Synthesis, characterization, and catalytic oxidation of styrene, cyclohexene, allylbenzene, and cis-cyclooctene by recyclable polymer-grafted Schiff base complexes of vanadium(IV)

Article abstract of DOI:10.1080/00958972.2018.1434516

Schiff base-functionalized chloromethylated polystyrenes, PS-[Ae-Eol] (I), PS-[Hy-Eda] (II) and PS-[HyP-Eda] (III), were synthesized by reacting 2-(2-aminoethoxy)ethanol (Ae-Eol), N-(2-hydroxyethyl)ethylenediamine (Hy-Eda), and N-(2-hydroxpropyl)ethylened

Full text of DOI:10.1080/00958972.2018.1434516

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
Synthesis, characterization, and catalytic oxidation
of styrene, cyclohexene, allylbenzene, and cis-
cyclooctene by recyclable polymer-grafted Schiff
base complexes of vanadium(IV)
Vijay Kumar Singh, Abhishek Maurya, Neha Kesharwani, Payal Kachhap,
Sweta Kumari, Arun Kumar Mahato, Vivek Kumar Mishra & Chanchal Haldar
To cite this article: Vijay Kumar Singh, Abhishek Maurya, Neha Kesharwani, Payal Kachhap,
Sweta Kumari, Arun Kumar Mahato, Vivek Kumar Mishra & Chanchal Haldar (2018): Synthesis,
characterization, and catalytic oxidation of styrene, cyclohexene, allylbenzene, and cis-cyclooctene
by recyclable polymer-grafted Schiff base complexes of vanadium(IV), Journal of Coordination
Chemistry, DOI: 10.1080/00958972.2018.1434516
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Journal of Coordination Chemistry, 2018
https://doi.org/10.1080/00958972.2018.1434516
Synthesis, characterization, and catalytic oxidation of styrene,
cyclohexene, allylbenzene, and cis-cyclooctene by recyclable
polymer-grafted Schiff base complexes of vanadium(IV)
Vijay Kumar Singh, Abhishek Maurya, Neha Kesharwani, Payal Kachhap, Sweta Kumari,
Arun Kumar Mahato, Vivek Kumar Mishra and Chanchal Haldar
department of applied Chemistry, indian institute of technology (indian school of mines), dhanbad, india
ABSTRACT
ARTICLE HISTORY
received 4 october 2017
accepted 18 January 2018
Schiff base-functionalized chloromethylated polystyrenes, PS-[Ae-
Eol] (I), PS-[Hy-Eda] (II) and PS-[HyP-Eda] (III), were synthesized by
reacting 2-(2-aminoethoxy)ethanol (Ae-Eol), N-(2-hydroxyethyl)
ethylenediamine (Hy-Eda), and N-(2-hydroxpropyl)ethylenediamine
(HyP-Eda) with oxidized chloromethylated polystyrene. Oxidized
chloromethylated polystyrene (PS-CHO) was prepared by oxidation
of chloromethylated polystyrene (PS) with sodium bicarbonate
in DMSO. By reacting DMSO solution of [VO(acac)2] with polymer-
anchored Schiff base ligands I, II, and III, vanadium(IV) complexes
PS-[VIVO(Ae-Eol)] (1), PS-[VIVO(Hy-Eda)] (2), and PS-[VIVO(HyP-Eda)]
(3) were prepared. Structure and bonding of I, II, and III as well as
corresponding vanadium complexes 1, 2, and 3 were confirmed by
FT-IR, UV–vis spectroscopy, SEM, EDX, AAS, TGA, EPR, etc. Polymer-
anchored vanadium(IV) complexes 1, 2, and 3 show, efficient
catalysis toward oxidation of styrene, cyclohexene, allylbenzene,
and cis-cyclooctene in the presence of hydrogen peroxide. Optimized
reaction conditions for the oxidation of these alkenes was achieved
by changing various reaction parameters (like amount of catalyst,
amount of oxidizing agent, volume of solvent, etc.). Polymer-grafted
1, 2, and 3 can be reused multiple times without depletion of their
activity.
KEYWORDS
Chloromethylated
polystyrene; vanadium(iV)
complex; ePr; catalyst;
oxidation of alkenes
CONTACT Chanchal haldar
chanchal0007@gmail.com, chanchal@iitism.ac.in
supplemental data for this article can be accessed at https://doi.org/10.1080/00958972.2018.1434516.
© 2018 informa uK limited, trading as taylor & francis Group

2
V. K. SINGH ET AL.
1. Introduction
In synthetic organic chemistry, catalytic oxidation products of alkenes such as carbonyl
compounds, epoxides, and diols are the major feedstock among the most effective synthetic
intermediates in fundamental research as well as industrial commodities [1–4]. Particularly
oxidation of cyclohexene, styrene, cis-cyclooctene, allylbenzene, etc. produces versatile and
useful intermediates widely used in polymers, pharmaceuticals, fine chemicals, and biological
materials [5–8]. Certain herbs and spices, like basil, cinnamon, nutmeg, ginger, black pepper,
clove, tarragon, etc. carry naturally occurring allylbenzenes (such as eugenol, estragole, and
safrole) in high concentration, which can be a promising green alternative source of renew-
able raw materials for chemical industry. Oxidation products of allylbenzenes are widely
used in pharmacological, cosmetic, food, and fragrance industries [9–12] as well as direct
synthetic intermediates in the production of biologically active compounds [13].
2-Cyclohexene-1-ol and 2-cyclohexene-1-one formed through the allylic oxidation of
cyclohexene are applied in the manufacture of spices, medication, pesticides, and insect
pheromones [14, 15]. Oxidation of styrene is one of the most important research tasks for
converting hydrocarbons into other valuable commodities such as benzaldehyde, styrene
epoxide, and formaldehyde [16–20]. Styrene oxide is widely used for the synthesis of epoxy
resin-diluting agents, ultraviolet absorbents, flavoring agents, etc.; benzaldehyde is a precious
chemical mainly used in perfumery, pharmaceuticals, dyestuffs, and agrochemicals [21].
Likewise, the oxidation products of cyclooctene have widespread applications in industrially
important pharmaceuticals and fine chemicals [22]. Conventionally, stoichiometric amount
of oxidants such as permanganates, chromium reagents, ruthenium(VIII) oxide or activated
DMSO were used to carry out the oxidation of alkenes. From the last few decades, metal
complexes of Ti, V, Cr, Mn, Co, Ni, Cu, Mo, Ru, and Re were utilized for oxidation of alkenes in
the presence of peroxides, peracids, and other oxidizing reagents [4, 23–30]. Vanadium com-
plexes in high oxidation states have been widely used as effective catalysts in oxidation
reactions of industrial as well as academic interest in the presence of a suitable oxidant [26,
30–34]. Moreover, the involvement of vanadium haloperoxidase, a vanadium-containing
enzyme in a variety of biological processes, stimulated the application of vanadium-con-
taining enzymes in various catalytic organic transformations [1–4, 23–38]. Flexible coordi-
nation number, easily interconvertible high oxidation states between +4 and +5, Lewis acidic
nature of the vanadium center along with high affinity toward oxygen make high valent
vanadium complexes promising agents for the catalytic oxidation of alkenes.
Some drawbacks associated with the utilization of homogeneous catalysts need to be
addressed, such as use of over stoichiometric ratio of costly oxidizing agents, stability of the
catalysts, easy separation from the reaction mixture, recyclability as well as leaching of the
metal during the reaction. In general, irreversible conversion of monomeric species into
dimeric and polymeric species during the catalytic cycle along with difficulties in recovering
and recycling of homogeneous catalysts discourages its application in industrial chemistry
[39].
Covalent attachment of homogeneous catalysts on the surface of solid support allows
easy catalyst separation, recyclability, and thermal stability and it can be considered as an
effective strategy to make green and sustainable catalytic processes [40]. Furthermore,
anchoring the homogeneous catalysts into the solid supports can effectively enhance reac-
tivity of the catalysts as well as TOF of the catalytic reaction [41].

