Selective catalytic oxidation of sulfides to sulfoxides or sulfones over amorphous Nb2O5/AC catalysts in aqueous phase at room temperature

Article abstract of DOI:10.1016/j.catcom.2019.04.013

Selective oxidation of sulfides into sulfoxides or sulfones is an important transformation in organic synthesis. Activated carbon supported Nb-based catalysts were prepared by the impregnation method and then characterized by the XRD, TEM and XPS. The characterization results indicate Nb species is amorphous Nb₂O₅. The catalysts were applied for the catalytic oxidation of sulfides in aqueous phase with H₂O₂ as the oxidant. Sulfides could be converted into the corresponding sulfoxides or sulfones with high conversion and selectivity at room temperature. The recycling experiments showed the Nb₂O₅/AC catalysts exhibited good reusability. They could be recycled for 10 times without obvious loss of activity and selectivity.

Full text of DOI:10.1016/j.catcom.2019.04.013

Accepted Manuscript
Selective catalytic oxidation of sulfides to sulfoxides or sulfones
over amorphous Nb2O5/AC catalysts in aqueous phase at room
temperature
Junjie Zhang, Tingting Jiang, Yuliang Mai, Xi Wang, Jiazhi Chen,
Bing Liao
PII:
S1566-7367(19)30127-X
DOI:
https://doi.org/10.1016/j.catcom.2019.04.013
CATCOM 5681
Reference:
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
23 January 2019
2 April 2019
20 April 2019
Please cite this article as: J. Zhang, T. Jiang, Y. Mai, et al., Selective catalytic oxidation
of sulfides to sulfoxides or sulfones over amorphous Nb2O5/AC catalysts in aqueous
phase at room temperature, Catalysis Communications, https://doi.org/10.1016/
j.catcom.2019.04.013
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ACCEPTED MANUSCRIPT
Selective catalytic oxidation of sulfides to sulfoxides or sulfones over amorphous Nb O /AC
2
5
catalysts in aqueous phase at room temperature
*
Junjie Zhang zjjzhang@mail.ustc.edu.cn, Tingting Jiang, Yuliang Mai, Xi Wang, Jiazhi Chen,
Bing Liao
Guangdong Provincial Key Laboratory of Industrial Surfactant, Guangdong Research Institute of
Petrochemical and Fine Chemical Engineering, Guangdong Academy of Sciences, Guangzhou
510665, People’s Republic of China
*
Corresponding author.

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Abstract
Selective oxidation of sulfides into sulfoxides or sulfones is an important transformation in
organic synthesis. Activated carbon supported Nb-based catalysts were prepared by the
impregnation method and then characterized by the XRD, TEM and XPS. The characterization
results indicate Nb species is amorphous Nb O . The catalysts were applied for the catalytic
2
5
oxidation of sulfides in aqueous phase with H O as the oxidant. Sulfides could be converted into
2
2
the corresponding sulfoxides or sulfones with high conversion and selectivity at room temperature.
The recycling experiments showed the Nb O /AC catalysts exhibited good reusability. They could
2
5
be recycled for 10 times without obvious loss of activity and selectivity.
Keywords: Catalytic oxidation; Sulfide; Nb O /AC; Aqueous phase; H O
2
2
5
2

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1. Introduction
Sulfoxides and sulfones are important intermediates for the construction of high value-added
fine chemicals and biologically active molecules such as pharmaceuticals, agrochemicals and
chiral auxiliaries[1]. Thus the selective oxidation of sulfides to sulfoxides or sulfones is one of the
important transformations in organic synthesis and has attracted much attention[2].
Various oxidants, such as KMnO [3], NaClO[4], sodium metaperiodate[5], bromine[6], nitric
4
acid[7] and m-chloroperbenzoic acid[8], have been used for the oxidation of sulfides. But these
processes involve non-atom-economic and expensive stoichiometric reagents, generating a large
of wastes. Aqueous hydrogen peroxide (H O ) is a low-cost and environment-friendly oxidant, for
2
2
the by-product is only water. Thus many procedures with H O as the oxidant have been
2
2
developed for the oxidation of sulfides[9]. For the H O mediated catalytic oxidation of sulfides, a
2
2
number of solvents, such as methanol[10], ethanol[11], dichloromethane[12], acetone[13],
acetonitrile[14] and water[15], have been applied. Among the above-mentioned solvents, water is
environment-friendly solvent. Considering the environmental concerns, the development of
methods for the catalytic oxidation of sulfides in aqueous phase with H O as the oxidant is highly
2
2
desired.
Polyoxometalates and metal complexes, such as Fe O @SiO @CH (CH ) NH -[P W O -
+
3
3
4
2
2
2 2
2
18 68
IV
-
(
HOSn OH) ] nanocatalyst[16], (NH ) (MimAm) [Mo132O372(CH COO) (H O) ][17],
3 11 4 2 40 3 30 2 72
Fe O @SiO -FeQ [18] and Fe O @SiO /Mn(III) chiral salen complex[19], have been applied as
3
4
2
3
3
4
2
the catalysts for H O -mediated aqueous phase catalytic oxidation of sulfides into sulfoxides or
2
2
sulfones. But, the preparation procedures for the above-mentioned catalytic systems were
complicated and disfavored the practical application. Metal oxides-based catalysts have been

