Polyphosphoric acid-zirconia pillared clay composite catalytic system for efficient multicomponent one pot synthesis of tetrahydropyridines under environmentally benign conditions

Article abstract of DOI:10.1016/j.molcata.2014.02.024

A series of zirconia pillared clay-polyphosphoric acid (PPA) composites are synthesized by adopting different preparative strategies. Initially, PPA is intercalated to the clay matrix with and without the use of structure expanding agent (CTAB). Subsequently, the PPA-clay composite is pillared with Zr-polycation to form the composite materials. In an alternate approach, Zr-pillared clay is synthesized by insertion of Zr-polycation, which is then used for dispersion of PPA moiety in the pillared clay matrix. The synthesized composites are characterized by XRD, FTIR, UV-vis, TGA, EDX, FE-SEM and sorptometric techniques. XRD study indicated an expansion in the clay structure after intercalation of PPA as well as the Zr-polycations. The FTIR spectra exhibit characteristic vibrational features of the clay sheet as well as PPA moiety indicating the structural stability of the composite materials. The phosphorous content in the composite samples is analyzed using EDX study. FE-SEM study indicated morphological changes upon intercalation of PPA and Zr-polycations to the clay matrix. The catalytic activity of the composite catalysts has been examined for the synthesis of tetrahydropyridines under environmental benign conditions by one pot multicomponent condensation of β-dicarbonyl compounds, substituted anilines and substituted benzaldehydes. The PPA intercalated clay pillared with Zr polycations (PPA-ZrP) is found to be highly efficient for the synthesis of structurally diverse substituted tetrahydropyridines under mild conditions.

Full text of DOI:10.1016/j.molcata.2014.02.024

Journal of Molecular Catalysis A: Chemical 387 (2014) 103–111
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Polyphosphoric acid–zirconia pillared clay composite catalytic system
for efficient multicomponent one pot synthesis of
tetrahydropyridines under environmentally benign conditions
Purabi Kar, B.G. Mishra∗, S.R. Pradhan
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 8 January 2014
Received in revised form 22 February 2014
Accepted 24 February 2014
Available online 3 March 2014
A series of zirconia pillared clay–polyphosphoric acid (PPA) composites are synthesized by adopting
different preparative strategies. Initially, PPA is intercalated to the clay matrix with and without the
use of structure expanding agent (CTAB). Subsequently, the PPA–clay composite is pillared with Zr-
polycation to form the composite materials. In an alternate approach, Zr-pillared clay is synthesized by
insertion of Zr-polycation, which is then used for dispersion of PPA moiety in the pillared clay matrix.
The synthesized composites are characterized by XRD, FTIR, UV–vis, TGA, EDX, FE-SEM and sorptometric
techniques. XRD study indicated an expansion in the clay structure after intercalation of PPA as well as
the Zr-polycations. The FTIR spectra exhibit characteristic vibrational features of the clay sheet as well as
PPA moiety indicating the structural stability of the composite materials. The phosphorous content in the
composite samples is analyzed using EDX study. FE-SEM study indicated morphological changes upon
intercalation of PPA and Zr-polycations to the clay matrix. The catalytic activity of the composite catalysts
has been examined for the synthesis of tetrahydropyridines under environmental benign conditions by
one pot multicomponent condensation of ␤-dicarbonyl compounds, substituted anilines and substituted
benzaldehydes. The PPA intercalated clay pillared with Zr polycations (PPA–ZrP) is found to be highly
efficient for the synthesis of structurally diverse substituted tetrahydropyridines under mild conditions.
Keywords:
Polyphosphoric acid
Zirconia pillared clay
Composite
Tetrahydropyridines
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
active components such as oxides, metals and organometallic com-
plexes [3,7–10]. In recent years there have been efforts to exploit
Clay materials are one of the most primordial and economic
materials that have been used as adsorbents and as catalyst to carry
out several acid catalyzed reactions. Although clays are used as
catalysts, they possess certain disadvantages such as low thermal
stability, loss of BrØnsted acidic sites and collapsing of interlayer
space at higher temperature [1,2]. To overcome these limitations,
pillaring of clays with inorganic cationic nanoclusters has been
done to enhance the thermal stability, pore volume, surface area
and acidity of the clay. Pillared clays are a class of microporous
materials with high surface area and acidic property. The interca-
lation of the inorganic cationic clusters imparts thermal stability
generating new micropores and acidic sites in the clay materials.
In the past years, these materials have been increasingly used as
catalysts for several important catalytic reactions [3–6]. Pillared
clays also provide a robust matrix for dispersion of catalytically
the acidic properties of pillared clay for synthesis of biologically
important molecules under environmentally benign conditions.
