Physicochemical characterization and catalytic applications of MoO 3-ZrO2 composite oxides towards one pot synthesis of amidoalkyl naphthols

Article abstract of DOI:10.1016/j.catcom.2011.04.014

A series of MoO₃-ZrO₂ composite oxide catalysts were prepared by coprecipitation and impregnation methods and characterized by XRD, Raman, UV-Vis, TEM and sorptometric techniques. Characterization studies indicated the presence of te

Full text of DOI:10.1016/j.catcom.2011.04.014

Catalysis Communications 12 (2011) 1255–1259
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Catalysis Communications
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Short Communication
Physicochemical characterization and catalytic applications of MoO3–ZrO2 composite
oxides towards one pot synthesis of amidoalkyl naphthols
⁎
Satish Samantaray, G. Hota, B.G. Mishra
Department of Chemistry, National Institute of Technology, Rourkela-769 008, Orissa, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of MoO3–ZrO2 composite oxide catalysts were prepared by coprecipitation and impregnation
methods and characterized by XRD, Raman, UV–Vis, TEM and sorptometric techniques. Characterization
studies indicated the presence of tetragonal zirconia phase and well dispersed MoO3 species as isolated and
polymolybdate clusters in the composite oxide. The MoO3(20 mol%)–ZrO2 material was used as efficient
catalyst for synthesis of amidoalkyl naphthols under solvent free conditions using conventional as well as
microwave heating. The results obtained clearly showed that the composite oxide catalyst was recyclable and
highly efficient for the reaction giving good yield and purity of the products.
Received 5 February 2011
Received in revised form 7 April 2011
Accepted 12 April 2011
Available online 22 April 2011
Keywords:
MoO3
ZrO2
© 2011 Elsevier B.V. All rights reserved.
Amidoalkyl naphthol
Raman
TEM
1. Introduction
as high thermal stability, extreme hardness, stability under reducing
conditions, and surface acidic and basic functions. These properties render
The use of heterogeneous catalytic methods for synthesis of fine
chemicals is a promising field of research with potential application in
pharmaceutical and related fine chemical industries [1,2]. Many fine
chemicals are synthesized under homogeneous conditions using Lewis
acidic salts, mineral and organic acids as catalysts. However, homoge-
neous acid catalyzed processes are inherently associated with problems
such as catalyst handling and recyclability, product purification and
corrosion. In recent years, heterogeneous catalysts are increasingly used in
place of homogeneous catalysts for synthesis of fine chemicals [1–5].
Among heterogeneous catalysts employed, heteropoly acids, zeolites,
sulfated metal oxides, clays and composite oxides are the most widely
investigated catalysts [1–6]. Among the composite oxides, WO3 and MoO3
based oxides are the most extensively investigated catalysts. These oxides
show strong surface acidic and redox properties and varied surface
molecular structures [6,7]. MoO3 based heterogeneous catalysts have
been investigated for industrial catalytic process such as hydro-
desulfurization [8], metathesis [9], oxidation and oxidative dehydrogena-
tion reactions [10]. Molybdenum species supported on a variety of oxide
supports have been reported in literature [11–19]. The flexibility of the
MoO3 material to form different molecular structures and the ability of Mo
atoms to acquire various oxidation states are the main features which are
invoked to explain its catalytic activity. Among different supported MoO3
materials, the MoO3/ZrO2 is the most promising heterogeneous catalyst
[11–16]. Zirconia as support possesses all the required characteristics such
zirconia as a promising support for many important catalytic processes
[16,20–22]. The MoO3/ZrO2 catalyst has been investigated for several
industrially important reactions such as cumene dealkylation, ethanol
conversion, ammoxidation of picoline and toluene, synthesis of methox-
ymethyl benzene, methylcyclopentane conversion, methanol oxidation
reactions and methane combustion reaction [11–16]. Although there are
investigations on the catalytic applications of MoO3–ZrO2 for industrially
important reactions, however, the applicability of this important material
to organic synthesis involving biologically important molecules is yet to be
explored.