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widely used in the oxidation reactions[20] and are suitable for the industrial application. However,
the metal oxides have been rarely applied for the catalytic oxidation of sulfides in aqueous phase.
Rajabi et al.[21] developed a procedure for the oxidation of sulfides into sulfoxides catalyzed by
SBA-15 supported iron oxide nanoparticles in aqueous medium. Choudary et al.[22] reported the
selective catalytic oxidation of sulfides into sulfoxides or sulfones over tungstate-exchanged
Mg-Al-LDH in aqueous medium. But the process for the formation of sulfones needed long
reaction time (12 h). Thus, it is still a challenge to realize the selective catalytic oxidation of
sulfides into sulfoxides or sulfones in aqueous phase. New and efficient methods are needed to be
developed for the catalytic oxidation of sulfides into sulfoxides or sulfones.
Nb-based catalysts have been applied in the catalytic oxidation of alcohols[23], epoxidation
of cycloolefins[24] and oxidation of sulfides[25] with H O as the oxidant. But Nb O -based
2
2
2
5
catalysts have been rarely used for the catalytic oxidation of sulfides, especially in the aqueous
phase. Activated carbon is usually used as the support for its high surface area and good
adsorption property. Here, activated carbon supported amorphous Nb O catalysts were prepared
2
5
and applied for the H O mediated catalytic oxidation of sulfides in aqueous phase at room
2
2
temperature. Sulfoxides or sulfones could be efficiently and selectively produced just by changing
the amount of H O added and reaction time. Furthermore, the catalysts possessed excellent
2
2
reusability.
2
2
. Experimental
.1 Catalyst preparation
The activated carbon supported Nb O catalysts were prepared by a modified
x
y
incipient-wetness impregnation method. Typically, appropriate amounts of activated carbon were

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added into the aqueous solution of niobium oxalate under stirring, and the resulting solution was
o
sonicated for 30 min. Then the slurry was dried in air at 110 C for 12 h after being aged for 24 h
o
at room temperature. Finally, the sample was calcined at 300 C for 4 h under N atmosphere. The
2
loading amount of Nb was 10wt%. Nb O /AC catalysts with loadings of 2wt% and 5wt% Nb were
x
y
prepared by the same method just by changing the addition amount of niobium oxalate.
2.2 Catalyst characterization
The N adsorption-desorption experiments were performed on a Quantachrome Autosorb-IQ
2
o
instrument at -196 C. The X-ray diffraction (XRD) patterns of the catalysts were measured using
a Bruker D8 ADVANCE diffractometer with a Cu Kα radiation source (λ=1.5418 Å). The
transmission electron microscopy (TEM) images were taken on a FEI Titan G2 60-300
microscope operated at 120 kV. The X-ray photoelectron spectroscopy (XPS) was recorded on a
Thermo ESCALAB 250XI spectrometer with a Al Kα radiation source. The ATR infrared spectra
were recorded on a ThermoFisher Nicolet iS10 spectrophotometer using a Diamond ATR
accessory.
2.3 Catalytic reactions and products analysis
The catalytic reactions were performed in a 25 mL round-bottomed flask equipped with a
reflux condenser and a magnetic stirrer. In a typical procedure, the flask was charged with methyl
phenyl sulfide (2 mmol), catalyst (50 mg), deionized water (10 mL) and 30wt% H O aqueous
2
2
solution (H O /methyl phenyl sulfide molar ratio=1-3.5). The flask was placed in an oil bath and
2
2
o
then was heated to 35 C under stirring. After completion of the reaction, the catalyst was
separated by centrifugation. The reactant and products were extracted from the solution with
dichloromethane (4×6 mL). The organic phase was collected for analysis. The products were

