A Mn(III) polyoxotungstate in the oxidation of organosulfur compounds by H2O2 at room temperature: An environmentally safe catalytic approach

Article abstract of DOI:10.1039/c5cy01564b

The tetrabutylammonium (TBA) salt of a Keggin-type polyoxometalate (POM), with the chemical formula TBA₄H₂[BW₁₁Mn(H₂O)O₃₉]·H₂O, TBABW₁₁Mn, was evaluated as a catalyst in the oxidation by hydrogen peroxide of several organosulfur compounds, namely benzothiophene (BT), 2-methylbenzothiophene (2-MBT), 3-methylbenzothiophene (3-MBT), dibenzothiophene (DBT), 4-methyldibenzothiophene (4-MDBT) and 4,6-diethyldibenzothiophene (4,6-DEDBT), in acetonitrile at room temperature. All compounds were oxidized to their corresponding sulfones with high conversion and selectivity. Following the excellent results achieved, the BW₁₁Mn/H₂O₂ in CH₃CN system was tested in the oxidation of model fuels (MFs) consisting of a mixture of BTs and DBTs in hexane (MF1 containing mainly BTs and MF2 containing predominantly DBTs). In both cases, the organosulfur compounds from the model fuels were fully converted into their corresponding sulfones. Envisaging the development of a promising desulfurization procedure, the extraction of sulfur compounds from MF2 was attempted after the catalytic oxidation process. More than 98 mol% was removed using an ethanol/H₂O mixture.

Full text of DOI:10.1039/c5cy01564b

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DOI: 10.1039/C5CY01564B
A Mn(III) Polyoxotungstate in the Oxidation of Organosulfur Compounds by H2O2 at
Room Temperature: An Environmentally Safe Catalytic Approach
a,
#
a,
a
a,
#
Tiago A.G. Duarte , Sónia M.G. Pires , Isabel C.M.S. Santos , Mário M.Q. Simões *,
M. Graça P.M.S. Nevesa,*, Ana M.V. Cavaleirob, José A.S. Cavaleiroa
aQOPNA, Departamento de Química, Universidade de Aveiro, 3810-193 Aveiro, Portugal
cCICECO, Departamento de Química, Universidade de Aveiro, 3810-193 Aveiro, Portugal
#these two authors contributed equally to this work
*Corresponding author: Mário M.Q. Simões
Tel.: +351 234 370 713; Fax: +351 234 370 084; Eꢀmail address: msimoes@ua.pt
*Corresponding author: M. Graça P.M.S. Neves
Tel.: +351 234 370 710; Fax: +351 234 370 084; Eꢀmail address: gneves@ua.pt
Abstract
The tetrabutylammonium (TBA) salt of a Kegginꢀtype polyoxometalate (POM), with the
chemical formula TBA4H2[BW11Mn(H2O)O39]ꢁH2O, TBABW11Mn, was evaluated as catalyst
in the oxidation by hydrogen peroxide of several organosulfur compounds, namely
benzothiophene (BT), 2ꢀmethylbenzothiophene (2ꢀMBT), 3ꢀmethylbenzothiophene (3ꢀMBT),
dibenzothiophene
(DBT),
4ꢀmethyldibenzothiophene
(4ꢀMDBT),
and
4,6ꢀ
diethyldibenzothiophene (4,6ꢀDEDBT), in acetonitrile at room temperature. All compounds
were oxidized to the corresponding sulfones with high conversion and selectivity. Following
the excellent results achieved, the BW11Mn/H2O2 in CH3CN system was tested in the
oxidation of model fuels (MF) consisting of a mixture of BTs and DBTs in hexane (MF1
containing mainly BTs and MF2 containing predominantly DBTs). In both cases the
organosulfur compounds from the model fuels were fully converted into the corresponding
sulfones. Envisaging developing a promising desulfurization procedure, the extraction of the
sulfur compounds from MF2 was attempted after the catalytic oxidation process. More than
98 mol % was removed using an ethanol/H2O mixture.