Compounds containing 1,3-amino-oxygenated functional groups are
frequently found in biologically active natural products and potent drugs
such as nucleoside antibiotics and HIV protease inhibitors [23]. It has been
reported that amidoalkyl naphthols can be converted to biologically active
aminoalkyl naphthol derivatives by amide hydrolysis reaction [24]. Thus
the synthesis of amidoalkyl naphthols is of paramount importance in
organic synthesis. 1-amidoalkyl 2-naphthols are synthesized by multi-
component condensation of aryl aldehydes, 2-naphthol, and acetonitrile
or amide in the presence of acid catalysts [25]. Several Lewis and Brønsted
acidic catalysts have been used for this reaction which include among
others Ce(SO4)2, Iodine, K5CoW12O403H2O, p-TSA, FeHSO4 and Sulfamic
acid [25–28]. Recently, montmorillonite K10, has been used as an efficient
heterogeneous catalyst for synthesis of amidoalkyl naphthols [29]. The
other heterogeneous catalysts used for this reaction include FeCl3/SiO2
and HClO4/SiO2 [30,31]. In this work, we have reported the application of
the MoO3–ZrO2 materials as heterogeneous catalyst for synthesis of
structurally diverse amidoalkyl naphthols under environmentally benign
conditions.
⁎
Corresponding author. Tel./fax: +91 661 2462651.
E-mail address: brajam@nitrkl.ac.in (B.G. Mishra).
1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.04.014

1256
S. Samantaray et al. / Catalysis Communications 12 (2011) 1255–1259
2. Experimental
2.1. Preparation of the materials
dried at 120 °C for 12 h and calcined at 400 °C for 1 h. For synthesis
under microwave irradiation, the reaction mixture was irradiated at
900 W for the specified time with an intermittent cooling interval of
2 min after every 1 min of microwave irradiation.
The MoO3–ZrO2 catalysts were prepared by coprecipitation method
using zirconium oxychloride and ammonium heptamolybdate (S.D.
Fine Chemicals, India, 99.9%) as precursor salts and liquid ammonia as
precipitating agent. Initially, 500 ml of double distilled water was
adjusted to pH 9.0 by addition of liquid ammonia. To this solution
required amount of precursor salt solution (0.5 M) was added drop wise
(20 ml/h) under constant stirring and pH condition. The resulting
aqueous mixture was stirred for 12 h, filtered and washed 5–6 times in
double distilled water (till Cl− free). The obtained material was dried at
120 °C for 12 h in a hot air oven and calcined at 500 °C for 2 h to obtain
the MoO3–ZrO2 catalyst. Using this procedure MoO3–ZrO2 material with
2, 5, 10, 20 and 50 mol% MoO3 were prepared. The MoO3–ZrO2 catalysts
were referred to as xMoZr in the subsequent text, where x represents
the mol% of MoO3 present in the composite oxide. For the sake of
comparison, the MoO3–ZrO2 catalysts were also prepared by wet
impregnation method. Required amount of ammonium heptamolyb-
date was added to ZrO2 aqueous suspension and stirred for 6 h. The
aqueous suspension was heated with constant stirring to remove water,
dried at 120 °C for 12 h followed by calcinations at 500 °C for 2 h. The
MoO3–ZrO2 catalysts prepared by this method were referred to as
xMoZrI in the text, where x represents the mol% of MoO3 in the
composite oxide and I stands for the impregnation method.
3. Results and discussion
3.1. Characterization of the MoO3−ZrO2 catalyst
The XRD patterns of the xMoZr and xMoZrI catalyst are presented in
Figs. 1 and 2. Pure ZrO2 shows intense and well defined diffraction peaks
at 2θ values of 28.3° and 31.5° corresponding to the reflections from the
ꢀ
ꢁ
111 and (111) planes of monoclinic zirconia phase (JCPDS-ICDD file
no. 83–0940) and at 30.2° corresponding to the reflections from the
(101) plane of tetragonal phase (JCPDS-ICDD file no. 81–1545) (Fig. 1a).
The percentage tetragonal phase in the ZrO2 sample is calculated to be
58%. The 2MoZr catalyst contains 69% tetragonal phase (Table 1, Fig. 1b).
