Selective oxidation of sulfides to sulfoxides using hydrogen peroxide over Au/CTN-silica catalyst

Article abstract of DOI:10.1016/j.catcom.2015.09.015

Chitosan (CTN) can be used as potential support to create environmentally friendly catalysts; however, chitosan's poor diffusion properties and low surface area limit its application. Direct deposition method on inorganic supports might be a feasible and simple solution, using core (silica nanosphere)-shell (chitosan) material as support, a series of Au/CTN-silica catalysts were prepared by inverse microemulsion system. These catalysts were active in catalyzing the oxidation of sulfides to sulfoxides with relative good conversion and high selectivity. This study confirms that the key factor governing catalytic performance of this catalyst should be the interaction between gold nanoparticles and N atoms.

Full text of DOI:10.1016/j.catcom.2015.09.015

Selective oxidation of sulfides to sulfoxides using hydrogen peroxide over
Au/CTN-silica catalyst
Fang Wang, Cheng Liu, Gui Liu, Wenxiu Li, Junhua Liu
PII:
DOI:
Reference:
S1566-7367(15)30074-1
doi: 10.1016/j.catcom.2015.09.015
CATCOM 4439
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
14 July 2015
18 September 2015
21 September 2015
Please cite this article as: Fang Wang, Cheng Liu, Gui Liu, Wenxiu Li, Junhua Liu,
Selective oxidation of sulfides to sulfoxides using hydrogen peroxide over Au/CTN-silica
catalyst, Catalysis Communications (2015), doi: 10.1016/j.catcom.2015.09.015
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ACCEPTED MANUSCRIPT
Selective oxidation of sulfides to sulfoxides using hydrogen
peroxide over Au/CTN-silica catalyst
a
b
b
b
b,c*
Fang Wang , Cheng Liu , Gui Liu , Wenxiu Li , Junhua Liu
a
College of Sciences, Nanjing Tech University, Nanjing 211816, China
b
c
College of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210023,China
Jiangsu Provincial Key Laboratory of Palygorskite Science and Applied Technology, Huaiyin
Institute of Technology, Huaian 223003, China
Abstract: Chitosan (CTN) can be used as potential support to create environmentally friendly
catalysts; however, chitosan’s poor diffusion properties and low surface area limit its application.
Direct deposition method on inorganic supports might be a feasible and simple solution, using
core (silica nanosphere)-shell (chitosan) material as support, a series of Au/CTN-silica catalysts
were prepared by inverse microemulsion system. These catalysts were active in catalyzing the
oxidation of sulfides to sulfoxides with relative good conversion and high selectivity. This study
confirms that the key factor governing catalytic performance of this catalyst should be the
interaction between gold nanoparticles and N atoms.
Keywords: Chitosan; Silica nanosphere; Core-shell; Nano-gold; Oxidation of sulfides.
1
. Introduction
In recent years, interest in sulfoxide chemistry is increasing; the chemoselective preparation of
sulfoxides and sulfones represents an extremely important synthetic process in organic chemistry
1]. So various efforts have been made in development of new synthetic methods for sulfoxide
[
synthesis [2-6], however, many of these procedures utilize environmentally unfavorable reagents,

Corresponding author. Tel: +86-25-85891767
E-mail address: liujh2007@aliyun.com (J. Liu); qufuwangfang@aliyun.com (F. Wang).
1

