CsxH3-xPW12O40 nanoparticles dispersed in the porous network of Zr-pillared Α-zirconium phosphate as efficient heterogeneous catalyst for synthesis of spirooxindoles

Article abstract of DOI:10.1016/j.mcat.2017.12.013

Layered α-zirconium phosphate (α-ZrP) was prepared by reflux method using ZrOCl_2.8H₂O and H₃PO₄ as precursor. The α-ZrP material was pillared with Zr-oxyhydroxy nanoclusters to prepare Zr-pillared α-zirconium phosphate (ZZP). An enhancement in interlayer spacing, surface area and porosity was noticed as a result of Zr-pillaring. The cesium exchanged phosphotungstic acid (Cs_xH_3-xPW₁₂O₄₀) nanoparticles were dispersed in the porous matrix of ZZP material to prepare Cs_xH_3-xPW₁₂O₄₀-ZZP nanocomposite systems. The nanocomposites were characterized using XRD, FTIR, UV–vis-DRS, TGA-DTA, XPS, N₂ sorption, TPD, FESEM and HRTEM techniques. The expansion in layer structure of α-ZrP upon pillaring and its subsequent retention in the composite material was noticed from XRD. UV–vis and FTIR study indicated structural integrity of the Cs_xH_3-xPW₁₂O₄₀ nanoclusters in the ZZP interlayer. The surface area of the composite materials was in the range of 80–130 m² g^?1. The composite materials contained significantly higher amount of medium and strong acidic sites compared to the ZP and ZZP materials. Microscopic study suggested the presence of hierarchical nanospheres with diameter between 150 and 200 nm. The presence of well dispersed Cs_xH_3-xPW₁₂O₄₀ nanoparticles with size between 8 and 15 nm was confirmed from HRTEM study. The nanocomposite materials were used as efficient heterogeneous catalyst for synthesis of spirooxindoles by multicomponent one pot condensation of isatin, malononitrile, naphthol/1,3-diketones. Structurally diverse spirooxindole derivatives were synthesized in high yield and purity in short span of time using the Cs_xH_3-xPW₁₂O₄₀-ZZP nanocomposites as catalyst under mild conditions.

Full text of DOI:10.1016/j.mcat.2017.12.013

Molecular Catalysis 446 (2018) 58–71
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
CsxH3-xPW12O40 nanoparticles dispersed in the porous network of
Zr-pillared ␣-zirconium phosphate as efficient heterogeneous catalyst
for synthesis of spirooxindoles
Sagnika Pradhan, B.G. Mishra∗
Department of Chemistry, National Institute of Technology, Rourkela 769008, Odisha, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 19 June 2017
Received in revised form 8 December 2017
Accepted 12 December 2017
Layered ␣-zirconium phosphate (␣-ZrP) was prepared by reflux method using ZrOCl2.8H2O and H3PO4
as precursor. The ␣-ZrP material was pillared with Zr-oxyhydroxy nanoclusters to prepare Zr-pillared ␣-
zirconium phosphate (ZZP). An enhancement in interlayer spacing, surface area and porosity was noticed
as a result of Zr-pillaring. The cesium exchanged phosphotungstic acid (CsxH3-xPW12O40) nanoparticles
were dispersed in the porous matrix of ZZP material to prepare CsxH3-xPW12O40-ZZP nanocomposite
systems. The nanocomposites were characterized using XRD, FTIR, UV–vis-DRS, TGA-DTA, XPS, N2 sorp-
tion, TPD, FESEM and HRTEM techniques. The expansion in layer structure of ␣-ZrP upon pillaring and
its subsequent retention in the composite material was noticed from XRD. UV–vis and FTIR study indi-
cated structural integrity of the CsxH3-xPW12O40 nanoclusters in the ZZP interlayer. The surface area of
the composite materials was in the range of 80–130 m2 g−1. The composite materials contained sig-
nificantly higher amount of medium and strong acidic sites compared to the ZP and ZZP materials.
Microscopic study suggested the presence of hierarchical nanospheres with diameter between 150 and
200 nm. The presence of well dispersed CsxH3-xPW12O40 nanoparticles with size between 8 and 15 nm
was confirmed from HRTEM study. The nanocomposite materials were used as efficient heterogeneous
catalyst for synthesis of spirooxindoles by multicomponent one pot condensation of isatin, malonon-
itrile, naphthol/1,3-diketones. Structurally diverse spirooxindole derivatives were synthesized in high
yield and purity in short span of time using the CsxH3-xPW12O40-ZZP nanocomposites as catalyst under
Keywords:
Phosphotungstic acid
␣-ZrP
Spirooxindole
UV–vis-DRS
HRTEM
mild conditions.
© 2017 Elsevier B.V. All rights reserved.
1. Introduction
als by modification of the interlayer space and layer composition of
the ZrP material. Several organomodified ZrP materials have been
Zirconium phosphates (ZrP) are a class of versatile materials
used in many advanced applications such as proton conducting
membrane, ion exchanger, solid state gas sensor, adsorbent and
as substrate for immobilization of biological materials [1–3]. These
materials possess surface acidic functionality which are water tol-
erant, thermally stable and hence are suitable for application in
acid catalyzed organic transformations [4]. Among different struc-
tural forms of zirconium phosphates, ␣-zirconium phosphate is the
most widely investigated material in catalysis [1,2]. In recent years,
efforts have been devoted to prepare engineered catalytic materi-
reported in literature and studied for selective drug delivery and
catalysis [1,3]. The other important characteristics of ZrP material
are the exfoliation of the layers in polar solvents and presence
of exchangeable interlayer protons. These properties have been
exploited to ion exchange the protons with surfactant molecules,
inorganic cationic nanoclusters as well as bulky organic moieties
to form expanded structures with porosity and high surface area
[2,5–10]. The pillared ␣-ZrP materials prepared by intercalation of
inorganic cationic clusters of Al, Ti, Cr, Fe-Cr, Zr and Si into the
interlayer space are a class of promising materials for catalysis
[8–17]. The intercalation of these inorganic nanoclusters imparts
structural rigidity and significantly improves the surface area and
pore volume of ␣-ZrP. The pillared materials also exhibit enhanced
surface acidity which is reflected in their higher catalytic activity
∗
Corresponding author.
E-mail addresses: brajam@nitrkl.ac.in, bgmcat@rediffmail.com (B.G. Mishra).
https://doi.org/10.1016/j.mcat.2017.12.013
2468-8231/© 2017 Elsevier B.V. All rights reserved.

