Synthesis and characterization of Si-Zr-Mo nanocomposite as a rapid and efficient catalyst for aromatization of Hantzsch 1,4-dihydropyridines

Article abstract of DOI:10.1016/j.crci.2012.10.005

Nanocomposite of Silica-zirconia-molybdate designated as Si-Zr-Mo was prepared via the reaction of the in situ generated zirconium-tetra octanoxide [Zr(Oct)₄] through condensation of zirconium-tetra-n-butoxide and 1-octanol in a sol-gel method with sulfuric acid and tetraethylorthosilicate (TEOS) followed by grafting of MoO₄^2- on modified silico zirconia nanocomposite under reflux conditions. The prepared nanocomposite was characterized using inductively coupled plasma (ICP), N₂ sorption isotherms, transmission electron microscopy (TEM), and FT-IR spectroscopy. The as prepared nanocomposite had a surface area and pore dimension of 140 m ²/g and 1.48 nm, respectively. The morphology of sulphated silico zirconia nanocomposite after immobilization MoO₄^2- has been changed from nanoparticles to nanaorods. It was found that the synthesized nanocomposite successfully catalyze the oxidative dehydrogenation of 1,4-dihydropyridines (1,4-DHPs) with 92-100% conversion and 80-100% selectivity toward the desired products.

Full text of DOI:10.1016/j.crci.2012.10.005

C. R. Chimie 16 (2013) 176–182
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´
Full paper/Memoire
Synthesis and characterization of Si-Zr-Mo nanocomposite as a rapid and
efficient catalyst for aromatization of Hantzsch 1,4-dihydropyridines
Masoomeh Sharbatdaran a,b, Leila Jafari Foruzin a, Faezeh Farzaneh a,
Majid Mojtahedzadeh Larijani b
,
*
a Department of Chemistry, Alzahra University, Vanak, Tehran, 1993891176, Iran
b Agricultural, Medical and Industrial Research School, NSTRI, Karaj, 31485/498, Iran
A R T I C L E I N F O
A B S T R A C T
Article history:
Nanocomposite of Silica-zirconia-molybdate designated as Si-Zr-Mo was prepared via the
reaction of the in situ generated zirconium-tetra octanoxide [Zr(Oct)4] through condensa-
tion of zirconium-tetra-n-butoxide and 1-octanol in a sol–gel method with sulfuric acid and
Received 15 July 2012
Accepted after revision 11 October 2012
Available online 29 November 2012
2À
tetraethylorthosilicate (TEOS) followed by grafting of MoO4 on modified silico zirconia
nanocomposite under reflux conditions. The prepared nanocomposite was characterized
using inductively coupled plasma (ICP), N2 sorption isotherms, transmission electron
microscopy (TEM), and FT-IR spectroscopy. The as prepared nanocomposite had a surface
area and pore dimension of 140 m2/g and 1.48 nm, respectively. The morphology of
Keywords:
Silica-zirconia-molybdate
Nanocomposite
Oxidative aromatization
1,4-dihydropyridine
Dehydrogenation
2À
sulphated silico zirconia nanocomposite after immobilization MoO4 has been changed
from nanoparticles to nanaorods. It was found that the synthesized nanocomposite
successfully catalyze the oxidative dehydrogenation of 1,4-dihydropyridines (1,4-DHPs)
with 92–100% conversion and 80–100% selectivity toward the desired products.
´
ß 2012 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.
1. Introduction
agents such as alkaline or acidic reagents also have a
significant effect on the catalyst activity. The most
Many efforts have been devoted for the preparation of
dehydrogenation catalysts with nanoparticle structures
[1]. The study of sulfated metal oxides has become an
active area of research because of the high catalytic activity
of organic reactions manifested by such compositions [2].
