CaO-ZrO2 nanocomposite oxide prepared by urea hydrolysis method as heterogeneous base catalyst for synthesis of chromene analogues

Article abstract of DOI:10.1016/j.molcata.2016.10.031

A series of CaO–ZrO₂ nanocomposite oxides containing 2–50?mol% of CaO were synthesized employing urea as a mild hydrolyzing agent. The nanocomposite systems were characterized using XRD, Fourier analysis, TPD, Raman, XPS, TGA-DSC, FESEM, HRTEM,

Full text of DOI:10.1016/j.molcata.2016.10.031

Accepted Manuscript
Title: CaO–ZrO2 nanocomposite oxide prepared by urea
hydrolysis method as heterogeneous base catalyst for
synthesis of chromene analogues
Author: Sagnika Pradhan Vivekananda Sahu B.G. Mishra
PII:
S1381-1169(16)30454-X
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2016.10.031
MOLCAA 10093
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
9-6-2016
26-10-2016
28-10-2016
Please cite this article as: Sagnika Pradhan, Vivekananda Sahu, B.G.Mishra, CaO–ZrO2
nanocomposite oxide prepared by urea hydrolysis method as heterogeneous base
catalyst for synthesis of chromene analogues, Journal of Molecular Catalysis A:
Chemical http://dx.doi.org/10.1016/j.molcata.2016.10.031
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<AT>CaO-ZrO2 nanocomposite oxide prepared by urea hydrolysis method as heterogeneous base
catalyst for synthesis of chromene analogues
<AU>Sagnika Pradhan, Vivekananda Sahu, B.G. Mishra*
##Email##brajam@nitrkl.ac.in##/Email##
<AU>
<AFF>Department of Chemistry, National Institute of Technology, Rourkela-769008, Odisha,
India
<PA>*Corrresponding author.
<ABS-Head><ABS-HEAD>Graphical abstract
<ABS-P>
<ABS-P><xps:span class="xps_Image">fx1</xps:span>
<ABS-HEAD>Highlights► CaO-ZrO2 nanocomposite oxides prepared by urea hydrolysis method
► The Ca2+ substitute for Zr4+ to form a solid solution in a limited composition range ► Creation
of new basic sites of higher strength due to Ca2+ incorporation to ZrO2 ► Presence of O2- vacancy
and distortion in oxygen sublattice due to Ca2+ incorporation ► The CaO-ZrO2 composite highly
efficient catalyst for synthesis of chromene analogues
<ABS-HEAD>Abstract
<ABS-P>A series of CaO-ZrO2 nanocomposite oxides containing 2-50 mol% of CaO were
synthesized employing urea as a mild hydrolyzing agent. The nanocomposite systems were
characterized using XRD, Fourier analysis, TPD, Raman, XPS, TGA-DSC, FESEM, HRTEM, and
BET surface area measurement techniques. XRD study indicated selective stabilization of the
tetragonal zirconia phase in the composite oxide. Upto 20 mol% CaO, the Ca2+ ions substituted for
Zr4+ ions in the zirconia lattice to form a substitutional solid solution. Beyond 20 mol% CaO, the
presence of a mixed phase system consisting of the solid solution phase, CaO and
nonstoichiometric Zr0.93O2 phase was observed. The composite oxides contain crystallites with size
<20 nm as observed from Fourier and HRTEM analysis. TPD study revealed a significant
enhancement in the basicity of zirconia as a result of Ca2+ incorporation. XPS study confirmed
presence of different lattice oxygen as potential basic sites. Raman spectral data indicated the
presence of oxygen vacancy and distortion in oxygen sublattice due to Ca2+ incorporation to
zirconia. Microscopic investigation of the composite oxide suggested a gradual morphological
change from rod shape particles to flake like and subsequently to cubic morphology with increase
in CaO content in the composite. The CaO-ZrO2 nanocomposite oxides were used as an efficient
and recyclable heterogeneous base catalyst for synthesis of chromene analogues. Structurally
diverse 2-amino-4H-chromenes and 2-amino-2-chromenes were synthesized in high yield and
purity under mild condition using CaO-ZrO2 material as catalyst and multicomponent
condensation approach.
<KWD>Keywords: 4H-chromene; 2-amino-2-chromene; CaO-ZrO2; Raman; Urea hydrolysis
<H1>1. Introduction
Catalytic application of novel heterogeneous base catalyst in fine chemical synthesis is a promising
field of research with potential application in pharmaceuticals and related fine chemical industries
[1-5]. Among different heterogeneous base catalysts used in fine chemical synthesis, the alkali and
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alkaline earth metal oxides are quite promising because they are inexpensive, easy to prepare and
display strong surface basic sites capable of promoting a variety of base catalyzed reactions [1-3].
However, the low surface area and deactivation of these oxide materials by surface passivation are
important factors which influence their catalytic activity. In recent years, there is significant effort
to design novel heterogeneous base catalyst with high surface area, structural stability and
molecularly defined basic sites. The mesoporous hybrid material containing organic basic moieties
tethered to the internal surface are the most widely investigated new generation base catalyst [6].
Recently, the surface and structural modification of zirconia using alkali and alkaline earth metal
ions such as Cs+, Na+, Ca2+ and Mg2+ ions has been actively pursued to generate thermally robust
high surface area heterogeneous base catalyst with strong surface basicity [7-14]. For example,
hard templating approach has been used to prepare sodium incorporated mesoporous zirconia with
thermally stable tetragonal crystalline frameworks and superbasicity (27.0 in Hammet scale). The
sodium modified mesoporous tetragonal zirconia exhibit excellent catalytic activity for synthesis
of dimethyl carbonate [7]. The exchange of surface –OH group of hydrous zirconia with Cs+ ions
have been carried out to prepare a novel class of heterogeneous base catalyst. The Cs modified
zirconia nanoparticles exhibit enhanced surface basicity and activity for transesterification of
vegetable oil compared to alkaline earth metal oxides and LDH materials [8]. The incorporation of
Ca2+ ions into the zirconia lattice to generate strong surface basic function has been a subject of
intense study in literature [11-18]. The CaO forms a substitutional solid solution with ZrO2
[13,15,18]. The incorporation of Ca2+ ions into the ZrO2 lattice increases the iconicity of the
crystal [11,13,15]. The nonstoichiometric solid solution phase exhibit strong surface basicity, high
thermal stability and stable active sites for base catalyzed reactions. The CaO-ZrO2 mixed oxide
systems have been studied as catalyst for dehydration of 2,3-butanediol, synthesis of dimethyl
carbonate and transesterification a variety of oils including waste cooking oil, refined rapeseed oil
and soybean oil [11, 15-18]. The CaO-ZrO2 mixed oxide system has also been used as efficient
support for metallic nanoparticles for application as bifunctional catalysts [12, 19-22]. Ni Metal
nanoparticles loaded on CaO-ZrO2 has been studied extensively as catalyst for reforming reaction
including dry reforming of methane and ethanol stem reforming [12, 19, 21]. The presence of Ca2+
ions in ZrO2 lattice inhibits coke formation and enhances the stability and life time of the catalyst.
Similarly, the Ru metal supported on CaO-ZrO2 mixed oxide exhibit excellent catalytic activity for
selective oxidation of alcohols with molecular oxygen [22]. The basicity of the CaO-ZrO2 material
can be tuned by adjusting the Ca/Zr ratio and proper choice of preparative method [15]. The
reported studies suggest promising potential of CaO-ZrO2 material for base catalyzed reactions.
However, the applicability of this important catalytic material for base catalyzed fine chemical
synthesis is yet to be explored. In order to explore the applicability of CaO-ZrO2 material in the
domain of fine chemical synthesis, in this work, we have prepared a series of CaO-ZrO2
nanocomposite materials using urea hydrolysis method and studied their catalytic application for
synthesis of a wide variety of biologically important chromene analogues.
The chromene moiety represents an important class of chemical units which are found in many
natural products [23]. Chromene analogues particularly the 2-amino-2-chromenes and 2-amino-
4H-chromene exhibit interesting pharmacological properties besides their use as value added
products in chemical industries [23-27]. The 4H-chromene derivatives exhibit a wide spectrum of
biological activities such as anticancer, anticoagulant, antibacterial, and fungicidal properties
[27,28].The 2-amino-chromenes are widely employed as pigments, cosmetics and potential
agrochemicals [23,4,5]. Due to the widespread application, the development of facile and clean
protocol for synthesis of these compounds has been a subject of intense study in literature. The
2