1

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1. Introduction
The search for new and efficient catalytic systems for the oxidation of organosulfur
compounds into the corresponding sulfoxides and sulfones is a research area of great
relevance nowadays. Firstly, sulfoxides and sulfones, besides being important intermediates
in organic synthesis, are also known biological active molecules.1ꢀ5 Secondly, the oxidative
desulfurization (ODS) of fuels constitutes a process with very high potential, in terms of
efficiency and environmental sustainability, thus gathering the interest of the scientific
community.6ꢀ12
Polyoxometalates (POMs) are a class of metalꢀoxygen cluster anions that have been
applied in several fields, namely catalysis, medicine and materials science.13ꢀ19 This derives
from the high diversity of the known structures, and the large range of physical and chemical
properties that make them fit for a wide domain of applications.20ꢀ23 One of the most
important fields in which these metalꢀoxygen cluster anions have been applied is oxidative
catalysis.13,14,24ꢀ27 POMs evidence an all set of important properties, like strong Brønsted
acidity, good redox properties under mild conditions, tunable solubility in different solvents
(according to the counterꢀion) and, not less important, high thermal and oxidative stability,
which make them very interesting catalysts. The soꢀcalled Kegginꢀtype polyoxometalates,
namely [XM12O40]n− (M = Mo, W; X = P, Si, B, etc.), the lacunary [XM11O39]n−, and other
related species, are among the most studied in catalysis. In particular, in the case of the
transition metal monosubstituted Kegginꢀtype polyoxometalates [XM11M’(H2O)O39]n− (M’ =
transition metal), their success as catalysts is associated also with the easy tuning of structure
and properties, as the POM properties can be modulated by the introduction of diverse M’ and
varying the X atoms. In our previous work, the transition metal monosubstituted Kegginꢀtype
polyoxometalates were used successfully as catalysts in the oxidation of diverse types of
organic compounds with H2O2.28ꢀ36
Several recent reviews mentioned the role of polyoxometalates in ODS related
processes.7,9,14,18,19 Among others, the catalytic efficiency of the lacunary and transition
metalꢀsubstituted Kegginꢀtype polyoxotungstates in the oxidation of Sꢀcompounds has
already been demonstrated in several reports.27,37ꢀ46 Our previous experience with the use of
the undecatungstoboromanganate(III) anion, [BW11Mn(H2O)O39]6– (BW11Mn), in oxidative
catalysis has shown that it is an efficient and robust catalyst for the oxidation with H2O2 of
several types of substrates,28,31,47ꢀ49 suggesting that it could be a good candidate to be used for
the oxidation of organosulfur compounds. Moreover, our research group has developed a
2

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catalytic oxidation system of organosulfur derivatives under mild conditions using
metalloporphyrin complexes.50ꢀ52 Thus, based on our experience on the catalytic oxidation of
recalcitrant organosulfur compounds and on the catalytic applications of POMs, and taking
advantage of the fact that manganese POMs have been poorly studied in this context,45 the
oxidation, by the environmentally safe oxidant H2O2, of several benzothiophenes and
dibenzothiophenes (Figure 1), namely benzothiophene 1 (BT), 3ꢀmethylbenzothiophene 2 (3ꢀ
MBT), 2ꢀmethylbenzothiophene
methyldibenzothiophene 5 (4ꢀMDBT), and 4,6ꢀdiethyldibenzothiophene 6 (4,6ꢀDEDBT), is
studied in the presence of the tetrabutylammonium (TBA) salt
3
(2ꢀMBT), dibenzothiophene
4
(DBT), 4ꢀ
TBA4H2[BW11Mn(H2O)O39]ꢁH2O (TBABW11Mn). Furthermore, the potential of this
BW11Mn/H2O2 in CH3CN catalytic system for ODS is demonstrated using two distinct model
fuels (MF), mixtures of four BTs and DBTs in hexane. Finally, in order to evaluate the
potential of this desulfurization procedure, the ultimate removal of the sulfones is also
assessed under green and sustainable conditions.
R2
R2
R2
[O]
[O]
R1
R1
R1
S
S
S
O
O
O
1 R1=H, R2=H
1a R1=H, R2=H
1b R1=H, R2=H
2 R1=H, R2=CH3
3 R1=CH3, R2=H
2a R1=H, R2=CH3
3a R1=CH3, R2=H
2b R1=H, R2=CH3
3b R1=CH3, R2=H
[O]
[O]
S
S
S
R2
R2
R2
R1
R1
R1
O
O
O
4 R1=H, R2=H
4a R1=H, R2=H
4b R1=H, R2=H
5 R1=CH3, R2=H
6 R1=R2=CH2CH3
5a R1=CH3, R2=H
6a R1=R2=CH2CH3
5b R1=CH3, R2=H
6b R1=R2=CH2CH3
Figure 1. Benzothiophenes and dibenzothiophenes studied and the corresponding oxidation products.