Silica/zirconia is known to have extensive catalytic
activities in processes such as isomerization [1], alkylation
[3], esterification [2] and those involving oxygenation of
hydrocarbons [4]. The synthetic methods to such solid
acids mainly differ in the type of precursor, hydrolyzing
agent and sulfating agent [1–5]. Various zirconium
compounds, such as Zr(NO3)4, ZrCl4, zirconium isoprop-
oxide, zirconyl chloride, zirconium oxychloride and
sometimes zirconia itself, have been used for the
preparation of these solid acids [6–8]. The hydrolyzing
commonly used sulfating species are H2SO4 and
(NH4)2SO4. Some sulfur compounds like H2S and SO2 have
also been used [7,9]. Heterogenizing homogeneous catal-
ysis by covalently anchored metal complexes based on
chemically modified support materials have been devel-
oped these days [10,11].
Dehydrogenation reactions have been the subject of
extensive studies for obtaining derivatives which can be
used in pharmaceutical industries. Among the reactions
which have not yet been industrially developed, dehydro-
genation of 1,4-DHPs could be an interesting choice due to
their antioxidant effects that may contribute to their
pharmacological activities [12]. The oxidative dehydroge-
nation (aromatization) of 1,4-DHPs to the corresponding
pyridines has been extensively studied in view of the
pertinence of the reaction to the metabolism of the calcium
channel blocking drugs used in the treatment of various
cardiovascular disorders [13]. Furthermore, the oxidation
of Hantzsch 1,4-DHPs is one of the ubiquitous issues in
* Corresponding author.
E-mail address: faezeh_farzaneh@yahoo.com (F. Farzaneh).
1631-0748/$ – see front matter ß 2012 Acade´mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
http://dx.doi.org/10.1016/j.crci.2012.10.005

M. Sharbatdaran et al. / C. R. Chimie 16 (2013) 176–182
177
inorganic chemistry. In recent years several groups have
reported various methods for aromatization including
oxidation with ferric nitrate on a solid support [14], ceric
ammonium nitrate [15], vanadomolybdo phosphate het-
eropolyacids [16], 4-phenyl-1,2,4-triazole-3,5-dione [17]
and palladium on carbon [18]. However, most of these
reactions required an extended period of time for
completion, utilize strong oxidants in large excess and
affording only modest yields of the products. Therefore, it
is still difficult to design a catalytic oxidative dehydroge-
nation (ODH) system with high yield and good selectivity
due to the tendency of the formed pyridine derivatives
toward further oxidation. Many catalytic systems had been
investigated by scientists in an attempt to find an optimum
catalyst [19–21]. The most studied systems for ODH of 1,4-
DHPs were based on vanadium [19] and Co complex as
catalyst [22,23].
(0.5 g) for 3 h. The solid product was then decanted by
centrifugation, washed with deionized water and dried at
100 8C.
2.2. Catalytic dehydrogenation
2.2.1. Preparation of 1,4-DHPs
1,4-DHPs were successfully prepared using the previ-
ously reported method [26].
2.2.2. Oxidative dehydrogenation of 1,4-DHP with H2O2
In a typical procedure, 1,4-DHP (30 mg, 0.09 mmol) and
catalyst (30 mg) were added to EtOH (15 ml), H2O2 (2 ml,
30%) and the mixture was refluxed at 60 8C until the
complete disappearance of 1,4-DHS based on TLC. The
products were then subjected to GC and GC-MS analyses.
Among various transition metals, molybdenum has not
previously been used in ODH reactions. Attempts to
synthesize and characterize of Si-Zr-Mo nanocomposite
by sol–gel method and investigation of catalytic activity
toward the dehydrogenation of 1,4-DHPs both in solvent
and solventless will be discussed in this presentation.
2.2.3. Solventless dehydrogenation of 1,4-DHPs
Dehydrogenation reactions were performed in
a
stirring round bottom flask fitted with a water-cooled
condenser at atmospheric pressure under reflux condi-
tions. A mixture of benzaldehyde (0.2 g, 2 mmol), ethyl
acetoacetate (0.5 g, 4 mmol) and ammonium acetate (0.2 g,
3 mmol) and catalyst (0.1 g) was stirred at 90 8C until the
reaction was completed. The products were extracted by
ethyl acetate and dried over anhydrous Na2SO4. Two
isomers of products were isolated by column chromatog-
raphy on silica gel (60–120 mesh) and elution with ethyl
acetate–chloroform mixture (1:4). The products were
identified by GC and GC-MS analyses.
