-
photungstic heteropolyacid; SALDI, surface-assisted laser
desorption/ionization; NMR, nuclear magnetic resonance;
TPD NH , temperature programmed desorption of ammonia;
3
SEM, scanning electron microscopy; SEI, secondary electron
imaging; GLC, gas–liquid chromatography; DMF, dimethyl-
formamide; BJH, Barrett–Joyner–Halenda method; BET,
Brunauer–Emmett–Teller method.
775

776
BRYZHIN et al.
action between the heteropolyacid and the surface of compounds of the thiophene series [2, 32]; second,
mineral carriers is an urgent problem that arises when
creating such systems [17–19].
the oxidation of thiophene can lead to the formation of
sulfate anion, styrene, carboxylic acids, and carbon
dioxide [2, 33–35]. The latter products, especially sul-
furic acid, can adversely affect the catalyst and reduce
its stability. Therefore, the catalysts were tested under
intentionally severe conditions. In addition to the
model process, all the heterogeneous catalyst compo-
sitions were tested in the process of oxidative desulfur-
It is known that ILs are widely used as homoge-
neous desulfurization catalysts [20, 21] and extractants
[10, 22]. Nevertheless, their high cost limits the possi-
bility of large-scale use; therefore, a method for their
immobilization on the surface was proposed to reduce
the consumption of ionic liquids [4, 23, 24]. The SILP
catalyst is a layer of an ionic liquid deposited on the sur- ization of petroleum diesel provided by OAO
face of porous materials by physical adsorption [25]; in
this case, both the IL itself (cation and/or anion) and
additional metal-containing components located in
the IL layer can exhibit catalytic activity. As a result,
the obtained hybrid compositions have the advantages
of both homogeneous (high activity and selectivity
under mild conditions) and heterogeneous systems
(stability and ease of separation of products) [26]. As a
result, heterogeneous systems of the SILP type are
widely used in a number of catalytic processes, includ-
ing oxidative desulfurization [4, 24, 27–29].
Var’eganneft’.
A side reaction of decomposition of hydrogen per-
oxide under the action of metal-containing catalysts is
among the factors preventing deep oxidation of the
substrate. To increase the efficiency of oxidizer use, we
applied the approach of fractional loading of hydrogen
peroxide. This method is based on the fact that the
kinetic reaction order of catalytic decomposition of
H2O2 depends, according to the published results of
studies [36, 37], on its initial concentration, while the
order of the main reaction with respect to the oxidant
remains unchanged. Therefore, to increase the effi-
ciency of use, hydrogen peroxide must be added in
small portions, i.e., its initial concentration should be
lowered, thereby slowing down the side decomposi-
tion reaction. This approach is well known [38, 39]
and has been successfully implemented in our previous
studies [4, 40–42].
The performed studies made it possible to carry out
a comparative analysis of SILP catalysts and an analo-
gous heterogeneous catalyst sample without ionic liq-
uid, as well as to evaluate the effect of the structure of
the imidazole-containing cation on the stability of
heteropolyanions, the textural characteristics of the
samples, their acidity and activity in oxidation of the
model substrate, and the process of ODS of petroleum
diesel.
In our previous study [4], we proposed an efficient
method for the synthesis of heterogeneous catalysts
based on phosphotungstic or phosphomolybdic acids
and 4-(3′-ethylimidazolium)-butanesulfonate. The
aim of this work was to establish the effect of the struc-
ture of the imidazole cation on the stability of phos-
photungstate anions on the surface of silica gel and the
activity of the obtained hybrid materials of the SILP
type in the oxidation of thiophene with hydrogen per-
oxide. This compound was chosen as a model sub-
strate because of its highest resistance to oxidation
among other sulfur-containing heterocycles [30].
Samples of SILP catalysts were synthesized from
imidazolium butanesulfonate with ethyl or vinyl sub-
stituents and phosphotungstic acid (Scheme 1). The
choice of reagents was based on the published data,
according to which vinyl ILs, in comparison with ethyl
ILs, are capable of forming intra- and intermolecular
hydrogen bonds [31]. We assumed that such an addi-
tional interaction would help stabilize both the IL and
anions on the silica gel surface. Moreover, we tested,
for comparison, a sample synthesized under similar
conditions from phosphotungstic acid and silica gel
without using organic additives.