2. Experimental Section
2.1. Reagents and synthetic procedures
Acetonitrile (Panreac), 30% (w/w) aqueous hydrogen peroxide (SigmaꢀAldrich) and all other
reagents and solvents obtained from commercial sources were used as received or distilled
and dried using standard procedures. TBA4H2[BW11Mn(H2O)O39]ꢁH2O was prepared and
characterized according to previously described procedures.48 The obtained compound was
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characterized by elemental analysis (C, H, N, W, Mn), thermogravimetry and infrared
spectroscopy and the results were in agreement with those previously published.47,48
2.2. Typical procedure for the oxidation reactions
The TBABW11Mn catalyzed oxidation reactions were carried out as follows: the substrate
(0.3 mmol); the catalyst, corresponding to the appropriate substrate/catalyst (S/C) molar ratio
150 or 300; the required amount of 30% (w/w) aqueous H2O2 (61 ꢀL, 0.6 mmol for a
H2O2/substrate (O/S) molar ratio equal to 2; 122 ꢀL, 1.2 mmol for a H2O2/substrate (O/S)
molar ratio equal to 4; 183ꢀL, 1.8 mmol for a H2O2/substrate (O/S) molar ratio equal to 6);
2.0 mL of acetonitrile and the internal standard (chlorobenzene, 0.3 mmol) were stirred in a
close vessel at room temperature (22ꢀ24ºC) for 3 hours in the absence of light. To monitor the
sulfoxidation reactions, small aliquots from the reaction mixture were directly injected into
the GCꢀFID apparatus at regular intervals. The reactions were stopped after 3 h of reaction, or
when a conversion plateau was reached. Blank reactions were performed for all the substrates,
confirming that almost no oxidation occurs in the absence of catalyst (conversion below 5%
for all the substrates, except for DBT that gives 12% of conversion) after 3 hours.
Experiments done to mimic the oxidation of model fuels (MF) were performed as follows:
four substrates (0.03 mmol of each organosulfur compound, a total of 0.12 mmol) and the
catalyst were dissolved in 500 ꢂL of CH3CN, and the total reaction volume was completed
with 1.5 mL of hexane. Addition of H2O2 and the completion of the reaction were done as
indicated above. Experiments designated as MF1 contain substrates 1-4 and MF2 contain
substrates 3-6, both MF with 3.8x103 ppm of sulfur.
2.3. Instruments and Methods
The GCꢀFID and GCꢀMS analyses were performed as previously described.51,52 The
conversion and selectivity values were estimated from the corresponding chromatographic
peak areas, using chlorobenzene as internal standard. To confirm the structure of the achieved
products, the retention times in the GCꢀFID were compared with those of authentic samples
identified previously,51,52 and the reaction mixtures were also injected in the GCꢀMS.
3. Results and Discussion
3.1. Oxidation of the organosulfur compounds in acetonitrile
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The catalytic performance of the tetrabutylammonium salt of
a
Kegginꢀtype
undecatungstoboromanganate(III) anion, TBA4H2[BW11Mn(H2O)O39]ꢁH2O, was evaluated in
the sulfoxidation of recalcitrant benzothiophenes and dibenzothiophenes by H2O2 under
homogeneous conditions at room temperature. The catalytic tests were performed with
substrates 1ꢀ6 (Figure 1) and the results obtained after 3h (except when indicated) are
summarized on Table 1. For all the substrates, only the corresponding sulfones are observed
at the end of the reactions. Sulfoxides could only be detected in some cases at the beginning
of the reactions (up to 30 min). From the results obtained, it is possible to conclude that good
to excellent conversions, ranging from 90 to 100%, were attained with most of the substrates
for both S/C molar ratios tested (150 and 300). As expected, the use of a higher amount of
TBABW11Mn catalyst (S/C molar ratio = 150) affords the best results for each substrate. In
particular, the full oxidation of DBT 4 and 4ꢀMDBT 5 are attained after 60 min for S/C = 150.
