Fe3O4?BNPs?SiO2-SO3H as a highly chemoselective heterogeneous magnetic nanocatalyst for the oxidation of sulfides to sulfoxides or sulfones

Article abstract of DOI:10.1039/c9ra06221a

To achieve green chemistry goals and also to reduce the cost of catalysts as well as to avoid producing toxic wastes and show the importance of separation and recycling of catalysts from the reaction medium, in this work, we describe the preparation and characterization of magnetic acidic boehmite nanoparticles as a heterogeneous catalyst, which is called Fe₃O₄?BNPs?SiO₂-SO₃H. This catalyst works efficiently in the selective oxidation of sulfides to sulfoxides or sulfones in the presence of H₂O₂ as a green oxidant. It can easily be separated from the reaction medium by using an external magnet and it was recycled 6 times without loss of magnetic catalytic properties.

Full text of DOI:10.1039/c9ra06221a

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Fe3O4@BNPs@SiO2–SO3H as a highly
chemoselective heterogeneous magnetic
nanocatalyst for the oxidation of sulfides to
sulfoxides or sulfones
Cite this: RSC Adv., 2019, 9, 36103
ab
Mohammad Ghanbari Kermanshahia and Kiumars Bahrami
*
To achieve green chemistry goals and also to reduce the cost of catalysts as well as to avoid producing toxic
wastes and show the importance of separation and recycling of catalysts from the reaction medium, in this
work, we describe the preparation and characterization of magnetic acidic boehmite nanoparticles as
a heterogeneous catalyst, which is called Fe3O4@BNPs@SiO2–SO3H. This catalyst works efficiently in the
selective oxidation of sulfides to sulfoxides or sulfones in the presence of H2O2 as a green oxidant. It can
easily be separated from the reaction medium by using an external magnet and it was recycled 6 times
without loss of magnetic catalytic properties.
Received 9th August 2019
Accepted 17th October 2019
DOI: 10.1039/c9ra06221a
rsc.li/rsc-advances
sites and increasing catalytic activity, it also causes poor
recovery and low reusability.
Introduction
Most of the time separation of homogeneous catalysts is
a problem, especially when toxic and noble metals are used. For
this reason, the use of heterogeneous catalysts is proposed as
a better route, because they can be easily separated from the
reaction medium.15 But, two issues of heterogeneous catalytic
systems include a loss of activity and their selectivity towards
homogeneous catalytic systems.16 However, the use of nano-
scale supports can be very benecial by increasing the active
sites of heterogeneous catalysts.17
Selective oxidation of suldes into sulfoxides or sulfones is one
of the most signicant and fascinating reactions in organic
chemistry1 because they are valuable synthetic intermediates for
the pharmaceutical and chemical industry.2 A literature survey
revealed that sulfoxide derivatives are reported to have an
important role in enzyme activation,3 are used as solvents,4 and
have a wide range of pharmacological activity.5 Also, sulfones
and sulfoxides have many applications in the elds of organic
synthesis,6 agrochemistry7 and medical syntheses.8
In the past few decades, a wide range of materials have been
used as catalytic supports. Graphene,18 carbon nanotubes,19
zeolite,20 MOF-5,21 and also some nanoparticles like TiO2 NPs22 are
included among these. However, in most cases, special conditions
are needed to produce these nanoparticles, such as high temper-
ature and inert gases. Boehmite nanoparticles are considered as
one of the most suitable, stable and recyclable materials due to
their high mechanical and thermal stability, availability, low cost
of raw materials, high surface area (>120 m2 gꢀ1), and green
properties.23,24 Many uses of boehmite have been reported in
recent years, some of which include use as an absorbent, ceramic
enhancer, catalyst, vaccine assistant, cosmetic product, and
infrequently a support for heterogeneous catalysts.25,26 Boehmite
nanoparticles can be easily synthesized in an aqueous medium
using NaOH and Al(NO3)3$9H2O at appropriate conditions and
room temperature.27 The aluminum oxide-hydroxide boehmite (g-
AlOOH) mineral is a component of aluminum ore bauxite which
has a surface full of hydroxyl groups.24,28
Recently, researchers have developed numerous techniques
for the oxidation of sulfoxides and sulfones.9 Among possible
oxidation pathways, the use of hydrogen peroxide for oxidation
of suldes is more preferable10 because it has several pros like
low-price, secure storage, being readily accessible, and soluble
in a large number of organic solvents as well as in water, and
producing water as a green by-product.11–14 In recent years,
many catalysts including homogeneous and heterogeneous
ones for oxidation of suldes have been announced. Although
these methods have a lot of potential, the reactions suffer from
low yields, long run times and poor catalyst recovery, and
consume too much oxidant in comparison with substrates.15
One of the features of homogeneous catalysts is that they can
be dissolved in the reaction medium. Although solubility in
homogeneous catalytic systems leads to enhancement of active
aNanoscience and Nanotechnology Research Center (NNRC), Razi University,
Kermanshah 67149-67346, Iran. E-mail: kbahrami2@hotmail.com; k.bahrami@
razi.ac.ir; Fax: +98 83 34274559
As a result of the disadvantages mentioned in previous
researches and in continuation to our reported works in this
eld29 and the use of heterogeneous nanocatalysts in organic
bDepartment of Organic Chemistry, Faculty of Chemistry, Razi University, Kermanshah
67149-67346, Iran
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reactions,30 in this study we introduced a novel type of acidic magnetic elds (Scheme 1). Preparation of Fe3O4@BNPs@SiO2–
catalyst based on a magnetic core–shell heterogeneous catalyst SO3H is shown in Scheme 2.