The Al-pillared clay has been found to be effective for the disper-
sion of the sulphated tin oxide (STO) nanoparticles. The STO/Al-P
materials have been used as an efficient catalyst for synthesis of
dihydropyrimidinones, coumarins and thiochromans. The syner-
gism between the acidic sites of the pillared clay and the STO
particles has been invoked to explain the higher catalytic activity
observed for this material [3]. Similarly, chromia pillared clays have
also been used as an efficient support to disperse silicotungstic acid
(STA) nanoparticles. The STA/Cr-P materials show superior catalytic
activity for the synthesis of dihydropyridines [11].
Polymeric acid display molecularly defined active sites suit-
able for a variety of chemical transformation. However, they
possess inherent disadvantages such as low surface area and
difficulty in separation and recovery when they are used in pris-
tine form. In order to enhance their surface area, stability, and
selectivity and to avoid corrosive property it is desirable to hetero-
genize these materials by dispersing in inorganic hard matrices by
∗
Corresponding author. Tel.: +91 0661 2462651; fax: +91 0661 2462659.
E-mail addresses: brajam@nitrkl.ac.in, bgmcat@rediffmail.com (B.G. Mishra).
http://dx.doi.org/10.1016/j.molcata.2014.02.024
1381-1169/© 2014 Elsevier B.V. All rights reserved.

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P. Kar et al. / Journal of Molecular Catalysis A: Chemical 387 (2014) 103–111
forming nanocomposites. In the present investigation, we have
utilized the pillared clay as a host matrix to disperse polyphos-
phoric acid and studied their catalytic activity for synthesis of
tetrahydropyridines. Polyphosphoric acid has promising applica-
tion potential as catalyst in acid catalyzed organic transformations.
Compared to other homogeneous corrosive acids, it is less cor-
rosive, mild and exhibit dehydrating properties, which has been
successfully exploited to carry out acylation and cyclization reac-
tion of a variety of substrates [12–14]. PPA is also used as a
catalyst for synthesis of heterocyclic compounds. The synthe-
sis of 2-aryl/alkyl substituted benzimidazoles, benzoxazoles and
benzothiazoles as well as purine derivatives has been accom-
plished by using PPA as catalyst [12,13]. Although there are
reports on the catalytic activity of PPA, its applicability is limited
due to disadvantages such as high viscosity and difficulty in
separation and handling. In order to extend its effectiveness
and to improve its recyclability, PPA has been supported over
porous materials having high surface area such as SiO2 [14].
The PPA–SiO2 has been used as efficient catalyst for the multi-
component one pot synthesis of polyhydroquinoline derivatives,
1,8-octahydroxanthenes, N,Nꢀ-alkylidenebisamide derivatives and
structurally diverse 3-benzoylisoxazoles [14–17]. Haniyeh Norouzi
et al. have investigated the catalytic activity of the alumina sup-
ported PPA for the synthesis of 14-aryl-14H-dibenzo xanthenes
[18]. Sachdev et al. have successfully synthesized SBA-15 supported
PPA by direct as well as impregnation method and the efficiency
of the prepared catalyst has been examined for the acylation of
naphthalene in liquid phase using acetyl chloride as acylating agent
[19].
Piperidine motifs are found in the basic skeleton of various
natural alkaloids and pharmaceuticals [20]. Synthesis of vari-
ous substituted piperidines has received considerable attention
in recent years because of their antibacterial, anti-inflammatory,
anticonvulsant and antimalarial properties [21]. Furthermore these
moieties are highly active in inhibiting the action of cancer caus-
ing enzymes farneysl transferase [22]. Hence different strategies
are being developed for the synthesis of substituted tetrahydropy-
ridines motifs. One pot multicomponent condensation reaction
(MCR) is found to be most favourable way for the synthesis of
these complex organic molecules within a very short period of
time. MCR has certain advantages such as atom economy, shorter
reaction time and eluding difficult purification processes. Ear-
lier literature shows that lot of effort are being devoted by the
researchers to develop a simple and easy protocol for the synthe-
sis of substituted tetrahydropyridines by using various catalysts
such as l-proline/TFA, bromodimethylsulfonium bromide (BDMS),
tetrabutylammonium tribromide (TBATB), molecular iodine, ceric
ammonium nitrate (CAN), ZrOCl2·8H2O, picric acid and bismuth
nitrate (Bi(NO3)3·5H2O) [22–29]. Although these catalysts have
been found to be effective for the multicomponent condensation,
they possess limitations in terms of greater reaction time, homoge-
nous nature of the reaction, requirement of higher amount of
catalyst and cost ineffectiveness. To overcome such drawbacks we
have employed PPA–Zr-pillared clay composite materials as cata-
lyst for synthesis of various substituted tetrahydropyridines under
environment benign conditions. In this study, different synthetic
strategies are adopted to maximize the dispersion of the PPA in
the clay and pillared clay matrix. The resulting composite mate-
rials are characterized by XRD, IR, FE-SEM, EDX, TGA, UV–vis and
sorptometric techniques.
industries, Japan) was used for the preparation of the pillared clay
catalysts. The cation exchange capacity of the clay is 120 mequiv
(100 g clay)−1 which is determined using the method reported
by Fraser and Russel [30]. Polyphosphoric acid (PPA) (85% P2O5),
zirconium oxychloride (ZrOCl2·8H2O), CTAB were procured from
Loba Chemie Pvt. Ltd., India. Double distilled water prepared in the
laboratory was used in the synthesis procedure.