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solvents and catalysts that bring out the matter of eco-efficiency in our environmentally conscious
times. Hence, there is still a need for the development of new, efficient, highly selective, and
widely applicable methods for this transformation under mild reaction conditions.
Nano-gold catalysis has attracted considerable attention in recent years, which showed various
catalytic activities. It has been widely accepted that the catalytic performance of nano-gold is
dependent on size of gold nanoparticles (GNPs) and the interaction between gold and supports.
Articles have showed that some functional groups, such as –NH , -SH, -COOH etc., are necessary
2
for the controlling of GNPs size. As the presence of these groups, some natural polysaccharides,
such as alginate, carrageenan and especially chitosan (CTN), have stimulated widespread interests
to use them as supports to create environmentally friendly catalysts, these materials are renewable
resource, chemically stable but biodegradable [7]. Chitosan is a copolymer of linked β, (1→4),
2-amino-2-deoxy-D-glucan and 2-acetamidodeoxy-D-glucan (Scheme 1), the presence of
hydroxyl and amino groups, which are excellent functional groups for the anchoring of GNPs.
Scheme 1 Chemical Structure of Chitosan
The poor diffusion properties of chitosan, its weak mechanical properties and low surface area
limit its application; researchers have provided many methods to solve these problems, such as
CO supercritical drying method [7-9], direct deposition method on inorganic supports [10-11].
2
However, these methods have encountered with rigorous experimental conditions, high
energy-consumptions or poor dispersion of chitosan on inorganic supports. Obviously, the good
dispersion of chitosan on silica nanosphere was necessary for the deposition of GNPs. Direct
deposition method might be a feasible and simple way, however, previous researchers are failed to
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achieve this goal by this method. In this article, we have deposited chitosan on silica nanosphere
by an inverse microemulsion system and used chitosan/silica nanosphere supports to capture
GNPs. Firstly, we synthesized some silica nanosphere, then a certain amount of chitosan-acetic
acid solution was added dropwise to the mixture of a large amount of acetone and silica
nanosphere with vigorous stirring, uniform deposition of chitosan on silica nanosphere ensured the
formation of core (silica nanosphere)-shell (chitosan)-type supports, and then dispersed gold
nanoparticles (GNPs) were obtained on the supports. In this catalyst, silica nanosphere as the core
ensures the GNPs are dispersed equably, and improve chitosan's mechanical stability, surface area
and diffusion properties. To the best of our knowledge, it is the first time to construct core-shell
typed Au/CTN-silica catalyst by inverse microemulsion system, and this catalyst showed high
catalytic activity and selectivity for the oxidation of sulfides to sulfoxides. Meanwhile, only a few
articles present the oxidation of sulfides over gold-based catalysts so far [12-13].
2
. Experimental
2.1. Materials
H O (30% in water) and other reagents were all obtained from commercial sources without any
2
2
further purification.
2
2
.2. Catalyst preparation
.2.1 Preparation of silica nanosphere
Silica nanosphere was prepared according to one modified procedures [14]. 0.5 g of
cetyltrimethyl ammonium bromide (CTAB) was dissolved in a system composed of 70 mL of H O,
2
0.8 mL of aqueous ammonia and 20 mL ethanol. After the mixture was vigorously stirred for 0.5 h
at room temperature, 2.5 mL of tetraethyl orthosilicate (TEOS) was quickly dripped into the
mixture. The resulting mixture was vigorously stirred at room temperature for 6 h. A white
precipitate was obtained, filtered, washed with pure water, and dried in air at 60 ℃ for 24 h.
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CTAB and other organic components in the products were eventually removed by calcination in
air at 550 ℃ for 5 h.
2.2.2 Preparation of Au/CTN-silica catalyst
The Au/CTN-silica catalyst was prepared as follows: Firstly, 3 g chitosan was dissolved in 100
ml 2% acetic acid solution, then 0.5 g silica nanosphere and 4 ml above solution was sequentially
added into the large amounts of acetone, and 0.35 ml 4 M NaOH solution was added when the
stirring time was 4h. The inverse microemulsion system was vigorous stirring for 24 h at room
-7
temperature. After drying, 0.2 g CTN-silica powder was transferred into 20 ml 9.1×10
M
HAuCl solution and refluxed at 80 ℃ for 10 h; the catalyst was filtrated, washed with ethanol
4
and dried at 60 ℃ in vacuum (Au content: 0.15 mmol %).
2.3. Catalyst characterization
The morphologies of obtained Au/CTN-silica catalysts were examined by transmission electron
microscope (TEM) (JEM-2010) and scanning electron microscopy (SEM) (JSM-7600F). The
