Nano Pt-TiO2 for an efficient one-pot photocatalytic synthesis of quinaldines from anilines and ethanol

Article abstract of DOI:10.1007/s11164-011-0415-7

Nanosized platinum particles loaded on the TiO₂ nanoparticles were prepared to assess its photocatalytic activity in simple one-pot synthesis of quinaldines from anilines in ethanol using UV light. The catalyst was characterized by X-ray diffra

Full text of DOI:10.1007/s11164-011-0415-7

Res Chem Intermed (2012) 38:761–773
DOI 10.1007/s11164-011-0415-7
Nano Pt–TiO2 for an efficient one-pot photocatalytic
synthesis of quinaldines from anilines and ethanol
•
Kaliyamoorthy Selvam Bojja Sreedhar
Meenakshisundaram Swaminathan
•
Received: 19 August 2011 / Accepted: 27 September 2011 / Published online: 14 October 2011
Ó Springer Science+Business Media B.V. 2011
Abstract Nanosized platinum particles loaded on the TiO2 nanoparticles were
prepared to assess its photocatalytic activity in simple one-pot synthesis of qui-
naldines from anilines in ethanol using UV light. The catalyst was characterized by
X-ray diffraction (XRD), high-resolution transmission electron microscopy, X-ray
photoelectron spectra (XPS), Brunauer–Emmer–Teller surface area, atomic force
microscope and diffuse reflectance spectra. XRD patterns revealed that the crystal
structure of Pt–TiO2 resembled anatase phase of TiO2. The UV–Vis spectra indi-
cated an increase in absorption of visible light when compared to TiO2. XPS
analysis reveals that platinum particles are present mainly in metallic form. Fur-
thermore, TEM analysis showed non-spherical-shaped Pt–TiO2 nanoparticles of the
diameter 10–30 nm. Upon irradiation in the presence of Pt–TiO2, aniline and oxi-
dation products derived from ethanol undergo condensation–cyclization to afford
quinaldines. Higher efficiency of Pt–TiO2 than Au–TiO2 in the conversion of aniline
to quinaldines is due to the higher work function of Pt.
Keywords Quinaldine ꢀ Photocatalytic cyclization ꢀ Pt–TiO2 ꢀ
Combined redox reaction ꢀ Aniline
Introduction
Quinoline and its derivatives has been the subject of much research due to their
importance in various applications and their widespread biochemical significance. A
K. Selvam ꢀ M. Swaminathan (&)
Department of Chemistry, Annamalai University, Annamalainagar 608 002, India
e-mail: chemres50@gmail.com
B. Sreedhar
Inorganic and Physical Chemistry Division, Indian Institute of Chemical Technology,
Hyderabad 500 007, India
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K. Selvam et al.
large variety of quinoline derivatives have been used as antimalarial, anti-
inflammatory, antiasthmatic, antibacterial, antihypertensive, and tyrokinase PDGF–
RTK inhibiting agents [1–3]. A number of synthetic methods such as Skraup,
Doebner–Von Miller, Friedlander, and Combes reactions have been developed for
the preparation of quinolines [4–6]. However, many of these methods are not fully
satisfactory with regard to operational simplicity, cost of the reagent, and isolated
yield. Thus, the research is focused on a better and improved methodology.
We have recently reported Au–TiO2 for selective photocatalytic synthesis of
quinaldines from aniline and nitrobenzene under UV light [7, 8]. These reactions are
attractive from the viewpoint of converting light energy to chemical energy as well
as yielding quinaldines that are of synthetic importance.
Since the work function and the oxidation potential of Pt are greater than those of
Au, the current study is aimed at modifying prepared TiO2 by doping with platinum
and to evaluate its efficiency in the photocatalytic synthesis of quinaldines from
anilines under UV light. Kraeutler and Bard [9] have reported the photocatalytic
preparation of Pt–TiO2 from aqueous solution of chloroplatinic acid
(H2PtCl6ꢀ6H2O) and used for decarboxylation of acetic acid and for the photo-
Kolbe reaction. By using platinized TiO2 photocatalysts in the absence of O2,
generally, hydrogen evolution proceeds as a counter reaction of oxidation by h?,
e.g., primary and secondary alcohols are converted into aldehydes and ketones with
the evolution of H2 [10, 11]. Earlier we reported the defluoridation of PFBA using
Pt–TiO2-P25. This is the first report on Pt–TiO2 in the photocatalytic synthesis of
quinaldines from anilines and ethanol.