most efficient route for synthesis of 2-amino 4H-chromene involve the multicomponent
condensation of aldehyde, malononitrile and 1,3-dicarbonyl compounds using basic catalyst
[26,27]. Similarly, the 2-amino-2-chromenes are synthesized by base catalyzed multicomponent
condensation (MCC) of aldehyde, malononitrile and -naphthol [24,25]. Several homogeneous
and heterogeneous catalytic systems including piperazine, Potassium phthalimide,
tetrabutylammonium fluoride, I2/K2CO3, basic ionic liquid, acid hydrolysate of tendons, Silica
bonded N-propylpiperazine, amino functionalized silica gel, and DMAP (4-
dimethylaminopyridine) functionalized polyacrylonitrile fiber catalyst have been used for the
synthesis of these two classes of compounds [23-35]. However, most of the methods require longer
time, stoichiometric reagents, and toxic solvents providing moderate to good yields of the product.
Moreover, many procedures employ homogeneous catalyst and supported reagents which suffer
from drawback such as recovery, recyclability, leaching and stability of the active sites. In our
previous work, we have demonstrated the use of alkali and alkaline earth metal oxide based
nanomaterials as catalyst for synthesis of 2-amino-2-chromene [4,5]. In continuation of our interest
to develop zirconia based heterogeneous catalyst with strong acid-base property for application in
organic synthesis [36-38], in this work, we have used CaO-ZrO2 nanocomposite oxide as an
efficient and recyclable heterogeneous catalyst for synthesis of 2-amino-4H-chromene and 2-
amino-2-chromenes using MCC approach.
<H1>2. Experimental
Calcium nitrate (Ca(NO3)2.4H2O), zirconyl chloride (ZrOCl2.8H2O) and urea (N2H4CO) were
procured from Merk India Limited. All the reagents used in this study were of AR grade (> 99.9%
purity) which were used directly in the experiments without further purification. Double distilled
water prepared in the laboratory was used in composite oxide preparation.
<H2>2.1 Preparation of CaO (x mol%)-ZrO2 nanocomposite oxides (xCa-Zr-O)
The xCa-Zr-O nanocomposite oxides containing different mol% CaO were prepared by hydrolysis
and co-condensation of calcium nitrate and zirconyl chloride salt solution using urea as a mild
hydrolyzing agent. Required amount of 0.5 M calcium nitrate and zirconyl chloride precursor salt
solutions were mixed with equimolar quantity of urea solution. The resulting solution was refluxed
for 12 h at 100oC. The obtained white precipitate was filtered and washed several times with hot
water to remove the chloride ions (AgNO3 test). The solid material was subsequently dried in a hot
air oven at 120oC for 12 h and calcined at 700oC for 2 h to obtain the xCa-Zr-O nanocomposite
oxides. The choice of the calcination temperature is based on earlier reports where the
incorporation of Ca2+ ions into the zirconia lattice has been observed for calcination temperature in
the range of 700-800oC [11, 14-16,18]. Moreover, at these calcination temperatures the evolution
of strong basic sites has been noticed on CaO-ZrO2 catalyst surface [15]. Using this procedure,
xCa-Zr-O composite oxides containing x =2, 5, 10, 20, 50 mol% of CaO were prepared. The
CaO(x mol%)–ZrO2 nanocomposite materials were referred to as xCa-Zr-O in the subsequent text
where x represents the mol% of CaO present in the composite oxide. The prepared catalytic
materials were stored in a vacuum desiccator in air tight borosilicate glass container.
<H2>2.2 Characterization techniques
The XRD patterns of the pure ZrO2, CaO and xCa-Zr-O nanocomposite oxides were recorded
using a Rigaku Ultima IV multipurpose X-ray diffraction system. The XRD measurements were
carried out in the 2θ range of 20–70o with a scan speed of 2 degrees per minute using CuKα (k =
1.5418Å) radiation. The specific surface area of the samples was determined by BET method using
3

