Peroxomolybdate supported on water soluble polymers as efficient catalysts for green and selective sulfoxidation in aqueous medium

Article abstract of DOI:10.1016/j.molcata.2016.09.026

Peroxomolybdenum(VI) species immobilized on linear water soluble polymers formulated as, [MoO(O₂)₂(sulfonate)]–PS [PS?=?poly(sodium vinyl sulfonate)] (PSMo) and[Mo₂O₂(O₂)₄(carboxylate)]–PA [PA?=?poly(sodium acrylate)](PAMo) served as efficient and recyclable catalysts for selective oxidation of organic sulfides to sulfoxide in water with 30% H₂O₂. The new polymer anchored complex, PSMo has been prepared by reacting H₂MoO₄ and 30% H₂O₂ with macromolecular ligand, poly(sodium vinyl sulfonate) and characterized by elemental analysis, spectral studies (FTIR, Raman, ¹³C NMR, ⁹⁵Mo NMR, UV–vis), SEM, EDX and TGA-DTG analysis. The catalytic protocol effects clean and facile oxidation of a wide range of sulfides to afford pure sulfoxides with high TOF and excellent chemoselectivity at ambient temperature,with an easy work-up procedure. The catalysts can be regenerated in-situ and recycled at least up to ten consecutive reaction cycles without significant loss of activity and are amenable for ready scalability. The catalytic procedure thus provides an ecologically sustainable alternative as it involves water as solvent, H₂O₂ as green oxidant, a recyclable non-polluting catalyst, and is absolutely free from halide, organic co-solvent or co-catalyst. This is the first catalytic protocol for selective aqueous oxidation of sulfides using a catalytic system based on peroxometal species supported on water soluble polymers.

Full text of DOI:10.1016/j.molcata.2016.09.026

Journal of Molecular Catalysis A: Chemical 425 (2016) 21–30
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Peroxomolybdate supported on water soluble polymers as efficient
catalysts for green and selective sulfoxidation in aqueous medium
Jeena Jyoti Boruah1, Kabirun Ahmed1, Srewashi Das, Sandhya Rani Gogoi, Gangutri Saikia
, Mitu Sharma, Nashreen S. Islam∗
Department of Chemical Sciences, Tezpur University, Napaam, Tezpur 784028, Assam, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 10 June 2016
Received in revised form 19 August 2016
Accepted 16 September 2016
Available online 16 September 2016
Peroxomolybdenum(VI) species immobilized on linear water soluble polymers for-
mulated
as,
[MoO(O2)2(sulfonate)]–PS
[PS = poly(sodium
vinyl
sulfonate)]
(PSMo)
and[Mo2O2(O2)4(carboxylate)]–PA [PA = poly(sodium acrylate)](PAMo) served as efficient and recy-
clable catalysts for selective oxidation of organic sulfides to sulfoxide in water with 30% H2O2. The
new polymer anchored complex, PSMo has been prepared by reacting H2MoO4 and 30% H2O2 with
macromolecular ligand, poly(sodium vinyl sulfonate) and characterized by elemental analysis, spectral
studies (FTIR, Raman, 13C NMR, 95Mo NMR, UV–vis), SEM, EDX and TGA-DTG analysis. The catalytic
protocol effects clean and facile oxidation of a wide range of sulfides to afford pure sulfoxides with
high TOF and excellent chemoselectivity at ambient temperature,with an easy work-up procedure. The
catalysts can be regenerated in-situ and recycled at least up to ten consecutive reaction cycles without
significant loss of activity and are amenable for ready scalability. The catalytic procedure thus provides
an ecologically sustainable alternative as it involves water as solvent, H2O2 as green oxidant, a recyclable
non-polluting catalyst, and is absolutely free from halide, organic co-solvent or co-catalyst. This is the
first catalytic protocol for selective aqueous oxidation of sulfides using a catalytic system based on
peroxometal species supported on water soluble polymers.
Keywords:
Peroxomolybdenum(VI) catalyst
Water soluble polymers
Chemoselective sulfoxidation
Non-polluting oxidation catalyst
Selective aqueous sulfoxidation
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
cesses that use safer green reagents and oxidants, as well as benign
non-polluting reaction media [15–32].
Selective oxidation of organic sulfur containing compounds to
sulfoxides is one of the pivotal reactions in organic synthesis, which
holds immense interest from both chemical as well as biologi-
cal perspectives [1–9]. Sulfoxides and sulfones serve as versatile
synthetic precursors for production of a range of fine chemi-
cals and biologically active molecules including pharmaceuticals,
agrochemicals and chiral auxiliaries [4–9]. In addition, oxidative
desulfurization of crude oil based on sulfide oxidation, is emerging
as a sustainable alternative to the traditional catalytic hydrodesul-
furization process [10–14]. Currently, with the ever increasing
environmental concerns, research efforts are being oriented to
development of simple, atom economical catalytic oxidation pro-
Among the multitude of available organic oxidants, aqueous 30%
H2O2 with its high oxygen content, has been identified as an ideal
atom efficient and environmentally acceptable terminal oxidant
[13–16,27–30]. Consequently, scores of promising methodologies
for selective sulfide oxidation with H2O2 using both homogeneous
and heterogeneous transition metal catalysts have been reported
in recent years [13–19,32–46]. Nevertheless, there still remains
plenty of opportunities to design new alternative catalysts which
can perform optimally in safer reaction media under mild condi-
tion, as some of the available protocols still rely upon volatile and
toxic organic solvents, or require harsh reaction conditions, long
reaction time, a promoter or a co-catalyst, or suffer from poor recy-
clability, which might compromise the ecological sustainability of
an otherwise efficient process.
Water is indeed the ultimate green natural solvent, being inex-
pensive, non-toxic, readily available with great redox stability
and heat capacity [47–53]. It is therefore not surprising that we
have been witnessing a dramatic upsurge in interest on water
based organic synthesis [15,18,22,36,49–58]. Concomitant with the
∗
Corresponding author at: Dept. of Chemical Sciences, Tezpur University,
Napaam, Tezpur-784028, Assam, India. Tel.: +91 3712267007 (O)/+91 9435380222
(Off.); fax: +91 3712267006.
E-mail addresses: nsi@tezu.ernet.in, nashreen.islam@rediffmail.com
(N.S. Islam).
1
These authors contributed equally to this work.
http://dx.doi.org/10.1016/j.molcata.2016.09.026
1381-1169/© 2016 Elsevier B.V. All rights reserved.

22
J.J. Boruah et al. / Journal of Molecular Catalysis A: Chemical 425 (2016) 21–30
growing importance of aqueous phase organic reactions, the devel-
opment of water-tolerant catalysts and water soluble ligands to
support such transformations has become an area of intense inves-
tigation [50–52]. We have reported in recent years, several new
catalysts and eco-compatible synthetic methodologies involving
polymer immobilized peroxo derivatives of vanadium [59], molyb-
denum [17,42] and tungsten [20,42] for oxidative transformations,
such as sulfide oxidation and oxidative bromination, using aque-
ous H2O2. Significantly, a set of peroxoniobium complexes has been
shown to be excellent recoverable catalysts for clean conversion of
organic sulfides with 30% H2O2 to the corresponding sulfoxide or
sulfone with high selectivity and functional group compatibility in
water [18].
sulfonate) (PS) as recyclable catalysts. For practical application of
the water soluble macroligand, apart from high hydrophilicity it
should ideally possess an adequate molecular weight distribution,
easy and cost-effective route of preparation, chemical stability, and
high affinity to bind metal ions [79,80]. Apart from fulfilling all these
requirements the chosen polymer supports are biocompatible and
non-toxic as evident from their extensive use in development
of pharmaceutical formulations [80]. As far as we are aware,
this is the first report on application of a peroxometal derivative
supported on water soluble polymer as catalyst for sulfide oxida-
tion.
