Cubic Ag2O nanoparticle incorporated mesoporous silica with large bottle-neck like mesopores for the aerobic oxidative synthesis of disulfide

Article abstract of DOI:10.1039/c4ra14802a

Highly stable, environmentally benign cubic Ag₂O nanoparticles dispersed on mesoporous silica with large bottle-neck like mesopores were synthesized and characterized by BET surface area analysis, HR TEM, EDX, FTIR and powder XRD studies. The Ag₂O nanoparticles were homogeneously distributed having an average size of 20-40 nm. The activity of the catalyst was probed through an efficient aerobic chemoselective oxidation of thiols to disulfides in water under atmospheric oxygen as the cheapest oxidant. Alkyl, aryl and imine containing symmetrical disulfides can be easily obtained in high yields under mild reaction conditions with no over oxidized product. The efficiency of the catalyst was further demonstrated as the highly sensitive imine bond was well sustained under these mild reaction conditions. Moreover, unlike the other literature precedents, in this present work, single crystal structures of both simple symmetrical disulphide as well as imine containing disulphides are reported for the first time. The catalyst is very much water compatible and can be recycled and reused for at least five cycles. The standard leaching experiment proved that the reaction was heterogeneous with this recyclable catalyst.

Full text of DOI:10.1039/c4ra14802a

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Cubic Ag2O nanoparticle incorporated mesoporous
silica with large bottle-neck like mesopores for the
aerobic oxidative synthesis of disulfide†
Cite this: RSC Adv., 2015, 5, 6323
Paramita Das,a Suman Ray,a Asim Bhaumik,b Biplab Banerjeeb
a
*
and Chhanda Mukhopadhyay
Highly stable, environmentally benign cubic Ag2O nanoparticles dispersed on mesoporous silica with large
bottle-neck like mesopores were synthesized and characterized by BET surface area analysis, HR TEM, EDX,
FTIR and powder XRD studies. The Ag2O nanoparticles were homogeneously distributed having an average
size of 20–40 nm. The activity of the catalyst was probed through an efficient aerobic chemoselective
oxidation of thiols to disulfides in water under atmospheric oxygen as the cheapest oxidant. Alkyl, aryl
and imine containing symmetrical disulfides can be easily obtained in high yields under mild reaction
conditions with no over oxidized product. The efficiency of the catalyst was further demonstrated as the
highly sensitive imine bond was well sustained under these mild reaction conditions. Moreover, unlike
the other literature precedents, in this present work, single crystal structures of both simple symmetrical
disulphide as well as imine containing disulphides are reported for the first time. The catalyst is very
much water compatible and can be recycled and reused for at least five cycles. The standard leaching
experiment proved that the reaction was heterogeneous with this recyclable catalyst.
Received 18th November 2014
Accepted 15th December 2014
DOI: 10.1039/c4ra14802a
www.rsc.org/advances
(thiolsulnates), disulde S-dioxides (thiolsulfonates), and
sulfonic acids.6 Consequently, considerable research has gone
Introduction
into controlling the initial oxidation to avoid over oxidations.
Moreover, most of the thiols are toxic, have unpleasant odour
and act as catalyst poisons. Therefore, from biological and
practical points of view sweetening of catalyst poisons thiols2e to
low volatile disuldes led us to investigate the introduction and
applications of new member of this category of reagents in
oxidation of thiols to the corresponding disuldes under
neutral and mild conditions.
Reagents such as of MnO2,7 dichromates,8 borohydride
exchange resin,9 DMSO/alumina,10 CBr4/K2CO3/18-crown-6/
benzene,11 NaIO3/alumina,12a molybdate sulfuric acid/NaNO2,12b
tetramethylammonium uorochromate,13 cerium(IV) salt,14
copper salt,15 rhenium–sulfoxide complex,16a rhodium(I) com-
plex,16b have been utilized for oxidation of thiols to disuldes.