N2 adsorption/desorption at 77 K on a Quantachrome autosorb gas sorption system. The composite
oxide samples were degassed at 150oC for 2 h prior to the sorptometric studies. FESEM pictures
were taken using Nova Nano SEM/FEI microscope. The powder sample was placed on carbon tape
followed by gold spitting for three minutes prior to the FESEM analysis. The TEM images of the
20Ca-Zr-O composite oxide was recorded using TECNAI 300kV equipment using carbon coated
copper grids of 300 mesh size as substrate. The micro- Raman spectra were obtained on a Horiba
Jobin-Yvon spectrometer using a 17 mW He–Ne laser light source (excitation wavelength 632.8
nm). The X-ray photoelectron spectra of 20Ca-Zr-O sample was recorded using SPECS make
(Germany) spectrophotometer with 150 mm hemispherical analyzer at band pass energy of 12 eV.
Monochromatic Al K radiation of 1486.74 eV was used as X-ray source. Binding energy
corrections due to electrostatic charging was made relative to the C1s peak taken as 284.6 eV.
Thermogravimetry analysis of the 20Ca-Zr-O and 50Ca-Zr-O composite oxides was performed
using a Perkin- Elmer TGA-7 apparatus in air atmosphere (30 ml/min) with a linear heating rate
(10oC/min) from room temperature to 600oC. The number of basic sites on the xCa-Zr-O
composite oxides was estimated by CO2 TPD experiments using Chembec 3000, Quantachrome
instrument with a TCD detector. Prior to CO2 adsorption, approximately 200 mg of the sample was
degassed at 250oC under flowing He for 2 h. The sample was then allowed to cool to the room
temperature under flowing He. The sample was exposed to flowing CO2 gas (100%) for 1h, after
which it is purged with He gas for 30 min to remove the excess physisorbed carbon dioxide
molecules. The CO2-TPD profiles were recorded by rising the temperature from 300 K to 923 K at
a rate of 10 K min-1. The 1H NMR spectra were recorded with Bruker 400 MHz NMR
spectrometer using TMS as internal standard.
<H2>2.3 Catalytic activity study
The catalytic activity of the xCa-Zr-O nanocomposite oxides was evaluated for the
multicomponent one pot synthesis of 2-amino 4H-chromenes and 2-amino-2-chromenes. Prior to
the catalytic tests, the xCa-Zr-O nanocomposite catalyst materials were pretreated by heating at
450oC for 2 h under flowing N2 (40 ml/min) to remove any adsorbed CO2 and H2O species.
<H3>2.3.1 Synthesis of 2-amino 4H-chromenes
In a typical procedure, aromatic aldehyde (2 mmol), malononitrile (2 mmol), and dimedone (2
mmol) was dissolved in 3 ml of aqueous ethanol (1:1 ratio) to which 20Ca-Zr-O catalyst (50 mg)
was added. The reaction mixture was stirred at 50oC for 1 h. The progress of the reaction was
monitored by thin‐layer chromatography (TLC). After completion of the reaction, as indicated by
TLC, 05 ml of ethanol was added to the reaction mixture and it was filtered. The crude product
was recovered from the ethanolic solution and recrystallized from hot methanol to afford the pure
product.
<H3>2.3.2. Synthesis of 2-amino-2-chromenes
The synthesis of 2-amino-2-chromenes was carried out by multicomponent one pot condensation
of aromatic aldehydes, malononitrile, and α-naphthol in presence of 20Ca-O-Zr catalyst. A 1:1
mixture of polyethylene glycol (PEG-400, average molecular mass: 380-420 g/mol) and water was
used as solvent. In a typical procedure, reaction mixture containing benzaldehyde (2 mmol),
malononitrile (2 mmol), α-naphthol (2 mmol) and 20Ca-Zr-O (50 mg) catalyst in PEG–water (3
ml; 1:1 mixture) was stirred at 50oC for 1h. Upon completion of the reaction as monitored by TLC,
the reaction mixture was diluted with water (5 ml) and the obtained solid compound was removed
by filtration. The reaction product was separated by dissolution in hot ethanol (5ml) followed by
4

simple filtration to separate the catalyst. The crude product was recovered from the ethanolic
solution and recrystallized to afford the pure product.
<H1>3. Results and discussion
<H2>3.1 Characterization of the xCa-Zr-O nanocomposite oxides
The XRD patterns of the xCa-Zr-O nanocomposite oxides along with pure ZrO2 material are
presented in Fig 1. Zirconia prepared by urea hydrolysis method exhibit intense and broad
diffraction peaks with d spacing values of 3.12, 2.92, 2.82, 2.52, 2.19, 1.8, 1.65 and 1.53 Å. These
peaks correspond to the presence of a mixture of monoclinic and tetragonal phases (Fig. 1a)
(JCPDS-ICDD File no. 83-0940 and 81-1545). The percentage tetragonal phase in the zirconia
sample is 58% calculated using the Toroya's method [38]. Addition of 2 mol% of CaO into the
zirconia matrix results in selective stabilization of the tetragonal phase of zirconia. The 2Ca-Zr-O
sample exhibit characteristic reflection from the tetragonal phase of zirconia. The tetragonal phase
is preserved in all composite oxide materials. No new crystalline phase corresponding to either
CaO or CaZrO3 could be detected upto 20 mol% CaO content in the composite oxide system. In
contrast to this observation, the 50Ca-Zr-O material, in addition to the characteristic t-ZrO2
reflections, display less intense but well defined diffraction peaks with d spacing of 3.01, 2.27,
2.08, 1.90 and 1.87 Å. The peaks at 2.27, 2.08 and 1.87 Å corresponding to the presence of non-
stoichiometric zirconia (Zr0.93O2) whereas the peaks at 3.01 and 1.90 Å correspond to the CaO
phase (JCPDS-ICDD File no. 81-1319 and 74-1226). The tetragonal reflections shift gradually to
lower 2θ value with increase in CaO content in the composite oxide (Fig. 2I). The cell parameters
and cell volume are calculated using the software POWD. The plot of cell volume vs CaO content
exhibits a liner relationship satisfying the Vegards Law (Fig. 2II). This observation indicates an
expansion in the zirconia lattice as a result of substitution of Ca2+ (ionic radius =1.0 Å) for Zr4+
(Ionic radius =0.84 Å). The Ca2+ ion being larger in size is responsible for expansion of the
zirconia lattice. The CaO-ZrO2 composite oxide thus contains a substitutional solid solution and
retains the tetragonal lattice of zirconia. The tetragonal phase of zirconia is known to be selectively
stabilized in presence of aliovalent dopants in ZrO2 lattice [38-40]. It has been observed that for an
oversize dopant such as Ca2+ the oxygen vacancy resides near the Zr4+ ions [39,40]. The selective
stabilization of the tetragonal phase in is study can be ascribed to the presence of oxygen vacancy
which prevents the phase transformation of tetragonal phase by blocking the dislocation motion.
The XRD study indicate that upto 20Ca-Zr-O the composite oxide material contain a solid solution
phase whereas the 50Ca-O-Zr material is a mixed phase system consisting of the solid solution
phase, a cation deficient nonstoichiometric zirconia phase and CaO.
The microstructural characteristics of the xCa-Zr-O materials is studied using Fourier line profile
analysis of the broadened XRD patterns by Warren and Averbach method [41] using the software
BRAEDTH [42]. The Fourier analysis is carried out by taking the ZrO2 T (111) and T (220) peaks
for all composite oxide samples. The plots of the volume weighed distribution function (PV) and
size coefficient (AS) as a function of Fourier length are presented in Fig. 3I and II respectively.
Pure ZrO2 show a wide range of distribution function, the crystallite size distribution being
extended beyond 40 nm. This observation suggests that the zirconia sample contain polycrystalline
particles with larger average crystallite size. Contrary to this observation, the 2Ca-Zr-O, 5Ca-Zr-O,
10Ca-Zr-O and 20Ca-Zr-O composite materials exhibit a comparable narrow size distribution
function (< 30 nm). The crystallite size and rms strain of the xCa-Zr-O material are calculated
from the AS~L plot and are presented in Table 4. The calculated crystallite size data indicates that
the xCa-Zr-O material upto 20 mol% CaO, contain smaller crystallites compared to ZrO2 and
5

