Preparation and characterization of Ca-modified Co/Al2O3 and its catalytic application in the one-pot synthesis of 4H-pyrans

Article abstract of DOI:10.1007/s11164-020-04139-2

Abstract: In this research, Co/Ca–Al₂O₃ composite as a heterogeneous nanocatalyst was synthesized by the co-precipitation method. The effect of Ca addition on the textural properties and the catalytic activity was investigated. The r

Full text of DOI:10.1007/s11164-020-04139-2

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-020-04139-2
Preparation and characterization of Ca‑modifed Co/
Al2O3 and its catalytic application in the one‑pot synthesis
of 4H‑pyrans
Sakineh Khaledi, et al. [full author details at the end of the article]
Received: 7 February 2020 / Accepted: 27 March 2020
© Springer Nature B.V. 2020
Abstract
In this research, Co/Ca–Al2O3 composite as a heterogeneous nanocatalyst was syn-
thesized by the co-precipitation method. The efect of Ca addition on the textural
properties and the catalytic activity was investigated. The results show that the spe-
cifc surface area and the particle size of the catalyst were infuenced by the addition
of Ca as well as the catalytic activity of the Co/Al2O3 oxide catalyst was increased.
The catalyst was characterized by XRD, SEM, EDS, TEM and BET surface area
analysis. The catalytic activity of Co/Ca–Al2O3 was investigated in the synthesis
of 4H-pyrans. The Co/Ca–Al2O3 showed better catalytic activity than Al2O3 and
Co/Al2O3. The catalyst can be recycled and reused without a signifcant structural
change or considerable decrease in its activity.
Graphic abstract
O
R
Al
Ca
Co
H3C
O
H3CH2CO
CN
O
EtO
H3C
CHO
O
1a-1l
NH2
CN
CN
+
O
R
R
O
H3C
H3C
CN
O
H3C
H3C
O
2a-2m
NH2
Keywords Co/Ca–Al2O3 nanocatalyst · 4H-pyrans · Multicomponent reactions ·
Co-based catalyst · Heterogeneous catalysis
Vol.:(0123456789)
1 3

S. Khaledi et al.
Introduction
4H-pyrans and their derivatives are found in the structure of a series of heterocy-
clic compounds with biological and pharmacological properties [1]. It has been
reported that these compounds have signifcant biological activities such as anti-
bacterial [2], antiviral [3] and anticancer activities [4]. Furthermore, some of the
4H-pyrans have been used for the treatment of Alzheimer’s and Parkinson’s dis-
ease [5]. A number of pyran derivatives are photoactive compounds and used in
light-emitting diodes and fuorescent materials [6, 7].
Multicomponent reactions (MCRs) are considerable interest in organic and
combinatorial chemistry [8]. One-pot synthesis of complex molecules with sev-
eral functional groups through applying simple molecules is one of the bene-
fts of multicomponent reactions. Besides these features, the MCRs have some
advantages such as simplicity of reactions without separation of intermediates,
reducing time and saving energy [9–11]. One type of MCRs is the synthesis of
4H-pyran derivatives by the one-pot, three-component condensation reaction of
aldehydes and malononitrile with active methylene compounds. Various cata-
lysts have been reported for the multicomponent synthesis of 4H-pyrans such as
1-methylimidazolium tricyanomethanide [12], P4VPy-CuI [13], Ca(OTf)2 [14],
gold nanoparticles [15], ionic liquids [16], SnCl2 [17], l-valine [18], PEG-400
[19], SO3H-dendrimer [20], KF-Al2O3 [21], sodium alginate [22], H5BW12O40
[23], thiamine hydrochloride [24], Mg/La [25], KOH/CaO [26], visible light [27,
28] and meglumine [29].
In recent years, many research has been carried out to use heterogeneous cata-
lysts because of their easy separation and recyclability [30]. One important type
of heterogeneous catalysts is metal oxide nanoparticles that are widely used as
a catalyst or as a support [31]. Due to the high catalytic activities, metal oxides
have found many applications as a catalyst. They have been used in various reac-
tions, such as oxidation [32, 33], hydrogenation [34], alkylation [35] and esterif-
cation [36].
A survey shows that numerous reactions have been examined in the pres-
ence of cobalt-based catalysts [37–39]. Cobalt catalysts are generally deposited
on suitable supports to improve the activity and reusability of the catalyst [40].
Based on previous reports, the interactions between Co nanoparticles and support
have a great efect on the physicochemical properties of the Co catalysts [41].
Among Co-based catalysts, Co/Al2O3 catalyst has been used widely in steam
reforming of ethanol [41] and Fischer–Tropsch reaction [42]. Al2O3 has been
used as a support in commercial catalysts due to its thermal stability for industrial
applications, high surface area and large pores [43, 44]. One method to increase
the catalytic stability of cobalt-based catalysts is to add additives such as transi-
tion metals, alkaline and alkali earth metals [45]. Moreover, Ca has been used
as a promoter in various catalysts because it can improve the reducibility of the
metal species and increase the metal dispersion and metal–support interactions
[40, 41]; in addition, Ca is a metal with basic properties. Since the γ-Al2O3 has
inherent acidity, its application as a catalyst support in acid-sensitive reactions
1 3