However, some of the reported reagents suffer from disadvan-
tages such as toxicity, high price of some reagents, instability,
hygroscopicity, low selectivity, long reaction time, difficulty of
preparation, need for a large excess of the reagent and the over-
oxidation of the disulde to thiosulnates and derivatives. Thus
a milder, more selective and inexpensive reagent is still in
demand. Solutions to these problems lead to the development
of a good number of other oxidants, such as pyridinium
chlorochromate,17 tributylammonium halochromates/silica
gel,18a pyridinium uorochromate,18b quinolinium dichroma-
te,19a N-phenyltriazolinedione19b etc. The Burgess reagent20 has
also been used for obtaining disuldes from thiols, with the
Disuldes are readily formed through covalent cross-linking
and are one of the most important organic sulfur compounds
possessing an exclusive chemistry both in biological1 (DNA
cleavage properties, stabilization of peptides in proteins, and
drug delivery systems) and chemical2 (protecting groups,
vulcanizing agents, and oils for rubber and elastomers)
processes. Disuldes are used as key intermediates for the
synthesis of sulnyl, sulfenyl compounds3 and sulphur con-
taining heterocycles.4 As disuldes are relatively more stable to
organic reactions such as oxidation, alkylation and acylation
compared to the corresponding free thiols, the thiol groups can
conveniently be protected as a disulde during oxidation of
certain other functional groups. The desired thiol can be
generated from the disulde either by reduction or by other
sulfur–sulfur bond cleavage reactions such as CN–, OH– or
hydrazines.5 Oxidation of thiols to the corresponding disuldes
is a characteristic functional group transformation. This disul-
de may possibly be further oxidized to give disulde S-oxides
aDepartment of Chemistry, University of Calcutta, 92 APC Road, Kolkata-700009,
India. E-mail: cmukhop@yahoo.co.in; Tel: +91-9433019610
bDepartment of Materials Science, Indian Association for the Cultivation of Science,
Jadavpur, Kolkata 700 032, India. E-mail: msab@iacs.res.in
† Electronic supplementary information (ESI) available. CCDC 1024098 and
1024099. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c4ra14802a
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high cost of this reagent being the main limitation of the highly sensitive imine bonds are well tolerated under this mild
method. However, with the increasing environmental concern, reaction condition. For this present purpose we have synthe-
chemists have developed different catalysts with molecular sized several silver incorporated silica catalysts with different
oxygen as the primary oxidant which include diaryl tellurides weight percentages of silver and denoted as MSAGN-x (x ¼ wt%
under photosensitized conditions,21 Co(II)-Schiff base (Co-salo- of silver). Among them MSAGN-10 performs most efficiently as
phen),22a Mn(III)-Schiff base,22b vanadium polyoxometalate,23 20 an oxidation catalyst for this present synthesis.
mol% NaI/10 mol% FeCl3 (ref. 24) and 15 mol% Ni-nano-
particles.25 The oxidative dimerization of thiols with O2 was also
possible in several ionic liquids,26a–c using laccases as catalyst,26d
Results and discussions
Characterization of the catalysts
under sonication with Et3N in DMF.26e Using neutral Al2O3
under ball milling condition27a or using subcritical water under
high pressure at 100 ꢀC without catalyst.27b Despite some
progress, most of these methods suffer from a number of
disadvantages, including the use of toxic and/or expensive
oxidants or catalysts. Some of the reagents employed are
commercially unavailable and their preparation is tedious. Also,
some catalysts suffer from low catalytic efficiency. In addition,
many of the methods reported require the use of volatile
organic solvents. Thus, there is still an interest in developing
clean, fast, inexpensive, environmentally harmless oxidative
methods that would produce the desirable disuldes in high
yields.
N2 adsorption analysis. N2 adsorption–desorption isotherm
of the silver incorporated mesoporous silica catalyst is shown in
Fig. 1. The isotherm is a typical type IV isotherm with desorp-
tion hysteresis loop, which could be attributed to the meso-
porous nature of the sample.36 The presence of hysteresis loop is
due the presence of bottle-neck like large mesopore in the
material. The BET surface area calculated from this isotherm is
579.05 m2 gꢁ1 which is reasonably high considering the loading
of heavy metal Ag in the sample surface. The pore volume
estimated for this sample is 0.4273 cc gꢁ1. Corresponding pore
size distribution of the sample estimated by the non-local
density functional theory (NLDFT) model is shown inset of
Fig. 1. Estimated pore dimensions for the sample MSAGN-10 are
found to be 2.9 nm and 6.95 nm. Peak pore dimension of 2.9 nm
could be assigned due to the use of template CTAB in the
sample preparation, whereas peak at 6.95 is assigned due to the
presence of interparticle voids. Under the present synthesis
conditions interparticle pores are generated, as conrmed from
the hysteresis in the N2 sorption isotherm and TEM data. Size of
these interparticle pores are bigger than 4 nm and this helps in
the formation of Ag2O nanoparticles at the interparticle voids.
