Manganese (III) salen supported onto hydrous zirconia: Synthesis, characterization, and solvent free aerobic oxidation of styrene

Article abstract of DOI:10.1080/15533174.2013.841220

Solid catalysts comprising manganese salen and hydrous zirconia were synthesized and characterized, and catalytic activity was evaluated by carrying out solvent-free liquid phase aerobic oxidation of styrene. As solid catalysts show high conversion of sty

Full text of DOI:10.1080/15533174.2013.841220

Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry (2015) 45, 539–545
Copyright © Taylor & Francis Group, LLC
ISSN: 1553-3174 print / 1553-3182 online
DOI: 10.1080/15533174.2013.841220
Manganese (III) Salen Supported Onto Hydrous Zirconia:
Synthesis, Characterization, and Solvent Free Aerobic
Oxidation of Styrene
KETAN PATEL, VARSHA BRAHMKHATRI, and ANJALI PATEL
Chemistry Department, Faculty of Science, M. S. University of Baroda, Vadodara, India
Received 27 June 2013; accepted 1 September 2013
Solid catalysts comprising manganese salen and hydrous zirconia were synthesized and characterized, and catalytic activity was
evaluated by carrying out solvent-free liquid phase aerobic oxidation of styrene. As solid catalysts show high conversion of styrene
with good selectivity toward benzaldehyde and styrene epoxide. The recovery of the catalyst was readily accomplished by simple
filtration. The recycled catalyst can be used up to four cycles without any loss in conversion as well as selectivity.
Keywords: manganese-salen, hydrous zirconia, styrene/styrene oxide, benzaldehyde
Introduction
homogeneous medium. Further, deactivation results in the
formation of m-peoroxo dimer and other polymeric spe-
Catalytic oxidation of alkenes into more valuable epoxides as cies[13,14] mainly because of ligand oxidation.
well as oxygen containing carbonyl compound is one of the
To overcome the above mention problems Mn(III)salen
important synthetic reaction. Oxidation of styrene is an complex are supported onto different types of supports, such
important organic transformation as the obtained benzalde- as functionalized polymeric ionic liquid,[15] organic poly-
hyde is a very valuable chemical having widespread applica- mers,[16,17] polymeric membranes,[18] mesoporous materi-
tions in perfumery, dye stuffs and agrochemicals,[1] whereas als,[19–26] ion exchange resins,[27] layered double
epoxides can be easily transferred to a large variety of com- hydroxides,[28,29] zeolite,[30] and pillared clay.[31]
pounds via regio- and stereoselective ring opening reac-
The Literature survey shows that the oxidation of sty-
rene using supported Mn(III)salen was carried in different
tions.[2–4]
Typical oxidants compatible with a majority of oxidation organic solvent using various types of oxidants such as
system are various alkyl hydroperoxide, hypochlorite, or NaClO, CPBA, and peroxides. Although few reports are
iodosylbenzene. A problem associated with these oxidants is available in the literature describing the aerobic oxidation
their low active oxygen content. The major drawback associ- of styrene in different solvent system such as dichlorome-
ated with these oxidants is the waste generation. Hence, from thane, toluene, benzene under homogeneous and heteroge-
an environment and economical point of view, molecular neous medium.[4,32]
oxygen should be the preferred oxidant and considering its
high active oxygen content and no by-products.
It was observed in the literature that no reports are avail-
able in the use of metal hydroxide for supporting Mn(III)
Among number of complexes, Mn-salen catalysts are pre- salen. Among all-metal hydrous oxides, hydrous zirconia has
ferred since manganese itself is a relatively nontoxic metal, drawn great attention. This may be due to the fact that its
and manganese complexes are superior for the selective oxi- surface hydroxyl groups are able to undergo chemical inter-
dation of olefins, chiefly because of fewer side reactions over action with active compounds. A literature survey also show
manganese complexes. Manganese Schiff base has been that no reports are available on solvent free aerobic oxidation
widely described as good epoxidation catalysts with different of styrene using Mn(III)salen supported onto hydrous zirco-
oxidants.[5–7] Especially Mn(III)salen complexes have been nia. Hence, in the present work an attempt was made to use
proven excellent for their selectivity toward epoxidation.[5–12] hydrous zirconia as a support for Mn(III)salen as well as to
However these complexes are also suffer from the same tradi- carry out solvent free aerobic oxidation of styrene.