Experimental
Materials and methods
All chemicals were of the highest purity available and were used as received
without further purification. Hexachloroplatinic acid (H2PtCl6ꢀ6H2O) (Aldrich) was
used as the Pt source. Nano TiO2 was prepared by the hydrolysis of titanium
tetraisopropoxide (Himedia 98.0%), in isopropanol solution (1.6 M) by the addition
of distilled water (isopropanol:water 1:1). The resulting colloidal suspension was
stirred for 4 h. The gel obtained was filtered, washed, and dried in an air oven at
100 °C for 12 h. The sample was calcinated at 400 °C in a muffle furnace for 1 h.
The metal loaded TiO2 catalyst was prepared by photoreduction of metal ions on the
TiO2 as per the procedure reported in our previous article [7]. Pt(x)–TiO2 samples,
containing different platinum loadings [x (wt%) = Pt/(TiO2 ? Pt)100; x = 0.5,
1.0,1.5, 2.0] were prepared .
Apparatus
X-ray diffraction (XRD) patterns of TiO2 and Pt–TiO2 powder samples were
obtained using a Model D/Max 2550 V with Cu anticathode radiation. The
diffractograms were recorded in 2h range between 10 and 80° in steps of 0.02°
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Nano Pt–TiO2 for an efficient one-pot photocatalytic synthesis
763
with count time of 20 s at each point. The crystalline phase can be determined
from integration intensities of anatase (101), rutile (110), and brookite (120)
peaks and the average crystallite sizes were determined according to the Debye–
Scherrer equation using the full width half maximum (FWHM) data of each
phase.
Kk
D ¼
ð1Þ
b cos h
where D is the crystal size of the catalyst, k is the X-ray wavelength (0.154 nm),
b is the FWHM of the catalyst, K = 0.89 and h is the diffraction angle.
The phase formation, particle size, surface morphology, and crystallinity of pure
and loaded catalysts were examined using a transmission electron microscope
(TEM) (Model JEOL TEM-3010) operated at 300 keV. The samples for TEM
analysis were prepared by dispersion of the catalysts in ethanol under sonication and
deposition on a copper grid. High-resolution TEM (HR-TEM) measurements were
carried out using a JEOL-JEM-2010 UHR instrument operated at an acceleration
voltage of 200 kV with a lattice image resolution of 0.14 nm. X-ray photoelectron
spectra (XPS) of the catalysts were recorded in an ESCA-3 Mark II spectrometer
(VG Scientific Ltd., England) using Al Ka (1486.6 eV) radiation as the source. The
spectra were referenced to the binding energy of C (1 s) (285 eV).
The DRS of all the catalysts were recorded in Shimadzu UV 2450 model UV–Vis
spectrophotometer in the range of 800–190 nm equipped with an integrating sphere
and using powdered BaSO4 as a reference. Reflectance spectra were converted to
the absorbance spectra using Kubelka–Munk equation
2
ð1 ꢁ R Þ
1
1
FðR ðkÞÞ ¼
ð2Þ
1
2R
The specific surface areas of the catalysts were determined using a Micromeritics
ASAP 2020 sorption analyzer. The samples were degassed at 423 K for 12 h and
analysis was performed at 77 K with N2 gas as the adsorbate. The Brunauer–
Emmett–Teller (BET) multipoint method least-square fit provided the specific
surface area.
To study the morphology of catalyst, atomic force microscope (AFM) JSPM-
5200TM, JEOL model was used. This AFM uses a silicon tip with a radius of
20 mm and a low-resonance frequency cantilever that has a manufacturer’s spring
constant of 35–65 N m-1. Scans of 1.8 9 1.8 lm were obtained for each sample.
The images were recorded in the noncontact mode. All recordings were made in air
under ambient conditions to produce 2D and 3D images.
For GC analysis, a Perkin-Elmer GC-9000 with a capillary column of DB-5
and flame ionization detector was used. GC/MS analysis was carried out using
Varian 2000 Thermo with the following features: capillary column VF5MS (5%
phenyl–95% methyl polysiloxane), 30-m length, 0.25-mm internal diameter, 0.25-
lm film thickness, temperature of column range from 50 to 280 °C (10 °C min-1
)
and injector temperature 250 °C, attached with mass spectrometer model SSQ
7000.