It is pertinent to mention that ink-bottle or large bottle neck
like mesoporous silica with H2 type hysteresis loop has para-
mount importance as catalyst in organic synthesis, since the
large transition states are easily accommodated inside the large
pores and also the pore mouth is large enough to allow easy
diffusion of organic molecules.
No doubt that, oxidation by molecular oxygen (O2) in air as
stoichiometric oxidant offers one of the most environmentally
benign and ideal oxidation processes in organic synthesis.28
However for the oxidation with molecular oxygen the use of a
catalyst becomes inevitable. The traditional catalysts for cata-
lytic oxidation can be classied into the following categories: (a)
supported precious metals29 (b) metal oxides and supported
non-precious metals30 and (c) mixtures of metal oxides and
precious metals.31 Generally speaking, metal oxides and non-
precious metals exhibited somewhat low efficiency and shot
life span in spite of their availability. Among supported precious
metals, silver has been drawing more and more attentions due
to its availability and high activity for some oxidation reaction at
low temperatures.32 However, most of metal oxides supports are
mainly structurally nonuniform, the size and the shape of the
obtained metal nanoparticles are hard to be controlled. The
catalytic performance of the catalysts is directly related to the
particle-size and the reaction tends to occur easily on smaller
particles by virtue of the increased surface area.33 If the metal
oxide nanoparticles can be conned within a nanoporous host
material, this may restrict the size to which the metal oxide
nanoparticles can grow.32a,b Mesoporous silica34 has been found
to be the promising templates to control the shape and size of
the occluded metal/metal oxide nanoparticles.32c–e Actually, it's
great high surface area is in favor for a high dispersion of the
active component on the surface of the supports as well as
increased availability of active sites, which should show a
favorable impact on catalytic activity.35
In this work, we present our efforts obtained in the mild
transformation of thiols to disuldes in aqueous media using
Ag incorporated high surface area mesoporous silica catalyst
(MSAGN) with bottle-neck like large mesopore and the cheapest
oxidant on earth (atmospheric oxygen). It can be used for
preparing aromatic, heteroaromatic and aliphatic symmetrical
Fig. 1 N2 adsorption–desorption isotherm of MSAGN-10 at 77 K.
disuldes. It is pertinent to mention that thiols containing NLDFT pore size distribution is shown in the inset.
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Fig. 2 TEM image of MSAGN-10.
Fig. 4 Wide angle powder XRD pattern of MSAGN-10.
TEM analysis. TEM images of MSAGN-10 are shown in Fig. 2.
A closer analysis of this electron microscopic data reveals that
there is 3–10 nm pores in silica matrix. Some of the represen-
tative pores are shown in the TEM images (red dotted circled)
(Panel a). This value matches well with the value obtained from
N2 sorption analysis. In the TEM image (Panel b) we have circled
one of the representative Ag2O nanoparticles which are having
average size 20–40 nm and they are present in the matrix
homogeneously.37
EDS elemental analysis. The chemical characterization for
the material was carried out by EDX analysis. EDX analysis
revealed that three elements –O, Si and Ag, existed in the
material (Fig. 3). EDX chemical analysis conrmed that the
expected silver incorporated mesoporous silica has successfully
developed in this study. From the EDX elemental analysis a
loading of silver of 0.886 mmol gꢁ1 on silica surface was
determined.
Wide angle powder XRD pattern. Wide angle powder XRD
pattern revealed the presence of cubic Ag2O nanoparticles in the
mesoporous silica matrix. Crystalline planes corresponding to
this cubic phase have been indexed in Fig. 4. This indexing
agrees very well with the JCPDS 12-0793 (ref. 38) corresponding
to a cubic phase of Ag2O.