tional problem such as they can not be recovered in
A series of catalysts containing 20–40% loading of Mn(III)
salen (Mn(salen)) onto hydrous zirconia (ZrO2) were synthe-
sized. The synthesized new catalysts was characterized by var-
ious physicochemical techniques such as chemical stability,
elemental analysis, Fourier transform infrared spectroscopy
(FT-IR), diffuse reflectance (DR-UV), thermal gravimetric
Address correspondence to Anjali Patel, Chemistry Department,
Faculty of Science, M. S. University of Baroda, Vadodara 390
002, India. E-mail: aupatel_chem@yahoo.com

540
Patel et al.
analysis (TGA), surface area measurement (BET Method),
scanning electron microscopy (SEM), and electron spin reso-
nance (ESR). The catalytic activity was evaluated for aerobic
oxidation of styrene. Among the series of the catalysts, 30%
loaded complex (Mn(salen)30/ZrO2) was found to be excel-
lent. Further study was carried out using this catalyst by vary-
ing different parameters such as amount of the catalyst and
reaction time. The present catalyst can be regenerated and
reused up to four cycles without loss any appreciable loss in%
conversion and selectivity.
Table 1. EDX values of supported catalyst.
Element
EDX
Theoretical
O
34.90
7.91
5.29
34.38
7.54
5.05
Cl
Mn
Zr
51.90
52.29
analysis for Mn(salen)30/ZrO2 are in good agreement with
the theoretical values. Theoretical: O, 34.90; Cl, 7.91; Mn,
5.29; Zr, 51.90; observed: O, 34.38; Cl, 7.54; Mn, 5.05; Zr,
51.29 (Table 1).
Experimental
Materials
Characterization
All the chemicals used were of A. R. grade. Zirconium oxy-
chloride (ZrOCl2.8H2O) (Loba Chemie, Mumbai),
MnCl2.4H2O, salicylaldehyde, ethylene diamine, ethanol,
methanol, dichloromethane, styrene, styrene oxide, and benz-
aldehyde were obtained from Merck and used as received.
Elemental analysis was carried out using the JSM 5610 LV
EDAX-SEM analyzer. Thermogravimetric analysis was car-
ried out on the Mettler Toledo Star SW 7.01 up to 600ꢀC in
air with the heating rate of 5ꢀC/min. FT-IR spectra of the
sample were obtained using the KBr wafer on the Perkin
Elmer instrument. The UV-DR of samples was recorded at
ambient temperature on Perkin Elmer 35 LAMDA instru-
ment using the 1cm quartz cell using barium sulfate as a refer-
ence. The surface morphology of the support and supported
catalyst were studied by SEM analysis using JEOL SEM
instrument (Model-JSM-5610LV) with scanning electron
electrode at 15 kV. Scanning was done at 1 mm range and
image was taken at a magnification of 100£ for ZrO2 and
Mn(salen)30/ZrO2. The specific surface area was calculated
from adsorption-desorption isotherm using BET method on
Micromeritics spectrophotometer (Model: Series ASAP
2020). The ESR spectra were recorded on a Varian E-line
Century series X-band ESR spectrometer (room temperature
and scanned from 2000 to 4000 Gauss).
Synthesis of the Catalyst
The synthesis of the catalyst was carried out in three steps.
Synthesis of the support, hydrous Zirconia
Hydrous zirconia was synthesized following the same proce-
dure reported by Bhatt and Patel,[33] by adding an aqueous
ammonium solution to an aqueous solution of ZrOCl2¢8H2O
and pH of the solution was adjusted to 8.5. The resulted pre-
cipitates were aged at 100ꢀC over a water bath for 1 h, fil-
tered, washed with distilled water until chloride free water
was obtained, and dried at 100ꢀC for 10 h. The obtained
material is designated as ZrO2.
Synthesis of the complex, [Mn(salen)Cl]
The Mn(III) salen was synthesized by the method reported by
Freie et al.[2] The salen ligand (Schiff Base) was synthesized
by refluxing the ethanolic solution of the ethylenediamine
and salicyladehyde in a mole ratio of 1:2 for 1 h.