EXPERIMENTAL
Reagents and Auxiliary Materials
We used the following reagents manufactured by
Sigma Aldrich: thiophene (99%), hydrogen peroxide
(50%), 1,4-butane sultone (99%), 1-ethylimidazole
(95%), 1-vinylimidazole (95%), and phosphotungstic
acid (99%). Solvents were obtained from commercial
sources without further purification. The support for
the preparation of catalysts was a Perlkat 97-0 silica gel
from BASF.
R = C2H5, C2H3
R
3−
PW12O
40
N
N C4H8SO3H
SiO2
3
Preparation of Catalysts
Synthesis of Zwitterionic Compounds
Scheme 1. Structure of synthesized SILP composites.
Zwitterionic compounds (ZIs) were synthesized
The use of thiophene as a model substrate is of according to the procedure (Scheme 2) previously
interest for the following two reasons: first, it has a low described in [4]. An imidazole derivative (9.4 g of
electron density on the sulfur atom and, consequently, vinylimidazole or 9.6 g of ethyl imidazole) was dis-
is most difficult to oxidize among the organosulfur solved in acetonitrile (100 mL) and mixed with 15 g of
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HETEROGENEOUS CATALYSTS SILP WITH PHOSPHOTUNGSTIC ACID
1,4-butane sultone (imidazole : sultone molar ratio 1 : (NCH2CH2CH2CH2SO3, NCH2CH3),
777
51.6
124.9
1.1). The mixture was purged with argon and then
boiled under reflux with vigorous stirring for 24 h. The
precipitate formed was filtered off, washed thrice with
acetone, and dried under a vacuum at 100°C for 16 h.
The structure of the obtained zwitterions was con-
firmed by NMR spectroscopy on a JNM ECX instru-
ment (JEOL, Japan) at a frequency of 400 MHz. The
data for two ZI samples are given below:
(NCH2CH2CH2CH2SO3,
NCH2CH3),
(NCHNCHCH), 125.3 (NCHNCHCH), 134.6
(NCHNCHCH);
4-(3′-vinylimidazolium)-butanesulfonate: 1H NMR
(400 MHz, D2O) 1.58–1.67 (2H, m, NCH2CH2CH2–
CH2SO3), 1.87–1.96 (2H, m, NCH2CH2CH2CH2SO3),
2.91 (2H, t, J = 7.6 Hz, NCH2CH2CH2CH2SO3), 4.16
(2H, t, J = 8.4 Hz, NCH2CH2CH2CH2SO3), 5.26–
5.30, (1H, dd, N–CH=CHH), 5.63–5.69, (1H, dd,
N–CH=CHH), 6.96–7.04 (1H, dd, N–CH=CH2),
7.47 (1H, m, NCHNCHCH), 7.65 (1H, m,
NCHNCHCH), 8.94 (1H, s, NCHNCHCH);
13C NMR (D2O) 21.0 (NCH2CH2CH2CH2SO3), 28.0
(NCH2CH2CH2CH2SO3), 49.3 (NCH2CH2CH2CH2SO3),
50.1 (NCH2CH2CH2CH2SO3), 109.4 (N–CH=CH2)
119.6 (NCHNCHCH), 122.9 (NCHNCHCH), 128.3
4-(3′-ethylimidazolium)-butanesulfonate: 1H NMR
(400 MHz, D2O) 1.51 (3H, t, J = 7.2 Hz, NCH2CH3),
1.83–1.72 (2H, m, NCH2CH2CH2–CH2SO3), 2.11–
2.03 (2H, m, NCH2CH2CH2CH2SO3), 2.93 (2H, t,
J = 7.1 Hz, NCH2CH2CH2CH2SO3), 4.32–4.24 (4H,
m, NCH2CH3, NCH2CH2CH2CH2SO3), 7.81 (2H, s,
13
NCHNCHCH–), 9.03 (1H, s, NCHNCHCH); C
NMR
(D2O)
14.3
(NCH2CH3),
23.5
(NCH2CH2CH2CH2SO3), 29.7 (NCH2CH2CH2CH2SO3),
45.7 (NCH2CH2CH2CH2SO3),
51.3 (NCHNCHCH), 134.5 (N–CH=CH2).