JOURNAL OF COORDINATION CHEMISTRY
3
Among various inorganic, organic, and hybrid solid supports, chloromethylated polysty-
rene cross-linked with divinylbenzene-anchored heterogeneous catalysts emerged as an
important research area in the field of catalytic oxidation of various organic substrates. Major
advantages of using chloromethylated polystyrene as a solid support are easy availability,
low price, higher thermal stability, and easy functionalization. In this connection, we have
described herein an easy synthesis for polymer-anchored vanadium(IV) complexes of
2-(2-aminoethoxy)ethanol, N-(2-hydroxyethyl)ethylenediamine, and N-(2-hydroxpropyl)
ethylenediamine. Interestingly, reaction of oxidize chloromethylated polystyrene (PS-CHO)
with 2-(2-aminoethoxy)ethanol, N-(2-hydroxyethyl)ethylenediamine, and N-(2-hydroxpropyl)
ethylenediamine produces polymer-anchored monobasic tridentate Schiff base ligands with
OON and ONN donors. Polymer-anchored vanadium(IV) complexes efficiently oxidized a
wide variety of alkenes under moderate reaction condition in a short reaction time. Use of
a readily available, cheap, and environmentally green oxidizing agent like hydrogen peroxide
makes the catalytic oxidation of alkenes more acceptable for academic and industrial pur-
poses. Catalytic oxidation of alkenes by polymer-anchored vanadium complexes is available
in the literature [38, 42–46]. However, uncomplicated and straightforward synthesis of pol-
ymer-anchored vanadium(IV) Schiff base complexes along with effective catalytic efficiency
toward a wide range of alkenes makes the current polymer-anchored vanadium complexes
labor-saving, cost-effective, energy-saving, and ecofriendly catalysts for catalytic oxidation
of alkenes.
2. Experimental
2.1. Materials
Merrifield’s peptide resin (1% cross-linked with divinyl benzene, 2.5–4.0 mmol/g Cl−1 loading,
Sigma–Aldrich, USA), 2-(2-aminoethoxy)ethanol 98% (Alfa Aeser, Great Britain), 2-(2-ami-
noethylamino)ethanol 99% (Sigma–Aldrich, USA), 1-(2-aminoethylamino)propan-2-ol 96%
(TCI Chemicals, Japan), styrene 99% (Alfa Aeser, Great Britain), cyclohexene 99% (Alfa Aeser,
Great Britain), allylbenzene 98% (Alfa Aeser, Great Britain), cis-cyclooctene 95% (Alfa Aeser,
Great Britain), V2O5 (Loba Chemie, India), acetylacetone (SRL, India), Nujol (paraffin wax heavy
liquid, SRL, India), and 30% H2O2 (Merck Chemicals, India) were used as received. All other
solvents and reagents were of AR grade. HPLC grade solvents were used in the GC analysis.
[VIVO(acac)2] was prepared as described [47].
2.2. Physical method and analysis
FT-IR spectra were recorded by ATR mode on an Agilent 600 series FT-IR spectrophotometer.
Electronic spectra of the polymer-anchored metal complexes were recorded in Nujol on a
SHIMADZU UV-1800 spectrophotometer. Surface morphology and elemental composition
of polymer-supported compounds were analyzed by SEM and EDX on a HITACHI S-3400N
instrument after coating the polymer surface with thin film of gold to block the surface
charging and thermal damage by the electron beam. Percentage of metal loading into the
polymericmatrixwasconfirmedbyLABINDIAAA8000AtomicAbsorptionSpectrophotometer.
Solid-state EPR spectra of the polymer-anchored metal complexes were recorded on a Bruker
EMX X-band spectrometer operating at 100-kHz field modulation at room temperature.

