Aluminium Oxide Supported on SBA-15 Molecular Sieves as Potential Lewis Acid Catalysts for Epoxide Ring Opening Using Aniline

Article abstract of DOI:10.1007/s10562-018-2366-8

Abstract: A series of aluminium oxide (Al₂O₃)-supported SBA-15 molecular sieves were prepared using a one-step wet-impregnation method. Powder X-ray diffraction, nitrogen adsorption/desorption, infrared spectroscopy and ammonia TPD were used to investigate the structures and chemical natures of the surface-bound species. The FT-IR studies of metal-impregnated SBA-15 materials revealed strong covalent interaction of Al₂O₃ on SBA-15 materials with strong Lewis acidic properties, evident from ammonia-TPD studies. The metal oxide-supported SBA-15 catalysts are active for epoxide ring opening with aniline at room temperature, and showed remarkably high stability and selectivity towards mono-alkylated products (about 86%) viz., 1-(phenylamino)propan-2-ol and 2-(phenylamino)propan-1-ol. The catalytic activities remained intact after several recycles. The observed activities and selectivities were compared with other metal oxide-loaded SBA-15 catalysts obtained by similar preparation methods. Graphical Abstract: Aluminium oxide supported SBA-15 molecular sieves were prepared using a one-step wet-impregnation method. The materials showed strong Lewis acidic sites and promising catalytic activity for epoxide ring opening with aniline at room temperature. [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-018-2366-8

Catalysis Letters
https://doi.org/10.1007/s10562-018-2366-8
Aluminium Oxide Supported on SBA-15 Molecular Sieves as Potential
Lewis Acid Catalysts for Epoxide Ring Opening Using Aniline
Rekha Yadav1 · Akhila Muralidhar2 · A. Shamna2 · P. Aghila2 · Lakshmiprasad Gurrala3 · Ayyamperumal Sakthivel2
Received: 8 February 2018 / Accepted: 15 March 2018
© Springer Science+Business Media, LLC, part of Springer Nature 2018
Abstract
A series of aluminium oxide (Al2O3)-supported SBA-15 molecular sieves were prepared using a one-step wet-impregnation
method. Powder X-ray diffraction, nitrogen adsorption/desorption, infrared spectroscopy and ammonia TPD were used to
investigate the structures and chemical natures of the surface-bound species. The FT-IR studies of metal-impregnated SBA-
15 materials revealed strong covalent interaction of Al2O3 on SBA-15 materials with strong Lewis acidic properties, evident
from ammonia-TPD studies. The metal oxide-supported SBA-15 catalysts are active for epoxide ring opening with aniline
at room temperature, and showed remarkably high stability and selectivity towards mono-alkylated products (about 86%)
viz., 1-(phenylamino)propan-2-ol and 2-(phenylamino)propan-1-ol. The catalytic activities remained intact after several
recycles. The observed activities and selectivities were compared with other metal oxide-loaded SBA-15 catalysts obtained
by similar preparation methods.
Graphical Abstract
Aluminium oxide supported SBA-15 molecular sieves were prepared using a one-step wet-impregnation method. The
materials showed strong Lewis acidic sites and promising catalytic activity for epoxide ring opening with aniline at room
temperature.
Keywords Epoxide ring opening · Kinetics · β-Amino alcohol · Al2O3-SBA-15 · Molecular sieves
Akhila Muralidhar and A. Shamna have equally contributed to this
work.
Extended author information available on the last page of the article
Vol.:(0123456789)
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R. Yadav et al.
1 Introduction
2 Experimental
Catalysts play an important role in day-to-day life by contribut-
ing to various chemical and environmental processes [1, 2]. In
particular, heterogeneous catalysts e.g. zeolite and zeolite-like
molecular sieves are extensively used in various petrochemical
processes such as fluidised catalytic cracking (FCC) [3], refin-
ing, Friedel–Crafts alkylation, hydroisomerisation, methanol
to gasoline, polymerisation, condensation, etc. [3–6]. How-
ever, the small pore size of zeolites restricts their use for bulky
molecular transformations. The pore size constraints of zeo-
lite materials were addressed by development of mesoporous
molecular sieves possessing wider pore diameters in the range
of 2–50 nm, high surface area (1000–1500 m2 g‒1), uniform
framework channels, excellent sorption capacity (1 cm3 g‒1),
etc. [7–9]. In particular, a substantial amount of work has
focused on the Santa Barbara Amorphous (SBA-15) materials
developed by Stucky et al. [9], due to their unique character-
istic features, including excellent thermal and hydrothermal
stability, thick pore walls, highly ordered structural regular-
ity and simple synthesis method and reproducibility [10–12].