In continuation of our efforts to develop simple and sustainable
routes to accomplish selective sulfoxidation in aqueous medium,
in the present work we have attempted to explore the scope of
generating new peroxomolybdenum (pMo) based catalysts using
water soluble polymers (WSP) as supports. We have envisaged that
the combination of WSP support with a catalytically active pMo
species would add advantages of polymeric reagent (stability and
phase homogeneity to the catalyst and oxidant, for instance) to
the inherent oxidant ability of the pMo species to afford immo-
bilized recoverable catalysts usable in aqueous medium. Previous
research has adequately proved that pMo complexes serve as sto-
ichiometric oxidants or catalysts to activate H2O2 in a variety of
organic oxidative transformations [17,22,60–65]. Utility of water
soluble macromolecular metal complexes in diverse fields includ-
ing biomedical [66,67] and environmental applications [67–69] has
been well-documented in the literature [70]. However, the con-
cept of application of soluble non-cross-linked polymers as support
for attaching active metal peroxo complexes in catalyst design
appears to have received scant attention, although the use of sol-
uble polymers to recover catalyst in synthetic approaches related
to peptide synthesis was introduced by Merrifield and Letsinger in
the 1960 [71–73], which paved the way toward developing immo-
bilized homogeneous catalysts [74,75]. It is pertinent to mention
that we have previously gained an access to series of well-defined
and stable peroxometal containing macro complexes by anchoring
peroxo derivatives of vanadium, molybdenum or tungsten to the
pendant functional groups of a variety of WSP matrices [59,76,77].
As some of these compounds exhibited unique oxidant activity as
well as bio-relevant properties [59,76–78] very distinct from the
corresponding neat peroxo complexes, the study of peroxometal
anchored soluble macro complexes has subsequently been recog-
nised as a promising new field [34,78].
2. Results and discussion
2.1. Catalyst preparation and characterization
Preparation of polymer anchored metal complexes using sol-
uble polymeric ligands, which are essentially polychelatogens,
often offers the convenience of adopting synthetic protocols used
for obtaining their monomeric analogues. The new supported
oxodiperoxomolybdenum catalyst, PSMo has been prepared con-
veniently by the reaction of molybdic acid, H2MoO4 with 30%
H2O2 using commercially available polymer, poly(sodium vinyl sul-
fonate) in aqueous medium, as shown in Scheme 1. The macro
complex PAMo has been obtained earlier using an analogous syn-
thetic protocol [76]. The pH of ca. 5 was found to be optimal for
the desired synthesis. It was also important to limit the amount of
water to that contributed by the hydrogen peroxide and sodium
hydroxide solution used. The catalyst was finally isolated as solid
by solvent induced precipitation using acetone. The water solu-
ble compound PSMo like the polyacrylate bound catalyst PAMo,
remains stable in the solid state for several weeks stored dry in
closed containers at ambient temperature.
The catalyst PSMo was characterized by elemental analysis and
a combination of spectroscopic techniques viz., FTIR, 13C NMR, 95Mo
NMR, UV–vis, as well as SEM- EDX and TGA-DTG analysis.
The presence of pMo species in the compound in its oxodiper-
oxo form, [MoO(O2)2] was indicated by elemental analysis data
2−
showing the Mo:O2
ratio as 1:2. The molybdenum loading on
the polymer, estimated on the basis of Mo content obtained from
elemental analysis, atomic absorption spectroscopy (AAS) and EDX
spectral analysis was found to be 1.02 mmol g−1 of the poly-
meric support [Table S1 (Supplementary information)]. The value
is observed to be slightly lower than the metal loading of the cata-
lyst PAMo (1.45 mmol g−1 of support). The occurrence of Mo in its
+6 oxidation state in the immobilized compound was confirmed
by the magnetic susceptibility measurement which revealed the
diamagnetic nature of the compound.
Herein, we present a new catalytic protocol for H2O2 mediated
selective oxidation of thioethers in aqueous medium using diper-
oxomolybdenum(VI) species anchored to non-cross linked soluble
polymer resins, poly(sodium acrylate) (PA) or poly (sodium vinyl
Scheme 1. Synthesis of soluble polymer anchored pMo complexes. (a) PSMo and (b) PAMo [76]. PS = poly(sodium vinyl sulfonate), PA = poly(sodium acrylate) and “
” represents polymer chain.

J.J. Boruah et al. / Journal of Molecular Catalysis A: Chemical 425 (2016) 21–30
23
Fig. 1. Scanning electron micrographs of (a) PS and (b) PSMo. EDX spectrum of (c) PSMo.
2.1.1. SEM and energy dispersive X-ray (EDX) analysis
The scanning electron micrograph shows that the smooth and
even surface of the pristine polymer after the incorporation of pMo
species, undergoes considerable morphological changes in PSMo
revealing that the metal ions are distributed across the external
surface of the polymer [Fig. 1(a) and (b)]. Moreover, Energy disper-
sive X-ray spectroscopic analysis, carried out by focusing multiple
regions over the surface of the polymeric catalyst clearly showed
the presence of Mo, Na, C, O and S in the compound [Fig. 1(c)].
The composition of the compound as obtained from EDX analy-
sis data was in close agreement with the elemental analysis values
[Table S1 (supplementary)].
2.1.2. IR, Raman and electronic spectral studies
The FTIR and Raman spectra of the catalyst PSMo displayed
the expected spectral pattern to indicate successful anchoring of
pMo species to the polymer chain via pendant sulfonate group. As
seen in Fig. 2, the IR spectrum of the neat poly(sodium vinyl sul-
fonate) showed the ꢀsym vibration of sulfonate anion at 1040 cm−1
in addition to a strong broad band representing S O stretching
at 1190 cm−1 [81,82]. The weak intensity ꢀasym(CH2) and bending
mode of CH2 group of the polymer chain occurred at 2954 and
1446 cm−1, respectively [81,83,84]. In the spectrum of PSMo, apart
from the band at 1042 cm−1, a broad band with a distinct absorp-
tions at 1210 and 1170 cm−1 appeared which may be ascribed to the
splitting of S O stretching vibrations originating from complexed
sulfonate group [81,84–86].
Fig. 2. FT-IR spectra of (a) PS, (b) PSMo and (c) PSMo after 10th reaction cycle.
The spectrum also showed diagnostic features depicting the
presence of terminal oxo and side-on bound peroxo groups of
the [MoO(O2)2] moiety with an intense absorption at 960 cm−1
and well resolved bands at 860, 610 and 530 cm−1 corresponding
to ꢀ(Mo O), ꢀ(O O), ꢀasym(Mo O2) and ꢀsym(Mo O2) frequen-
cies, respectively [87,88]. The IR results have been corroborated
further by the complementary Raman spectrum (Fig. 3) which
exhibited the well resolved bands corresponding to ꢀ(Mo O),
ꢀ(O O), ꢀasym(Mo O2) and ꢀsym(Mo O2) vibrations at 974, 880,
650 and 553 cm−1, respectively [89]. It also enabled identifica-
tion of ꢀsym(S O) as a sharp absorption at 1080 cm−1 as well
as a broad band located in the 1150–1290 region with a peak

24
J.J. Boruah et al. / Journal of Molecular Catalysis A: Chemical 425 (2016) 21–30
Fig. 5. TG-DTG plot of PSMo.
Fig. 3. Raman spectrum of PSMo.
ronment suggesting inter alia, that the compound did not undergo
hydrolysis and remained stable in solution. The stability of the cat-
alyst PAMo in aqueous solution has been established previously
[76].
2.1.4. Thermal analysis
The TGA-DTG profile of the catalyst, PSMo is shown in Fig. 5,
which revealed that the compound undergoes multistage decom-
position on heating up to a final temperature of 750 ◦C. Similar to
other pMo compounds immobilized on WSP, including the catalyst
PAMo [76], and unlike some monomeric peroxomolybdenum com-
pounds [93], the catalyst PSMo did not explode on heating. After
the initial dehydration step occurring with the liberation of water
molecules of crystallization in the temperature range of 74–101 ◦C,
the next decomposition stage has been observed in the range of
110–199 ◦C, with the corresponding weight loss of 6.04%, due to the
loss of peroxo groups from the complex. The absence of peroxide
group in the decomposition was also verified by its IR spectrum.