S. Pradhan, B.G. Mishra / Molecular Catalysis 446 (2018) 58–71
59
for acid catalyzed reactions [8–15]. In addition to their application
as heterogeneous acid catalyst, the pillared ␣-ZrP materials due
to their high surface area and uniform porosity have been used
as support for catalytically active phases. For example, the alu-
mina pillared ␣-ZrP material has been used an efficient support
for the Ni-Mo hydrodesulfurization catalyst [16] and Cu catalyst
for reduction of NO with propane [17]. Although there are reports
on the catalytic application of the pillared ␣-ZrP material, there is
no study dealing with the catalytic application of these materials
for synthesis of biologically important molecules. In this work, we
have used the porous matrix of Zr-pillared ␣-ZrP material as carrier
for catalytically active CsxH3-xPW12O40 nanoparticles and studied
their catalytic application for synthesis of biologically important
spirooxindoles.
The metal insoluble salts of heteropoly acids (HPAs) obtained
by partially exchanging the labile protons of HPA with large mono-
valent ions such as Cs+, NH4+, K+, Ag+ are attractive materials
for application in heterogeneous catalysis [18–23]. These mate-
rials possess improved physicochemical characteristics such as
high surface area, evolved porosity and enhanced acidity com-
pared to the parent HPA. In recent years, the Cs+ exchanged
phosphotungstic acid (CsxH3-xPW12O40) has been studied widely
as a promising heterogeneous catalyst for acid catalyzed reactions.
The CsxH3-xPW12O40 material exhibits superior catalytic activ-
ity for Beckmann rearrangement, alkylation/acylation reactions,
hydration of olefins, hydrocracking of extra-heavy oil, transesterifi-
cation reactions, esterification of fatty acids and biodiesel synthesis
[19–24]. These materials are known to exhibit composition depen-
dent surface properties. Particularly, the CsxH3-xPW12O40 material
with x = 2.0–2.5, display higher catalytic activity than the parent
H3PW12O40 [20,21]. The catalytic utility of CsxH3-xPW12O40 mate-
rials has been further enhanced by dispersing them on high surface
area support [24–27]. The CsxH3-xPW12O40 material dispersed over
catalytically active functional supports such as ZrO2, SnO2 have
been studied recently for unsymmetrical ether synthesis and car-
bonolysis and etherification of glycerol [25–27]. A synergistic effect
between the acidic sites of the support and the active phase has
been invoked to explain the higher catalytic activity of the sup-
ported system.
Indole has been widely discovered as a pharmacophore, which
represents many natural isolates of diverse biological activity [28].
In the past, several natural products containing indole nucleus
particularly 3-substituted indoles and spirooxindoles have been
isolated [28,29]. The heterocyclic spirooxindole ring is found in a
number of pharmaceutical compounds including horsfiline, alston-
isine, coerulescine, spirotryprostatins A and elacomine [30]. It
has also been observed that spirooxindole compounds containing
substituted fused 4H-chromenes display a wide range of useful
biological properties, including spasmolitic, diuretic, anticoagu-
lant, anticancer, and antianaphylactic activities [31–33]. In view
of the therapeutic properties and biological relevance, the syn-
thesis of these compounds has received considerable attention
from synthetic chemists. The most effective route for synthesis
of spirochromene derivatives involves the catalytic multicompo-
nent condensation of isatin, active methylene compound, and
1,3-dicarbonyl compounds [34]. Several catalytic protocols have
been developed in the recent past for the synthesis of these bio-
logically important compounds which include the use of InCl3,
sodium stearate, sulfamic acid, triethylbenzylammonium chloride
(TEBA), Fe3O4@SiO2-imid-PMA, amino-functionalized SBA-15, ZnS
nanoparticles, nano MgO and guanidine functionalized magnetic
Fe3O4 nanoparticles as catalyst [33–40]. However, many proto-
cols use homogeneous catalysts at elevated temperatures giving
moderate to good yield of the product. Although the investi-
gated heterogeneous catalytic protocols offer advantages in term of
reusability of the catalyst, some protocols involve complex catalytic
system requiring multiple steps and costly chemicals for their syn-
thesis. Keeping in mind the merits and demerits of the developed
methods; we feel that there is scope to develop novel heteroge-
neous catalytic protocol for synthesis of spirooxindole under mild
conditions. In this work, we have used CsxH3-xPW12O40 nanoparti-
cles dispersed in the porous matrix of Zr-pillared ␣-ZrP material as
efficient catalyst for synthesis of structurally diverse spirooxindole
derivatives.
2. Materials and methods
Zirconyl chloride (ZrOCl2·8H2O), phosphoric acid (H3PO4),
phosphotungstic acid (H3PW12O40) and cesium carbonate (Cs2CO3)
were procured from Hi media Chemicals Pvt. Ltd., India. All chem-
icals used in this study were of AR grade (>99.9% purity) which
was used directly in the experiments without further purification.
Deionized water prepared in the laboratory was used for material
synthesis.
2.1. Preparation of ˛-zirconium phosphate (ZP)
The ␣-zirconium phosphate material was prepared using
the procedure described in the literature [7,8]. Briefly, 10 g of
ZrOCl2·8H2O was added to 100 ml of 12 M H3PO4 solution and
refluxed at 100 ◦C for 24 h. The obtained white precipitate was fil-
tered, washed repeatedly with hot water to remove Cl− ions and
centrifuged at 5000 rpm. The solid material was subsequently dried
in a hot air oven at 90 ◦C for 24 h to obtain the ZP material.
2.2. Preparation of Zr-pillared ˛-zirconium phosphate (ZZP)
2.2.1. Preparation of Zr-pillaring solution
A 0.1 M ZrOCl2·8H2O solution was prepared by dissolving the
required amount of the salt in deionized water. The solution was
subsequently refluxed for 24 h to prepare the pillaring solution.
2.2.2. Pillaring process
2 g of ZP was dispersed in 200 ml of 0.1 M n-butylamine solu-
tion and stirred for 24 h to prepare a well dispersed exfoliated
colloidal suspension. Required amount of Zr-pillaring solution
(ZP:ZrOCl2·8H20 molar ratio 1:30) was then added dropwise
(50 ml h−1) to the colloidal suspension and refluxed at 100 ◦C for
24 h under constant stirring. After completion of the intercala-
tion process, the solid material was recovered by centrifugation
at 5000 rpm and washed repeated with hot water to remove the
amine and chloride ions. The solid material was subsequently dried
in a hot air oven at 80 ◦C for 12 h and calcined at 350 ◦C for 2 h to
obtain the ZZP catalyst.
2.3. Preparation of CsxH3-xPW12O40 dispersed over Zr-pillared
˛-zirconium phosphate (CPxZZP)
The 20 wt% CsxH3-xPW12O40 supported over ZZP material was
prepared by ion exchange of H+ with Cs+ ions followed by wet
impregnation. In a typical procedure, 2 g of ZZP material was dis-
persed in 20 ml water by sonication for 20 min. To this aqueous
dispersion, 0.4 g of H3PW12O40 was added and stirred for 2 h. Sto-
ichiometric amount of 0.5 mmol Cs2CO3 solution was then added
dropwise to this suspension and stirred for 6 h at room temper-
ature. The temperature was then raised to 90 ◦C and was heated
continuously under stirring to remove the excess water. The result-
ing material was dried in air at 120 ◦C for 6 h followed by calcination
at 350 ◦C for 2 h to obtain the CsxH3-xPW12O40/ZZP material. Using
this procedure 20 wt% Cs1H2PW12O40 and Cs2H1PW12O40 sup-
ported over ZZP material were prepared. These supported materials