−
C4H8SO3
C2H5
N
N
C2H5
N
N
N
N
O
O
O
S
−
C4H8SO3
N
N
CH2 CH
CH2 CH
Scheme 2. Synthesis of zwitterionic compounds.
Synthesis of ILs and preparation of SILP samples
silica gel from an aqueous alcohol solution (silica gel :
acid mass ratio 10 : 1) [17]; the process lasted for 12 h,
and then the resulting granules were dried under a vac-
uum as described above.
To protonate the sulfonate group, 0.7 g of a zwitte-
rionic compound (4-(3′-ethylimidazolium)-butane-
sulfonate or 4-(3′-vinylimidazolium)-butanesulfonate
was added to a concentrated aqueous solution of 2.9 g
of phosphotungstic heteropolyacid (PTA) (ZI : PTA
molar ratio 3 : 1). The mixture was vigorously stirred
for 6 h at room temperature; the obtained ILs in the
form of white crystalline substances were dried in air.
A silica gel was kept in an aqueous solution of IL
(support : IL mass ratio 10 : 1) at room temperature for
12 h. Next, the solution layer was decanted, and the
catalytic samples were dried under a vacuum at 60°C
until obtaining a constant weight.
Characterization of Catalysts
The obtained ionic liquids were identified by IR
spectroscopy in KBr pellets on an Infralum FT-801
FT-IR spectrophotometer (SIMEKS, Russia) oper-
ated in the range of 4000–400 cm–1.
To determine the composition of heteropolyanions
on the surface of the support, the mass spectrometry
method with surface-assisted laser desorption/ioniza-
tion (SALDI) was used. The mass spectra of the sam-
ples were recorded in the RN Pep Mix operation mode
on an Ultraflex instrument (Bruker, Germany)
equipped with a nitrogen laser (wavelength 337 nm,
energy 110 μJ) and a time-of-flight mass analyzer. The
spectra were recorded using a reflectron operating in
the mode of detection of negatively charged ions.
Thus, the samples designated as vin-W and et-W
were obtained from 4-(3′-vinylimidazolium)-butane-
sulfonate and 4-(3'-ethylimidazolium)-butanesulfon-
ate, respectively (Scheme 1).
Synthesis of catalyst without ionic liquid
The catalyst sample designated as W was prepared Cluster ions were identified from isotopic distribution
by depositing a phosphotungstic heteropolyacid on using the IsoPro simulator software.
KINETICS AND CATALYSIS Vol. 61 No. 5 2020

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BRYZHIN et al.
Elemental analysis was performed using a CE1106 raphy (GLC) was used for the quantitative analysis of
automatic CHN analyzer (Carlo Erba Instruments, the organic phase; samples (0.5 μL) were taken with a
Italy).
Data on the acidic properties of the catalyst com-
time interval of 1 h; dodecane served as an internal
standard. The chromatographic analysis was per-
formed on a Kristall 2000 instrument (Khromatek,
Russia) equipped with a Zebron ZB-1 capillary col-
umn with dimensions of 30 m × 0.32 mm × 0.5 μm
(Phenomenex, United States). The products of the
process were identified using the H and C NMR
spectroscopy methods. The NMR spectra were
recorded on an Avance-600 instrument (Bruker, Ger-
many, 600 MHz) at room temperature. To reuse the
catalysts, the reaction mixture was decanted and the
catalyst samples were washed with isooctane.
For oxidative desulfurization, diesel fuel produced
by OAO Var’eganneft’ (20 mL, total sulfur content
1080 ppm), catalyst (0.04 g), and H2O2 (0.4 mL, 50%)
were thermostated for 4 h in a glass reactor (60°C)
under vigorous stirring. The mixture was cooled to
room temperature and washed with 5 mL of dimethyl-
formamide (DMF) in a separatory funnel. Dimethyl-
formamide was chosen because it is the most effective
extractant for the removal of oxidation products of
organosulfur components of diesel fuel [18]. Prelimi-
nary experiments on fuel treatment with DMF with-
out oxidation showed that the extraction of sulfur did
not exceed 20%. Next, the fuel sample was placed into
the reactor, and a fresh portion of the oxidizer
(0.4 mL) and the catalyst, which was used in the pre-
vious stage and washed with isooctane, were added.