The pure silica-based SBA-15 framework, however, lacks
surface acidity, basicity or redox properties, which limits its
catalytic applications. The introduction of hetero-elements into
the guest framework of SBA-15 resulted in a wide variety of
applications, including catalysis, absorbance and potential
support [13–15]. In particular, the introduction of a metal
oxide such as aluminium oxide (Al2O3) into the framework
by a post-synthesis method facilitates the generation of Lewis
acidic sites, which have shown potential acid catalysis in vari-
ous reactions [12, 16, 17].
2.1 Synthesis of Al‑SBA‑15
The SBA-15 was synthesized as per literature [11, 24]
described elsewhere with molar gel composition 6.89×10−4
P123 triblock copolymers: 0.24 HCl:0.041 TEOS:7.88
H2O. Aluminum oxide is introduced into SBA-15 using wet
impregnation method [25]. Calculated amount of Al(NO3)3
required to get 5, 10 and 20 wt% of Al2O3 on SBA-15 sup-
port (1.25 g) was introduced and the resultant materials was
calcined at 550 °C for 6 h in an air oven. The sample pre-
pared with 5, 10 and 20 wt% Al2O3 were represented as
Al-SBA-15-5, Al-SBA-15-10, and Al-SBA-15-20 respec-
tively. Similarly, 20 wt% of iron oxide and zinc oxide were
loaded using Fe(NO3)3 as iron precursor and Zn(NO3)3 as
zinc precursor and are represent as Fe-SBA-15-20 and Zn-
SBA-15-20 respectively [26].
2.2 Characterization
All the samples were systematically characterized using ana-
lytical and spectroscopic techniques. The wide angle powder
X-ray diffraction patterns were obtained with a Rigaku Mini
flex 600 diffractometer using nickel-filtered Cu-Kα radiation
(λ = 1.54184 Å) and a liquid nitrogen cooled germanium
solid-state detector. The diffracto-grams were recorded in
the 2θ ranges of 5°–60° with a scan speed and step size of
4°/min and 0.02 respectively. While low angle powder X-ray
diffraction patterns of different samples were recorded using
18 kW XRD Rigaku (2500 V), Japan with Cu-Kα radiation
(λ=1.54184 Å) in the 2θ ranges of 0.5°–6° with scan speed
and step size of 0.5° and 0.02 min−1 respectively.
At this junction, it is important to note that epoxides are
widely utilised as synthetic intermediates and are considered
as ‘Spring-loaded’ rings for nucleophilic ring opening result-
ing in versatile intermediates in organic synthesis [18–23]. The
epoxide ring opening yields β-substituted alcohols, which have
tremendous application as organic intermediates [23, 24]. It
will be interesting to develop a versatile heterogeneous catalyst
for such processes. Thus, the present work focuses on the prep-
aration of aluminium oxide supported on SBA-15 and utilises
it for β-amino alcohol formation by epoxide ring opening with
aniline under ambient reaction conditions. For comparison,
ZnO and Fe2O3 loaded SBA-15 were prepared using similar
processes and their catalytic activities were compared under
identical conditions.
FT-IR spectra were obtained using Perkin-Elmer FT-IR
SPECTRUM TWO in the range 400–4000 cm−1 using KBr
pellets. The textural properties (BET surface area, micropore
area (t-plot method), BJH average pore volume etc.) of the
samples were derived from N2 adsorption–desorption meas-
urements carried out at − 196 °C using automatic micropore
physisorption analyser (Micromeritics 2020) after the sam-
ples were degassed at 150 °C for at least 10 h prior to each
run. Thermogravimetry-analysis (TGA) was carried out in
a thermal analyser (Perkin-Elmer STA6000) in air. About
0.5–0.6 mg of the sample was taken in the alumina pan and
heated in air in temperature range of 50–800 °C at the rate
of 20 °C min−1.