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analyzed by GC using an Agilent 7890A gas chromatograph equipped with a 19091N-113
capillary column and a flame ionization detector. Toluene was used as the internal standard for the
quantitative analysis of sulfides. The product distributions were calculated by the peak area.
3
3
. Results and discussion
.1 Characterization of catalysts
Figure S1 shows the N adsorption-desorption isotherms of AC and 10%Nb O /AC. As can
2
x
y
be seen from the figure, the N adsorption-desorption isotherms of AC and 10%Nb O /AC exhibit
2
x
y
type-IV isotherm with H4 hysteresis loop. The results indicate the presence of mesoporous
character in support AC and the mesoporous structure of AC is retained after being supported by
Nb O [26]. The Brunauer-Emmett-Teller (BET) specific surface area of AC and 10%Nb O /AC
x
y
x
y
2
-1
was 971 and 777 m g , respectively. The AC with high surface area is favorable for the dispersion
of Nb O . The slight decrease of the surface area of Nb O /AC is probably attributed to the
x
y
x
y
blocking of cavities by the Nb O [27].
x
y
The catalysts were characterized by the X-ray diffractometer to investigate the crystallite
phases. The XRD patterns of AC and Nb O /AC with different Nb loadings are shown in Figure
x
y
o
S2. The XRD pattern of support AC exhibits a major diffraction peak around 26.5 , which is
assigned to graphitic C[28]. In the XRD patterns of 2%Nb O /AC, 5%Nb O /AC and
x
y
x
y
1
0%Nb O /AC, there were no new peaks as compared to that of AC support. These results indicate
x y
that Nb O nanoparticles are highly dispersed on the support AC or they exist in form of
x
y
amorphous phase[29].
To further confirm the crystallite phases of Nb O /AC, the high-resolution TEM image of
x
y
1
0%Nb O /AC was taken. As shown in Figure S3, the Nb O nanoparticles were highly dispersed
x
y
x
y

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on the support AC. Figure 1 shows the high-resolution TEM image of Nb O /AC and the
x
y
corresponding selected area electron-diffraction pattern. No crystal lattice of Nb O was observed
x
y
in the TEM image. Besides, there were no diffraction spots or rings related to Nb O in the
x
y
selected area electron-diffraction pattern of 10%Nb O /AC. These results indicate that the Nb O
y
x
y
x
nanoparticles are amorphous[30], which is consistent with the XRD analysis result.
Figure 1. The TEM image and the selected area electron-diffraction pattern of 10%Nb O /AC
x
y
X-ray photoelectron spectroscopy (XPS) analysis was performed to determine the chemical
constitution of Nb O /AC. The XPS spectrum of the Nb 3d of 10%Nb O /AC is shown in Figure
x
y
x
y
S4. The peaks of Nb 3d spectrum were located at 207.5 eV and 210.3 eV, which are corresponded
to Nb 3d5/2 and Nb 3d3/2 spin states, respectively[31]. These results indicate niobium is in the +5
oxidation state and is present in the form of Nb O .
2
5
3.2 Catalytic oxidation of methyl phenyl sulfide
Methyl phenyl sulfide was chosen as the model substrate. A series of activated carbon (AC)
supported Nb-based catalysts were prepared and investigated in the catalytic oxidation of methyl
phenyl sulfide. The reaction mixtures were analyzed by GC-MS and the spectra are shown in
Figure S5. The results indicate the formation of methyl phenyl sulfoxide and methyl phenyl
sulfone. The reaction results for the catalytic oxidation of methyl phenyl sulfide over various

