Ag?CeO2 nanoparticles with “rice ball” configuration as an efficient and heterogeneous nanocatalyst for the selective oxidation of sulfides to sulfones with 30% H2O2

Article abstract of DOI:10.1007/s00706-020-02668-5

Abstract: Ag?CeO₂ with a rice-ball configuration as a heterogeneous and highly efficient catalyst was described for activation of H₂O₂ in the selective oxidation of aromatic and aliphatic sulfides to their corresponding sulfones. Ag nanoparticles in the CeO₂–Ag interface increase the oxygen vacancy defects on the surface of CeO₂ and oxygen vacancy defects promote the reduction of Ce⁴⁺ to Ce³⁺ to keep the electroneutrality. Generated Ce³⁺ species act as the active sites in the interface of CeO₂–Ag to promote the oxidation of sulfides to sulfones. Compatibility with various aromatic and aliphatic sulfides, excellent selectivity, high yield of product, simple experimental procedure, and mild reaction conditions are some of the precious advantages of Ag?CeO₂/H₂O₂ catalyst system. Graphic abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s00706-020-02668-5

Monatshefte für Chemie - Chemical Monthly
https://doi.org/10.1007/s00706-020-02668-5
ORIGINAL PAPER
Ag@CeO2 nanoparticles with “rice ball” confguration as an efcient
and heterogeneous nanocatalyst for the selective oxidation of sulfdes
to sulfones with 30% H2O2
Rouhallah Parnian1 · Ebrahim Soleimani1 · Kiumars Bahrami1 · Somayeh Ghorbani1
Received: 9 September 2019 / Accepted: 10 August 2020
© Springer-Verlag GmbH Austria, part of Springer Nature 2020
Abstract
Ag@CeO2 with a rice-ball confguration as a heterogeneous and highly efcient catalyst was described for activation of H2O2
in the selective oxidation of aromatic and aliphatic sulfdes to their corresponding sulfones. Ag nanoparticles in the CeO2–Ag
interface increase the oxygen vacancy defects on the surface of CeO2 and oxygen vacancy defects promote the reduction
of Ce4+ to Ce3+ to keep the electroneutrality. Generated Ce3+ species act as the active sites in the interface of CeO2–Ag to
promote the oxidation of sulfdes to sulfones. Compatibility with various aromatic and aliphatic sulfdes, excellent selectiv-
ity, high yield of product, simple experimental procedure, and mild reaction conditions are some of the precious advantages
of Ag@CeO2/H2O2 catalyst system.
Graphic abstract
O
Ag@CeO2 cat., 30% H2O2
R
S
R
R
S
R
2
1
2
1
CH3CN, 25 °C
O
R1, R2 = alkyl, aryl
Up to 99% isolated yield
Mild conditions
Reusable catalyst
Keywords Thiols · Confguration · Sulfones · Heterogeneous catalysis · Lewis acids
Introduction
is the most convenient method. Due to the availability of
sulfde derivatives, this method is remarkably popular for the
synthesis of these biologically active molecules. Sulfones
are important synthetic intermediates for the synthesis of
diferent chemically and biologically active molecules. The
studies are mostly focused on the oxidation of sulfdes to
sulfoxides as compared to the oxidation of sulfdes to sul-
fones. In addition to the green nature, H2O2 is a relatively
weak electrophile and an ideal oxidant to oxidize the sulfde
derivatives. Besides, oxidation of sulfdes into sulfones has
been extensively studied which H2O2 was frequently utilized
in combination with diferent catalysts such as MoO3 [8],
dioxo-molybdenum(VIO) complex [9], 1,3,4-triazo-2,4,6-
triphosphorine-2,2,4,4,6,6-tetrachloride [10], carboxyl-dec-
orated graphene oxide sheets [11], MWW-type titanosilicate
Organosulfur compounds, specially sulfoxides and sulfones,
as two important products of sulfde oxidation are signifcant
moieties with medicinal and catalytic applications [1–5].
