Alumina supported nanoruthenium as efficient heterogeneous catalyst for the selective H2O2 oxidation of aliphatic and aromatic sulfides to sulfoxides

Article abstract of DOI:10.1016/j.molcata.2010.09.008

Highly stable polyvinylpyrrolidone (PVP) capped ruthenium nanoparticles (RuNPs) supported on γ-Al₂O₃ in CH₃CN serve as efficient heterogeneous catalysts for the H₂O₂ oxidation of sulfides into the cor

Full text of DOI:10.1016/j.molcata.2010.09.008

Journal of Molecular Catalysis A: Chemical 332 (2010) 128–137
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Alumina supported nanoruthenium as efficient heterogeneous catalyst for the
selective H2O2 oxidation of aliphatic and aromatic sulfides to sulfoxides
P. Veerakumara, Zong-Zhan Lub, M. Velayudhamb, Kuang-Lieh Lub, S. Rajagopala,∗
a Department of Physical Chemistry, School of Chemistry, Madurai Kamaraj University, Madurai, 625 021, India
b Institute of Chemistry, Academia Sinica, Taipei, 115, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 June 2010
Received in revised form 4 September 2010
Accepted 6 September 2010
Available online 15 September 2010
Highly stable polyvinylpyrrolidone (PVP) capped ruthenium nanoparticles (RuNPs) supported on ␥-Al2O3
in CH3CN serve as efficient heterogeneous catalysts for the H2O2 oxidation of sulfides into the correspond-
ing sulfoxides in excellent yields. The synthesized catalyst I is well characterized by XRD, HRTEM, BET,
H
2 chemisorption, SEM–EDX, AFM, FT-IR, and UV–vis spectral techniques. The catalyst I can be recovered
and reused for several cycles without loss of any activity.
© 2010 Elsevier B.V. All rights reserved.
Keywords:
PVP
␥-Al2O3
30% H2O2
RuNPs
Sulfides
Sulfoxides
1. Introduction
area and well defined porosity. Apart from ruthenium, other metal
nanocatalysts, e.g., Ag [13], Pd [14], Cu [15], Au [16], Ni [17] and Pt
[18] on alumina support were used as the catalysts for oxidation
reactions.
Heterogeneous catalysis has been employed as an important
chemical technology because of its inherent operational advan-
tages such as ease of handling, separation, and recovery for the
reuse of catalysts [1]. Ruthenium, in the nanoform, is known
for the past one decade to show fascinating catalytic activity for
the oxidation reactions [2]. Ruthenium is less expensive com-
pared to Au, Pd, Pt, and Rh which have been extensively used
as catalysts for organic transformations. [3,4]. Nanocrystalline
gamma-phase (␥) alumina is among many polytypes of alumina
that find extensive applications as a catalyst and catalytic support
[5,6]. The number of ruthenium-catalyst systems used, employ-
ing oxygen (or air) as the sole oxidant, is limited. For example,
[RuCl2(p-cymene)]2 complex on carbon [7] and Ru/C [8] have
been shown to catalyze the oxidation of alcohols, Ru/CeO2 [9]
and Ru/Al2O3 [10,11], for the oxidation of alcohols and amines.
Ru-hydroxyapatite (nano-RuHAP) [12] has been used for effective
oxidation reactions using NaIO4 as an oxidant. The most useful
properties provided by alumina as catalyst support are high surface
The oxidation of organic sulfides to the corresponding sulfoxides
is one of the important functional group transformations in organic
synthesis [19]. Organic sulfoxides are also important intermediates
for the asymmetric synthesis of biologically active compounds [20].
The oxidation reaction of organic sulfides is also of potential use in
the decontamination of toxic chemicals and facile electron transfer
reactions [21,22]. The metal-salen complexes (metal = Mn, Cr, Fe,
Ru, Co, V and Ti) have extensively been used, particularly in our
laboratory, as efficient homogenous catalysts for the selective oxy-
genation of organic sulfides and sulfoxides [23–28]. Very recently
efficient oxidation of sulfides to sulfoxides with H2O2 catalyzed
by core–shell (SiO2@WO42−) nanoparticles has been reported [29].
The uncatalyzed H2O2 oxidation of sulfide into sulfoxide or sulfone
at room temperature (RT) is a slow reaction [30]. Herein we report a
novel and simple method for the oxidative conversion of sulfides to
sulfoxides using H2O2 as the oxidant and RuNPs loaded on ␥-Al2O3
as the catalyst I, (Ru (PVP)/␥-Al2O3). H2O2, an environmentally
benign oxidant, is frequently used by many researchers. However,
H2O2 alone has to be used in a controlled manner to reduce the pos-
sibility of an over-oxidation reaction. Therefore, there is a need to
improve the utility of H2O2 for the selective conversion of sulfides
to sulfoxides. The title reaction is represented in Eq. (1).
∗
Corresponding author. Tel.: +91 452 2458246; fax: +91 452 2459105.
E-mail addresses: rajagopalseenivasan@yahoo.com, spveerakumar@gmail.com
(S. Rajagopal).
1381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.09.008

P. Veerakumar et al. / Journal of Molecular Catalysis A: Chemical 332 (2010) 128–137
129
Scheme 1. Two-step process for the synthesis of catalyst I.
tainer. The details of the synthesis of the catalyst I are given in
Scheme 1.
The catalyst support, ␥-Al2O3, was dried at 120 ◦C overnight
prior to impregnation and surface area >40 m2/g (BET) is the prop-
erty which makes ␥-Al2O3 appealing as a catalyst support. The
␥-alumina supplied by Sigma–Aldrich with an average particle size
of <50 nm (TEM) is used as the support in this study.
(1)
2. Experimental
2.1. Materials
RuCl3·H2O
2.3. Characterization
Powder X-ray diffraction (XRD) patterns were recorded using
a
XPERT-PRO diffractometer operated at
a
voltage of 40 kV
˚
(38.0–42.0%
Ru
basis),
1,2-propanediol,
and a current of 30 mA with Cu K␣ radiation (ꢁ = 1.54060 A).
High resolution transmission electron microscopy (HRTEM) and
selected area electron diffraction (SAED) patterns were performed
using a JEOL 3011, 300 kV instrument with a UHR pole piece.