The Mn(III) salen complex was synthesized following the
procedure given in the literature,[2] by refluxing the ethanolic
solution of salen ligand and methanolic solution of
MnCl2.4H2O in a mole ratio of 1:1 for 2 h. During the reac-
tion the bright yellow color of the solution changes to dark
brown. The dark brown solution was concentrated to obtain
a dark brown solid. The obtained dark brown solid (yield D
73%) was designated as Mn(salen).
Catalytic Reaction
Oxidation of styrene
The catalytic activity was evaluated for the solvent free
oxidation of styrene using molecular oxygen as an oxi-
dant. Oxidation reaction was carried out in a batch type
reactor operated under atmospheric pressure. In a typical
reaction, a measured amount of catalyst was added to a
three-necked flask containing styrene (100 mmole) at
80ꢀC. The reaction was started by bubbling O2 into the
reaction medium and was continuously stirred on mag-
netic hot plate.
In the oxidation of styrene, after completion of reaction
the reaction mixture was allowed to cool to room tempera-
Supporting of the Mn(salen) onto ZrO2
A series of catalysts containing Mn(salen) (20–40%) was syn- ture and then 10% aqueous solution Na2CO3 was added with
thesized by impregnating ZrO2 (1gm) with an ethanolic solu- constant stirring. The resultant mixture (organic and aque-
tion of Mn(salen) (0.2–0.4 g/20–40 mL of ethanol) and dried ous) was allowed to stand for 15–20 min in order to separate
at 60ꢀC for 3 h in oven and impregnated at 100ꢀC for 10 h. the two distinct layers. The aqueous layer was collected and
The obtained materials were designated as Mn(salen)20/ concentrated HCl was added slowly with constant stirring.
ZrO2, Mn(salen)30/ZrO2, and Mn(salen)40/ZrO2.
No white precipitate of benzoic acid was observed. The
The catalytic reaction was optimized with 30% loaded [Mn remained organic layer was extracted with dichloromethane
(salen)Cl] complex. The observed values for the elemental and analyzed on a gas chromatograph (Nucon 5700 model)

Manganese (III) Salen Supported Onto Hydrous Zirconia
541
having a flame ionization detector and BP-1 capillary column supporting onto the hydrous zirconia. Apart from this the
(30 m, 0.25 mm i.d.).
catalyst shows the considerable shift in C-O (1542 cm¡1
)
Further the product identification was done by compari- vibration band. This indicates the interaction of oxygen of C-
son with authentic samples and finally by a combined gas O of Mn(salen) to the H of hydroxyl group of ZrO2. Hence,
chromatography mass spectrometer (Hewlett-Packard) using there must be strong hydrogen bond between Mn(salen) and
HP-1 capillary column (30 m, 0.5 mm i.d.) with EI (70 eV).
ZrO2 through surface hydroxyl groups of zirconia
The percentage conversion of the substrate and the per- (Scheme 1).
centage-selectivity of the products in the epoxidation reaction
are calculated as
It is known that Mn(salen) is not thermally stable (Fig-
ure 3b). In free complex, at 130ꢀC there is a cleavage of Mn-
Cl bond and release of chlorine as hydrogen chloride and at
300ꢀC the free complex completely oxidises to give a dark
manganese oxide. The TGA of support ZrO2 (Figure 3a)
shows initial weight loss up to 210ꢀC which is due to loss of
adsorbed water of crystallization. TGA of Mn(salen)30/ZrO2
(Figure 3b) shows first weight loss of 7% at 50ꢀC is due to the
adsorbed water molecule of hydrous zirconia. The second
weight loss of 7% at 170ꢀC is due to the cleavage of Mn-Cl
bond. The final weight loss of 14% at 430ꢀC is due to the
decomposition of Mn(salen) on ZrO2. The weight of the non-
removable residue for Mn(salen)30/ZrO2 is 4.1648mg
(78.15%). This shows that supporting of Mn(salen) on the
surface of hydrous zirconia increases the thermal stability of
Mn(salen). An increase in thermal stability indicates the pres-
ence of strong interaction between Mn(salen) and support.
BET surface area of ZrO2 and the Mn(salen)30/ZrO2 was
found to be 170 m2g¡1 and 234.4 m2g¡1 respectively. The
larger surface area of the 30% Mn(salen)/ZrO2 compared to
that of the support was because of supporting of Mn(salen)
and it is as expected.[36] The increases in the surface area of
the supported catalyst may be due to the high dispersion of
Mn(salen) onto the surface of ZrO2.