2.3 Catalytic Studies
The different metal oxide loaded SBA-15 were studied for
propylene oxide ring opening using aniline (Scheme 1). In a
1 3

Aluminium Oxide Supported on SBA-15 Molecular Sieves as Potential Lewis Acid Catalysts for…
NH2
OH
H
N
H
N
O
Al/Zn/Fe-SBA-15
OH
Di-alkylated products
Scheme 1 Schematic representation of ring opening of propylene oxide with aniline using metal-oxide loaded SBA-15 catalysts
(P6mm) of the SBA-15 material [27]. The intense peak cor-
responding to plane (100) (2θ of 0.67) indicates the long-
range ordering. The calcined SBA-15 impregnated with
20 wt% metal oxides showed much broader peaks centred
at 2θ of 0.89, 0.91 and 1.0 for Al, Zn and Fe respectively
(Fig. 1). The prominent similarity in the X-ray diffrac-
tion peaks suggests the retention of hexagonal structure
even after a large amount of metal oxide introduction. The
broadening in the peaks owing to the introduction of large
amounts of bulk oxide inside the mesoporous channels
results in shift of X-ray reflection towards the higher angles.
The higher-angle XRD patterns of all the samples showed a
broad peak ranging from 2θ=15°–30° due to the presence
of amorphous silica walls, and the absence of any peaks
corresponding to metal oxides (Fig. 2A) [17, 29]. The above
observation clearly indicates that even at very high loading
(20 wt%) there is no X-ray reflection for Al2O3, suggesting
that metal oxides are uniformly distributed in the SBA-15
channels. Similarly, no segregation of metal oxides on the
sample prepared using zinc oxide and/or iron oxide-loaded
samples (Fig. 2B) suggests well dispersed metal oxides
on the SBA-15 surface.
Fig. 1 Low-angle powder XRD of (a) Al-SBA-15-20, (b) Zn-
SBA-15-20, and (c) Fe-SBA-15-20
typical reaction, aniline and propylene oxide (10 mmol each)
were taken in a round bottom flask with 60 mg of catalyst
under constant stirring without solvent for one hour at 45 °C.
The reaction was also studied in different experimental con-
ditions including temperature, reactant ratios, different epox-
ide as substrate. After reaction, the products were extracted
with calculated amount of propanol and analysed using gas
chromatography (Agilent 7890A series) equipped with HP-5
capillary column.
FT-IR spectra of Al2O3 loaded SBA-15 and different
metal-loaded SBA-15 samples were recorded in the infra-
red region from 400 to 4000 cm−1 (Fig. 3). The broad band
centred around 3400 cm−1 in all materials was due to the
stretching vibration of surface hydroxyl group [10]. The
presence of C–H bond stretching was confirmed by peaks
appearing around 2900 cm−1, which are characteristic of the
polymer hydrocarbon [30, 31]. All these peaks disappeared
after calcination (Fig. 3B), indicating the complete removal
of template. The as-synthesised Al-SBA-15 samples showed
(Fig. 3A) a sharp peak around 1300 cm−1 corresponding to
nitrate species derived from the aluminium nitrate precur-
sor. The three typical Si–O–Si peaks centred at 1100 cm−1
(asymmetric stretching), 800 cm−1 (symmetric stretching),
and 450 cm−1 (bending) were assigned to the condensed
silica network. The vibrational bands appearing around 1087
and 1096 cm−1 in both as-prepared and calcined sample cor-
respond to as-symmetric stretching of Si–O–Si and Si–O–Al
respectively. For the metal oxide-loaded materials, the band
3 Result and Discussion
The low-angle XRD patterns of calcined parent SBA-15
(Fig. 1) and different metal-loaded SBA-15 materials are
displayed in Fig. 1. The low-angle XRD pattern of calcined
SBA-15 exhibits three well resolved peaks at 2θ of 0.67°,
1.3° and 1.5°, indexed to diffraction planes (100), (110) and
(200), respectively (not shown here) [27, 28]. The result-
ing Bragg reflections confirmed the hexagonal symmetry
1 3

R. Yadav et al.
Fig. 2 Wider-angle A powder XRD pattern of (a) Al-SBA-15-5, (b) Al-SBA-15-10, (c) Al-SBA-15-20 and B powder XRD pattern of (a) Al-
SBA-15-20, (b) Zn-SBA-15-20, (c) Fe-SBA-15-20
Fig. 3 FT-IR spectra for calcined A (a) SBA-15, Al2O3 loaded SBA-15 materials (b) Al-SBA-15-5, (c) Al-SBA-15-10, (d) Al-SBA-15-20 and B
different metal oxide loaded SBA-15 (a) Al-SBA-15-20, (b) Zn-SBA-15-20, (c) Fe-SBA-15-20
at 960 cm−1 is ascribed to Si–O–X (X = Al, Zn and Fe)
stretching vibration (Fig. 3), indicating metal oxide cova-
lently incorporated into the SBA-15 framework [26, 32, 33].