2. Experimental
2.1. Materials
All chemicals were purchased from Merck and used
without further purification. Solvents were dried and
distilled under nitrogen prior to use according to a
standard procedure. Zirconium octanoxide was synthe-
sized from zirconium-tetra-n-butoxide and 1-octanol by
the alcohol interchange method, according to the general
procedure for the preparation of metal alkoxides and was
purified by vacuum distillation [24]. The reaction and
manipulation of zirconium octanoxide synthesis were
carried out under an atmosphere of dry nitrogen, using
standard Schlenk techniques.
2.3. Instrumentation
Infrared spectra were performed (KBr pellets) on a
Bruker Tensor 27FT-IR spectrometer. Chemical analysis of
samples was carried out with Varian 150AX inductively
coupled plasma (ICP) emission spectrometer. Electron
microscopy was performed on a Philips EM208S, trans-
mission electron microscope (TEM). Surface areas, pore
volume and pore size distributions were obtained from the
N2 isotherms which determined at 77 K using Quanta-
chrome Nova 2200, Version 7.11 Analyzer. The products
were analyzed by GC and GC-MS using Agilent 6890 Series,
with FID detector, HP-5, 5% phenylmethylsiloxane capil-
lary and Agilent 5973 Network, mass selective detector,
HP-5 MS 6989 Network GC system, respectively.
2.1.1. Preparation of sulfated silica-zirconia nanocomposite
The sulfated silica-zirconia nanocomposite was pre-
pared in three steps. Initially, Zr(Oct)2SO4 was prepared
by drop-wise addition of sulfuric acid (4.9 mmol, 0.26 ml,
96%) to Zr(Oct)4 solution in 0.6:1 molar ratio at room
temperature. Zr-Si mixed oxide was prepared with TEOS,
EtOH, AcOH and distilled water. The corresponding
molar ratios of xZr(Oct)2SO4:yTEOS: 7(x + y) EtOH:
10(x + y)H2O: 2(x + y)AcOH in which the desired experi-
ment is x = 8.2, y = 82 mmol. EtOH, TEOS, H2O and AcOH
were mixed by stirring. In the next step the solution of
Zr(Oct)2SO4 (0.1 M, in 82 ml 1-octanol) was added to
TEOS solution under vigorous stirring [25]. The resulting
sol was stirred for 3 h. The produced gel was decanted by
centrifugation with 3900 r/min for 15 min and drying at
180 8C.
3. Results and discussion
3.1. Characterization of Si-Zr-Mo nanocomposite
Si-Zr-Mo nanocomposite was prepared according to the
procedure presented in Fig. 1. The exothermic reaction
between sulfuric acid and Zr(Oct)4 affords the sulfated
zirconia (Scheme 1, path a). Subsequent treatment with a
mixture of TEOS, EtOH, acetic acid and distilled water gives
the corresponding Zr-Si mixed oxide (Scheme 1, path b).
Upon addition of Na2MoO4 to Zr-Si mixed oxide, Si-Zr-Mo
nanocomposite is generated presumably via reaction
2.1.2. Preparation of silica-zirconia-molybdate
MoO3 (1.5 g, 7.7 mmol) dissolved in NaOH (10 mL, 5 M)
was initially titrated with HCl (0.5 M) until pH = 7, followed
by refluxing of the solution with sulfated silica-zirconia gel
2À
between Zr-SO4 and MoO4 (Scheme 1, path c). In fact,
the adsorption capacity of Mo to the nanocomposite

178
M. Sharbatdaran et al. / C. R. Chimie 16 (2013) 176–182
Fig. 1. The procedure for preparing sulfated silica-zirconia nanocomposite.