Preparation and characterization of Ca‑modifed Co/Al2O3…
has limitations. The alkaline or alkaline earth metal oxides can modify γ-Al2O3
and make it suitable as a support for many catalysts [44]. In continuation of our
interest in regards to the synthesis and applications of green catalysts in multi-
component reactions [46–48], herein, we report the preparation and characteri-
zation of Co3O4/CaO–Al2O3 (named Co/Ca–Al2O3) through the co-precipitation
method as a heterogeneous and recyclable nanocatalyst. Then, the catalytic activ-
ity of the synthesized nanocatalyst was evaluated on the synthesis of 4H-pyran
derivatives (Scheme 1).
Experimental
All chemicals and solvents were purchased from Merck and Sigma-Aldrich Com-
panies. All reagents were used without any further purifcation. The prepared
4H-pyrans were identifed by a comparison of their melting points and IR and
NMR data with those reported in the literature. The progress of the reactions was
monitored by TLC using silica gel SIL G/UV 254 plates. The melting points were
measured with an Electrothermal 9100 apparatus and are uncorrected. FT-IR spec-
tra were recorded with a JASCO 6300 spectrometer in KBr pellets in the range
1
400–4000 cm−1. H NMR spectra were attained using a Bruker Ultrashield spec-
trometer at 400 MHz by TMS as an internal standard in DMSO-d6 or CDCl3. Pow-
der X-ray difraction patterns were recorded on a Bruker, D8 ADVANCE with Co
Kα (α=1.5406 Å) at a voltage of 40 kV and a current of 40 mA. The morphology
of the catalyst was studied by scanning electron microscopy (HITACHI S-4160).
Transmission electron microscopy (TEM) was performed on a TEM Philips EM
208S instrument. The BET surface area and the BJH pore size analysis of the sam-
ples were measured from nitrogen adsorption–desorption analysis at liquid nitrogen
temperature (77 K) using a BELSORP-mini II instrument (MicrotracBEL; Japan).
Scheme 1 Synthesis of 4H-pyran derivatives
1 3