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catalysts are shown in Table 1. When AC was used as the catalyst, the conversion of methyl
phenyl sulfide was only 9.7% (entry 1). The conversion of methyl phenyl sulfide increased
gradually with the increase of the Nb loading (entries 2-4). The 10%Nb O /AC showed the highest
x
y
conversion (99.7%), which is probably due to more active catalytic sites provided by the
0%Nb O /AC. Besides, the selectivities towards methyl phenyl sulfoxide and methyl phenyl
1
x
y
sulfone were 92.0% and 8.0%, respectively. In the absence of H O , no catalytic activity was
2
2
observed for the catalytic oxidation of methyl phenyl sulfide over Nb O /AC (entry 5). According
x
y
to the characterization results, niobium exists in the form of Nb O . Thus commercial Nb O was
2
5
2
5
also evaluated in the catalytic oxidation of methyl phenyl sulfide, and the conversion was only
5
7.8% (entry 6). These results indicate supported Nb O is beneficial for the catalytic oxidation of
2 5
methyl phenyl sulfide.
a
Table 1. Catalytic oxidation of methyl phenyl sulfide over various catalysts
b
Conversion
%)
Product distribution (%)
Entry
Catalyst
(
Methyl phenyl sulfoxide Methyl phenyl sulfone
1
2
3
4
5
6
AC
9.7
46.8
90.4
99.7
-
100.0
98.1
92.4
92.0
-
0
2%Nb O /AC
1.9
7.6
8.0
-
x
y
5%Nb O /AC
x
y
10%Nb O /AC
x
y
c
10%Nb O /AC
x
y
d
Nb O5
57.8
98.6
1.4
2
a Reaction conditions: methyl phenyl sulfide (2 mmol), H O (10 mL), 30% H O (2 mmol),
2
2
2
o
catalyst (50 mg), 35 C, 120 min. b Based on GC. c Without H O . d 0.027 mmol.
2
2

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H O plays an essential role in the catalytic oxidation of methyl phenyl sulfide, thus the effect
2
2
of the amount of H O was investigated. As shown in Figure S6, the conversion of methyl phenyl
2
2
sulfide increased gradually with the increase of the amount of H O , reaching 99% when 3
2
2
equivalences of H O were added. The selectivity to methyl phenyl sulfoxide slowly decreased at
2
2
the beginning and then decreased rapidly, while the selectivity to methyl phenyl sulfone showed
the opposite trend. When the addition amount of H O was less than 2 equivalences, methyl
2
2
phenyl sulfoxide was produced as the main product. Methyl phenyl sulfone became the main
product when more than 3 equivalences of H O were added. Under the conditions of 3
2
2
equivalences of H O and 150 min reaction time, 99% conversion of methyl phenyl sulfide and
2
2
96.6% selectivity to methyl phenyl sulfone were obtained. These results indicate selective
catalytic oxidation of methyl phenyl sulfide to methyl phenyl sulfoxide or methyl phenyl sulfone
over Nb O /AC could be realized by changing the amount of H O added and reaction time.
2
5
2
2
For the practical applications of heterogeneous catalysts, their lifetime is an important factor.
Therefore, the recyclability of 10%Nb O /AC was examined for the selective catalytic oxidation
2
5
of methyl phenyl sulfide. After each run, the 10%Nb O /AC was separated from the reaction
2
5
system by centrifugation. The Nb O /AC was used for the next run after being washed by three
2
5
times with dichloromethane. The results of the reuses of Nb O /AC in the catalytic oxidation of
2
5
methyl phenyl sulfide are shown in Figure 2. The Nb O /AC demonstrated excellent recyclability.
2
5
After 10 successive runs, the conversion was still above 90%, and the selectivity to methyl phenyl
sulfoxide was 97%. Throughout the reuse tests, there were no obvious changes for the conversion
and the selectivity towards methyl phenyl sulfoxide. According to the ICP analysis, the Nb
contents in the fresh and 10th recovered Nb O /AC catalysts were determined to be 9.56 wt% and
2
5

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7.42 wt%, respectively, indicating the leaching of Nb. The content of Nb species in the filtrate of
the reaction after catalyst separation for the first run was 6.51 ppm. With the filtrate as the reaction
system, the conversion of methyl phenyl sulfide was 22.2% (Table S1). The results indicate the
leached Nb species also partly contributed to the total catalytic activity. The XRD pattern of used
Nb O /AC is shown in Figure S7, no new peaks appeared compared to that of fresh Nb O /AC.
2
5
2
5
The results indicate the Nb O /AC catalyst is relatively stable under the reaction conditions.
2
5
Figure 2. Reuses of 10%Nb O /AC in the catalytic oxidation of methyl phenyl sulfide
2
5
3
.3 Selective oxidation of sulfides to sulfoxides or sulfones over Nb O /AC
2 5
Since 10%Nb O /AC displayed a high catalytic performance and excellent recyclability in
2
5
the selective catalytic oxidation of methyl phenyl sulfide, the substrate scope of the protocol was
subsequently extended to a series of sulfides. The selective catalytic oxidation of sulfides to
sulfoxides or sulfones over 10%Nb O /AC was performed at room temperature (RT) with 30%
2
5
H O as the oxidant. Table 2 shows the results for the selective catalytic oxidation of sulfides to
2
2
sulfoxides over Nb O /AC. All the aromatic and aliphatic sulfides could be selectively oxidized
2
5