’
crystals of the catalysts were studied by X-ray diffraction (XRD) (X Pert PRO PANalytical).
Surface composition was determined by X-ray photoelectron spectroscopy (XPS), using a
K-Alpha-surface Analysis system with X-Ray Monochromatisation. The fourier
transformed-infrared spectroscopy (FT-IR) spectra of samples were obtained on a Bruker Tensor
27 spectrometer. Elemental analysis was determined by X-ray fluorescence spectrometry (XRF)
(Magix PW 2403 XRF).
2.4 Catalyst test
Catalytic reactions were carried out in a batch reactor, a typical procedure was as follows: 0.02
g catalyst, 6 mmol substrate and 5 ml solvent were added into a 100 ml round-bottom flask
equipped with a condenser and stirred for 5 min at room temperature, after addition of hydrogen
peroxide, the reaction was started by immersing the flask in the oil bath kept at 60 ℃, then
carried out with vigorous stirring for a certain time. The products were analyzed by gas
chromatography (GC 9560) with a DB-1 capillary column, the nature of the products were also
determined by gas chromatography-mass spectrometer (GC-MS).
3
. Results and discussion
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3
3
.1. Catalyst characterization
.1. Catalyst characterization
The XRD patterns of Au/CTN-silica catalyst showed that CTN was very well dispersed on
silica nanosphere and the gold crystals were loaded on CTN-silica nanosphere (Fig.1a, b, d). For
Au/CTN-silica catalyst, the featured four peaks were indexed to the (1 1 1), (2 0 0), (2 2 0) and (3
1
1) reflection of face centered cubic (fcc) crystalline Au (JCPDS, Card no. 7440-57-5), which
indicated the formation of gold crystal loaded on silica nanosphere (Fig. 1c). Fig.2 shows the
qualitative identification of the structure vibration in the samples. The typical Si-O-Si bending,
Si-O-Si symmetric stretching and Si-O-Si asymmetric stretching associated with the condensed
silica network were present in all samples, corresponding to the vibration band at 470, 800 and
-1
-1
1
,100 cm [15], respectively. An absorption peak at around 970 cm was attributed to stretching
-1
vibration of Si–OH groups. The intense band at 3430 cm was assigned to -OH group asymmetric
-1
stretching, the peak around 1630 cm was attributed to the stretching and bending vibrations of
-1
adsorbed H O [16]. The peak at 1597 and 1650 cm was assigned for the N-H bending of the
2
primary amine and C=O stretch of the secondary amide (Fig. 2a, c). In fact, the deformation
-1
-1
vibration of NH group at 1597 cm disappeared and a new band at 2036 cm appeared after
incorporation of the GNPs (Fig. 2d). This new band assigned to the NH group vibration of the
amine group that shifted to higher wave numbers when interacting with the GNPs, which was
contrary with the previous reports [17].
Fig.1 XRD patterns of (a) Silica nanosphere; (b) CTN-silica; (c) Au/CTN-silica catalyst; (d) CTN.
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Fig.2 FT-IR spectra of (a) CTN; (b) silica nanosphere; (c) CTN-silica; (d) Au/CTN-silica catalyst.
Fig.3 shows a representative TEM picture of CTN-silica supports and Au/CTN-silica catalyst.
TEM revealed that chitosan regimented equably around the silica nanosphere, and core-shell-type
supports were obtained (Fig.3a). More precise information about the shape and size distribution of
GNPs was obtained from TEM and HADDIF-STEM measurements, a careful analysis of the TEM
images revealed that gold nanoparticles were highly dispersed in core-shell type support with the
mean particle diameter of 25.3 nm (Fig.3b, c). SEM further testified that chitosan was deposited
equably around the silica nanosphere (Fig.3d). BET showed that the Au-CTN-silica catalyst had a
2
2
larger surface area (958 m /g), it was much larger than chitosan itself (0.65 m /g), which made it
possible to exhibit high catalytic activity for organic reactions.
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Fig.3 TEM images of (a) CTN (shell)-Silica (core), (b) Au/CTN-silica catalyst and (c) HADIFF-STEM, (d) SEM
of Au/CTN-silica catalyst.
Fig.4 XPS spectra of N1s of (a) CTN-silica, (b) Au/CTN-silica and (c) Au 4f of Au/CTN-silica.
Fig.4 shows N 1s and Au 4f XPS spectra of Au/CTN-silica sample. In general, the banding
3
energy at 399.1-399.7 eV in N 1s is attributed to sp nitrogen [18], the peak at energy 399.4 eV are
associated with -NH [19], so the peak at 399.7 eV is assigned to -NH-C=O-. In Au/CTN-silica
2
catalyst, the N 1s signal could be deconvoluted into three peaks at 399.7 eV, 400.2 eV and 400.8
eV, which were assigned to the to -NH-C=O- groups, GNPs bonded NH and NH-C=O- groups,
2
there were with 0.8 eV and 1.1 eV positive shift corresponding to the CTN-silica, respectively
(
Fig. 4a, b), it might due to the presence of an interaction between Au NPs and the N atoms.
Clearly, the positive shift of the N 1s binding energy suggested that the electron of N atoms was
7