50Ca-Zr-O materials. The rms strain exhibit an inverse trend for the composites. The higher lattice
strain observed in case of xCa-Zr-O composites is due to the presence of local and random lattice
defects due to their smaller crystallite size.
In order to study the thermal transformation behavior, the TG-DSC analysis of the hydroxy
precipitate is carried out in air atmosphere. The TG-DSC profiles of 10Ca-Zr-O and 50Ca-Zr-O
material are resented in Fig. 4. The 10Ce-Zr-O material exhibit a major weight loss of 22 wt% in
the temperature range of 50-250oC. Two weight loss regions could be discerned in this temperature
range due to removal of physisorbed and coordinated water molecules. Both the weight losses are
endothermic in nature. At higher temperature, however, the weight loss is more gradual in the TG
profile without any well-defined inflection point. The DSC curve of 10Ca-Zr-O material exhibit an
exothermic peak in the temperature range of 500-550oC. This peak can be assigned to the
dehydration and dehydroxylation of the hydroxy precipitate to the corresponding 10Ce-Zr-O
composite oxide. The TG profile of 50Ca-Zr-O material, on the other hand, shows three weight
loss upto 350oC. The weight loss below 150oC is due to removal of physisorbed water whereas the
high temperature loss accounts for the removal of different coordinated water molecules. Beyond
this temperature, a gradual but significant weight loss is observed in the range 450-600oC. In the
DSC profile, this material exhibit three endothermic and two exothermic peaks. The DSC and TG
profile of 50Ca-Zr-O precipitate precursor indicate that it is microscopically inhomogeneous
leading to uneven distribution of both ions. The uneven distribution of the ions may lead to local
segregation of the two hydroxide phases which transform to a multiphase oxide system in the
temperature range of 450-600oC. The TG analysis of 50Ca-Zr-O is supported by the XRD study
where the presence of a mixed phase system consisting of the CaxZr1-xO2-x solid solution, a cation
deficient nonstoichiometric zirconia phase (Zr0.93O2) and pure CaO phase is observed.
The X-ray photoelectron spectra of the 20Ca-Zr-O material are presented in Fig. 5. The survey
spectra in Fig. 5a display the characteristic spectral features due to the presence of Ca, Zr, C and O
species in the composite material. In the Zr 3d spectral region, the 20Ca-Zr-O material exhibits a
broad doublet with maxima at 182.0 eV and 184.2 eV. These peaks corresponds to the
photoelectron emission from the Zr3d5/2 and Zr3d3/2 energy states characteristics of Zr(IV) species
in oxide environment [36,43]. The C1s region a board and intense XPS peak was observed at 284.6
eV along with a broad intense shoulder at 289.7 eV. The former peak is assigned to the
adventitious carbon whereas the high binding energy peak is due to the presence of a minor
amount of carbonate species on the surface of the composite [44]. The O1s high resolution
spectrum is broad and complicated due to the overlapping contribution from different types of
oxygen species present in the sample. The broad spectrum can be deconvoluted into three regions
with binding energy maxima at 529.8, 531.2 and 532.2 eV. These peaks can be assigned to the
lattice oxygen of the CaxZr1-xO2-x solid solution phase and oxygen from CaO and surface carbonate
species. Pure CaO and Ca(OH)2 is known to exhibit O 1s peak ~ 531.2- 531.6 eV whereas for
CaCO3 the O1s peak occur at 532.5 eV [45]. For pure ZrO2 the lattice oxygen binding energy
appears at 530.1 eV [36]. It has been observed that when Zr4+ ions are doped into the CaO lattice
the O1s peak shifts progressively to lower binding energy. Hence assigning the lower binding
energy peak to the solid solution phase is in line with the literature report [43]. In this study,
neither the formation of cubic CaZrO3 phase nor the crystalline CaO phase was detected in XRD
study of 20Ca-Zr-O. It is likely that the 20Ca-Zr-O sample may contain a small portion of Ca2+
ions as amorphous CaO/Ca(OH)2 phase which escaped XRD detection. These species are
responsible for the peak at 531.2 eV. In the Ca2p high resolution spectra, the prominent peak at
346.1 eV along with a satellite at 350.6 eV corresponds to the photoelectron emission from the
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Ca2p3/2 and Ca2p1/2 states. These peaks are characteristics of Ca2+ ions in oxide environment
[43-47]. However, unlike pure CaO, where a well-defined valley is observed between the two
peaks, in this study these two peaks are broad and unresolved from one another. This may be due
to the fact that a small fraction of Ca2+ may exist in carbonate environment [44,45]. The CaCO3
show Ca2p doublet at 347.4 and 351.0 eV. The presence of Ca2+ ions in the solid solution phase
and a small fraction as surface carbonate species can be inferred from the XPS study.
The Raman spectra of the xCa-Zr-O composite materials along with pure ZrO2 are presented in
Fig. 6. The Raman technique is an effective method for microstructural analysis of crystalline
oxide phases. Particularly, the Raman technique has been highly effective for characterization of
different polymorphic forms of zirconia in composite materials [22,48, 49]. Pure ZrO2 prepared by
urea hydrolysis method exhibit intense Raman bands with maxima at 177, 187, 220, 331, 381, 500,
536, 559 and 615 cm-1 corresponding to the monoclinic phase and 146, 269,314, 475 and 638 cm-
1corresponding to the tetragonal phase of zirconia [22, 49]. In contrast to this observation, the xCa-
Zr-O composite oxide exhibit broad, asymmetric and low intense Raman bands corresponding to
the t-ZrO2 only [16,22]. The broadening of the peaks may be ascribed to the presence of large
amount of oxygen vacancy created due to incorporation of Ca2+ ions into the ZrO2 lattice [16,22].
Tetragonal zirconia belongs to P42/nmc space group and is expected to exhibit six Raman active
vibration modes having A1g + 3Eg + 2B1g symmetry [48,49]. Among these vibrational modes, the
symmetric A1g vibration appears around 600 cm-1 and is often too weak to be observed. In the
present study for the xCa-Zr-O composite, the broad Raman bands observed at 146 and 318 cm-1
can be assigned to 2 B1g modes and the bands at 269, 456 and 642 cm-1to the 3Eg modes,
respectively. The asymmetric nature of the Raman bands indicates distortion in the oxygen sub-
lattice caused by the insertion of the Ca2+ ions into the zirconia lattice.
The surface basic property of the xCa-Zr-O composite materials was studied using the CO2-TPD
technique. The TPD profiles of the xCa-Zr-O composite materials are presented in Fig.7. The TPD
profiles for the composite oxides contain three CO2 desorption regions in the temperature range of
< 400 K, 450-750 K and >750 K. Pure ZrO2 prepared by urea hydrolysis method exhibit
significant CO2 desorption below 400 K (Fig. 7a). This low temperature TPD peak can be assigned
to the presence of weak basic sites on zirconia surface. The addition of Ca2+ ions into the zirconia
lattice improves the CO2 retention at high temperature indicating generation of new basic sites of
higher strength (Fig. 7 b-e). For the 5Ca-Zr-O, 10Ca-Zr-O and 20Ca-Zr-O materials a strong CO2
desorption peak is noticed in the range of 400-750 K. Simultaneously, the low temperature
desorption peak below 400 K is significantly suppressed. With increase in Ca2+ content in the
composite, the high temperature TPD peak progressively shifts towards the higher temperature
side. For 50Ca-Zr-O material, a single prominent desorption peak is observed with peak maxima at
790 K (Fig, 7e). The CO2-TPD method has been used earlier for characterization of basic sites of
CaO-ZrO2 composite materials. The basicity of the CaO-ZrO2 material depends on various factors
such as the preparation method, the crystallographic phase of zirconia, CaO content and
calcinations temperature [11, 15-18, 22]. The tetragonal phase of zirconia and a higher calcination
temperature facilitates the incorporation of Ca2+ ions into the zirconia lattice [18,22]. The presence
of Ca2+ ions in the vicinity of O2- ions improves the basicity of the lattice oxygen on the surface.
Moreover, the incorporation of Ca2+ to ZrO2 lattice leads to the formation of Ca-O-Zr linkages and
uneven charge distribution which is responsible for generation of new basic sites [13]. In the
present study, the desorption peak in the temperature range of 450-750 K can be assigned to the
generation of new basic sites due to solid solution formation. The TPD peak observed at 790 K for
50Ca-Zr-O material can be ascribed to the formation of CaO crystallites which has been detected
7

