Selective and solventless oxidation of organic sulfides and alcohols using new supported molybdenum (VI) complex in microwave and conventional methods

Article abstract of DOI:10.1002/aoc.5781

A new dioxo-molybdenum (VI) complex supported on functionalized Merrifield resin (MR-Mo) has been synthesized and characterized by elemental, scanning electron mcroscopy, energy-dispersive X-ray analysis, TGA, Brunauer–Emmett–Teller method, powder-X-ray d

Full text of DOI:10.1002/aoc.5781

Received: 14 January 2020
DOI: 10.1002/aoc.5781
Revised: 8 April 2020
Accepted: 17 April 2020
F U L L P A P E R
Selective and solventless oxidation of organic sulfides and
alcohols using new supported molybdenum (VI) complex in
microwave and conventional methods
Siva Prasad Das1
| Jeena Jyoti Boruah1,2
1Department of Chemistry, School of
Science, RK University, Bhavnagar
Highway, Kasturbadham, Rajkot, Gujarat,
360020, India
2Department of Chemistry, Moridhal
College, Moridhal, Dhemaji, Assam,
787057, India
A new dioxo-molybdenum (VI) complex supported on functionalized Merri-
field resin (MR-Mo) has been synthesized and characterized by elemental,
scanning electron mcroscopy, energy-dispersive X-ray analysis, TGA,
Brunauer–Emmett–Teller method, powder-X-ray diffraction, Fourier trans-
form infrared, X-ray photoelectron spectroscopy and DRS–UV–vis analysis.
The virgin Merrifield resin (MR) was functionalized by carbonylation followed
by Schiff base formation with ethanolamine (MR-SB). Experimental data
Correspondence
Siva Prasad Das and Jeena Jyoti Boruah,
Department of Chemistry, School of
Science, RK University, Bhavnagar
Highway, Kasturbadham, Rajkot-360020,
Gujarat, India.
2+
showed that the Schiff base coordinated with the MoO2 moiety via O- and
N-atoms. The catalytic activity of MR-Mo was explored under solventless con-
ditions toward the oxidation of organic sulfides and alcohols using 30% aque-
ous H2O2 as oxidant. The oxidation reactions were conducted under
microwave and conventional methods. The microwave-assisted oxidation reac-
tions were found to be many times faster than the conventional methods. The
oxidation reactions were selective and formed sulfoxides or aldehydes as the
sole product with superior TOF values among the molybdenum (VI)-based
complexes. Besides these, the MR-Mo was purely heterogeneous in nature
and can be recycled for at least five reaction cycles without the loss of catalytic
efficiency and product selectivity.
Email: siva.spd@gmail.com; siva.das@
rku.ac.in; jeena.jyoti@gmail.com; jeena.
jyoti@rku.ac.in
Funding information
University Grants Commission, Grant/
Award Number: No. F. 5-30/2013-14/
MRP/NERO/427
K E Y W O R D S
alcohol oxidation, heterogeneous catalysis, Merrifield resin, microwave reaction, sulfide
oxidation, supported molybdenum (VI) complex
1 | INTRODUCTION
upgrades the starting materials into high-value-added
products.4–6 In particular, the aldehydes or ketones
There has been continuous upsurge in research interest
toward the development of catalytic systems for the selec-
tive oxidation of organic sulfides and alcohols as the oxi-
dation products are versatile intermediates in various
organic transformations and widely used in organic syn-
thesis and industrial chemistry.1–3 The oxidation process
formed from the selective oxidation of alcohols are essen-
tial and important reactions7 and they are widely used as
solvents, perfumes, intermediates of dyes and polymeric
precursors as well as pharmaceuticals.8 On the other
hand, the oxidation products of organic sulfides are key
intermediates in pharmaceutical and other fine chemical
industries.9,10 In particular, the sulfoxides are useful
building blocks and form a wide range of chemically and
biologically active molecules such as antiulcer (proton
Siva Prasad Das and Jeena Jyoti Boruah contributed equally to this
work.
Appl Organomet Chem. 2020;e5781.
wileyonlinelibrary.com/journal/aoc
© 2020 John Wiley & Sons, Ltd.
1 of 18
https://doi.org/10.1002/aoc.5781

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DAS AND BORUAH
pump
inhibitors),
antibacterial,
antifungal,
halogenated solvents, the use of additives, the generation
of huge amounts of waste materials and corrosion of
industrial materials are other disadvantages associated
with most of the protocols.18,31,32 A detailed list per-
taining to the use of molybdenum (VI) based complexes
in alcohol and sulfide oxidation is given in the Supple-
mentary data (Table S3 and S4). Notably, most of the pro-
tocols suffered from some of the drawbacks mentioned
above. Thus, our primary goal was to develop catalytic
systems that avoids these problems.
antiatherosclerotic, antihypertensive and cardiotonic
agents as well as psychotropics and vasodilators.11,12 In
addition to these, oxidative desulfurization is a technique
used to obtain ultra-low-sulfur-containing fuels. In the
typical process, the sulfur contaminants in the fuels are
oxidized into sulfones and subsequently easily separated
by extraction, distillation or decomposition.13 Thus, the
selective oxidation of sulfides and alcohols is a useful pro-
cess in organic chemistry and hence there are many
reports available for these transformations.
The process of “heterogenization of homogeneous
catalysts” has gained increasing prominence among
researchers owing to its striking advantages.33 In the typi-
cal process, the homogeneous catalysts are anchored on
polymers or other supports. This imparts the combined
advantages of homogeneous and heterogeneous catalysis
to the catalyst. Besides this, the process provides stability
to catalysts, easy recovery of catalyst from the reaction
mixture, simple product separation, etc. These advan-
tages are attractive considering the current environmen-
tal demands and green chemistry practices. In this regard
the use of Merrifield resin, which is a chloromethylated
polystyrene cross-linked with divinylbenzene, introduced
by RB Merrifield in the year 1963,34 is still popular as an
insoluble support. This polymeric support is insoluble,
provides chemical and mechanical robustness, available
commercially, cheap and undergoes easy and higher
degrees of functionalization.35,36
The use of microwave irradiation in synthetic organic
chemistry, known as microwave-assisted organic synthe-
sis (MAOS), is a relatively new and quickly developing
area and draws much research attention. This method
offers superior performance to conventional heating
methods. In many cases, the organic reactions under
microwave conditions are much faster and are completed
within several minutes or even seconds whereas the same
reaction under conventional heating requires many hours
at reflux temperature. In addition, easier workup, better
product purification, solventless conditions, waste mini-
mization and higher conversions and yields are other
notable features of MAOS.37 Thus, MAOS complies with
many of the postulates of “green chemistry”.38
The oxidation of alcohols and sulfides is usually
achieved using stoichiometric amounts of organic or
inorganic oxidants. For example, the oxidation of alco-
hols has been carried out with chromate, permanganate,
etc. based reagents.14,15 The use of these reagents usually
suffers from various drawbacks such as the generation of
huge amounts of toxic byproducts or heavy metal wastes,
difficulty in workup and requirements for larger excesses
of oxidizing agents.11,16,17 Moreover, with most of the
reagents the alcohols or sulfides are overoxidized into
carboxylic acids or sulfones, respectively. Therefore, the
search for a selective and environmentally acceptable
protocol toward oxidation of alcohols and sulfides is still
going on.
In view of effective oxygen content (47%), waste-
avoiding oxidant as water being the only byproduct, low
production cost, ease of transportation, storage and oper-
ation, aqueous H2O2 is found to be an ideal green inor-
ganic oxidant.18 However, in neutral or weak acidic
media, H2O2 possesses low standard redox potential and
hence most often it has been used with HCl, H2SO4,
CH3COOH, HClO4 and peracids.12,18 Although these pro-
tocols have potential in various oxidation reactions, they
suffer from severe limitations such as the corrosive
nature of the acids used, homogeneous reaction mixture,
difficulty in workup along with the requirement for addi-
tional strong bases for neutralization of the used acids.
Hydrogen peroxide is relatively a weak electrophile,
so over recent decades, a plethora of catalytic systems
based on various transition metals such as iron, vana-
dium, tungsten, molybdenum, titanium, rhenium, ruthe-
nium, scandium, zirconium, manganese, etc. have been
With a view to developing of an environmentally
benign and heterogenized homogeneous catalyst, in this
work, we are reporting the synthesis and characterization
of a dioxomolybdenum (VI) complex supported on pre-
functionalized Merrifield resin (MR-Mo). The most
advantageous aspect of the protocol is its simplicity in
synthetic steps and use of readily available commercial
chemicals. In addition, a less crowded coordination envi-
ronment facilitates easily accessible active molybdenum
(VI) centers. Thus we show that MR-Mo acts as a hetero-
geneous catalyst toward selective oxidation of organic
developed for oxidation of sulfides or alcohols with
11,19–30
H2O2.
Although these reported transition metal
ion complexes in conjunction with H2O2 as oxidant were
found to be efficient and selective and showed reproduc-
ibility, a common problem encountered in most of the
oxidation processes was the concomitant decomposition
18
of H2O2. This causes the requirement of a large excess
of H2O2 to achieve a full conversion in reactions. Besides
this, the homogeneous nature, removal or recovery of
catalysts from the reaction mixture, the requirement for