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into sulfoxides. The conversions of sulfides were all more than 90%, and the selectivities to
sulfoxides were in the range of 81.9% to 94.5%. For the aromatic sulfide with an
electron-withdrawing group and the aliphatic sulfide with long chain, excess H O was needed to
2
2
realize the selective catalytic oxidation of sulfides to sulfoxides.
a
Table 2. Selective catalytic oxidation of sulfides to sulfoxides over Nb O /AC
2
5
b
Product distribution (%)
Entry
Substrate
Conversion (%)
Sulfoxide
93.4
Sulfone
6.6
1
2
3
4
5
6
Methyl phenyl sulfide
99.7
93.9
95.7
97.7
99.8
91.4
c
Ethyl phenyl sulfide
84.8
15.2
5.5
4-Methoxythioanisole
94.5
d
4-Chlorothioanisole
83.0
17.0
9.4
Diethyl sulfide
90.6
e
n-Dibutyl sulfide
81.9
18.1
a Reaction conditions: sulfide (2 mmol), H O (10 mL), 30% H O (2 mmol), 10%Nb O /AC (50
2
2
2
2
5
mg), RT, 2 h. b Based on GC. c 3 h. d 7 mmol H O , 1 h. e 6 mmol H O .
2
2
2
2
The results for the selective catalytic oxidation of sulfides to sulfones are shown in Table 3. A
range of aromatic and aliphatic sulfides were selectively converted into sulfones when 3.5
equivalences of H O were used. The conversions of the sulfides were more than 99% and the
2
2
selectivities to sulfones were in the range of 86.4% to 100%. For aromatic sulfides, the formation
rate of sulfone for the substrate with an electron-donating group was faster than that for the
substrate with an electron-withdrawing group. This is consistent with the literature results[18].
The sulfides with high electron density on the sulfur atoms have better reaction activity. For

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aliphatic sulfides, the short-chain substrate was more easily oxidized into sulfone, which might be
due to the little steric hindrance.
a
Table 3. Selective catalytic oxidation of sulfides to sulfones over Nb O /AC
2
5
b
Product distribution (%)
Entry
Substrate
Conversion (%)
Sulfoxide
6.4
Sulfone
93.6
99.5
98.9
86.4
100
1
2
3
4
5
6
Methyl phenyl sulfide
Ethyl phenyl sulfide
4-Methoxythioanisole
4-Chlorothioanisole
Diethyl sulfide
100
99.5
100
99.6
100
100
0.5
1.1
13.6
0
n-Dibutyl sulfide
9.0
91.0
a Reaction conditions: sulfide (2 mmol), H O (10 mL), 30% H O (7 mmol), 10%Nb O /AC (50
2
2
2
2
5
mg), RT, 3 h. b Based on GC.
3.4 Reaction pathway and proposed mechanism
In order to explore the reaction pathway, the catalytic oxidation of methyl phenyl sulfide over
0%Nb O /AC as a function of time was carried out. The reaction results are shown in Figure 3.
1
2
5
The concentration of methyl phenyl sulfide gradually decreased with time, and complete
conversion was achieved within 30 min. Methyl phenyl sulfoxide and methyl phenyl sulfone were
produced as the products. For methyl phenyl sulfoxide, its concentration rapidly increased at the
beginning and then slowly decreased, reaching a maximum value at 10 min. The concentration of
methyl phenyl sulfone increased with time. The volcano curve for methyl phenyl sulfoxide
indicates it is formed as the intermediate. Besides, the slope for methyl phenyl sulfide was larger