Further increase in MoO3 content to 5 mol% and above resulted in the
complete formation of tetragonal phase. No separate crystalline phases
corresponding to MoO3 and Zr(MoO4)2 are detected in the XRD patterns
of the coprecipitated samples even for sample with higher MoO3
loading. This may be due to the presence of MoO3 phase as amorphous
or molecular-type species without well defined crystallinity in the
coprecipitated sample. Pure MoO3 shows well defined diffraction peaks
at 2θ values of 23.3°, 25.6°, 27.2° 33.6° and 49.2° corresponding to the
reflections from the (002), (012), (120), (202) and (400) planes
respectively (JCPDS-ICDD file no. 84–1360) (Fig. 1e). The selective
stabilization of the tetragonal phase was also noted for xMoZr-I samples,
however, the phase transformation is less pronounced. The 10MoZrI,
20MoZrI and 50MoZrI materials contain 69, 81 and 82% tetragonal
phase, respectively. Moreover at higher Mo content the presence of
crystalline MoO3 is clearly observed in the X-ray diffraction patterns of
the 20MoZrI and 50MoZrI samples. The selective formation of the
tetragonal phase can be ascribed to the reduction in the grain boundary
area which prevents the mobility of the ions during phase transforma-
tion. Srinivasan et al. has reported that in sulfated zirconia catalyst the
tetragonal phase is stabilized due to the preferential segregation of the
sulfate ions along the grain boundary [19]. It has also been reported that
there is a critical crystallite size of zirconia below which the tetragonal
phase is stabilized [11]. In the present study, it is believed that the
2.2. Characterization techniques
The XRD patterns of the samples were obtained using Philips PAN
analytical diffractometer fitted with a Ni filtered CuKα1 radiation. The
percentage tetragonal phase in the composite oxide was estimated
using the method described elsewhere [11]. The UV–Visible spectra
were recorded using a Shimadzu spectrophotometer (UV-2450) with
BaSO4 coated integration sphere. The confocal micro-Raman spectra
were recorded on a Horiba Jobin-Yvon LabRam HR spectrometer using a
17 mW internal He–Ne laser source with an excitation wavelength of
632.8 nm. The specific surface areas were determined by BET method
using N2 adsorption/desorption at 77 K employing Quantachrome
Autosorb gas sorption equipment. The composite oxide samples were
degassed at 120 °C for 12 h prior to the sorptometric studies.
Transmission electron micrographs of the samples were recorded
using PHILIPS CM 200 equipment using carbon coated copper grids. The
surface acidity of the MoZr catalysts was determined by employing
titration method [5]. The IR spectra were recorded on a Perkin-Elmer IR
spectrophotometer as KBr pellets. 1H NMR spectra were obtained using
a Bruker spectrometer at 400 MHz using TMS as internal standard. All
the reaction products are known compounds and are identified by
comparing their physical and spectral characteristics with the literature
reported values.
e
d
c
b
a
2.3. Synthesis of amidoalkyl naphthols
Typically, a mixture of β-naphthol (1 mmol), benzaldehyde
(1 mmol), benzamide (1.1 mmol) and 20MoZr (100 mg) was heated
at 80 °C under stirring for the required amount of time. The progress of
the reaction was checked periodically using TLC. After completion of the
reaction, 10 ml of methanol was added to the reaction mixture and the
heterogeneous catalyst wasfiltered. The filtrate was concentrated under
vacuum and the crude residue was purified by crystallization from
ethanol: water (1:1) to afford pure N-[(2-Hydroxynaphthalen-1-yl)
phenylmethyl] benzamide. MP. 241–242 °C; IR (KBr, cm−1): 3420,
3061, 3022, 1629, 1572, 1511, 1435, 1348, 1271, 1049, 822, 696; 1H NMR
(CDCl3, 400 MHz): δ=10.3 (s, 1H), 9.0 (d, 1H), 8.1 (d, 1H), 7.9 (d, 2H),
7.84(d, 1H), 7.8 (d, 1H), 7.6 (t, 1H), 7.4–7.5 (m, 3H), 7.2–7.4 (m, 8H). The
used catalyst was regenerated by washing with methanol (3×10 ml),
Fig. 1. X-ray diffraction patterns of (a) ZrO2, (b) 2MoZr, (c) 5MoZr (d) 10 MoZr and
(e) MoO3.