The nitrogen adsorption/desorption was performed by using a
Micromeritics ASAP 2010 sorptometer for high-resolution low
surface area studies. The same apparatus was used to perform
H2 chemisorption measurements. Detailed measurements of sur-
face area were determined from nitrogen adsorption isotherm
by the Brunauer–Emmett–Teller (BET) method and determina-
tion of the pore size distribution from adsorption branch by
the Barrett–Joyner–Halenda (BJH) method. SEM–EDX observations
were carried out on a Hitachi S-3400N electron microscope. The
energy dispersive X-ray analysis (Thermo SuperDry II) is used to
carry out semi-quantitative elemental analysis of the samples. EDX
observations were carried out by a cathodoluminescence detec-
tor. Ultraviolet–visible absorption spectra were recorded using a
SPECORD S100 diode-array spectrophotometer. FT-IR spectra were
recorded using 8400S Shimadzu FT-IR spectrometer in the region
of 4000–400 cm−1 with a spectral resolution of 2 cm−1 using dry
KBr at room temperature. Atomic force microscopy (AFM) (APE
Research nanotechnology, AFM A100 SGS), working at 100 kV, was
used to measure the size of nanoparticles. The samples for AFM
measurements were prepared by evaporating a drop of the dilute
aliquot solution on a thin glass plate. 1H NMR data for the sulfoxides
were acquired on a Bruker 300 MHz NMR spectrometer with CDCl3
as the solvent. GC analyses were carried out using a Shimadzu 17
A model capillary gas chromatography (SE-30 10%; carbowase and
ZB-55%, with FID detector).
polyvinylpyrrolidone (PVP, Mw ∼ 40,000), ␥-Al2O3, methyl
phenyl sulfide (MPS), para-substituted phenyl methyl sulfides
(p-methoxy, p-methyl, p-flouro, p-chloro, p-bromo, and p-nitro),
diphenyl, diethyl, di-n-propyl, di-n-butyl, and di-sec-butyl sulfide
were purchased from Aldrich and used as such. HPLC grade ace-
tonitrile and 30% H2O2 were used as received. All the glassware
were cleaned with aqua regia (v/v, HCl/HNO3, 3:1; caution: aqua
regia is very toxic and should be handled carefully!)
2.2. Synthesis of Ru (PVP)/ꢀ-Al2O3 catalyst
Metal colloids dispersed liquid medium can be divided into
four categories, solvent-stabilized [31], surfactant-stabilized [32],
ligand-stabilized [33] and polymer-stabilized [34]. Metal colloids
of the first three categories are usually unstable when they are
used as catalysts even when the reaction conducted in ambi-
ent conditions. Thus, the polymer-stabilized metal colloids are
the most useful catalytic systems for organic transformation.
Very stable PVP-capped RuNPs were synthesized by reduction of
Ru3+ with 1,2-propanediol in the presence of PVP according to
Chen et al. [35] Briefly, RuCl3 (0.1434 g, 5.25 × 10−4 M), and PVP
(0.5828 g, 5.25 × 10−3 M, as monomeric unit) were dissolved in
1,2-propanediol (100 mL) under stirring to form a dark red solu-
tion and refluxed. The color of the solution changed from dark
red to yellow and then turned to deep green and finally to dark
brown. The dark brown colored solution was then left to cool to
room temperature. Scheme 1 illustrates the two-step process that
we have used for the synthesis of the RuNPs supported on the
alumina: (1) synthesis of the ruthenium colloids, and (2) impreg-
nation of the RuNPs onto the alumina nano surface using PVP as
the stabilizer. The 1,2-propanediol was removed later by dilut-
ing the suspension with 450 mL of 0.3 M NaNO3 aqueous solution.
The black solid was collected by filtration, washed with distilled
water to remove the Na+ and Cl− ions, dried under vacuum at
RT and calcinated at 500 ◦C for 8 h and stored in a closed con-
2.4. Selective oxidation
The general procedure for the sulfoxidation reaction is as fol-
lows: Catalyst I (1 wt%, 0.5–2.0 mmol) and sulfide (1.0 mmol) were
dissolved in CH3CN (3 mL) at 298 K. To this mixture, 30% H2O2
(1.0 mmol) was added dropwise slowly. The mixture was stirred

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P. Veerakumar et al. / Journal of Molecular Catalysis A: Chemical 332 (2010) 128–137
catalyst I (Fig. 1) containing reflections from ␥-Al2O3, Ru, and RuO2
were detected by XRD. Fig. 1 also indicates a very little aggrega-
tion on the catalyst surface. Minor ruthenium reflections peaks
emerging from catalyst I indicate that Ru forms very small parti-
cles dispersed on the support [36]. The Ru colloids showed a diffuse
peak which was separated into three independent peaks located at
2ꢂ = 38.5◦, 42.8◦ and 44◦ stand for Ru (1 0 0), Ru (0 0 2) and Ru (1 0 1),
respectively [37].
The average crystal size of the RuNPs calculated by the Scher-
rer formula is in agreement with the average diameter (5–6 nm)
determined by HRTEM. (vide infra).
3.2. HRTEM results
Representative HRTEM images of the catalyst I (Fig. 2a and b)
show the presence of numerous dark and well-dispersed RuNPs on
the alumina support. These dark spots correspond to the RuNPs.
The individual RuNPs have clear edge and are quite uniform in size
as shown in Fig. 2c. The RuNPs are crystalline and the observed lat-
tice fringes are shown in Fig. 2d. The average diameter of RuNPs
estimated from the Gaussian fit of the particle size distribution his-
tograms is about 5–6 nm. SAED (Selective Area Electron Diffraction)
analysis also confirmed that the RuNPs have an average diameter
of about 5–6 nm.
At low magnification all particles appear as smooth-surfaced
and perfect spheres (Fig. 2a). At higher magnification, the particles
are single crystals exhibiting lattice fringes characteristic of Ru0.