DAS AND BORUAH
3 of 18
sulfides and alcohols to sulfoxides and aldehydes, respec-
tively. The reactions are conducted by microwave irradia-
tion as well as conventional methods using aqueous
H2O2 as an oxidant under solventless conditions. We also
compared our protocol with the reported works and,
interestingly, we found that our demonstrated work
offered superior TOF among molybdenum (VI)-based
complexes toward oxidation of organic sulfides and
alcohols.
spectra of the compounds were recorded in solid state
using BaSO4 as a reference. The spectra were recorded in
the range from 240 to 800 nm with the help of a Hitachi
U-3400 spectrophotometer equipped with an integrating
sphere of 60 mm inner diameter. The scanning electron
mcroscope (SEM) images of the samples were recorded
with the help of a Jeol JSM-6390LV SEM with an
attached EDX detector. Dinitrogen adsorption/desorption
measurements were carried out with the help of a Quan-
tachrome model Nova 4200e porosimeter at 77.3 K. The
powder X-ray diffraction (XRD) patterns of the com-
pounds were collected over the 2θ range of 5–70ꢀ with a
Rigaku X-ray diffractometer (Miniflax, UK) using Cu Kα
radiation (λ = 1.54 Å). X-ray photoelectron spectroscopy
(XPS) was performed in a multiprobe system (Omicron
Nanotechnology, Germany) equipped with a dual Mg/Al
X-ray source and a hemispherical analyzer operating in
constant analyzer energy mode. The X-ray source
(Mg Kα) with a power of 300 W was operated at 15 kV.
The spectra were calibrated using the C 1 s peak by set-
ting the binding energies of the C 1 s line at 284.8 eV.
The TG–DTG measurements of the compounds were per-
formed with a Shimadzu TGA-50 system over the tem-
perature range of 30–700ꢀC under nitrogen using an
2 | EXPERIMENTAL
2.1 | Materials
All of the chemicals were commercial products and used
as supplied, unless otherwise stated. The source of
chemicals was as follows: 4-chlorobenzyl alcohol,
4-fluorobenzyl alcohol, 4-methoxybenzyl alcohol,
4-bromobenzyl alcohol, 4-hydroxybenzyl alcohol, methyl
phenyl sulfide (MPS), dibutyl sulfide, diphenyl sulfide
and allylphenyl sulfide were purchased from Alfa Aesar,
Mumbai, India. Ethanolamine, benzyl alcohol, dimethyl
sulfide, 4-nitrobenzyl alcohol, sodium bicarbonate,
hydrogen peroxide (30% w/v aqueous solution), dimethyl
1
aluminum pan at a heating rate of 10ꢀC min−1. The H
sulfoxide
(DMSO),
ethyl
acetate,
acetone,
NMR spectra of the substrates and products were
recorded at room temperature in CDCl3 or DMSO-d6
using a Bruker, Avance 400 MHz spectrophotometer. The
solvent resonance peak was used as an internal standard.
GC analysis was carried out on a CIC gas chromatograph
model 2010 equipped with a flame ionization detector
and an SE-52 packed column (length 2 m, 1/80 inch o.d.)
using nitrogen as the carrier gas (30 ml min−1).
tetrahydrofurane (THF), hexane and methanol (MeOH)
were purchased from SD Fine-Chem Limited, Mumbai,
India. Merrifield resin (2% crosslinked, 2–4 mmol Cl per
g of polymeric support) was purchased from Alfa Aesar,
Mumbai, India and used without further purification. Sil-
ica gel (60–120 mesh) for column chromatography and
thin-layer chromatography (TLC) plates (TLC silica gel
60 F254) were purchased from Molychem, Mumbai, India
and Merck Limited (India), respectively. The compound
[MoO2(acac)2] was prepared from ammonium hep-
tamolybdate tetrahydrate and acetylacetone by following
a reported procedure.39
2.3 | Functionalization of the Merrifield
resin
2.2 | Physical measurements
The Merrifield resin (MR) was
functionalized in two steps as follows.
The compounds were analyzed for C, H and N content
using a Perkin-Elmer 2400 series II CHN analyzer and
the molybdenum content was determined using a
Thermo iCE 3000 series atomic absorption spectropho-
tometer (model analyst 200). The C, H, N and Mo con-
tents along with Cl content in the compounds were also
determined by energy-dispersive X-ray (EDX) analysis.
The infrared spectra of the compounds were recorded in
the range of 4000–400 cm−1 using the KBr disk method
in a Nicolet model 410 Fourier transform infrared (FTIR)
spectrophotometer. The UV–visible diffuse reflectance
2.3.1 | Carbonylation of MR (MR-CO)
The carbonyl functionalization of MR was achieved by
adopting a reported procedure.40 In the typical procedure,
1.0 g of MR was pre-swelled in DMSO–THF (5:1) at room
temperature for 24 h. Subsequently, the carbonyl func-
tionalization was done by refluxing a mixture of 1.0 g of
MR and 0.50 g, 5.95 mmol of sodium bicarbonate
(NaHCO3) in 50 ml DMSO for 24 h. The resulting mix-
ture was cooled and the solid polymeric beads filtered off.