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K. Selvam et al.
Photocatalytic synthesis of quinaldines
In a typical experimental run, 50 mg Pt–TiO2 was suspended in 25 mL of an
absolute ethanolic solution containing 25 mM of the aniline and irradiated by a
365-nm medium-pressure mercury lamp (Sankyo Denki, Japan; intensity
I = 1.381 9 10-6 Einstein L-1 s-1) after purging with N2 for 30 min. N2
bubbling (flow rate = 6.1 mL s-1) and magnetic stirring of the suspension were
continued throughout the reaction while the temperature was maintained at
30 1 °C. Progress of the reaction was monitored by TLC. Product analysis
was performed by GC-MS analysis and yields reported were estimated on the
basis of GC. The isolation was performed by column chromatography on a
silica gel column by eluting with a co-solvent of hexane and ethyl acetate (volume
ratio: 8:2).
Results and discussion
Catalyst characterization
Pt-loaded TiO2 (1.5 wt% Pt) catalyst was characterized by XRD, BET surface area,
AFM, TEM, and diffuse reflectance spectroscopy (DRS). The XRD pattern of
prepared TiO2 is identical with the standard pattern of anatase (JCPDS 01-078-2486
C), and the rutile lines (01-089-0553 C) are absent. The peaks at 25.43, 37.92,
48.03, 53.97, 55.05, 62.70, 68.80, 70.39, and 75.05° are the diffractions of the (101),
(004), (200), (105), (211), (204), (116), (200), and (215) crystal planes of anatase
TiO2, respectively. This clearly reveals that the TiO2 used for loading is of anatase
phase. The XRD patterns of the prepared Pt–TiO2 samples match that of the
undoped TiO2 and are identical with the JCPDS pattern of anatase (Fig. 1). This
shows that photo deposition of the platinum does not modify the basic crystal
structure of the TiO2 used (tetragonal, a = 0.37845 nm; c = 0.95143 nm, body
centered). The diffraction patterns of the Pt–TiO2 samples do not show XRD peaks
of metallic platinum at 39.76 (111), 67.71 (220), etc. This may be because of either
homogeneous dispersion of discrete platinum deposits in the nanoscale on the
surface of the TiO2 or because of very low platinum content. The average crystalline
size of Pt–TiO2 was calculated using the Scherrer equation. Pt-doped TiO2 has
12.1 nm, which is slightly higher than the size of prepared TiO2 (11.3 nm). Crystals
of Pt-doped TiO2 showed higher size than pure TiO2 at the same calcination
temperature.
In Fig. 2, an AFM 3D image of a dense Pt-loaded TiO2 nanoparticle is shown.
Pt–TiO2 particles are well dispersed. It seems that Pt particles in the TiO2 layer are
either over-coated or sandwiched. A cross-section image through a single particle
reveals almost quite flat and non-spherical shape despite the close packing of the
particles. The target nanoparticle sits about 20 9 20 nm above the surrounding
matrix. The apparent width and height of the Pt-loaded TiO2 nanoparticle are 0.66
and 0.97 nm, respectively.
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Nano Pt–TiO2 for an efficient one-pot photocatalytic synthesis
765
Fig. 1 X-ray diffraction patterns of a TiO2 and b Pt–TiO2
High-resolution TEM images at two different regions give a clear differentiation
of Pt loading in TiO2 crystallites (Fig. 3a, b). Dark spots on TiO2 surface in Fig. 3a,
b indicate the presence of Pt nanocrystals. The particle sizes of Pt–TiO2
nanoparticle have been analyzed in HR-TEM and shown in Fig. 3c. Pt–TiO2
particle sizes are in the range of 5–25 nm with an average particle size of 14.8 nm.
Furthermore, Pt metal particles are randomly dispersed on the TiO2 surface. This
reveals that Pt mainly exists as Pt0, which is in agreement with previous studies
[12]. Further analysis of HR-TEM images reveal that the Pt particles have diameter
in the range of 3–12 nm and they are randomly located on the crystal surface.
Figure 3d, e show the lattice fringes of Pt and TiO2 and SAED pattern of Pt–TiO2.
The distance between the fringes is measured by an analysis software attached on
the instrument. The statistical d-values are 0.225 nm for the Pt particles and
0.351 nm for the TiO2 crystals, indicating that the observed fringes are for the Pt
(111) plane and TiO2 (101) anatase facets, respectively.