S. Samantaray et al. / Catalysis Communications 12 (2011) 1255–1259
1257
to 10% and higher, a broad band with maxima at 280 nm was observed
for all the samples. The peak observed at 280 nm for sample with
higher loading thus can be assigned to the polymolybdate clusters.
Although the bulk type MoO3 peak at 320 nm peak is absent for the
samples with higher loading, it is possible that this absorption feature
can be masked by the broad band at 280 nm. This is more likely to
occur for the 50MoZr sample. However, since the XRD study does not
show crystalline MoO3 phase, the bulk type MoO3 may be present in
small quantities in these samples. The UV–Vis spectra of the MoO3–
ZrO2 materials prepared by impregnation method are presented in
Fig. 3, panel II. All the impregnated catalysts show the 226 nm peak. In
addition, the 20MoZrI and 50MoZrI materials show a prominent peak
at 315 nm corresponding to the presence of bulk type molybdate
species (Fig. 3II).
f
e
d
c
b
The Raman spectra in the range of 700–1200 cm−1 for the xMoZr
and xMoZrI catalysts are shown in Fig. 4. Pure MoO3 (Fig. 4 inset)
shows two sharp and intense bands at 816 and 990 cm−1 due to the
crystalline MoO3 phase [16]. These bands are also observed for the
20MoZrI catalyst indicating the presence of crystalline MoO3 in bulk
state in this sample. Contrary to this observation, the coprecipitated
samples show broad Raman features with altered peak position. The
Raman spectroscopic investigations for the supported MoO3 system
provides good insight into the dispersion, interaction, symmetry and
the type of molecular Mo species present on the support [12,14]. In
the present study, the 2MoZr catalyst shows two bands of low
intensity at 837 and 910 cm−1. These peaks have been be assigned to
the presence of isolated molybdate species present on the surface of
zirconia [17]. With increase in MoO3 content, the Raman band at
910 cm−1 was found to shift to higher wavenumber. The shift to
higher wavenumber can be ascribed to the formation of oligomeric
and polymolybdate species. Upon increase in MoO3 content, more
number of surface sites is occupied by the molybdate ions resulting in
the formation of polymolybdates by linkage between adjacent
monomolybdate species [12]. The Raman peak observed in the
range of 940–960 cm−1 can be assigned to the oligomeric molybdate
clusters present on the surface of zirconia. Supported heptamolybdate
a
Fig. 2. X-ray diffraction patterns of (a) 10MoZr, (b) 10MoZrI, (c) 20MoZr, (d) 20MoZrI,
(e) 50MoZr and (f) 50MoZrI.
formation of a well dispersed layer of MoO3 on the surface and grain
boundary region helps in the stabilization of the tetragonal phase.
The UV–Vis spectra of the xMoZr and xMoZrI materials are
presented in Fig. 3 (panels I and II). Pure ZrO2 prepared by precip-
itation shows a sharp and intense band at 211 nm (Fig. 3I a).
Zirconium dioxide is a direct band gap insulator, which shows
interband transitions at ~215 nm [6]. The absorption band observed at
211 nm is assigned to the O2− →Zr4+ charge transfer transition. Pure
MoO3 shows two well defined absorption bands at 240 and 320 nm
(Fig. 3II d). The peak at 240 nm has been assigned to the ligand–metal
charge transfer transition (LMCT), O2− →Mo6+ from isolated MoO4
species with tetrahedral symmetry whereas the peak at 320 nm can
be assigned to bulk polymolybdate species present in the sample
[15,17]. Depending on the coordination preference and local symme-
try of the Mo (VI) ions different absorption bands are observed in the
electronic spectrum of MoO3 [17]. The MoO3–ZrO2 composite oxides
show absorption features which are different from the individual
oxide components. For samples with low MoO3 loading (2 and 5 wt.%)
a prominent band was observed at 226 nm along with a broad
shoulder at 280 nm. The sharp and narrow absorption band at 226 nm
can be assigned to the tetrahedrally coordinated isolated molybdate
species with structural distortion. The broad peak at 280 nm can be
assigned to small nanosize molybdate clusters dispersed on the
surface of the zirconia. Liu et al. have assigned the peak at ~300 nm to
the presence of heptamolybdate clusters for MoO3/ZrO2 catalyst [15].