.initial mol %/ ¡ .final mol %/
Conversion. %/ D
Selectivity. %/ D
£ 100
Initial mol %
product formed.mol/
Substrate converted.mol/
£ 100
Results and Discussion
DR-UV of Mn(salen) show two characteristic absorption
bands at 432 and 500 nm (Figure 1a) corresponding to
metal-ligand charge transfer and d-d transition respec-
tively.[34] DR-UV of Mn(salen)30/ZrO2 show two absorption
bands at 432 and 500 nm, which are identical with free Mn
(salen) complex (Figure 1b), indicates no change in the local
environment of the Mn(III) coordination centre after
supporting.
Figure 2 shows FT-IR spectra of Mn(salen), ZrO2, and
Mn(salen)30/ZrO2. The characteristic absorption frequencies
for Mn(salen), ZrO2, Mn(salen)30/ZrO2, and reused catalyst
are presented in Table 2. The main characteristic bands of
Mn(salen) are observed at 1331cm¡1 (n(C-N)) and
1630 cm¡1 (n (C-C, CDN)) and are in good agreement with
reported value.[35]
Catalytic Reaction
The FT-IR spectra of ZrO2 shows a broad band in the
region of 3400 cm¡1. This is attributed to asymmetric
hydroxo (-OH) stretches. Two types of bending vibrations
are observed at 1600 and 1370cm¡1, indicating the presence
of (H-O-H) and (O-H-O) bending, and vibrations, respec-
tively. It also shows a weak bending band at 600 cm¡1 attrib-
uted to the presence of Zr-O-H bond. The FT-IR spectra of
Mn(salen)30/ZrO2 shows bands at 1333 cm¡1 and
1628 cm¡1 corresponding to C-N stretching vibration and C-
Oxidation of styrene involves the formation of two products:
(a) epoxide and (b) benzaldehyde. To ensure the catalytic
activity, the same reaction was carried out without catalyst.
It was found that no oxidation takes place. The support,
ZrO2 was also used as catalyst for oxidation of styrene and
no conversion was observed.
Effect of % loading
C, CDN. The presence of these bands in Mn(salen)30/ZrO2 The aerobic oxidation of styrene was carried using unsup-
confirms that Mn(salen) keeps its structure undegraded after ported i.e. parent Mn(salen) and Mn(salen) supported
Fig. 1. DR-UV of (a) Mn(salen), (b) Mn(salen)30/ZrO2, and (c) recycled catalyst.

542
Patel et al.
Sch. 1. Interaction of Mn(salen) with surface of ZrO2.
multilayer formation of Mn(salen) onto support surface,
all the catalytically active sites may not be accessible to
the substrate molecule, which results in decreases in
conversion.
Fig. 2. FT-IR spectra of (a) Mn(salen), (b) ZrO2, and (c) Mn
(salen)30/ZrO2.
onto ZrO2 with different loading. The obtained results are
presented in Table 3. As the% loading of the Mn(salen)
complex (10–30%) increases, conversion also increases, as
expected. The catalyst with 30% loading shows maximum
conversion as well as selectivity toward epoxide. Further,
increases in% loading from 30% to 50% results in decrease
in conversion. This can be explained as follows. Up to
30% loading, the entire support surface is covered by
monolayer of Mn(salen) complex. Hence, substrate is
accessible to maximum active sites, which are responsible
for maximum conversion. After 30% loading, due to
Table 2. Band assignment of (a) ZrO2, (b) Mn(salen), (c) Mn
(salen)30/ZrO2, and (d) recycled catalyst.
Wavenumber (cm¡1
)
Mn
(salen)
Mn(salen)30/
ZrO2
Reused
Catalyst
Assignment
ZrO2
n(-OH)
3400
1600
600
—
—
—
—
—
—
1630
1535
1331
3407
1602
603
1628
1542
1333
3406
1600
602
1628
1543
1332
d(H-O-H)
d(Zr-O-H)
n(C-C), n(CDN)
n(C-O)
Fig. 3. TGA curve of (a) support (ZrO2) and (b) Mn(salen) and
Mn(salen)30/ZrO2.
n(C-N)

Manganese (III) Salen Supported Onto Hydrous Zirconia
543
Effect of reaction time
Table 3. Effect% loading of Mn(salen) on to ZrO2.