The electronic state of Pt in the catalysts was verified by XPS. The XPS survey
spectrum (Fig. 4) of the Pt–TiO2 indicates the peaks of elements Ti, O, C, and Pt
(trace). The carbon peak is attributed to the residual carbon from the sample and
adventitious hydrocarbon from XPS instrument itself. The Ti2p peak at 461 eV,
O1 s peak at 534 eV and Pt4f peak at 70.3 eV reveal the presence of Ti, O, and Pt
elements in Pt–TiO2. High-resolution XPS spectrum (inset Fig. 4) from 60 to 80 eV
confirms the existence of Pt. Figure 4 shows the binding energies of Pt4f7/2
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K. Selvam et al.
Fig. 2 Atomic force microscope images of Pt–TiO2
(70.3 eV) and Pt4f5/2 (74.0 eV) and the splitting of the 4f doublet (3.7 eV). These
binding energies indicate that platinum is present in metallic state.
The N2 adsorption/desorption isotherms of the synthesized Pt-loaded TiO2
(Fig. 5a) exhibited a hysterisis loop, typical of type IV pattern representing the
predominant mesoporous structure according to the classification of IUPAC.
Surface area measurements, made by the BET method, provide the specific surface
areas of Pt–TiO2 and prepared TiO2 as 64.96 and 74 m2 g-1, respectively. The pore
size distribution obtained by the BJH method (Fig. 5b) is narrow and monomodel
indicating the good quality of the mesoporous catalyst. The pore size distribution of
the Pt-loaded TiO2 sample thus confirms the mesoporosity. The average pore radius
˚
of Pt–TiO2 obtained from Fig. 5b is 52.1 A. The single-point adsorption total pore
3
volume of pores less than 1010.720 A radius at P/Po = 0.99 is 0.24 cm g
-1
˚
.
The diffuse reflectance spectrum of 1.5% Pt-doped shows an increased absorption
in the visible region (Fig. 6). The reflectance data reported as F(R) values have been
obtained by application of the Kubelka–Munk algorithm. The band gap of the doped
oxide has been deduced from the Tauc plot of [F(R) hm]1/2 versus photon energy
(Fig. 7) as 3.02 eV. This value is less than the band gap energy of TiO2 (3.2 eV).
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Nano Pt–TiO2 for an efficient one-pot photocatalytic synthesis
767
Fig. 3 High-resolution transmission electron microscope analysis: a and b images of Pt–TiO2, c particle
size distribution of Pt–TiO2, d lattice fringes of Pt–TiO2, and e SAED pattern of Pt–TiO2
Photocatalytic synthesis of quinaldine from anilines
Initially, prepared nano-TiO2 was used for the quinaldine synthesis under different
conditions. Irradiation of aniline and TiO2 in ethanol with 365 nm UV light
produced cyclized product of quinaldine. Neither the irradiation of ethanolic
solution of aniline alone nor the solution of aniline and the catalyst without light
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K. Selvam et al.
Fig. 4 X-ray photoelectron spectra of Pt–TiO2
gave any product. Hence, both light and TiO2 are essential for the formation of
quinaldine. Photocatalytic activity of Pt–TiO2 for the synthesis of quinaldine 2 from
aniline 1 and ethanol was investigated. With prepared TiO2 (Table 1, entry 1), 6-h
photo irradiation was required to achieve 75% consumption of 1, affording 2 in only
circa 50% and 2,3-dimethylindole (15%) in addition to small amounts of other
reduction products (10%). The structure of product was identified by GC/MS. The
percentage yield was determined by comparison with the retention times of
authentic samples and by co-injection with the authentic compounds. The main
product identified was quinaldine. With Pt (1.5%)–TiO2 98% conversion of 1
occurred in 4 h irradiation giving 80% of 2 and 14% of 2,3-dimethylindole
(Table 1, entry 4). This shows that Pt–TiO2 promotes rapid and selective quinaldine
production. The percentage yields of the product with 0.5 (70%) and 1 wt% (74%)
of platinum content are less when compared to 1.5 wt% Pt (Table 1, entries 2 and
3). The same reaction when performed without catalyst for 3 h gave no product.