Fourier transform infrared spectroscopy. FTIR spectra of Ag/
SiO2 (MSAGN-10) in the region of 400–4000 cmꢁ1 are shown in
Fig. 5. In the IR spectra, water bands observed at around 3459
and 1634 cmꢁ1 corresponding to stretching and bending
vibrations indicate the hygroscopic nature of the powdered
samples. Strong bands observed at 1095 and 969 cmꢁ1 were
assigned to the stretching vibration of Si–O–Si. The bands at 802
and 468 cmꢁ1 were for the internal bonds of the tetrahedral SiO4
structural units.39
Recently, disulde bond formation reactions have engrossed
substantial research interest, because of the abundance of this
disulde functionality in biological relevant compounds.
Herein, the disulde bond formation reaction was studied with
several catalysts under various reaction conditions. The oxida-
tion of benzenethiol to diphenyl disulde was chosen as model
reaction to evaluate the catalytic activity of different catalysts
(Table 1). Formation of disulde bond was realized by FTIR
analysis of the products. From the FTIR spectra in Fig. 6 it is
clear that the peak at 2567.1 cmꢁ1 corresponding to –S–H bond
in the spectrum (a) has disappeared in the spectrum (b), and a
new peak at 462.8 cmꢁ1 appeared corresponding to the newly
formed –S–S– bond in spectrum (b). The progress of this reac-
tion can be visually monitored in naked eye as the reactant
thiophenol is liquid and the product diphenyl disulde is a
white crystalline solid.
Owing to very importance of these compounds and keeping
in mind the possibility of the disulde to undergo over oxida-
tion to disulde S-oxides, disulde S-dioxides, and sulfonic
acids, it becomes a tremendous task to control the reactions by
appropriate choice of the reaction conditions as well as the
catalysts. Initially different common oxidation catalysts were
used with appropriate stoichiometric oxidants as shown in
Table 1. Some of them showed good activity for oxidation of
thiol in the initial stages of reaction. However with the
advancement of reaction the selectivity for disulde decreases
due to the formation of side products like disulde S-oxides,
Fig. 3 EDS spectrum of MSAGN-10.
Fig. 5 FTIR spectra of MSAGN-10 catalyst.
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Fig. 7 Wide selectivity to diphenyldithiol vs. time plots for the oxida-
tion of thiophenol in the presence of molecular oxygen catalyzed by
MSAGN-10.
Fig. 6 FTIR spectra of (a) thiophenol and (b) diphenyl disulfide.
Table 1 Optimization of reaction conditiona
In order to fully establish the role of Ag/SiO2 (MSAGN-10) in
the present case, a control reaction was performed which
showed that mesoporous silica, without any added metal, was
not able to catalyze the disulde formation reaction (entry 11).
Interestingly, also in the absence of air (entry 14) the oxidation
reaction could not proceed. Further, when bulk Ag2O was used
as the catalyst under identical reaction conditions, only a very
poor yield of diphenyldithiol was observed (entry 10). These
results suggest that porosity and high surface area of silver
incorporated mesoporous silica are crucial for this catalytic
reaction.
The reaction was also studied with varied percentage of silver
on silica and the results are represented in Table 2. From the
Table 2 it is found that 10% by weight of Ag on silica exhibits
best performance for the present synthesis.
The reusability of the silver incorporated mesoporous silica
catalyst (MSAGN-10) in the disulde bond formation reaction
was also examined by using thiophenol as a reference substrate
in aqueous medium. The kinetics of the fresh catalyst was
evaluated as shown in Fig. 8. The yield did not increase
substantially aer 8 h. Also we had performed the kinetic study
with the recycled catalyst for the next consecutive four runs. The
Solvents
(4 mL)
Entry
Catalyst
Yieldb (%)
TON
1
2
3
4
5
6
7
8
I2/O2
CuI/NBS
CuI/O2
CeO2/O2
FeCl3/O2
Cu(OAc)2
CuBr/O2
Cu–SiO2/O2
AgNO3/O2
Ag2O/O2
AcOH
Toluene
H2O
62
69
77
20
49
62
61
74
55
64
0
12.4
13.8
15.4
4.0
H2O
Toluene
Toluene
Toluene
H2O
Toluene
H2O
H2O
H2O
H2O
H2O
9.8
12.4
12.2
14.8
11.0
12.8
0
102.7
95.9
0
9
10
11
12
13
14
SiO2/O2
MSAGN-10/O2
MSAGN-10/tBuOOH
MSAGN-10/N2
91
85
0
a
Reaction conditions: thiophenol (1 mmol), different catalysts
(0.1 mmol for homogeneous catalysts and 20 mg for heterogeneous
b
catalysts), different solvents, 5 h, reux. Isolated yields.