in XRD study for this sample. At high CaO content, the CaO crystallites segregates along the grain
boundary area which is responsible for the high temperature CO2 desorption [18]. Table 1 provides
a summary of the number of basic sites observed for the composite materials. The number of basic
sites increases with CaO content upto 20 mol% beyond which there is a marginal decrease for
50Ca-Zr-O material. The TPD study clearly indicates that the CaO-ZrO2 composite materials
contain strong surface basic sites. The specific surface area of the Ca-Zr-O composite materials is
presented in Table 1. Pure ZrO2 prepared by urea hydrolysis method exhibit specific surface area
of 58.2 m2/g. In general, the nanocomposite oxides exhibit higher surface area compared to the
pure zirconia material. Highest surface area of 91.2 m2/g is obtained for the 20Ca-Zr-O material.
The Ca-Zr-O composite oxides prepared by urea hydrolysis method exhibit more number of strong
basic sites (450-750 K) and higher surface area compared to CaO-ZrO2 materials prepared by
impregnation, coprecipitation, birch-templating and combustion synthesis methods
[11,14,15,18,20]. The urea hydrolysis method seems to be effective for synthesis of high surface
area Ca-Zr-O composite oxides with strong surface basicity. Urea is a weak Bronsted base
(pKb=13.8) whose hydrolysis rate can be controlled by varying the temperature [50]. The mild
nature of urea is helpful in terms of controlled hydrolysis and precipitation of the metal cations
resulting in materials with high surface area. The slow hydrolysis rate also ensures uniform
precipitation of the two components leading to compositional homogeneity and formation of a
uniform solid solution (as evident from elemental mapping study in HRTEM). These factors
contribute to the high surface area and basicity of Ca-Zr-O composite samples prepared by urea
hydrolysis method.
The FESEM images of the xCa-Zr-O material are presented in Fig. 8. Pure ZrO2 prepared by urea
hydrolysis method contain submicron size elongated rod like particles (Fig. 8a). Upon
incorporation of Ca2+ into zirconia lattice morphological changes are clearly noticed in the FESEM
study. The 5Ca-Zr-O and 10Ca-Zr-O materials contain agglomerated particles of irregular shape
with size in the submicron range (Fig. 8b and 8c). With further increase in CaO content the
appearance of flake like particles is observed for 20Ca-Zr-O and 50Ca-Zr-O materials (Fig. 8d and
8e). The flake like particles are formed by controlled growth along the X and Y planar direction.
The pure CaO, on the other hand, contain particles with distinct cubic morphology (Fig. 8f).
Considerable intergrowth among the cubic particles is noticed from the FESEM study. The TEM
image of the 20Ca-Zr-O material along with the EDX pattern and elemental mapping analysis is
presented in Fig. 9. The presence of flake like nanostructures can be inferred from the TEM study
(Fig. 9a). The high resolution image of the 20Ca-Zr-O material indicate the presence of quasi
spherical particles with size in the range of 5-11 nm attached to each other through the grain
boundary region (Fig. 9b). The Fourier analysis data is found to be complementing the TEM
observation. The EDX pattern of the 20Ca-Zr-O material is presented in Fig. 9c. The presence of
Ca, Zr and O of required stoichiometry is inferred from the EDX analysis. The elemental mapping
analysis images of the 20Ca-Zr-O composite materials are presented in Figs 9d-g. Uniform
distribution of both Ca2+ and Zr4+ ions through the specimen sample can be noticed from the
elemental mapping study.
<H2>3.2 Catalytic application of the xCa-Zr-O nanocomposite oxides for base catalyzed synthesis
of chromene analogues
The catalytic activity of the xCa-Zr-O composite oxides is evaluated for the base catalyzed one pot
multicomponent synthesis of 2-amino 4H-Chromenes and 2-amino-2-chromenes. The 2-amino-5-
8