60
S. Pradhan, B.G. Mishra / Molecular Catalysis 446 (2018) 58–71
are referred to as CPxZZP in the subsequent text, where x repre-
sent the number of moles of Cs+ ions replacing the H+ ions from
H3PW12O40
.
2.4. Characterization techniques
The powder XRD patterns of the CPxZZP materials were
recorded with a Rigaku Ultima IV multipurpose X-ray diffraction
system using CuK␣ (␭ = 1.5418 Å) radiation in the 2␪ range of 2–50◦
with a scan speed of 2◦ min−1. The specific surface areas were
determined by BET method using N2 adsorption/desorption at 77 K
on a Quantachrome autosorb gas sorption system. The composite
samples were degassed at 150 ◦C for 6 h prior to the sorptometric
studies. Thermogravimetric analysis was performed using a Perkin-
Elmer TGA-7 apparatus in air atmosphere (30 ml min−1) with a
linear heating rate (10 ◦C min−1) from room temperature to 700 ◦C.
Microscopic analysis was carried out using FESEM (Nova Nano
SEM/FEI microscope) and HRTEM (TECNAI 300kV) techniques. The
UV–vis-DR spectra of the composite materials were recorded using
a Jasco V-650 spectrometer. The FTIR spectra were obtained in
transmittance mode using a Perkin-Elmer infrared spectrometer
with a resolution of 4 cm−1, in the range of 400–4000 cm−1. The X-
ray photoelectron spectra of CP2ZZP sample was recorded using
SPECS make (Germany) spectrophotometer with 150 mm hemi-
spherical analyzer at band pass energy of 12 eV. Monochromatic
AlK␣ radiation of 1486.74 eV was used as X-ray source. Binding
energy corrections due to electrostatic charging was made rela-
tive to the C1s peak at 284.6 eV. The number of acidic sites was
estimated by ammonia TPD experiments using a Micromeritics
AutoChem II 2920 chemisorption apparatus equipped with a TCD
detector. Prior to NH3 adsorption, approximately 250 mg of sample
was treated at 350 ◦C for 2 h under flowing He and then cooled to
room temperature under He environment. Subsequently, the sam-
ple was exposed to flowing ammonia gas mixture (5% NH3 in He)
for 1 h, and then purged by He gas for 30 min to remove excess
physisorbed ammonia. The NH3-TPD profiles were recorded from
room temperature to 700 ◦C at a rate of 10 ◦C min−1. The 1H NMR
spectra were recorded with Bruker 400 MHz NMR spectrometer
using TMS as internal standard.
Fig. 1. Low angle XRD patterns of (a) ZP, (b) ZZP, (c) CP1ZZP and (d) CP2ZZP materials.
considerable expansion of the interlayer structure is observed. The
ZZP material exhibits a broad XRD peak with d spacing value of
34.0 Å (Fig. 1b). The intercalation of the Zr-oligomeric clusters into
the ␣-ZrP interlayer has moved apart the ZrP sheets resulting in
an enhanced interlayer space. The low angle XRD feature has been
found to be preserved in the CPxZZP material indicating the reten-
tion of the layer structure (Fig. 1c &d). The CPxZZP materials exhibit
basal spacing in the range of 27.5–29.0 Å. Assuming the layer
thickness of ␣-ZrP sheets to be 6.5 Å [1], these materials exhibit
interlayer gallery height between 21.0 and 22.5 Å. The observed
result suggests a clear enhancement in the interlayer free space as a
2.5. Catalytic study for synthesis of spirooxindoles
The synthesis of spirooxindole was carried out by one pot
condensation of isatin, naphthol/1,3-diketones and malononitrile
using CPxZZP catalyst. In a typical procedure, 1 mmol each of isatin,
␣- naphthol and malononitrile was dissolved in 5 ml of acetoni-
trile to which 50 mg of CP2ZZP catalyst was added. The reaction
mixture was stirred for 1.5 h at 60 ◦C. The progress of the reac-
tion was monitored using TLC. After completion of the reaction,
the catalyst particles were filtered from the reaction mixture and
the crude product was recovered. The crude product was further
recrystallized from hot ethanoic solution to get the pure product.
3. Results and discussion
3.1. Characterization of CsxH3-xPW12O40 dispersed over
Zr-pillared ˛-zirconium phosphate (CPxZZP) materials
The XRD patterns of the CPxZZP materials together with the ZP,
ZZP, H3PW12O40 and Cs2H1PW12O40 (Cs2PWA) are presented in
Figs. 1 and 2. In the 2␪ range of 2–10◦, the ZP material does not
show any well-defined XRD peak. The peak at 2␪ value of 11.8◦
corresponds to reflection from the (002) planes with an interlayer
spacing of 7.5 Å (Fig. 1a). This value is in agreement with earlier lit-
erature reports and indicates alignment of the ␣-ZrP sheets along
the axial direction [1,2]. Upon pillaring with the Zr-polycations,
Fig. 2. XRD patterns of (a) ZP, (b) ZZP, (c) CP1ZZP, (d) CP2ZZP, (e) Cs2PWA and (f)
PWA materials.

S. Pradhan, B.G. Mishra / Molecular Catalysis 446 (2018) 58–71
61
Table 1
Physicochemical characteristics of CPxZZP materials.
Material
Specific surface area (m2 g−1
)
Pore volume (cm3 g−1
)
Crystallite size of CsxPWA (nm)
NH3 desorption (mmol g−1
)
Total acidity (mmol g−1
)
<250 ◦
C
250–450 ◦
C
>450 ◦
C
ZP
ZZP
CP1ZZP
CP2ZZP
46.8
133.9
96.4
0.09
0.19
0.11
0.14
–
–
18.1
9.2
0.12
0.10
0.06
0.08
0.07
0.14
0.20
0.24
0.04
0.08
0.15
0.16
0.23
0.32
0.41
0.48
107.7
Fig. 3. FTIR spectra of (a) ZP, (b) ZZP, (c) CP1ZZP, and (d) CP2ZZP materials.
result of intercalation of Zr-oligomeric clusters. Zr4+ions are known
ondary cubic crystalline structure (space group-Pn3m, JCPDS ISDD
No-75-2125) (Fig. 2f). The XRD profile of Cs2H1PW12O40 (Cs2PWA)
material is presented in Fig. 2e. No peaks corresponding to the
free acid could be detected for the Cs2PWA sample. The peaks are
usually broad and shifted to higher 2␪ values in comparison to
the H3PW12O40. Similar XRD features are also noted for CP1ZZP
and CP2ZZP materials (Fig. 2c & d). The shift in the peak position,
in comparison to the parent H3PW12O40 phase, can be assigned
to the formation of a distinct body-centered cubic structure of
Cs3PW12O40 salt consistent with earlier observation [19,42]. It has
been reported that the Cs exchanged H3PW12O40 with different
degree of substitutions contain a core of ultrafine crystallites of
Cs3PW12O40 salt over which the free acids are deposited epitaxially
[19,42]. In the present study, the similarity in the XRD features of
CP1ZZP and CP2ZZP materials with Cs2PWA phase indicates that the
CPxZZP materials contain Cs3PW12O40 salt as a distinct phase. The
average crystallite size of the supported CsxPWA phase is calculated
from the Fourier line profile analysis of the broadened XRD pro-
files using Warren-Averbach method using Bredth software [43].
The volume average crystallite size distribution of the supported
CsxPWA phase along with pure Cs2PWA is presented in Fig. S1.
The crystallite size distribution is quite broad for CP1ZZP materials
whereas comparable narrow distribution is observed for Cs2PWA
and CP2ZZP materials. The average crystallite sizes calculated from
the initial slope of As ∼ L plot (Fig. S2) are presented in Table 1. The
8+
to form stable tetrameric clusters [Zr4(OH)8(H2O)16
]
with size
8.9 × 8.9 × 5.8 Å3 in dilute acidic solutions. At elevated temperature,
further hydrolysis and condensation of these tetrameric clusters
into larger clusters have been reported [41]. In the present study,
the increase in the basal spacing can be ascribed to the intercala-
tion of larger Zr-oligomeric clusters into the ␣-ZrP interlayer. In the
2␪ range of 20–50◦, the ␣-ZrP material displays a series of discrete
and well defined XRD peaks with d spacing values of 7.5, 4.46, 4.41,
3.54, 3.51, 2.63 and 2.61 Å (Fig. 2a). These peaks correspond to the
reflections from the (002), (−111), (200), (−113), (202), (020) and
(−311) planes of monoclinic Zr(HPO4)2(H2O) (space group- P21/n,
JCPDS-ISDD No.-70-0561). The ZZP and CPxZZP materials display
broad and less intense peaks in comparison to the ␣-ZrP phase
(Fig. 2b–d). This observation indicates a partial loss of crystallinity
due to pillaring and subsequent dispersion of CsxPWA particles.
During the pillaring process, complete exfoliation of ␣-ZrP sheets
takes place due to intercalation of n-butyl amine as a layer expan-
sion agent. When the sheets are reassembled in presence of the
Zr-oligomeric clusters the stacking of the ␣-ZrP sheets is favored
along the z-direction. However, the long range ordering along other
crystallographic directions is affected due to the inclusion of the
Zr-oligomeric clusters. Commercially procured H3PW12O40·xH2O
(PWA) shows highly intense and narrow XRD peaks with d spacing
value of 8.4, 4.86, 3.41, 2.96, 2.53, 2.32 Å corresponding to the sec-