These compounds exist in the structure of many natural
products and exhibit some biological properties [6, 7]. To
obtain the sulfoxides and sulfones, oxidation of sulfdes
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s00706-020-02668-5) contains
supplementary material, which is available to authorized users.
* Ebrahim Soleimani
e_soleimanirazi@yahoo.com
1
Department of Organic Chemistry, Faculty of Chemistry,
Razi University, 67149-67346 Kermanshah, Iran
Vol.:(0123456789)
1 3

R. Parnian et al.
zeolite [12] and etc. Recently Zhao et al. used a hybrid cata-
lyst based on SO3H-fuctionalized quaternary ammonium
and polyoxovanadometalate [13] for selective oxidation of
sulfdes to sulfones. Furthermore, urea-hydrogen peroxide
in the presence of phthalic anhydride was reported as an ef-
cient reagent for oxidation of aryl and alkyl sulfdes to their
corresponding sulfones [14]. Tosi et al. described a poly-
meric catalyst [MoO3(2,2′–bipy)]n for oxidation of sulfdes
to sulfoxides or sulfones using tert-butylhydroperoxide
(TBHP) as the stoichiometric oxidant [15].
Scheme 1
Results and discussion
The catalytic activity of Ag@CeO2 was initially investi-
gated in a standard reaction using benzyl phenyl sulfde
(1 mmol) as a model substrate. The efect of the solvent,
temperature, amount of the catalyst and H2O2 was explored
to establish a general reaction condition. The results are
summarized in Table 1. First, a reaction was carried out
with 3 equiv. 30% H2O2 in the absence of catalyst and
solvent (Table 1, entry 1). No signifcant oxidation was
observed and only a trace amount of corresponding sul-
fone was resulted. Then, equivalent reactions were carried
out with 1 mmol benzyl phenyl sulfde, 0.05 g Ag@CeO2,
and 3 equiv. 30% H2O2 in the various solvents such as
ethanol, methanol, toluene, chloroform, dichloromethane,
and acetonitrile at 25 °C for 5 min (Table 1, entries 2–7).
As shown (Table 1, entry 7), the best result was obtained
in CH3CN as the solvent, although the reaction could even
be successfully accomplished in the protic solvents such as
ethanol and methanol (Table 1, entries 2, 3).
Other oxidants are studied in detail by Jareb [16]. The
size and shape of nanomaterials appreciably impress the
physical, chemical, electrical, and optical properties [17].
For the oxidation reactions, environmentally benign proce-
dures such as use of the heterogeneous recoverable catalysts
and green oxidants are extremely important. In recent years,
biocompatible heterogeneous nanocatalysts have attracted
much attention in green chemistry pursuits [18–21]. The
use of silver nanoparticles as catalyst in the organic syn-
thesis has been developed rapidly in the past 10 years due
to their unique reactivity and selectivity, stability, as well
as recyclability. A broad range of reactions is catalyzed by
silver nanoparticles such as reduction reactions, oxidative
transformations, catalytic cycloaddition and cyclization
to complex products, and alkylation reactions [22]. Metal
oxides are extensively utilized as catalyst and support in the
heterogeneous catalysis.
CeO2 is able to supply or release oxygen and conse-
quently exist in two diferent oxidation states of+3 and+4.
In addition, surface defects and specially presence of oxygen
vacancies aford the catalytic activity in the metal oxides
[23]. To exploit the properties of Ag and CeO2, Ag@CeO2
with a new confguration called “rice ball” is a good can-
didate. Ag@CeO2 with “rice ball” confguration, including
numerous spherical ‘balls’ in which the center Ag metal is
surrounded by fne CeO2 particles, was frstly introduced
by T. Kayama et al. for soot oxidation with gaseous oxygen
at temperatures below 300 °C [24, 25]. The advantageous
efects resulting from the metal-support interface play a key
role in the development of similar catalysts. Therefore, K.