4 of 18
DAS AND BORUAH
The solid polymeric beads thus obtained were washed
with water (3 × 10 ml) and methanol (3 × 10 ml) and
finally dried in vacuo.
at 1:1000:1050. The progress of the reaction was moni-
tored by TLC and gas chromatography (GC). After com-
pletion of the reaction, the solid catalyst was separated
from the reaction mixture by filtration, washed with
methanol (5 × 5 ml), acetonitrile (5 × 5 ml) and ace-
tone (3 × 5 ml), dried under vacuum and reused. The
crude filtrate along with the combined catalyst wash-
ings were concentrated under reduced pressure and
extracted with ethyl acetate (3 × 10 ml). The combined
extract was dried over anhydrous Na2SO4, filtered and
the volume reduced to one-tenth under reduced pres-
sure. The crude products were further purified by col-
umn chromatography (silica gel). The pure products
were identified by 1H and 13C NMR analysis (along
with melting point determination in case solid
products).
2.3.2 | Schiff base functionalization on
Merrifield resin (MR-SB)
The Schiff base functionalization on the MR was
achieved by the reaction of MR-CO and ethanolamine
using methanol as solvent. In the typical process, 0.27 g,
4.42 mmol of ethanolamine (~2 equiv. of loaded carbonyl
group) was added to 1.0 g of MR-CO, which was pre-
swelled in 20 ml methanol for 24 h at room temperature.
The reaction mixture was subsequently refluxed for 48 h
in the presence of a drop of acetic acid under magnetic
stirring. After the reaction, the off-white color beads thus
obtained were separated by filtration, washed with meth-
anol (5 × 5 ml) and acetone (3 × 5 ml) and finally dried
under vacuum.
In an alternative method, keeping
a
similar
molar ratio of alcohol, H2O2 and MR-Mo, the reac-
tions were conducted under microwave radiation
(100 W). The progress of the reaction, product isola-
tion and characterization were performed as mentioned
above.
2.4 | Synthesis of molybdenum
(VI) complex supported on Merrifield resin
(MR-Mo)
2.5.2 | General procedure for oxidation of
sulfides
The
synthesis
of
Merrifield
resin-supported
molybdenum (VI) complex was achieved by refluxing a
mixture of MR-SB and MoO2(acac)2 in methanol–THF
(1:1). In the typical process, 1.0 g of MR-SB was pre-
swelled in 15 ml THF for 24 h at room temperature.
To this, a solution of 0.57 g, 1.75 mmol MoO2(acac)2
(~1.1 equiv. with respect to Schiff base) and 0.07 g,
1.67 mmol LiOH (monohydrate) (~1.05 equiv. with
respect to Schiff base) in 15 ml of methanol was added
and the mixture refluxed for 24 h. At this moment,
the polymeric beads became slightly brownish in color.
After the reaction, the solid products were
separated by filtration, washed with methanol
(5 × 10 ml) and acetone (3 × 5 ml) and finally dried
under vacuum.
In the representative procedure, a mixture of organic sul-
fide (2.5 mmol), H2O2 (2.75 mmol) and MR-Mo
(4.90 mg, 2.5 × 10−3 mmol of Mo) was magnetically
stirred at 70ꢀC for an specified time period as shown in
Table 5. The molar ratio of Mo:substrate:H2O2 was
maintained at 1:1000:1100. The progress of the reaction
was monitored by TLC and GC. After completion of the
reaction, the solid catalyst was separated from the reac-
tion mixture and reused as mentioned above in Sec-
tion 2.5.1. Similarly, the crude product isolation,
purification and identification were performed as men-
tioned above in Section 2.5.1.
In an alternative method, keeping a similar molar
ratio of sulfide, H2O2 and MR-Mo, the reactions were
conducted under microwave radiation (100 W). The pro-
gress of the reaction, product isolation and characteriza-
tion were performed as mentioned above.
2.5 | Catalytic activity
2.5.1 | General procedure for oxidation of
alcohols
2.5.3 | Recyclability of the catalyst
In the representative procedure, a mixture of alcohol
(2.5 mmol), H2O2 (2.625 mmol) and MR-Mo (4.90 mg,
2.5 × 10−3 mmol of Mo) were magnetically stirred at
50ꢀC for the specified time period as shown in Table 5.
The molar ratio of Mo:substrate:H2O2 was maintained
The recyclability test of MR-Mo was conducted sepa-
rately by taking benzyl alcohol and MPS as model sub-
strates for alcohol and sulfide oxidation reactions,
respectively. After completion of the reaction, the solid
catalyst MR-Mo was separated, washed and dried as

DAS AND BORUAH
5 of 18
respect to Schiff base), was added to deprotonate the alco-
hol group41,42 of MR-SB. In order to achieve the maxi-
mum molybdenum loading on the support, various
reaction parameters were screened and are described in
the Supplementary data (Figures S1–S3). The compounds
derived at each synthetic step were characterized by ele-
mental analyses, SEM–EDX, Brunauer–Emmett–Teller
(BET), powder-XRD, TGA, FTIR and diffuse reflectance
UV–vis spectroscopy and are discussed in detail below.
Additionally, the compound MR-Mo was further charac-
terized by AAS and XPS.
The elemental analysis data of the synthesized com-
pounds provided vital information about the loading of
the Schiff ligand as well as metal on the support. The
analytical data for the compounds are shown in Table 1.
It is seen from the table that 7.77% of chlorine was
reduced in MR after formation of MR-CO, i.e. 79% of -
CH2Cl was converted into -CHO during the
carbonylization step. In MR-SB, the nitrogen content
was 2.21%, which was absent in MR-CO. This indicated
the formation of Schiff base in MR-SB by the reaction of
MR-CO and aminoethanol. The molybdenum content
determined by AAS in MR-Mo was 4.86%. This corre-
sponds to 0.51 mmol Mo/g of polymeric support. This
value is consistent with the data obtained from EDX
analysis.
The changes in surface morphology of the polymeric
beads at different synthetic steps were studied by scan-
ning electron microscopy (SEM) and are presented in
Figure 1. From the micrographs it is seen that the virgin
MR undergoes distinctive morphological changes during
the formation of MR-Mo. The surfaces of MR
(Figure 1a) were lightly roughened after formation of
MR-CO (Figure 1b) and MR-SB (Figure 1c). This
suggested a uniform surface functionalization of the MR
beads. However, in MR-Mo (Figure 1d), a few randomly
oriented depositions were observed, which suggested fur-
ther roughening of the polymeric beads. The EDX spec-
trum of MR-Mo (Figure 1e) showed the compositional
SCHEME 1 Synthetic pathway for MR-Mo. “ ” represents
polymeric support
mentioned above in Section 2.5.1. The recovered catalyst
was reused by adding it to a fresh reaction mixture
maintaining a molar ratio of Mo:substrate:H2O2 at
1:1000:1050 (for alcohol oxidation reaction) or Mo:sub-
strate:H2O2 at 1:1000:1100 (for organic sulfide oxidation
reaction using methyl phenyl sulfide as substrate). The
progress of the reaction was monitored by TLC and
GC. After completion of the reaction, the process was
repeated at least up to the fifth reaction cycle.
3 | RESULTS AND DISCUSSION
3.1 | Synthesis and characterization
The synthesis of MR-Mo was done in three separate
steps as shown in Scheme 1 and the detail methods are
outlined in Section 2. The base, LiOH (~1.05 equiv. with
TABLE 1
Analytical data for MR, MR-CO, MR-SB and MR-Mo
Data obtained from elemental analysis (%) (Data obtained from EDX
analysis, %)
Compound
MR
C
H
N
Cl
Mo
Metal loadinga (mmol/g of polymer)
83.18 (83.29)
87.16 (87.11)
84.92 (84.97)
79.51 (79.43)
6.99 —
7.23 —
7.51 —
6.74 —
—
— (9.80)
— (2.03)
— (1.88)
— (1.77)
—
—
MR-CO
MR-SB
MR-Mo
—
—
—
2.21 (2.16)
2.04 (2.10)
—
—
4.86b (4.79) 4.78c
0.51
aMetal loading = (observed molybdenum % × 10)/(atomic weight of molybdenum).
bMolybdenum % obtained from AAS.
cMolybdenum content after 5th reaction cycle.