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S. Pradhan, B.G. Mishra / Molecular Catalysis 446 (2018) 58–71
Cs2PWA material contains crystallite with average size of 4.8 nm.
With increase in Cs content in the supported material the crystallite
size of the CsxPWA phase is found to decrease.
transfer transition can be ascribed to strong interaction between
the ZZP support and the CsxPWA clusters through W O bond
resulting in grafting of these species in the intra-particle pores as
well as the open surfaces of the ␣-ZrP sheets. The shift in the band
edge to lower wavelength can be ascribed to quantum confine-
ment effect due to small crystallite size of the CsxPWA particles. It
has been observed earlier that the Cs exchanged heteropoly acids
contains small crystallites; display enhanced surface area and pos-
rity [19–24]. This observation is further corroborated by TEM and
Fourier analysis data where the crystallite size of the CsxPWA is
observed in the range of 8–15 nm.
The FTIR spectra of ZP, ZZP and CPxZZP materials are presented
in Fig. 3. In the stretching frequency region, the ZP material exhibits
discrete peaks at 3591 and 3512 cm−1, corresponding to the PO
H
stretching vibration of structural hydroxyl groups (Fig. 3I a). The
peak at 3150 cm−1 can be assigned to O H stretching of inter-
layer water molecules [7]. After pillaring with the Zr-polycations
and subsequent dispersion of CsxPWA species, broad and intense
IR bands are observed in this spectral region (Fig. 3I b–d). The broad-
ness of the stretching bands is due to the presence of different types
of O H species originating from the ␣-ZrP layers, Zr-pillars and
the dispersed CsxPWA species interacting with each other through
The thermal transformation behavior of the CP2ZZP material
along with pure ZP and ZZP materials is studied using combined
TG-DTA technique (Fig. 5). The ␣-ZrP material shows 13% weight
loss in the temperature range of 100–250 ◦C. In this temperature
range, two endothermic peaks are observed in the DTA profile cor-
responding to the removal of the physisorbed interlayer water
molecules as well as the coordinated water molecules (Fig. 5I)
[14]. The water molecules in the coordination sphere of the phos-
phate ions are more strongly bonded and hence desorbs at higher
temperature. Beyond 250 ◦C, the weight loss is more gradual up
to 470 ◦C. The weight loss of 6.5% observed in the temperature
range of 470–600 ◦C can be assigned to the degradation of the
layer structure of ␣-ZrP leading to formation of pyrophosphates
(Fig. 5I). Such thermal transformation behavior has been noted ear-
lier for the ␣-ZrP material [6]. The ZZP and CP2ZZP material exhibits
similar low temperature weight loss feature with the exception
that the weight loss is more pronounced for ZZP material (35 wt%)
(Fig. 5II). This observation indicates a significant improvement in
water retention capacity of the ␣-ZrP matrix as a result of pil-
laring. The pillaring process increases the interlayer spacing and
porosity of the ␣-ZrP which is responsible for the improved water
retention capacity. In the high temperature region, the ZZP mate-
rial exhibits an exothermic peak at 320 ◦C due to the combustion of
the residual intercalated n-butyl amine (Fig. 5II). Beyond 400 ◦C, the
gradual weight loss feature corresponds to the thermal transforma-
tion of the pillars [14,15]. In addition to the low temperature weight
loss feature, the CP2ZZP material exhibits a second weight loss of
3 wt% between 500 and 600 ◦C. This weight loss corresponds to the
decomposition of the primary structure of the Keggin ions (Fig. 5III)
[24]. The TGA-DTA study indicates that the CP2ZZP material is stable
up to 500 ◦C.
The oxidation state of the elements and their chemical envi-
ronment was probed using XPS study. The XPS spectra of CP2ZZP
material is presented in Fig. 6. The survey spectrum displays the
characteristic features of Zr, P, W, Cs, O and S present in the compos-
ite sample (Fig. 6a). In the high resolution Zr 3d spectrum, two peaks
are observed at 182.2 eV and 184.4 eV (Fig. 6b). These peaks corre-
spond to the photoelectron emission from 3d3/2 and 3d5/2 energy
states of tetravalent Zr(IV) species [47]. In the W4f region, a doublet
is observed at 35.8 and 37.9 eV corresponding to the 4f7/2 and 4f5/2
energy states of W6+ species of the Keggin ions (Fig. 6c) [23]. The O1s
spectrum is centered at 531.8 eV and is quite broad and asymmetric
(Fig. 6d). The peak can be deconvoluted into three spectral regions
with maxima at 531.2, 532.1 and 533.2 eV. The peak at 531.2 eV
hydrogen bonding. In the spectral region of 1500–2000 cm−1 all
,
materials exhibit a strong IR band at 1625–1630 cm−1, the intensity
of which increases upon pillaring and modification with CsxPWA
species (Fig. 3II). This band has been assigned to the O H bending
vibration of structural water as well as water molecules present in
the primary coordination sphere of the Zr-nanopillars [41]. In the
finger print region of 400–1400 cm−1 the ␣-ZrP material shows
,
a series of discrete IR bands at 1251, 1050, 962 and 595 cm−1
(Fig. 3III a). The peaks at 1251 and 595 cm−1 can be assigned to the
P
OH stretching and in plane deformation vibrations whereas the
1050 and 962 cm−1 peaks are characteristic vibrations of phosphate
groups [7,14,15]. In this spectral region, the ZZP material retains all
the vibrations of ␣-ZrP (Fig. 3III b). After dispersion of the CsxPWA
species, new IR bands are observed at 1070, 885 and 800 cm−1
corresponding to the ␯as(P O), ␯(W Oc W) and ␯(W Oe W)
vibrations from the Keggin heteroply structure (Fig. 3III c & d)
[19,20,25]. The FTIR study confirms the structural integrity of the
CsxPWA and ␣-ZrP species in the composite materials.
The UV–vis-DR spectra of CPxZZP materials along with the
pure components are presented in Fig. 4. Pure ␣-ZrP material
exhibits a low intense UV band with maxima at 233 nm due to
the O → P charge transfer transition (Fig. 4I a). Upon pillaring with
Zr-oxyhydroxy clusters a strong UV absorption feature is noticed
at 208 nm along with a broad shoulder at 290 nm (Fig. 4I b). The
208 nm peak can be assigned to O2− → Zr4+ charge transfer tran-
sition of the zirconia pillars present in the ␣-ZrP interlayer [41].
Zirconia is a direct band gap insulator which shows an interband
transition in the UV region of the spectrum. Among the polymor-
phic form of ZrO2, the octacoordinated tetragonal phase shows
absorption maxima in the range of 205–215 nm whereas the hep-
tacoordinated monoclinic phase shows maxima at 245 nm [44,45].
With decrease in the number of coordinating O2− ions from 8 to
6 the absorption maxima progressively shifts towards the lower
energy side. Thus the peak at 208 nm can be assigned to the pres-
ence of octacoordinated t-ZrO2 nanoclusters as pillars [41]. The
290 nm peak arises due to presence of defect centers in the t-ZrO2
nanocrystallites. Such features have been observed earlier for ZrO2
nanocrystallites which have been ascribed to the presence of local-
ized state between valence and conduction bands [45]. The UV–vis
spectra of pure H3PW12O40 show three absorption maxima with
absorption edge near 400 nm (Fig. 4II b). The absorption maxima
at 215, 260 and 310 nm can be assigned to the P5+ ←− O2− and
W6+ ←− O2− charge transfer from the edge and corner shared WO6
octahedra of H3PW12O40 [46]. The essential absorption features
of the phosphotungstic acid are retained in the Cs2PWA material
(Fig. 4II a). The individual absorption peaks have broadened prob-
ably due to the structural rearrangement due to Cs+ ion exchange.
The CPxZZP material on the other hand exhibit distinct absorption
features (Fig. 4I c & d). The absorption edge has shifted towards
the higher energy side (commence near 350 nm) along with a sig-
nificant reduction in intensity of the W6+ ←− O2− charge transfer
transition. The decrease in the intensity of the W6+ ←− O2− charge
arises due to the combined contribution from W
O W and the
ZrO2 pillars. ZrO2 is known to exhibit O1s peak at 530.2–530.7 eV
[48]. Hence, it is likely that the O from the ZrO2 can contribute to this
peak. The Keggin ions are known to exhibit an O1s peak at 532.6 eV
due to photoemission from W
O P and terminal oxygen (W O)
of the Keggin ion structure [49]. This peak appeared at 532.1 eV
in the present study. This is probably due to the strong interac-
tion of the terminal oxygens with the Zr4+ ions of the ␣-ZrP sheets.
Zr4+ being more electropositive can increase the electron density
around the terminal oxygen lowering their binding energy. This
is further corroborated from UV study where a significant decrease