(a)
(b)
Zr(Oct)4+H2SO4
Zr(Oct)2(SO4 )2+ 2OctOH
O
O
O
O
O
TEOS + H2O + EtOH + AcOH
Zr(Oct)2(SO4 )2
Si
Zr
S
O
2-
2-
MoO4
SO4
O
O
O
O
O
O
O
O
O
O
Si
Zr
Mo
(c)
Si
Zr
S
2Na+
O
O
Scheme 1. Synthesis of Si-Zr-Mo nanocomposite.
depends on the number of SO42À molecules bonded to the
Zr atom in the nanocomposite. The weight percentages of
Si, Zr and Mo in the nanocomposite were found to be 5.69,
1.17 and 18.58, respectively. Although the amount of S in
the nanocomposite before Mo adsorption was 0.4%, but no
significant amount of sulphur was detected by ICP
technique after Mo adsorption.
the first one is as nanoparticles but for last one as
nanorods. Interestingly, different shapes of particles which
are evident in the micrographs of the samples may be due
2À
to the reaction between SO4
and MoO42À, through
covalent bonding, with molybdenum oxygen’s and zirco-
nium. Moreover, the amount of adsorption of Mo control
the size and shape of the particles [27].
À2
The TEM images of SO4À2/ZrO2–SiO2 and Si-Zr-Mo
nanocomposite are shown in Fig. 2a, b.
The FT-IR spectra of Zr(Oct)4, Zr(Oct)2SO4, TEOS, SO4
/
ZrO2–SiO2 and Si-Zr-Mo are shown in Fig. 3a–e, respec-
tively, in the range of 400–4000 cmÀ1. No absorption was
found in the range of 1750–2700 cmÀ1. The FT-IR spectra of
As seen in this figure, the morphology before and after
2À
addition of MoO4 are completely different, in which for
Fig. 2. The TEM image of (a) SO4À2/ZrO2–SiO2 nanocomposite, (b) Si-Zr-Mo nanocomposite.

M. Sharbatdaran et al. / C. R. Chimie 16 (2013) 176–182
179
Fig. 4. a: N2 adsorption-desorption isotherm for SO4À2/ZrO2–SiO2
À2
nanocomposite dried at 180 8C; b: pore size distribution of SO4
ZrO2–SiO2 nanocomposite dried at 180 8C.
/
bonds present in inorganic chelating bidentate sulfate [3].
Fig. 3c shows the bands displaying at 800, 1070 and
1225 cmÀ1 due to the stretching vibrations of Si–O and a
band at 520 cmÀ1 belonging to bending vibration for Si–O
of TEOS [2]. The broad bond appearing at 1000–1500 cmÀ1
indicated in Fig. 3d is probably due to vibration of Zr–O–Si
present between ZrO2 and SiO2 parts [2]. As seen in Fig. 3e,
the appearance of two bands at 907 and 960 cmÀ1 is
attributed to stretching modes of Mo–O bonds of Mo
particles present on the nanocomposite [28,29]. Compari-
son of the FT-IR spectra of Fig. 3b, d with that of Fig. 3e of
Fig. 3. The FT-IR spectra of (a) Zr(Oct)4, (b) Zr(Oct)2SO4, (c) TEOS, (d)
SO4À2/ZrO2–SiO2 nanocomposite, (e) Si-Zr-Mo nanocomposite.
zirconium octanoxide (Fig. 3a) shows vibrations at 500,
610, 725 cmÀ1
, corresponding to Zr–O bending and
stretching vibrations. The bands appearing at 1000–
1300, 1300–1400 and 2800–2900 are attributed to the
C–O, CH3 or CH2 and C-H stretchings of alkoxy groups,
respectively. A broad band displaying at 3400 cmÀ1 is due
to the OH stretching associated with alcohol residue
present in zirconium alkoxide. The FT-IR spectrum of
sulfated zirconia (Fig. 3b) indicates the vibration bands of
2À
the nanocomposite indicates that the MoO4 has been
exchanged with SO42À anions because no vibration due to
the presence of SO42À was observed. These observation are
in good agreement with the ICP results.
The nitrogen adsorption/desorption isotherms and pore
size distribution of nanocomposite are shown in Fig. 4a, b
respectively. Nitrogen adsorption/desorption analyses
were performed in order to investigate the textural
2À
SO4
group in the region of 900–1200 cmÀ1 and
asymmetric and symmetric stretching vibrations of S–O
Table 1
The effect of reaction time and amounts of catalyst on dehydrogenation of DHP.