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DAS AND BORUAH
FIGURE 1 Scannung electron microscopy
images of (a) MR, (b) MR-CO, (c) MR-SB and
(d) MR-Mo. Energy-dispersive X-ray spectrum
of (e) MR-Mo
analysis and confirmed the presence of carbon, nitrogen,
oxygen, chlorine and molybdenum on the compound.
The absence of lithium in the compound suggested the
formation of a neutral molybdenum (VI) complex. More-
over, a diamagnetic nature of the compound was found
from the magnetic susceptibility measurements, which is
also consistent with the formation of a charge-neutral
molybdenum complex with +6 oxidation state.
The powder XRD patterns of the compounds, viz.
MR, MR-CO, MR-SB and MR-Mo were recorded at 2θ
values from 5 to 70ꢀ. Figure 2 shows the powder XRD
patterns for MR, MR-SB and MR-Mo. Each powder
XRD pattern of the compounds shows an identical broad
peak centered at ca. 20ꢀ. No sharp peak was observed. On
the basis of the existing literature,43–46 such a type of
broad diffraction pattern is characteristic for the PS-DVB
resin. In addition to this, absence of sharp peaks in the
diffraction pattern indicated the amorphous nature of the
molybdenum (VI) moiety in MR-Mo.
The XPS spectra of MR-Mo were recorded and are
presented in Figure 3, which provides the surface elec-
tronic properties. The Mo 3d spectrum (Figure 3a) shows
two well-resolved peaks at 232.7 and 235.8 eV. On the
basis of available literature report, the peaks correspond
to Mo (3d3/2) and Mo (3d5/2).47–50 These peaks are attrib-
utable to the molybdenum centers in +6 oxidation state.
Vital information about the coordination of the molybde-
num (VI) center with the support came from the compar-
ison of O 1 s and N 1 s spectra of MR-SB and MR-Mo.
The O 1 s spectrum of MR-SB (Figure 3b) showed a peak
at 531.2 eV, which is attributable to the attached hydroxyl
group of Schiff base moiety.51,52 However, in MR-Mo,
the O 1 s spectrum showed two well-resolved peaks at
530.1 and 532.3 eV attributable to Mo=O and

DAS AND BORUAH
7 of 18
FIGURE 2 Powder X-ray diffraction patterns of (a) MR,
(b) MR-SB and (c) MR-Mo
coordinated C–O−, respectively.51–53 The absence of a
peak at 531.2 eV and the blue shift of 1.1 eV indicated
that all of the surface-bound hydroxyl groups of Schiff
base moiety are deprotonated and coordinated with the
molybdenum (VI) center in MR-Mo. In addition, the
absence of any other peak at 527.0 eV53 indicated the
complete removal of acac ligand from the molybdenum
(VI) precursor during the formation of MR-Mo. There
was a 1.7 eV blue shift (399.5 eV in MR-SB to 401.2 eV in
MR-Mo) in the N 1 s peak position (Figure 3c) after
metal incorporation, which confirmed the coordination
of imine nitrogen with MoO22+ moiety in MR-Mo.51,52,54
To investigate the surface area, pore volume and pore
size of the synthesized compounds, N2 absorption and
desorption measurements were used at liquid nitrogen
temperature. The isotherms showed (Figure S4, Supple-
mentary data) typical Type II adsorption of a IUPAC
standard.55,56 This type of adsorption behavior is charac-
teristic of macroporous or nonporous material.57 The sur-
face areas and pore volume were calculated by following
the BET method58 and Barrett–Joyner–Halenda model,59
respectively, in the nitrogen isotherms and the results are
shown in Table 2. By comparing the data, it was found
that the surface area (from 12.5 m2 g−1 in MR to
3.9 m2 g−1 in MR-Mo), pore volume and pore size were
gradually decreased after each step of functionalization.
Similar types of observation were reported31,46,60 with
MR-based supported metal complexes where the func-
tionalization and subsequent metal loading blocked the
pores of the polymeric support, resulting in lower surface
area, pore volume and pore size.
FIGURE 3 X-ray photoelectron spectroscopy spectra of
(a) Mo 3d [in (i) MR-Mo and (ii) MR-Mo after fifth reaction cycle
(for the oxidation reaction of benzyl alcohol)] and (b) O 1 s and
(c) N 1 s energy regions
molybdenum (VI) with MR-SB. The spectra for the com-
pounds, viz. MR, MR-CO, MR-SB and MR-Mo, are
shown in Figure 4 (Figure S5, Supplementary data, pre-
sents full range spectra) and the most significant peaks
are tabulated in Table S1. (Supplementary data). The
characteristic medium intensity peak at 1261 cm−1
(Figure 4a), in MR, assigned for the ν(C–Cl) mode of the
-CH2Cl group disappeared (or the intensity decreased
Comparison of the FTIR spectra of the synthesized
compounds provided crucial information about the
chemical changes that take place during the func-
tionalization of MR as well as the binding mode of

8 of 18
DAS AND BORUAH
TABLE 2
The surface area, pore volume and pore size of the
polymeric compounds
a
b
SBET
Vtot
Pore radius
(Å)
Compound (m2 g−1
)
(cm3 g−1
0.13
)
MR
12.5
7.5
51.7
30.0
27.5
13.4
MR-CO
MR-SB
MR-Mo
0.08
6.8
0.07
3.9
0.03
aBrunauer–Emmett–Teller surface area.
bTotal pore volume.
significantly) in MR-CO.31,46,61,62 Interestingly, new
peaks appeared in the spectrum of MR-CO at 1734, 2830
and 2722 cm−1 which may be attributed to ν(C=O), ν(C–
H)aldehydic,asym and ν(C–H)aldehydic,sym, respectively.46,63–65
This confirmed the transformation of -CH2Cl into the -
CHO group during the carbonylation step. This observa-
tion is in consistent with the data obtained from elemen-
tal analysis where the chlorine content in MR-CO was
found to reduce significantly.
The FTIR spectrum of MR-SB (Figure 4c) provides
clear information about the formation of the Schiff base.
In comparison with MR-CO, the IR spectrum of MR-SB
showed new peaks at 1631 and 1064 cm−1, which are
attributable to ν(C=N) and ν(C–O), respectively.42,63 On
the other hand, the ν(C=O) and ν(C–H)aldehydic peaks
present in MR-CO vanished in MR-SB. This confirmed
the formation of Schiff base in MR-SB by the reaction of
the -CHO (of MR-CO) and -NH2 (of ethanolamines)
group. In addition to this, a broad and medium-intensity
peak appeared in MR-SB at 3318 cm−1 which can be
assigned to ν(O–H) vibration of the attached ethanol-
amine moiety.42,66
The comparison of FTIR spectra of MR-SB and MR-
Mo provided vital information about the attachment of
the molybdenum (VI) center with the support. The FTIR
spectrum of MR-Mo (Figure 4d) showed new sharp
peaks at 955 and 918 cm−1. These are characteristic peaks
for the asymmetric and symmetric vibrations of
2+
dioxomolybdenum
(VI),
MoO2
moiety,
Thus, this
67,68
FIGURE 4 Fourier transform infrared spectra of (a) MR,
i.e. ν(O=Mo=O)asym and ν(O=Mo=O)sym.
(b) MR-CO, (c) MR-SB, (d) MR-Mo and (e) MR-Mo after fifth
2+
confirmed the presence of MoO2 moiety in the MR-
Mo. It is pertinent here to mention that the coordination
of the Schiff bases, through its nitrogen atom of imine
group, with the metal is expected to lower the
ν(C=N)imine absorption frequency more than the
corresponding noncoordinated form.41,42 Indeed, the
ν(C=N)imine peak in MR-Mo was shifted to lower wave
number (Δν = 16 cm−1) after metal incorporation.
Another striking feature of the spectrum is the absence of
the ν(O–H) peak which was otherwise broad and
medium intensity in MR-SB. This indicates the
reaction cycle
deprotonation of the -OH group of the ethanolamine dur-
ing coordination.41,42,66 These observations confirmed the
coordination of molybdenum (VI) center with the imine
and alcohol group.
The diffuse reflectance UV–visible spectra of MR-SB
and MR-Mo further support the complex formation.
The spectra were recorded by taking BaSO4 as the refer-
ence. Each of the spectra displayed two well-resolved