It is seen, from Table 5 that with increase in reaction time
the conversion also increases. Initially, the increase in the
conversion after 8 h is slow. This may be due to the fact
that the alkene is consumed during the reaction, as a result
the amount of the reactant decreases, which then requires
time to reversibly bind with the oxidant. Further, the rate
of desorption of the products formed from the catalyst sur-
face is faster as a result the overall rate of the reaction
slows down resulting into slow increase in the conversion
with time.
Selectivity (%)
Catalysta
Conversion (%) Benzaldehyde Epoxide
b
ZrO2
—
62
60
67
54
47
—
45
76
53
70
74
—
55
24
47
30
26
Mn(salen)c
Mn(salen)20/ZrO2
Mn(salen)30/ZrO2
Mn(salen)40/ZrO2
Mn(salen)50/ZrO2
aConversion based on styrene; Styrene, 100 mmol; amount of catalyst,
25 mg; Oxidant, O2 1 atm; reaction time 4 h; temperature, 80ꢀC. bamount of
ZrO2 19.3 mg. camount of catalyst (i.e., parent Mn(salen)) 5.7 mg.
Test for leaching and heterogeneity
Any leaching of the active species from the support makes
the catalyst unattractive and hence it is necessary to study
the stability as well as leaching. The catalyst was filtered
after completion of reaction and the filtrate was character-
ized for UV-VIS spectroscopy. For comparison, UV-VIS
spectra of Mn(salen) complex in ethanol were also
recorded (Figure 4). The absence of any characteristic
peaks in filtrate (Figure 4) indicates that there is no leach-
ing of Mn(salen) and the catalyst remains completely insol-
uble under reaction condition and could be reused.
Further leaching of the Mn(salen) from Mn(salen)30/
ZrO2 was confirmed by carrying out an analysis of the
used catalyst (EDAX) as well as the product mixture
(AAS). Analysis of the used catalyst did not show any
appreciable loss in manganese content as compared to the
fresh catalyst. Analysis of the product mixture also showed
that if any Mn was present it was below the detection limit,
which corresponded to less than 1 ppm. These observations
strongly suggest no leaching of the Mn species from the
support. Further, for the rigorous proof of heterogeneity, a
The control experiments with ZrO2 and Mn(salen) were
also carried out under optimized conditions with O2 and
results are presented in Table 3. It can be seen from
Table 3 that ZrO2 is inactive toward the epoxidation of
styrene indicating the catalytic activity is due to only Mn
(salen). The same reaction was carried out by taking the
active amount (5.7 mg) of Mn(salen). It was found that the
active catalyst gives 62% conversion with 55% selectivity
toward epoxide and 45% selectivity toward benzaldehyde.
Almost the same activity and selectivity was obtained for
supported catalyst. This indicates that Mn(salen) is the real
active species. Thus, we were successful in supporting Mn
(salen) onto ZrO2 without any significant loss in activity
and hence in overcoming the traditional problems of
homogeneous catalysis.
Effect of the amount of the catalyst (Mn(salen)30/ZrO2)
The effect of amount of the catalyst was studied keeping all test was carried out by filtering catalyst from the reaction
other parameters (temperature, reaction time) constant and mixture at 80ꢀC after 2 h and allowed the filtrate to react
the obtained results are shown in Table 4. With increase in up to 4 h. The reaction mixture of 2 h and filtrate was ana-
the amount of catalyst the amount of active species (Mn) lyzed on a gas chromatogram. Results are presented in
increases as a result % conversion increases. Further, Table 6. No change in % conversion as well as % selectiv-
the increase in amount the number active sites increases, ity was found. On the basis of the results, it can be con-
the epoxide is readily converted to the benzaldehyde. To the cluded that there is no leaching of the Mn(salen) from the
known industrial importance of epoxide the amount of support and the present catalysts is truly heterogeneous in
catalyst is optimize to 25 mg.
nature and fall into the category C.[37]
Table 4. Effect of amount of the catalyst.
Table 5. Effect of reaction time.