The compound subsequently undergoes two decomposition pro-
cesses accompanied by a weight loss of 43.63% in the temperature
range commencing at 379 ◦C which continued up to 750 ◦C. On the
basis of the thermal degradation pattern documented for poly(vinyl
sulfonate) sodium salt, these decompositions have been ascribed
to loss of sulfonate group with concomitant breakup of the poly-
mer matrix [95]. It has been shown previously that the loss of
sulfonate group is accompanied by release of ethylene, water, SO2
and CS2 from the compound [95]. The residue remaining at the
end of the degradation process of the catalyst was characterized
to be a hydrated oxomolybdenum species which showed the typ-
ical ꢀ(Mo O) and ꢀ(OH) bands in the IR spectrum and was devoid
of absorptions attributable to peroxo and the polymeric ligand of
the starting catalyst. Significantly, the degradation pattern of PSMo
closely resembled the pattern exhibited by the peroxotungsten
containing analogue reported previously by us [77]. The TGA-DTG
profile of the compound was thus found to be in conformity with
the composition and formula assigned to the compounds.
Fig. 4. 95Mo NMR spectrum of (a) PSMo and (b) PSMo after 10th reaction cycle in
D2O.
at 1205 cm−1 which may be ascribed to complexed sulfonate
group [76,77,81,90,91]. The absorptions representing ꢀasym(CH2)
and shear mode of vibration of CH2 group of the polymer support
were observed as strong band at 2956 cm−1 and a relatively weaker
intensity band at 1492 cm−1, respectively [92].
The electronic spectrum of the compound recorded in aque-
ous solution displayed weak intensity broad band at 310–330 nm,
which is characteristic of peroxo to metal (LMCT) transition of a
diperoxomolybdate species [Fig. S1 (Supplementary information)]
[76,93].
2.1.3. 13C and 95Mo NMR
The 13C NMR spectrum of the neat poly (sodium vinyl sulfonate)
shows characteristic resonances due to the chain carbon atoms of
CH2 and CH groups at 30.97 and 54.51 ppm, respectively. In the
spectrum of PSMo, positions of these peaks showed no significant
shift compared to parent polymer, with CH2 and CH signals appear-
ing at 30.98 and 54.53 ppm, respectively indicating that the chain
carbon atoms remain practically unaffected after incorporation of
pMo moieties into the polymer matrix [Fig. S2 (Supplementary
information)]. The observation is not unusual [76,77], considering
that the metal anchoring occurs via pendant sulfonate groups and
hence, are well separated from the carbon atoms of the polymer
chain.
On the basis of the above collective evidences, a structure of the
type shown in Fig. 6 has been proposed for the catalyst PSMo.
The 95Mo NMR has been recognised as a sensitive and highly
useful technique to study the nature of peroxomolybdenum species
in aqueous solution [94]. 95Mo NMR spectrum of PSMo provided an
unambiguous evidence in support of the presence of pMo species
in its oxodiperoxo form by displaying (Fig. 4) a single resonance
at −230 ppm (relative to [MoO4]2−), as has been observed previ-
ously in case of the catalyst PAMo at −225 ppm. These signals are in
the range expected for a diperoxo molybdenum species [76,89,94].
Identification of a single peak in the spectrum further confirmed the
existence of Mo(VI) in the compound in a single co-ordination envi-
2.2. Catalytic activity of the supported complexes, PSMo and
PAMo
2.2.1. Oxidation of sulfides to sulfoxides
The macro complexes PSMo and PAMo were examined for their
catalytic efficiency in sulfoxidation reaction with 30% H2O2 as ter-
minal oxidant under aqueous conditions. In order to optimize the
reaction condition an exploratory experiment has been first con-
ducted employing thioanisole as the representative substrate and
PSMo as the catalyst, maintaining Mo: substrate molar ratio at

J.J. Boruah et al. / Journal of Molecular Catalysis A: Chemical 425 (2016) 21–30
25
at 1: 4 for subsequent reactions. A blank experiment conducted in
absence of the catalyst under otherwise analogous reaction con-
ditions, showed very low conversion of MPS to sulfoxide (ca. 13%)
within the stipulated reaction time, (Table 1, entry 15), confirming
the indispensability of the catalyst in obtaining the targeted prod-
uct at a remarkably faster rate. Furthermore, we have investigated
the activity of the supported pMo catalyst vis-à-vis unsupported
peroxo molybdate generated in situ, by adding Na2MoO4 to the
reaction mixture containing H2O2 under optimized reaction con-
dition, maintaining essentially the same Mo: MPS ratio of 1: 1000.
The distinctly superior activity displayed by the supported catalyst
relative to the neat pMo species (Table 1, entry 14) made it apparent
that immobilization of pMo species on the WSP support indeed led
to an enhancement of the catalytic efficiency pMo derivatives both
in terms of yield, as well as product selectivity. We have also per-
formed the oxidation reactions under identical condition using the
free polymer PA or PS in lieu of the pMo supported catalysts. The
yield obtained in each case was nearly the same as that obtained
from the blank experiment conducted in absence of the catalyst
(Table 1, entry 16). The results clearly demonstrated that the cho-
sen pristine polymers have no observable effect on the course of
the oxidation reaction.
Next, we considered it imperative to examine the performance
of the catalysts in a variety of solvents other than water, since sol-
vents have been known to have immense influence on the activity
and selectivity of catalysts in sulfoxidation reaction. The solvent
effect has been assessed in the oxidation of methyl phenyl sulfide
using a series of common organic solvents viz., methanol, ethanol,
acetonitrile, chloroform and dichloromethane. Interestingly, apart
from water, the methodology for sulfoxidation has been found to
be compatible in presence of other polar protic organic solvents
such as methanol and ethanol, although not very effective in non-
polar solvents (Table 1, entries 8 and 9). A good conversion was
also obtained in acetonitrile although without product selectivity
(Table 1, entry 10). It is notable that despite the insolubility of the
catalysts in neat organic solvents, in presence of aqueous H2O2 of
the reaction mixture, both the catalysts dissolve completely in the
water miscible solvents viz., methanol, ethanol and acetonitrile,
Fig. 6. Proposed structure of PSMo (“
” represents polymer chain).
1: 1000, and under continuous stirring at ambient temperature.
The details of the study are summarized in Table 1. To our plea-
sure, under these conditions MPS was completely and selectively
transformed into sulfoxide within 90 min of reaction time with a
reasonably good TOF (Table 1, entry 1). Our assessment of the effect
of the catalyst amount on the rate and selectivity of the reaction
revealed that, increasing the amount of catalyst led to a substantial
acceleration of thioanisole oxidation as expected, but rendered a
decrease in the resulting TOF (Table 1, entry 3). On the other hand,
the catalyst worked well even at Mo: substrate ratio of 1: 2000
with 2 equivalents of H2O2 affording a good TOF, albeit the reac-
tion time increased considerably. Subsequently, the influence of
the amount of oxidant on the reaction profile was examined. It was
gratifying to note that increase in the amount of the oxidant led to
a significant enhancement of the rate of the reaction, enabling us to
achieve over eight fold improvement of TOF (Table 1, entry 7) sim-
ply by using 4 equivalents of H2O2, without altering other reaction
parameters. Thus, for achieving best results in aqueous medium
without compromising the selectivity, we have decided to main-
tain a Mo: substrate molar ratio of 1: 1000 and substrate: H2O2
Table 1
a
Optimization of reaction conditions for PSMo catalyzed selective oxidation of methyl phenyl sulfide (MPS) to sulfoxide by 30% H2O2
.
Entry
Molar ratio (Mo:MPS)
H2O2 (equiv.)