Table 2 Effect of silver loadings on the yield of the disulfidea
disulde S-dioxides, and sulfonic acids. Therefore, at the end of
the reaction, when no thiophenol were le, the yield of the
disulde was not satisfactory. Compared to the catalysts
mentioned in Table 1, the present Ag/SiO2 (MSAGN-10) afforded
better yield and very high selectivity for disulde formation.
Selectivity did not decrease even aer the reaction was allowed
to continue for 8 h (Fig. 7). Being heterogeneous and ease of
separation of the catalyst from the reaction mixture, Ag/SiO2
(MSAGN-10) was chosen as a right choice of catalyst for the best
yield of disulde.
Entry
wt% of silver (x)
Yieldb (%)
TON
1
2
3
4
2
5
10
20
21
58
91
81
23.7
65.5
102.7
91.4
a
Reaction conditions: thiophenol (1 mmol), 20 mg MSAGN-x (x ¼ 2, 5,
10, 20 wt%), water, 5 h, reux in air. b Isolated yields.
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Fig. 8 Kinetics plot demonstrating the recycling efficiency of the
mesoporous MSAGN-10 catalyst.
results of kinetic plots suggested that the catalyst retained
almost same efficiency as that of fresh catalyst.
With the optimized conditions in hand, to further extend the
scope and generality of this protocol the applicability of various
thiols was next assessed for this reaction. Both electron-
withdrawing (Scheme 1, entries 2–5) and electron donating
substituents (Scheme 1, entries 6–9), in the aromatic ring, gave
the desired products with good to excellent yields. The presence
of substituents at the ortho position did not exert any obvious
effect on the yield of the reaction (Scheme 1, entries 5, 6 and 9).
It was pleasing to nd that sterically bulky naphthalene-2-thiol
also reacted very efficiently with no side reactions (Scheme 1,
entry 9). The reaction was also compatible with aliphatic thiols
bearing alkyl chains (Scheme 1, entry 12), and alicyclic thiols
(Scheme 1, entry 11). Differently substituted thiophenol
compounds were studied here, and the MSAGN-10 catalyst
exhibited good catalytic activity in all cases with no over
oxidized product in aqueous medium.
Scheme
disulphides.
2
Substrate scope and synthesized imine containing
It is pertinent to mention that highly sensitive imine bond is
well sustained under this mild reaction condition. Therefore,
when the compounds (3) were subjected to reux in presence of
MSAGN-10 catalyst, compound (4) was formed in excellent yield
without the rapture of C]N bond (Scheme 2). This type of
compounds have novel application as template for the growth
metal oxide nano particle on their surface,40 as precursors of
sulphur containing heterocycles like benzothiazoles41 and
benzospirothiazoles41,42 etc.
At the outset of our work, to the best of our knowledge there
was no literature precedent in which a single crystal structure of
simple symmetrical disulphide was reported. In most of the
earlier works the structure determination mainly relied on FTIR
analysis. The conversion of –S–H to –S–S– could be realized
from the IR spectrum as the peak at 2567 cmꢁ1 corresponding
to –S–H of thiophenol disappeared and reappeared at 463 cmꢁ1
corresponding to –S–S– in the product, indicating the presence
of a disulphide. Gratifyingly, we conrmed the structure of
diphenyl disulphide compound (2a) unambiguously through an
X-ray single crystal analysis (CCDC 1024098) (Fig. 9). Further-
more we also presented X-ray single crystal structure of
compound (4f) (CCDC 1024099) to prove unambiguously that
the imine bond was intact aer formation of the –S–S– bond
from the corresponding compound (Fig. 10). The crystal data
Scheme 1 Substrate scope and synthesized disulphides.
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Table 3 Detailed analysis data of the X-ray crystal structure of the
compound 2a and 4f
and details concerning data collection and structural rene-
ment are given in Table 3.
A probable mechanistic pathway for this disulde bond
formation reaction is shown in Scheme 3. Here the reaction
mechanism can be expressed as eqn (1)–(3).