Furthermore, for BT 1 and 4,6ꢀDEDBT 6, a significant difference between the two S/C ratios
was observed. It must be emphasized that this is a rare example of a Kegginꢀtype
polyoxotungstate catalyst efficiently used at room temperature in a sulfoxidation process of
recalcitrant sulfur compounds. Usually the use of these POMs as catalysts in sulfoxidation
processes requires temperatures around 50ꢀ80 ºC, or even higher. Mizuno and collaborators
have obtained quite good results in the homogeneous oxidation of various kinds of sulfides at
room temperature with H2O2 in the presence of [γꢀPW10O38V2(ꢀꢀOH)2]3−, but the
sulfoxidation of dibenzothiophene was only obtained at 50 ºC.41 A similar situation was
reported quite recently, with hybrid composite materials based on the parent Keggin anions as
catalysts.53 Also, the monoꢀlacunary polyoxotungstate [PW11O39]7ꢀ showed high catalytic
activity on the oxidation of BT in a emulsion at 30 ºC, but the corresponding transition metal
[PW11MO39]n–anions (M = Ti, Mn, Fe, Co, Ni, Cu) had much lower activity.45 Thus, the
efficiency demonstrated here by BW11Mn at room temperature is quite remarkable.
Noteworthy, BW11Mn is especially efficient with the recalcitrant DBTs 4 and 5. In fact, this
constitutes a very promising result, since the most difficult or the most recalcitrant
organosulfur compounds to treat through an ODS procedure are DBTs. Besides, all the
previous studies indicate that the catalyst is stable, either under similar conditions or under
more drastic conditions than that used in the present work (e. g., H2O2/substrate molar ratio up
to 7, at 80 ºC).48
Figure 2 illustrates the kinetic profile of the reaction for each substrate in the catalytic
assays with BW11Mn for S/C molar ratio of 300. This puts in evidence the higher efficiency
of BW11Mn for the oxidation of DBTs. Considering the results at 30 or 60 min of reaction,
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the higher conversion values were attained for DBTs 4, 5 and 6, decreasing in this order,
whereas the worst result was for the unsubstituted BT 1.
Table 1. Results obtained for the oxidation of substrates (1-6) with H2O2 catalyzed by the manganese
polyoxometalate (BW11Mn).a
S/C
molar ratio
Conversion
(%)b
Time
(min)
Substrate
Product
300
47
180
1
(BT)
150
300
97
90
180
180
(1b)
2
(3-MBT)
150
100
180
(2b)
(3b)
(4b)
(5b)
(6b)
300
150
300
93
180
180
120
3
(2-MBT)
100
100
4
(DBT)
150
300
150
300
150
100
98
60
180
60
5
(4-MDBT)
100
72
180
180
6
(4,6-DEDBT)
98
aReaction conditions: the substrate (0.3 mmol), the catalyst corresponding to a substrate/catalyst (S/C) molar
ratio of 150 and 300, the internal standard (chlorobenzene, 0.3 mmol) and the 30% (w/w) H2O2 (6 equiv.) were
stirred in 2.0 mL of CH3CN at room temperature. bConversion values determined by GCꢀFID.
This shows that the results may only be partially related to the increase of the electron
density at the S atom, which varies in the order 3ꢀMBT < BT < 2ꢀMBT < DBT < 4ꢀ
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MDBT.6,54,55 Yet, the distinction between BTs and DBTs is clear. In the case of
benzothiophenes the presence of the methyl substituents seems to have a positive effect in the
course of the sulfoxidation reactions, independently of its position (2 and 3 are easily oxidized
if compared with 1). Moreover, when the kinetic profile of the oxidation of BT (1) is
compared for different O/S molar ratios (Figure 3), the 6:1 molar ratio gives rise to higher
conversion than 4:1 and 2:1 after 180 min of reaction. Based on this results for the most
recalcitrant substrate studied here, the O/S 6:1 molar ratio was used for all the organosulfur
compounds (1-6). This slight excess is not unusual in the oxidation of this kind of substrates
by hydrogen peroxide and catalyzed by POMs, since O/S molar ratios of 50:1,56 23:1,57
12:1,58,59 11:1,60 10:1,61 8:1,62,63 or 6:1,53 can be found in the very recent literature.