4
V. K. SINGH ET AL.
Agilent7890-BgaschromatographfittedwithHP-5capillarycolumn(30 m × 0.32 μm × 0.25 μm)
and FID detector was used to analyze the reaction products. Percent conversion was calcu-
lated using calibrated plot. Identities of the reaction products were confirmed by a thermo
ISQ QD single quadrupole mass analyzer coupled with a Trace 1300 GC system.
2.3. Preparations
2.3.1. Preparation of oxidized chloromethylated polystyrene
Oxidized chloromethylated polystyrene was prepared by following the reported procedure
[48] (Scheme 1). Chloromethylated polystyrene beads (2.5 g, 1% cross-linked with divinyl
benzene, 2.5–4.0 mmol/g Cl−1 loading, and 50–100 mesh) were allowed to swell in DMSO
(25 mL) for 12 h. A solution of sodium bicarbonate (1.125 g) was added to the above sus-
pension and the reaction mixture was heated at 155 °C for 6 h with stirring. After cooling
the reaction mixture to room temperature, oxidized chloromethylated polystyrene beads
were filtered, washed with DMSO, followed by hot methanol and dried in an oven at 120 °C
for 24 h. Presence of 1700 cm−1 IR stretching band in the oxidized chloromethylated poly-
styrene proves the oxidation of chloromethylated polystyrene (PS) to the oxidized chlo-
romethylated polystyrene (PS-CHO).
2.3.2. Preparation of PS-[Ae-Eol] (I), PS-[Hy-Eda] (II) and PS-[HyP-Eda] (III)
PS-[Ae-Eol] (I), PS-[Hy-Eda] (II), and PS-[HyP-Eda] (III) were prepared by following the method
described in Scheme 1. In DMSO, oxidized chloromethylated polystyrene (0.500 g) was
reacted with 2-(2-aminoethoxy)ethanol (Ae-Eol), N-(2-hydroxyethyl)ethylenediamine
(Hy-Eda) and N-(2-hydroxpropyl)ethylenediamine (HyP-Eda) (1.0 g each), respectively, at
130 °C for 6 h. Then, dark polymer-bound compounds PS-[Ae-Eol] (I), PS-[Hy-Eda] (II), and
PS-[HyP-Eda] (III) were filtered and washed with hot DMSO followed by hot methanol to
remove unreacted and surface adsorbed (Ae-Eol), (Hy-Eda), and (HyP-Eda). The polymer
beads were dried in an oven at 120 °C for 24 h.
2.3.2.1. Data of PS-[Ae-Eol] (I). Recovery yield 95%; FT-IR Stretching frequency (ATR mode,
cm−1): 1643(νC=O), 1601(νC=N), 3395(νO–H).
Scheme 1. Proposed reaction methodology for the preparation of polymer-anchored ligands and metal
complexes.

JOURNAL OF COORDINATION CHEMISTRY
5
2.3.2.2. Data of PS-[Hy-Eda] (II). Recovery yield 96%; FT-IR Stretching frequency (ATR mode,
cm−1): 1649(νC=O), 1602(νC=N), 3358(νO-H).
2.3.2.3. Data of PS-[HyP-Eda] (III). Recovery yield 95%; FT-IR Stretching frequency (ATR
mode, cm−1): 1652(νC=O), 1602(νC=N), 3373(νO–H).
2.3.3. Preparation of PS-[VIVO(Ae-Eol)] (1), PS-[VIVO(Hy-Eda)] (2) and PS-[VIVO(HyP-
Eda)] (3)
A DMSO solution (15 mL) of [VIVO(acac)2] (7.54 mmol, 2.0 g) was mixed with pre-swelled
(12 h in 15 mL DMSO) PS-[Ae-Eol] (I), PS-[Hy-Eda] (II), and PS-[HyP-Eda] (III), respectively, and
allowed to heat at 130 °C for 72 h with slow and smooth stirring. Then, dark metal com-
plex-bound polymer beads were filtered and washed with hot DMSO followed by hot meth-
anol and dried in an oven at 120 °C for 24 h.
2.3.3.1. Data of PS-[VIVO(Ae-Eol)] (1)
Recovery yield: 98%; FT-IR Stretching frequency (ATR mode, cm−1): 1632(νC=O), 1604(νC=N),
3298(νO–H), 964(νV=O). UV–vis (Nujol, nm): 851, 756, 539, 362, 326.
2.3.3.2. Data of PS-[VIVO(Hy-Eda)] (2)
Recovery yield: 97%; FT-IR Stretching frequency (ATR mode, cm−1): 1636(νC=O), 1586(νC=N),
3234(νO–H), 940(νV=O). UV–vis (Nujol, nm): 969, 766, 691, 551, 354, 297.
2.3.3.3. Data of PS-[VIVO(HyP-Eda)] (3)
Recovery yield: 97%; FT-IR Stretching frequency (ATR mode, cm−1): 1630(νC=O), 1603(νC=N),
3342(νO–H), 929(νV=O). UV–vis (Nujol, nm): 882, 528, 377, 337, 290.
2.4. Catalytic activity
Polymer-bound 1–3 were used as catalysts for oxidation of styrene, cyclohexene, allylben-
zene, and cis-cyclooctene. For the best catalytic performance, optimized reaction conditions
were achieved by changing the various reaction parameters such as catalyst amount, oxidant
amount, solvent amount, and nature of solvent while taking styrene as the representative
alkene. In a typical reaction, 10-mL methanolic solution of styrene (0.52 g, 5 mmol) was
reacted with dilute aqueous solution of 30% H2O2 (1.14 g, 10 mmol) in the presence of pre-
swelled catalysts 1–3 (0.015 g) at 70 °C for 6 h. Catalysts were swelled in methanol for 12 h
before using in oxidation reactions. In a fixed time interval, a small portion of reaction mixture
was withdrawn and analyzed in a gas chromatograph fitted with a HP-5 capillary column
and a FID detector. Percent conversion was estimated using a calibration plot. Catalytic
oxidation of cyclohexene, allylbenzene, and cis-cyclooctene under optimized reaction con-
ditions was also examined and analyzed through GC periodically. Identities of the reaction
products were established by GC-MS analysis using a thermo ISQ QD single quadrupole
mass analyzer system equipped with a TG-5MS column (30 m × 0.32 μm × 0.25 μm); percent
conversions of the substrates were calculated using the following equation.
Area of substrate
% Conversion of substrate = 100 −
× 100
Total area of substrate + Area of products