The electron diffraction pattern of the representative ruthenium
nanoparticle is also shown in Fig. 2d; it exhibits bright spot with
rings with the d-spacings of hcp ruthenium metal. Considerable
broadening of the diffraction rings suggests that the particles are
very small and of high crystallinity. These results are in good agree-
ment with the corresponding XRD data. When high-energy X-ray
diffraction is used for the characterization of RuNPs it is realized
that the nanoparticles are only 5 nm in size and are heavily disor-
dered [38]. Calcination at 500 ◦C for 8 h caused some broadening
of the particle size distribution and the particles had crystal faces
with no evidence of any amorphous surface layer (Fig. 2d). Bedford
Fig. 1. The XRD patterns of the catalyst I. The catalyst was calcinated at 500 ◦C in air
for 8 h. Fresh catalyst (black line), and used catalyst (red line). (᭹), Ru; (ꢀ), RuO2;
(ꢀ), (-Al2O3. (For interpretation of the references to color in this figure legend, the
reader is referred to the web version of the article.)
for 1–2 h at room temperature. The resulting solution was diluted
with CHCl3, washed initially with water and then with saturated
brine solution. The organic layer was dried over anhydrous MgSO4,
filtered and concentrated under reduced pressure. The crude prod-
uct was purified by flash column chromatography on silica gel using
petroleum ether and ethyl acetate (2:1) as eluent. The solvent was
evaporated and the product was analyzed using 1H NMR and FT-IR
techniques.
3. Results and discussion
3.1. XRD results
The X-ray diffraction pattern of the catalyst I is shown in Fig. 1.
For the metallic ruthenium, the diffraction peak is around 2ꢂ = 44◦
˚
which is exactly consistent with the d value (2.11 A) of standard
ruthenium metal data file (JCPDS No. 06-0663). The pattern of the
Fig. 2. HRTEM images of ruthenium catalyst I. (a), (b), (c), and (d) correspond to the different images and electron diffraction pattern of the representative ruthenium
nanoparticle is also shown in inset (d). The bars represent (a, b) 50 nm, (c) 20 nm, and (d) 5 nm.

P. Veerakumar et al. / Journal of Molecular Catalysis A: Chemical 332 (2010) 128–137
131
Table 1
Physicochemical properties of the (-Al2O3 and catalyst I.
Sample
Ru content (wt%)
BET surface
area (m2/g)
Langmuir surface
area (m2/g)
t-plot micropore
Average pore
diameter (nm)
Pore volume
area (m2/g)
(cm3/g)
␥-Al2O3
Catalyst I
–
1.0
26.5
24.2
42.4
40.0
0.19
0.14
33
20
0.13
0.05
et al. [39] showed, by calcination at higher temperatures, 600 and
700 ◦C, significant broadening of the particle shape with increas-
ing particle size from round ones (small particles) to flat, elongated
ones (large particles). It has high thermal stability up to 700 ◦C of
small RuNPs which has already been reported [40].
impregnated on the surface of the ␥-Al2O3. The RuNPs were well
dispersed on the outside surface and no obvious aggregation was
observed, whereas unsupported RuNPs were likely to aggregate
immediately.
From the measurements, the BET surface area, Langmuir surface
area, pore volume and average pore diameter were calculated and
the data are given in Table 1.
3.3. BET surface area studies
The change in textual property of ␥-Al2O3 and RuNPs impreg-
nated on ␥-Al2O3 (catalyst I) are given in the supporting
information. A sharp inflection of the adsorption and desorption
isotherms, in particular around P/Po = 0.9712 (for N2 at 77 K), indi-
cates a forced closure of the hysteresis loop. In Fig. 3a the inset
figure shows the details of the isotherm in the P/Po range 0.8–1.0
(dotted square) emphasizing the TSE (tensile strength effect) at
P/Po = 0.97. The (4V/A) term used in the estimation of pore average
sizes corresponds to the assumed cylindrical model of pores. How-
ever, this assumption of cylindrical model of pores is also cited in
BJH estimates of pore volume and surface area distributions. Fig. 3b
shows the adsorption and desorption isotherm value of catalyst I
at P/Po = 0.9711. However, pores with diameters smaller than 4 nm
show no hysteresis and are completely filled and emptied at sim-
ilar pressures, resulting in a reversible adsorption and desorption
isotherm [42]. The results may point to localization of the RuNPs
mainly at the outer surface of the ␥-Al2O3 and only some of them
may be present inside the ␥-Al2O3 pores. The average pore diam-
eter, pore volume and surface area of the ␥-Al2O3 decrease after
impregnation with RuNPs (Table 1). Okal et al. [43] reported that
the impregnation of ␥-Al2O3 with an aqueous solution of RuCl3
caused a 25% decrease in BET surface area and some drop of the pore
volume. Mieth and Schwarz [44] pointed out that the partial dis-
solution of the support during impregnation may have significant
consequences on the catalyst activity. Similar effect was observed
in other systems such as Pd/Al2O3 [45] and Pt/Al2O3 [46]. The
adsorption studies help us to conclude that due to impregnation of
RuNPs on ␥-Al2O3 the surface area, pore diameter and pore volume
of catalyst I decreased.
The N2 adsorption and desorption isotherms for ␥-Al2O3 and
catalyst I were measured at 77 K. Prior to the adsorption mea-
surement, the samples were pretreated under helium gas flow
at 300 ◦C for 4 h. Total surface areas were calculated accord-
ing to the BET and Langmuir methods [41]. Alumina exhibited a
Brunauer–Emmett–Teller (BET) surface area of 26.5 m2/g, including
micropore area of 0.19 m2/g and external surface area of 29.2 m2/g.
The catalyst I gave a BET surface area of 24.2 m2/g, average micro-
pore area of 0.14 m2/g and external surface area of 26.3 m2/g. The
decrease in surface area is mainly due to the reduction of micropore
surface area. The results help us to conclude that the RuNPs were
Fig. 3. Adsorption–desorption isotherm for (a) (-Al2O3 and (b) catalyst I.
Fig. 4. Hydrogen chemisorption isotherm of catalyst I.

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P. Veerakumar et al. / Journal of Molecular Catalysis A: Chemical 332 (2010) 128–137
Fig. 5. SEM pictures of catalyst I: (a) 40 K× magnification, (b) 100 K× magnification and (c) EDX analysis of catalyst I.
