The effects of thermal treatment of ZnO–ZnCr2O4 catalyst on the particle size and product selectivity in dehydrocyclization of crude glycerol and ethylenediamine

Article abstract of DOI:10.1134/S0023158416050177

The ZnO–ZnCr₂O₄ (Zn–Cr–O) sample obtained by decomposition of Zn-Cr hydrotalcite precursor was subjected to the thermal treatment at different temperatures and the physico-chemical properties of the Zn–Cr–O system were compared with

Full text of DOI:10.1134/S0023158416050177

ISSN 0023-1584, Kinetics and Catalysis, 2016, Vol. 57, No. 5, pp. 602–609. © Pleiades Publishing, Ltd., 2016.
Published in Russian in Kinetika i Kataliz, 2016, Vol. 57, No. 5, pp. 607–614.
The Effects of Thermal Treatment of ZnO–ZnCr2O4 Catalyst
on the Particle Size and Product Selectivity in Dehydrocyclization
of Crude Glycerol and Ethylenediamine1
Reema Sarkaria, Vankudoth Krishnaa, Medak Sudhakara, Tumula Venkateshwar Raoa,
Aytam Hari Padmasrib, Darbha Srinivasc, and Akula Venugopala, *
aCatalysis Laboratory, Inorganic and Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology,
Hyderabad-500 007, Telangana, India
bDepartment of Chemistry, University College for Women, Osmania University,
Koti, Hyderabad 500 095, Telangana, India
cCatalysis Division, CSIR-National Chemical Laboratory, Pune 411 008, India
*e-mail: akula@iict.res.in
Received October 7, 2015
Abstract—The ZnO–ZnCr2O4 (Zn–Cr–O) sample obtained by decomposition of Zn-Cr hydrotalcite pre-
cursor was subjected to the thermal treatment at different temperatures and the physico-chemical properties
of the Zn–Cr–O system were compared with its catalytic behavior in dehydrocyclization of crude glycerol
and ethylenediamine (EDA). Upon high temperature treatment of Zn–Cr–O the Cr6+ ions underwent auto-
reduction to form stable Cr3+ species and the particle size of both ZnO and ZnCr2O4 increased dramatically.
Thermal effect did not influence the intermolecular cyclisation of EDA to form pyrazine. By contrast, an
inversely proportional dependence was found between the rate of formation of 2-methylpyrazine and the par-
ticle size of Zn–Cr–O whereas the rate of 2-pyrazinylmethanol was directly proportional to the particle size.
Keywords: crude glycerol, dehydrocyclization, 2-methylpyrazine, Zn-Cr-O, ESR, XRD
DOI: 10.1134/S0023158416050177
1. INTRODUCTION
drocyclization of bio-glycerol and EDA to proceed
with high selectivity towards 2MP [10]. It is also
reported that reductive pretreatment of Zn–Cr–O
afforded better conversions of both bio-glycerol and
EDA at moderate temperatures [11]. Liu et al reported
maximum selectivity of 42.9% to 2MP along with
53.1% to 2-pyrazinylmethanol (2PM) on modified
SiO2–Al2O3 catalysts [12]. In the present study cata-
lytic activity of the Zn–Cr–O samples calcined at high
temperatures was evaluated in dehydrocyclization of
glycerol and EDA. The catalyst samples were analyzed
by electron spin resonance (ESR), transmission elec-
tron microscopy (TEM), CHNS analysis, scanning
Crude glycerol obtained from the bio-diesel indus-
try can be converted to fine chemicals, fuels and fuel
additives [1]. Recent years have seen an increased
interest in the effective conversion of bio- glycerol to
useful compounds [2]. One of the key processes in the
utilization of glycerol is the synthesis of 1, 2-propane-
diol by hydrogenolysis of bio-glycerol over supported
noble metal catalysts at high H2 pressures [3, 4]. Pyra-
zinamide obtained from 2-methylpyrazine (2MP) is
considered as an important antitubercular drug inter-
mediate [5, 6]. Conventionally, 2-MP is synthesized
by dehydrocyclization of 1,2-propanediol and eth-
ylenediamine (EDA) over Pd modified Zn–Cr–O
catalyst [7, 8]. A novel route for utilization of bio-glyc- electron microscopic analysis (SEM), Fourier trans-
erol in conjunction with EDA for the production of
2MP is described that involves dehydrocyclization
over ZnO–ZnCr2O4 (Zn–Cr–O) catalyst [9]. The
kinetic and mechanistic studies revealed that a combi-
nation of acid-base pair site is desirable for the dehy-
form infrared spectroscopy (FT-IR), X-ray diffraction
analysis (XRD) and ultra-violet diffuse reflectance
spectroscopy (UV-DRS). To rationalize catalytic
behavior of the Zn–Cr–O system, the relationships
between dehydrocyclization rates and the spectro-
scopic data were considered.