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than that for methyl phenyl sulfone within 10 min, which indicates the formation of methyl phenyl
sulfone is slow and is the rate-determine step. According to the above results, the reaction pathway
for the catalytic oxidation of sulfides over 10%Nb O /AC was proposed and is shown in Scheme
2
5
S1. The oxidation of sulfides to sulfones is a consecutive reaction. With H O as the oxidant,
2
2
sulfides were first rapidly converted into sulfoxides, and sulfoxides were then further slowly
oxidized to sulfones.
Figure 3. The time course of catalytic oxidation of methyl phenyl sulfide over 10%Nb O /AC.
2
5
Reaction conditions: methyl phenyl sulfide (2 mmol), H O (10 mL), 30% H O (6 mmol),
2
2
2
o
1
0%Nb O /AC (50 mg), 35 C.
2 5
For the catalytic oxidation of sulfides with H O as the oxidant, the reactions might proceed
2
2
via a free-radical or nonradical mechanism. To clarify the mechanism for the catalytic oxidation of
sulfides over the 10%Nb O /AC, the radicals trapping experiments were carried out. Isopropanol
2
5
and 1,4-benzoquinone act as free radical scavengers for hydroxyl radical (·OH) and superoxide
-
2
radical (·O ), respectively[32]. They were added into the reaction mixture to investigate their

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effects on the catalytic performance of 10%Nb O /AC in the oxidation of methyl phenyl sulfide.
2
5
As shown in Table S2, the addition of isopropanol and 1,4-benzoquinone had almost no effect on
both the conversions and the product distributions in the catalytic oxidation of methyl phenyl
sulfide over 10%Nb O /AC. These results imply that the catalytic oxidation of sulfides over
2
5
Nb O /AC follows a nonradical mechanism.
2
5
The formation of oxygen-based species is pivotal in the catalytic systems with H O as the
2
2
oxidant. To identify the active species, ATR infrared spectra of Nb O /AC and Nb O /AC treated
2
5
2
5
with H O or H O were collected. The characterization results are shown in Figure 4. For
2
2
2
-1
Nb O /AC and Nb O /AC treated with H O, no bands were observed from 1200 to 600 cm . A
2
5
2
5
2
-1
new band at about 874 cm appeared for the Nb O /AC after being treated with H O , indicating
2
5
2
2
the formation of Nb(OO) species[33]. This is consistent with the literature results that the contact
of amorphous Nb O with H O could generate the niobium peroxo species[34].
2
5
2
2
Figure 4. The ATR infrared spectra of Nb O /AC and Nb O /AC treated with H O or H O
2
2
5
2
5
2
2
Based on the above results, Nb(OO) was proposed as the possible active species. Thus, the

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possible mechanism for the catalytic oxidation of sulfides to sulfoxides or sulfones over
Nb O /AC with H O as the oxidant was proposed. The active species Nb(OO) is first formed by
2
5
2
2
the interaction of Nb O /AC with H O . Then, the transfer of electrophilic oxygen from the
2
5
2
2
Nb(OO) species to sulfide affords sulfoxide, and the Nb O /AC catalyst is regenerated. The
2
5
sulfoxide could further react with another mole of Nb(OO) species to form sulfone.
. Conclusions
Efficient catalytic oxidation of sulfides with H O as the oxidant in aqueous phase at room
4
2
2
temperature was achieved by amorphous Nb O /AC. The excellent catalytic performance was due
2
5
to highly dispersed Nb O . Selective synthesis of sulfoxides or sulfones was realized by changing
2
5
the amount of H O added and reaction time. The Nb O /AC catalyst could be used up to 10 times
2
2
2
5
without notable decrease in the catalytic activity. The catalytic oxidation of sulfides over
Nb O /AC follows a nonradical mechanism with Nb(OO) as the possible active species. Given the
2
5
mild reaction conditions and easily prepared catalysts with excellent catalytic performance and
recyclability, this protocol is suitable for the industrial application of the conversion of sulfides to
sulfoxides and sulfones.
Acknowledgements
This work is financially supported by the GDAS' Project of Science and Technology
Development (2019GDASYL-0302016, 2017GDASCX-0852, 2017GDASCX-0705 and 2017GD-
ASCX-0116).

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Highlights
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AC supported amorphous Nb O catalysts were prepared.
2 5
Sulfides could be converted into sulfoxides or sulfones with high conversion and selectivity.
The Nb O /AC catalyst demonstrated excellent recyclability.
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5
The catalytic oxidation of sulfides over Nb O /AC follows a nonradical mechanism.
2
5

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Graphical abstract