The mixture was stirred for another 4 h at 60°C, and
the procedure of extraction of oxidation products was
repeated. For comparison, the same process was car-
ried out in one step with hydrogen peroxide (0.8 mL).
The residual sulfur content was determined on an
ASE-2 X-ray fluorescence spectrometer (Burevestnik,
Russia) with a measurement error of 0.5%.
posites was obtained by the method of temperature
programmed desorption of ammonia (TPD NH3) on
a USGA-101 sorption analyzer (Unisit, Russia). The
sample was preheated in an inert atmosphere at 150°C
and then cooled to room temperature. Ammonia
adsorption was carried out at 60°C for 30 min; NH3
was diluted with nitrogen in a ratio of 1 : 1. Physically
sorbed ammonia was blown off in a flow of dried
helium at 100°C for 1 h. Temperature programmed
desorption experiments were carried out in the tem-
perature range from 60 to 400°C in a flow of dried
helium (flow rate 30 mL/min). The heating rate was
8°C/min.
1
13
The acidic properties of aqueous solutions of ZIs
were determined using an HI 8314 portable pH meter
(Hanna Instruments, Germany).
The surface topography was investigated by scan-
ning electron microscopy (SEM) using a JSM-6000
NeoScope electron microscope (JEOL, Japan) with
an EX-230 integrated X-ray analyzer for energy dis-
persive analysis (EDA) of the particle size distribution.
Microscopy studies were performed in the high-vac-
uum mode with an accelerating voltage of 15 kV. The
secondary electron imaging (SEI) mode was used in
signal detection.
The tungsten content on the catalyst surface was
determined photometrically using pyrocatechol,
which forms a stable complex compound with tung-
state in the presence of sodium sulfite and sodium
hydroxide [43]. Electronic spectra were recorded on a
UV-2101PC instrument (Shimadzu, Japan) by mea-
suring the optical density at 350 nm.
Adsorption measurements were carried out on an
ASAP 20000N automatic sorption analyzer
(Micromeritics, United States). Before measure-
ments, the samples were exposed to vacuum pressure
at 150°C for 2 h.
RESULTS AND DISCUSSION
Characterization of Catalysts
The IR spectra of individual ILs prepared using
two ZIs and phosphotungstic acid in KBr pellets are
shown in Fig. 1. Assignment of bands and identifica-
tion of compounds were carried out using the pub-
lished data [44, 45].
As seen from Fig. 1, both spectra contain bands in
the range of 3200–2800 cm–1, which correspond to
stretching vibrations of the C–H bonds in the aro-
matic heterocycle and in the alkyl and vinyl fragments.
The bands in the region of 1600–1500 cm–1 character-
ize the stretching vibrations of the imidazolium ring. A
Catalytic Experiments and Analysis of Products
To prepare a model mixture, the calculated
amount of thiophene was dissolved in isooctane to
obtain a concentration of 1 wt % of the substrate.
Experiments on thiophene oxidation were carried
out in a thermostatically controlled glass reactor with
a jacket. A model mixture (10 mL), 50% hydrogen
peroxide (0.4 mL), and a catalyst (0.1 g) were placed in
the reactor at room temperature, and then the mixture
was heated to 60°C and stirred for 4 h. For catalysts relatively strong band at 1650 cm–1 in the spectrum of
based on heteropolyacids, the optimal temperature of vin-W can correspond to ν(C=C) vibrations in the
the process (60°C) has been determined in our previ- vinyl fragment. In addition, the bands in the region of
1173–1165 cm–1 in the spectra of both samples can be
attributed to δ(H–C–C) and δ(H–C–N) bending
ous studies [4, 40, 42]. Under conditions of fractional
loading of H2O2, the oxidizing agent was added in 0.2-mL
increments every two hours. Gas–liquid chromatog- vibrations in the heterocycles and to stretching vibra-
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HETEROGENEOUS CATALYSTS SILP WITH PHOSPHOTUNGSTIC ACID
tions of the S–O and S=O bonds in the sulfo groups
779
(1160–1170 and 1350–1380 cm–1, respectively). The
bands of stretching vibrations in heteropolyanion
PW12 are observed in the range of 1100–800 cm–1. The
P–O bond is characterized by absorption bands at
1080 and 500 cm–1, which can be attributed to stretch-
ing and bending vibrations, respectively. The bonds of
W with the O atoms of the WO6 group, as well as the
W–O–W bridging bonds linking WO6 octahedra that
have a common angle or a common face, are charac-
terized by absorption bands at 975, 895, and 806 cm–1,
respectively. Thus, the IR spectroscopy data confirm
the presence of ionic liquids, relevant imidazoliosul-
fonates, and Keggin-type heteropolyanions in the
analyzed samples.