Efficiency of this 1.5 wt% Pt–TiO2, in the synthesis of quinaldines from aniline, is
compared with the efficiency of Au–TiO2 reported for the same reaction (Table 1,
entry 6) [7]. Among the two catalysts (1.5%) Pt–TiO2 is more efficient as it gives
80% yield in neat ethanol. When the platinum content was increased to 2 wt%, the
product yield decreased to 74.0% (Table 1, entry 5). Hence, 1.5 wt% of platinum
was found to be the optimum level.
As reported earlier [7], the higher efficiency of Pt–TiO2 (Table 1, entry 4) is due
to the trapping of electrons by Pt on the excited TiO2. This electron trapping
enhances the charge separation between electron and hole [13] and the hole
accelerates the oxidation of the alcohol to aldehyde, thus facilitating the rapid
condensation between 1 and the aldehyde. The trapped electrons by the platinum
particles are consumed efficiently by the reduction of H? formed during the
oxidation of the alcohol [14].
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Nano Pt–TiO2 for an efficient one-pot photocatalytic synthesis
769
Fig. 5 (a) Brunauer–Emmett–Teller isotherm of Pt–TiO2. (b) pore size distribution of Pt–TiO2
This process was applied for the synthesis of various substituted quinaldines.
Photoirradiation of alcohol solutions that contained substituted anilines and the
Pt–TiO2 catalyst successfully afforded the corresponding quinaldines (Table 2). In
the case of p-toluidine, the main product was 2,6-dimethylquinoline. m-Toluidine
and o-toluidine produced 2,7-dimethylquinoline (84%) and 2,8-dimethylquinoline
(72%), respectively. Irradiation of 3,5-dimethylaniline and 2,5-dimethylaniline led
to the formation of 2,5,7-trimethylquinoline and 3,5,7-trimethylquinoline. When
p-anisidine was used, 6-methoxyquinaldine was formed in slightly higher yield. In
the case of 4-chloro- and 4-fluoroanilines, the yield of quinaldines was very low.
This is attributable to photoinduced dehalogenation. It is interesting to observe that
p-phenetidine has the conversion efficiency of 98% in the presence of irradiated
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K. Selvam et al.
Fig. 6 Diffuse reflectance spectroscopy spectra of a bare TiO2 and b Pt–TiO2
Fig. 7 Tauc plot of 1.5% Pt-doped TiO2
Pt–TiO2. This similar reaction with Au–TiO2 is reported earlier and the results are
as follows. The percentages of quinaldine formed with Au–TiO2 for o-toluidine,
o-anisidine, 3,5-dimethylaniline 3,5-Dimethoxylaniline, and p-phenetidine are
68,78, 76, 74, and 88 respectively. With all substituted anilines Pt–TiO2 is more
efficient than Au–TiO2. This may be due to the higher work function and
the oxidation potential of Pt than Au.
GC–MS chromatograms recorded at different reaction times of the photocatalytic
conversion of aniline in ethanol are presented (Fig. 8). These chromatograms
reveal the formation of 2-methyl-1,2,3,4-tetrahydroquinoline, 1-phenyl-2-(phenyla-
mino)propan-1-one, 1-phenyl-2-(phenylamino)propan-1-ol, N,N-dimethylaniline,
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Nano Pt–TiO2 for an efficient one-pot photocatalytic synthesis
771
Table 1 Comparison of catalytic efficiencies for the photocatalytic synthesis of quinaldinea
Entry Catalyst
Prepared TiO2
Time (h) Products yield (%)
Byproduct (%) Conversionb (%)
1
2
3
4
5
6
a
6
4
4
4
4
4
Quinaldine (50)
10
8
75.0
89.0
92.0
98.0
92.0
90.0
2,3-Dimethylindole (15)
Quinaldine (70)
Pt (0.5%) TiO2
Pt (1.0%) TiO2
Pt (1.5%) TiO2
Pt (2.0%) TiO2
Au (1.0%) TiO2*
2,3-Dimethylindole (11)
Quinaldine (74)
5
2,3-Dimethylindole (13)
Quinaldine (80)
4
2,3-Dimethylindole (14)
Quinaldine (74)
5
2,3-Dimethylindole (13)
Quinaldine (75)
5
2,3-Dimethylindole (10)
All reactions were performed with a 25 mM alcoholic solution of a reactant 50 mg of catalyst
suspension, I0 = 1.381 9 10-3 Einstein L-1 s-1
b
Remaining unreacted amine
* From [16]