which is called Fe3O4@BNPs@SiO2–SO3H. These magnetic
catalysts not only show high efficiency and selectivity in oxida-
Experimental
tion of suldes but also can be separated and recovered effi-
ciently from the reaction medium by simply applying external
Materials were purchased from Merck and Fluka and were used
without any additional purication. All reactions were moni-
tored by TLC. Melting points were determined using a Stuart
Scientic SMP2 apparatus. FT-IR spectra were determined with
1
a PerkinElmer 683 instrument. H NMR and 13C NMR spectra
were recorded with a Bruker (300 MHz) spectrometer in CDCl3
as solvent. TGA was carried out with a STA PT-1000 Linseis
instrument (Germany) under air atmosphere at a heating rate of
10 C minꢀ1. FESEM and EDX measurements were performed
ꢁ
using a TESCAN- MIRA3 operated at 26 kV with an electron gun
lament of tungsten. TEM images were carried out with
a Zeiss-EM10C (Germany) operating at 100 kV with samples on
a formvar 300 mesh carbon-coated Cu grid. XRD patterns were
obtained using a STOE STADI-P diffractometer (Cu K-a1 radia-
Scheme 1 Oxidation of sulfides to sulfoxides and sulfones.
ꢀ
tion wavelength ¼ 1.54060 A).
Scheme 2 Preparation of Fe3O4@BNPs@SiO2–SO3H as a magnetic Fig.
1
FT-IR spectra of (a) boehmite nanoparticles (b) Fe3O4@-
acidic nanocatalyst.
BNPs@SiO2 and (c) Fe3O4@BNPs@SiO2–SO3H.
Scheme 3 Reagents and conditions: molar ratio of substrates to H2O2 (1 : 1 : 3) in the presence of 0.075 g of catalyst in EtOH at 25 ꢁC.
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Fig.
4
TGA-DTA curve of (a) Fe3O4@BNPs and (b) Fe3O4@-
BNPs@SiO2–OSO3H.
General procedure to produce sulfoxides
To a mixture of suldes (1 mmol) and Fe3O4@BNPs@SiO2–
SO3H (0.075 g, 0.7 mmol) in ethanol (2 mL) at 25 ꢁC was added
30% aqueous H2O2 (0.3 mL). The progress of the reaction was
monitored by TLC (EtOAc/n-hexane ¼ 2/8). Aer completion of
the reaction, the magnetic catalyst was separated by an external
magnet and washed with ethanol, then the product was
extracted with EtOAc (4 ꢂ 5 mL). The organic layer was dried
over anhydrous magnesium sulfate, and evaporated to afford
the sulfoxides in good yields (90–98%).
Fig. 2 FESEM images of (a) Fe3O4@BNPs and (b) Fe3O4@BNPs@SiO2–
SO3H.
General procedure to produce sulfones
Suldes (1 mmol) were added to a mixture of 30% H2O2 (0.4 mL)
and Fe3O4@BNPs@SiO2–SO3H (0.1 g, 0.1 mmol) in ethanol (2
mL) and the mixture was then stirred at 50 ꢁC for the time
specied. The progress of a reaction was monitored by TLC
(EtOAc/n-hexane ¼ 2/8). Aer completion of the reaction the
Fig. 3 TEM image of Fe3O4@BNPs@SiO2–SO3H.