Solvent
Temperature
Time (min)
Isolated yield (%)
1a:1b
TONb
TOFc(h−1
)
1
2
3
4
5
6
7
8
1:1000
1:1000
1:500
1:1500
1:2000
1:1000
1:1000
1:1000
1:1000
1:1000
1:1000
1:1000
1:1000
1:1000
–
1
2
2
2
2
3
4
2
2
2
2
2
2
4
4
4
H2O
H2O
H2O
H2O
H2O
H2O
H2O
MeOH
EtOH
CH3CN
CHCl3
CH2Cl2
H2O
H2O
H2O
H2O
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
90
60
45
85
100
25
10
80
65
100
80
80
20
10
10
10
98
96
94
97
98
97
98
97
97
98
8
13
90
16
13
14
100:00
100:00
100:00
100:00
100:00
100:00
100:00
100:00
100:00
61:39
87:13
84:16
100:00
68:32
100:00
100:00
980
960
470
1455
1960
970
980
970
970
980
80
653
960
627
1027
1176
2328
5880
727
895
588
60
9
10
11
12
13
14d
15e
16f
130
900
160
–
98
60 ◦
RT
C
2700
960
–
RT
RT
–
–
–
Note: Values in bold signify optimized reaction condition.
a
All the reactions were carried out with 5 mmol of substrate in 5 mL of solvent. Catalyst amount = 4.90 mg for 0.005 mmol of Mo.
TON (turnover number) = mmol of product per mmol of catalyst.
TOF (turnover frequency) = mmol of product per mmol of catalyst per hour.
Using Na2MoO4 as catalyst (1.03 mg, 0.005 mmol of Mo).
Blank experiment without any catalyst.
Reaction in presence of free poly(sodium vinyl sulfonate) (4.42 mg).
b
c
d
e
f

26
J.J. Boruah et al. / Journal of Molecular Catalysis A: Chemical 425 (2016) 21–30
Table 2
Selective oxidation of sulfides to sulfoxides with 30% H2O2 catalyzed by PSMo and PAMoa.
PSMo
PAMo
Entry
1
Substrate
Time (min)
Isolated
yield (%)
TONb
TOFc
Time (min)
Isolated
yield (%)
TONb
TOFc (h−1
)
(h−1
)
10
10
10
98
980
960
970
5880
5760
5820
15
15
15
98
980
950
960
3920
3800
3840
96d
97e
95d
96e
2
5
96
960
11520
10
93
930
5580
3
4
8
8
97
96
970
960
7275
7200
12
12
96
95
960
950
4800
4750
5
10
94
98
97
95
97
96
98
940
980
970
950
970
960
980
5640
3920
1164
1425
448
15
97
96
93
93
96
94
97
970
960
930
930
960
940
970
3880
2307
797
930
372
313
363
6
15
25
7
50
70
8
40
60
9
130
150
150
155
180
160
10
11f
384
392
12g
11h
90
90
8.1
11h
89
89
8.0
a
All reactions were carried out in 5 mmol substrates, 20 mmol 30% H2O2 and catalyst (0.005 mmol of Mo) in 5 mL H2O at RT, unless otherwise indicated.
TON (turnover number) = mmol of product per mmol of catalyst.
TOF (turnover frequency) = mmol of product per mmol of catalyst per hour.
Yield of 10th reaction cycle.
Scale up data (7.5 g of MPS).
b
c
d
e
f
Reaction condition: 5 mmol substrate, 20 mmol 30% H2O2 and catalyst (0.005 mmol of Mo) in 5 mL methanol at RT.
g
Reaction condition: 5 mmol substrate, 20 mmol 30% H2O2 and catalyst (0.05 mmol of Mo) at 65 ◦C in refluxing methanol.
affording homogeneity of the catalytic process. From our survey
ies although, a significant increase in TOF could be attained at a
moderately higher temperature as shown in (Table 1, entry 13).
Having established the optimized reaction conditions for selec-
tive sulfoxidation of the model substrate, we extended the study
to a range of structurally diverse sulfides such as dialkyl, diaryl,
aryl alkyl, aryl vinyl, aryl alcohol as well as dibenzothiophene
(DBT). The results depicted in Table 2 reveal that excellent yield
with complete selectivity could be obtained in presence of each
of the catalysts, PSMo and PAMo, across the series of substrates
examined, affording the corresponding sulfoxide as the sole prod-
uct. Significantly, no over oxidation of sulfoxide to sulfone was
noted under the investigated conditions. Thus the catalytic proto-
col, being able to stop the oxidation at the sulfoxide level, provided
complete control over degree of oxidation of the substrate. The
catalyst PSMo, supported on poly (sodium vinyl sulfonate) with
monomeric oxodiperoxomolybdenum moieties was observed to
display consistently superior activity compared to the catalyst
PAMo having dimeric pMo units. The distinct influence of the co-
ordination sphere on catalytic activity of these supported pMo
catalysts is thus apparent from the data.
of the solvent effect on sulfoxidation, water emerged to be the best
solvent as indicated by the maximum TOF obtained along with 100%
selectivity with respect to sulfoxide, despite the poor solubility of
the organic substrates in water. Such observation is not unusual as
it has been amply demonstrated by Sharpless et al. [52], followed
by reports from some other laboratories [96,98], that for reactions
to occur optimally in neat water, solubility of the organic reagents
is not a fundamental requirement. Furthermore, our results are in
accord with previous findings which showed that selective oxi-
dation of sulfides to sulfoxides is highly favoured in polar protic
solvent with ability to form H-bond [18,19,22,96–98].
It is worthy to note that in the present work all transformations
occurred at the natural pH of the reaction medium. Notwithstand-
ing previous literature [99–101], including our own work [18,42]
showing considerable improvement of catalytic activity of per-
oxometallates in oxidative transformations in presence of acidic
additives, we refrained from adjusting the pH as we endeavoured
to bring about the desired transformations under mild condition,
without involving any acid and by avoiding or minimizing the use
of other hazardous auxiliaries. Also, in view of the environmentally
benign aspect, we have chosen room temperature for our stud-
The methodology worked well for both aliphatic as well as aro-
matic substrates, albeit the rates of oxidation showed variations

J.J. Boruah et al. / Journal of Molecular Catalysis A: Chemical 425 (2016) 21–30
27
depending on the nature of the substrate and substituents attached.
Previous work has already established that sulfide oxidation by
H2O2 generally occurs via an electrophilic addition reaction of oxy-
gen atoms, due to which sulfides with higher electron density on
sulfur atom are expected to react faster [102,103]. The observed
sequence in the present study (Table 2, entries 1–10) indicating
the oxidation of aliphatic sulfides to be more facile in compari-
son to aromatic sulfides as well as other conjugated systems viz.,
allylic or vinylic sulfides is thus in accord with the decreasing nucle-
ophilicity of the tested substrates, owing to conjugation of sulfur
lone pair of electrons of the aromatic and allylic sulfides. It was
therefore not surprising that the protocol enabled us to attain the
highest TOF value of 11,520 h−1 (for PSMo) for dimethyl sulfide
oxidation (Table 2, entry 2), whereas the oxidation of diphenyl sul-
fide and benzyl phenyl sulfides (Table 2, entries 10 and 11) were
rather sluggish and DBT being least nucleophilic and a relatively
inert refractory sulfide, manifested no sulfoxidation reaction under
the optimized condition in water even after 12 h of reaction time.
Nevertheless, it was possible to selectively oxidize these substrates,
even DBT in presence of each of the developed catalysts by replac-
ing H2O with methanol as medium of reaction and using a higher
amount of catalyst (Mo: MPS ratio of 1:100). The reaction was con-
ducted at a moderately elevated temperature of 65 ◦C in case of DBT
oxidation in order to enhance the reaction rate (Table 2, entry 12).
One of the significant attributes of the catalysts is their excel-
lent chemoselectivity displayed exclusively toward sulfur group
of substrates containing other oxidation prone functional groups
such as allylic, vinylic and alcoholic sulfides. As seen from the
data of (Table 2, entries 8 and 9), allylic and vinylic sulfides were
chemoselectively oxidized to the corresponding sulfoxide without
epoxidation of C C. Similarly, the alcoholic and benzylic sulfides
yielded the targeted sulfoxides without affecting any other func-
tional group transformation (Table 2, entries 7 and 11) under the
standard reaction conditions testifying to the excellent functional
group tolerance of the catalysts. It is important to highlight herein
that we have also confirmed the feasibility of the protocol for
scaled-up synthetic applications by conducting the oxidation of
thioanisole at ten-fold scale under the standard reaction conditions
(Table 2, entry 1e).