R
H
R
EtO2C
Me
CO2Et
Me
EtO2C
Me
CO2Et
Me
Conditionsa
R = Ph
N
N
H
Entry
Time
Catalyst
Conversion
(%)
Selectivity
(%)
(min)
(g)
1
2
3
4
5
6
7
3
3
0.003
0.007
0.015
0.03
0.03
0.03
–
40
62
100
100
100
100
100
100
100
3
67
3
100
75
2
1
45
600
25

180
M. Sharbatdaran et al. / C. R. Chimie 16 (2013) 176–182
Table 2
The effect of R group on dehydrogenation of DHP derivativesa.
Entry
R
Solvent
Time
Conversion
(%)
Selectivity
(%)
(min)
1
2
3
H
EtOH
EtOH
EtOH
1
2
3
100
100
100
100
100
100
Me
ph
a
Conditions: DHPs (0.09 mmol), catalyst (30 mg), H2O2 (1.5 ml).
Table 3
The effect of solvent on dehydrogenation of DHP derivatives.
Entry
R
Time
(h)
Solvent
Conversion
(%)
Selectivity
(%)
1
2
3
H
12
24
24
CCl4
97
19
50
100
100
100
Ph
Ph
CCl4
CH3COOH
Catalyst (0.03 g), substrate (0.01 g).
Fig. 5. Effect of time on the aromatization of 1,4-DHP with Si-Zr-Mo
nanocomposite.
with R = H and Me are quantitatively converted to the
corresponding pyridine derivatives in and 2 min,
respectively (entries 1 and 2, Table 2). The observed rate
enhancements are in accord with the decreasing steric
hindrance at the substituted secondary carbon.
properties of the resulting nanocomposite. The isotherm
indicated a typical microporous character. The calculation
of surface area and pore size distribution was based on BET
and BJH method [30]. The surface area, pore size
distribution, pore diameter and pore volume are
140 m2/g, 1.48 nm, 1.188 nm and 0.075 cc/g, respectively.
1
The effect of other solvents on oxidative aromatization
reaction of 1,4-DHPs are presented in Table 3. Compared to
carbon tetrachloride as solvent, reaction has proceeded
more efficiently in acetic acid (entries 2 and 3, Table 3)
perhaps due to the higher solubility of the oxidant or
refluxing at higher temperature. Moreover, the acid
catalytic effect of this solvent on DHP containing the basic
amine functional group may not be overlooked. Notably,
the unsubstituted DHP proceeds much faster than DHP
substituted with Ph (entries 1, 2 and 3, Table 3) [31,32].
The solventless dehydrogenation of 1,4-DHPs was
carried out using Si-Zr-Mo nanocomposite based on the
Scheme 2, with benzaldehyde, ethyl acetoacetate and
ammonium acetate as starting materials at 90 8C.
In order to optimize the reaction conditions, the effect
of the amount of catalyst and reaction time was studied
(Table 4). As illustrated in this table, increasing the amount
of catalyst from 30 mg to 100 mg the conversion has been
increased from 53 to 92% with selectivity 80% and 20%
toward the formation of 1a and 1b respectively (Table 4,
entries 1–3). The reaction time was also extended to 15, 18,
21, and 24 h (Table 4, entries 3–6) with conversion 48, 65,
78 and 92% respectively.
3.2. Catalytic activity
In the next step, the oxidative aromatization of the
prepared nanocomposite was studied using H2O2 as
oxidant and EtOH as solvent. The results of the optimiza-
tion of reaction conditions based on the time and the effect
of amounts of catalyst are given in Table 1. It was found
that increasing the amount of catalyst from 3 mg to 30 mg
increases the conversion from 40 to 100% with the
formation of the corresponding pyridine derivative with
100% selectivity (entries 1–4, Table 1). The completion of
reaction within 3 min seems remarkable. It was also found
that increasing time of reaction from 1 to 3 minutes, the
conversion increases from 45 to 100% with 100% selectivity
respectively (entry 6, 5 and 4, Table 1 and Fig. 5). As seen in
Table 1, 25% of DHP is converted within 600 min in the
oxidative aromatization reaction in the absence of catalyst
(entry 7, Table 1). Therefore, catalyst has a key role in the
oxidative dehydrogenation of DHP.