1
1365
1551
2945
3146
1656
514
1172
1160
2
1380
1560
895
976
1080
806
3500 3000 1800 1600 1400 1200 1000 800 600
Wavenumber, cm–1
Physicochemical parameters of the obtained solid
samples and catalyst support are given in Table 1. A
substantial decrease in the specific surface area and
volume of pores of the support was achieved as a result
of the synthesis of SILP catalysts; according to the
SEM data, the formation of dense layers of ILs uni-
formly distributed over the silica gel surface is
observed (Fig. 2). On the contrary, modification of
the surface with HPA without ILs (catalyst W) does
not lead to a noticeable change in the specific surface
area and volume of pores. We believe that this can be
explained by decomposition of PTA (Table 2) with the
formation of low-molecular-weight tungstates and
WO3 and W2O6 tungsten oxides, which have molecules
with sizes much smaller than the size of heteropolya-
cid molecules and do not have a substantial effect on
the textural characteristics of the support. Neverthe-
less, the surface of this sample and the surface of et-W
have a similar rough loose structure, in contrast to the
structure of catalyst vin-W, the SEM images of which
clearly show the formation of large IL agglomerates
(Figs. 2 and 3). Such a difference in the behavior of ILs
on the surface may be due to the tendency of vinyl
derivatives to form hydrogen bonds between cations
and anions both within and between molecules [31].
Fig. 1. IR spectra of ILs based on H PW O with
3
12 40
(1) 4-(3′-vinylimidazolium)-butanesulfonate and (2) 4-(3′-
ethylimidazolium)-butanesulfonate.
to [17, 47], on its loading and a stable phase is formed
only at loads above 20 wt %, while the content of the
active phase on the catalyst surface in our study did not
exceed 10 wt % (Table 1). Sample et-W based on 4-
(3'-ethylimidazolium)-butanesulfonate contains
a
wide range of heteropolyanions with preferential pres-
ervation of the structure of the initial phosphotungstic
acid precursor. However, HPAs on the surface of sam-
ple vin-W are partially decomposed with the formation
of high-molecular-weight polytungstates (Table 2).
This difference in stability is determined, in our opin-
ion, by the acidity of the medium used in the synthesis
of ILs and solid catalysts. According to the measure-
ment results, the acidity of an aqueous solution of
4-(3′-ethylimidazolium)-butanesulfonate is higher
than that of 4-(3′-vinylimidazolium)-butanesulfon-
ate, which can be explained by the low solubility of the
latter due to the presence of strong intermolecular
interactions. Taking into account that the syntheses of
ILs were carried out in water and HPA is stable in
aqueous solutions only in an acidic medium in a nar-
row pH range [1, 17, 48], the role of ZIs consists in
According to the study results obtained by mass spec-
trometry in the SALDI mode, the catalysts differ in the
composition of ions, the structure of which changes
depending on the type of the IL used (Table 2).
Table 1. Textural characteristics of the catalyst support and
samples, and the content of tungsten and nitrogen in them
according to the elemental analysis data
As a result of the synthesis of catalyst W (without an
ionic liquid), the heteropolyacid is completely decom-
posed with the formation of low-molecular-weight
tungsten oxides (Table 2). On the other hand, the mass
spectra of SILP samples (et-W and vin-W) contain
peaks that correspond to the following fragments of
Vpore
cm3/g
,
Dpore
nm
,
Ssp, m2/g
Samples
W, wt % N, wt %
PWO6−, PW2O5−, PW3O1−2, PW4O1−4, PW5O1−5,
PTA:
W
vin-W
et-W
297
150
104
300
9
11
10
10
0.69
0.39
0.32
0.75
9
0
−
−
−
6.5
2.8
0
0.78
0.45
0
and
Despite similar pro-
PW6O17, PW7O19,
PW12O40.