Furthermore, the oxidations occurred with high utilization
efficiency of hydrogen peroxide (defined as: 100 × mol of H2O2
consumed in the formation of the oxidised product/mol of H2O2
converted) [Text S1 (Supplementary information)] [45]. The H2O2
efficiency in the oxidations was found to be in the range of 90–94%
in presence of each of the catalysts PSMo and PAMo. The mild reac-
tion condition of the protocol with the requirement of reasonably
short reaction time appears to be responsible for the observed high
H2O2 efficiency. In case of DBT oxidation, relatively higher reaction
temperature employed and longer reaction time required perhaps
facilitates direct H2O2 decomposition leading to considerably lower
H2O2 efficiency (ca. 58%).
It is pertinent to mention that we have also explored the possi-
bility of employing the same catalysts to obtain selectively sulfone
from sulfide in aqueous medium, by varying the reaction condi-
tions. Using MPS as the representative substrate, we could finally
obtain pure sulfone with 50% H2O2 as oxidant in lieu of 30% H2O2,
maintaining substrate: H2O2 molar ratio of 1: 8 and Mo: MPS ratio
of 1: 500. The reaction however required nearly 24 h for completion
and the resulting TOF attained was about 20 h−1. On the other hand,
when the reaction was conducted in acetonitrile instead of water,
MPS was selectively and completely converted to sulfone within
4 h, affording a TOF of 253 h−1, under otherwise analogous reaction
condition. In the non-protic solvent acetonitrile, sulfone formation
appears to be relatively more facile probably owing to the inability
of acetonitrile to impart stability through hydrogen bond formation
to the sulfoxide formed initially from sulfide. In fact, acetonitrile
Fig. 7. Recyclability of PSMo and PAMo for the selective oxidation of MPS to sul-
foxide.
proved to be the best solvent to obtain pure sulfone from sulfide in
some previous studies as well [17,21,45,103].
2.2.2. Recyclability of the catalyst
For practical utility of a catalytic process the longevity of the
catalyst and its potential for regeneration and reusability are of
fundamental importance. Albeit a homogeneous catalyst usually
exhibits superior activity and selectivity vis-à-vis its heteroge-
neous counterpart, its recovery however, often involves tedious
separation processes from organic products and lowered activity
of recovered catalyst [50,53]. Interestingly however, it has been
demonstrated that employing water soluble catalysts and water as
the reaction medium it is possible to combine catalytic efficiency
with easy recovery of the catalyst through simple phase separation,
owing to poor solubility of the organic products in water [50,53].
Recovery of a soluble polymeric catalyst is often achieved by dilut-
ing the homogeneous solution with appropriate solvent to induce
precipitation of the polymeric species which subsequently enables
its isolation from the resulting heterogeneous mixture by filtration
or centrifugation [104]. To our satisfaction, in the present study, the
catalysts could be effectively regenerated in situ for reuse in subse-
quent cycles of reaction, simply by H2O2 treatment of the aqueous
extract of the spent reaction mixture after separation of the organic
product, followed by addition of a fresh batch of substrate after
each catalytic cycle. Recycling experiments were conducted under
optimized reaction condition using MPS as model substrate. The
regenerated catalysts could be reused for at least ten reaction cycles
with the activity and selectivity remaining nearly unaltered [Fig. 7
and Table 2 (entry 1d)]. It was also possible to recover the cata-
lysts as solid by addition of acetone to aqueous extract of the spent
reaction mixture after replenishing the peroxide. The recovered
catalyst could be reused in subsequent cycles of reaction without
further conditioning. The approach of in situ recycling of the cat-
alyst is definitely more convenient as the troublesome separation
and purification steps usually associated with soluble catalyst could
greatly be avoided. The redundancy of these steps further helped in
limiting the use of organic solvents during the process. It is perti-
nent to mention that, using a similar approach we have previously
achieved excellent results in case of recovery of homogeneous per-
oxoniobium as well as soluble polymer anchored peroxovanadium
catalysts used in H2O2 induced oxidations [18,59]. It is important
to highlight in this context that the overall TOF of ca. 57,530 h−1
obtained using PSMo as catalysts, after ten cycles of MPS sulfox-
idation in the present study, is nearly 7 and 4 fold higher than
the corresponding values obtained after six reaction cycles in case
of our recently reported homogeneous pNb catalysts [18] as well
as heterogeneous polymer supported molybdenum(VI) catalysts
[17], respectively. Although a direct comparison of the observed
results with many of the other available methodologies is difficult

28
J.J. Boruah et al. / Journal of Molecular Catalysis A: Chemical 425 (2016) 21–30
in absence of TON or TOF values in most these reports, neverthe-
less, the efficiency of the catalysts PSMo and PAMo appear to be
indeed remarkable, particularly considering the truly mild reaction
conditions under which the reactions have been accomplished.
In order to further ascertain that the catalysts retain their struc-
tural integrity during the catalytic cycles, the regenerated catalysts
isolated from spent reaction mixture by treatment with acetone,
was dried and subsequently subjected to characterization by ele-
mental and spectral analysis. The FTIR spectra of the recovered
catalysts showed the signature peaks corresponding to metal-
peroxo as well as pendant functional groups of the polymer support
as has been observed in the original catalyst (Fig. 2). The 95Mo
NMR spectral pattern of the regenerated catalysts was identical
with the corresponding spectrum of fresh catalyst (Fig. 4). Neither
the metal loading nor the peroxide content of the recovered cata-
lysts showed any significant decrease compared to the respective
starting catalyst as revealed by elemental analysis and EDX spec-
tral data. Moreover, the filtrate obtained after the isolation of the
regenerated catalyst, subsequent to completion of the reaction, was
treated with a fresh lot of H2O2 and MPS and the reaction was
monitored over a period of 1 h. The reaction afforded about 12%
conversion after stipulated time, which is comparable to the value
obtained in absence of the catalyst in the blank experiment. The
absence of Mo in the filtrate after isolation of the catalysts was
also ascertained by ICP analysis. Results of our studies thus con-
firm that there was no metal leaching out of the polymer during
the catalytic process, indicating that the catalysts are structurally
robust being capable of remaining intact during repeated cycles of
oxidation.
3. Experimental
3.1. Materials and methods
Molybdic acid, poly (sodium vinyl sulfonate) (Mw = 4000),
methyl phenyl sulfide (MPS), ethyl phenyl sulfide (EPS), dimethyl
sulfide (DMS), Methyl p-tolylsulfide (MpTS), dibutylsulfide (DBS),
dibenzothiophene (DBT), dihexylsulfide (DHS), phenylvinylsul-
fide (PVS), 2- (phenylthio)ethanol (PTE), diphenylsulfide (DPS),
benzyl phenyl sulfide (BPS) and allyl phenyl sulfide (APS) were
purchased from Sigma-Aldrich Chemical Company, Milwaukee,
USA. Poly (sodium acrylate) (Mw = 2100) (Fluka). Hydrogen per-
oxide and sodium sulfate were obtained from E. Merck, India.
Acetone, acetonitrile, methanol, ethylacetate, petroleum ether
(boiling range 60–80 ◦C), diethyl ether, dichloromethane, silica gel
(60–120 mesh), sodium hydroxide are the products of Rankem,
India. [Mo2O2(O2)4(carboxylate)]–PA[PA = poly(sodium acrylate)]
(PAMo) was obtained according to the method described in our
earlier paper [76]. The water used for solution preparation was
deionized and distilled.