In the present study, such clusters can exist in the zirconia matrix in a
well dispersed state. When the percentage MoO3 loading is increased
6−
4−
(Mo7O ) and octamolybdate (Mo8O ) clusters show Raman band
24
26
in the range of 935–960 cm−1 due to the terminal Mo=O stretch of
the distorted MoO6 octahedron [12]. In the present study, for higher
MoO3 loading it is likely that such clusters are present in the ZrO2
matrix. From the Raman analysis it can be concluded that at low MoO3
content the predominant molecular species are isolated molybdates
whereas at higher loading oligomeric clusters and polymolybdate
species are present on zirconia surface. The transmission electron
micrograph of the ZrO2 and 20MoZr material is shown in Fig. 5. Pure
ZrO2 show particle size in the range of 40–50 nm. Addition of MoO3 to
the zirconia matrix resulted in the formation of 20–30 nm size
particles for the 20MoZr materials. The crystallite size of the ZrO2 and
20MoZr materials was also calculated by Fourier line shape analysis of
the broadened X-ray profiles [32]. The calculated crystallite sizes for
ZrO2 and 20MoZr are 37 and 20 nm respectively. A good agreement
was observed between the Fourier analysis data and TEM study. The
Table 1
Physicochemical characteristics and catalytic activity of xMoZr catalysts.
Catalyst
Surface area (m2/g)
% tetragonal phase
Acidic sites (mmol/g)
Yielda (%)
Reaction rateb (mmol h−1 m−2×10−2
)
TOFb (h−1
)
ZrO2
45
62
82
95
90
78
58
69
100
100
100
100
0.17
0.26
0.35
0.41
0.47
0.42
NRc
30
46
65
89
68
–
–
2MoZr
5MoZr
10MoZr
20MoZr
50MoZr
5.3
5.9
7.1
12
14
16
19
16
10.0
8.9
Reaction conditions: 2-chlorobenzaldehyde:β-naphthol:urea 1:1:1.3, Temperature 80 °C, solvent free conditions.
a
Isolated yield for N-[(2-Hydroxynaphthalen-1-yl)2-chlorophenylmethyl]urea (Table 2, Entry 2) for a reaction time of 1 h.
Calculated with respect to 2-Chlorobenzaldehyde conversion using gas chromatography.
No reaction.
b
c

1258
S. Samantaray et al. / Catalysis Communications 12 (2011) 1255–1259
d
c
f
e
d
c
b
a
b
a
Fig. 3. UV–Vis spectra of the MoO3–ZrO2 composite oxide materials: (a) ZrO2, (b) 2MoZr, (c) 5MoZr, (d) 10MoZr, (e) 20MoZr and (f) 50MoZr in panel I and (a) 10MoZr-I, (b)
20MoZr-I, (c) 50MoZr-I and (d) MoO3 in panel II.
composite oxide materials show smaller particle size compared to the
pure zirconia.
2-chlorophenylmethyl]urea at 80 °C under solvent free condition for 1 h
of reaction time. The physicochemical properties of the composite oxide
materials are also given in Table 1. The composite oxides shows higher
surface area and acidity compared to pure zirconia. Addition of MoO3 to
zirconia surface has a profound influence on the acidity of the material.