2.1. Preparation of the catalyst
2.1.1. Preparation of PPA dispersed in parent clay matrix
(PPA–clay)
2 g of clay material was dispersed in 50 mL of deionized water
to form clay slurry. The slurry was sonicated for 15 min for better
dispersion of the clay platelets. 0.4 mL of PPA was added to the clay
suspension and the suspension was allowed to stir for 6 h. After stir-
ring for the required amount of time, the excess water was removed
by heating under vacuum. The resulting material was air dried and
grinded to obtain the polycrystalline powder of PPA–clay.
2.1.2. Preparation of PPA intercalated clay using CTAB as a
structure expansion agent (PPA–CTAB-clay)
5 g of montmorillonite clay was dispersed in 100 mL of double
distilled water and stirred for 2 h at room temperature followed
by sonication for 15 min to prepare clay slurry. 5 mmol of CTAB
was completely dissolved in 50 mL of distilled water under 20 min
of sonication. The CTAB solution was added dropwise to the clay
slurry under continuous stirring at room temperature. The result-
ing suspension was stirred for 12 h. The ensuing solid material was
filtered, washed three times with distilled water and then air dried
at 110 ◦C overnight to obtain CTAB intercalated clay material. 2 g of
CTAB-clay was subsequently dispersed in 50 mL of distilled water
to which required amount of PPA (0.4 mL) was added. The result-
ing slurry was kept under continuous stirring for 12 h which was
centrifuged, washed and dried overnight at 120 ◦C to obtain the
CTAB-PPA–clay.
2.1.3. Preparation of PPA dispersed in Zr-pillared clay matrix
(ZrP–PPA)
The 0.1 M Zr-pillaring solution was prepared by dissolving
required amount of ZrOCl2·8H2O in 500 mL of water. The pillar-
ing solution was subjected to heat treatment at 60 ◦C for 24 h. 5 g
of clay was dispersed in 250 mL of water to form a clay suspen-
sion. To ensure well dispersion, the clay suspension was stirred
at room temperature for 2 h followed by sonication for 15 min. The
pillaring solution was added dropwise to the clay suspension at the
rate of 50 mL/h under continuous stirring at room temperature. The
mixture was stirred at room temperature for 24 h which was sub-
sequently filtered and washed six times with deionised water to
remove the chloride ions. The obtained gel was air dried at 120 ◦C
followed by calcination at 500 ◦C for 2 h to obtain the Zr-pillared
clay. 2 g of as synthesized zirconia pillared clay was dispersed in
50 mL of distilled water and was sonicated for 15 min. 0.4 mL of
PPA was added to the Zr-P clay slurry under vigorous stirring. Stir-
ring was continued for 6 h at room temperature. The solvent was
evaporated by moderate heating under vacuum and the obtained
solid particles were subsequently air dried at 120 ◦C to obtain the
ZrP–PPA clay.
2.1.4. Preparation of PPA clay pillared with Zr-polycation
(PPA–ZrP)
The Zr-pillaring solution was prepared as per the procedure
described in Section 2.1.3. To a 2 g clay suspension in 50 mL water,
0.4 mL of PPA was added and stirred at room temperature for 6 h.
100 mL Zr-pillaring solution was added dropwise to the same pot
at the rate of 50 mL/h under continuous stirring. The mixture was
2. Materials and methods
tet
The Na-montmorillonite (Na0.35K0.01Ca0.02
)
(Si3.89Al0.11
)
oct
(Al1.60Fe0.08Mg0.32
)
O
10
(OH)2·nH2O
(Kunipia-F,
Kunimine

P. Kar et al. / Journal of Molecular Catalysis A: Chemical 387 (2014) 103–111
105
Fig. 1. XRD patterns of (a) parent clay, (b) air dried Zr-P, (c) calcined Zr-P, (d) PPA–clay, (e) PPA–ZrP and (f) ZrP–PPA.
Table 1
then allowed for stirring at room temperature for another 12 h. The
Basal spacing and surface area of different clay–PPA composite catalysts.
Surface area (m2/g)
˚
pillared clay particles were centrifuged and washed with deionized
water. The resulting solid is air dried at 110 ◦C overnight, and heated
at 300 ◦C for 2 h to obtain the PPA–ZrP clay material.