The textural properties of different metal oxide-loaded
SBA-15 samples measured by N2 sorption and the cor-
responding isotherms are displayed in Fig. 4. Figure 4A
depicts the N2 adsorption desorption isotherm of SBA-15
and Al2O3-supported SBA-15 materials. The BET surface
area of the parent SBA-15 was found to be in the range of
769 m2 g−1 with a pore volume of 1.1 cm3 g−1. A well-
defined adsorption step at relative high pressure of 0.6–0.8
was observed, corresponding to capillary condensation of
N2, indicating uniformity of the pores. According to the
IUPAC classification, all isotherms are of type IV and
exhibit a hysteresis loop, typical of large-pore mesoporous
solids. The isotherms of different metal loaded SBA-15
feature well-defined hysteresis loops [34–36]. The specific
structural parameters are given in Table 1. The BET surface
area of the modified samples decreased compared with that
of the calcined SBA-15, which suggested the incorporation
of Al2O3 particles into the pore channels of SBA-15. Simi-
larly, Fig. 4B depicts the N2 adsorption desorption isotherm
of different metal oxide supported SBA-15 materials. All
isotherms are of type IV and exhibit a hysteresis loop indi-
cating the presence of uniform mesopores.
Figure 5A displays the pore size distribution of SBA-15
and Al2O3-loaded SBA-15 materials. The parent SBA-15
shows a uniform distribution of pore size in the range of
1 3

Aluminium Oxide Supported on SBA-15 Molecular Sieves as Potential Lewis Acid Catalysts for…
Fig. 4 N2 adsorption isotherm A (a) SBA-15, Al2O3 loaded SBA-15 materials (b) Al-SBA-15-5, (c) Al-SBA-15-10, (d) Al-SBA-15-20 and B (a)
SBA-15, different metal oxide-loaded SBA-15 (b) Al-SBA-15-20, (c) Zn-SBA-15-20, (d) Fe-SBA-15-20
5.3 nm, whereas Al-SBA-15 shows a broad distribution of
pore size centred at 5.2 nm, possessing inter-particle mes-
oporosity derived from the impregnated Al2O3 present in
the Al-SBA-15 samples. The pore size distribution displays
a bimodal distribution typical of cage-type mesoporous
structures. Compared to calcined SBA-15, a decrease in
the primary mesoporous cage structure diameter (5.2 nm)
is noted for metal oxide-loaded samples. Similarly, Fig. 5B
shows the pore size distribution for different metal oxides. A
sharp decrease in surface area and pore volume is observed
for different metal oxide-loaded materials, with pore sizes
of 5.2, 5.0 and 5.2 nm for Al-SBA-15-20, Zn-SBA-15-20
and Fe-SBA-15-20 respectively compared to parent calcined
SBA-15 material [12].
Table 1 Textural properties of Al2O3 loaded SBA-15 materials
Catalyst
Surface
area
Acidity (mmol)
Pore volume (cm3/g)
(m2/g)
BET
Moderate Strong t-plot
micropore
BJH
SBA-15
Al-SBA-15-5 364
769
–
1.05
–
–
0.52
–
0.018
0.015
0.026
1.100
0.792
0.562
Al-
SBA-15-10
Al-
SBA-15-20
Fe-
SBA-15-20
Zn-
SBA-15-20
386
355
431
406
1.58
0.33
1.11
4.92
2.45
2.50
0.015
0.598
0.021
0.762
0.047
0.587
The surface acidity of different metal oxide-loaded SBA-
15 (Fig. 6) was qualitatively measured using ammonia
Fig. 5 Pore size distribution of materials A (a) SBA-15, Al2O3 loaded SBA-15 materials (b) Al-SBA-15-5, (c) Al-SBA-15-10, (d) Al-SBA-15-20
and B (a) SBA-15, different metal oxide loaded SBA-15 (b) Al-SBA-15-20, (c) Zn-SBA-15-20, (d) Fe-SBA-15-20
1 3

R. Yadav et al.
Fig. 6 TPD of A Al-SBA-15-5, B Al-SBA-15-20, C Zn-SBA-15-20 and D Fe-SBA-15-20
TPD. The NH3-TPD profile for all catalysts shows two
distinct broad peaks in the temperature range of 100–400
and 400–800 °C, which are assigned to weak and to mod-
erate-to-strong acidic sites respectively [37]. The samples
Al-SBA-15-5 and Al-SBA-15-20 possess similar types of
peaks; however, the areas under the curves are different.