oxo-5,6,7,8-tetrahydro-4H-chromenes were synthesized by one pot condensation of aryl aldehydes,
dimedone and malononitrile (Scheme 1). Initially, the condensation of benzaldehyde, dimedone
and malononitrile was taken as a model reaction and xCa-Zr-O materials with different
composition were evaluated as heterogeneous base catalyst at 50oC employing aqueous ethanol
(1:1) as reaction media. The results of these catalytic experiments are presented in Table 2. Pure
zirconia prepared by urea hydrolysis method provided an isolated yield of 32.6% after 3h of
reaction time. The weak basic sites of zirconia are ineffective in expediting the base catalyzed the
multicomponent reaction. Addition of CaO into the zirconia matrix improves the yield of the 4H-
chromene simultaneously reducing the reaction time. Best catalytic results were obtained when
20Ca-Zr-O composite oxide was used as heterogeneous base catalyst (Table 2). The 20Ca-Zr-O
material provides an isolated yield of 82.6% within 1 h of reaction time. The catalytic results
presented in Table 2 are well correlated with the basicity data presented in Table 1. As observed
from the TPD study, the incorporation of Ca2+ ions into zirconia creates new basic sites
simultaneously modulating the basicity of the oxygen sublattice. The enhancement in basicity is
responsible for the improved catalytic performance of the Ca-Zr-O composite oxides for 4H-
chromene synthesis. Since the composite oxide materials exhibit different surface area and
basicity, in order to compare their catalytic performance, the reaction rates are calculated by
estimating the benzaldehyde conversion in the reaction mixture using gas chromatography. The
rate of reaction with respect to unit surface area and mass is presented in Table 2. Among all Ca-
Zr-O composite oxide materials, the 20Ca-Zr-O material exhibit highest reaction rate indicating its
superior performance for the base catalyzed reaction. For the sake of comparison, the 20Ca-Zr-O
material is also prepared by co-precipitation (P) method using liquid ammonia as precipitating
agent and impregnation (I) method. The 20Ca-Zr-O material prepared by urea hydrolysis method
displays higher reaction rate per unit surface and provides better isolated yield of the 2-amino 4H-
chromene compared to the 20Ca-Zr-O-I and 20Ca-Zr-O-P materials. The reaction rate per unit
surface depends on the preparation method (Table 2). Structurally, the 20Ca-Zr-O composite
materials prepared by urea hydrolysis and coprecipitation methods contain a solid solution phase
whereas the 20Ca-Zr-O-I material contains a mixture of monoclinic and tetragonal zirconia phase
along with crystalline CaO/Ca(OH)2 phases (Fig. S1). The urea hydrolysis method is particularly
useful for preparation of composite oxides with compositional homogeneity, higher surface area
and basicity. In order to compare the catalytic activity of the 20Ca-Zr-O catalyst with other
zirconia based alkali and alkaline earth composite oxides, 20M-Zr-O (where M = Ba and Mg)
materials are prepared by urea hydrolysis method. The catalytic activity of the 20M-Zr-O catalyst
varies in the order 20Ca-Zr-O > 20Mg-Zr-O > 20Ba-Zr-O (Table 2). Based on the catalytic activity
data presented in Table 2, the 20Ca-Zr-O composite oxide material is chosen for further catalytic
study. In order to optimize the reaction protocol, the base catalyzed reaction us performed by
varying the catalyst weight, temperature and reaction media. The results of these optimization
experiments are presented in Fig. 10. For reaction involving 2 mmol of the reactants, 50 mg of the
catalyst is ideal for efficient condensation of the three components (Fig. 10I). Further increase in
catalyst weight does not have a marked impact on the yield of the product. Similarly, the yield of
the product increases with increase in temperature before attending saturation at 50oC (Fig. 10II).
After optimizing the catalyst weight and temperature, we used different solvents to study the effect
of reaction media on the base catalyzed condensation reaction. The condensation reaction is
expedited in presence of polar solvent. Among different polar solvent used ethanol provide better
yield of the product. The poor yield observed in case of water is attributed to the limited solubility
and poor accessibility of the reactants to the catalyst surface. The best results were obtained when
9