Material
Basal spacing (A)
Clay
12.8
19.0
15.0
18.5
19.2
18.8
30.2
176.0
17.5
38.0
95.2
86.3
Zr-P
PPA–clay
PPA–CTAB-clay
PPA–ZrP
ZrP–PPA
2.2. Characterization of the catalysts
The X-ray diffraction patterns of the clay material were recorded
using a Rigaku Ultima-IV diffractometer using Ni filtered CuK␣1
˚
(ꢀ = 1.5405 A) radiation. The XRD measurements were carried out in
the 2Â range of 1–15◦ with a scan speed of 1 degree per minute using
Bragg–Brantano configuration. The IR spectra of different clay sam-
ples (as KBr pellets) were obtained in transmittance mode by using
Perkin-Elmer Infrared spectrometer with a resolution of 4 cm−1
in the range of 400 cm−1 to 4000 cm−1. The UV–vis absorbance
spectra of the sample were recorded using Shimadzu spectrom-
eter model 2450 with BaSO4 coated integration sphere in the range
of 200–800 nm. Simultaneous thermogravimetric and differential
scanning calorimetric analysis of the samples was performed on
Netzsch, Model 449C apparatus in air atmosphere with linear heat-
ing rate (10 ◦C per min) from room temperature to 600 ◦C. The
specific surface areas of the samples were determined by BET
method using N2 adsorption/desorption at 77 K on a Quantachrome
Autosorb gas sorption system. The samples were degassed at 120 ◦C
for 5 h prior to the sorptometric studies. Acidity of the prepared cat-
alysts was determined by the nonaqueous titration method using
the procedure reported elsewhere [4]. FESEM studies were per-
formed by using Nova NanoSEM/FEI microscope. Prior to FESEM
analysis the powder sample is placed on carbon tape followed
by carbon coating. 1H NMR spectra were recorded with Bruker
400 MHz NMR spectrometer using TMS as internal standard.
tetrahydropyridines was carried out by stirring a mixture of aryl
aldehyde (2 mmol), ethylacetoacetate (1 mmol), aniline (2 mmol)
and 50 mg of different synthesized catalysts in acetonitrile solvent
at 50 ◦C. The reaction was monitored by TLC. Catalytic activity was
also examined at different temperatures and in different solvent
media. After the completion of the reaction, the reaction mixture
was filtered to separate the catalyst. The final product was recov-
ered from the acetonitrile solution and recrystallized from ethanol
to acquire the pure product. All the products obtained are known
compounds and are characterized by comparing their physical and
spectral characteristics with those reported in literature [22–29].
3. Results and discussions
3.1. Characterization of PPA–clay nanocomposite materials
The XRD patterns of the parent clay along with the Zr-pillared
clay and clay–PPA composite materials in the 2Â range of 3–15◦ are
presented in Fig. 1. The corresponding basal spacing values calcu-
lated from the XRD profile are presented in Table 1. The parent clay
shows a sharp and intense peak at 2Â = 6.8◦ corresponding to the
reflection from the {0 0 1} planes of the layered material. The basal
˚
2.3. Catalytic activity study
spacing of the parent clay is calculated to be 12.8 A (Fig. 1a). The
intercalation of Zr-polyoxocationic clusters leads to the shifting of
d0 0 1 peak to the lower 2Â value indicating an expansion in the clay
structure due to pillaring (Fig. 1b). The as synthesized Zr-pillared
The catalytic activity of the composite materials was evalu-
ated for the synthesis of tetrahydropyridines. The synthesis of

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P. Kar et al. / Journal of Molecular Catalysis A: Chemical 387 (2014) 103–111
Fig. 2. FTIR spectra of (a) parent clay, (b) PPA–clay, (c) CTAB-PPA–clay, (d) ZrP–PPA and (e) PPA–ZrP (Panel I and Panel II in the range of 4000–3000 cm−1 and 1700–400 cm−1
,
respectively).
˚
the peak broadening is the formation of a well dispersed interca-
lated PPA–clay nanocomposite. Since PPA is intercalated to the clay
matrix in swelled condition, it is likely that PPA molecules reside
in the interlayer region. The presence of PPA molecules can disturb
the stacking arrangement of the clay sheets along the [0 0 1] axis,
which give rise to the disordered structure. The presence of PPA
in the interlayer region is further evident from the shifting of peak
towards lower 2Â value for the parent clay intercalated with PPA
clay shows a basal spacing of 20.2 A. On thermal treatment for 2 h at
500 ◦C, the Zr-pillared clay shows a slight shift in the peak position
towards the higher 2Â value (Fig. 1c). This is due to conversion of
the polyoxocations to oxide nanoclusters.