The molybdenum content of the synthesized compounds
was determined by atomic absorption spectroscopy (AAS) using
Thermo iCE 3000 series atomic absorption spectrophotometer
model analyst 200, inductively coupled plasma optical emission
spectrophotometer (ICP-OES) (Perkin Elmer, OPTIMA 2100 DV)
analysis, as well as gravimetrically as molybdenum oxinate [107],
MoO2(C9H6ON)2. Elemental analyses for C, H and N was performed
on an elemental analyzer (Perkin-Elmer 2400 series II). Peroxide
content was estimated volumetrically by procedures described in
earlier papers [17,18]. The sodium content was measured by AAS
and also with an ionometer (ORION VERSASTER). The IR spectra
were recorded by making pressed pellets of samples with KBr
using Perkin-Elmer spectrum 100 FTIR spectrophotometer. Raman
spectra of the compounds were recorded using a Renishaw InVia
Raman microscope equipped with an argon ion laser with an exci-
tation wavelength of 514 nm and a laser maximum output power
of 20 mW. The 13C NMR spectra were recorded on a JEOL JNM-
ECS400 spectrometer at a carbon frequency of 100.5 MHz, 131072
X-resolution points, number of scans 8000, 1.04 s acquisition time,
2.2.3. The proposed catalytic cycle
On the basis of the afore mentioned experimental findings, we
propose the catalytic cycle for selective oxidation of sulfides to sul-
foxide shown in Fig. 8, using PSMo as representative. The first step
of the reaction is likely to be the facile transfer of electrophilic oxy-
gen from the active oxodiperoxomolybdenum(VI) species of the
catalyst to sulfide to afford sulfoxide (reaction a) converting itself
into a dioxomonoperoxo intermediate. The monoperoxo molybde-
num derivative eventually combines with H2O2 and reverts back
to form the original catalyst thus completing the catalytic cycle
(reaction b). The proposed reaction cycle is in accord with the pre-
vious literature [15,17,20,61,64,105,106] on reactivity of pMo and
peroxotungsten compounds where the formation of an inactive
monoperoxo Mo(VI) and W(IV) intermediate during substrate oxi-
dation by a more reactive diperoxo species of these metals has been
well documented.
and 2.0 s relaxation delay with the 1H NMR decoupling method
95
in D2O. The
Mo NMR spectra were recorded in a Bruker AV
400 MHz FT-NMR spectrometer at a molybdenum frequency of
26.07 MHz with samples in a 10 mm spinning tube with a sealed
coaxial tube containing D2O, which provided the lock signal. The
chemical shift data are recorded as negative values of ppm (␦) in
the low-frequency direction with reference to 1 M Na2MoO4·2H2O
solution at 298 K. Thermogravimetric analysis (TGA) was carried
out on a SHIMADZU TGA-50 system at a heating rate of 10 ◦C min−1
under N2 atmosphere using an aluminium pan. Scanning elec-
tron microscopy (SEM) characterizations of the compounds were
obtained using a JEOL JSM-6390LV Scanning Electron Micrograph
attached with an energy-dispersive X-ray detector. Scanning was
done in the 10–20 ␮M range, and images were taken at a mag-
nification of 15–20 kV. Data were obtained using INCA software.
Standardization of the data analysis is an integral part of the
SEM-EDX instrument employed. Magnetic susceptibilities of the
complexes were measured by the Gouy method, using Hg[Co(NCS)]
as the calibrant.
3.2. Synthesis of [MoO(O2)2(sulfonate)]–PS [PS = poly(sodium
vinyl sulfonate)] (PSMo)
To a solution of molybdic acid (1.84 g, 11.50 mmol) dissolved
in 10 mL of 30% H2O2 maintaining the temperature at 30–40 ◦C,
1.5 g of poly(sodium vinyl sulfonate) (PS) was added with constant
stirring. The resulting mixture was stirred for an hour in an ice
Fig. 8. Proposed catalytic cycle.

J.J. Boruah et al. / Journal of Molecular Catalysis A: Chemical 425 (2016) 21–30
29
bath. The pH of the reaction solution at this stage was recorded
to be ca. 2, which was adjusted at 5 by dropwise addition of 8 M
NaOH solution. On addition of about 50 mL of pre-cooled acetone
to the reaction solution with constant stirring, a red colored pasty
product separated out which on repeated treatment with acetone
turned into a microcrystalline solid. The compound was separated
by centrifugation, washed 3–4 times with cold acetone and dried
in vacuo over concentrated sulfuric acid. The compound was sub-
sequently dried by heating up to 70 ◦C under nitrogen atmosphere.
The metal loading calculated from the observed molybdenum con-
tent is 1.02 mmol g−1 of polymer for [MoO(O2)2(sulfonate)]–PS
[Table S1 (Supplementary information)]. FT-IR (KBr pellet, cm−1):
1042, 1210, 1170 (S O), 960 (M O), 860 (O O), 610 and 530
(M O2). Raman (cm−1): 1080, 1205 (S O), 974 (M O), 880 (O O),
650 and 553 (M O2).
the selective oxidation of organic sulfides to the corresponding sul-
foxide with 30% H2O2 in aqueous medium. Compatibility of the
catalysts with relatively safer organic solvents such as methanol
and ethanol is an additional significant feature of the catalysts,
which has been used effectively to obtain high purity sulfoxide from
less reactive dibenzothiophene in impressive yield. Most impor-
tantly, the catalysts are simply prepared from readily available
non-toxic starting materials and are sufficiently robust to afford
easy in-situ recyclability for sulfoxidation up to a minimum of ten
reaction cycles with consistent activity selectivity profile. More-
over, ease of work-up, excellent chemoselectivity, ready scalability
are additional green attributes of this novel methodology making
it potentially attractive for practical applications. Our findings thus
show that linear soluble polymers can be considered as a green
and viable alternative to insoluble cross linked polymer support for
designing sustainable peroxometal based catalysts for reactions in
aqueous medium. The work on other applications of this highly effi-
cient catalyst system to other important aqueous phase oxidative
transformations including oxidation of biomass derived substrates
are now underway in our laboratory.
3.3. General procedure for catalytic oxidation of sulfides to
sulfoxides
In a typical reaction, organic substrate (5 mmol) was added to
a solution of catalyst [PAMo (3.44 mg) or PSMo (4.9 mg), contain-
ing 0.005 mmol of Mo] and 30% H2O2 (2.26 mL, 20 mmol) in 5 mL
of water. The molar ratio of substrate: H2O2 and that of catalyst
(Mo): substrate was maintained at 1: 4 and 1: 1000, respectively.
The reaction was conducted at room temperature under mag-
netic stirring. The reaction progress was monitored by thin layer
chromatography (TLC) and GC. After completion, the product and
unreacted organic substrates were extracted with diethyl ether,
dried over anhydrous sodium sulfate and distilled under reduced
pressure to remove excess solvent. The crude product obtained was
purified by column chromatography on silica gel with ethyl acetate-
hexane (1: 9 v/v) as the eluent. The product obtained was charac-
terized by a combination of IR, 1H NMR, 13C NMR spectroscopy and
melting point determination (for solid products) [Text S2 (Supple-
mentary information)].
Acknowledgements
The authors gratefully acknowledge the Department of Science
and Technology, New Delhi, India, for providing financial support.
We are also grateful to the University Grants Commission, New
Delhi, for providing Moulana Azad National Fellowship to K.A.
Notes and references
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.molcata.2016.09.
026.
References
3.4. Procedure for regeneration of the catalyst
[1] S. Caron, R.W. Dugger, S.G. Ruggeri, J.A. Ragan, D.H.B. Ripin, Chem. Rev. 106
(2006) 2943–2989.
[2] M. Tanaka, H. Yamazaki, H. Hakusui, N. Nakamichi, H. Sekino, Chirality 9
(1997) 17–21.
[3] H. Cotton, T. Elebring, M. Larsson, L. Li, H.H. Sorensen, S. VonUnge,
Tetrahedron: Asymmetry 11 (2000) 3819–3825.
[4] H.L. Holland, Chem. Rev. 88 (1988) 473–485.
[5] I. Fernandez, N. Khiar, Chem. Rev. 103 (2003) 3651–3706.
[6] S. Patai, Z. Rappoport, Synthesis of Sulfones, Sulfoxides and Cyclic Sulfides, J.
Wiley, Chichester, 1994.
[7] M.C. Carreno, Chem. Rev. 95 (1995) 1717–1760.
[8] R. Bentley, Chem. Rev. 105 (2005) 609–624.
[9] J. Legros, J.R. Dehli, C. Bolm, Adv. Synth. Catal. 347 (2005) 19–31.