1
The article is published in the original.
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603
2. EXPERIMENTAL
2.1. Catalyst Preparation
sured using −18/+23 sieved (BSS) particles. The car-
bon mass balance was done based on the inlet and out-
let concentration of the organic moiety. Prior to the
reaction, the calcined catalyst (about 0.2 g) was
reduced in a flow of 5% H2 and 95% Ar (30 mL min−1)
at 400°C for 5 h. The catalytic activities were measured
under strict kinetic control. An aqueous glycerol solu-
tion (20 wt% in H2O) was used with a glycerol to EDA
The Zn-Cr-O sample was prepared by a simple co-
precipitation method using Zn(NO3)2 · 6H2O and
Cr(NO3)3 · 9H2O (Sigma-Aldrich) with Zn : Cr = 2 : 1
(mole ratio) to produce a double hydroxide with the
hydrotalcite structure. The Zn–Cr–HT sample was
prepared at a constant pH of 9 using a mixture of 2 M
NaOH + 1 M Na2CO3 (base mixture) as a precipitat-
ing agent. The gel was washed thoroughly, filtered and
oven-dried for 12 h at 120°C and calcined in static air
at different temperatures 450, 550, 650 and 750°C for
5 h. The calcined Zn–Cr–O samples were denoted as
ZC450, ZC550, ZC650 and ZC750 respectively.
mole ratio of 1 :1 , at a flow rate of 5 mL h−1 (10 mmol
glycerol + 10 mmol EDA + 200 mmol H2O), along
with N2 as a carrier gas at a flow rate of 1800 mL h−1.
The feed mixture mole ratio is glycerol : EDA : H2O :
N2= 1 : 1 : 20 : 8. The product mixture was analyzed by
gas chromatograph (Shimadzu, GC-17A) via a flame
ionization detector (FID) using a ZB-5 capillary col-
umn at a ramping rate of 10°C min−1 from 60 to
280°C. The mass balance for all the measurements was
>95%. The samples were analyzed by GC-MS
(QP5050A Shimadzu) using a ZB-5 capillary column
with EI mode.
2.2. Catalyst Сharacterization
The surface properties of the Zn–Cr–O samples
calcined at different temperatures were assessed by N2
adsorption at –196°C using an Autosorb Tristar 3000
(Micromeritics, United States) physical adsorption
apparatus. The specific surface areas were calculated
applying the BET method. The reduced forms of Zn–
Cr–O catalysts were characterized by powder XRD
analysis using a Rigaku Miniflex X-ray diffractometer
with Ni-filtered CuKα radiation (λ = 0.15406 nm) at
30 kV and 15 mA. X-ray diffractograms were recorded
from 20 to 80° at a scanning rate of 2° (2θ) min−1. The
SEM-EDX analysis was carried out using a JEOL-
JSM 5600 instrument. For the TEM analysis the sam-
ples were dispersed in methanol solution and sus-
pended on a 400-mesh, 3.5 mm diameter Cu grid and
the images were taken using a JEOL JEM 2010 high-
resolution transmission electron microscope. The
ESR analysis of the Zn–Cr–O samples were per-
formed at room temperature using an X-band
JEOL/JES-FA200 spectrometer equipped with an
oscillator operating at frequency υ = 9.029 GHz. The
UV-visible diffuse reflectance spectra were recorded
on a UV-2600 Shimadzu UV-visible spectrometer in
the wavelength range of 185–800 nm.