cedures used in the syntheses of catalysts (et-W, vin-
W, and W), the heteropolyacid structure can be dam-
aged by strong interaction between the heteropolyan-
ions and the surface of the support [4, 19, 46]. In addi-
tion, the stability of PTA on silica depends, according
Silica gel
Specific surface area S was determined by the BET method, and
sp
pore diameter D
and pore volume V
were determined by
pore
pore
the BJH method.
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780
BRYZHIN et al.
Perlkat 97-0 silica gel
W
20 μm
vin-W
20 μm
et-W
(а)
(c)
(b)
20 μm
20 μm
(d)
Fig. 2. SEM of (a) Perlkat 97-0 silica gel and catalysts (b) W, (c) vin-W, and (d) et-W.
Table 2. Composition of ions on the surface of the catalysts
according to the results of analysis by the SALDI method in
the mode of detection of negative ions*
regulating the acidity in the process of prepration of
metal-containing ILs. In addition, HPAs could also
decompose at the stage of immobilization of ILs on
silica gel; however, the nature of interaction of proton-
ated ILs and metal-containing anions with the surface
of silica (Scheme 1) is apparently different from the
interaction of free PCA with silica, which leads to an
increase in the stability of heteropolyanions in the fol-
lowing sequence of samples: et-W > vin-W > W.
Absolute intensity,
Catalyst
Ions
m/z
arb. u.
W
WO3
232
464
311
86
54
W2O6
et-W
PWO6
15
The sample acidity parameters measured by the
TPD NH3 method are given in Table 3.
PW2O5
PW3O12
PW4O14
PW5O15
PW6O17
PW12O40
PWO6
479
62
As a result of the formation of the active phase
layer, the total acidity increases; moreover, et-W is dis-
tinguished by a significant number of acid sites, which
is apparently associated with their availability due to
the uniform distribution of the ionic liquid over the
surface (according to the SEM data). It also follows
from Table 3 that a temperature range up to 190°C can
be distinguished for catalysts, in which ammonia
desorbs from weak Brønsted acid sites composed,
probably, of hydroxylated tungsten oxides. Desorption
of NH3 in the range of 190–250°C can be associated
with stronger Brønsted acid sites, which are related to
acidic fragments of HPAs or sulfo (–SO3H) groups of
ILs. For catalysts W and et-W, the presence of Lewis
acid sites (Tdes above 250°C) can also be noted, which
is possibly caused by the partial decomposition of
HPA anions during the analysis. The absence of these
sites in catalyst vin-W may be due to the island plant-
ing of this IL, which leads to a greater stability of
metal-containing fragments and their lower accessi-
bility for the adsorbate. As will be shown below, et-W
turned out to be the most active catalyst, since acid
sites play, as is known, an important role in the activa-
tion of both organosulfur compounds and hydrogen
peroxide. We believe that it is the strong Brønsted acid
775
102
46
991
1191
1407
2879
311
23
8
12
vin-W
510
5410
870
5720
5700
2830
2440
115
118
130
W2O6
464
479
PW2O5
W3O9
696
W4O12
928
W5O15
1160
1392
1407
1623
2879
W6O18
PW6O17
PW7O19
PW12O40
* The mass spectra of PTA heteropolyacid are published in our
previous study [42].
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HETEROGENEOUS CATALYSTS SILP WITH PHOSPHOTUNGSTIC ACID
781
sites that make the main contribution to the catalytic bent with acidic (electrophilic) characteristics is more
properties of this sample.
reactive with respect to sulfur compounds that are
present in petroleum owing to their high nucleophilic-
ity [12], and the addition of acids to the reaction mix-
ture promotes the deeper oxidation of organosulfur
compounds in homogeneous and biphasic systems
[51, 52]. The model of double activation of thiophene
on a catalyst containing a Brønsted ionic liquid and
ammonium tungstate was proposed in [53]; the aro-
maticity of the substrate was violated due to the forma-
tion of a hydrogen bond with an IL. In our study, no
organic products of thiophene oxidation were found
The isotherms of nitrogen adsorption on the cata-
lysts are shown in Fig. 4. The isotherms are of type IV
with an H1 hysteresis loop, which indicates a meso-
porous surface structure. In addition, a sharp kink at a
sufficiently high relative pressure (P/P0 > 0.6) indicates
capillary condensation in uniform cylindrical pores.