3.4. Hydrogen chemisorption studies
Identification lines for the major emission energies of Al metal from
the catalyst I are displayed and these correspond with peaks in the
spectrum, thus giving confidence that ruthenium has been correctly
identified (see supporting information).
Fig. 4 presents the hydrogen chemisorption on catalyst I carried
out at 77 K using Micromeritics 2020 equipment with use of glass
volumetric adsorption system described elsewhere [47]. This study
provides hydrogen/metal (H/M) data. (H/M is the ratio of the num-
ber of adsorbed hydrogen atoms to the total number of metal atoms
multiplied by 100%.) Each measurement was performed three times
and the average value H/M = 1:1 was used. The total amount of
adsorbed hydrogen was obtained by extrapolating the linear high-
pressure part of the isotherm to zero pressure. H2 chemisorption
at 77 K is helpful to determine the percent dispersion and average
Ru particle size to be 17% and 5.1 nm respectively. The equilibrium
adsorption measurements were made for equilibrium pressures of
0–850 mm Hg range. Average RuNPs size was estimated using the
relation l (chem) = 5/Sd, where S is the metal surface area and d is
the density of ruthenium [48,49].
The reversible hydrogen isotherms (Fig. 4) show a linear depen-
dence on pressure in the 200–850 mm Hg range but the uptake
decreased slowly at 200 mm Hg (see supporting information).
Sayari et al. [50] found that at room temperature reversible H2
chemisorption occurs in appreciable amounts on the Ru crystal-
lites with sizes within the limited range of 5–6 nm. Zupanc et al.
[51] explained this finding by a modification in the surface ener-
getics of the Ru crystallites with the change of their size. However,
only very small change in the mean size of RuNPs was detected by
the hydrogen chemisorption.
3.6. UV–vis absorption spectral data
UV–vis absorption spectroscopy was also used to establish the
complete reduction of Ru3+ to Ru0 state [52,53]. The Ru3+ is mixed
with PVP in 1,2-propanediol and heated to the temperatures indi-
cated in Scheme 2.
Ru3+ (dark red) was reduced, step-by-step, to Ru0 (dark brown)
during the reaction: (a) the reaction mixture at the beginning of the
reaction; (b) reaction solution obtained at approximately 20 min of
the reaction; (c) at approximately 30 min (d and e), at 50–90 min;
(f) at the end of the reaction. The UV–vis absorption spectra of the
solutions are displayed in Fig. 6 and color changes with the change
of temperature is shown in Scheme 2.
One of the well-known reduction methods for the preparation
of metal colloid is by employing 1,2-propanediol as a solvent and
reducing agent, and the polymer like PVP as the stabilizing agent.
3.5. SEM–EDX observations
SEM–EDX shows typical results of catalyst I deposited on a car-
bon strip by means of SEM. Fig. 5a represents the view of the
sample at 40k× magnification which stands for examining the area
of 9.6 × 9.6 mm2 surface. These objects consist of tiny particles,
confirmed by SEM results.
Analysis through energy dispersive X-ray (EDX) spectrometers
confirmed the presence of elemental ruthenium and aluminum sig-
nals from the catalyst I (Fig. 5). The vertical axis displays the number
of X-ray counts whilst the horizontal axis displays energy in keV.
Fig. 6. UV–vis absorption spectra of the PVP-Ru in a 1,2-propanediol solution at
different stages.

P. Veerakumar et al. / Journal of Molecular Catalysis A: Chemical 332 (2010) 128–137
133
Scheme 2. Color changes of the reaction solution during the course of the Ru nanocolloids formation with 1,2-propanediol taken as solvent.
Table 2
3.7. AFM analysis
Relationship between the oxidation states of Ru in the Ru-PVP in 1,2-propanediol
with the absorbance data.
AFM provides real topographical image of sample surface. Since
ruthenium nanocatalysts exhibit physical properties different from
bulk ruthenium and the properties largely dependent on their size
and structure, a good structural characterization becomes possible.
A good dispersion of the nanoparticles is possible to image them
individually using AFM. Further, this technique enables measur-
ing the size of the particle, considering the particle height rather
than its width because the particle may be distorted by the AFM
tip geometry. Fig. 7 provides three-dimensional structural infor-
mation, which is not available with conventional light-scattering
measurements. Fig. 7a and b shows three-dimensional overview of
the fresh catalyst I in a scan size of 1.7 × 1.7 ␮m.
Oxidation state
Temp (◦C)
Color
Wavelength (nm)
Ru3+
Ru2+
Ru+
RT
Dark red
Yellow
Green
391
345
328
80–120
120–180
>200
Ru0
Dark brown
No characteristic peak
The standard reduction potential of Ru3+ to Ru0 is relatively high
(E0 = 0.3862 V) [54]. The redox properties of 1,2-propandiol is sim-
ilar to 1,2-ethanediol [55] but it is less toxic [56].
The UV–vis spectrum of the freshly prepared Ru3+ solution in
1,2-propanediol exhibited a broad absorption with maximum at
391 nm. It is well known that colloidal dispersion of metals exhibit
broad regions of absorption in the UV–vis range. These are due to
the excitation of plasma resonances or interband transitions, and
they are thus a very characteristic property of the metallic nature
of the particles (Fig. 6b). This peak is shifted to a shorter wave-
length, 345 nm, after 20 min of reduction (Fig. 6c). At this reduction
time the solution turned to light yellow color. The intensity of the
peak decreases at 328 nm and the solution turned dark green by
further heating. After about 110 min, the absorption peak disap-
peared completely, which indicated that Ru3+ was entirely reduced
to Ru0 (Fig. 6f and g). The increasing scattering absorbance at wave-
lengths between 300 and 800 nm with time revealed the formation
of ruthenium colloids [57]. Each color change represents a change
in the oxidation state of ruthenium and reduction takes place step-
wise from Ru3+ state to Ru0 state and the details are collected in
Table 2.
The nanoparticles have an average size of 50–100 nm and are
separated from each other. The particle size of nanoruthenium was
determined using HRTEM as shown in Fig. 2.