S. Khaledi et al.
Inductively coupled plasma optical emission spectrometry (ICP‐OES) analysis was
performed using a Spectro Genesis ICP-OES instrument.
Catalyst preparation
Co/CaO–Al2O3 nanocatalyst was prepared by the co-precipitation method. In a
typical procedure, appropriate amounts of Co(NO3)2·4H2O, Ca(NO3)2·4H2O and
Al(NO3)3·9H2O were dissolved in 100 mL distilled water. Loading amounts of Co
and Ca were 15 wt% and 10 wt %, respectively. Then, an aqueous solution of NaOH
and Na2CO3 (0.15:0.08 molar ratio) was prepared as a precipitating factor. The
nitrate salts’ solution was heated to 70 °C. Subsequently, the solution of precipitant
was added slowly into the aqueous solution of nitrate salts with vigorous stirring at
70 °C until the pH of the solution reached 8. The resulting suspension was aged at
70 °C for 24 h, then fltered and washed with distilled water until the pH becomes
7. The resulting solid was dried at 100 °C overnight and then calcined at 600 °C for
6 h.
General procedure for synthesis of 4H‑pyrans
Co/CaO–Al2O3 hybrid nanocatalyst (0.02 g), aldehyde (1.0 mmol), malononitrile
(1.0 mmol), 1,3-diketone compounds (1.0 mmol) and ethanol (5 mL) were mixed
and stirred at refux conditions (monitored by TLC). After completion of the reac-
tion, the catalyst was removed by fltration and washed with hot ethanol. The fltrate
was cooled to room temperature to give a solid product, and the crude product was
purifed by recrystallization with ethanol/water.
Results and discussion
Today, the use of heterogeneous catalysts has attracted much attention for the syn-
thesis of biologically active molecules. Among various heterogeneous catalysts,
metal oxide nanoparticles are widely used as a catalyst in many chemical processes.
Cobalt as an inexpensive, nontoxic transition metal has attracted many research
attention in sustainable chemistry. Co/Ca–Al2O3 as a metal oxide nanocatalyst was
prepared by the co-precipitation method using nitrate salts as precursors of Co,
Ca and Al in basic solution. After precipitation, the obtained powder was dried at
100 °C overnight and then calcined at 600 °C for 6 h. The prepared catalyst is desig-
nated here as Co/Ca–Al2O3 and characterized by XRD, SEM, EDS, TEM and BET
analyses.
Characterization of Co/Ca–Al2O3 catalyst
The XRD patterns of the Al2O3, Co3O4/Al2O3 (named Co/Al2O3), fresh Co/
Ca–Al2O3 and used catalyst at 2θ=10°–90° are shown in Fig. 1. The XRD of Al2O3
and Co/Al2O3 is used for comparison. There are some peaks in 2θ=32.10, 34.85,
1 3

Preparation and characterization of Ca‑modifed Co/Al2O3…
Co O
3
4
Al O
2
3
(A)
(B)
(C)
(D)
Fig. 1 XRD patterns of catalysts
35.70, 45.30, 46.20, 61.00, 67.15 (Fig. 1a) that correspond to the crystal planes
(220), (311), (400) and (440) which are attributed to cubic γ-Al2O3 (JCPDS 00-047-
1770). Figure 1b shows the XRD patterns for the Co/Al2O3 catalysts with 15 wt%
cobalt which were prepared by the co-precipitation method. The difraction patterns
show peaks at 2θ=37.50, 45.50, 67.15 corresponding to γ-alumina, and the peaks
at 2θ=31.44, 37.07, 45.09, 55.66, 59.65, 65.57 indexed to spinal structure of Co3O4
correspond to the crystal planes (220), (311), (400), (422), (511) and (440), respec-
tively (JCPDS 01-078-1969) [42]. XRD pattern for the Co/Ca–Al2O3 (Fig. 1c) is
much similar to Co/Al2O3, with some decrease in the peak intensity in the modifed
catalyst, which results from the dispersion of cobalt oxide nanoparticles within the
catalyst support with the addition of Ca. Moreover, no obvious characteristic dif-
fraction peaks of CaO can be detected; this means that CaO was highly dispersed
in an amorphous form on the surface of Al2O3. The most intense peak of Co3O4 at
around 2θ=37 overlaps with a γ-alumina difraction peak; thus, it cannot be used to
Table 1 Structural parameters calculated from N2 adsorption/desorption isotherms and XRD analysis
Catalyst
BET surface area Total pore volume Average pore diam- Crystallite size (nm)
(m2/g)
(cm3/g)
eter (nm)
Al2O3
Co/Al2O3
Co/Ca–Al2O3
154
185
169
0.33
0.65
0.49
106
59
28
8.2
22.5 (19.2)a
18.5 (19.5)a
aCalculated from the (2θ=31.44) difraction plane of Co3O4 with Debye–Scherrer equation
1 3