However, there is no appreciable difference in the intensity or
broadness of the peak indicating the calcined Zr-pillared clay is
stable up to 500 ◦C. The d0 0 1 value for the calcined Zr-P material
˚
is found to be 19.0 A. From the XRD study it is illustrated that heat
treatment up to 500 ◦C does not noticeably affect the crystallinity
and stacking pattern of the pillared clay material. Fig. 1d–f shows
the XRD patterns of different PPA–clay composite materials. Inter-
calation of PPA in general leads to the broadening of the peaks as
compared to the respective clay materials. The broadening of the
peak implies loss of crystallinity to a certain degree upon PPA inter-
calation. This may be attributed to mainly two factors. First, because
˚
(PPA–clay). The d0 0 1 value observed for PPA–clay is 15 A (Table 1).
The XRD pattern of the PPA–ZrP composite is shown in Fig. 1e.
The XRD peak is broadened indicating a decrease in the crystalline
character as compared to pure Zr-P. However, the noticeable point
is that when the PPA–clay is crosslinked using Zr-polycation a basal
˚
spacing value (19.2 A) similar to the Zr-pillared clay was obtained
(Table 1 and Fig. 1e). This observation is indicative of the fact that
the pillaring process imparts structural rigidity to the PPA–clay
structure and the PPA molecules are trapped in the lateral space
between the pillars. In case of ZrP–PPA materials, which is pre-
pared by the dispersion of PPA molecules in the micropores of Zr-P,
of acidic nature of PPA, it can hydrolyze the Al
O Si bond of the
clay sheet which may result in disorder clay structure and conse-
quent loss in crystallinity. However, considering the noncorrosive
nature of PPA compared to other mineral acid this can occur to a
small extent. The second important factor which can contribute to
˚
basal spacing value of 18.8 A is observed (Fig. 1f).
Table 2
Physicochemical characteristics and catalytic activity of the clay–PPA composite materials.
Material
Acidic sitesa (mmol g−1
)
Phosphorous content (wt%)
Yieldb (%)
Rate (mmol h−1 g−1
)
Rate (mmol h−1 m−2 × 10−3
)
TOFc (h−1
)
PPA–clay
PPA–CTAB-clay
PPA–ZrP
0.36
0.46
0.73
0.63
9.6
2.6
7.1
8.5
30.2
36.2
83.3
65.2
2.4
2.9
6.7
5.2
6.9
3.8
3.5
3.0
6.7
6.2
9.1
8.2
ZrP–PPA
Reaction condition: benzaldehyde:aniline:ethylacetoacetate 2:2:1, temperature 50 ◦C, 5 mL of acetonitrile, reaction time 5 h.
a
Calculated from non-aqueous titration.
Refers to pure and isolated yield.
Calculated with respect to the benzaldehyde conversion in the reaction mixture.
b
c

P. Kar et al. / Journal of Molecular Catalysis A: Chemical 387 (2014) 103–111
107
hydroxyl group and the water molecules present in the inter-
layer space of the clay materials, respectively [31]. Strained water
molecules present in the first coordination sphere of the interlayer
cations contribute significantly to the peak at 1636 cm−1 (Fig. 2a,
Panel II) [29]. For the parent clay, three bands are observed at the
finger print region at 915, 845 and 805 cm−1, which are attributed
to the bending vibration modes of Al Al OH, Al Mg OH and
Mg Mg OH groups, respectively, in the octahedral layer of the
montmorillonite clay [31,32]. After the intercalation of the PPA to
the clay, noticeable changes are observed in the stretching region
of the FTIR spectra. The OH stretching peak for PPA intercalated
clay (Fig. 2b) is very broad that ranges from 3000 to 3600 cm−1
.
The broadening of the OH band can be ascribed to the forma-
tion of a variety of hydrogen bonding in presence of PPA inside the
clay interlayer. The OH group of PPA can form hydrogen bond
with the OH group of the clay sheet as well as the coordinated
water molecule. Since the clay structure contain OH groups dif-
fering in bond strength due to their location and coordination, it is
expected that a wide range of hydrogen bonds of different strength
can form in clay interlayer. These hydrogen bonds are responsible
for broadening of the OH peak [33]. PPA in its bulk state shows
a prominent peak at 1015 cm−1 due to the stretching vibrations of
P
P
O
O
P bond. The asymmetric stretching and bending vibration of
P absorb IR radiation at 924 and 484 cm−1, respectively [34].
These characteristic peaks with slight shift are observed for the
clay–PPA composite materials which support the intercalation of
PPA into the clay materials. For CTAB-PPA–clay, in addition to the
characteristic peaks corresponding to the clay lattice and PPA, well
distinguished peaks for CTAB are observed at 2851 and 2920 cm−1
after intercalation of PPA.
Fig. 3. UV–vis spectra of (a) parent clay, (b) PPA–clay, (c) ZrP–PPA and (d) PPA–ZrP.
These peaks are ascribed to symmetric and asymmetric stretch-
ing vibrations of C CH2 present in the cetyl group of CTAB [35].