3.8. FT-IR analysis
The ␥-alumina is chemically inert in 1,2-propanediol under the
conditions used in this study and thus used as a support for Ru
catalyst. The catalyst I is characterized using FT-IR spectroscopy
and the spectrum is shown in Fig. 8.
The FT-IR fingerprints of the PVP indicate that the polymer is
present on the surface of the RuNPs serving as a good stabiliz-
ing agent. Compared to the spectrum of PVP, the resonance peak
of C O of PVP coordinated to RuNPs was shifted from1656 cm−1
to 1644 cm−1 while the C–N peaks at (1014 cm−1 and 1074 cm−1
)
were not changed. These changes suggest that the coordination of
nitrogen-containing heterocyclic ring of the PVP to the RuNPs sur-
Fig. 7. (a) AFM image of catalyst I in 2D and (b) the 3D representation.

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P. Veerakumar et al. / Journal of Molecular Catalysis A: Chemical 332 (2010) 128–137
Table 3
Oxidation of various sulfides H2O2 catalyzed catalyst Ia.
Entry
1
Substrate
Time (min)
120
Conversion (%)b
100
Yield (%)b,c
98, 92b, 98c
Selectivity (%)b,c
98
S
S
2
3
4
150
150
150
98
98
99
96
97
96
95
98
96
Cl
S
Br
F
S
S
5
6
180
120
99
98
90
92
95
98
O2N
H3C
S
S
3
100
75
99
99
97
99
99
99
H3CO
S
8f
S
9
110
100
99
95
92
96
97
94
S
10
S
S
11
12
100
100
>98
>98
100
100
99
98
S
13
100
100
>97
>97
100
100
99
97
S
14
a
Reaction conditions: sulfide (1 mmol), 30%H2O2 (1 mmol), catalyst I (1 wt%, 0.5–2 mmol), CH3CN (3 ml).
Determined by GC or 1H NMR using an internal standard technique on the crude reaction mixture.
Yield = No. of moles of sulfoxide/No. of moles of sulfide; Selectivity = Sulfoxide/(Sulfoxide + Sulfone).
b
c
d,eThe catalyst is reused repeatedly.
f
MeOH was used (70% yield in CH3CN medium).
face may be weaker compared to the carbonyl group [34]. Such
PVP coordination to the surface of the RuNPs was also proposed
by Bonet et al. [58] the spectrum also exhibits an O–H stretch-
ing mode, associated with water and hydroxide species centered
at ∼3470 cm−1 and ∼1620 cm−1 respectively. Though the signal
obtained for the FT-IR spectrum is predominantly from the RuNPs
there is some contribution from the surface of ␥-alumina as shown
in Fig. 8.
3.9. Selective oxidation of sulfides to sulfoxides
In this study, 14 organic sulfides are oxidized to sulfoxides by
using H2O2 and catalyst I (Table 3).
After the completion of the reaction, the catalyst was extracted
with CHCl3. The oxidation process was repeated for three consecu-
tive cycles with little loss of activity (entry 1). The data collected in
Table 3 show that the rate of oxidation of aromatic sulfides varied

P. Veerakumar et al. / Journal of Molecular Catalysis A: Chemical 332 (2010) 128–137
135
Table 4
Oxidation of MPS by H2O2 in the presence of catalyst Ia.
Entry
Solvent
Conversion (%)b
Yield (%)b,c
Selectivity (%)d
1
2
3
4
5
6
7
8
9
–
80
80
90
95
98
72
75
88
99
52
75
82
85
97
70
60
71
98
86
73
80
85
98
80
85
86
98e
H2O:CH3CN
MeOH
EtOH
CH3CN
CHCl3
CH2Cl2
1,4-Dioxane
MeOH
a
Reaction conditions: 3 mL of solvent; 1.0 mmol of MPS; 1 mmol, H2O2; 0.5 mmol
(catalyst I); RT with stirring.
b
Determined by GC on the crude reaction mixture.
Yield = No. of moles of sulfoxide/No. of moles of sulfide.
Selectivity = Sulfoxide/(Sulfoxide + Sulfone).
The substrate used was DPS.
c
d
e
3.11. Influence of different solvents with H2O2
Fig. 8. FT-IR spectra of (a) PVP (b) (-Al2O3 and (c) catalyst I after treatment at 500 ◦C.
We first investigated the oxidation reaction with MPS as model
substrate by using 30% H2O2 (1.0 mmol) as an oxidant. To a solution
of 3 mL of MPS (1.0 mmol) and catalyst I (1.0 mmol) taken in a sol-
vent (the reaction was carried out in different solvents) the oxidant
was added at room temperature. The results given in Table 4 show
that the reaction is sensitive to the change of solvent.
Generally sulfides are insoluble in water, but when a mixed sol-
vent (H2O:CH3CN) 1:1 (v/v) is used for oxidation the yield is 75%.
As far as the oxidation of MPS is concerned CH3CN is the best sol-
vent. On the other hand, CH3OH seems to be the better solvent for
the oxidation of DPS and it provides the highest yield (99%) and
selectivity is 98%. The data collected in Table 4 show that of all the
solvents, the selectivity and yields are better in CH3CN but lower
in the mixed solvent H2O:CH3CN (1:1) because the substrates are
poorly soluble in water specifically.
with the nature of substituents on the phenyl ring (entries 2–7). The
presence of electron withdrawing substituents on the phenyl ring
required slightly longer reaction times when compared to electron
donating substituents. Selectivity towards the formation of sulfox-
ide is higher than 90% in all cases and in most of the cases it is close
to 100%. The other product expected is the corresponding sulfone
but the yield of sulfone is negligible here. However, with increase
in time and H2O2, the formation of sulfone (i.e. overoxidation) has
been observed. As far as diphenyl sulfide (DPS) is concerned MeOH
seems to be better solvent than CH3CN. The alkyl sulfides are more
reactive than aryl sulfides and the reaction is little sensitive to steric
effect. The values of selectivity for the obtained products are pre-
sented in Table 3. Notably, the use of catalyst I at 1 wt% resulted
in the quantitative conversion of sulfides to the corresponding sul-
foxides with 1 mmol of 30% H2O2.
3.12. Influence of reaction time with MPS
The influence of the reaction time was investigated with catalyst
I and MPS as the substrate at RT and the results were summarized
in Table 5.