DAS AND BORUAH
9 of 18
The thermogram of MR-Mo showed four weight-
loss steps at the temperature ranges of 206–244,
300–351,
351–488
and
488–700ꢀC
with
the
corresponding weight losses of 10.03, 1.27, 31.69 and
46.80%, respectively. The compounds MR, MR-CO and
MR-SB were decomposed almost completely at 700ꢀC.
However, MR-Mo never underwent weight loss
completely up to 700ꢀC. We could not ascertain the
exact identity of the species formed at 700ꢀC in MR-
Mo, but based on literature reports the residue formed
oxo-molybdenum species and char residue.31,74 Indeed,
the IR spectrum of the residue (Figure S6
(Supplementary data)) recorded at 700ꢀC showed char-
acteristic peaks for oxo-molybdenum species and
devoid of any significant peaks for Schiff base and
polymeric support. The lack of degradation steps up to
206ꢀC for MR-Mo also indicated the high thermal sta-
bility of the compound, which is good for its catalytic
applications at higher temperature.
FIGURE 5 The TGA thermogram of MR (black), MR-CO
(green), MR-SB (red) and MR-Mo (blue)
peaks. On the basis of available literature reports, in
the case of MR-SB, the higher energy peak appearing
at 266 nm may be assigned to the π–π* transition of the
benzene ring of the polymeric support and imine moi-
ety and the lower energy peak appearing at 340 nm
may be assigned to the n–π* transition of the imine
moiety.69–71 However, after metal coordination, in the
case of MR-Mo, the π–π* transition remained unaf-
fected, but the n–π* transition was blue-shifted to
326 nm. This shifting suggests the coordination of
molybdenum (VI) center with the imine moiety.69–71
Bands for ligand to metal charge transfer in MR-Mo
On the basis of the above collective evidences, a
structure is proposed for MR-Mo as shown in
Scheme 1, where the dioxomolybdenum (VI) center is
attached to the polymeric support via the pendant
functional groups.
3.2 | Catalytic activity
2+
are expected for the MoO2 moiety. However, those
After successful synthesis and characterization of MR-
Mo, its catalytic activity was investigated toward the oxi-
dation of alcohols (Section 2.5.1) and organic sulfides
(Section 2.5.2). The oxidation reactions were conducted
by conventional as well as microwave methods as dis-
cussed below.
bands were absent in the spectrum as such types of
bands generally appear at higher concentrations of
metal complexes.
The TGA thermograms for the compounds are
shown in Figure 5 and data are presented in Table S2
(Supplementary data). Each of the thermograms
showed multiple stage thermal degradation within the
temperature range 30–700ꢀC. The virgin polymer MR
underwent three weight-loss steps at 290–368, 368–476,
and 476–700ꢀC. These decomposition steps are attrib-
uted to the decomposition of the chloromethylated
group (in the first step) and cross-linked polystyrene
backbone (in the second and third steps) in view of
the previous assignments.72,73 The thermogram of MR-
CO also showed three-stage thermal decomposition.
The thermogram of MR-SB showed the initial decom-
3.2.1 | Oxidation reactions by
conventional method
Alcohol oxidation
The oxidation of alcohols forms aldehydes, ketones or
carboxylic acids. The benzylic alcohols may form alde-
hydes and carboxylic acids. We studied the catalytic oxi-
dation of different types of substituted benzyl alcohols
using MR-Mo as a catalyst and 30% aqueous H2O2 as an
oxidant under solventless conditions. As the product
selectivity depends upon the employed reaction condi-
tions, the reaction conditions were optimized to achieve
the aldehydes as the exclusive products.
The reaction conditions were optimized by using ben-
zyl alcohol as model substrate. Various reaction parame-
ters such as (a) the amount of H2O2 as oxidant,
(b) solvents, (c) the amount of catalyst, (d) reaction
ꢀ
position step at ca. 197–225 C with the corresponding
weight loss of 11.17%. This weight loss value quite
close to the Schiff base loading on the support suggests
loss of the Schiff base at this step (on the basis of ele-
mental analysis, 2.21% N contents corresponds to
11.37% Schiff base). The subsequent decomposition
steps are attributed to the decomposition of the poly-
styrene backbone.

10 of 18
DAS AND BORUAH
TABLE 3
Optimization of reaction conditions for selective oxidation of alcohols to aldehydesa
Sample no. Molar ratio
Solvent
Temperature (ꢀC) Time (min) Yield (%) Product ratio a:b TOFb (h−1
)
c
Mo:Sc
S:H2O2
1
1:1000 1:0.5
1:1000 1:1.05
1:1000 1:1.5
1:1000 1:1.05
1:1000 1:1.05
1:1000 1:1.05
1:1000 1:1.05
1:1000 1:1.05
1:1000 1:1.05
1:1000 1:1.05
1:1000 1:1.05
Solventless
Solventless
Solventless
Chloroform
50
50
50
50
300
120
90
37
99
99
31
27
85
88
32
98
94
89
99
98
0
100:0
100:0
91:9
74
2
495
660
155
135
425
440
160
392
470
445
79
3
4
120
120
120
120
120
150
120
120
75
100:0
100:0
100:0
100:0
100:0
100:0
100:0
100:0
100:0
100:0
—
5
Dichloromethane 40
6
Acetonitrile
Water
50
50
50
30
70
90
50
50
50
50
50
50
50
7
8
Toluene
9
Solventless
Solventless
Solventless
Solventless
Solventless
Solventless
Solventless
Solventless
Solventless
Solventless
10
11
12
13
14d
15e
16f
17g
18h
1:100
1:500
1:1000
—
1:1.05
1:1.05
—
80
368
—
120
120
120
120
120
1:1.05
0
—
—
1:1000 1:1.05
1:1000
1:1000 1:1.05
63
0
100:0
—
315
—
—
4
100:0
20
aReaction conditions: all reactions were carried out with benzyl alcohol as substrate (2.5 mmol), MR-Mo (4.90 mg for 2.5 × 10−3 mmol of
Mo) and 5 ml solvent (unless otherwise indicated).
bTOF = (mmol of product)/[(mmol of catalyst) × (time)].
cS, Substrate.
dReaction conducted without added oxidant.
eReaction conducted without MR-Mo or blank reaction.
ftert-Butyl hydroperoxide (TBHP) (70% solution) used in lieu of H2O2.
gO2 balloon used in lieu of H2O2.
hMoO2(acac)2 (2.5 × 10−3 mmol) used in lieu of MR-Mo as catalyst.
temperature and (e) types of oxidant were screened to get
the best product selectivity and yield.
not completed even after 5 h (Table 3, entry 1). On the
other hand, the reaction conducted at the molar ratio of
1:1.5 caused loss of product selectivity and formed 6%
benzoic acid (Table 3, entry 3). Thus, from these prelimi-
nary investigations, the optimum molar ratio of
substrate–H2O2 for selectively oxidize benzyl alcohol to
benzaldehyde was found to be at 1:1.05.
In order to find out the impact of solvent on the alco-
hol oxidation activity, various solvents were screen by
keeping the molar ratio of benzyl alcohol:H2O2 at 1:1.05
and that of Mo:benzyl alcohol at 1:1000. We conducted
the reactions in various solvents such as chloroform, dic-
hloromethane, acetonitrile, water and toluene, along
with solventless conditions. From the reactions it was
Preliminary reactions were carried out with different
molar ratios of benzyl alcohol (substrate) to oxidant. This
is because excess oxidant may overoxidize the substrate,
resulting in loss of product selectivity. We chose molar
ratios of benzyl alcohol:H2O2 of 1:0.5, 1:1.05 and 1:1.5.
All of the reactions were conducted at a molar ratio of
Mo:benzyl alcohol of 1:1000. The results are shown in
Table 3. The best result in terms of product selectivity
and yield was achieved with molar ratio of 1:1.05
(Table 3, entry 2). At this molar ratio, benzaldehyde was
the sole product. The other two molar ratios were not
acceptable as the reaction at the molar ratio of 1:0.5 was