Table 2 Photocatalytic synthesis of various substituted quinaldines with Pt–TiO2a
Run Conditions
Products yield (%)
By product (%) Conversionb
1
Aniline
Quinaldine (80)
4
96
2,3-Dimethylindole (12)
2
o-Toluidine
2,8-Dimethylquinoline (72)
2,7-Dimethylquinoline (84)
2,6-Dimethylquinoline (74)
5-Methoxy-2-methylquinoline (76)
7-Methoxy-2-methylquinoline (78)
6-Methoxy-2-methylquinoline (80)
2,5,7-Trimethylquinoline (79)
3,5,7-Trimethylquinoline (70)
18
12
17
5
90
96
91
81
89
87
82
76
84
98
92
92
3
m-Toluidine
4
p-Toluidine
4
o-Anisidine
5
m-Anisidine
11
7
6
p-Anisidine
7
3,5-Dimethylaniline
2,5-Dimethylaniline
3
8
6
9
3,5-Dimethoxylaniline 5,7-Dimethoxy-2-methylquinoline (79)
5
10
11
12
a
p-Phenetidine
p-Chloroaniline
p-Fluoroaniline
6-Ethoxyquinaldine (91)
7
6-Chloro-2-methylquinoline (23)
6-Fluoro-2-methylquinoline (14)
69
78
All reactions were performed with a 25 mM alcoholic solution of a reactant 50 mg of Pt–TiO2 sus-
pension, I = 1.381 9 10-3 Einstein L-1 s-1, irradiation time = 4 h
b
Remaining unreacted amine
N-hydroxyaniline, 2-methyl-N-phenyl-1,2,3,4-tetrahydroquinolin-4-amine, 2,3-dimethyl-
1H-indole as intermediates during the quinaldine formation.
Based on intermediates obtained during the photoreaction and on the earlier
report on the quinaldine synthesis from nitrobenzene by Hakki et al. [15], formation
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K. Selvam et al.
Fig. 8 GC-MS chromatograms at different reaction times for the photocatalytic conversion of aniline
of quinaldine and 2,3-dimethylindole from aniline in ethanol can be explained by
the same mechanism proposed for Au–TiO2 [7]. The initial step in the reaction
which is necessary for all the reactions is the photocatalytic oxidation of the alcohol
to aldehyde by valance band holes on Pt–TiO2. This is followed by the condensation
of amine and aldehyde leading to a cyclized product. The photocatalytic formation
of aldehyde from alcohol was well established [16].
Analysis of the effect of catalyst concentration revealed that the percentage of
quinaldine increased from 55 to 80% with increasing amount of TiO2 from 0.5 to
1 g L-1 (Fig. 9). As there is a gradual decrease in the conversion efficiency of the
processes above 1 g L-1, the optimum catalyst concentration is 1 g L-1. The
decrease in efficiency at concentrations above 1 g L-1 may be due to the light
scattering by the catalyst particles. It was also found that the increase of aniline
concentration from 15 to 65 mM decreased the formation of quinaldine gradually
from 88 to 49%.
In conclusion, Pt–TiO2 enables efficient quinaldine production under photo
irradiation conditions. Pt–TiO2 is found to be more efficient than Au–TiO2 in the
conversion of aniline and most of the substituted anilines to quinaldines. As this
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Nano Pt–TiO2 for an efficient one-pot photocatalytic synthesis
773
Fig. 9 Effect of the Pt–TiO2 concentration on the photocatalytic conversion of aniline. Reaction
conditions: 25 mM ethanolic solution of a reactant; time of irradiation = 4 h; N2 flow rate = 6.1 mL s-1
I = 1.381 9 10-6 Einstein L-1 s-1
;
process uses cheap and reusable photocatalyst under mild ambient conditions, it has
the potential for a more sustainable quinaldine synthesis. The reaction presented
here may help to develop a new strategy towards the development of photocatalysis-
based organic synthesis.
Acknowledgments The authors thank University Grants Commission, New Delhi for GC-MS facility
provided by under Special Assistance Program. The author (M.S) thanks the Council of Scientific and
Industrial Research (CSIR), New Delhi, for the financial support through research grant No.21 (0799)/10/
EMR-II. One of the authors, K. Selvam is thankful to CSIR, New Delhi, for the award of Senior Research
Fellowship.
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