3.10. Product analysis
Initially the oxidation reaction was slow till 60 min but the reac-
tion is completed in 120 min (Table 5). When the progress of the
oxidation reaction was monitored for 30 min, the yield was 60% but
the maximum yield is obtained in 120 min. But the increase in the
selectivity is also significant, from 85% to 98%, compared with the
results obtained in 30 min (Table 5, entry 1 vs. entry 4). If the reac-
tion time is increased to 150 min there is no change in selectivity
as well as in yield.
The H2O2 oxidation of sulfides in the presence of the cata-
lyst I was monitored via 1H NMR to identity the products of the
reaction. The 1H NMR spectra for the products obtained during
the course of the reaction (entry1, 5, 8, 10 and 13 in Table 3)
are shown in Figs. S4–S15 (see supporting information). CDCl3 is
used as the solvent and TMS as an internal standard. The peak
appeared as a singlet at ı 2.73 ppm corresponds to methyl proton.
The aromatic region is also informative as the aromatic protons
of sulfoxide appear as multiplets centered at ı values 7.4–7.6 ppm
[59–63]. The FT-IR spectrum of the product obtained from the oxi-
dation of MPS shows S O stretching frequency at 1043 cm−1 and
no stretching frequency at 1150 cm−1 corresponding to the forma-
tion of sulfone is seen. The interesting aspect of this work is that the
substrate to H2O2 molar ratio of 1:1 was sufficient enough to give
98% conversion within 2 h. Increasing the amount of oxidant to 2:1
(oxidant:substrate) molar ratio causes the formation of a mixture of
sulfoxide and sulfones which is confirmed using 1H NMR and FT-IR
spectroscopy (see supporting information). The aqueous layer con-
taining the catalyst was washed with acetone three times, dried
in vacuum and preserved for the next run to check their catalytic
activity. This product analysis study demonstrates that sulfoxide is
the only product formed under the present reaction conditions.
Table 5
Influence of the reaction time with MPSa.
Entry
Time (min)
Conversion (%)b
Yield (%)b,c
Selectivity (%)d
1
2
3
4
5
30
60
90
120
150
73
89
92
98
98
60
70
85
98
98
85
90
95
98
98
a
Reaction conditions: 3 mL of solvent; 1.0 mmol of MPS; 1 mmol, H2O2; 0.5 mmol
(catalyst I); RT with stirring.
b
Determined by GC on the crude reaction mixture.
Yield = No. of moles of sulfoxide/No.of moles of sulfide.
Selectivity = sulfoxide/(sulfoxide + sulfone).
c
d

136
P. Veerakumar et al. / Journal of Molecular Catalysis A: Chemical 332 (2010) 128–137
Table 6
Oxidation of MPS using 30% H2O2 using different catalysts.
Catalysta
Size (nm)
Mol. (%)
Time (h)
Solvent
Conversion (%)
94
Yield (%)
2−
2−
LDH-WO4 [64]
SiO2@WO4 [29]
WO3/MCM-48 [65]
Nanobiocatalyst [66]
TiO2 [62]
␥-Al2O3
Catalyst I
RuCl3
–
4.4
2.0
1.0
5
1.25
0.5
0.5
0.5
0.5
1.5
4.0
H2O
88
82
50–60
30–100
90
6–8
<50
5–6
–
CH2Cl2:CH3OH
CH3OH
–
CH3CN
CH3CN
CH3CN
CH3CN
100
100
>99%
100
60
100
50
100
>99%
99
10 min
3.0
5.0
2.0
5.0
50b
98c
Traced
aReferences.
b,c,dReaction conditions as exemplified in the experimental procedure.
3.13. Comparison with other catalyst systems
In order to evaluate the efficiency of the catalyst, we have
compiled the data available on the H2O2 oxidation of MPS in the
presence of other catalysts in Table 6. It is clear from the data col-
lected in Table 6 that although the yields are similar, and mol%
and the size of catalyst I used in the present case are smallest, the
selectivity is better than the other systems, i.e., metal nanoparticles
are very attractive catalysts compared to bulk catalytic materials
due to their high surface-to-volume ratio. The additional advan-
tage with the present system is that the reaction is conducted at
room temperature.
To investigate the effect of size on the catalytic activity, different
supported catalysts used for the oxidation of organic sulfides are
collected in Table 6. In the present work when catalyst I is used for
the oxidation of MPS (as illustrated in Table 3) the reaction is carried
out using 0.5 mol% of catalyst I and 1.0 mmol of 30% H2O2. The data
in Table 6 clearly indicate that the catalyst I has the smallest size but
2−
exhibits comparable efficiency and selectivity with SiO2@WO4
[29] and nanocatalyst systems [64–66]. The novelty of this cata-
lyst is that less quantity of catalyst (0.5 mol%) is needed during the
sulfoxidation reaction compared to other catalysts (Table 6). There-
fore, the catalytic activity is influenced by the nature and size of the
nanocatalyst. Thus, ruthenium catalyst (0.5 mol%) in combination
with H2O2 provided the complete conversion of MPS to MPSO (98%
yield), indicating a very good catalytic performance in sulfoxida-
tion reactions. The major problem with the nanobiocatalyst [66]
(chloroperoxidase-coated magnetic nanoparticles) is the unsatis-
fied catalyst stability during operation and the recycling process
and the dramatically reduced activity in comparison with catalyst
I.
Scheme 3. Mechanism for the oxidation of sulfides using catalyst I in CH3CN.
the particles [68]. It is known that organic sulfides are oxidised by
H2O2 in a heterolytic process involving the nucleophilic attack of
the sulfur atom on the oxygen according to Scheme 4.
The rate of the oxidation of substrates increases with the
increased nucleophilicity on the sulfur atom. The oxygen atoms
of the H2O2 molecule bound on the surface of Ru(0) can interact
with sulfide to form an intermediate which readily forms the cor-
responding sulfoxide and water as the products. Aqueous H2O2 is
an ideal oxidant owing to its high effective-oxygen content, clean-
liness (it produces only water as by-product), and enough safety
in storage and operation. Therefore, the importance of H2O2 as a
“green” oxidizing agent has grown considerably. The progress of
the reaction is monitored using TLC and yield of products analyzed
by using GC method. Also, the catalyst I was reused up to three
times (entry1 in Table 3) and the catalytic activity decreased only
slightly.