6
V. K. SINGH ET AL.
3. Results and discussion
3.1. FT-IR and UV–vis spectral analysis
Selected FT-IR data of polymer-anchored Schiff base ligands PS-[Ae-Eol] (I), PS-[Hy-Eda] (II),
and PS-[HyP-Eda] (III) along with their corresponding vanadium complexes PS-[VIVO(Ae-Eol)]
(1), PS-[VIVO(Hy-Eda)] (2), and PS-[VIVO(HyP-Eda)] (3) are listed inTable 1. Due to poor loading
of the metal complexes into the polymeric chain, intensity of the FT-IR spectrum is relatively
weak, as displayed in Figures S1–S6. Oxidized polymer-anchored complexes show νC=O
stretching frequency at 1700 cm−1, while Schiff base-modified polymer beads exhibit νC=O
stretching frequency of 1643 to 1652 cm−1 along with characteristics νC=N stretching frequency
of 1601 to 1602 cm−1, suggesting the formation of Schiff base-modified polymer chain.
Shifting of the νC=N stretching frequency to lower wavenumber in the polymer-anchored
vanadium complexes signifies coordination of azomethine nitrogen to vanadium [49].
In all the ligands except I, –NH and –OH functional groups appear jointly as a broad band
from 3395–3358 cm−1, whereas corresponding vanadium complexes (2 and 3) show one
medium sharp band at 3243–3242 cm−1 due to νN–H. All the polymer-bound metal complexes,
PS-[VIVO(Ae-Eol)] (1), PS-[VIVO(Hy-Eda)] (2), and PS-[VIVO(HyP-Eda)] (3), show one strong and
sharp peak at 929–964 cm−1 assigned to νV=O stretch [49]. Recycled catalysts show a similar
IR spectral pattern except slightly reduced intensity (shown in Figures S7–S9), indicating no
change in their molecular skeleton.
Table 1. selected ft-ir data of polymer-anchored ligands and metal complexes.
S.No.
Compounds
νC=O
νC=N
νO–H
νN–H
νV=O
1
2
3
4
5
6
7
Ps-Cho
Ps-[ae-eol] (I)
Ps-[hy-eda] (II)
1700
1643
1649
1652
1632
1636
1630
–
–
–
–
–
–
–
1601
1602
1602
1604
1586
1603
3395
3358
3373
3298
–
merged with –oh
merged with –oh
Ps-[hyP-eda] (III)
Ps-[ViVo(ae-eol)] (1)
Ps-[ViVo(hy-eda)] (2)
Ps-[ViVo(hyP-eda)] (3)
–
–
3243
3342
964
940
929
–
Figure 1. uV–vis spectra of Ps-[ViVo(ae-eol)] (1), Ps-[ViVo(hy-eda)] (2), and Ps-[ViVo(hyP-eda)] (3) recorded
by dispersed in nujol.

JOURNAL OF COORDINATION CHEMISTRY
7
Electronic spectral data of the polymer-grafted 1, 2, and 3 were recorded by dispersing
in Nujol. Metal complex-anchored polymer beads show medium to low intensity absorbance
due to the poor loading of the metal complexes into the polymeric matrix. Electronic spectra
of 1, 2, and 3 are displayed in Figure 1 and selected absorbance bands are listed in Table S1.
All three polymer-anchored vanadium complexes show very low intensity but detectable
electronic transitions. Bands from 969 to 756 nm are assigned to d–d transitions. Ligand to
metal charge transfer bands may be observed from 691 to 528 nm. All three polymer-an-
chored vanadium complexes show n–π* transitions from 362 to 337 nm. Bands at 326 to
290 nm may be attributed to π–π* transitions. However, it is very difficult to predict the exact
nature of the electronic transitions, but the electronic spectra definitely confirm the presence
of metal complexes in the polymeric chain.
3.2. SEM and EDX analysis
Morphological changes during the preparation of polymer-grafted metal complexes were
recorded by field emission scanning electron micrograph (FE-SEM). Selected FE-SEM images
of pure and metal-anchored polymer beads are shown in Figure 2 and their corresponding
Figure 2. fe-sem images of (a) merrifield’s peptide resin (Ps), (B) oxidized chloromethylated polystyrene
(Ps-Cho), (C) Ps-[hy-eda] (II), and (d) Ps-[ViVo(hy-eda)] (2).

8
V. K. SINGH ET AL.
Figure 3. edX profile of (a) pure chloromethylated polystyrene (Ps), (B) oxidized chloromethylated
polystyrene (Ps-Cho), (C) Ps-[hy-eda] (II), and (d) Ps-[ViVo(hy-eda)] (2).
Table 2. tGa, edX, and aas data of Ps-[ViVo(ae-eol)] (1), Ps-[ViVo(hy-eda)] (2), and Ps-[ViVo(hyP-eda)] (3).
Metal loading (mmol g−1
)
S.No.
Complexes
(EDX)
TGA
(AAS)
(AAS)*
1
2
3
Ps-[VVo(ae-eol)] (1)
Ps-[VVo(hy-eda)] (2)
Ps-[VVo(hyP-eda)] (3)
4.19
3.02
2.18
1.6
5.2
1.9
0.41
0.61
0.21
0.38
0.55
0.19
*Values for recycled catalysts.
EDX plots are shown in Figure 3. Grafting of the vanadium complex resulted in roughening
of the upper surface of polymer beads. Energy dispersive X-ray analysis plot of pure polymer
bead (Figure 3(A)) shows carbon and chlorine signals, whereas an additional oxygen signal
is observed in the oxidized polymer bead (Figure 3(B)) along with carbon and chlorine signals,
indicating partial oxidation of polymer bead. Ligand-anchored polymer bead (Figure 3(C))
shows nitrogen content along with carbon and oxygen while metal complex-anchored pol-
ymer bead (Figure 3(D)) exhibits the signals of C, N, and O along with the signal of V.
Semi-quantitative measurement of polymer-anchored metal complexes by EDX indicates
4.19, 3.02, and 2.18 mmol g−1 vanadium content in 1, 2, and 3, respectively. Actual metal
loading into the polymeric matrix was confirmed by AAS technique (listed in Table 2 along
with the metal loading calculated from TGA and EDX analysis). All these plots and signals
suggest successful grafting of the vanadium complexes into the polymeric chain.
3.3. Thermogravimetric analysis
Thermal stability of polymer-anchored vanadium complexes was examined by TGA analysis
at a heating rate of 20 °C/minute in air from 40 to 750 °C. TGA plots along with DTA plots of
polymer-anchored metal complexes PS-[VIVO(Ae-Eol)] (1), PS-[VIVO(Hy-Eda)] (2), and
PS-[VIVO(HyP-Eda)] (3) are presented in Figure 4(A)–(C). Complex 1 is stable upto 418 °C,
whereas 2 and 3 show thermal stability upto 442 °C. TGA profile of 1, 2, and 3 proceeds with
an endothermic decomposition of 1.7, 1.4, and 3%, respectively, from 46 to 55 °C, due to

JOURNAL OF COORDINATION CHEMISTRY
9
Figure 4. tGa/dta plots of polymer-anchored complexes (a) Ps-[ViVo(ae-eol)] (1), (B) Ps-[ViVo(hy-eda)]
(2), and (C) Ps-[ViVo(hyP-eda)] (3).
desorption of entrapped moisture and gasses from the polymer cage. Complex 1 produces
two endothermic peaks, one at 387 °C because of the melting of the polymer, followed by
an endothermic peak from 418 to 550 °C associated with the breakdown of polymer