This observation implies the limited uptake of PPA in presence of
CTAB. CTAB being an amphiphile generates a hydrophobic environ-
ment in the clay matrix which is not conducive for the intercalation
The FTIR spectra of different clay materials are represented in
Fig. 2. For parent clay, a sharp intense band is observed at 3636 cm−1
along with a broad less intense peak at 3450 cm−1 in the stretching
frequency region of the FTIR spectrum (Fig. 2a, Panel I). These bands
are assigned to the O H stretching vibration from the structural
Fig. 4. Thermogravimetric profile of (I) Zr-P, (II) PPA–clay and (III) ZrP–PPA materials.

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P. Kar et al. / Journal of Molecular Catalysis A: Chemical 387 (2014) 103–111
of PPA which is predominantly hydrophilic in nature. This fact is
also evident from Table 2 which shows CTAB-PPA clay has less
phosphorous content as compared to other clay materials.
The UV–vis spectra of the parent clay along with other clay poly-
mer composite materials are presented in Fig. 3. The band observed
at 247 nm (Fig. 3a) is characteristic for montmorillonite clay and
attributed to the charge transfer transition for the structural iron
present in the octahedral layer of the clay mineral (Fe3+ ← O2−, OH−
or OH2) [4].
Upon incorporation of PPA into the clay matrix the band max-
ima was found to shift towards the higher wavelength side.
The PPA–clay shows absorption maximum at 265 nm which is
attributed to the change in coordination environment in the clay
interlayer in presence of the PPA moieties. The same shift in band
has also been observed for other composite materials. All the PPA
containing composites shows the UV absorption maxima in the
range of 255–265 nm. In case of the PPA–ZrP and ZrP–PPA an addi-
tional band is observed at 210 nm. This band can be ascribed to the
Zr4+ (4d) ← O2− (2p) charge transfer transition from the zirconia
nanopillars present in the clay interlayer [31].
The TGA profile of the air dried Zr-P, PPA–clay and ZrP–PPA clay
materials is depicted in Fig. 4. Three major weight loss regions
have been observed for the air dried Zr-pillared clay sample in
the range of 50–200 ◦C, 400–550 ◦C and >550 ◦C (Fig. 4, Panel I).
The removal of adsorbed water molecules present in the inter-
layer region of pillared materials is responsible for the weight loss
between 50–200 ◦C. However, the weight loss detected in the range
of 400–550 ◦C and >500 ◦C are accredited to the loss of structural
water molecules and dehydroxylation of the pillars as well as clay
sheets, respectively. The weight loss at high temperature region
(>550 ◦C) is more gradual without any well-defined inflection point
[4]. For ZrP–PPA three weight loss regions are also observed (Fig. 4,
Panel III). The weight loss between 35–120 ◦C is due to the release
of water molecules physically adsorbed into the pores of the clay
matrix. The second weight loss between 125–180 ◦C is due to
loss of water molecules coordinated to the pillars. These water
molecules are in a coordinated state and hence require higher tem-
perature for their removal. The third weight loss region observed
in the temperature range 430–580 ◦C is ascribed to the removal of
polyphosphoric acid. This weight loss is relatively sharp and can
be clearly differentiated from the high temperature weight loss
observed for air dried Zr-pillared clay sample.
The PPA–clay shows two major weight loss regions in the tem-
perature range of 75–175 ◦C and 180–250 ◦C. The low temperature
weight loss can be ascribed to the removal of physisorbed water
molecules present in the clay interlayer and attached to the clay
sheet. The weight loss in the region 180–250 ◦C is probably due
to the removal of polyphosphoric acid in its monomeric form.
Polyphosphoric acid is a linear polymer which contains mixture of
oligomers of orthophosphoric acid up to 14 phosphorous units [36].
Polyphosphoric acid undergo hydrolysis at a very sluggish rate in
presence of water to convert to its monomeric form i.e. orthophos-
phoric acid. The rate of hydrolysis becomes significant at higher
temperature in presence of excess of water [37]. It is possible that
the hydrophilic environment of the clay interlayer coupled with the
constrained interlayer region of the clay can promote the hydrol-
ysis of PPA to orthophosphoric acid (boiling point 160 ◦C) which
is released in the temperature range of 180–250 ◦C. This hydrol-
ysis process is suppressed to a greater extent in the Zr-pillared
clay matrix. Although the hydrolysis of PPA in Zr-P matrix can-
not be ruled out completely, a significant fraction of PPA remains
in polymeric form and are removed in the temperature range of
430–580 ◦C which is the normal boiling point range of PPA used in
this study.
Fig. 5. FE-SEM images (a) PPA–ZrP and (b) ZrP–PPA.
relatively smooth surface extending up to the edge of the crystal-
lites. The layer to layer orientation of the particles seems to be
the favored orientation which leads to the morphology observed
in Fig. 5a.