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transferred to gold atoms. Most of articles concluded that Au 4f7/2 at 84.0 eV was assigned to the
metallic Au [20, 21], in the Au 4f region of Au/CTN-silica catalyst, six peaks could be observed
mainly due to the Au 4f7/2 and Au 4f5/2 transitions. The signal at 84.3 eV indicated the presence of
+
3+
metallic gold species. Similarly, the peaks at 85.5 eV and 86.6 eV were assigned to Au and Au ,
respectively. From the XPS quantitative report, the atomic percentage of gold was estimated to be
0
3+
+
0
.5%, corresponding to 87.9% of Au , 3.6% of Au and 8.5% of Au (Fig. 4c), the reduction in
valence state of gold was due to the electron transfer of N atoms, which further testified the
interaction between Au NPs and N atoms.
3.2. Catalytic activity
The oxidation of sulfides to sulfoxides on Au/CTN-silica catalysts were investigated, sulfones
has been the main by-products during the oxidation of sulfides. Initially, the oxidation of diphenyl
sulfide was examined as a model reaction under different conditions; the results are shown in
Table 1. The Au/CTN-silica catalyst showed the highest catalytic activity and selectivity for
diphenyl sulfoxide in methanol, diphenyl sulfone was primary byproducts. Protic and aprotic
solvents have different polarity (i.e. different dielectric constants), these solvents are chosen to
form a single phase with organic substrates and hydrogen peroxide solution, so that the mass
transfer problems associated with dissolubility are avoided. The solvents used in the experiments
are CH OH, CH CN, ClCH CH Cl and toluene, of which CH OH and H O are protic solvents; the
3
3
2
2
3
2
other solvents are aprotic. The decrease order of dielectric constants is for protic solvents: MeOH
32.7) < H O (78.5), and for aprotic solvents: toluene (2.4) < ClCH CH Cl (10.4) < CH CN (37.5),
(
2
2
2
3
respectively. Obviously, for the aprotic solvents, the yield of diphenyl sulfoxide increased
gradually with increasing polarity of solvents, polarity of solvent was a crucial factor to the
activity and selectivity of catalyst, so only acetonitrile gave satisfactory results among these
solvents (Table 1, entries 1-3). However, the protic solvents obviously were in favor of oxidation
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of sulfides, the conversion of diphenyl sulfide was 96.2% and the selectivity of diphenyl sulfoxide
was 81.4% when the methanol was used as solvent (Table 1, entry 4). Meanwhile, the existence of
Au/CTN-silica catalyst was crucial because no product was found without catalyst (Table 1, entry
5). Considering the cost of the solvent, we chose methanol as solvent and hydrogen peroxide as
oxidant in next experiments.
a
Table 1. Selective oxidation of diphenyl sulfide using different solvents
b
Product Selectivity (%)
b
Entry
Solvent
Conversion (%)
Sulfoxide
100
Sulfone
0
1
2
3
4
Toluene
Dichloroethane
Acetonitrile
Methanol
0.7
12.4
87.1
96.2
86.7
70.6
81.4
-
13.3
29.4
18.6
-
c
d
5
Methanol
N.R.
6
H O
2
7.2
96.2
3.8
a
Reaction conditions: Au/CTN-silica catalyst 0.02 g, diphenyl sulfide 6 mmol, solvent 5 mL, H O 3 mL (30% in
2
2
b
c
d
water), 60℃, 3 h. Determined by GC. Blank experiment, without catalyst. N.R.: Reaction did not occur.
Next, we carried out oxidation of various sulfides under optimum conditions, the results are
listed in Table 2.
As shown in Table 2, various sulfides showed good reaction activities and selectivities for the
oxidation to corresponding sulfoxides. Obviously, methyl phenyl sulfide gave much higher yields
(94.2% yield) than diphenyl sulfide (Table 2, entries 1), meanwhile，we observed the significant
effects of electronic and steric factors of the substrates on the conversion of sulfide and selectivity
for sulfoxide. Generally, sulfides with electron-rich side groups (hence, higher electron density on
the -S- atom of the sulfide), such as 4-(methylthio)benzenamine and 4-methylthioanisole, were
oxidized at a higher conversion than the electron-deficient 4-nitrothioanisole (Table 2, entries 3–5).
In contrast, sulfides with greater steric hindrance showed lower conversion than those with less
steric hindrance (Table 2, entries 7–8 vs. entry 6). Some straight-chain sulfides, such as
dibenzylsulfane, showed good catalytic results for the corresponding sulfoxides. Meanwhile, it is
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interesting to note that both the conversion and selectivity for the oxidation of diphenyl sulfide
and 2-(phenylthio) benzenamine (Table 2, entry 10) are much lower than those of other sulfides,
which should be caused by the p system of this particular substrate [22]. Because the GNPs in
Au/CTN-silica catalyst were anchored by dispersed Si-OHs, so the catalyst was easily separated
from the reaction mixture, ICP analysis of filtrate confirmed that no leaching of Au species
occurred. It was shown that the recovered Au/CTN-silica catalyst could be reused (Table 2, entry
2), meanwhile, the catalyst was filtered off after ca. 50% conversion at the reaction temperature,
further treatment of the filtrate under similar reaction conditions did not afford to any products.
a
Table 2. Selective oxidation of various sulfides to sulfoxides over Au/CTN-silica catalyst
b
b
Conversion
%)
Selectivity
(%)
Entry
1
Substrate
Product
Time (h)
(
O
2
2
100
94.2
S
CH3
S
CH3
c
2
100
93.7
O
S
S
CH3
CH3
CH3
CH3
CH3
CH3
3
4
5
1
2
6
97.6
100
100
H2N
H2N
O
S
S
96.7
100
H3C
O2N
H3C
O
S
S
81.5
O2N
O
6
7
4
2
96.8
100
100
S
S
O
S
S
99.2
O
S
Cl
S
8
9
Cl
1
3
3
88.5
96.2
66.5
100
81.4
92.9
O
S
S
NH2
O
S
NH2
S
1
0
1
0