In the case of DBTs the steric hindrance appears as the main factor affecting the catalytic
results. Despite the differences in conversion at the beginning of the reaction, the presence of
a methyl substituent does not affect significantly the results attained after 180 min for 5 if
compared with 4; in the case of 6, possessing two ethyl substituents, a significant lower
conversion after 3 hours of reaction is detected, when a S/C molar ratio of 300 is used.
100
90
80
70
60
50
BT (1)
40
3ꢀMBT (2)
30
2ꢀMBT (3)
20
10
0
DBT (4)
4ꢀMDBT (5)
4,6ꢀDEDBT (6)
0
15
30
45
60
75
90
105 120 135 150 165 180
Time (min.)
Figure 2. Kinetic profile (conversion vs time) of the oxidation reactions with H2O2 for all the organosulfur
compounds tested (1-6) in the presence of BW11Mn (S/C molar ratio of 300) at room temperature in CH3CN.
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100
90
80
70
60
50
40
30
20
10
0
O/S (6:1)
O/S (4:1)
O/S (2:1)
0
30
60
90
120
150
180
Time (min)
Figure 3. Kinetic profile (conversion vs time) of the oxidation of BT (1) with different H2O2/substrate (O/S)
molar ratios in the presence of BW11Mn (S/C molar ratio of 150) at room temperature in CH3CN.
Recently, our group has demonstrated the efficiency of Mn(III) porphyrin complexes in the
oxidation of this type of substrates by hydrogen peroxide.52 The experiments using the
Mn(III) porphyrins and the BW11Mn anion were carried out using similar reaction conditions.
Thus, in both cases the reaction solvent was CH3CN, the oxidant was aqueous H2O2 and the
assays were carried out at room temperature (22ꢀ24 ºC). It is important to refer that for
Mn(III) porphyrins the oxidant is previously diluted and added to the reaction in small
aliquots (half of the substrate amount at each 15 min) to protect the Mn(III) porphyrin catalyst
from the oxidation conditions. For the studies employing BW11Mn, and due to its higher
chemical resistance, 30% (w/w) hydrogen peroxide is used directly and added all at once at
the beginning of the reactions. This may explain differences in the observation of intermediate
sulfoxides in the course of reactions with porphyrins when compared with this study.
Comparatively, Mn(III) porphyrins proved to be more efficient for BTs sulfoxidation by
H2O2, whereas the MnPOM evidences better performance for DBTs.
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Scheme 1
Based on our previous studies with the BW11Mn/H2O2 system in the oxidation of
anthracene to 9,10ꢀanthraquinone31 and on what is known for oxidations with
polyoxotungstates,14 we suggest a possible mechanism for the oxidation of the studied sulfur
compounds, translated by equations (1)ꢀ(3), Scheme 1. Equation (1) represents the formation
of a complex, by the reaction of hydrogen peroxide with the POM, whereas equations (2) and
(3) correspond to the successive sulfoxidation reactions of a generic substrate XS. The exact
type of complex formed is unknown (in scheme 1 is shown the simplest possibility) but, as in
the case of the oxidation of anthracene31 it seems to be prone to a nucleophilic attack, in this
case by the S atoms of BTs and DBTs. The possible formation of a peroxocomplex through
coordination at a W atom (suggested in many other cases26,31) cannot be ruled out, but, as the
reactions took place at room temperature, its formation seems less plausible (in view of the W
coordinative saturation, it needs the breaking of, at least, one POM W–O bond). It has been
shown that the oxidation of BW11Mn(III) favors the formation of a Mn(IV)ꢀOH derivative.64
For this reason we have not considered a Mn(V)=O containing species, even if they have been
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identified in the oxidation of other POMs in different conditions.65,66 Also, the reactivity of
Mn and other metal substituted POMs often seems to be more in accordance with a peroxo or
hydroperoxo intermediate than with an oxo complex.14,24,67 Peroxo or hydroperoxo complexes