10
V. K. SINGH ET AL.
backbone as well as the ligand structure. This is supported by the fact that the TGA plot of
1 shows a massive weight loss at that temperature range (Figure 4(A)). DTA plot of 2 shows
one broad endothermic peak at 372 °C for slow melting of the polymer bead. Finally, decom-
position of the polymeric structure followed by decomposition of ligand structure is observed
for 2 from 442 to 549 °C. Existence of two endothermic peaks in that temperature range is
consistent with this interpretation (Figure 4(B)).
Complex 3 also shows one endothermic peak at 418 °C because of the melting of the
polymer bead. An exothermic peak at 444 °C and an endothermic peak at 487 °C originate
from the decomposition of polymer chain as well as the ligand, supported by the huge
weight loss (Figure 4(C)). An approximate vanadium loading was also calculated from residual
amount of vanadium oxide (V2O5); 1, 2, and 3 contain 1.6, 5.2, and 1.9 mmol/g of vanadium,
respectively (listed in Table 2).
3.4. EPR studies
To confirm the oxidation state of vanadium, room temperature X-band EPR spectra of
PS-[VIVO(Ae-Eol)] (1), PS-[VIVO(Hy-Eda)] (2), and PS-[VIVO(HyP-Eda)] (3) are recorded and
Figure 5. X-band ePr spectra of (a) Ps-[ViVo(ae-eol)] (1), (B) Ps-[ViVo(hy-eda)] (2), and (C) Ps-[ViVo(hyP-
eda)] (3) at room temperature.

JOURNAL OF COORDINATION CHEMISTRY
11
shown in Figure 5. Complexes 1, 2, and 3 show an axially symmetrical signal characteristic
of V4+ [49, 50]. Hyperfine lines appearing in Figures 5(A)–(C) are due to the coupling of a
single unpaired electron of V4+ to its own nucleus of spin value 7/2 (2nI + 1 = 8). All three
compounds show well-resolved hyperfine spectra in both parallel and perpendicular regions.
Out of expected 16 lines (8 in perpendicular orientation and 8 in parallel orientation), all of
the perpendicular transitions and 4 transitions in parallel orientation are observed. The
resolved hyperfine EPR spectra imply that the vanadium centers are well dispersed in the
polymeric matrix and practically no vanadium–vanadium interactions exist.
3.5. Catalytic activities studies
3.5.1. Oxidation of alkenes
Catalyst precursors PS-[VIVO(Ae-Eol)] (1), PS-[VIVO(Hy-Eda)] (2), and PS-[VIVO(HyP-Eda)] (3)
were used for oxidation of a number of alkenes, namely styrene, cyclohexene, allylbenzene,
and cis-cyclooctene in the presence of aqueous hydrogen peroxide. Oxidation of styrene
was considered as a model reaction with 1 as the representative catalyst. Previously, styrene
oxide (bep), benzaldehyde (bzh), benzoic acid (bza), phenylacetaldehyde (bzy), and 1-phe-
nylethane-1,2-diol (ped) were reported during the catalytic oxidation of styrene by H2O2
(shown in Scheme 2) [4, 51, 52].
In the present study, two new oxidation products, 2-hydroxy-1-phenylethanone (hep)
and 2-phenylacetaldehyde (pac), were identified along with the five expected products. For
maximizing the oxidation of styrene, a number of parameters such as amount of catalyst,
amount of oxidant, amount of solvent, and nature of the solvent were studied.
Figure 6(A) displays the effect of amount of catalyst on the oxidation of styrene. Three
different amounts of 1, 0.015, 0.020, and 0.025 g, were considered while reacting fixed
amounts of styrene (0.52 g, 5 mmol) with H2O2 (1.7025 g, 15 mmol) in 10 mL of methanol at
70 °C; 87.3% product conversion was observed in six hours by 0.015 g catalyst. Increasing
Scheme 2. main oxidation products of styrene catalyzed by 1, 2, and 3 in the presence of hydrogen
peroxide.

12
V. K. SINGH ET AL.
Table 3. Conversion % and tof values of catalytic oxidation of styrene, cyclohexene, allylbenzene, and
cis-cyclooctene using 1, 2, and 3 in 6 h under various reaction conditions.
Solvent
amount
(mL)
Amount
(mg)
% Conver-
sion
TOF (h−1
)
S.No.
Cat.
Substrates
Sub:Ox
Solvent
1
2
3
4
5
6
7
8
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
styrene
styrene
styrene
styrene
styrene
styrene
styrene
styrene
styrene
styrene
styrene
styrene
Cyclohexene
allylbenzene
Cis-cyclooc-
tene
15
20
25
15
15
15
15
15
15
15
15
15
15
15
15
1:3
1:3
1:3
1:2
1:4
1:4
1:4
1:4
1:4
1:4
1:4
1:4
1:4
1:4
1:4
10
10
10
10
10
5
15
5
5
5
5
5
5
methanol
methanol
methanol
methanol
methanol
methanol
methanol
acetonitrile
ethanol
thf
dmf
acetone
methanol
methanol
methanol
87.3
90.9
93.5
86.7
95.1
99.9
91.3
99.5
97.8
99.1
15.4
92.0
95.1
77.5
83.0
116.02
90.61
74.54
115.23
126.34
132.72
121.37
132.28
129.94
131.72
20.40
131.72
126.42
102.93
110.34
9
10
11
12
13
14
15
5
5
16
17
2
2
styrene
Cis-cyclooc-
tene
15
15
1:4
1:4
5
5
methanol
methanol
88.1
32.5
79.75
29.44
18
19
20
21
2
2
3
3
Cyclohexene
allylbenzene
Cyclohexene
Cis-cyclooc-
tene
15
15
15
15
1:4
1:4
1:4
1:4
5
5
5
5
methanol
methanol
methanol
methanol
79.1
58.7
92.8
99.2
71.66
53.16
236.74
253.05
22
23
3
3
styrene
allylbenzene
15
15
1:4
1:4
5
5
methanol
methanol
95.4
81.2
243.31
207.03
the catalyst amount from 0.015 to 0.020 g, percent conversion increases merely 3.6%. Further
increase in the catalyst amount from 0.020 to 0.025 g, percent conversion also increases
from 90.9 to 93.5%, which is quite marginal. Hence, 0.015 g catalyst was considered
optimal.
To optimize the amount of oxidant (30% H2O2), three different styrene to H2O2 ratios, i.e.
1:2, 1:3, and 1:4, were used for fixed amounts of styrene (0.52 g, 5 mmol) and catalyst pre-
cursor (0.015 g) in 10-mL methanol at 70 °C temperature for 6 h. 1:4 styrene to H2O2 ratio
produced 95.1% product conversion. It is clear from Figure 6(B) that 1:2 and 1:3 show only
86.7 and 87.3% conversion, respectively, so 1:4 ratio of styrene to H2O2 was set as optimal.
The impact of volume of solvent was checked by taking 5, 10, and 15 mL of methanol
while using fixed amounts of styrene (0.52 g, 5 mmol), 30% H2O2 (2.27 g, 20 mmol), and
catalyst precursor (0.015 g) at 70 °C temperature for 6 h (shown in Figure 6(C)).
In 5 mL of methanol, 0.015 g of catalyst exhibit maximum 99.9% conversion. Increasing
the solvent amount from 5 to 10 mL, percent conversion decreases from 99.9 to 95.01%.
Conversion continues to decline with increasing solvent from 10 to 15 mL, which may be
due to the less effective collision among the reactant molecules in more diluted solution.
Thus, 5 mL of methanol is considered the best.
Effect of solvent polarity on the catalytic oxidation of styrene was also examined. Six dif-
ferent solvents, methanol (MeOH), ethanol (EtOH), acetonitrile (ACN), tetrahydrofuran (THF),
acetone, and dimethyl formamide (DMF), were considered while keeping the other reaction
parameters fixed, i.e. styrene (0.52 g, 5 mmol), 30% H2O2 (2.27 g, 20 mmol), catalyst precursor