S. Khaledi et al.
Fig. 2 N2 adsorption/desorption isotherms of catalysts
Fig. 3 Pore size distribution of catalysts obtained by the BJH method
calculate the accurate size of the cobalt particles. Therefore, the average particle size
of Co3O4 was calculated based on both the cobalt oxide peak indicated at 2θ=31.44
and 2θ=37.07 and is listed in Table 1.
The textural properties of the catalysts were determined by N2 adsorption/des-
orption isotherms and pore size distributions obtained by the BJH method. The N2
adsorption–desorption isotherms and pore size distribution of Al2O3, Co/Al2O3
oxide and Co/Ca–Al2O3 are shown in Figs. 2 and 3. It can be seen in Fig. 2 that
1 3

Preparation and characterization of Ca‑modifed Co/Al2O3…
all samples exhibit type IV isotherms with an H3-type hysteresis loop according
to the IUPAC classifcation, which are in agreement with the mesoporous struc-
tures [49].
As shown in Table 1, after the addition of Co, the surface area and the pore
volume of the Al2O3 were increased. As the BET results show, adding of Co on
Al2O3 increased the BET specifc surface area and pore volume, from 154 m2/g
and 0.33 cm3/g for the Al2O3 support to 185 m2/g and 0.65 cm3/g for the Co/
Al2O3 catalyst. The Ca-modifed catalyst has specifc surface area and pore vol-
ume (169 m2/g, 0.49 cm3/g), smaller than those of the Co/Al2O3 catalyst. It can
be resulting from the blocking of some pores of the catalyst. Furthermore, the
addition of CaO results in a decrease in the specifc surface area of the catalysts;
this is due to the small specifc surface area of CaO [50]. Figure 3 shows the pore
size distribution of the synthesized catalysts. As can be seen, the Co/Al2O3 has a
pore size distribution with a pore diameter of about 59 nm. After Ca incorpora-
tion, the pore size distribution was changed to 28 nm, which indicates the distri-
bution of CaO on the surface of the catalyst.
(a)
(b)
(c)
O
Al
Ca
Co
(d)
Fig. 4 SEM images (a, b), FESEM mapping (c) and FESEM-EDX analysis (d) of the Co/Ca–Al2O3
1 3

S. Khaledi et al.
The morphology and chemical composition of the prepared catalyst were inves-
tigated by SEM, EDX and TEM. The SEM images in Fig. 4a show the morphol-
ogy of Co/Ca–Al2O3. The morphological structure unit of Co/Ca–Al2O3 is a rodlike
structure with the sizes of nanometer. Figure 4d shows the EDX spectra of the cata-
lyst. The spectra show the presence of only Co, Ca, Al and oxygen, which indicates
the purity of the synthesized Co/Ca–Al2O3 mixed metal oxide. The weight of the
Co, Ca, Al and oxygen content from EDX results is about 14.38, 7.45, 44.17 and
33.99%, respectively, on the catalyst surface. According to the elemental mapping
analysis (Fig. 4c), all of the elements have good distribution in the Co/Ca–Al2O3
catalyst. Figure 5 shows the TEM image for Co/Ca–Al2O3. The cobalt and calcium
oxides with various sized particles less than 100 nm were well dispersed on the
Al2O3 support.
Catalytic activity of Co/Ca–Al2O3
After characterization of Co/Ca–Al2O3 nanocatalyst, its catalytic activity was
studied in the synthesis of 2-amino-3-cyano-4H-pyran derivatives. To optimize
the reaction conditions, the condensation of 4-nitrobenzaldehyde, malononitrile
and ethyl acetoacetate was selected as the model reaction. The efciency of vari-
ous reaction parameters such as the efect of solvent, temperature and the amount
of the catalyst on the reaction time and yield of the product was investigated
(Table 2). First, the reaction was performed without the catalyst in neat condi-
tions; it was found that no desired product was obtained at room temperature and
at 100 °C after 2 h. The reaction was carried out by 0.01 g of catalyst; the corre-
sponding product was obtained in low yield. Then, the reaction was investigated
in diferent solvents. As shown in Table 2, among them, ethanol was the best
solvent for this reaction. In view of the fact that reaction temperature has a sig-
nifcant efect at the time of the reaction and the product yield, the model reaction
was performed at diferent temperatures. As shown in Table 2, the increase in
temperature resulted in a signifcant increase in the reaction rate and the product
yield; thus, the refux conditions were chosen for the reactions. In order to inves-
tigate the efect of the amount of the catalyst, the model reaction was performed
under optimal conditions with various amounts of catalyst from 0.01 to 0.1 g. The
Fig. 5 TEM images (a, b) of the Co/Ca–Al2O3
1 3