Catalytic Properties of the Composites
The catalytic properties of the samples were deter-
mined in the reactions of peroxide oxidation of thio-
phene and ODS of petroleum diesel provided by OAO
Var’erganneft’.
In Fig. 5 shows the results of testing catalysts W,
et-W and vin-W in the oxidation of a model mixture
containing 1 wt % thiophene, which is an organosulfur
compound that is most difficult to oxidize [2, 32]. An
obvious advantage can be seen in et-W, for which the
high conversion rate of the substrate correlates with
the number of strong Brønsted acid sites and the sta-
bility of heteropolyanions.
It is known from the literature about the effect of
the Brønsted acidity on the activity of heterogeneous
catalysts in the oxidation of organosulfur compounds;
in particular, neutralization of acidic protons gives rise
to a decrease in the rate of the oxidation reaction until
its complete cessation [49]. As was shown in [50], acid
groups can also serve as adsorption sites for thiophene
and its derivatives. In general, a catalyst or an adsor-
1
according to the H and 13C NMR spectroscopy
results and the data of chromatographic analysis. The
appearance of sulfate anions in the aqueous phase of
the reaction solution was established using the bar-
ium-sulfate method. Thus, the aromatic structure of
thiophene is destroyed as a result of the process with
2−
the formation of
and, possibly, CO2. It is well
SO4
known from the published data that the oxidation
mechanism involves peroxo complexes formed as a
result of the reaction of hydrogen peroxide with het-
eropolyanions [2, 16, 54]. During the ODS process,
nucleophilic attack by the organosulfur substrate on
the peroxo complex occurs (Scheme 3), giving rise to
the formation of unstable sulfoxide and thiophene sul-
fone, which are then converted into sulfate anions and
various oxygenates, such as styrene, benzoic acid, or
CO2 [33–35, 53].
S
O
H2O
W
W
O
H+
O
S
O
O
S
[O]
[O]
SO42− + CO2
H2O2
O
Scheme 3. Formation of peroxo complexes and their role in the mechanism of thiophene oxidation.
Despite the high activity of catalyst et-W (thio- another method for decreasing peroxide decomposi-
phene conversion 45%), the side reaction of hydrogen
peroxide decomposition under the action of metal-
containing catalysts is a factor that prevents the deep
oxidation of the substrate in its presence. The con-
sumption of hydrogen peroxide can be particularly
reduced by regulating the acidity of the reaction
medium through the number of Brønsted acid sites
tion [38, 39]; therefore, H2O2 was added in two por-
tions in this study. As seen from Fig. 6, this technique
increases the degree of conversion of thiophene,
which reaches 58% when the most active sample is
tested. In addition, it should be noted that the SILP
catalyst retains its activity for five operating cycles
[55]. The fractional loading of the oxidizing agent is (Fig. 6).
KINETICS AND CATALYSIS Vol. 61 No. 5 2020

782
Table 3. Acidity of catalysts and silica gel according to data of TPD NH3
Amount of desorbed ammonia, μmol NH3/g catalyst
BRYZHIN et al.
Sample
<190°C
190–250°C
>250°C
total
W
47
25
22
7
10
31
53
6
17
6
74
62
vin-W
et-W
79
7
154
20
Silica gel
1000
900
800
700
600
500
400
300
200
100
0
(а)
0
1
2
3
4
5
6
7
8
9
10
Е, keV
(b)
(c)
20 µm
20 µm
O K
S K
(d)
(e)
20 µm
20 µm
W K
P K
Fig. 3. (a) SEM–EDA spectrum and (b–e) distribution of elements on the surface of catalyst vin-W.