Selectivity (%)
Benzaldehyde Epoxide
Selectivity (%)
Benzaldehyde Epoxide
Amount Conversion
Time Conversion
Catalysta
(mg)
(%)
Catalysta
(h)
(%)
Mn(salen)30/
ZrO2
10
25
35
50
75
28
67
66
70
74%
58
53
61
68
75%
42
47
39
32
25%
Mn(salen)30/
ZrO2
2
4
8
12
16
23
67
76
79
81
60
53
65
80
81
40
47
35
20
19
aConversion based on styrene; Styrene, 100 mmol; Oxidant, O2 1 atm; reac-
tion time 4 h; temperature, 80ꢀC.
aConversion based on styrene; Styrene, 100 mmol; amount of catalyst,
25 mg; Oxidant, O2 1 atm; temperature, 80ꢀC.

544
Patel et al.
Fig. 4. DR-UV spectra of (a) Mn(salen) and (b) filtrate.
Recycling of the catalyst
Table 6. Heterogeneity test.
The catalysts, Mn(salen)30/ZrO2 was recycled to test their
activity as well as stability. To see any change in the structure
of catalyst after completion of reaction, the DR-UV of used
catalyst was recorded. Figure 1b and 1c illustrates the DR-
UV of fresh catalyst as well as used catalyst. No difference in
DR-UV indicates the stability of the used catalyst after the
reaction.
Selectivity (%)
Catalysta
Conversion (%)
23
Benzaldehyde
60
Epoxide
40
Mn(salen)30/
ZrO2 (2 h)
Filterate (4 h)
23
60
40
The FT-IR spectra of reused catalyst show all the bands of
C-N, C-C, CHN, and C-O stretching vibration. The presence
of these bands confirms that Mn(salen) keeps its structure
undegraded. This indicates that the catalyst is stable during
the reaction conditions.
aConversion based on styrene; Styrene, 100 mmol; amount of catalyst,
25 mg; Oxidant, O2 1 atm; reaction time 4 h; temperature, 80ꢀC.
The regeneration of the catalyst was carried out and
reused up to four cycles without any appreciable loss in cata-
lytic activity. The distribution for the selectivity of the prod-
ucts is remains the same. The results are shown in Table 7.
To determine the effectiveness of the catalyst, the synthe-
sized Mn-salen was supported on to neutral alumina, MCM-
41, and SBA-15. The reactions were carried out under opti-
mized conditions (Table 8).
Table 7. Recycling of the catalyst.
Selectivity (%)
Catalysta
Conversion (%)
67
Benzaldehyde
53
Epoxide
47
Mn(salen)30/
ZrO2 (4 h)
R1
R2
R3
R4
66
66
66
66
55
55
55
55
45
45
45
45
Conclusion
aConversion based on styrene; Styrene, 100 mmol; amount of catalyst,
25 mg; Oxidant, O2 1 atm; reaction time 4 h; temperature, 80ꢀC.
In the present article, we report the use of hydrous zirconia as
a support for Mn(salen). TGA indicates the stability of cata-
lyst up to 430ꢀC indicates the strong interaction between
complex and support. FT-IR and DR-UV indicates the struc-
ture of Mn(salen) did not get destructed after supporting on
the surface of ZrO2. SEM image indicates the existence of
highly charged species on the surface of support. The present
contribution reports the solvent free liquid phase aerobic oxi-
dation of styrene using Mn(salen) on ZrO2, which is the first
report made ever. The Mn(salen)30/ZrO2 gives 67% conver-
sion with 47% selectivity of styrene oxide and 53% selectivity
of benzaldehyde under mild reaction condition. The superior-
ity of the work also lies in regenerating and reusing the cata-
lyst with same % conversion, which indicates the stability of
the catalyst in reaction condition.
Table 8. Mn(salen) complex supported on to different supports.
Selectivity (%)
Catalyst
Conversion % Epoxide Benzaldehyde
Mn(salen)30/ZrO2
67
53
57
55
47
40
38
43
53
60
62
57
Mn(salen)30/Alumina
Mn(salen)30/MCM-41
Mn(salen)30/SBA-15
Reaction conditions: Styrene, 100 mmol; amount of catalyst, 25 mg; Oxi-
dant, O2, 1 atm; temperature, 80ꢀC; time, 4 h.

Manganese (III) Salen Supported Onto Hydrous Zirconia
545
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Funding
Mr. Ketan Patel and Ms. Varsha Brahmkhatri are thankful
to Department of Science and Technology (DST, New Delhi)
and BRNS, Mumbai, for providing financial support.
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