Crystal descriptions
Compound 2a
12H10S2
Compound 4f
Formula
C
C26H18Br2N2S2
582.34
Triclinic
ꢀ
P1
7.625(2)
7.802(2)
20.002(5)
87.587(3)
88.794(3)
83.332(3)
1180.7(5)
2
Formula weight
Crystal system
Space group
218.32
Orthorhombic
P2(1)2(1)2(1)
5.6420(4)
8.1176(6)
23.6684(17)
90
2RSH / 2RSc + 2Hc
2Hc + O2 / H2O2
(1)
(2)
(3)
˚
a [A]
˚
b [A]
˚
c [A]
H2O2 / H2O + 1/2O2
a [ꢀ]
b [ꢀ]
g [ꢀ]
90
90
3
˚
Following the reaction as in eqn (1), the thiol RSH moiety
adsorbed on the Ag2O nanoparticle on silica surface induces
oxidative addition of the metal center to give Ag–SR and Ag–H
species. Two RSc radicals then come out from the solid surface
and combine with each other in the uid phase, making
possible the formation of the disulde compound RS–SR.43 If
the recombination would be in the solid phase then sterically
bulky imine containing thiophenols (3) would not react effi-
ciently to give even more bulky product (4) because of steric
V [A ]
1084.00(14)
4
1.338
Z
D(calc) [g cmꢁ3
]
1.638
3.627
Absorption
0.446
coefficient m [mmꢁ1
]
Absorption correction Empirical
F(000)
Empirical
580.0
456.0
Crystal size [mm]
Temperature (K)
0.80 ꢂ 0.560 ꢂ 0.40
0.30 ꢂ 0.28 ꢂ 0.25
296
296
˚
Radiation [A]
MoKa (l ¼ 0.71073)
1.72, 26.65
MoKa (l ¼ 0.71073)
1.02, 23.710
ꢁ8 # h $ 8; ꢁ8 #
Theta min–max [ꢀ]
Collection range
ꢁ6 # h $ 7; ꢁ10 #
k $ 10; ꢁ26 # l $ 27 k $ 8; ꢁ22 # l $ 22
Reections collected
2191
3537
Independent
1947 (0.0335)
2323 (0.0336)
reections (Rint
)
R, wR2, S
CCDC number
0.0313, 0.1022, 0.799
1024098
0.0467, 0.1583, 0.714
1024099
repulsion with the catalyst surface. The Ag–H intermediate will
reduce molecular oxygen O2 and produce hydrogen peroxide,
which is decomposed into water and O2 according to eqn (3)
Fig. 9 ORTEP representation of 2a (CCDC 1024098).
Scheme
3 Probable mechanistic pathway for the mesoporous
MSAGN-10 catalyzed aerobic oxidative synthesis of disulphides using
molecular oxygen as oxidant.
Fig. 10 ORTEP representation of 4f (CCDC 1024099).
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and closing up the catalytic cycle. The reaction performed in N2 stability of the reused catalyst, TEM and IR analysis of reused
atmosphere did not show any catalytic activity. This result catalyst were examined. The results are shown in the ESI, Fig. S1
demonstrates that the catalyst was only active in the presence of and S2† respectively.
oxygen and again supports our possible catalytic cycle.
Conclusion
Catalyst stability and recycling
In conclusion, Ag2O nanoparticle dispersed on mesoporous
Hot-ltration test. To conrm the heterogeneous nature of
silica with large bottle neck like mesopores has been synthe-
catalyst and catalytic activity bound to the solid phase, a hot-
sized and characterized by using sophisticated analytical tech-
ltration test was performed. The thiophenol (1 mmol) and
niques like N2 sorption analysis, HR TEM in combination with
mesoporous MSAGN-10 (20 mg) were mixed together in water
EDS elemental analysis, FTIR and powder X-ray diffraction
(4 mL) on an oil bath under reux. The catalyst was ltered off
(from the hot reaction mixture) aer 1 h. The ltered reaction
mixture was then reuxed without the catalyst for the next 5 h;
no further formation of the corresponding product was
studies. The scope of the method is almost general since it is
successful for aliphatic, aromatic (with electron-rich and
electron-poor substituents), and heteroaromatic thiols. Addi-
tionally, it is compatible with the presence of other functional
observed, indicating that no homogeneous catalyst was
groups in the molecule. Particularly highly sensitive imine bond
involved. Atomic absorption spectroscopic analysis of the liquid
is well retained under this mild condition. Synthesis using
phase obtained by ltering (hot) the reaction mixture aer 5 h
revealed that no silver was leached from mesoporous MSAGN-10
and the prepared catalyst is heterogeneous in nature.