The established optimized reaction conditions were
then tested on DHPs with R = H, Me and Ph, prepared from
suitable starting materials (Table 2). Compared to oxida-
tive aromatization of DHP with R = Ph which proceeds to
completion within 3 min (entry 3, Table 2), those DHPs
A
possible mechanism of the Si-Zr-Mo catalyzed
aromatization of 1,4-DHPs with H2O2 is suggested in
Scheme 3. Initially, coordination of H2O2 with Mo of Si-Zr-
Mo nanocomposite I affords the corresponding proxo
CHO
Ph
O
O
CO2Et
Ph
EtO2C
CO2Et
EtO2C
+
+
+
NH4OAC
OEt
N
N
1b
1a
Scheme 2. Synthesis of conventional dihydropyridines in the presence of Si-Zr-Mo nanocomposite under reflux condition.

M. Sharbatdaran et al. / C. R. Chimie 16 (2013) 176–182
181
Table 4
Condensation of benzaldehyde, ethyl acetoacetate and ammonium acetate catalyzed by nanocomposites under different times of reaction and different
amounts of catalyst in solventless condition.
CHO
Ph
O
O
CO2Et
Ph
EtO2C
CO Et
EtO2C
2
+
+
+
NH4OAC
OEt
N
N
1b
1a
Entry
Catalyst
(mg)
Time
Conversion
(%)
Selectivity
(1a %)
Selectivity
(1b %)
(h)
1
2
3
4
5
6
30
70
24
24
24
21
18
15
53
80
92
78
65
48
47
63
80
77
77
67
47
37
20
23
23
33
100
100
100
100
Temperature: 90 8C; substrate: benzaldehyde (2 mmol), ethyl acetoacetate (3 mmol), ammonium acetate (2 mmol).
H2O2
O
O
O
O
O
O
O
O
O
O
O
H
Si
Zr
Mo
Si
Zr
Mo
OH
O
O
H
I
II
H
H2O
O
O
O
O
O
H3C
N
R
Si
Zr
Mo
OH
CO2Et
H3C
CO2Et
O
H
(R = H, Me, Ph)
R
N
EtO2C
H3C
CO2Et
CH3
Scheme 3. Proposed mechanism for the oxidative aromatization of Hantzsch 1,4-DHP with Si-Zr-Mo nanocomposite.
intermediate II [29]. As indicated in Scheme 3,
a
work showed that the addition of Mo plays a crucial role on
the particle size and morphology of the nanocomposite.
The prepared nanocomposite was tested as catalyst for
oxidative aromatization of 1,4-DHPs. A number of 1,4-
DHPs were quantitatively converted to the aromatic
analogous within 1 to 3 min. The attractive features of
this protocol are a simple reaction procedure, short
reaction time, easy product separation and purification.
concomitant proton and hydride transfer from N-H and
C-H of 1,4-DHP to nucleophilic M = O and electrophilic HO-
O oxygen’s of II, respectively leads to the corresponding
substituted pyridine and regeneration of the catalyst via
elimination of water [19].
4. Conclusion
Acknowledgment
The Si-Zr-Mo nanocomposite was prepared using
Zr(Oct)4, H2SO4, TEOS and sodium molybdate via sol–gel
method. The nanocomposite with surface area, pore size,
pore diameter and pore volume of 140 m2/g, 1.48 nm,
1.188 nm and 0.075 cc/g, respectively was obtained. This
The authors would appreciate the Alzahra University
and Agricultural, Medical and Industrial Research School
(AMIRS) for supporting this work.

182
M. Sharbatdaran et al. / C. R. Chimie 16 (2013) 176–182
[16] M.M. Heravi, F. Derikvand, S. Hassan-Pour, K. Bakhtiari, F.F. Bamohar-
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