Preparation and characterization of Ca‑modifed Co/Al2O3…
Table 2 Optimization of the reaction conditions for the preparation of 4H-pyran-3-carboxylate deriva-
tives
Entry
Catalyst (g)
Solvent
Temp. (°C)
Time (min)
Yield (%)a
1
2
3
4
5
6
7
8
–
–
–
–
–
–
H2O
H2O
r.t.
100
r.t.
Refux
100
Refux
Refux
Refux
r.t.
Refux
r.t.
Refux
Refux
Refux
Refux
r.t.
120
120
120
120
120
120
120
120
120
120
120
60
15
15
15
60
60
60
–
–
Trace
10
60
28
Trace
60
40
82
50
85
95
95
92
55
70
86
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.02
0.05
0.1
–
CHCl3
n-hexane
CH3CN
H2O
9
10
11
12
13
14
15
16
17
18
H2O
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
0.02
0.02
0.02
40
60
Reaction conditions: 4-nitrobenzaldehyde (1.0 mmol), malononitrile (1.0 mmol), ethyl acetoacetate
(1.0 mmol), solvent (3 mL) and required amount of the catalyst
aThe yields are related to the isolated products
results showed that increasing the catalyst from 0.01 to 0.02 g caused the reaction
time to considerably decrease. An increase in the catalyst amount to 0.1 g did not
have a considerable efect on the yield and the reaction time; therefore, 0.02 g of
Co/Ca–Al2O3 nanocatalyst was selected as the best amount of catalyst in respect
of reaction times and yields of the products.
The generality and efciency of the Co/Ca–Al2O3 as the catalyst in the syn-
thesis of 6-amino-5-cyano-4H-pyran-3-carboxylate derivatives were evaluated
by the reaction of malononitrile, various aldehydes and ethyl acetoacetate under
optimized conditions as shown in Table 3. As presented, aryl aldehydes with
electron-donating or electron-withdrawing groups efectively reacted with ethyl
acetoacetate, and the desired products were obtained in high yields.
After the synthesis of 6-amino-5-cyano-4-phenyl-2-methyl-4H-pyran-3-car-
boxylic acid ethyl esters, the efciency of Co/Ca–Al2O3 was studied for the syn-
thesis of tetrahydrobenzo[b]pyran derivatives by replacing ethyl acetoacetate with
dimedone. The condensation reaction was performed between 4-nitrobenzalde-
hyde, malononitrile and dimedone as a model to optimize the reaction conditions
(Table 4). As shown in Table 4, in the presence of 0.02 g of catalyst, the best
results were obtained in ethanol under refux conditions. The majority of this
1 3