Kaneda et al. prepared this catalyst using a diferent meth-
odology and investigated its efciency in the chemoselective
reduction reactions [26]. Recently, various catalysts with this
strategy by changing the metal or metal oxide in Ag@CeO2
were obtained. For example, K. Kaneda et al. developed a
core-gold/shell-ceria and core-Pd/shell-Ag nanomaterial and
explored their catalytic activity for selective semihydrogena-
tion of alkynes [27, 28]. The other CeO2 encapsulated noble
metals are comprehensively discussed by S. Song and cow-
orkers [29]. Herein, we use Ag@CeO2 as a heterogeneous
catalyst with increased stability and high catalytic activity
for activation of H2O2 to prepare sulfones from one-pot oxi-
dation of sulfdes (Scheme 1).
The reaction in the nonpolar solvents such as toluene
and chloroform resulted in a slight amount of the desired
product. In addition, reaction in the dichloromethane
medium as a moderately polar solvent resulted in low con-
versions (Table 1, entry 6). Furthermore, no signifcant
change in the yield of product was observed with increas-
ing the temperature to 50 °C (Table 1, entry 8). When the
amount of Ag@CeO2 and H2O2 (Table 1, entry 9) were
decreased, a decrease in the yield of product was observed.
The efect of oxidant on the reaction was also investi-
gated. The results showed that the reaction aforded high
yield in the presence of hydrogen peroxide instead of
molecular oxygen or air (Table 2). The high conversion of
this reaction is related to the high activity of H2O2 oxidant.
Under the optimized reaction conditions, we investigated
generality of the protocol on the preparation of sulfones
from a series of sulfdes with various aromatic and aliphatic
groups (R1 and R2). The results are collected in the Table 3.
Both symmetric and asymmetric sulfdes endured the oxi-
dation and resulted their corresponding sulfones in high-to-
excellent yields. All the reactions carried out with complete
selectivity for sulfone formation without any overoxidation
to sulfoxides. Furthermore, the electron donating and elec-
tron withdrawing groups properly tolerated the oxidation
reaction and resulted the corresponding sulfones.
1 3

Ag@CeO2 nanoparticles with “rice ball”confguration as an efcient and heterogeneous…
Table 1 Optimization of reaction conditionsa
O
S
O
S
Entry
Ag@CeO2/g
Solvent
Temperature /°C
Conversion /%
Yield /%b
1
2
3
4
5
6
7
8
9
-
–
25
25
25
25
25
25
25
50
50
5
90
80
15
20
Trace
80
70
10
15
18
99
99
93
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.03
EtOH
MeOH
Toluene
CHCl3
CH2Cl2
CH3CN
CH3CN
CH3CN
25
100
100
98
aAll the reactions were performed with 3 equivalents of 30% H2O2 except for entry 9 that 2 equivalents of 30% H2O2 was used
bIsolated yield
Table 2 The efect of the oxidant in the oxidation of benzyl phenyl
the similar reactions catalyzed by non-defective surfaces
[35, 38]. As a consequence, Ag nanoparticles in the
CeO2–Ag interface increase the oxygen vacancy defects
on the surface of CeO2 and oxygen vacancy defects pro-
mote the reduction of Ce4+ to Ce3+ that Ce3+ species act
as the active sites in the interface of CeO2–Ag to oxidize
the organic sulfdes to sulfones. As indicated, many use-
ful catalytic properties of CeO2 are attributed to the pres-
ence of Ce3+ species [39]. Besides, the structure of Ag@
CeO2 provides the access of substrates to the interface of
CeO2–Ag in contrast to core–shell materials that the shell
deactivates the core catalyst metal [24, 25].
sulfdea
Entry
Oxidant
Conversion /%
Yield /%b
1
2
3
Air
O2
H2O2
30
60
100
25
57
99
aThe reactions were performed with sulfde (1 mmol), 0.05 g Ag@
CeO2, air or molecular O2 (bubbling rate 20 cm3/min) and 30% H2O2
(3 equiv.) in 2 cm3 acetonitrile at room temperature
bIsolated yield
By changing the strength of Ag–CeO2 interaction the
properties of silver-containing catalysts can be controlled in
the selective oxidation of organic substances [40]. Therefore,
we explored a new confguration of Ag–CeO2 materials.