DAS AND BORUAH
11 of 18
found that the oxidation activities were lowest (in terms
of TOF) in the case of chloroform, dichloromethane and
toluene (Table 3, entries 4, 5 and 8). This may be due to
the immiscibility of the oxidant with these solvents. On
the other hand, the highest activity in terms of TOF
(495 h−1) was achieved under solventless conditions
(Table 3, entry 2). The values of TOF for the reactions
conducted in water and acetonitrile were moderate
(Table 3, entries 6 and 7). This may be due to dilution of
the oxidant concentration by the solvents. The use of
alcoholic solvents such as methanol or ethanol was
avoided as such types of solvents compete with the sub-
strate for oxidation. Thus, solventless conditions were
found to be optimum for selectively oxidizing benzyl
alcohol to benzaldehyde.
We also screened the oxidation reaction at different
temperatures, viz. 30, 50, 70 and 90ꢀC (Table 3, entries
2, 9–11) as the temperature has a great impact on the rate
of reaction. The reactions were conducted by keeping the
molar ratio of benzyl alcohol–H2O2 at 1:1.05 and that of
Mo–benzyl alcohol at 1:1000 under solventless condi-
tions. From these experiments, it was seen that the TOF
value increased with increasing temperature and reached
a maximum at 50ꢀC and thereafter the values declined.
Therefore, 50ꢀC was taken as the optimum reaction
temperature.
The TOF value is dependent on the amount of cata-
lyst. We conducted reactions at molar ratios of Mo:benzyl
alcohol of 1:100, 1:500 and 1:1000 (Table 3, entries
2, 12 and 13) by keeping the molar ratio of benzyl alco-
hol:H2O2 at 1:1.05 under solventless conditions at 50ꢀC.
It was seen that the reaction time decreased with higher
amount of catalysts. However, the TOF values decreased
sharply with increasing catalyst amount. Therefore, the
molar ratio of Mo:benzyl alcohol at 1:1000 was taken as
the optimum ratio. We also screened the oxidation reac-
tions with 70% aqueous tert-butyl hydroperoxide (TBHP)
or O2 balloon as a source of oxidant using benzyl alcohol
as substrate. Experiments showed that the reaction with
TBHP gave 63% yield in 2 h (Table 3, entry 16), which is
much lower than the reactions in H2O2. On the other
hand, there was no reaction of benzyl alcohol in an O2
atmosphere (Table 3, entry 17). These observation led us
to conduct the oxidation reactions using H2O2 as oxidant.
Thus, on the basis of above screened reaction condi-
tions, the optimum reaction condition for selective oxida-
tion of benzyl alcohol to benzaldehyde was Mo–
substrate–H2O2(30%) at 1:1000:1050 under solventless
conditions at 50ꢀC. The optimum reaction conditions are
shown in Scheme 2.
SCHEME 2 Optimum reaction conditions for selective
oxidation of alcohol
reaction was conducted without the added oxidant
(Table 3, entry 14) and the other reaction was conducted
without the added catalyst (Table 3, entry 15). Both of the
reactions were unsuccessful within the stipulated time
period. This showed that the catalyst MR-Mo is not
active in its original form (Table 3, entry 14) and the cata-
lyst played an important role in the oxidation process
(Table 3, entry 15). Further, another separate reaction
was conducted using MoO2(acac)2, the precursor of MR-
Mo, as catalyst under optimum reaction conditions.
Notably, the precursor offered very poor yield (4%) and
TOF (20 h−1) (Table 3, entry 18) within the stipulated
2+
period, which showed that the MoO2
moiety on
supporting in MR-Mo increases its catalytic activity
many times.
After gaining the optimum reaction condition, we
applied the catalyst MR-Mo to oxidize various
substituted benzyl alcohols, primary alcohols and second-
ary alcohols. All of the reactions were conducted under
the optimum reaction conditions and the results are pres-
ented in Table 5. The data showed that MR-Mo effi-
ciently and selectively oxidized all of the benzyl alcohols
to the corresponding aldehydes with reasonably good
TOF. No overoxidation to carboxylic acid was found
under the applied reaction conditions. Apart from this,
the substituents, viz. -F, -Cl, -Br, -OMe, -OH and -NO2,
were well tolerated during the oxidation process. Under
optimum reaction conditions, the oxidation of primary
alcohols and secondary alcohols was found to be much
slower than that of benzyl alcohols. One notable feature
of the catalyst is its ability to oxidize benzyl alcohol selec-
tively at relatively higher amounts (10 g scale; Table 5,
entry 1e), which shows the potential of the catalyst in
large-scale applications.
Sulfide oxidation
The sulfide oxidation activity of the catalyst was studied
with different organic thioethers. In the preliminary reac-
tion, the reaction conditions such as (a) amount of H2O2,
(b) solvents, (c) amount of catalyst, (d) reaction tempera-
ture and (e) type of oxidants were screened using MPS as
model substrate.
We have also conducted two separate blank reactions
under optimum condition using benzyl alcohol as sub-
strate. Under optimum reaction conditions, the first
Like the optimization steps in alcohol oxidation reac-
tions, a series of reaction were conducted with MPS as
substrate by changing various reaction parameters. The