Fig. 5 XRD patterns of (a) Fe3O4@BNPs and (b) Fe3O4@BNPs@SiO2–
SO3H.
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vigorous stirring. The resulting black mixture was continuously
stirred for 2 h at 90 ꢁC under N2 atmosphere. Aer that, the
mixture was allowed to cool at room temperature and then
Fe3O4 NPs was separated by using an external magnet and
washed several times with deionized water and kept in an oven
at 50 ꢁC for 24 h. In the next steps, Fe3O4 NPs (1 g) was dispersed
in a solution of Al(NO3)3$9H2O (1 g) in deionized water (25 mL);
then, a solution of 0.649 g of NaOH in 10 mL of distilled water
was added to the mixture drop by drop under vigorous stirring.
Aer that the mixture was sonicated in an ultrasonic bath for
3 h at 25 ꢁC. Finally, the produced Fe3O4@BNPs was separated
Fig. 6 EDX pattern of Fe3O4@BNPs@SiO2–SO3H.
catalyst was separated from the reaction mixture by an external
magnet and washed with ethanol. Next, the residue was
extracted with EtOAc (4 ꢂ 5 mL) and aer that the organic layer
was dried over anhydrous magnesium sulfate. Eventually, by
evaporation the organic solvent sulfones were obtained in
satisfactory yields (90–98%).
Preparation of Fe3O4@BNPs@SiO2–SO3H
First, a mixture of FeCl3$6H2O (1.55 g) and FeSO4$7H2O (1.05 g)
in deionized water (25 mL) was magnetically stirred for 3 min at
room temperature under N2 atmosphere. Aerwards, 25 mL
NaOH (5%) was added very slowly into the mixture under
Fig. 7 VSM analysis of (a) Fe3O4@BNPs (b) Fe3O4@BNPs@SiO2–SO3H. Fig. 8 Mapping pattern of Fe3O4@BNPs@SiO2–SO3H.
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with an external magnet and washed with ethanol three times high yields (90–98%) with shorter reaction times (5–45 min).
and dried in an oven at 60 C for 2 h.27,31
Also, as a result of our work this magnetic nanocatalyst can
ꢁ
Fe3O4@BNPs (0.5 g) was dispersed in water (5 mL) and easily be separated from the reaction medium in the shortest
ethanol (25 mL) in an ultrasonic bath for 30 min. Under time possible.
continuous stirring, PEG (2.68 g), ammonia solution (5 mL) and
The FT-IR technique was used to affirm the connection of
TEOS (1 mL) were respectively added into the suspension, and various functional groups on a catalyst's surface. As seen in the
continuously reacted for 38 h at room temperature. Then the IR spectrum of initial boehmite (Fig. 1a), the three frequencies
obtained Fe3O4@BNPs@SiO2 was separated by using an appearing in the 470.17, 604.71 and 801.24 cmꢀ1 regions are
external magnet and washed with ethanol and distilled water related to the Al–O stretching frequency. Also, OH/OH
and the obtained Fe3O4@BNPs@SiO2 was dried at room hydrogen bonds between layers of the boehmite nanoparticles
temperature.
at 1084.80 and 1165.41 cmꢀ1, and a nitrate impurity or bending
The obtained Fe3O4@BNPs@SiO2 (0.5 g) was dispersed in vibration of O–H at 1621.52 cmꢀ1 also appeared. In the Fe3-
dry CH2Cl2 (5 mL) by sonication for 30 min. Chlorosulfonic acid O4@BNPs@SiO2 IR spectrum (Fig. 1b), all of the stretching
(0.75 mL) was added dropwise over a period of 30 min and the vibrations of the boehmite nanoparticles are seen with a slight
mixture was stirred for 4 h at room temperature. The resulting displacement and the stretching frequency of Fe–O is seen at
Fe3O4@BNPs@SiO2–SO3H was separated by using an external 596.21 cmꢀ1. In the FT-IR spectrum of the nal catalyst,
magnet and washed several times with dry CH2Cl2 and meth- absorption bands due to hydroxyl groups at 3385.09 cmꢀ1, Al–O
anol and CH2Cl2 respectively to remove the unreacted materials. at 470.52 and 802.96 cmꢀ1, –Si–O at 949.28 cmꢀ1 and –nitrate
The nal product was dried at room temperature overnight impurities at 1621.52 cmꢀ1 were identied. Also the band at
(Scheme 3).