The presence of oxygen vacancy defects is the key of
generation of Ce3+ ions as the active species in the surface
of CeO2. In general, the oxygen vacancy defects (VO) are
generated by leaving electrons that reduce Ce4+ to Ce3+ in
CeO2−x [30]. Actually, when the oxygen content is decreased
in the CeO2, the oxygen vacancies defects are formed. The
oxidation state of oxygen in the CeO2 is 2- and higher den-
sity of oxygen vacancies increases the fraction of Ce3+ to
keep the electroneutrality [31, 32]. Therefore, the oxygen
vacancies accompany the reduction of adjacent Ce4+ to
Ce3+ [32]. As confrmed by independent groups [33, 34],
Ag nanoparticles activate the surface lattice oxygen. Indeed,
Ag surface increases the lattice oxygen, mobility of CeO2,
generating oxygen vacancies. Defects such as oxygen vacan-
cies and step edges are considered as the most reactive sites
on the surface of metal oxides [35]. Vacancy defects bind
the substrates and adsorbates more strongly than other oxide
sites [36, 37].
Proposed mechanism
A possible reaction mechanism for the oxidation of sulfdes
into the corresponding sulfones using H2O2 in the presence
of Ag@CeO2 catalyst is shown in Scheme 2. The cross
section of catalyst is shown. First, H2O2 coordinates to
the Lewis acidic sites in the Ag–CeO2 interface due to the
increased concentration of Ce3+ cations as well as strong
interaction of Ag surface with surface defects of CeO2 [36].
Then, polarized coordinated H2O2 molecule on the Ag@
CeO2 interact with a sulfde molecule through an oxygen
transfer pathway to produce a sulfoxide molecule by loss
of water and repetition of this cycle generates the sulfone
product by releasing the second water molecule [11, 31].
In addition, the reactions catalyzed by defective sur-
faces beneft from lower energy barrier as compared to
1 3

R. Parnian et al.
Table 3 Selective oxidation of sulfdes to sulfonesa
Entry
R1
R2
Time/min
Yield /%b
M.p. (Lit.)/°C
1
2
3
4
5
6
7
8
9c
C6H5
5
7
99
98
97
99
97
96
95
96
99
148 (146–147 [30])
130 (120–125 [31])
156–158 (156–158 [8])
150 (150–152 [8])
CH2(C6H5)
4-ClC6H4
CH2(C6H5)
4-BrC6H4
8
CH2(C6H5)
C6H5
5
CH2(4-MeC6H4)
4-MeC6H4
6
143–154 (143–145 [31])
177 (177–178 [32])
110–111 (111 [32])
125–126 (125–126 [30])
237 (234–237 [14])
CH2(C6H5)
CH2(C6H5)
9
CH2(4-BrC6H4)
CH2(4-NO2C6H4)
11
8
C6H5
C6H5
C6H5
6
10
n-C4H9
n-C4H9
6
98
46 (44–46.5 [30])
aThe reactions were performed with sulfde (1 mmol), 0.05 g Ag@CeO2, and 30% H2O2 (3 equiv.) in 2 cm3 acetonitrile at room temperature
bIsolated yield
cR1 =R2
The recycling of Ag@CeO2
Scheme 2
For a heterogeneous catalyst simplicity of separation proce-
dure, stability in the course of reaction and minimum drop in
the reactivity after successive runs are signifcantly impor-
tant. Therefore, we investigated the reusability of Ag@CeO2
as a heterogeneous and recoverable catalyst. After comple-
tion of the reaction of benzyl(phenyl)sulfane (Table 3, entry
1), the reaction mixture was diluted by 10 cm3 of acetonitrile
and heated at 50 °C for 10 min. Then, the catalyst was easily
separated from the mixture by fltration and washed thor-
oughly with acetonitrile (3×10 cm3) and water (2×10 cm3).