a mixed solvent system comprising of ethanol and water (1:1 molar ratio) is used (Fig. 10III).
After optimizing the reaction conditions, we explored the scope and limitation of the developed
catalytic method, by using different substituted aldehydes in the optimized protocol (Table 3). The
yield of the 4H-chromene is found to be marginally better in case of aromatic aldehydes containing
electron withdrawing groups. However, a variety of aromatic aldehydes reacted efficiently under
the optimized catalytic conditions to provide the corresponding 4H-chromenes in high yield and
purity.
Encouraged by the results of the catalytic experiments, we further studied the applicability of xCa-
Zr-O composite oxide materials as heterogeneous base catalyst for synthesis of 2-amino-2-
chromenes by one pot condensation of -naphthol, aryl aldehydes and malononitrile (Scheme 1).
Initially, the reaction conditions are optimized by taking the condensation of benzaldehyde, -
naphthol and malononitrile as a model reaction. It is observed that for a reaction involving 2 mmol
of reactants, 50 mg of the catalyst is sufficient for the condensation of the three components. The
condensation reaction is carried out in different solvents. Table 4 provides a comparison of the
catalytic activity of the composite materials with Ca based catalysts in different reaction media.
Among different solvents studied for the reaction, the polar solvents provide better yield of the
product compared to nonpolar solvents. Among different polar solvents studied at 50oC, best
results are obtained when a mixture of PEG and water (1:1) is used. For solvents like water,
chloroform and ethanol, the formation of the 2-amino-2-chromenes require longer time with poor
yield of the product at 50oC. When the reaction is carried out under reflux condition, the product
yield is found to improve for these solvents (Table 4). The use of this mixed solvent system
considerable reduces the reaction temperature and time. Among the composite oxide, the 20Ca-Zr-
O material is catalytically more active compared to 10Ca-Zr-O, CaO and CaCO3 which is mainly
ascribed to the enhanced basicity of this composite oxide. In order to confirm that the reaction is
truly heterogeneous in nature, the concentration Ca2+ ions in the used solvent was measured using
atomic absorption spectroscopy (AAS). No appreciable concentration of Ca2+ ions could be
detected in the used solvent indicating that the leaching of the Ca2+ does not takes place during the
reaction. The stability of the Ca2+ ions in the Ca-Zr-O catalyst could be ascribed to the formation
of the solid solution phase as a result of incorporation of Ca2+ ions into the zirconia lattice. After
optimizing the reaction conditions, the applicability of the 20Ca-Zr-O catalyzed multicomponent
condensation protocol is extended to the synthesis of structurally diverse 2-amino2-chromenes by
using different substituted aryl aldehydes (Table 5). Under identical reaction conditions, aryl
aldehydes bearing electron withdrawing and donating groups reacted efficiently to give the
corresponding 2-amino-2-chromenes in high yield and purity. The recyclability of the 20Ca-Zr-O
catalyst is tested for three consecutive cycles for synthesis of 2-amino-2-chromene. After
completion of each cycle, the catalyst particles are filtered, washed with excess of water and later
with 05 ml of ethanol and dried in hot air oven. The catalyst particles are regenerated by heat
treatment at 500oC for 2 h. The 20Ca-Zr-O catalyst retains its catalytic activity upto three cycles
without any significant decrease in yield of the product (Table 5, Entry 1, yields, 88.6%, 1st; 87%,
2nd; 85%, 3rd). The recyclability test indicates stable activity of the 20Ca-Zr-O material for
synthesis of 2-amino2-chromenes. The heterogeneous catalytic protocol developed in this work
offer distinct advantage in terms of easy recovery and recyclability of the catalyst compared to
many literature reported homogeneous catalyst methods such as the piperazine, Potassium
phthalimide, CTACl, triethyl amine, tetrabutylammonium fluoride, I2/K2CO3, basic ionic liquid,
acid hydrolysate of tendons [24, 25,29-31, 33-35]. Moreover, although the yield obtained in this
method are comparable with the earlier methods, the present protocol is advantageous in terms of
10

the use of lower catalyst amount, low reaction temperature (in many studies reflux condition and
aqueous media has been used) and use of mixed solvent system.
A plausible mechanism for synthesis of the chromene analogues over the xCa-Zr-O composites is
presented in scheme 2. Initially, the Knoevenagel condensation reaction between the aryl aldehyde
with malononitrile leads to the formation of arylidenemalononitrile. The basic sites of the xCa-Zr-
O catalyst can activate the malononitrile to promote the Knoevenagel condensation. The
nucleophilic addition of the enolic form of the dimedone to arylidenemalononitrile affords the
intermediate I. The formation of the enolic form of the dimedone can be facilitated on the O2- basic
sites on the catalyst surface. The intermediate I is subsequently cyclized by nucleophilic attack of
the OH group on the CN moiety, which undergo tutomerization to provide the 2-amino 4H
chromenes. Similarly, during synthesis of 2-amino-2-chromene, the ortho C-alkylation of -
naphthol takes place by reaction with arylidenemalononitrile to give the intermediate II. The ortho
C-alkylation is favored in presence of xCa-Zr-O catalyst due to the perpendicular adsorption of the
-naphthol molecule over the catalyst surface. It has been observed that the heterogeneous base
catalysts are highly selective to ortho alkylation of phenol moieties. The phenolic molecules
adsorb perpendicularly to the catalyst surface to minimize the repulsion between electron rich
surface sites and the aromatic ring [51]. The intermediate II reacts in a similar fashion as that of
intermediate I by following the nucleophilic addition and tutomeriztion steps on the xCa-Zr-O
surface to produce the 2-amino-chromenes.
<H1>4. Conclusion
In this study, we have prepared a series of CaO-ZrO2 nanocomposite oxides and explored their
catalytic application for synthesis of chromene analogues. The urea hydrolysis method provided
well dispersed CaO-ZrO2 composite oxide materials with compositional homogeneity. The
incorporation of Ca2+ ions into the zirconia lattice leads to the formation of an oxygen deficient
solid solution phase which retains the tetragonal crystal structure of zirconia. Upto 20 mol% CaO
content in the composite oxide a homogeneous solid solution phase is formed whereas the
formation of a mixed phase system is noticed beyond 20 mol%. Fourier and TEM analysis
indicated formation of nanocrystallites of the composite oxide with size less than 20 nm. The
generation of new basic sites of higher strength due to the presence of Ca2+ ions in ZrO2 lattice
could be inferred from the TPD study. XPS study indicated the presence of different lattice oxygen
as potential basic sites. The morphological reorganization of the particles with increase in CaO
content is inferred from FESEM study. The CaO-ZrO2 nanocomposite oxides exhibit excellent
catalytic activity for synthesis of chromene analogues. Structurally diverse 2-amino-2-chromenes
and 2-amino-4H-Chromenes were synthesized under mild condition using the xCa-Zr-O composite
materials as catalyst. The heterogeneous catalytic protocol developed in this work is advantageous
in terms of simple experimentation, use of recyclable catalyst, mild reaction condition and high
yield and purity of the products.
Acknowledgement
We would like to thank the Board of Research in Nuclear Sciences (BRNS), Mumbai for financial
support through the grant No. 37(2)/14/23/2015/BRNS.
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<Figure>Fig. 1. X-ray diffraction patterns of (a) ZrO2, (b) 2Ca-Zr-O, (c) 5Ca-Zr-O, (d) 10Ca-Zr-
O, (e) 20Ca-Zr-O and (f) 50Ca-Zr-O materials.
<Figure>Fig. 2.
(I)XRD patterns indicating an gradual shift in the peak position of TZ (111)
peak to lower 2θ value with CaO content (a) ZrO2, (b) 2Ca-Zr-O, (c) 5Ca-Zr-O, (d)
10Ca-Zr-O, (e) 20Ca-Zr-O and (f) 50Ca-Zr-O, (II) variation of cell volume with CaO
content for the xCa-Zr-O nanocomposite materials.
<Figure>Fig. 3.
<Figure>Fig. 4.
Fourier line profile plots for the ZrO2 and Ca-Zr-O nanocomposite material.
TG profiles of 10Ca-Zr-O (panel I) and 50Ca-Zr-O (panel II) along with the
DSC profiles (a) 10Ca-Zr-O and 50Ca-Zr-O (panel III).
<Figure>Fig. 5.
XPS spectra of 20Ca-Zr-O material.
<Figure>Fig. 6.
O.
Raman Spectra of (a) ZrO2, (b) 5Ca-Zr-O, (c) 10Ca-Zr-O and (d) 20Ca-Zr-
<Figure>Fig. 7.
CO2 TPD profiles of (a) ZrO2, (b) 5Ca-Zr-O, (c) 10Ca-Zr-O, (d) 20Ca-Zr-O
and (e) 50Ca-Zr-O materials.
<Figure>Fig. 8. FESEM images of (a) ZrO2, (b) 5Ca-Zr-O, (c) 10Ca-Zr-O, (d) 20Ca-Zr-O, (e)
50Ca-Zr-O and (f) CaO.
<Figure>Fig. 9. TEM images (a, b), EDX pattern (c) and elemental analysis study (d-g) of 20Ca-
Zr-O material.
15