with d0 metals are electrophilic in nature, contrarily to complexes with d8 metals, which are
nucleophilic.68 BW11Mn(III), with a d4 configuration, is possibly intermediate in character,
and it is reasonable to consider it liable to a nucleophilic attack. The presence of protons in
the catalyst chemical formula (protonation) may increase its nucleophilic character, as
reported for Ti(IV) POMs.14
3.2. Oxidation of the organosulfur compounds and desulfurization of the model fuels
Based on the promising results achieved for the recalcitrant organosulfur compounds
described above, the catalytic performance of BW11Mn was evaluated for the oxidation of
two model fuels (MF), consisting of a mixture of substrates 1-4 (MF1) and 3-6 (MF2), in
hexane. The reactions occurred in homogeneous conditions in a CH3CN/hexane (1:3)
medium. The solvent mixtures are very common in this type of studies, as can be profusely
found in the literature.38,40,57ꢀ59,61ꢀ63,69ꢀ71
9.50
5.90
6.65 6.74
(A)
(B)
(C)
(D)
11.37
5.90
9.40
9.40
8.78
8.54
6.74
6.74
6.65
6.65
11.37
11.37
8.78
8.78
8.54
8.54
5.90
5.90
9.40
Figure 4. Typical GCꢀFID chromatograms illustrating MF1 oxidation reaction profile with H2O2 in the presence
of BW11Mn for a S/C molar ratio of 150. (A) Initial reaction mixture before the addition of H2O2; (B) After 60
min of reaction; (C) After 120 min of reaction; (D) After 180 min of reaction. The model fuel MF1 is a solution
of 1 (Rt = 5.90 min), 2 (Rt = 6.65 min), 3 (Rt = 6.74 min), and 4 (Rt = 9.50 min) in hexane (0.03 mmol each).
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Figure 4 illustrates the GCꢀFID monitoring profile for the oxidation of MF1 by H2O2 in the
presence of BW11Mn, after 60, 120 and 180 min of reaction. The disappearance along the
time of the four original signals (corresponding to the initial compounds) was accompanied
by the appearance of four new peaks, indicating that the only products were the sulfones 1b
(Rt = 8.54 min), 2b (Rt = 8.78 min), 3b (Rt = 9.40 min), and 4b (Rt = 11.37 min).52 Only a
minor amount of BT 1 (Rt = 5.90 min) remains in solution after 180 min of reaction. More
than 99% of the starting MF1 is fully converted into the corresponding oxidized products and,
in accordance to the aboveꢀmentioned results, the BW11Mn catalyst seems to be more
efficient for the DBTs oxidation. In fact, after 60 min of reaction, the DBT 4 (Rt= 9.50 min) is
hardly observed in the corresponding chromatogram (Figure 4B), whereas all the BTs are still
present. The different rates of sulfone formation follow closely the described in 3.1.
The course of the oxidation of model fuel MF2, composed by three DBTs (4, 5 and 6) and
one BT (3), is illustrated on Figure 5. The chromatogram registered before H2O2 addition
shows four peaks that correspond to the initial compounds (Figure 5A), and the
chromatogram obtained after 3 hours of reaction (Figure 5B) shows four different peaks that
match the corresponding sulfones.52 Due to the partial overlap of some of the starting peaks
with those corresponding to the oxidized products, the chromatograms on Figure 5 may not
demonstrate clearly the total conversion of the starting substrates in MF2. Therefore, the
unambiguous identification of each peak was performed by GCꢀMS: the peak at 9.44 min
+
ꢀ
corresponds to the sulfone 3b (M = 180); the peak at 11.39 was identified as the product 4b
+
+
ꢀ
ꢀ
(M = 216); finally the peaks at 11.65 and 12.68 were assigned to the sulfones 5b (M = 230)
+
ꢀ
and 6b (M = 272), respectively.
The oxidation of hexane did not take place in the model fuels. It is important to note that
polyoxometalates may be good catalysts for the oxidation of several types of organic
compounds, including hydrocarbons.14,26 In particular, the BW11Mn/H2O2 system in
acetonitrile has been used in the oxidation of cycloalkanes,49 cisꢀcyclooctene,47 anthracene,31
styrene,28 and indene,36 among others. However, none of these reactions occurs at room
temperature. This point may need consideration in studies with real fuels at higher
temperatures.