3. RESULTS AND DISCUSSION
3.1. Characterization of Catalysts
3.1.1. FTIR Analysis. The FTIR spectra of ZnO-
ZnCr2O4 (Zn–Cr–O) calcined at various tempera-
tures are given in Fig. 1. Analysis of vibrational spectra
of ZnCr2O4 spinel is made by the factor group analysis,
which predicts four infrared active modes due to F1u
symmetry species for ZnCr2O4 spinel. Two strong
absorption bands ν1 and ν2 at 640 and 501 cm-1 can be
ascribed to the vibrations of octahedral chromium [9–
11]. A sharpening of these absorption bands is
observed with increasing calcination temperature of
Zn–Cr–O. The other two low frequency bands ν3 and
ν4 (not shown because they appear below 400 cm–1)
arise due to vibrations involving both zinc tetrahedral
and chromium octahedral groups of ZnCr2O4 spinel
[13]. The intensity of the absorption band at 3600–
3400 cm–1 attributable to stretching vibration of
hydroxyl group is decreased as the calcination tem-
perature of Zn-Cr-O was increased from 450 to 750°C.
This could be explained by the condensation of adja-
cent hydroxyl groups at higher temperature. A small
absorption band at 1620 cm–1 is due to bending defor-
mation mode of hydroxyl groups [14]. The vibrational
band at 937cm-1 indicates the presence of Cr6+ species,
the intensity of this band is gradually decreased with
increasing calcination temperature. These results
clearly demonstrate that Cr6+ species are auto reduced
at higher calcination temperatures [15, 16].
2.3. Activity Measurements
The dehydrocyclization activities of Zn–Cr–O
catalysts calcined at different temperatures were per-
formed at 375°C and atmospheric pressure in a fixed-
bed vertical quartz reactor (i.d = 8 mm, length =
450 mm) placed in a two zone furnace operated in a
down flow mode. In the first zone maintained at
300°C the reaction mixture was preheated whereas in
the second zone containing the catalyst bed the reac-
tor temperature was set at 375°C. Temperatures in the
both zones were monitored by a temperature control-
ler-cum-programmer using a K-type thermocouple. reduced Zn–Cr–O catalysts registered at room tem-
Glycerol (Fluka) and EDA (SDFCL, India) were perature are shown in Fig. 2. The spectra showed vari-
used. Nitrogen (IOLAR-I grade, BOC, India) was ations in line widths in both fresh and reduced forms
used as a carrier gas. The catalytic activities were mea- (Table 1). The fresh Zn–Cr–O showed broad lines
3.1.2. ESR analysis. The ESR spectra of fresh and
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604
REEMA SARKARI et al.
revealed that the ESR line width varied between 39
Transmittance, %
and 65 mT [19]. The peak-to-peak line width of Cr3+
species is in accord with reported data [17, 19].
90
1
80
The broad line width of ESR signal of Zn–Cr–O of
fresh catalysts can be explained on the basis of strong
spin-spin exchange dipolar broadening whereas the
decrease in line widths of the reduced Zn–Cr–O can
be the result of spin-spin exchange narrowing. Broad
ESR signal is an indication of anisotropy broadening
[20]. All the fresh samples exhibited very broad sym-
metric signal of a Lorenzian shape, arising from Cr3+
ions interacting with Zn (g ~1.97). Upon thermal
treatment at high temperatures the Cr6+ species are
auto-reduced to form stable Cr3+ entities. The fraction
of Cr3+ present in the samples can be estimated from
the ESR spectra that showed a higher concentration of
Cr3+ species in ZC750. Hence taken the ESR spec-
trum of ZC750 as a standard, the fraction of Cr3+ ions
in ZC450, ZC550 and ZC650 samples can be assessed.
The fraction of Cr3+ makes up 14.4, 19.7 and 54.2% of
chromium present in ZC450, ZC550 and ZC650 sam-
ples, respectively.
2
3
70
60
50
40
30
20
10
0
4
4000
3000
2000
1000
Wavenumber, cm–1
Fig. 1. FTIR spectra of fresh Zn–Cr–O catalysts calcined
at different temperatures: (1) ZC450, (2) ZC550,
(3) ZC650 and (4) ZC750.
assigned to Cr3+ species. There is a narrowing of the
Cr3+ signal formed in fresh and reduced catalysts upon
an increase in the calcination temperature. This could
be due to stronger spin exchange (line width narrow-
ing) at slightly higher concentration of Cr3+ ions cre-
ated by conversion of Cr6+ to Cr3+ species in the
reduced form. This suggestion is consistent with the
data derived from the FTIR analysis (Fig. 1). Changes
in the shape of ESR spectra can be attributed to varia-
tions in the configuration of the Cr species [17]. The
ESR line width of β-phase Cr3+ was reported to
increase from about 80 to 200 mT when chromia-alu-
mina catalysts were reduced in H2 at 500°C [18]. A
study by Forni et al. on the ZnO–ZnCr2O4 catalyst
3.1.3. UV–Visible DRS analysis. UV-visible DR
spectra of Zn–Cr–O catalysts are collected in Fig. 3.