12 of 18
DAS AND BORUAH
TABLE 4
Optimization of reaction conditions for selective oxidation of sulfides to sulfoxidesa
Sample no.
Molar ratio
Solvent
Temperature (ꢀC)
Time (min)
Yield (%)
Product ratio
TOFb (h−1
)
c
Mo:Sc
1:1000
1:1000
1:1000
1:1000
1:1000
1:1000
1:1000
1:1000
1:1000
1:1000
1:1000
1:100
S:H2O2
a: b
1
1:0.5
Solventless
Solventless
Solventless
Chloroform
Dichloromethane
Acetonitrile
Water
70
70
70
70
40
70
70
70
30
50
90
70
70
70
70
70
300
30
25
30
30
30
30
30
30
30
30
20
25
30
30
30
44
98
99
18
11
96
62
21
84
93
91
99
99
81
0
100:0
100:0
94:6
88
2
1:1.10
1:1.5
1960
2376
360
3
4
1:1.10
1:1.10
1:1.10
1:1.10
1:1.10
1:1.10
1:1.10
1:1.10
1:1.10
1:1.10
1:1.10
—
100:0
100:0
97:3
5
220
6
1920
1240
420
7
100:0
100:0
100:0
100:0
100:0
100:0
100:0
100:0
—
8
Toluene
9
Solventless
Solventless
Solventless
Solventless
Solventless
Solventless
Solventless
Solventless
1680
1860
1820
297
10
11
12
13
14d
15e
16f
1:500
1188
1620
—
1:1000
1:1000
1:1000
1:1.10
24
94:6
480
aReaction conditions: all reactions were carried out with methyl phenyl sulfide (MPS) as substrate (2.5 mmol), MR-Mo (4.90 mg for
2.5 × 10−3 mmol of Mo) and 5 ml solvent (unless otherwise indicated).
bTOF = (mmol of product)/[(mmol of catalyst) × (time)].
cS, Substrate.
dTBHP (70% solution) used in lieu of H2O2.
eO2 balloon used in lieu of H2O2.
fMoO2(acac)2 (2.5 × 10−3 mmol) used in lieu of MR-Mo as catalyst.
results are summarized in Table 4. The optimum molar
ratio of MPS:H2O2 for the selective oxidation of MPS to
the corresponding sulfoxide was 1:1.1 (Table 4, entry 2).
It was seen that a lower molar ratio of MPS:H2O2 at 1:0.5
did not complete the reaction even after 5 h (Table 4,
entry 1). On the other hand, a higher molar ratio of MPS:
H2O2 at 1:1.5 caused faster reaction with concomitant
formation of 6% sulfone (Table 4, entry 3), i.e. loss of
selectivity. Like in alcohol oxidation, a similar observa-
tion was made with the change in catalyst amount
(Table 4, entries 2, 12 and 13). The molar ratio of Mo:
MPS at 1:1000 was optimum for a high TOF (Table 4,
entry 2). Thus, after screening the amount of Mo, MPS
and H2O2, the optimum molar ratio of Mo:MPS:H2O2
was found to be 1:1000:1100 (Table 4, entry 2).
molar ratio of Mo:MPS:H2O2 at 1:1000:1100. The highest
TOF was achieved for the reaction under solventless con-
ditions (Table 4, entries 2). The reactions in acetonitrile,
water, dichloromethane, chloroform and toluene did not
improve the TOF (Table 4, entries 4–8). This may be due
to the immiscibility of the oxidant with the solvent in the
cases of dichloromethane, chloroform and toluene. On
the other hand, the reactions in acetonitrile and water
caused dilution of the oxidant concentration. A similar
The reaction temperature and the use of different sol-
vents on the reaction were also studied by taking the
SCHEME 3 Optimum reaction conditions for selective
oxidation of organic sulfides

DAS AND BORUAH
13 of 18
TABLE 5
Catalytic oxidation of alcohols and sulfides by MR-Mo with 30% H2O2 as oxidant
Reaction type
Entry
Substrate
Product
Time (min)
120 (15)
120d
Isolated Yield (%)
TOFc (h−1
)
Alcohol oxidationa
1
99 (98)
98
495 (3920)
490
120e
98
490
2
3
4
5
6
7
150 (17)
150 (17)
150 (17)
170 (20)
140 (17)
180 (20)
98 (97)
97 (98)
99 (99)
99 (98)
98 (97)
97 (97)
392 (3423)
388 (3458)
396 (3494)
349 (2940)
420 (3423)
323 (2910)
8
9
240 (30)
270 (30)
96 (88)
94 (80)
240 (1760)
209 (1600)
10
11
360 (30)
330 (30)
94 (72)
95 (76)
156 (1440)
172 (1520)
(Continues)

14 of 18
DAS AND BORUAH
TABLE 5 (Continued)
Reaction type
Entry
Substrate
Product
Time (min)
30 (5)
30d
Isolated Yield (%)
TOFc (h−1
)
Sulfide oxidationb
12
98 (99)
97
1960 (11880)
1940
30e
98
1960
13
14
40 (5)
98 (98)
99 (98)
1470 (11760)
495 (4900)
120 (12)
15
45 (8)
98 (99)
1307 (7425)
16f
17
35 (5)
97 (98)
98 (99)
1663 (11760)
2352 (13200)
25 (4.5)
aReaction conditions: unless otherwise stated, all reactions were performed solventless at 50ꢀC using 2.5 mmol of substrate, 4.9 mg of
MR-Mo (contain 2.5 × 10−3 mmol of Mo) and 2.625 mmol of 30% aqueous H2O2. The values within the parentheses are for the reactions
conducted in microwave method (100 W).
bReaction conditions: unless otherwise stated, all reactions were performed solventless at 70ꢀC using 2.5 mmol of substrate, 4.9 mg of
MR-Mo (contain 2.5 × 10−3 mmol of Mo) and 2.75 mmol of 30% aqueous H2O2. The values within the parentheses are for the reactions con-
ducted using the microwave method (100 W).
cTOF = (mmol of product)/[(mmol of catalyst) × (time)].
dYield at the fifth reaction cycle.
eYield at 10 g scale reaction.
fReaction conducted at 35ꢀC.
observation was made in the case of alcohol oxidation
reactions (Table 4, entries 6 and 7). Under solventless
conditions by keeping the molar ratio of Mo:MPS:H2O2
at 1:1000:1100, we also conducted reactions at different
temperatures, viz. 30, 50, 70 and 90ꢀC (Table 4, entries
2, 9–11). The TOF value was increased with increasing
temperature and reached the highest value at 70ꢀC;
thereafter the values were decreased. Reactions were also
conducted with TBHP (70% solution) or O2 balloon as
oxidant using MPS as substrate. Under identical reaction
conditions, the reaction in TBHP was found to be slower
than with H2O2 (Table 4, entry 14). On the other hand,
there was no reaction in O2 balloon (Table 4, entry 15).
Thus, we chose H2O2 as the oxidant for the sulfide oxida-
tion reactions. Interestingly, the catalytic reaction con-
ducted using MoO2(acac)2 as catalyst [maintaining a
molar ratio of Mo:MPS:H2O2(30%) = 1:1000:1100 under
solventless condition at 70ꢀC] was nonselective and
offered much lower TOF (Table 4, entry 16), which shows
the merit of supported MoO22+ moiety in MR-Mo.

DAS AND BORUAH
15 of 18
Thus, on the basis of the above screened reaction con-
compared with the conventional method. Interestingly,
no over-oxidation of alcohols or sulfides to carboxylic
acids or sulfones, respectively, was observed with any of
the substrates under study. All of the oxidation reactions
were clean, solventless, selective and provided pure prod-
uct. It is notable that the oxidation of thioethers took
place much faster than that of alcohols, which may be
due to the higher nucleophilicity of former than the
latter.
ditions, the optimum reaction conditions for selective oxi-
dation of methyl phenyl sulfide to methyl phenyl
sulfoxide were Mo:substrate:H2O2(30%) at 1:1000:1100
under solventless conditions at 70ꢀC. The optimum reac-
tion conditions are shown in Scheme 3.
The substrate scopes of the catalyst were studied
under the optimum reaction conditions and the results
are presented in Table 5. The catalyst selectively and effi-
ciently oxidized different types of sulfides such as ali-
phatic or aromatic sulfides into sulfoxides with high TOF
(Table 5, entries 12–17). No overoxidation to sulfone was
observed in any cases. Interestingly, the protocol was
effective at relatively higher amounts (10 g scale; Table 5,
entry 12e) and selectively formed sulfoxide.
3.2.3 | Regeneration
The regeneration of MR-Mo was studied using benzyl
alcohol as substrate at a molar ratio of Mo:substrate:
H2O2 at 1:1000:1050. The results are shown in Figure 7a.
In the typical procedure, after completion of the reaction,
the catalyst was separated from the reaction mixture by
3.2.2 | Oxidation reactions under
microwave method
We performed the oxidation of alcohols and thioethers in
microwave irradiation by keeping the optimized reaction
conditions as mentioned in Sections 3.2.1.1 and 3.2.1.2.
The results are summarized in Table 5. For the sake of
obtaining the best TOF in microwave irradiation, the oxi-
dation reactions were also conducted at 70, 100, 180 and
300 W microwave power by choosing benzyl alcohol and
MPS as representative substrates for alcohols and
thioethers, respectively. Figure 6 shows the correlation
between applied microwave power and yield. The use of
100 W microwave power was found to be optimum for
both the oxidation reactions. The TOF values for all of
the oxidation reactions (alcohol and thioether) under
microwave conditions were increased many times
FIGURE 6 Yield of (a) methyl phenyl sulfide (MPS) and
(b) benzyl alcohol oxidation reaction conducted under optimized
reaction conditions for 5 and 15 min, respectively at different
microwave powers
FIGURE 7 Catalyst recycling for (a) alcohol oxidation and
(b) sulfide oxidation