1096.38 cmꢀ1 corresponds to OH/OH hydrogen bonds
between layers of boehmite. And, the bands observed at
577.15 cmꢀ1 can be attributed to the Fe–O stretching frequency.
Aer sulfonation of the Fe3O4@BNPs, a broad adsorption bond
appeared in the 3200–3600 cmꢀ1 range (Fig. 1c).27
Results and discussion
Aer preparation of the catalyst, the Fe3O4@BNPs-SiO2–SO3H
was extensively studied with techniques such as Fourier trans-
form infrared spectroscopy (FT-IR), transmission electron
microscopy (TEM), eld emission scanning electron microscopy
(FESEM), thermogravimetric analysis (TGA), energy-dispersive
X-ray spectroscopy (EDX), X-ray diffraction spectroscopy (XRD),
vibrating-sample magnetometer (VSM) and pH analysis.
Synthetic steps for preparation of the catalyst are shown in
Scheme 2.
A. Ghorbani-Choghamarani and co-workers reported
boehmite silica sulfuric acid for oxidation of suldes to sulf-
oxides with H2O2 as an oxidant at room temperature.32 It was
found that in comparison with this same study, our studies led
to enhancement in the selectivity and caused a gain products in
FESEM was applied to study and conrm the structure and
morphology of the synthesized catalysts. As Fig. 2a shows, the
particles are nearly spherical with an average diameter of 30–
45 nm and uniformly scattered. Aer acidication of the Fe3-
O4@BNPs, the morphology of the particles was preserved
(Fig. 2b).
TEM was used to determine the exact size of the particles
which was 20–40 nm (Fig. 3). According to TEM images, BNPs
are shells, and Fe3O4 nanoparticles are cores.
TGA was used to study thermal behaviours of the nanocatalysts
in oxidative (air) media. TGA curves for BNPs@Fe3O4 and acidic
magnetic boehmite are shown in Fig. 4a and b. The TGA curve for
BNPs@Fe3O4 only showed about 5% weight loss which displays
Table 1 Effect of amount of H2O2, Fe3O4@BNPs@SiO2–SO3H and type of solvent on oxidation of sulfides
Yield (%)
Catalyst
(g)
Entry
30% H2O2 (mmol)
t (min)/T (ꢁC)
Solvent
Sulfoxides
Sulfones
1
2
3
4
5
6
7
8
2
2
2
2
3
4
3
3
3
3
3
3
3
4
4
—
12 h/RT
10/RT
10/RT
5/RT
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
Toluene
H2O
20
50
70
90
98
—
40
60
80
85
—
—
—
—
60
—
—
—
—
—
60
—
10
15
10
60
80
90
98
20
0.025
0.05
0.075
0.075
—
0.075
0.075
0.075
0.075
0.05
0.075
0.1
2/RT
24 h/50ꢁ
2/RT
2/RT
2/RT
2/RT
9
CH3CN
DMF
10
11
12
13
14
15
20/50ꢁ
15/50ꢁ
10/50ꢁ
10/50ꢁ
10/RT
EtOH
EtOH
EtOH
EtOH
EtOH
0.1
0.1
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Table 2 Oxidation of sulfides in presence of Fe3O4@BNPs-SiO2–SO3H with H2O2 in ethanol
Sulfoxidesa
Sulfonesb
Entry
1
Substrate
Time (min)
Yield (%)
94
MPref.
Time (min)
Yield (%)
92
MPref.
5
29–3038
10
10
83–8539
2
3
5
2
98
98
69–7138
98
94
125–12626
146–14841
120–12140
10
4
5
8
98
95
135–13638
15
20
96
95
149–15126
175–17743
10
13742
6
20
92
16242
45
96
203–20542
7
8
10
10
95
94
Oil44
25
30
90
96
25–2845
17146
212–21443
9
10
10
96
94
3047
20
20
96
92
44–461
10
Oil46
Oil1
11
12
13
15
20
10
92
92
92
151–15248
110–11143
Oil49
30
40
25
90
90
92
Oil1
110–11242
Oil50
14
10
94
20151
20
94
185–18742
Reaction conditions: sulde (1 mmol), H2O2 (0.3 mL), catalyst (0.075 g) at 25 ꢁC in EtOH. b Reaction conditions: sulde (1 mmol), H2O2 (0.4 mL),
a
catalyst (0.1 g) at 50 ꢁC in EtOH.