The presence of Cr6+ and Cr3+ species in freshly cal-
cined Zn–Cr–O catalysts can be identified by UV–
visible DR spectroscopy. The spectra of Zn–Cr–O
samples exhibited two absorption bands located
around 280 and 360 nm. The origin of these bands is
due to O2p→Cr6+ charge transfer transitions. Particu-
larly, the bands at 280 nm and 360 nm are results of the
1
charge transfer transitions of A1→1T2 (1t1→ 7t2 and
6t2→ 2e) and 1A1→ 1T2 (1t1→ 2e), respectively [21–
23]. Since no absorption band around 455 nm was
observed in any of the Zn-Cr-Osamples, the absence
of dichromates due to polymerization of Cr6+ species
can be suggested [24]. With increasing calcination
(а)
(b)
ESR intensity
ESR intensity
4
4
25000
20000
15000
10000
5000
15000
10000
3
3
2
5000
2
1
0
0
1
–5000
–10000
–15000
–20000
–25000
–5000
–10000
–15000
200 250 300 350 400 450 500
Magnetic field, mT
200 250 300 350 400 450 500
Magnetic field, mT
Fig. 2. (a) ESR spectra of fresh and (b) reduced Zn–Cr–O catalysts calcined at different temperatures: (1) ZC450, (2) ZC550,
(3) ZC650 and (4) ZC750.
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THE EFFECTS OF THERMAL TREATMENT
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Table 1. The physicochemical properties of Zn–Cr–O samples calcined at various temperatures
Cr3+ ESR linewidth, mT
Crystallite size, nma
g value
Catalyst
S
BET, m2/g
ZnCr2O4 (311)
fresh
reduced
fresh
reduced
ZnO (100)
ZC450
ZC550
ZC650
ZC750
38.9
36.6
26.2
21.6
157.8
104.8
66.7
63.6
61.5
47.5
28.4
1.9725
1.9739
1.9754
1.9758
1.9740
1.9743
1.9756
1.9758
19.8
27.3
28.7
31.2
15.9
18.1
24.1
36.0
36.6
a
Crystallite size measured from XRD patterns using the Scherrer formula.
temperature, there is a blue shift in absorption bands
around 260 and 380 nm in the spectra of Zn–Cr–O
samples. A gradual shift of the absorption bands to
lower wavelength from ZC450 to ZC750 can be ratio-
nalized in terms of the decreasing concentration of
Cr6+ ions in samples treated at higher calcination tem-
peratures. This observation is in good agreement with
3.1.4. TEM analysis. The TEM images of freshly
calcined ZC450, ZC550, ZC650 and ZC750 catalysts
are shown in Fig. 4. The average particle size of the
catalysts was measured by choosing around 10 parti-
cles. The average mean particle diameter of ZC450,
ZC550, ZC650, ZC750 samples was found to be 24.0,
32.8, 40.7 and 50.4 nm respectively. These results sug-
gest that the calcination temperature has significantly
FTIR and ESR results, which imply an increased con- increased the size of particles in the Zn–Cr–O system
version of Cr6+ to Cr3+ ions at higher calcination tem-
peratures. A blue shift in the spectra is therefore in line
with the ESR results discussed above [23]. The pres-
ence of Cr3+ species in the Zn-Cr-O samples con-
firmed by a broad band located at around 565–585 nm
that is due to d–d transitions of Cr3+ (A2g → T2g) in
octahedral Cr2O3 entities [11, 15, 23)]. The red shift of
this absorption band observed in the spectra of the
sample subjected to a high temperature heating can be
explained by increasing concentration of Cr3+ species.
that contained both ZnO and ZnCr2O4 phases. The
ESR results obtained with the Zn–Cr–O samples
provide further evidence that the decrease in line
widths of Cr3+ species can be associated with increas-
ing size of ZnCr2O4 particles (Fig. 5).