3.14. Mechanism for the oxidation of organic sulfides to sulfoxides
Generally, thiols (–SH) interact strongly with RuNPs [67]. Here
we presume that organic sulfides also bind fairly well with Ru
surface. A possible reaction mechanism for the H2O2 oxidation of
sulfides to sulfoxides using catalyst I is proposed in Scheme 3.
Initially, the sulfur atom of the substrate is likely to attach with
the surface of the ruthenium nanoparticles. It is proposed that the
interaction between the metal and sulfur is due to metal-to-sulfur
charge transfer as the result of thiol–metal bonds at the surface of
Scheme 4. Nucleophilic attack of the sulfur to oxygen atom of H2O2.

P. Veerakumar et al. / Journal of Molecular Catalysis A: Chemical 332 (2010) 128–137
137
4. Conclusions
[24] R. Sevvel, S. Rajagopal, C. Srinivasan, N.M.I. Alhaji, A. Chellamani, J. Org. Chem.
65 (2000) 3334–3340.
[25] V.K. Sivasubramanian, M. Ganesan, S. Rajagopal, R. Ramaraj, J. Org. Chem. 67
(2002) 1506–1514.
[26] N.S. Venkataramanan, S. Premsingh, S. Rajagopal, K. Pitchumani, J. Org. Chem.
68 (2003) 7460–7470.
[27] N.S. Venkatramanan, G. Kuppuraj, S. Rajagopal, Coord. Chem. Rev. 249 (2005)
1249–1268.
[28] A. Mary Imelda Jeyaseeli, S. Rajagopal, J. Mol. Catal. A: Chem. 309 (2009)
103–110.
[29] B. Sreedhar, P. Radhika, B. Neelima, N. Hebalkar, A.K. Mishra, Catal. Commun.
10 (2008) 39–44.
[30] D.N. Dybtsev, A.L. Nuzhdin, H. Chun, K.P. Bryliakov, E.P. Talsi, V.P.K. Kim, Angew.
Chem. Int. Ed. 45 (2006) 916–920.
[31] O. Vidoni, K. Philippot, C. Amiens, B. Chaudret, O. Balmes, J.O. Malm, J.O. Bovin,
F. Senocq, M.J. Casanove, Angew. Chem. Int. Ed. 38 (1999) 3736–3738.
[32] A. Nowicki, V.L. Boulaire, A. Roucouxa, Adv. Synth. Catal. 349 (2007) 2326–
2330.
[33] C. Pan, K. Pelzer, K. Philippot, B. Chaudret, F. Dassenoy, P. Lecante, M.-J.
Casanove, J. Am. Chem. Soc. 123 (2001) 7584–7593.
This work provides important insight into the heterogeneous
catalysis on the H2O2 oxidation of sulfides in the presence of cat-
alyst I, which is one of the most convenient, cheap and green
methods for the synthesis of various sulfoxides. The vital role of
catalyst is surface binding and the surface binding facilitates the
interaction of substrate (thioether) with the oxidant (H2O2) result-
ing in the efficient formation of the selective oxidized products.
The synthesized catalyst I was characterized using XRD, HRTEM,
BET, H2 chemisorption, SEM–EDX, AFM, FT-IR, and UV–vis spectral
techniques and the characterization of products was done by 1H
NMR, FT-IR, and GC analysis. This nanocatalyst may prove useful in
the development of selective oxidation catalyst for the oxidation of
other organic substrates.
[34] M. Zawadzki, J. Okal, Mater. Res. Bull. 43 (2008) 3111–3121.
[35] W. Chen, J.R. Davies, D. Ghosh, M.C. Tong, J.P. Konopelski, S. Chen, Chem. Mater.
18 (2006) 5253–5259.
Acknowledgements
[36] I. Balint, A. Miyazaki, K. Aika, J. Catal. 207 (2002) 66–75.
[37] I. Balint, A. Miyazaki, K. Aika, Chem. Commun. (2002) 630–631.
[38] A.M. Karim, V. Prasad, G. Mpourmpakis, W.W. Lonergan, A.I. Frenkel, J.G. Chen,
D.G. Vlachos, J. Am. Chem. Soc. 131 (2009) 12230–12239.
[39] N. Bedford, C. Dablemont, G. Viau, P. Chupas, V. Petkov, J. Phys. Chem. C 111
(2007) 18214–18219.
The authors thank UGC for a JRF position under Meritorious
Fellowship Scheme and DST for the financial support.
Appendix A. Supplementary data
[40] J. Okal, Catal. Commun. 11 (2010) 508–512.
[41] S. Lowell, J.E. Shields, M.A. Thomas, M. Thommes, Characterization of Porous
Solids and Powders: Surface Area, Pore Size and Density, Springer, 2004, ISBN
1402023022.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molcata.2010.09.008.
[42] J.C. Groen, L.A.A. Peffer, J. Perez-Ramirez, Micropor. Mesopor. Mater. 60 (2003)
1–17.
[43] J. Okal, M. Zawadzki, L. Kepinski, L. Krajczyki, W. Tylus, Appl. Catal. A: Gen. 319
(2007) 202–209.
[44] J.A. Mieth, J.A. Schwarz, J. Catal. 118 (1989) 218–226.
[45] I. Balint, A. Miyazaki, K. Aika, Chem. Mater. 13 (2001) 932–938.
[46] J.R. Regalbuto, A. Navada, S. Shadid, M.L. Bricker, Q. Chen, J. Catal. 184 (1999)
335–348.
[47] E. Bus, J.T. Miller, J.A. van Bokhoven, J. Phys. Chem. B 109 (2005) 14581–14587.
[48] J.G. Goodwin Jr., J. Catal. 68 (1981) 227–232.
[49] B.J. Chen, G. Goodwin Jr., J. Catal. 158 (1996) 228–235.
[50] A. Sayari, H.T. Wang, J.G. Goodwin Jr., J. Catal. 93 (1985) 368–374.