In case of the PPA–ZrP composite, the PPA molecules are present
in the interlayer space which is sandwiched between the clay
sheets. The protons present in the PPA molecules can form an elec-
trical double layer between the clay sheets facilitating the face
to face interaction. Where as in case of ZrP–PPA, the pillaring of
the clay with the Zr-polycations provides structural rigidity to the
clay lattice. Upon dispersion of the PPA in the Zr-P matrix, the PPA
molecules occupy the micropores as well as the external surface.
The presence of the PPA and its subsequent interaction with the
faces and edges of the Zr-P crystallites distorts the lamellar struc-
ture of the clay platelets which results in particles with folded
morphology (Fig. 5b). The surface area of the composite materials
was determined by N2 sorption. Pure clay shows Type-I adsorp-
tion isotherm according to the BDDT classification indicating the
material to be microporous in nature (figure not shown). Pillaring
with the Zr-pillared clay significantly increases the microporous
character of the clay material. There is a significant increase in the
The FE-SEM image of PPA–ZrP and ZrP–PPA composite materi-
als is presented in Fig. 5. The PPA–ZrP composite material shows

P. Kar et al. / Journal of Molecular Catalysis A: Chemical 387 (2014) 103–111
109
R2
NH
N
O
CHO
NH2
O
O
50 oC, 50 mg PPA-Zr-P
acetonitrile 5mL
R
2
2
H3C
R
R1
R2
R1
R1
R2
Scheme 1. Synthesis of tetrahydropyridines by multicomponent one pot condensation of arylaldehydes, substituted aniline and ␤-dicarbonyl compounds.
surface area and pore volume after pillaring with the Zr-polycation.
The pillared clay–PPA composite shows lower value of surface area
compared to the pure Zr-pillared clay due to the blockage of the
micropores by the polymeric material.
composite systems along with the yield of THP obtained after
5 h of reaction. Among all the composite materials the PPA–ZrP
shows highest yield of the product. This material also exhibits
higher surface area and acidic property compared to other com-
posite materials. Since the composite materials display different
physicochemical characteristics, the reaction rates are calculated
in terms of unit surface area and per gram of the material which
is presented in Table 2. It is observed that among all the compos-
ite catalyst, the PPA–ZrP shows higher reaction rate per gram of
the material. The higher reaction rate per unit surface observed
for the PPA–clay composite material is due to the higher density
of phosphorous atom per unit area compared to other catalyst.
This material contains higher percentage of phosphorous atom as
observed from the EDAX analysis. However, in terms of total num-
ber of acidic site exposed to the surface, the PPA–clay display less
number of sites compared to other catalyst. This is due to the fact
that the PPA blocks the micropore of the clay material significantly
decreasing its exposed surface. Moreover, due to the diffusional
3.2. Catalytic studies for synthesis of tetrahydropyridines
The catalytic activity of the clay–polymer composite systems is
evaluated for the synthesis of tetrahydropyridines (THP) by one-
pot multicomponent condensation of arylaldehydes, substituted
aniline and ␤-dicarbonyl compounds at 50 ◦C using acetonitrile as
solvent (Scheme 1).
At first emphasis is led on optimizing the reaction condition.
Multicomponent reaction of benzaldehyde, aniline, and ethylace-
toacetate served as paradigm reaction for optimization. Initially,
different clay–PPA composite materials synthesized in this work
are evaluated for their activity using the model reaction. Table 2
shows the physicochemical characteristics of different clay–PPA
Fig. 6. Effect of catalyst amount, temperature and solvent on percentage yield of the tetrahydropyridine synthesized by one-pot multicomponent reaction of benzaldehyde,
aniline and ethylacetoacetate.

110
P. Kar et al. / Journal of Molecular Catalysis A: Chemical 387 (2014) 103–111
Table 3
to that of methyl acetoacetate (Table 3). The minimum yield is
observed when benzoyl acetone is used as ␤-dicarbonyl compound
in the optimized protocol. The higher yield observed ethylace-
toacetate can be attributed to the greater acidic character of the
alpha protons which facilitate the formation of the enamine. In
case of benzoyl acetone the COC6H5 group has electron with-
drawing tendency and hence display weak acidic protons compared
to its ester counterparts. However, for substituted aromatic alde-
hydes the electronic effect of the electron withdrawing or electron
donating groups do not have much impact over the yield of the
product. Among all aldehydes, the yield of the THP is less for
p-nitrobenzaldehyde as compared to other substituted benzalde-
hydes. This is due to greater stability of the imine as the conjugation
is extended to the NO2 group. After completion of the reaction,
the catalyst particles are filtered from the reaction mixture. The
catalyst is washed three times with 10 mL portion of ethanol and
subsequently heat treated at 300 ◦C for 2 h to regenerate the cata-
lyst. The recyclability of the catalyst is studied for three consecutive
cycles without any significant loss in activity (Table 3, entry 11,
yields, 83%, 1st; 80%, 2nd; 78%, 3rd).