3.1.5. SEM and XRD analysis. SEM images dis-
played changes in morphology of ZnCr2O4 particles
caused by high temperature treatments of the Zn–Cr–
O system (Fig. 6). One can readily recognize a com-
posite structure of ZnCr2O4 particles and agglomera-
tion of Zn–Cr–O entities on their surface. XRD anal-
ysis of reduced Zn–Cr–O revealed the presence of
ZnO and ZnCr2O4 phases in the catalysts (Fig. 7).
Table 1 indicates that the crystallite size of the parti-
cles is increased with increasing calcination tempera-
ture. Particle sizes are found to be inversely propor-
tional to ESR linewidth (Fig. 5), which is in good
agreement with an earlier report [20].
Intensity, a.u.
258
360
2.7
2.4
2.1
1.8
585
3.2. Catalyst Activity Measurements for Vapor Phase
Dehydrocyclization of Glycerol and EDA
281
363
4
3
2
The results of dehydrocyclization of bio-glycerol
and EDA in the presence of Zn-Cr-O samples are
shown in Fig. 8. The primary useful product in
the dehydrocyclization is 2-methylpyrazine along
with 2-pyrazinylmethanol as a major byproduct. In
addition, minor amounts of pyrazinaldehyde, pyra-
zine and (2,3- 2,5- 2,6-) dimethylpyrazines were
found among the products (scheme).
1
565
200 300 400 500 600 700 800
Wavelength, nm
Fig. 3. UV-DRS pattern of fresh Zn-Cr-O catalysts cal-
cined at different temperatures: (1) ZC450, (2) ZC550,
(3) ZC650 and (4) ZC750.
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REEMA SARKARI et al.
100 nm
(b)
100 nm
(a)
100 nm
(d)
100 nm
(c)
Fig. 4. TEM images of Zn-Cr-O catalysts calcined at different temperatures: (a) ZC450, (b) ZC550, (c) ZC650 and (d) ZC750.
(а)
(b)
ΔHpp
ZnCr2O4 crystallite size, nm
ΔHpp
ZnCr2O4 crystallite size, nm
35
40
70
160
140
120
100
80
ΔH
ΔH
ZnCr2O4
ZnCr2O4
35
30
25
20
15
30
60
50
40
30
20
25
20
15
60
40
20
450
550
650
750
450
550
650
750
Calcination temperature, °С
Calcination temperature, °С
Fig. 5. ΔH values and ZnCr O crystallite size of fresh (a) and reduced (b) Zn–Cr–O catalysts as a function of calcination tem-
pp
2 4
perature.
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THE EFFECTS OF THERMAL TREATMENT
607
20 μm
20 μm
(a)
(b)
20 μm
20 μm
(c)
(d)
Fig. 6. SEM images of Zn–Cr–O catalysts calcined at different temperatures: (a) ZC450, (b) ZC550, (c) ZC650 and (d) ZC750.
Dehydrocyclization of glycerol and EDA for the synthesis of 2-methylpyrazine
Dehydrocyclization of glycerol and EDA for the synthesis of 2-methylpyrazine
Step I
Step II
Dimerization followed
by internal rearrangement
OH H2N
+
OH H2N
N
N
N
N
N
N
−2H2O, −2H2
+
O
HO
OH
Disproportionation
Intermolecular cyclization
N
+
N
N
N
N
N
N
N
Scheme.
Conversions of both glycerol and EDA decreased rate of formation of 2-pyrazinylmethanol in the pres-
with increasing calcination temperature of Zn–Cr–O ence of samples preheated at high temperatures is due
samples. As the calcination temperature was increased
from 450 to 750°C the EDA conversion decreased
from 40.0 to 88.1%, while the glycerol conversion
declined from 30.0 to 83.7%. The 2MP formation rate
drastically decreased from 45.3 to 22.7 μmol-s−1m−2,
on the contrary, the rate of 2-pyrazinylmethanol sig-
to reduction in the rate of homo coupling of 2-pyraz-
inylmethanol that leads to a cyclic transition state
involved in the formation of 2MP and 2-pyrazinalde-
hyde [Scheme, step II]. Selectivity towards pyrazine
remains nearly constant independent of the calcina-
tion temperature of Zn–Cr–O catalyst. These results
nificantly increased when the calcination temperature suggest that rate of formation of 2MP and 2-pyrazinyl
of Zn–Cr–O was increased (Table 2). The enhanced methanol is dependent on the particle size of Zn–Cr–
KINETICS AND CATALYSIS Vol. 57 No. 5 2016

608
Intensity, a.u.