Compared to other metal oxides, Al2O3-impregnated sam-
ples possess greater numbers of relatively weak acidic sites.
The number of moderate acidic sites follows in the order
Al-SBA-15-20>Zn-SBA-15-20>Fe-SBA-15. Similarly, the
number of moderate-to-strong acidic sites follows the same
order i.e. Al-SBA-15-20>Zn-SBA-15-20>Fe-SBA-15-20.
Hence, in comparison to other materials, Al-SBA-15-20
exhibits a greater number of weak, as well as moderate-to-
strong, acidic sites.
is carried out at high temperatures using conventional
acid–base catalysts. Hence, it becomes difficult to synthesise
β-amino alcohols from low-boiling epoxides. In the present
work, different metal oxide-impregnated materials were used
for epoxide ring opening at lower temperatures [19, 20]. In
all cases, mono-alkyl derivatives of aniline–1-(phenylamino)
propan-2-ol and 2-(phenylamino)propan-1-ol were obtained
as major products. Figure 7 shows the aniline conversion
using Al-SBA-15-5 catalyst at different temperatures. The
conversion of aniline increases with increase in temperature,
with increased selectivity towards the di-alkylated product;
however, above 45 °C only a slight increase in the conver-
sion was evident. Thus, for further studies, all the reactions
were carried out at 45 °C. The maximum conversion evident
at 45 °C was 69.5, with 85.9 and 14% selectivity towards
mono-alkylated and di-alkylated products, respectively.
The concentration of aniline to propylene oxide was
varied at 45 °C to study the effect of the ratio. The cata-
lytic activity of Al-SBA-15-5 with varying ratio (aniline/
propylene oxide) is displayed in Fig. 8. It is evident that
the use of lower propylene oxide concentration results in
moderate conversion, with exclusive formation of mono-
alkylated product. With an increase in the aniline/propylene
4 Catalytic Studies
Metal-loaded SBA-15 catalyst was used for epoxide ring
opening using amine for the preparation of β-amino alco-
hols (Scheme 1). The products are important intermediates
in fine-chemical industries. Normally epoxide ring opening
1 3

Aluminium Oxide Supported on SBA-15 Molecular Sieves as Potential Lewis Acid Catalysts for…
Table 2 Effect of different amount of metal loading on cal-SBA-15
Catalyst
Conversion Mono-alkylated Di-alkylated
Without catalyst
Cal-SBA-15
Al-SBA-15-5
Al-SBA-15-10
Al-SBA-15-20
Fe-SBA-15-20
Zn-SBA-15-20
2.4
33.4
69.5
58.8
48.1
38.6
51.0
100
100
–
–
86.0
91.2
92.9
93.9
94.9
86.2
14.0
8.8
7.1
6.1
5.1
13.8
Recycle Al-SBA-15-5 65.3
Reaction conditions: aniline:propylene oxide=1:1 (10 mmol each),
catalyst=60 mg, T=45 °C, t=1 h
Table 2 summarises the catalytic activities of Al2O3, ZnO
and Fe2O3 loaded SBA-15 materials studied at 45 °C with an
aniline/propylene ratio of 1:1. Without catalyst, only 2.40%
aniline conversion was observed, which increased to about
33.4% with parent calcined SBA-15. The pure SBA-15 hav-
ing surface hydroxyl group which have weak acidity thus
leads to considerable conversion with exclusive formation of
mono-N-alkylated products. The introduction of Al2O3, ZnO
and Fe2O3 facilitates to generate moderate-to-strong acidic
sites (Fig. 6), thus facilitate improved conversion with the
formation of mono-alkylated and considerable di-alkylated
products. The conversion level decreased with an increase
in Al2O3 concentration, and a maximum was reached with
Al-SBA-15-5 (about 69.5%). The decrease in the conver-
sion with increase in the Al content might be due to hin-
dering of the active sites of the catalyst and to the greater
number of moderate-to-strong acidic sites. The reaction was
further carried out on different metal-loaded SBA-15 materi-
als. The conversion followed the order Al-SBA-15-5>Al-
SBA-15-10 > Al-SBA-15-20 ≈ Zn-SBA-15-20 > Fe-
SBA-15-20. The higher conversion observed in case of
Al-SBA-15-5 might be due to presence of more number of
moderate acidic site which facilitates better conversion than
other catalyst. The comparable conversion obtained on Al-
SBA-15-20 and Zn-SBA-15-20 may be due to presence of
strong acidic sites along with moderate acidic sites and thus
facilitate secondary reaction. The observed low conversion
on Fe-SBA-15-20 might be due to the presence of less mod-
erate acidic sites. The presence of relatively strong acidic
sites on Al-SBA-15-20 and Zn-SBA-15-20 facilitates the
secondary reaction and thus favour considerable amount of
dialkylated product formations. After the catalytic reaction,
the catalyst was washed with methanol, dried and reused
under optimum reaction conditions. It was evident that the
catalytic activity remains intact even after the recycling exer-
cise, with a slight decrease in conversion.