[10] Y. Ding, W. Zhu, H. Li, W. Jeng, M. Zhang, Y. Duan, Y. Chang, Green Chem. 13
(2011) 1210–1216.
[11] H. Li, L. He, J. Lu, W. Zhu, X. Jiang, Y. Wang, Y. Tan, Energy Fuels 23 (2009)
1354–1357.
[12] W. Zhu, G. Zhu, H. Li, Y. Chao, Y. Chang, G. Chen, C. Han, J. Mol. Catal. A:
Chem. 347 (2011) 8–14.
[13] M.R. Maurya, A. Arya, A. Kumar, M.L. Kuznetsov, F. Avecilla, J. Costa Pessoa,
Inorg. Chem. 49 (2010) 6586–6600.
[14] V. Hulea, F. Fajula, J. Bousquet, J. Catal. 198 (2001) 179–186.
[15] B.M. Choudary, B. Bharathi, C.V. Reddy, M.L. Kantam, J. Chem. Soc. Perkin
Trans. 1 (2002) 2069–2074.
[16] K. Sato, M. Hyodo, M. Aoki, X.-Q. Zheng, R. Noyori, Tetrahedron 57 (2001)
2469–2476.
[17] J.J. Boruah, S.P. Das, S.R. Ankireddy, S.R. Gogoi, N.S. Islam, Green Chem. 15
(2013) 2944–2959.
The recyclability of each of the catalysts was examined by using
MPS as the substrate adopting the following procedures. For selec-
tive sulfoxidation, the catalyst could be regenerated and recycled
in situ. After completion of the reaction run, the aqueous part of the
reaction mixture was treated with 30% H2O2 (2.26 mL, 20 mmol)
followed by a fresh batch of substrate and the reaction was con-
ducted under optimized condition in the same manner as reported
in the first run (Section 3.5). The progress of the reaction was mon-
itored by thin layer chromatography (TLC) and GC. The process has
been repeated for ten cycles of reaction.
In an alternative procedure, the recovered catalyst could also
be isolated as solid and reused in subsequent cycles of oxidation.
The aqueous part of the spent reaction mixture was transferred to
a beaker and treated with 30% H2O2 (2.26 mL) in order to replenish
the peroxide content of the catalyst state. To this solution pre-
cooled acetone was added with constant stirring until the solid
catalyst separated out which was isolated following the same work
up procedure mentioned under Section 3.2. The dried solid catalyst
was used in subsequent reaction cycle conducted under optimized
condition.
[18] S.R. Gogoi, J.J. Boruah, G. Sengupta, G. Saikia, K. Ahmed, K.K. Bania, N.S.
Islam, Catal. Sci. Technol. 5 (2015) 595–610.
4. Conclusions
[19] S.P. Das, J.J. Boruah, N. Sharma, N.S. Islam, J. Mol. Catal. A: Chem. 356 (2012)
36–45.
[20] S.P. Das, J.J. Boruah, H. Chetry, N.S. Islam, Tetrahedron Lett. 53 (2012)
1163–1168.
[21] S. Doherty, J.G. Knight, M.A. Carroll, J.R. Ellison, S.J. Hobson, S. Stevens, C.
Hardacre, P. Goodrichb, Green Chem. 17 (2015) 1559–1571.
In conclusion, we successfully developed a simple, cost-effective
and environmentally clean catalyst system comprising diperoxo
molybdenum moiety immobilized onto non-cross linked water sol-
uble polymer matrices, which exhibited remarkable efficiency in

30
J.J. Boruah et al. / Journal of Molecular Catalysis A: Chemical 425 (2016) 21–30
[22] R.D. Chakravarthy, V. Ramkumar, D.K. Chand, Green Chem. 16 (2014)
2190–2196.
[65] Y.M.A. Yamada, H. Tabata, M. Ichinohe, H. Takahashi, S. Ikegami,
Tetrahedron 60 (2004) 4087–4096.
[23] B. Karimi, M. Ghoreishi-Nezhad, J.H. Clark, Org. Lett. 7 (2005) 625–628.
[24] M. Amini, M.M. Haghdoostb, M. Bagherzadehb, Coord. Chem. Rev. 257
(2013) 1093–1121.
[66] B.R. Twaites, C. de las, H. Alarcón, D. Cunliffe, M. Lavigne, S. Pennadam, J.R.
Smith, D.C. Górecki, C. Alexander, J. Control. Release 97 (2004) 551–566.
[67] N. Murthy, J. Campbell, N. Fausto, A.S. Hoffman, P.S. Stayton, J. Control.
Release 89 (2003) 365–374.
[68] A.N. Pustam, S.D. Alexandratos, React. Funct. Polym. 70 (2010) 545–554.
[69] I.M. Okhapkin, E.E. Makhaeva, A.R. Khokhlov, Adv. Polym. Sci. 195 (2006)
177–210.
[70] N.S. Islam, J.J. Boruah, J. Chem. Sci. 127 (2015) 777–795.
[71] R.B. Merrifield, Angew. Chem. Int. Ed. Engl. 24 (1985) 799–810.
[72] I. Letsinger, T.E. Wagner, J. Am. Chem. Soc. 88 (1966) 2062–2063.
[73] D.E. Bergbreiter, Chem. Rev. 102 (2002) 3345–3384.
[74] A.D. Pomogailo, Catalysis by Polymer Immobilised Metal Complexes,
Gordon and Breach Science Publishers, Amsterdam, 1998.
[75] D. Wohrle, in: H.R. Kricheldorf (Ed.), Handbook of Polymer Synthesis, Marcel
Dekker, New York, 2005, pp. 666–742.
[25] D.H. Koo, M. Kim, S. Chang, Org. Lett. 7 (2005) 5015–5018.
[26] G. Strukul, A. Scarso, in: M.G. Clerici, O.A. Kholdeeva (Eds.), Environmentally
Benign Oxidants, in Liquid Phase Oxidation via Heterogeneous Catalysis,
John Wiley & Sons, Inc, Hoboken, New Jersey, 2013, pp. 1–20.
[27] R. Noyori, M. Aoki, K. Sato, Chem. Commun. (2003) 1977–1986.
[28] B.S. Lane, K. Burgess, Chem. Rev. 103 (2003) 2457–2474.
[29] C.W. Jones, Applications of Hydrogen Peroxide and Derivatives, Royal
Society of Chemistry, Cambridge, 1999.
[30] M.V. Gómez, R. Caballero, E. Vázquez, A. Moreno, A. Hoz, Á. Díaz-Ortiz,
Green Chem. 9 (2007) 331–336.
[31] G. Strukul, Catalytic Oxidations with Hydrogen Peroxide as Oxidant, Kluwer
Academic Publishers, Dordrecht, 1992.
[32] S.K. Karmee, L. Greiner, A. Kraynov, T.E. Müller, B. Niemeijera, W. Leitner,
Chem. Commun. 46 (2010) 6705–6707.
[76] J.J. Boruah, D. Kalita, S.P. Das, S. Paul, N.S. Islam, Inorg. Chem. 50 (2011)
8046–8062.
[33] F. Gregori, I. Nobili, F. Bigi, F. Maggi, G. Predieri, J. Mol. Catal. A: Chem. 286
(2008) 124–127.
[34] V. Conte, B. Floris, Dalton Trans. 40 (2011) 1419–1436.
[35] S. Hussain, D. Talukdar, S.K. Bharadwaj, M.K. Chaudhuri, Tetrahedron Lett.
53 (2012) 6512–6515.
[77] S.P. Das, S.R. Ankireddy, J.J. Boruah, N.S. Islam, RSC Adv. 2 (2012) 7248–7261.
[78] C.C. McLauchlan, B.J. Peters, G.R. Willsky, D.C. Crans, Coord. Chem. Rev. 301
(2015) 163–199.
[79] B.L. Rivas, E.D. Pereira, M. Palencia, J. Sánchez, Prog. Polym. Sci. 36 (2011)
294–322.
[36] F. Rajabi, S. Naserian, A. Primo, R. Luque, Adv. Synth. Catal. 353 (2011)
2060–2066.
[37] M. Bagherzadeh, R. Latifi, L. Tahsini, M. Amini, Catal. Commun. 10 (2008)
196–200.