JOURNAL OF COORDINATION CHEMISTRY
13
Figure 6. (a) effect of amount of 1 on the oxidation of styrene. (B) effect of amount of h2o2 on the oxidation
of styrene. (C) effect of amount of solvent (methanol) on the oxidation of styrene. (d) effect of nature of
solvent on the oxidation of styrene. for reaction conditions see text.
(0.015 g), 5 mL solvent, 70 °C temperature, and 6 h reaction time. Figure 6(D) shows that of
the six solvents, quantitative (more than 99%) oxidation of styrene was observed in MeOH,
EtOH, ACN, and THF while in acetone catalytic oxidation proceeds up to 92%; DMF is not a
suitable solvent for the catalytic oxidation of styrene with only 15.4% of product observed.
Hence, easily available and cheaper methanol was used to perform all other catalytic oxida-
tion reactions. Various reaction conditions for the catalytic oxidation of styrene are summa-
rized in Table 3 in which serial number 6 represents the optimized conditions, which is
5 mmol (0.52 g) styrene, 20 mmol (2.27 g) 30% H2O2, 0.015 g catalyst precursor, 5 mL meth-
anol, 70 °C temperature, and 6-h reaction time.
Catalytic oxidation of styrene under optimized reaction conditions by 1 exhibits 99.9%
conversion with TOF value 132.72 h−1 and by 3 shows 95.4% conversion with TOF value
243.31 h-1, while using 2 only 88.1% conversion is achieved with TOF value 79.75 h−1 (shown
in Figure 7). Conversion % along with the % selectivity of various oxidation products of
styrene catalyzed by 1, 2, and 3 under optimized reaction conditions is shown in Table S2.
1 exhibits acetophenone as a major product with selectivity of 34.10%, whereas 2 and 3
produced benzaldehyde as a major product with selectivity 31.9 and 39.13%, respectively.
A blank reaction under the same optimized reaction conditions shows only 15% product
conversion. Although catalytic oxidation of styrene can be found in a number of reports [4,

14
V. K. SINGH ET AL.
53–60], comparing with the existing literature data (shown in Table S3), 1, 2, and 3 can be a
better choice over the reported catalysts, in terms of overall % conversion, reaction time,
and relatively moderate reaction conditions.
Liquid phase catalytic oxidation of cyclohexene was tested by 1, 2, and 3 under similar
optimized reaction conditions as described in the oxidation of styrene, i.e. 5 mmol cyclohex-
ene, 0.015 g catalyst precursor, and 20 mmol (2.27 g) 30% H2O2 reacted in 5-mL methanol
at 70 °C temperature for 6 h. Catalytic oxidation of cyclohexene by 1, 2, and 3 in the presence
of hydrogen peroxide produces 2-cyclohexene-1-ol (che), cyclohexeneepoxide (och) and
cyclohexane 1,2-di-ol (chd) as reported in the literature [61–64] along with a fourth product,
i.e. 2-hydroxycyclohexanone (hch) (shown in Scheme 3). Oxidation products were confirmed
by GC-MS and quantified by GC.
Figure 8 compares the catalytic efficiency of 1, 2, and 3 for the oxidation of cyclohexene.
Conversion % along with the % selectivity of the individual oxidation products is listed in
Table S4. Figure 8 shows that 1 and 3 are more efficient toward catalytic oxidation of cyclohex-
ene than 2.
Catalyst 1 exhibits 95.1% conversion with 126.42 h−1 TOF and 3 shows 92.8% conversion
with 236.74 h−1 TOF. Least catalytic activity was observed by 2 which shows only 79.1%
conversion with TOF of 71.66 h−1. In terms of % selectivity, again catalysts 1 and 3 produce
2-cyclohexene-1-ol as a major product with 43.29 and 43.40% selectivity, respectively, while
catalyst 2 produces cyclohexane 1,2-diol as a major oxidation product with 47.19% selec-
tivity. Under similar optimized reaction conditions, a blank reaction shows only 4.7% con-
version. Although 2 functions little less effectively in comparison to 1 and 3, with respect to
the available literature [4, 53, 61, 65–67] (shown in Table S5), 1, 2, and 3 emerge as better
candidates for the catalytic oxidation of cyclohexene.
Catalytic oxidation of allylbenzene by 1, 2, and 3 in the presence of 30% H2O2 is evaluated.
Oxidation of allylbenzene produces benzaldehyde (bzh), benzoic acid (bza), 1-phenylprop-
2-en-1-one (peo), 2-phenylacetaldehyde (pac), and 3-phenylacrylaldehyde (cal) as oxidation
Scheme 3. main products obtained by catalytic oxidation of cyclohexene using 1, 2, and 3.
Scheme 4. oxidation products of allylbenzene catalyzed by 1, 2, and 3 in the presence of hydrogen
peroxide.