Finally, dried in an oven at 50 °C for 1 h. The efciency of
the recycled catalyst was examined up to fve consecutive
runs, and its catalytic efciency remained almost the same
(Fig. 1).
The XRD spectra for typical CeO2 and Ag@CeO2 nano-
particles before and after utilization in oxidation confrms
the anchoring of Ag nanoparticles on the surface of CeO2.
1 3

Ag@CeO2 nanoparticles with “rice ball”confguration as an efcient and heterogeneous…
Fig. 1 Recyclability of Ag@
CeO2 for the oxidation of
benzyl(phenyl)sulfane as a
model substrate
XRD pattern of the recycled catalyst after run 5th shows
the difraction peaks of Ag2O along with Ag(0) and CeO2.
Fresh catalyst only shows Ag(0) and CeO2 difraction peaks.
Therefore, partial oxidation of Ag(0) to Ag2O may afect the
efciency of Ag@CeO2 (Figs. S1, S2, and S3).
X-ray difractometer with Cu Kα radiation (λ=1.54050 Å)
at 40 kV voltage and 40 mA current.
Synthesis of the Ag@CeO2 catalyst
Ag@CeO2 was synthesized according the literature pro-
cedures [24]. Typically, a premixed solution of 5.049 g
Ce(NO3)3.6H2O (11.628 mmol) and 2.963 g AgNO3
(17.44 mmol) in 15 cm3 water was immediately added to
NH4OH solution (3.56 g of 25% ammonia solution, diluted
by 10 cm3 water) under vigorous stirring (350 rpm) at room
temperature. After addition of aqueous ammonia, the solu-
tion was stirred for 1 min. Then, the black co-precipitate
was heated with steam in an autoclave at 120 °C for 10 min.
Finally, the resultant golden-brown like solid was separated
by centrifugation and calcined at 500 °C for 5 h in the air.
Conclusion
In conclusion, Ag@CeO2 was found to be an ideal hetero-
geneous catalyst for activation of H2O2 in selective oxida-
tion of aromatic and aliphatic sulfdes to their corresponding
sulfones. The unique structure of the catalyst provides active
sites due to the synergic efect of silver metal and CeO2.
Compatibility with various aromatic and aliphatic
sulfdes, short reaction times, high yield of product, ease of
reaction procedure, and mild reaction conditions are some of
the signifcant advantages of the present method.
General procedure for the preparation of sulfones
To a mixture of sulfde (1 mmol) and Ag@CeO2 (0.05 g) in
2 cm3 acetonitrile, 0.3 cm3 30% H2O2 (3 equiv.) was added,
and the mixture was stirred at room temperature until the
complete consumption of sulfde (monitored by TLC). After
completion of the reaction, 10 cm3 acetonitrile was added
to the reaction mixture and the mixture heated for 10 min
at 50 °C. After that, the catalyst was separated from the
reaction mixture by fltration and the organic residue was
extracted with 20 cm3 EtOAc, washed with 10 cm3 distilled
water, dried over MgSO4, and recrystallized in a mixture of
EtOH and H2O to obtain the pure product.
Experimental
All chemicals were purchased from Merck or Fluka chemi-
cal companies and were used without further purifcation.
The known products were identifed by comparison of their
melting points and spectral data with those reported in the
literature. Melting points were determined in a capillary
tube and are not corrected. 1H NMR and 13C NMR spectra
were recorded on a Bruker-300 spectrometer using TMS
as an internal standard and in CDCl3 as solvent at ambi-
ent temperature. The reaction progress was followed by
TLC using Merck 0.2 mm silica gel 60F-254 Al-plates.
The crystal structure of Ag@CeO2 catalyst was analyzed
by X-ray Difractometer (XRD) on a Bruker D8-advance
Acknowledgements We are grateful to the Razi University Research
Council for fnancial support.
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R. Parnian et al.
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