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a
Reaction conditions: Au/CTN-silica catalyst 0.02 g, sulfide 6 mmol, methanol 5 mL, H O 3 mL (30% in water),
2
2
b
c
6
0℃. Determined by GC. Recycled experiment.
4
. Conclusions
In this article, Au/CTN-silica catalyst was prepared by one-pot synthesis method, this catalyst
showed high catalytic activity and good sulfoxide selectivity for the oxidation of sulfides using
H O as oxidant. The interactions between the GNPs and Si-OH groups guaranteed good
2
2
dispersions of GNPs, and resulted in excellent catalytic performance. It is an environmental
friendly, recycled and economical catalytic system.
Acknowledgements
This work is financially supported by the National Natural Science Foundation of China
(21303085), the Natural Science Foundation of Jiangsu Province (BK20130901, BK20130930)
and the Priority Academic Program Development of Jiangsu Higher Education Institutions.
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Selective oxidation of sulfides to sulfoxides using hydrogen
peroxide over Au/CTN-silica catalyst
Fang Wang, Cheng Liu, Gui Liu, Wenxiu Li, Junhua Liu
Graphical abstract
1
3

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Research Highlights
▶
Core (silica)-shell (chitosan) materials were constructed by the inverse
microemulsion system. ▶ Au/CTN-silica catalyst was prepared using above core-shell
materials as supports. ▶ This catalyst showed good catalytic activity and selectivity
for the oxidations of sulfides to sulfoxides. ▶ Key factor governing catalytic property
was Au NPs-support interactions.
1
4