[38] H. Egami, T. Katsuki, J. Am. Chem. Soc. 129 (2007) 8940–8941.
[39] C. Yang, Q.P. Jin, H. Zhang, J. Liao, J. Zhu, B. Yu, J.G. Deng, Green Chem. 11
(2009) 1401–1405.
[40] I. Sheikhshoaie, A. Rezaeifard, N. Monadi, S. Kaafi, Polyhedron 28 (2009)
733–738.
[80] V.G. Kadajji, G.V. Betageri, Polymers 3 (2011) 1972–2009.
[81] B.L. Rivas, G.V. Seguel, K.E. Geckeler, J. Appl. Polym. Sci. 85 (2002)
2546–2551.
[82] A. Pourjavadi, H. Ghasemzadeh, Polym. Eng. Sci. 47 (2007) 1388–1395.
[83] Y. Feng, A. Schmidt, R.A. Weiss, Macromolecules 29 (1996) 3909–3917.
[84] R.M. Silverstein, G.C. Bassler, T.C. Morrill, Spectrometric Identification of
Organic Compounds, John Wiley and Sons, Inc, Hoboken, New Jersey, 1991.
[85] M.G.A.E. Wahed, K.A.E. Manakhly, N.E. Kososy, Mater. Chem. Phys. 41 (1995)
117–122.
[41] G.P. Romanelli, P.I. Villabrille, C.V. Cáreres, P.G. Vázquez, P. Tundo, Catal.
Commun. 12 (2011) 726–730.
[42] J.J. Boruah, S.P. Das, R. Borah, S.R. Gogoi, N.S. Islam, Polyhedron 52 (2013)
246–254.
[43] K. Kamata, T. Hiranoa, N. Mizuno, Chem. Commun. (2009) 3958–3960.
[44] K. Sato, M. Hyodo, M. Aoki, X.-Q. Zheng, R. Noyori, Tetrahedron 57 (2001)
2469–2476.
[45] V. Hulea, A.-L. Maciuca, F. Fajula, E. Dumitriu, Appl. Catal. A 313 (2006)
200–207.
[86] F.H. Cristovan, C.M. nascimento, M.J.V. Bell, E. Laureto, J.L. Duarte, I.F.L. Dias,
W.O. Cruz, A. Marletta, Chem. Phys. 326 (2006) 514–520.
[87] N.J. Cristovan, A.C. Dengel, W.P. Griffith, Polyhedron 8 (1989) 1379–1386.
[88] A.B.P. Lever, Harry B. Gray, Acc. Chem. Res. 11 (1978) 348–355.
[89] A.C. Dengel, W.P. Griffith, R.D. Powell, A.C. Skapski, J. Chem. Soc. Dalton
Trans. (1987) 991–995.
[90] Y. Fan, C.J. Cornelius, J. Mater. Sci. 48 (2013) 1153–1161.
[91] H.G.M. Edwards, D.R. Brown, J.R. Dale, S. Plant, J. Mol. Struct. 595 (2001)
111–125.
[46] K.L. Prasanth, H. Maheswaran, J. Mol. Catal. A: Chem. 268 (2007) 45–49.
[47] D.C. Rideout, R.J. Breslow, Am. Chem. Soc. 102 (1980) 7816–7817.
[48] U.M. Lindstrom, Organic Reactions in Water, Black wellPublishing, Oxford,
2007.
[92] P. Kar, A. Nayak, Y.P. Bhoi, B.G. Mishra, Microporous Mesoporous Mater. 223
(2016) 176–186.
[93] C. Djordjevic, N. Vuletic, B.A. Jacobs, M.L. -R, E. Sinn, Inorg. Chem. 36 (1997)
1798–1805.
[49] C.-J. Li, L. Chen, Chem. Soc. Rev. 35 (2006) 68–82.
[50] U.M. Lindstrom, Chem. Rev. 102 (2002) 2751–2772.
[51] M.-O. Simon, C.-J. Li, Chem. Soc. Rev. 41 (2012) 1415–1427.
[52] S. Narayan, J. Muldoon, M.G. Finn, V.V. Fokin, H.C. Kolb, K.B. Sharpless,
Angew. Chem. Int. Ed. 44 (2005) 3275–3279.
[53] X.-Y. Shi, J.F. Wei, J. Mol. Catal. A: Chem. 280 (2008) 142–147.
[54] W.A. Herrmann, B. Cornils, Angew. Chem. Int. Ed. Engl. 36 (1997) 1049–1067.
[55] X.-M. Zeng, J.-M. Chen, A. Yoshimura, K. Middletonb, V.V. Zhdankin, RSC
Adv. 1 (2011) 973–977.
[56] B. Yu, C.-X. Guo, C.-L. Zhong, Z.-F. Diao, L.-N. He, Tetrahedron Lett. 55 (2014)
1818–1821.
[57] Y. Imada, T. Kitagawa, H.-K. Wanga, N. Komiya, T. Naota, Tetrahedron Lett.
54 (2013) 621–624.
[94] V. Nardello, J. Marko, G. Vermeersch, J.M. Aubry, Inorg. Chem. 34 (1995)
4950–4957.
[95] D.D. Jiang, Q. Yao, M.A. McKinney, C.A. Wilkie, Polym. Degrad. Stab. 63
(1999) 423–434.
[96] S.K. Maiti, S. Banerjee, A.K. Mukherjee, K.M.A. Malik, R. Bhattacharyya, New
J. Chem. 29 (2005) 554–563.
[97] T. Patonay, W. Adam, A. Levai, P. Kover, M. Nemeth, E.-M. Peters, K. Peters, J.
Org. Chem. 66 (2001) 2275–2280.
[98] E. Baciocchi, M.F. Gerini, A. Lapi, J. Org. Chem. 69 (2004) 3586–3589.
[99] O.A. Kholdeeva, Eur. J. Inorg. Chem. (2013) 1595–1605.
[100] H.S. Schultz, H.B. Freyermuth, S.R. Buc, J. Org. Chem. 28 (1963) 1140–1142.
[101] O. Bortolini, F. Di Furia, G. Modena, R. Seraglia, J. Org. Chem. 50 (1985)
2688–2690.
[58] N.M. Gresley, W.P. Griffith, A.C. Laemmel, H.I.S. Nogueira, B.C. Parkin, J. Mol.
Catal. A: Chem. 117 (1997) 185–198.
[59] D. Kalita, S. Sarmah, S.P. Das, D. Baishya, A. Patowary, S. Baruah, N.S. Islam,
React. Funct. Polym. 68 (2008) 876–890.
[102] M. Bonchio, V. Conti, M.A. De Conciliis, F. Di Furia, F.P. Ballistreri, G.A.
Tomaselli, R.M. Toscano, J. Org. Chem. 60 (1995) 4475–4480.
[103] A.-L. Maciuca, C.-E. Ciocan, E. Dumitriu, F. Fajula, V. Hulea, Catal. Today 138
(2008) 33–37.
[60] A. Fuerte, M. Iglesias, F. Sánchez, A. Corma, J. Mol. Catal. A: Chem. 211 (2004)
227–235.
[61] M.H. Dickman, M.T. Pope, Chem. Rev. 94 (1994) 569–584.
[62] B. Tamami, H. Yeganeh, Eur. Polym. J. 35 (1999) 1445–1450.
[63] A.F. Ghiron, R.C. Thompson, Inorg. Chem. 28 (1989) 3647–3650.
[64] H. Mimoun, Catal. Today 1 (1987) 281–295.
[104] D.J. Gravert, K.D. Janda, Chem. Rev. 97 (1997) 489–509.
[105] K.A. Joergensen, Chem. Rev. 89 (1989) 431–458.
[106] W.R. Thiel, J. Eppinger, Chem.-Eur. J. 3 (1997) 696–705.
[107] J. Bassett, R.C. Denney, G.H. Jeffery, J. Mendham, VOGEL’s Textbook of
Quantitative Inorganic Analysis Including Elementary Instrumental
Analysis, 4th ed., Longman Group Ltd, London, 1978, pp. 472–473.