<Figure>Fig. 10. Effect of reaction parameters on yield of the model reaction. (I) Effect of catalyst
Material
Specific
surface area
(m2/g)
Crystallite
sizea
Strain
<e2>1/2
CO2 desorption (mmol/g)
Total
Basicityb
(mmol/g)
(nm)
< 400 K
450-750 K
---
0.230
0.370
0.418
0.075
> 750 K
---
---
--
0.220
0.485
ZrO2
58.2
68.9
82.6
91.2
68.6
16.9
7.5
11.4
13.5
20.4
4.82 x 10-3 0.104
2.16 x 10-3 ---
3.46 x 10-3 ---
3.93 x 10-3 ---
5.47 x 10-3 ---
0.104
0.230
0.370
0.630
0.560
5Ca-Zr-O
10Ca-Zr-O
20Ca-Zr-O
50Ca-Zr-O
amount (aqueous ethanol (1:1), 50oC), (II) effect of temperature (aqueous ethanol (1:1), 50
mg catalyst) and (III) effect reaction media (50 mg catalyst, 50oC).
<Figure>Scheme 1. Synthesis of chromene analogues catalyzed by xCa-Zr-O composite materials.
<Figure>Scheme 2. Plausible mechanism for synthesis of chromene analogues over the xCa-Zr-O
catalyst
<Table>Table 1. Physicochemical Characteristics of xCa-Zr-O composite materials.
a. Calculated form Fourier line profile of broadened XRD profile, b. Calculated from CO2 TPD profile
<Table>Table 2. Catalytic activity of xCa-Zr-O materials towards the synthesis of 4H-chromenes.
Yielda,b
Reaction ratec
(mmolh-1g-1)
Reaction rate
Catalyst
Time
(min)
(mmolh-1m-2 x
10-3)
2.0
6.7
8.1
9.4
7.3
7.1
6.8
--
ZrO2
180
90
60
60
90
90
120
90
32.6
55.8
65.2
82.6
73.3
71.6
65.8
76.5
45.3
4.6
5Ca-Zr-O
10Ca-Zr-O
20Ca-Zr-O
50Ca-Zr-O
20Ca-Zr-O-P
20Ca-Zr-O-I
20Mg-Zr-O
20Ba-Zr-O
15.6
27.0
33.8
20.0
19.4
13.6
20.8
9.4
120
--
a. Reaction conditions: benzaldehyde (2mmol), malononitrile (2 mmol), dimedone (2 mmol) and 50 mg catalyst in 3 ml of
aqueous ethanol (1:1 ratio) at 50oC for 1 h, b. Refer to pure and isolated yield, c. Calculated with respect to the
benzaldehyde
conversion in the reaction mixture analyzed using gas chromatography
16

<Table>Table 3. 20Ca-Zr-O catalyzed synthesis of structurally diverse 4H-chromenes.
Sl No
R
H
4-NO2
4-Cl
4-OCH3
3-NO2
4-Br
4-OH
4-Me
2-NO2
4-F
Time (min)
Yielda (%)
82.6
1
2
3
4
5
6
7
8
9
60
60
60
90
60
90
90
90
60
60
90.2
88.5
83.2
87.3
85.4
81.6
76.5
91.8
10
82.1
a. Reaction conditions: aryl aldehydes (2 mmol), malononitrile (2 mmol), dimedone (2
mmol) and 50 mg of 20Ca-Zr-O catalyst in 3 ml of aqueous ethanol (1:1 ratio) at 50oC
<Table>Table 4. Catalytic activity of Ca-Zr-O materials towards the synthesis of 2-amino-2-
chromenes.
Solvent
Ethanol
Catalyst
20Ca-Zr-O
20Ca-Zr-O
20Ca-Zr-O
20Ca-Zr-O
10Ca-Zr-O
CaCO3
Temperature
50oC
Time (min)
Yielda,b (%)
59.6
180
180
180
60
60
60
60
90
90
60
Chloroform
Water
50oC
42.2
51.5
88.6
73.5
65.1
78.2
84.0
76.8
50oC
PEG + Water (1:1)
PEG + Water (1:1)
PEG + Water (1:1)
PEG + Water (1:1)
Ethanol
50oC
50oC
50oC
CaO
50oC
20Ca-Zr-O
20Ca-Zr-O
20Ca-Zr-O
reflux
reflux
reflux
Chloroform
Water
80.5
a. Reaction conditions: benzaldehyde (2 mmol), malononitrile (2 mmol), α-naphthol (2 mmol) and 50 mg of catalyst in 3 ml of
solvent, b. Refer to pure and isolated yield
<Table>Table 5.
20Ca-Zr-O catalyzed synthesis of structurally diverse 2-amino-2-chromenes.
Sl No
R
H
Time (min)
Yielda (%)
88.6
1
2
3
4
5
6
7
8
9
60
60
60
90
60
90
60
90
120
2-NO2
4-NO2
4-OCH3
2-Cl
3-NO2
4-Cl
4-Br
4-F
89.5
91.2
83.6
87.0
84.5
90.4
80.5
82.5
17

10
4-CH3
120
81.0
a. Reaction conditions: 2 mmol of reactants with molar ratio 1:1:1, 50 mg of 20Ca-Zr-O catalyst,
3 ml of PEG–water (3 ml; 1:1 mixture) as solvent, temperature 50oC, b. Refer to pure and isolated
yield
TDENDOFDOCTD
18