The number of acidic sites was found to increase with the MoO3 content
up to 20% MoO3. In the composite oxide, the surface low coordinated
Mo atoms can act as Lewis acidic sites whereas the surface hydroxyl
group on the catalyst surface can provide Bronsted acidic sites. For well
dispersed molybdate species, the presence of a greater number of
surface active Mo sites is responsible for higher acidity of the composite
material. When the molybdenum content was increased to 50% the
acidity was found to decrease marginally probably due to the formation
of larger polymolybdate clusters with a less active surface area. Similar
observation has been noted for supported MoO3 catalyst in the earlier
studies of MoO3/ZrO2 and MoO3/MCM-41 system [12,18]. Among the
binary oxide the 20MoZr catalyst display highest catalytic activity which
is ascertained from the reaction rate and turnover frequency data. The
20MoZr catalyst was chosen for further catalytic experiments in the
present study. Various reaction parameters such as the catalyst amount,
reaction stoichiometry, temperature and mode of heating are varied to
obtain the optimized protocol. It is observed that for a reaction involving
1 mmol of the reactants, 100 mg of the catalyst was ideal for efficient
condensation of the reactants at 80 °C under solvent free condition. The
optimum reactant ratio was found to be 1:1:1.3 for benzaldehyde,
naphthol and urea, respectively. In case of reaction involving benzamide
the reactant ratio was found to be 1:1:1.1. The effect of mode of energy
supply was studied by conducting the reaction under solvent free
conditions both using a conventional oil bath and microwave oven. The
results obtained in these studies are presented in Table 2. It was
observed that the use of microwave as energy source considerably
shortens the reaction time. For both protocols involving microwave
and conventional heating, a wide variety of substituted benzaldehyde
reacted efficiently to give the amidoalkyl naphthol in high yield and
purity. Compared to the earlier reported heterogeneous catalytic
protocols (yields ~70–90%, time: 2–20 h) [29–31], the present method
3.2. MoO3–ZrO2 catalyzed synthesis of amidoalkyl naphthols
The xMoZr materials were used as an efficient and recyclable catalyst
for the synthesis of structurally diverse 1-amidoalkyl 2-naphthols under
mild conditions (Scheme 1). Table 1 shows the catalytic activity of the
composite oxides for the synthesis of N-[(2-hydroxynaphthalen-1-yl)
f
e
d
c
b
a
Fig. 4. Raman Spectra of (a) 2MoZr, (b) 5MoZr, (c) 10 MoZr and (d) 20MoZr (In the
inset (e) 20MoZrI, and (f) MoO3).

S. Samantaray et al. / Catalysis Communications 12 (2011) 1255–1259
1259
Table 2
Solvent free synthesis of amido alkyl naphthols using 20MoZr catalyst (Scheme 1).
Sl.
R
R1
Conventional synthesis
Microwave assisted
No
Time (min)
Yield (%)
Time (min)
Yield (%)
1
2
3
4
5
6
7
8
H
2-Cl
4-Cl
NH2
NH2
NH2
NH2
NH2
NH2
NH2
C6H5
C6H5
C6H5
C6H5
90
60
60
90
60
90
60
120
120
120
90
85
89
88
82
86
81
88
80
84
82
81
5
4
4
6
3
6
4
6
4
6
4
79
84
82
76
80
78
86
76
78
75
82
4-OCH3
3-NO2
4-OH
4-NO2
H
2-Cl
4-OCH3
4-NO2
9
10
11
catalysts prepared by the coprecipitation method show improved
physicochemical characteristics compared to impregnation method.
The presence of MoO3 in ZrO2 matrix modifies the phase composition of
zirconia. The MoO3 species are present predominantly in the form of
isolated and cluster molybdate on zirconia surface for the coprecipitated
samples, whereas the impregnated sample shows the presence of bulk
type MoO3. The 20MoZr materials were found to be quite efficient for
multicomponent one pot synthesis of structurally diverse amidoalkyl
naphthol. The protocols developed using the MoO3–ZrO2 catalyst was
advantageous in terms of simple experimentation, preclusion of toxic
solvents, recyclable catalyst, high yield and purity of the products.
Acknowledgment
The authors thank CSIR, New Delhi for the financial support.
Fig. 5. Transmission electron micrograph of (a) ZrO2 and (b) 20MoZr materials.
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Scheme 1. One pot synthesis of amidoalkyl naphthol using xMoZr catalyst.