JOURNAL OF COORDINATION CHEMISTRY
15
Figure 7. impact of 1, 2, and 3 on catalytic oxidation of styrene. reaction conditions: styrene (0.52 g,
5 mmol), catalyst precursor (0.015 g), 30% h2o2 (2.27 g, 20 mmol), 70 °C temperature, and 6-h reaction
time.
Figure 8. influence of 1, 2, and 3 on catalytic oxidation of cyclohexene. reaction conditions: cyclohexene
(5 mmol), catalyst precursor (0.015 g), 30% h2o2 (2.27 g, 20 mmol), 70 °C temperature, and 6h reaction
time.
products (shown in Scheme 4). Identity of the products was confirmed by GC-MS
analyses.
The impact of 1, 2, and 3 on the catalytic oxidation of allylbenzene as a function of time
has been studied and presented in Figure 9. Figure 9 shows that 1 and 3 are more influential
than 2 toward catalytic oxidation of allylbenzene, but this time 3 functions slightly better
than 1. Catalyst 3 shows 81.2% conversion with TOF value of 207.03 h−1 and catalyst 1 shows
77.5% conversion with TOF 102.93 h−1. Catalyst 2 leads to only 58.7% conversion accompa-
nied by TOF value of 53.16 h−1. Detailed % conversion and product selectivity of the different
oxidation products of allylbenzene are documented in Table S4.

16
V. K. SINGH ET AL.
Figure 9. effect of 1, 2, and 3 on catalytic oxidation of allylbenzene. reaction conditions: allylbenzene
(5 mmol), catalyst precursor (0.015 g), 30% h2o2 (2.27 g, 20 mmol), 70 °C temperature, and 6-h reaction
time.
With 40.47, 32.77, and 32.50% of selectivity, benzaldehyde appears as a major product
during the catalytic oxidation of allylbenzene by catalysts 3, 1, and 2, respectively. From
Table S4, it is clear that all three catalysts show similar trends in the product distribution
during the oxidation of allylbenzene: benzaldehyde > benzoic acid > 1-phenylprop-2-en-1-
one > 2-phenylacetaldehyde > 3-phenylacrylaldehyde. A blank reaction under the same
optimized reaction conditions exhibits only 2.5% product conversion. Catalytic oxidation of
allylbenzene reported in the literature is limited [68–72]. However, with respect to the avail-
able literature (shown in Table S7), 1, 2, and 3 appear as potential candidates for the catalytic
oxidation of allylbenzene under moderate reaction conditions.
Under the optimized reaction conditions, 1, 2, and 3 were employed for the catalytic
oxidation of cis-cyclooctene (Figure 10). Homogeneous or heterogeneous catalytic oxidation
of cis-cyclooctene produces cyclooctene oxide (ocn) as a major product. Apart from this, a
number of oxidation products, cyclooctane-1,2-diol, 2-cycloocten-1-one, 2-cycloocten-1-ol,
cyclooctanol, and cyclooctanone, were reported [73–80]. In the current study, cyclooctene
oxide (ocn) is a major product along with small percents of cyclooctane-1,2-diol (cod) and
2-cycloocten-1-one (coe) (shown in Scheme 5).
Identity of the oxidation products is confirmed by GC-MS analyses. GC-MS chromatograph
exhibits split cyclooctene oxide peak. Fragmentation pattern of each of the split peaks
matches with the standard fragmentation pattern of cyclooctene oxide (by library search)
at 70 eV. Hence, formation of two isomeric structures, cis- and trans- isomer of cyclooctene
oxide, during the catalytic oxidation of cis-cyclooctene is proposed. Under the optimized
reaction conditions, % conversion and % selectivity of the isolated products of catalytic
oxidation of cis-cyclooctene with H2O2 in the presence of 1, 2, and 3 are listed in Table S5.
Catalyst 3 shows a maximum of 99.2% conversion with TOF of 253.05 h−1, and catalyst 1
produces 83.0% product conversion with TOF of 110.34 h−1; only 32.5% product conversion
is observed by 2 with TOF value of 29.44 h−1. These values are quite high in comparison to
the data reported by Glaser et al. [74], Nomiya et al. [75], Cai et al. [78], and Carreon et al. [80],

JOURNAL OF COORDINATION CHEMISTRY
17
Figure 10. impact of 1, 2, and 3 on catalytic oxidation of cis-cyclooctene. reaction conditions: cis-
cyclooctene (5 mmol), catalyst precursor (0.015 g), 30% h2o2 (2.27 g, 20 mmol), 70 °C temperature, and
6-h reaction time.
Scheme 5. Various oxidation products of cis-cyclooctene catalyzed by 1, 2, and 3 in the presence of
hydrogen peroxide.
whereas Nakagaki et al. [79] and Saberyan et al. [77] reported few catalysts of close catalytic
properties to those of 1, 2, and 3 (Table S9). Only 1% product conversion is observed during
the blank reaction.
A comparative % conversion vs. reaction time plot of catalytic oxidation of alkenes (sty-
rene, cyclohexene, allylbenzene, and cis-cyclooctene) by 1, 2, and 3 is shown in Figure
11(A)–11(C). Although the three catalysts function for oxidation of styrene, cyclohexene,
allylbenzene, and cis-cyclooctene, overall functionality of 1 and 3 is much better than 2.
Under the optimized reaction conditions, in the presence of 3, catalytic oxidations of styrene,
cyclohexene, allylbenzene, and cis-cyclooctene show 95.4, 92.8, 81.2, and 99.24% conversion,
respectively. Catalyst 1 produces 99.9, 95.1, 77.5, and 83.0% conversion for the oxidation of
styrene, cyclohexene, allylbenzene, and cis-cyclooctene, respectively, under the same opti-
mized reaction conditions, whereas only 88.1, 79.1, 58.7, and 32.5% conversion was achieved
by 2.
Table 3 indicates that among styrene, cyclohexene, allylbenzene, and cis-cyclooctene, 1
is more effective toward the oxidation of styrene and cyclohexene, while 3 is more effective
toward oxidation of allylbenzene and cis-cyclooctene. In conclusion, 3 is more versatile than
1 and 2 in terms of oxidation of various alkenes.

18
V. K. SINGH ET AL.
Figure 11. Conversion % against reaction time for the catalytic oxidation of styrene, cyclohexene,
allylbenzene, and cis-cyclooctene by (a) Ps-[ViVo(ae-eol)] (1), (B) Ps-[ViVo(hy-eda)] (2), and (C) Ps-
[ViVo(hyP-eda)] (3) under optimized reaction conditions.