S. Pradhan, B.G. Mishra / Molecular Catalysis 446 (2018) 58–71
63
Fig. 4. UV–vis-DRS spectra of (a) ZP, (b) ZZP, (c) CP1ZZP, and (d) CP2ZZP (panel I) and (a) Cs2PWA and (b) H3PW12O40 (panel II).
Fig. 5. TG-DTA profiles of (I) ZP, (II) ZZP and (III) CP2ZZP materials.

64
S. Pradhan, B.G. Mishra / Molecular Catalysis 446 (2018) 58–71
Fig. 6. XPS spectra of CP2ZZP material.
in W6+ ←− O2− charge transfer transition has been observed for the
supported species. The third O1s peak at 533.2 eV can be assigned
to the phosphate ions of the ␣-ZrP sheets in line with the litera-
ture observation [47,50]. In the P2p region, a broad peak centered
at 133.8 eV is characteristics of pentavalent tetracoordinated phos-
phorous (P5+) (Fig. 6e). This peak arises due to the phosphate ions of
the ␣-ZrP sheets and PO4 species of the Keggin ions [47]. The occur-
rence of Cs+ ions in CP2ZZP material is confirmed from the spectral
doublet at 724.2 and 738.2 eV corresponding to the photoemission
from the 3d5/2 and 3d3/2 energy states of Cs+ ions (Fig. 6f) [19].
The surface area and pore volume of the CPxZZP materials is
evaluated using N2 sorption at 77 K. The N2 sorption isotherms of
the CPxZZP material together with ZP and ZZP materials are pre-
sented in Fig. 7. The sorption isotherm for ZP material exhibits
behavior between those of type-II and IV isotherm according to the
BDDT classification (Fig. 7a) [12–15]. The amount of N2 adsorption
at low pressure range is quite less indicating very little porosity
in micropore range. The observed hysteresis is H3 type according
to the IUPAC classification indicating the presence of slit shaped
pores or pores with space between parallel plates [51]. The pore

S. Pradhan, B.G. Mishra / Molecular Catalysis 446 (2018) 58–71
65
Fig. 7. N2 sorption isotherm and pore size distribution curves of ZP (a, e), ZZP (b, f), CP1ZZP (c, g) and CP2ZZP (d, h) materials.
size distribution calculated using NLDFT method indicates a broad
distribution profile with pore size in the range of 1.5–4 nm (Fig. 7e).
Upon pillaring with Zr-polycation, the initial nitrogen adsorption
is rapidly enhanced indicating an increase in the microporosity
(Fig. 7b). The ZZP material show narrow pore size distribution with
maxima at 1.9 nm (Fig. 7f). The adsorption features of CsxZZP mate-
rials are similar to that of ZZP material (Fig. 7c & d). The pore
size distribution is broader compared to ZZP probably due to gen-
eration of secondary pore structures in presence of the CsxPWA
moieties (Fig. 7g & h). It has been reported that the ion exchanged
heteropoly acid display high surface area and porosity due to the
reorganization of the interstitial voids in the ion exchanged sam-