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thermal stability of the magnetic boehmite. In Fig. 4b and with 0.1 g of catalyst (98%) (Table 1, entry 14). Different
a gradual and subcutaneous weight loss (ꢃ13%) occurred solvents such as acetonitrile, toluene, water, ethanol and DMF
ꢁ
between 50 to 400 C, which is related to the removal of water, also were studied and the best results were obtained in ethanol
organic solvent, TEOS and acid groups from the catalyst's surface. (Table 1, entry 5).
ꢁ
A second weigh loss of ꢃ5% occurred from 400–600 C, and is
In addition, the next important factor that affected the yield
and reaction time, as well as reaction selectivity, was the
related to the boehmite phase transformation to g-alumina.33,34
To analyze the crystalline structure of the synthesized cata- amount of H2O2. The optimum amount of H2O2 was 3 mmol for
lysts, an XRD technique was used (Fig. 5). The XRD pattern also sulfoxide formation and 4 mmol for sulfone preparation (Table
contains peaks at 2q values of 30.62ꢁ (220), 35.67ꢁ (311), 54.42, 1, entries 5 and 14).
57.82ꢁ (511) and 63.42ꢁ (440), which agree with the cubic
In the following studies, the effect of temperature was
structure of Fe3O4 NPs35 (Fig. 5a). It is worth noting that the considered. It is interesting to note that by controlling the
structure of the catalyst was maintained aer modication of amount of H2O2, the product will be changed. It was observed
Fe3O4@BNPs with silica where some peaks at 2q ¼ 20–27 are that the sulfoxide yield was reduced at 50 ꢁC (Table 1, entry 6). It
related to the amorphous silica and peaks at 2q values of 31.82ꢁ should be noted that for the oxidation of sulde to sulfone, the
ꢁ
(110), 44.72ꢁ (131) and 55.25ꢁ (151) can verify the existence of best result was achieved at 50 C (Table 1, entry 14).
boehmite36 (Fig. 5b). An increase in peak intensities in the XRD
The generality and scope of this protocol was assessed for
pattern could be due to the increase in average of particle size the oxidation of various suldes with H2O2 in the presence of
from 16.56 nm in magnetic boehmite to 29.55 nm in the nal Fe3O4@BNPs@SiO2–SO3H and the results are listed in Table
catalyst (aer silica coating on the boehmite as well as added 2. As shown in Table 2, Fe3O4@BNPs@SiO2–SO3H effectively
sulfonate groups).37
catalyzed the oxidation reaction of suldes bearing both
Elemental analysis of the Fe3O4@BNPs@SiO2–SO3H was electron-donating or electron-withdrawing groups with H2O2
investigated by EDX where the presence of 1Fe, Si, C, O, Al and S (3 mmol) to afford desired products in excellent yields (92–
in the catalyst conrmed the successful synthesis of Fe3O4@- 98%) and short times (5–20 min) in EtOH. With allylic
BNPs@SiO2–SO3H (Fig. 6).
suldes only the sulfur atom was oxidized (Table 2, entries
VSM analysis showed superparamagnetic behavior of the 10, and 13). Furthermore, in the presence of alcohol,
prepared Fe3O4@BNPs and Fe3O4@BNPs@SiO2–SO3H in Fig. 7. carboxyl and ketone groups could not intervene with the
It can be seen that the saturation magnetization values of Fe3- production derived from oxidation of suldes (Table 2,
O4@BNPs and Fe3O4@BNPs@SiO2–SO3H were 42.52 and 18.21 entries 11, 12 and 14). As expected, oxidation of suldes to
emu g ꢀ1, respectively. Functionalization of the catalyst's sulfoxides required lower reaction times (Table 2). And as
surface reduces its magnetic properties, but the magnetic can be seen in Table 2, oxidation reactions of suldes to
properties of the nal catalyst are still signicant.
sulfones in the presence of 0.1 g of the catalyst required more
Another analysis used to conrm the existence and distri- H2O2 (4 mmol) and higher reaction times (10–40 min) in
ꢁ
bution of elements in the catalyst's structures was a mapping EtOH at 50 C.
technique (Fig. 8) and it displayed very well all of the elements
in the Fe3O4@BNPs@SiO2–SO3H structure.