[51] C. Zupanc, A. Hornung, O. Hinrichsen, M. Muhler, J. Catal. 209 (2002) 501–514.
[52] X. Yan, H. Liu, K.Y. Liew, J. Mater. Chem. 11 (2001) 3387–3391.
[53] Y. Zhang, J. Yu, H. Niu, H. Liu, J. Colloid Interface Sci. 313 (2007) 503–510.
[54] R.C. Weast, CRC Handbook of Chemistry and Physics, 70th ed., CRC Press, Boca
Raton, FL, 1989, D-154.
[55] C. Bock, C. Paquet, M. Couillard, G.A. Botton, B.R. MacDougall, J. Am. Chem. Soc.
126 (2004) 8028–8037.
[56] N. Dimitratos, J.A.L. Sanchez, S. Meenakshisundaram, J.M. Anthonykutty, G.
Brett, A.F. Carley, S.H. Taylor, D.W. Knight, G.J. Hutchings, Green Chem. 11
(2009) 1209–1216.
[57] A. Miyazaki, I. Balint, K. Aika, Y. Nakano, J. Catal. 204 (2001) 364–371.
[58] F. Bonet, K.T. Elhsissen, K. Vijaya Sarathy, Bull. Mater. Sci. 23 (2000) 165–168.
[59] T.S. Smith, V.L. Pecoraro, Inorg. Chem. 41 (2002) 6754–6760.
[60] D.H. Koo, M. Kim, S. Chang, Org. Lett. 7 (2005) 5015–5018.
[61] G. Du, J.H. Espenson, Inorg. Chem. 44 (2005) 2465–2471.
[62] K.J. Adaikalasamy, N.S. Venkataramanan, S. Rajagopal, Tetrahedron 59 (2003)
3613–3619.
[63] S. Maksoud, A.B. Daniele, Sorokin, Green Chem. 10 (2008) 447–451.
[64] B.M. Choudary, B. Bharathi, Ch.V. Reddy, M.L. Kantam, J. Chem. Soc. Perkin Trans.
1 (2002) 2069–2074.
[65] B. Karimi, M. Ghoreishi-Nezhad, J.H. Clark, Org. Lett. 7 (2005) 625–628.
[66] W. Wang, Y. Xu, D.I.C. Wang, Z. Li, J. Am. Chem. Soc. 131 (2009) 12892–12893.
[67] N. Chakroune, G. Viau, S. Ammar, L. Poul, D. Veautier, M.M. Chehimi, C. Man-
geney, F. Villain, F. Fievet, Langmuir 21 (2005) 6788–6796.
[68] T. Tsukatani, H. Fujihara, Langmuir 21 (2005) 12093–12095.
References
[1] A.L. Nuzhdin, D.N. Dybtsev, V.P. Fedinb, G.A. Bukhtiyarova, Dalton Trans. (2009)
10481–10485.
[2] J.M. Basset, J.P. Candy, C.C. Santini, in: M. Beller, C. Bolm (Eds.), Transition Metals
for Organic Synthesis, 2, Wiley-VCH, Weinheim, 1998, p. 387.
[3] K. Yamaguchi, N. Mizuno, Angew. Chem. Int. Ed. 42 (2003) 1480–1483.
[4] Y. Na, S. Park, S.B. Han, H. Han, S. Ko, S. Chang, J. Am. Chem. Soc. 126 (2004)
250–258.
[5] B. Tang, J. Ge, L. Zhuo, G. Wang, J. Niu, Z. Shi, Y. Dong, Eur. J. Inorg. Chem. (2005)
4366–4369.
[6] L.J. Alvarez, L.E. Leon, J.F. Sam, M.J. Capitan, J.A. Odriozola, J. Phys. Chem. 99
(1995) 17872–17876.
[7] E. Choi, C. Lee, Y. Na, S. Chang, Org. Lett. 4 (2002) 2369–2371.
[8] S. Mori, M. Takubo, K. Makida, T. Yanase, S. Aoyagi, T. Maegawa, Y. Monguchi,
H. Sajiki, Chem. Commun. (2009) 5159–5161.
[9] F. Vocanson, Y.P. Guo, J.L. Namy, H.B. Kagan, Synth. Commun. 28 (1998)
2577–2582.
[10] K. Yamaguchi, N. Mizuno, Angew. Chem. Int. Ed. 41 (2002) 4538–4542.
[11] K. Yamaguchi, N. Mizuno, Chem. Eur. J. 9 (2003) 4353–4361.
[12] C.M. Ho, W.Y. Yu, C.M. Che, Angew. Chem. Int. Ed. 43 (2004) 3303–3307.
[13] L. Zhang, C. Zhang, H. He, J. Catal. 261 (2009) 101–109.
[14] J. Grunwaldt, M. Caravati, A. Baiker, J. Phys. Chem. B 110 (2006) 25586–25589.
[15] C. Evangelisti, G. Vitulli, E. Schiavi, M. Vitulli, S. Bertozzi, P. Salvadori, L.
Bertinetti, G. Martra, Catal. Lett. 116 (2007) 57–62.
[16] L. Xu, C. He, M. Zhu, S. Fang, Catal. Lett. 114 (2007) 202–205.
[17] L. Zhang, X. Wang, B. Tan, U.S. Ozkan, J. Mol. Catal. A: Chem. 297 (2009) 26–34.
[18] S. Kim, S. Ihm, Ind. Eng. Chem. Res. 41 (2002) 1967–1972.
[19] I. Fernandez, N. Khiar, Chem. Rev. 103 (2003) 3651–3705.
[20] M.C. Carreno, Chem. Rev. 95 (1995) 1717–1760.
[21] S. Balakumar, P. Thanasekaran, E. Rajkumar, K. John Adaikalasamy, S. Rajagopal,
R. Ramaraj, T. Rajendran, B. Manimaran, K.L. Lu, Org. Biomol. Chem. 4 (2006)
352–358.
[22] E. Rajkumar, S. Rajagopal, Photochem. Photobiol. Sci. 7 (2008) 1407–1414.
[23] A. Chellamani, P. Kulanthaipandi, S. Rajagopal, J. Org. Chem. 64 (1999)
2232–2239.