16 of 18
DAS AND BORUAH
centrifugation, washed with acetonitrile and acetone and
finally dried in vacuo. The recovered catalyst was used
for another reaction cycle by maintaining the optimum
reaction conditions and the process was continued. We
studied the activity of MR-Mo up to five reaction cycles.
The results showed that the catalyst did not lose its activ-
ity and product selectivity even after five reaction cycles
(Table 5, entry 1e). In order to further ascertain the stabil-
ity and robustness of the catalyst, FTIR (Figure 4e) and
XPS (Figure 3a) spectra of the recovered catalyst were
recorded after the fifth reaction cycle. The spectra of the
recovered catalyst were almost identical to those of the
fresh one. Moreover, the AAS analysis of the used cata-
lyst did not show any significant loss of molybdenum
content in the support (Table 1).
Similar to alcohol oxidation reaction, the catalyst was
also studied for regeneration in the sulfide oxidation reac-
tion using MPS as the substrate in the optimum reaction
condition. The results are shown in Figure 7b. Interest-
ingly, the catalyst showed regeneration at least up to the
fifth reaction cycle without loss of activity and product
selectivity (Table 5, entry 8e).
Similar to alcohol oxidation reaction, the catalyst was
studied for metal leaching and heterogeneity using MPS
as model substrate for the sulfide oxidation reaction. The
results revealed the absence of metal leaching from the
catalyst as well as pure heterogeneity of the catalytic
process.
3.2.5 | Comparison with other catalysts
In order to compare the efficiency of MR-Mo with that
of other reported catalysts, two separate comparison
tables were made and are shown in Tables S3 and S4
(Supplementary data). Both tables incorporate the best-
known data from the literature on catalytic activity based
on molybdenum (IV) complexes toward the oxidation of
benzyl alcohol and methylphenylsulfide. Interestingly, in
the case of alcohol oxidation, it is seen from Table S3
(Supplementary data) that MR-Mo offered superior cata-
lytic activity over the reported catalysts. Similarly, in the
case of sulfide oxidation, MR-Mo offered the best cata-
lytic activity in terms of TOF and product selectivity as
well as amount of oxidant used over the reported proto-
cols (Table S4). Notably, Table S4 entries 3 and 4 had
higher TOF values than MR-Mo; however, the protocols
were lacking of product selectivity and used excess
amounts of oxidant, respectively. In a nutshell, MR-Mo
offered better TOF, product yield and selectivity in alco-
hol and sulfide oxidation reactions than the reported
compounds, which demonstrates the importance of the
protocol.
3.2.4 | Leaching study and heterogeneity
of reaction
The leaching study of the catalyst was performed under
optimum reaction conditions (as in Scheme 2) using ben-
zyl alcohol as substrate. In the typical process, the molyb-
denum content in the recovered MR-Mo was determined
by AAS after five reaction cycles (as mentioned in regener-
ation step). As compared with the fresh catalyst (4.86%),
the analysis revealed 4.78% of molybdenum content in the
recovered MR-Mo (Table 1), which accounted for the loss
of a very negligible amount of metal from the support.
This proved the absence of molybdenum leaching from
the support during the course of the reaction.
The heterogeneity of the catalyst, MR-Mo was stud-
ied using benzyl alcohol as substrate. For this, a separate
reaction was conducted under optimum reaction condi-
tions (as in Scheme 2) without the added substrate,
i.e. benzyl alcohol for 120 min. After the reaction, the
solid catalyst was separated by filtration. To the filtrate, a
fresh lot of substrate and oxidant was added by
maintaining the optimum reaction conditions and the
reaction continued for another 120 min. It was observed
that the reaction did not take place within the timespan.
Moreover, the ICP-OES analysis of the filtrate did not
show the presence of molybdenum. Therefore, these
observations are in agreement with the pure heterogene-
ity of the catalytic process as well as absence of metal
leaching from the catalyst.
3.2.6 | Mechanism
On the basis of our observation, we would like to suggest
a common mechanism for the oxidation reactions as
shown in Figure 8. The catalyst, MR-Mo, consisting of
dioxomolybdenum (VI) moiety (I), is inactive as such in
the oxidation reactions (Table 3, entry 14). So, I reacts
with H2O2 via path (a) to form the peroxomolybdenum
(VI) moiety (II). The formation of active peroxometals
species from the reaction of oxo-metals and H2O2 is well
FIGURE 8 Proposed mechanism of alcohol and sulfide
oxidation catalyzed by MR-Mo

DAS AND BORUAH
17 of 18
documented in the literature.75–79 The species II is the
actual active species which oxidized the substrates to
their corresponding products (via path b). After the oxi-
dation, species II regenerates I and the cycles are
continued.
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In conclusion, we have designed a highly efficient hetero-
geneous molybdenum (VI) catalyst anchored on
functionalized Merrifield resin, MR-Mo. The synthesis of
the catalyst is simple and MR-Mo is applicable to the
selective oxidation of organic sulfides and alcohols with a
broad substrate scope using aqueous H2O2 as oxidant,
which is considered as a green oxidant. The oxidation
reactions were conducted under microwave and conven-
tional methods. The reactions by microwave method
were much faster than those with the conventional
method. The oxidation worked under solventless condi-
tions and did not require any other additives. In addition,
comparison of literature reports on molybdenum (VI)-
based catalysts showed that MR-Mo is the best catalyst
for the oxidation of sulfides and alcohols so far. The oxi-
dation products were isolated in pure form. In addition,
MR-Mo can be isolated from the reaction mixture by
simple filtration and reused at least up to the fifth reac-
tion run without loss of activity and product selectivity. A
possible mechanism for the catalytic system was also pro-
posed. Thus, in connection with the growing ecological
awareness in recent years the protocol complies with
many postulates of green chemistry.
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ACKNOWLEDGEMENTS
J.J.B. gratefully acknowledges the financial support from
the University Grants Commission, New Delhi, India
under Minor Research Project (no. F. 5-30/2013-14/
MRP/NERO/427). The authors are thankful to the
Sophisticated Analytical Instrumentation Centre, Tezpur
University, Sonitpur, Assam, India and the Centre of
Excellence, NFDD Centre, Saurashtra University, Rajkot,
Gujarat, India for providing the analytical facilities.
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ORCID
Siva Prasad Das https://orcid.org/0000-0002-4545-9857
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SUPPORTING INFORMATION
Additional supporting information may be found online
in the Supporting Information section at the end of this
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How to cite this article: Das SP, Boruah JJ.
Selective and solventless oxidation of organic
sulfides and alcohols using new supported
molybdenum (VI) complex in microwave and
conventional methods. Appl Organomet Chem.
2020;e5781. https://doi.org/10.1002/aoc.5781
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