JOURNAL OF COORDINATION CHEMISTRY
19
Table 4. Conversion % of catalytic oxidation of styrene, cyclohexene, allylbenzene, and cis-cyclooctene
using fresh and recycled 1, 2, and 3 under optimized reaction conditions.
Catalysts
1
2
3
S.No.
Substrates
% Conversion
1
2
3
4
5
6
7
8
styrene
styrene*
styrene**
99.9
92.5
90.3
95.1
90.7
89.4
77.5
74.1
73.5
83.0
82.7
80.3
88.1
84.0
83.6
79.1
75.0
74.6
58.7
56.3
55.4
32.5
27.3
26.9
95.4
91.6
90.2
92.8
86.9
86.2
81.2
75.6
75.1
99.2
94.1
93.5
Cyclohexene
Cyclohexene*
Cyclohexene**
allylbenzene
allylbenzene*
allylbenzene**
Cis-cyclooctene
Cis-cyclooctene*
Cis-cyclooctene**
9
10
11
12
*Values for first recycled catalysts.
**Values for second recycled catalysts.
3.5.2. Study of leaching of vanadium from the polymer-bound catalysts and test of
recyclability
Reusabilities of the polymer-anchored vanadium catalysts were checked. After the catalytic
reactions, 1, 2, and 3 were washed with methanol and dried in an oven at 120 °C up to 24 h.
Under the same optimized reaction conditions, used catalysts show % conversion close to
that of the fresh one up to 3 cycles. Detailed % conversion of fresh and recycled catalysts is
listed in Table 4. Slightly reduced activities in the first cycle may be explained by the fact
that a few physically adsorbed vanadium complexes were lost from the polymeric matrix
during the catalytic reactions. AAS analyses of the used catalysts (mentioned in Table 2)
show a little less but close vanadium loading in comparison to that of fresh catalysts, sug-
gesting partial leaching of vanadium center from the polymeric matrix. Hence, 1, 2, and 3
can be considered as recyclable and effective catalysts for the oxidation of alkenes.
FT-IR spectra of recycled catalysts 1, 2, and 3 are shown in Figures S7–S9. Comparable
FT-IR spectral patterns of the used catalysts to that of the fresh catalysts confirm the presence
of catalytic center in the used catalysts and support their recyclability.
4. Conclusion
Three chloromethylated polystyrenes cross-linked with divinylbenzene-functionalized Schiff
bases PS-[Ae-Eol] (I), PS-[Hy-Eda] (II), and PS-[HyP-Eda] (III) were synthesized by reacting
oxidized chloromethylated polystyrene with 2-(2-aminoethoxy)ethanol (Ae-Eol), N-(2-
hydroxyethyl)ethylenediamine (Hy-Eda), and N-(2-hydroxpropyl)ethylenediamine (HyP-Eda),
respectively. Corresponding monomeric vanadium(IV) complexes of the Schiff base ligands
were isolated by reacting [VO(acac)2] with I, II, and III. FT-IR, UV–vis spectral studies, and EDX
analyses confirm the successful anchoring of the vanadium complexes on the polymeric
chain. Vanadium loadings into the polymeric chain were calculated by AAS analysis. Catalysts
1, 2, and 3 showed thermal stability up to 430, 340, and 433 °C, respectively, as scrutinized
byTGA analyses. EPR analyses established magnetically dilute or well-dispersed attachments
of 1, 2, and 3 on the polymeric support. PS-[VIVO(Ae-Eol)] (1), PS-[VIVO(Hy-Eda)] (2), and

20
V. K. SINGH ET AL.
PS-[VIVO(HyP-Eda)] (3) efficiently catalyzed oxidation of a range of alkenes. Overall perfor-
mance of 2 is less than 1 and 3 toward the catalytic oxidation of alkenes. Between 1 and 3,
catalyst 1 showed better catalytic activity toward the oxidation of styrene and cyclohexene
(99.86% and 95.12% by 1 while 3 shows 95.42 and 92.84%, respectively). However, 3 showed
more effectiveness toward the catalytic oxidation of cis-cyclooctane and allylbenzene than
2. Under optimized reaction conditions, 99.24% conversion for cis-cyclooctene and 81.19%
conversion for allylbenzene were observed by 3 while 2 showed only 83.02 and 77.45%
conversion. Uncomplicated and straightforward synthesis of polymer-anchored monomeric
vanadium(IV) Schiff base complexes studied here along with their thermal stability, recycla-
bility without much loss of catalytic activity, and effective catalytic efficiency toward a wide
range of alkenes in the presence of an ecofriendly oxidizing agent, hydrogen peroxide, makes
them cost-effective and ecofriendly catalysts for the catalytic oxidation of alkenes.
Supplementary material
FT-IR, UV–vis data, and figures for the polymer-anchored ligands and metal complexes along with %
conversion, TOF, and % selectivity of individual oxidation products of various alkenes catalyzed by 1,
2, and 3 under optimized reaction conditions are submitted as Supporting Materials.
Acknowledgements
C.H. and V.K.M. are thankful to the Science and Engineering Research Board (SERB), Department of
Science andTechnology (DST), the Government of India, New Delhi for financial support (grant numbers.
SB/FT/CS-027/2014 and SB/EMEQ-055/2014). V.K.S., A.M., N.K., P.K., S.K., and A.K.M. are also wish to thank
IIT (ISM) Dhanbad for fellowship. C.H. is very thankful to Prof. Delia Haynes and Dr. Nikita Chaudhary,
Department of Chemistry and Polymer Science, Stellenbosch University, South Africa for recording the
EPR spectra. TGA analyses were provided by the SAIF, Cochin, India. Gas chromatograph (GC) and Gas
chromatograph-Mass-spectrometer (GC-MS) used in this study were procured from the grant given by
the Science and Engineering Research Board (SERB), Department of Science andTechnology (DST), grant
no. SB/FT/CS-027/2014 and grant no. SB/EMEQ-055/2014, respectively, Government of India, New Delhi,
India to CH. We also acknowledge central research facility IIT(ISM) Dhanbad for FE-SEM and EDX analysis.
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
This work was supported by Science and Engineering Research Board (SERB), Department of Science
and Technology (DST) [grant numbers SB/FT/CS-027/2014 and No.SB/EMEQ-055/2014].
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