66
S. Pradhan, B.G. Mishra / Molecular Catalysis 446 (2018) 58–71
Table 2
Table 3
Catalytic activity of CPxZZP materials towards synthesis of spirooxindoles.
Catalytic activity of CP2ZZP material for synthesis of structurally diverse
spirooxindoles.
Catalyst
Catalytic activity
Conversion of ␣-naphtholb
TOF (h−1
)
Sl No1,3-diketone/NaphtholProduct
Time (h)Yield (%)MP (◦C)
Time (h)
Yielda (%)
ZP
ZZP
CP1ZZP
CP2ZZP
5
5
2
1.5
35
52
75
86
39
55
79
88
6
7
19
25
1
1.5
86
223–224
a
Refers to pure ad isolated yield.
␣-Naphthol conversion in the reaction mixture calculated using gas chromatog-
b
raphy.
ples [19–24]. The surface area and pore volume of the materials
are presented in Table 1. The surface area varies in the order
ZP < CP1ZZP < CP2ZZP < ZZP.
2
3
1.5
81
75
231
240
The temperature programmed desorption profiles of CPxZZP
material along with ZP and ZZP materials are presented in Fig. 8.
Three ammonia desorption regions in the range of 100–200 ◦C,
200–350 ◦C and 350–450 ◦C could be observed for the ZP materials
(Fig. 8a). These desorption peaks can be assigned to the pres-
ence of weak, medium and strong acidic sites in this material.
The low temperature desorption peak is most prominent for this
material indicating the presence of a large fraction of weak acidic
sites. The total number of acidic sites calculated for this material
is 0.23 mmol g−1. The Zr-P material exhibits two broad asymmet-
ric desorption peaks in the range of 150–350 ◦C and 400–600 ◦C
(Fig. 8b). The low temperature peak can be deconvoluted into two
peaks. In comparison to ZP material, the ZZP material show an
incremental but significant rise in high temperature ammonia des-
orption indicating generation of new acidic sites of higher strength
(Table 1). As observed from UV–vis study, the ZZP material con-
tains zirconia nanoclusters with structural defects as pillars. These
pillars can act as potential new acidic sites. The CPxZZP materials
also exhibit two ammonia desorption peaks. However, a substan-
tial increase in the high temperature ammonia desorption peaks
are observed for these materials (Fig. 8c &d). This observation sug-
gests that dispersion of CsxPWA species in the ZZP lattice results
in generation of new medium and strong acidic sites. In order to
rule out the contribution of desorbed water and/or possible sur-
face decomposition products to this high temperature peak, blank
TPD experiments were conducted without ammonia adsorption for
CP2ZZP material (Fig. S3). The TPD profile of the blank experiment
does not show any appreciable desorption peak upto 600 ◦C. This
observation indicates that the high temperature NH3 TPD peak is
due to desorbed ammonia from the strong acidic sites of the com-
posite materials. The acidity of the CsxPWA materials has been
studied earlier using TPD and calorimetry techniques [19–24]. It
has been noticed that the Cs substitution increases the surface area,
porosity as well as number of accessible strong acidic sties on the
surface. Moreover, with increase in Cs content, the amount of strong
acidic sites is increased in the CsxPWA material reaching a maxi-
mum value with x = 2.00–2.5 [19,21]. Hence, the improvement in
the acidity observed for the composite material can be ascribed
to a synergistic effect between the acidity of the host lattice and
the CsxPWA material. The total number of acidic sites varies in the
order CP2ZZP > CP1ZZP > ZZP > ZP (Table 1).
2
4
5
1.5
82
80
236
2
230–231
6
1.5
88
269–270
7
8
9
2
2
2
82
85
78
277–278
260–262
271–272
The morphological features and particle sizes of the CPxZZP
materials are studied using FESEM and HRTEM techniques. The
FESEM images of the CPxZZP material together with the ZP and ZZP
materials are presented in Fig. 9. The ␣-ZrP material shows plate
like particles with typical length and breadth between 180 and
250 nm and 120–160 nm, respectively (Fig. 9a). The layer to layer
orientation of the ␣-ZrP platelets seems to be the favored orienta-
tion which is resulting into the plate like morphology. Pillaring with
Zr-oligomeric cluster significantly alters the particle morphology.
10
11
2
2
84
76
262–263
169–170

S. Pradhan, B.G. Mishra / Molecular Catalysis 446 (2018) 58–71
67
Fig. 8. TPD profiles of (a) ZP, (b) ZZP, (c) CP1ZZP, and (d) CP2ZZP materials.
The ZZP material contains agglomerated spherical particles with
3.2. Catalytic activity study for synthesis of spirooxindoles
diameter in the range of 30–60 nm (Fig. 9b). The insertion of Zr-
pillars imparts structural rigidity to the ␣-ZrP lattice. Moreover, the
presence of Zr-pillars reduces the face to face interaction and facili-
tates edge to face and edge to edge interactions resulting in particle
folding and generation of spherical particles. The spherical mor-
phology of the particles is retained in the CPxZZP materials (Fig. 9c
& d). The CP1ZZP and CP2ZZP materials show spherical particles
with diameter in the range of 150–200 nm. The transmission elec-
tron micrographs of the CP2ZZP material are presented in Fig. 10.
The low magnification image shows a typical spherical particle with
diameter 156 nm (Fig. 10a). The presence of folded ␣-ZrP sheets
and their hierarchical organization into the spherical structure can
be inferred from this TEM image. The CP2ZZP material contains
well dispersed Cs2PWA particles with size in the range of 8–14 nm
(Fig. 10a & b). The microscopic observation is further corroborating
the Fourier analysis data where presence of 8–15 nm CsxPTA parti-
cles has been observed. The EDX pattern of the CP2ZZP material is
presented in Fig. 10c. The presence of Cs, W, Zr, P and O of required
stoichiometry can be inferred from EDX study. The W/Zr atomic
ratio calculated from XPS study is 0.35, which is higher than the
value of 0.21 calculated from EDX measurement. This observation
suggests that the CS2PWA particles are present in a well dispersed
state on the surface of ZZP matrix. The elemental mapping anal-
ysis images of the CP2ZZP material are presented in Fig. 10d–h.
Uniform distribution of the Cs and W at microscopic level can be
inferred from the elemental mapping study. The EDS image clearly
indicates that the Cs2PWA nanoparticles are finely dispersed in the
ZZP matrix to form the CP2ZZP composite materials.
The catalytic activity of the CPxZZP materials has been studied
for the one pot synthesis of spirooxindoles by multi-component
condensation of isatin, malononitrile and naphthol/1,3-dicarbonyl
compounds (Scheme 1). Initially, the condensation of isatin, mal-
ononitrile and ␣-naphthol is taken as a model reaction and different
composite catalytic systems were studied for their catalytic activity
at 60 ◦C using 5 ml of acetonitrile as solvent (Table 2). Pure ␣-ZrP
material produced 35% product yield after 5 h of reaction time.
The weak acidic sites on ␣-ZrP material are unable to expedite
the spirooxindole synthesis. The product formation is marginally
improved after pillaring with Zr-polycation (Table 2). The CPxZZP
materials, on the other hand, show improved yield of the product
in short reaction time. Among the CPxZZP materials, the CP2ZZP
material exhibits 86% yield of the product in a short time span
of 1.5 h. Since the composite materials display different surface
area and acidity, we calculated the turnover frequency in h−1 by
estimating the ␣-naphthol conversion from the reaction mixture
using GC analysis (Fig. S4). Among the different catalyst formula-
tion studied for the reaction, the CP2ZZP material exhibits highest
turnover frequency indicating its suitability for this multicompo-
nent reaction (Table 2). The higher catalytic activity observed for
CP2ZZP material can be correlated to the improvement in number
of acidic sites which show ammonia desorption in the temperature
range of 250–450 ◦C. In order to evaluate the role of the support
(␣-ZrP) on the catalytic activity, the catalytic tests are also car-
ried out using unsupported Cs1H2PW12O40, Cs2H1PW12O40 and
H3PW12O40 materials. The results of these catalytic experiments

68
S. Pradhan, B.G. Mishra / Molecular Catalysis 446 (2018) 58–71
Fig. 9. FESEM images of (a) ZP, (b) ZZP, (c) CP1ZZP, and (d) CP2ZZP materials.
Scheme 1. CP2ZZP catalyzed multicomponent one pot synthesis of spriooxindoles.