Catalytic activity of PPA–ZrP composite materials for the synthesis of tetrahydropy-
ridines by multicomponent condensation of arylaldehydes, substituted aniline and
␤-dicarbonyl compounds.
Sl no
R
R1
R2
Yield (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
OCH3
OC2H5
C6H5
OCH3
OC2H5
C6H5
OCH3
OC2H5
C6H5
OCH3
OC2H5
C6H5
OCH3
OC2H5
C6H5
OC2H5
OC2H5
OC2H5
OC2H5
OC2H5
OC2H5
OC2H5
4-NO2
4-NO2
4-NO2
4-Cl
4-Cl
4-Cl
4-F
4-F
4-F
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
68.1
74.3
62.7
75.3
82.4
68.2
81.2
79.8
72.6
78.5
83.3
74.5
76.6
84.5
68.4
78.0
72.0
90.4
70.8
88.2
66.7
86.2
H
4-OCH3
4-OCH3
4-OCH3
H
H
H
H
H
H
H
4-Cl
2-Cl
3-OH
4-F
4-OCH3
4-NO2
2-OH
4. Conclusions
In this work, we have reported the synthesis, characterization
and catalytic application of novel polyphosphoric acid–zirconia
pillared clay nanocomposite systems. The PPA molecules are inter-
calated into the clay lattice which is subsequently pillared with
Zr-polycations to trap the PPA inside the clay interlayer. The expan-
sion of the clay structure upon PPA intercalation and subsequent
pillaring with Zr-polycation was confirmed from XRD study. The
characteristic vibrational features of the clay and PPA moiety
remain intact in the composite material. The clay material retains
the PPA molecules inside the clay interlayer as determined from
the EDX analysis. Various substituted THPs are synthesized in high
yield and purity by one pot multicomponent condensation of aro-
matic aldehydes, substituted aniline, and ␤ dicarbonyl compounds
using PPA–ZrP materials as heterogeneous catalyst. The PPA–ZrP
material is found to be highly efficient for the synthesis of struc-
turally diverse tetrahydropyridines. The protocol developed using
the PPA–ZrP composite catalyst was found to be advantageous
in terms of simple experimentation, shorter time, high yield and
purity of the products and recyclable catalyst.
constraint the internal sites remain inaccessible for catalysis. The
turn over frequency (TOF) calculated for all the composite materials
are presented in Table 2.
The PPA–ZrP shows highest TOF compared to other compos-
ite materials. The PPA–ZrP material has been selected for further
study for THP synthesis. The amount of catalysts in the reaction
mixture is varied between 25 mg and 100 mg. It is observed that for
reaction involving 2 mmol of benzaldehyde, the yield of the reac-
tion improves with catalyst amount up to 50 mg. Further increase
in the catalyst amount only marginally influences the yield of the
products (Fig. 6, Panel I).
Hence the catalyst amount was fixed at 50 mg. The effect of reac-
tion temperature is studied by varying the temperature in the range
of 30–60 ◦C. It is observed that with increase in temperature the THP
yield increases significantly up to 50 ◦C. Further increase in temper-
ature to 60 ◦C marginally improves the yield (Fig. 6, Panel II). Hence
in this study, the reaction temperature is fixed at 50 ◦C. In order to
study the effect of reaction media on the catalytic activity, solvents
with different polarity are used in the optimized reaction protocol.
It is noticed that for nonpolar solvent such as hexane the yield of the
product is very less. The yield is found to improve upon use of polar
solvent. Among different solvent tried for the reaction, comparable
yields of the THP are obtained in acetonitrile and ethanol. Hence
acetonitrile is used as a solvent for further study (Fig. 6, Panel III).
After optimizing the reaction conditions, we further investi-
gated the scope and limitation of the optimized protocol using
different ␤-dicarbonyl compounds, substituted aldehydes and sub-
stituted anilines (Table 3). It is observed that aniline substituted
with electron donating groups is more active for the synthesis of
THPs. The mechanistic pathway for formation of THP has been
reported earlier [25]. The aniline reacts with one mole each of
␤-dicarbonyl compound and arylaldehyde to form the correspond-
ing enamine and imine, respectively. Subsequently, the enamine
undergoes intermolecular Mannich reaction with imine to gen-
erate an intermediate. This intermediate reacts with aldehyde
followed by cyclization by intramolecular Mannich reaction to form
the THPs. The presence of electron donating group in the sub-
stituted aniline increases the nucleophilicity of the amine group
thereby facilitating the formation of enamine and imine. Upon vari-
ation of ␤-dicarbonyl compounds, it is noticed that the yield of
the products are better in case of ethylacetoacetate as compared
Acknowledgement
PK would like to thank NIT, Rourkela for senior research fellow-
ship.
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.molcata.2014.02.024.
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