KINETICS AND CATALYSIS Vol. 61 No. 5
2020

HETEROGENEOUS CATALYSTS SILP WITH PHOSPHOTUNGSTIC ACID
783
50
(а)
500
450
400
350
300
250
200
150
100
50
et-W
vin-W
W
45
40
35
30
25
20
15
10
5
0
0.2
0.4
0.6
0.8
1.0
Relative pressure P/P0
(b)
0
1
2
3
4
200
180
160
140
120
100
80
Time, h
Fig. 5. Curves for the conversion of thiophene. Reaction
conditions: 4 h, 60°C, 0.1 g of catalyst, and one-shot addi-
tion of 0.4 mL of H O .
2
2
et-W
60
vin-W
40
20
60
40
20
0
0
0.2
0.4
0.6
0.8
1.0
Relative pressure P/P0
(c)
300
250
200
150
100
50
1
2
3
4
5
Cycle number
Fig. 6. Stability of the SILP catalysts in the oxidation of
thiophene upon fractional addition of hydrogen peroxide
(0.2 + 0.2 mL). Reaction conditions: 4 h, 60°C, and 0.1 g
of catalyst.
0
0.2
0.4
0.6
0.8
1.0
Relative pressure P/P0
et-W
vin-W
W
100
80
60
40
20
0
Fig. 4. Nitrogen adsorption–desorption isotherms for the
samples of (a) W, (b) et-W, and (c) vin-W.
The results of ODS of petroleum diesel (initial sul-
fur content 1080 ppm) are shown in Fig. 7. All catalysts
showed high efficiency when the fractional loading of
hydrogen peroxide was used; however, only SILP
samples et-W and vin-W made it possible to obtain
diesel fuel with a sulfur content of 3 and 7 ppm,
respectively. In comparison with a number of other
known methods for conducting ODS [3, 56–60], the
proposed method allows one to obtain diesel fuel that
meets international environmental standards (total
sulfur content less than 10 ppm).
0.8 mL H2O2
0.4 + 0.4 H2O2
Fig. 7. Oxidative desulfurization of diesel fuel (1080 ppm
sulfur) with different ways of hydrogen peroxide loading
(0.4 + 0.4 and 0.8 mL). Reaction conditions: 60°C, and
0.04 g of catalyst.
KINETICS AND CATALYSIS Vol. 61 No. 5 2020

784
BRYZHIN et al.
11. Gao, J., Zhu, S., Dai, Y., Xiong, C., Li, C., Yang, W.,
CONCLUSIONS
and Jiang, X., Fuel, 2018, vol. 233, p. 704.
https://doi.org/10.1016/j.Fuel,2018.06.101
A method is proposed for the synthesis of heteroge-
neous SILP catalysts based on ionic liquids protonated
with phosphotungstic heteropolyacid, namely, ionic
liquids 4-(3′-ethylimidazolium)-butanesulfonate and
4-(3′-vinylimidazolium)-butanesulfonate. It is found
that the use of protonated ILs in the active phase of the
catalyst increases the stability of PTA heteropolyan-
ions on the surface of silica gel. The most active cata-
lyst is et-W, which shows high stability in thiophene
oxidation in five repeated reaction cycles. The frac-
tional loading of the oxidant promotes an increase in
the degree of conversion of thiophene and ODS of
diesel fuel. Diesel fuel with a sulfur content of less than
10 ppm is obtained using SILP catalysts.
12. Abdul-Kadhim, W., Deraman, M.A., Abdullah, S.B.,
Tajuddin, S.N., Yusoff, M.M., Taufiq-Yap, Y.H., and
Rahim, M.H.A., J. Environ. Chem. Eng., 2017, vol. 5,
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https://doi.org/10.1016/j.Fuel,2018.01.050
14. Wei, S., He, H., Cheng, Y., Yang, C., Zeng, G., Kang, L.,
Qian, H., and Zhu, C., Fuel, 2017, vol. 200, p. 11.
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FUNDING
The work was supported by the Russian Foundation for
Basic Research, project no. 19-33-90024. Equipment pur-
chased under the University Development Program of the
Moscow State University was used in the study.
17. Kozhevnikov, I.V., Chem. Rev., 1998, vol. 98, p. 171.
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Translated by O. Kadkin
https://doi.org/10.1016/j.fuproc.2011.04.030
KINETICS AND CATALYSIS Vol. 61 No. 5 2020