water as a solvent is of paramount importance for its sustain-
able green impact.44 Further, water and oxygen were the only by-
product which added to its attractiveness. High catalytic effi-
ciency of mesoporous MSAGN-10 reported herein for the
synthesis of disuldes may open up new opportunities in
heterogeneous catalysis for the synthesis of value-added disul-
des. Moreover, this catalytic oxidation system takes on the
clean, safety and operationally simple characteristic, and the
yields of the products are high, so the oxidation method meets
the needs of contemporary green chemistry and is suitable for
practical synthesis.
Effect of aerial oxygen or moisture towards the activity of the
catalyst. In the presence of MSAGN-10 (preheated at 100 ꢀC for 5
hours) the reaction of benzenethiol to diphenyl disulphide
occurred with 91% yield without formation of any other side-
products. The same reaction in the presence of MSAGN-10
aer being exposed to ambient atmosphere for 10 days
produced similar observation. Therefore, the catalyst had
considerable stability towards heat, air and moisture.
Catalyst recyclability. The reusability of the bottle-neck like
mesoporous MSAGN-10 catalyst was examined for benzenethiol
to diphenyl disulde formation reaction in water by using the
recycled catalyst. Aer completion of the reaction, the reaction
mixture was diluted with EtOAc (10 mL) and extracted with
EtOAc and the catalyst was separated by simple ltration and
was washed thoroughly with acetone and distilled water and
dried. The product was collected from the ethyl acetate mixture.
Experimental
Materials and instrumentation
The 1H- and 13C-NMR analyses were carried out on Bruker-
Advance Digital 300 MHz instruments in CDCl3 with TMS as
an internal reference. IR spectra were recorded in KBr pellets in
reection mode on a Perkin Elmer RX-1 FTIR spectrophotom-
eter. CHN analysis was performed using a Perkin-Elmer 2400
Series II CHN analyzer. X-ray single crystal analysis was per-
formed in a Bruker-AXS SMART APEX II diffractometer equip-
ped with a graphite monochromator. N2 sorption analysis was
performed on a Quantachrome Autosorb 1C surface area
analyzer at liquid nitrogen temperature (77 K). Prior to the
analysis the sample was degassed at 413 K for 3 h under high
vacuum. Transmission electron microscopic images were
recorded on a JEOL 2010 TEM operated at 200 kV in Indian
Association for the Cultivation of Science, Jadavpur, Kolkata
700032, India. Powder X-ray diffraction (PXRD) patterns of the
Ag-loaded samples were recorded on a Bruker AXS D-8 Advance
diffractometer operated at 40 kV voltage and 40 mA current.
ꢀ
The catalyst was heated at 150 C for 4 h and reused for next
catalytic cycle. The catalyst was recycled for ve times (Fig. 11).
The separated catalyst was dry under vacuum. To know the
Preparation of catalyst (MSAGN-10)
In a typical procedure, 390 g (390 mL) water and 317 g (400 mL)
MeOH were taken together in a 1 litre open beaker tted with a
magnetic stirrer. Then 3.52 g CTAB (cetyltrimethylammonium
bromide) was added at room temperature (30–35 ꢀC) and
stirred for 30 min. Aer a clear solution was obtained,
Fig. 11 Recycling efficiency of the silver incorporated mesoporous
bottle neck like silica catalyst (MSAGN-10) in the oxidation of
thiophenol.
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tetraethylorthosilicate (TEOS) was added drop wise from a
dropping funnel under stirring condition. Next, 133.34 mg (this
weight corresponds to MSAGN-10) AgNO3 was added and the
stirring was continued for another 5 min. Then 10 mL 0.4 N
NaOH solution was added drop wise taking the time period of
1 h. The stirring was continued for the next 8 h at room
temperature and then aged overnight (12–14 h) at room
temperature. It was ltered and washed thoroughly with
deionized water and dried ꢀat 35–40 ꢀC for 5 days. The dry
powder was calcined at 550 C for 6–8 h under static air.
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Acknowledgements
We thank the Council of Scientic and Industrial Research, New
Delhi for fellowship (SRF to both PD and SR, SPM to BB).
6330 | RSC Adv., 2015, 5, 6323–6331
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