REEMA SARKARI et al.
Conversion/Selectivity, %
ZnCr2O4
ZnO
XEDA
Xglycerol
S2-methylpyrazine
Spyrazine
S2-pyrazinemethanol
Sothers
100
90
80
70
60
50
40
30
20
10
4
3
2
1
10
20
30
40
50
60
70
80
2θ, deg
Fig. 7. XRD pattern of reduced Zn–Cr–O catalysts cal-
cined at different temperatures: (1) ZC450, (2) ZC550,
(3) ZC650 and (4) ZC750.
O catalysts. The process outlined above is an attractive
method which utilizes the by-product i.e. bio-glycerol
obtained during the production of bio-diesel.
450
550
650
750
Calcination temperature, °С
Fig. 8. The activity of Zn–Cr–O samples in the dehydro-
cyclization of glycerol and EDA measured at 375°C as a
function of calcination temperature. The samples were
4. CONCLUSIONS
ESR spectra showed the presence of Cr3+ as the
only paramagnetic species in both fresh and reduced
form of Zn–Cr–O catalysts. Upon reduction, a nar-
rowing in Cr3+ line widths is observed compared to the
signal registered for unreduced samples. This could be
possibly due to stronger spin exchange line width nar-
rowing at high concentration of Cr3+ ions caused by
reduction of Cr6+ to Cr3+ species. Upon thermal treat-
ment the BET-surface area of ZnO–ZnCr2O4 samples
decreases. The XRD results demonstrated an increase
in crystallite size of ZnO and ZnCr2O4 particles at high
calcination temperatures (Table 1). FTIR analysis
clearly showed that the high temperature treatment
resulted in decreasing intensity of the band around
3600 cm-1, which can be explained by a reduced popu-
lation of the surface hydroxyl groups. The UV-DRS
results revealed a blue shift in the absorption band
located around 260 nm indicating a loss of Cr6+ ions
with increasing calcination temperature. Conse-
preliminary reduced in 5% H /Ar at 400°C/5 h.
2
quently the absorption band at 580 nm assigned to
Cr3+ ions showed a red shift due to increase in the con-
centration of Cr3+ species. The Zn–Cr–O catalysts
calcined at lower temperatures are found to be active
and selective for dehydrocyclization of glycerol and
EDA for 2MP synthesis. On the contrary, Zn–Cr–O
treated at higher temperatures demonstrated
enhanced 2-pyrazinyl methanol yields. Crystallite size
of Zn–Cr–O strongly affects the catalytic activity. It
appears that ZnO–ZnCr2O4 mixed oxides with small
particle sizes are suitable for 2MP whereas Zn–Cr–O
particles with larger sizes are unfavorable for homo
coupling of 2-pyrazinylmethanol. To sum up, we have
developed a catalytic process comprising simple ZnO-
ZnCr2O4 mixed oxide for the synthesis of 2-meth-
Table 2. Comparison of dehydrocyclization activities of catalysts calcined at different temperatures
Specific rate, μmol s–1 m–2a
Catalyst
cCarbon, wt %
othersb
2-methylpyrazine 2-pyrazinylmethanol
pyrazine
ZC450
ZC550
ZC650
ZC750
47.5
32.6
22.9
18.7
a
4.6
10.6
19.1
21.8
6.5
6.2
6.1
5.1
6.7
5.9
5.8
5.7
1.95
1.56
1.54
0.82
Reaction temperature 375°C. Specific rates of 2-methylpyrazine, 2-pyrazinylmethanol, pyrazine and others are measured with respect to
b
2MP, 2-pyrazinylmethanol, pyrazine and others yields normalized by S
.
Others include 2-pyrazinaldehyde and (-2,6; -2,5; -2,3)-
BET
c
dimethylpyrazines. Carbon contents of the used catalysts (wt%.) recovered after 6 h continuous operation.
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THE EFFECTS OF THERMAL TREATMENT
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ylpyrazine, an intermediate compound for anti-TB
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ACKNOWLEDGMENTS
RS and VK thank CSIR New Delhi for fellowship.
AV thank for financial support from CSIR New Delhi
under INDUS-MAGIC program CSC-0123 is
acknowledged.
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