Fig. 7 Effect of reaction temperature on aniline conversion using Al-
SBA-15-5 catalyst. Reaction conditions: aniline: propylene oxide =
1:1 (10 mmol each), catalyst = 60 mg, t = 1 h
Fig. 8 Effect of change in ratio (aniline/propylene oxide) over Al-
SBA-15-5 catalyst. Reaction conditions: catalyst=60 mg, T=45 °C,
t=1 h
oxide ratio from 1:0.50 to 1:0.75, enhanced catalytic con-
version was observed, which increased further at 1:1 ratio
and reached a maximum of 69.48%, with 14.02% selectivity
towards di-alkylated products. Further increase in the ratio
to 1:1.25 resulted in a decrease in conversion (64.63%) with
a slight increase in the selectivity towards di-alkylated prod-
ucts. The decrease in the conversion with increase in the
propylene oxide might be due to the adsorption of propylene
oxide onto the active sites of the catalysts [20]. Hence, for
the further studies, a 1:1 ratio of aniline/propylene oxide
was used.
To further examine the catalytic strength of the Al-
SBA-15-5 catalyst, different epoxides were used, and
1 3

R. Yadav et al.
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Cyclohexene oxide
Epi-chloro-hydrine 33.2
1,2-Epoxyoctane 33.2
69.5
58.1
86.0
100.0
95.9
14.0
–
4.1
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This is the first time that aluminium oxide loaded SBA-15
materials have been applied to epoxide ring opening using
aniline under moderate reaction conditions. The FT-IR
studies revealed the covalent interaction of Al2O3 on SBA-
15 materials, which resulted in strong Lewis acidic sites,
evident from ammonia-TPD studies. The aluminium oxide
loaded onto SBA-15 materials showed a decrease in X-ray
crystallinity, surface area and pore volume, supporting
the aluminium oxide being impregnated in the channels
of mesoporous SBA-15 molecular sieves. The absence of
sharp peaks in high-angle XRD evidenced no segregation
of Al2O3. The catalyst showed excellent conversion, with
selective formation of mono-alkyated product. The catalyst
yielded best conversion within one minute, and the catalytic
activity remained constant after recycling.
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Aluminium Oxide Supported on SBA-15 Molecular Sieves as Potential Lewis Acid Catalysts for…
Affiliations
Rekha Yadav1 · Akhila Muralidhar2 · A. Shamna2 · P. Aghila2 · Lakshmiprasad Gurrala3 · Ayyamperumal Sakthivel2
3
* Ayyamperumal Sakthivel
Catalysis and Inorganic Chemistry Division, Council
of Scientific and Industrial Research-National Chemical
Laboratory (CSIR-NCL), Dr. Homi Bhabha Road,
Pune 411008, India
sakthivelcuk@cukerala.ac.in
1
Department of Chemistry, Inorganic Materials & Catalysis
Laboratory, University of Delhi, Delhi 110007, India
2
Department of Chemistry, School of Physical Sciences,
Central University of Kerala, Riverside Transit Campus,
Kasaragod, Kerala 671314, India
1 3