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6.74
(A)
(B)
(C)
11.39
12.68
Figure 5. Typical GCꢀFID chromatograms illustrating MF2 oxidation reaction profile with H2O2 in the presence
of BW11Mn for a S/C molar ratio of 150. (A) Initial reaction mixture before the addition of H2O2; (B) After 180
min of reaction; (C) After the extraction procedure. The model fuel MF2 is a solution of 3 (Rt = 6.74 min), 4 (Rt
= 9.50 min), 5 (Rt = 10.03 min), and 6 (Rt = 11.31 min) in hexane (0.03 mmol each).
Liquid phase oxidative desulfurization is basically a two stage process, in which sulfur
compounds are firstly oxidized to their corresponding sulfones, and subsequently removed
from the solution by extraction or other separation method.9,12 Proven the high efficiency of
the use of BW11Mn in the oxidation methodology, the next step studied was the extraction of
the oxidized products, in order to develop a promising desulfurization procedure. This was
performed with model fuel MF2. After a few attempts using different extraction solvents,
namely methanol, acetone, ethyl acetate, ethanol, water, and ethanol/H2O (2:1) or (1:1)
mixtures, the successful elimination of Sꢀcompounds from the hexane solution was achieved
using a mixture of ethanol/H2O (1:1).
In a typical procedure, 2 mL of the ethanol/H2O (1:1) mixture are added at the end of the
catalytic step to the reaction mixture, the system is stirred for 10 min and left to rest for
another 10 min. At the end, it is possible to observe that the addition of ethanol/H2O causes a
phase separation with the concomitant precipitation of sulfones in the interface of the two
phases (Figure 6). The GCꢀFID analysis of the hexane fraction after the described procedure
clearly put in evidence the success of the separation methodology (Figure 5C), since more
than 98 mol% of the organic compounds were removed. This process, besides being efficient
12

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DOI: 10.1039/C5CY01564B
and simple, using only ethanol and water as solvents, respects the environmental and safety
issues required to ODS.
hexane
hexane
ethanol/H2O
Figure 6. Reactors used during the desulfurization procedure of MF2, before extraction (left) and after
extraction (right)
4. Conclusions
In conclusion, with the catalytic experiments involving TBA4H2[BW11Mn(H2O)O39]ꢁH2O, the
BW11Mn/H2O2 in CH3CN catalytic system, a new environmentally friendly and efficient
oxidation procedure for BTs (1-3) and DBTs (4-6) at room temperature is disclosed. As far as
we know, this is the first example of a POM as catalyst operating at room temperature (22ꢀ
24ºC) with such efficiency in the oxidation of recalcitrant Sꢀcompounds. Under very mild
conditions, excellent organosulfur compounds conversion values were obtained (97ꢀ100% for
S/C molar ratio of 150). The high potential of BW11Mn for application in ODS processes was
verified through the results achieved in the oxidation of two model fuels by H2O2: one mainly
composed by BTs (1ꢀ4) and the other mainly by DBTs (3ꢀ6), both with conversions close to
100%, attained after 3 hours of reaction. Envisaging the simulation of the entire ODS
procedure, a simple, efficient and environmentally sustainable removal approach of the
sulfones from model oil (MF2) was also developed, using an ethanol/H2O (1:1) mixture to
successfully remove the sulfones from hexane. The GCꢀFID analysis after 20 min of
extraction showed that more than 98 mol% of the organosulfur compounds were removed
from the organic layer, corresponding to a decrease from 3.8x103 to 76 ppm of sulfur. In a
near future we intend to develop our studies by using POMꢀbased heterogeneous catalysts in
the oxidation of this kind of recalcitrant compounds, also in real fuel samples.
13

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DOI: 10.1039/C5CY01564B
Acknowledgements
Thanks are due to FCT/MEC for the financial support to QOPNA (FCT
UID/QUI/00062/2013) and CICECO (FCT UID/CTM/50011/2013), through national founds
and where applicable coꢀfinanced by the FEDER, within the PT2020 Partnership Agreement.
S.M.G. Pires thanks FCT for her PhD Grant (SFRH/BD/64354/2009).
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A manganese monosubstituted Keggin-type polyoxometalate as catalyst in the oxidation
of recalcitrant organosulfur compounds by hydrogen peroxide at room temperature.