S. Pradhan, B.G. Mishra / Molecular Catalysis 446 (2018) 58–71
69
Fig. 10. HRTEM images (a, b), EDX spectra (c) and elemental mapping study (d–h) of CP2ZZP material.
are presented in Table S1. For an identical loading amount, the
CP2ZZP material exhibits higher yield of the product compared to
the pure Cs2H1PW12O40 material. Moreover, the CsxH3-xPW12O40
species are more active compared to the parent H3PW12O40 mate-
rial. These observations suggest that the partial exchange of H+
ions with Cs+ ions is beneficial in terms of higher activity and the
␣-ZrP as support further enhances its catalytic activity by maxi-
mizing the dispersion of the active phase in its porous matrix. The
CP2ZZP material also exhibits higher catalytic activity compared to
the Cs2H1PW12O40 species supported over SiO2 and ZrO2 (Table
S1). The phosphate functionality of the support seems to be impor-
tant in enhanced adsorption of the polar reactant species on the
catalyst surface through van der Waal type interactions. Based on
the data described in Tables Table 2 and S1, the CP2ZZP material is
selected for further catalytic study. The CP2ZZP catalyzed reaction
protocol is optimized by varying the catalyst amount, solvent and
temperature (Fig. 11). For reaction involving 1 mmol of the reac-
tants, 50 mg of the catalyst is sufficient for condensation of the three
components. Further increase in catalyst quantity only marginally
improves the yield (Fig. 11II). The reaction temperature is varied
between 40 and 82 ◦C in an incremental step of 10 ◦C. The prod-
uct yield is found to improve significantly with temperature up to
60 ◦C. Increase in temperature beyond 60 ◦C, does not have much
impact on the yield of the product (Fig. 11I). To evaluate the effect of
reaction media on catalytic activity, solvents with different polar-
ity are used in the optimized protocol. In general, better yields of

70
S. Pradhan, B.G. Mishra / Molecular Catalysis 446 (2018) 58–71
Fig. 11. Optimization of reaction parameters for the model reaction using CP2ZZP catalyst (I) effect of temperature (acetonitrile, 50 mg catalyst) (II) Effect of catalyst amount
(acetonitrile, 60 ◦C), and (III) effect of reaction media (50 mg catalyst, 60 ◦C).
the product are noticed in polar solvents. Among different solvent
studied for the reaction, best yield of spirooxindole is obtained in
acetonitrile (Fig. 11III). Hence, acetonitrile is used as a solvent for
further study.
pyridine for the ZZP and CP2ZZP materials (Fig. S5). The FTIR spec-
tra show IR bands at 1439, 1489, 1544 and 1610 cm−1. The intense
peak observed at 1544 cm−1 corresponds to pyridine adsorbed on
Brønsted acidic site as pyridinium ions [52]. The peak at 1489 cm−1
is due to pyridine adsorbed on both Brønsted and Lewis acid sites
where as the peak at 1439 and 1610 cm−1 can be assigned to Lewis
acidic sites [52–55]. The host Zr-pillared ␣-ZrP lattice can contain
After optimizing the reaction conditions, we studied the scope
and limitation of the optimized protocol by employing istain,
malonitrile/ethyl cyanoacetate and 1-naphthol/2-naphthol in ace-
tonitrile media at 60 ◦C (Table 3). Under similar experimental
condition, all reactions proceed smoothly to give the corresponding
spiro(tetrahydrochromene- 4,3ꢀ-indole) in high yield and purity.
The CP2ZZP catalyzed reaction protocol is extended further to three
component coupling of istain, malonitrile/ethyl cyanoacetate and
dicarbonyl compounds. A variety of dicarbonyl compounds includ-
ing dimedone, ethyl acetoacetate and methyl acetoacetate reacted
efficiently under the optimized condition to form the correspond-
ing spirooxindole derivatives (Table 3). The recyclability of the
CP2ZZP catalyst is evaluated for three consecutive cycles. After each
catalytic cycle, the catalyst particles are filtered from the reaction
mixture, washed with ethanol (5 ml portions 3 times) and then
heat treated at 250 ◦C for 2 h. The CP2ZZP catalyst exhibits stable
catalytic activity for three consecutive cycles without any signifi-
cant decrease in the yield of the product (Table 3, entry 1, yields,
86.0%, 1st; 85%, 2nd; 83%, 3rd). The mechanism of spiroxyindole
formation has been described in literature [34,38]. The reaction
involves a cascade process, where the initial Knoevenagel con-
densation of isatin with malononitrile results in the formation of
isatylidene malononitrile. In the second step, the ortho C-alkylation
Zr
O P and coordinatively unsaturated zirconium ions as Lewis
acidic sites [52,53]. The isatin molecule can be activated on surface
low coordinated Zr4+ sites. The phosphate ions in the composite
catalyst can enhance the adsorption of the polar reactant species
through van der Waal interactions. Moreover, the ␣-naphthol can
be activated on the catalyst surface by interaction through the ␲
electron which can react with adsorbed isatylidene malononitrile
to form the product. These features of the composite catalyst can
expedite the catalytic reaction on its surface.
4. Conclusion
In this work, we have prepared a novel composite catalytic
system by dispersing cesium exchanged phosphotungstic acid
(CsxH3-xPW12O40) nanoparticles in the micropores of Zr-pillared
␣-zirconium phosphate (ZZP) and used as catalyst for synthesis of
structurally diverse spirooxinole derivatives containing chromene
moieties. The pillared ␣-ZrP materials displayed enhanced gallery
height, higher microporosity and acidity compared to the parent
␣-ZrP. The increased porous nature of the pillared material is suc-
cessfully utilized to disperse CsxH3-xPW12O40 nanoparticles. The
structural integrity and grafting of the CsxH3-xPW12O40 species
over the ZZP material is confirmed from FTIR and UV–vis-DRS
study. Study of the thermal transformation behavior of the compos-
ite materials revealed that these materials are stable upto 500 ◦C.
of ␣-naphthol takes place by reaction with the electrophilic C
C
double bond of isatylidene malononitrile. The final step involves the
nucleophilic addition of the phenolic OH group moiety to the cyano
moiety to form the spiroxyindole. The nature of acidic sites on the
composite sample has been probed using FTIR study of adsorbed