Catalytic studies
To investigate effects of various variables on an oxidation
reaction, we selected as a model the reaction of benzyl phenyl
sulde with H2O2 in the presence of Fe3O4@BNPs@SiO2–
SO3H, an acidic heterogeneous catalyst in ethanol. For this
purpose, factors such as solvent, amount of catalyst, amount
of H2O2 and temperature were evaluated. At rst, the model
reaction was carried out in the absence of the catalyst, which
aer a long time formed only 20% of the sulfoxide (Table 1,
entry 1).
Aer that, we focused on the effects of different catalyst
amounts on the yields and the reaction time. Catalytic activity
of Fe3O4@BNPs@SiO2–SO3H in the oxidation of various
suldes was examined with oxidation of benzyl phenyl sulde
(1 mmol) using 30% H2O2 in the presence of 0.025, 0.05, 0.075
and 0.1 g of Fe3O4@BNPs@SiO2–SO3H in ethanol at room
temperature and under vigorous stirring until the reaction was
complete (as observed by TLC). The best yield for sulfoxide
(98%) was obtained in the presence of 0.075 g of catalyst, and
Scheme 4 Proposed mechanism for the oxidation of sulfides to
sulfoxides or sulfones in the presence of Fe3O4@BNPs@SiO2–SO3H
further oxidation of the sulfoxide to its sulfone was achieved with H2O2 as a green oxidant.
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Table 3 Comparison of present methodology with literature data for the synthesis of benzyl phenyl sulfoxide and sulfone
Entry
Product
Conditionsref.
Time (min)
Yield (%)
1
2
3
4
5
6
7
8
H2O2/Fe3O4@SiO2-FeQ3/H2O/rt52
SBA-15-SA/H2O/rt53
90
15
10
5
6 h
2
10 h
2 h
60
94
98
98
85
98
98
94
83
98
70
94
H2O2/SBA-15-Pr-SO3H/CH3CN/40 ꢁC54
H2O2/Fe3O4@SSA/H2O/rt55
H2O2/Fe3O4-Salen of Cu(II)/EtOH/60 ꢁC56
This work
H2O2/MWCNTs-COOH/solvent-free/rt57
H2O2/AGO/CH3CN/rt58
9
10
11
H2O2/Mo132-MimAM/H2O/rt59
H2O2/SAPTPA40/CH3CN/70 ꢁC60
This work
5.5 h
10
According to the results of our study, the reaction tolerates
sensitive functional groups such as alkenes, alcohols, and
pyridine, and only the sulfur atom is selectively oxidized
(Scheme 3). A detailed mechanism for the oxidation of
suldes has been proposed which includes the interaction
between the catalyst and hydrogen peroxide. The OH moiety
of the catalyst forms a hydrogen bond with hydrogen peroxide
which increases the electrophilic ability of a peroxy oxygen
atom of hydrogen peroxide and assists the leaving group
(H2O) in departing from the reaction intermediate through
Fig. 9 Reusability of Fe3O4@BNPs@SiO2–SO3H in oxidation of benzyl
phenyl sulfide to the corresponding sulfoxide.
It is important to note that the reaction time was higher in
the presence of suldes with electron-withdrawing groups
such as nitro (Table 2, entry 6). With great pleasure, we saw
tetrahydrothiophene and 2-(benzylthio)-1H-benzo[d]imid-
azole as cyclic aliphatic and heterocycle suldes produced the
corresponding products in excellent yields (Table 2, entries 7
and 8). We also accomplished several competitive reactions
to show the chemoselectivity of the presented method.
Fig. 11 FESEM images of (a) fresh Fe3O4@BNPs@SiO2–SO3H and (b)
Fe3O4@BNPs@SiO2–SO3H after reuse 6 times.
Fig. 10 FT-IR spectra of the catalyst after one run and after six runs.
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them for the oxidation of suldes to both sulfoxides and
sulfones in EtOH. Here, various suldes reacted with both
electron-donating and -withdrawing groups, but suldes react-
ing with withdrawing groups took longer times and gave
smaller yields. The above-mentioned nanocatalyst showed high
catalytic activity for the sulde oxidation. The advantages of this
protocol include low reaction times, high yields, the conve-
nience of separating the catalyst from the reaction media,
relatively inexpensive and available raw materials, the use of
a green solvent, high acidity and catalyst recovery for at least 6
reuses without a dramatic drop in catalytic properties.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
We are thankful to the Razi University Research Council for
partial support of this work.
Notes and references
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