S. Pradhan, B.G. Mishra / Molecular Catalysis 446 (2018) 58–71
71
Microscopic investigation suggested the presence of well dispersed
CsxH3-xPW12O40 particles with size in the range of 8–14 nm in the
composite materials. The pillaring with Zr-polycations and subse-
quent dispersion of the CsxH3-xPW12O40 in the ␣-ZrP lattice results
in generation of new medium and strong acidic sites in the compos-
ite. The composite materials exhibited excellent catalytic activity
for synthesis of spirooxindoles containing fused chromene moieties
by multicomponent one pot condensation of isatin, malononitrile
and naphthol/1,3-dicarbonyl compounds. A variety of spirooxin-
dole derivatives are synthesized in high yield and purity within a
short time span of 1–2 h using acetonitrile as solvent. The catalytic
protocol developed in this work is found to be advantageous in
terms of simple experimentation, mild reaction conditions, recy-
clable heterogeneous catalytic system, less process time and high
yield and purity of the products.
[19] K. Narasimharao, D.R. Brown, A.F. Lee, A.D. Newman, P.F. Siril, S.J. Tavener, K.
Wilson, J. Catal. 248 (2007) 226–234.
[20] N.R. Shiju, H.M. Williams, D.R. Brown, Appl. Catal. B: Environ. 90 (2009)
451–457.
[21] H.J. Eom, D.W. Lee, S. Kim, S.H. Chung, Y.G. Hur, K.Y. Lee, Fuel 126 (2014)
263–270.
[22] K. Srilatha, R. Sree, B.L.A.P. Devi, P.S. Sai Prasad, R.B.N. Prasad, N. Lingaiah,
Bioresour. Technol. 116 (2012) 53–57.
[23] C.R. Kumar, S. Gatla, O. Mathon, S. Pascarelli, N. Lingaiah, Appl. Catal. A: Gen.
502 (2015) 297–304.
[24] A. Popa, V. Sasc, I.H. Antunovic, Microporous Mesoporous Mater. 156 (2012)
127–137.
[25] K.T.V. Rao, P.S.N. Rao, P.S. Sai Prasad, N. Lingaiah, Catal. Commun. 10 (2009)
1394–1397.
[26] M. Srinivas, G. Raveendra, G. Parameswaram, P.S. Sai Prasad, N. Lingaiah, J.
Mol. Catal. A: Chem. 413 (2016) 7–14.
[27] A. Srikanth, B. Viswanadham, V.P. Kumar, N. Rao Anipindi, K.V.R. Chary, Appl.
Petrochem. Res. 6 (2016) 145–153.
[28] R.J. Sundberg, The Chemistry of Indoles, Academic Press, New York, 1996.
[29] T.R. Garbe, M. Kobayashi, N. Shimizu, N. Takesue, M. Ozawa, H. Yukawa, J. Nat.
Prod. 63 (2000) 596–598.
[30] R.M. Williams, R.J. Cox, Acc. Chem. Res. 36 (2003) 127–139.
[31] N. Yu, J.M. Aramini, M.W. Germann, Z. Huang, Tetrahedron Lett. 41 (2000)
6993–6996.
[32] L. Bonsignore, G. Loy, D. Secci, A. Calignano, Eur. J. Med. Chem. 28 (1993)
517–520.
[33] A. Kamal, K.S. Babu, M.V.P.S.V. Vardhan, S.M.A. Hussaini, R. Mahesh, S.P. Shaik,
A. Alarifi, Bioorg. Med. Chem. Lett. 25 (2015) 2199–2202.
[34] G. Shanthi, G. Subbulakshmi, P.T. Perumal, Tetrahedron 63 (2007) 2057–2063.
[35] L.M. Wang, N. Jiao, J. Qiu, J.J. Yu, J.Q. Liu, F.L. Guo, Y. Liu, Tetrahedron 66 (2010)
339–343.
Acknowledgement
We would like to thank the Board of Research in Nuclear Sci-
ences (BRNS), Mumbai for financial support through the grant No.
37(2)/14/23/2015/BRNS.
Appendix A. Supplementary data
[36] M. Esmaeilpour, J. Javidi, M. Divar, J. Magn. Magn. Mater. 423 (2017) 232–240.
[37] G.M. Ziarani, A. Badiei, S. Mousavi, N. Lashgari, A. Shahbazi, Chin. J. Catal. 33
(2012) 1832–1839.
[38] A. Dandia, V. Parewa, A.K. Jain, K.S. Rathore, Green Chem. 13 (2011)
2135–2145.
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.mcat.2017.12.
013.
[39] B. Karmakar, A. Nayak, J. Banerji, Tetrahedron Lett. 53 (2012) 5004–5007.
[40] F.A. Tameh, J.S. Ghomi, M.M. Hashemi, H.S. Alavi, RSC Adv. 6 (2016)
74802–74811.
References
[41] P. Kar, A. Nayak, Y.P. Bhoi, B.G. Mishra, Microporous Mesoporous Mater. 223
(2016) 176–186.
[42] T. Okuhara, T. Nakato, Catalysis by porous heteropoly compounds, Catal. Surv.
Asia 2 (1998) 31–44.
[43] B.E. Warren, B.L. Averbach, J. Appl. Phys. 21 (1950) 595–599.
[44] S. Samantaray, B.G. Mishra, D.K. Pradhan, G. Hota, Ceram. Int. 37 (2011)
3101–3108.
[45] S. Samantaray, B.G. Mishra, J. Mol. Catal. A: Chem. 339 (2011) 92–98.
[46] G.R. Rao, T. Rajkumar, Catal. Lett. 120 (2008) 261–273.
[47] J.L. Colon, D.S. Thakur, C.Y. Yang, A. Clearfield, C.R. Martin, J. Catal. 124 (1990)
148–159.
[48] S. Pradhan, V. Sahu, B.G. Mishra, J. Mol. Catal. A: Chem. 425 (2016) 297–309.
[49] P.A. Jalil, M. Faiz, N. Tabet, N.M. Hamdan, Z. Hussain, J. Catal. 217 (2003)
292–297.
[50] R. Weingarten, Y.T. Kim, G.A. Tompsett, A. Fernandez, K.S. Han, E.W. Hagaman,
W.C. Conner Jr., J.A. Dumesic, G.W. Huber, J. Catal. 304 (2013) 123–134.
[51] S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, Academic Press,
London, 1982.
[52] F. Li, L.J. France, Z. Cai, Y. Li, S. Liu, H. Lou, J. Long, X. Li, Appl. Catal. B: Environ.
214 (2017) 67–77.
[53] G. Busca, V. Lorenzelli, P. Galli, A. Ginestra, P. Patrono, J. Chem. Soc. Faraday
Trans. 83 (1987) 853–864.
[1] G. Alberti, M. Casciola, U. Costantino, R. Vivani, Adv. Mater. 8 (1996) 291–303.
[2] Z. Wang, J.M. Heising, A. Clearfield, J. Am. Chem. Soc. 125 (2003)
10375–10383.
[3] Y. Tang, Y. Ren, X. Shi, Inorg. Chem. 52 (2013) 1388–1397.
[4] F. Zhang, Y. Xie, W. Lu, X. Wang, S. Xu, X. Lei, J. Colloid Interface Sci. 349
(2010) 571–577.
[5] S. Khare, R. Chokhare, J. Mol. Catal. A: Chem. 138 (2012) 353–354.
[6] K.D. Pont, J.F. Gerard, E. Espuche, Eur. Polym. J. 48 (2012) 217–227.
[7] H. Wang, W. Liu, W. Yao, K. Zhang, J. Zhong, R. Chen, Appl. Surf. Sci. 268 (2013)
179–187.
[8] J. Xiao, J. Xu, Y. Wu, Z. Gao, Appl. Catal. A: Gen. 181 (1999) 313–322.
[9] R. Chen, J. Wang, H. Wang, W. Yao, J. Zhong, Solid State Sci. 13 (2011) 630–635.
[10] A.J. Lopez, E.R. Castellon, J.S. Gonzalez, P.B. Garcıa, E. Felici, F. Marmottini,
Langmuir 16 (2000) 3317–3321.
[11] H. Mao, X. Lu, M. Li, J. Yang, B. Li, Appl. Surf. Sci. 276 (2013) 787–795.
[12] F.J.P. Reina, P.O. Pastor, E.R. Castellon, A.J. Lopez, J. Solid State Chem. 122
(1996) 231–238.
[13] K. Liu, X. Wang, S. Ding, Y. Li, W. Hua, Y. Yue, Z. Gao, J. Mol. Catal. A: Chem. 380
(2013) 84–89.
[14] N. He, Y. Yue, Z. Gao, Microporous Mesoporous Mater. 52 (2002) 1–9.
[15] J. Xu, Z. Gao, Microporous Mesoporous Mater. 24 (1998) 213–222.
[16] J.M. Robles, E.R. Castello, A.J. Lopez, J. Mol. Catal. A: Gen. 145 (1999) 169–181.
[17] R.H. Huesca, J.S. Gonzalez, P.B. Garcıa, P.M. Torres, E.R. Castellon, A.J. Lopez,
Appl. Catal. B: Environ. 29 (2001) 1–11.
[54] D. Cao, B. Yu, S. Zhang, L. Cui, J. Zhang, W. Cai, Appl. Catal. A: Gen. 528 (2016)
59–66.
[55] A.E.R.S. Khader, S.A. Ahmed, H.M. Altass, React. Kinet. Mech. Catal. 117 (2016)
745–759.
[18] L. Pesaresi, D.R. Brown, A.F. Lee, J.M. Montero, H. Williams, K. Wilson, Appl.
Catal. A: Gen. 360 (2009) 50–58.