S. Khaledi et al.
Table 3 Preparation of 4H-pyran-3-carboxylates derivatives catalyzed by Co/Ca–Al2O3
Entry ArCHO
Producta Time (min) Yield (%)b Mp (°C)
Found
Reported
1
2
3
4
5
6
7
8
4-NO2C6H4CHO
1a
1b
1c
1d
1e
15
35
25
30
20
45
25
15
30
40
20
25
95
87
90
95
92
83
90
95
95
88
90
85
178–180 173–175 [20]
145–147 141–144 [17]
171–173 167–169 [25]
210–212 217–219 [22]
180–182 179–181 [20]
216–218 215–217 [22]
183–185 188–190 [22]
177–179 173–175 [21]
208–210 212–214 [22]
205–207 211–212 [22]
198–200 206–209 [22]
168–170 173–175 [21]
4-CH3OC6H4CHO
2-HOC6H4CHO
2-CH3C6H4CHO
4-BrC6H4CHO
4-Dimethylaminobenzaldehyde 1f
C6H5CHO
4-ClC6H4CHO
3-NO2C6H4CHO
4-HOC6H4CHO
2-ClC6H4CHO
Furfural
1g
1h
1i
1j
1k
9
10
11
12
1l
Reaction conditions: aryl aldehyde (1 mmol), malononitrile (1 mmol), ethyl acetoacetate (1 mmol) and
Co/Ca–Al2O3 (0.02 g) in ethanol (3 mL) at refux conditions
aProducts characterized by comparison of their spectroscopic (NMR and IR) data and melting points
with those reported in the literature
bIsolated yields
reaction was investigated with the use of various aldehydes (Table 5). The desired
products were obtained in high yields at short time periods. The aromatic alde-
hydes with electron-withdrawing substituent reacted faster than aldehydes that
have electron-donating groups. Also, in the presence of dimedone in comparison
with ethyl acetoacetate, the pyran derivatives were obtained in shorter reaction
times.
The catalytic activity of the Co/Ca–Al2O3 in the synthesis of ethyl-6-amino-5-cy-
ano-4-(4-nitrophenyl)-2-methyl-4H-pyran-3-carboxylate was compared with Al2O3
and Co/Al2O3. The results are presented in Table 6. As indicated, the efciency of
Co/Ca–Al2O3 is better than others, according to the reaction time and the yield of
the product. When the aluminum oxide was used alone, the yield of the product was
only 35%. The model reaction was carried out in the presence of Co/Al2O3; the reac-
tion yield was improved to 50%, which shows the function of cobalt oxide in catalyst
activity. When the Ca-modifed catalyst was used, the reaction yield increased to
85%. The better efciency of the Ca-modifed catalyst is thought to be due to a com-
bination of factors such as the surface area of the catalyst, the acid–base properties
of the catalyst and the synergistic efect between active components of the catalyst
[45].
To show the merit of the present work in comparison with the results reported in
the literature, we compared the obtained results using Co/Ca–Al2O3 with a number
of catalysts reported on the synthesis of 4H-pyran derivatives (Table 7). As shown
in Table 7, the composite nanocatalyst is comparable to others, in respect of reaction
times and yields of the products.
1 3

Preparation and characterization of Ca‑modifed Co/Al2O3…
Table 4 Optimization of the reaction conditions for the preparation of tetrahydrobenzo[b]pyran deriva-
tives
Entry
Catalyst (g)
Solvent
Temp. (°C)
Time (min)
Yield (%)a
1
2
3
4
5
6
7
8
–
–
–
–
–
–
H2O
H2O
r.t.
100
r.t.
Refux
100
Refux
Refux
Refux
r.t.
Refux
r.t.
Refux
Refux
Refux
Refux
r.t.
120
120
120
120
60
60
60
60
60
60
60
10
5
–
–
Trace
23
50
40
10
65
60
90
75
85
90
92
90
77
80
88
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.02
0.05
0.1
–
CHCl3
n-hexane
CH3CN
H2O
9
10
11
12
13
14
15
16
17
18
H2O
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
5
5
30
30
30
0.02
0.02
0.02
40
60
Reaction conditions: 4-nitrobenzaldehyde (1.0 mmol), malononitrile (1.0 mmol), dimedone (1.0 mmol),
solvent (3 mL) and required amount of the catalyst
aThe yields are related to the isolated products
A probable mechanism for the formation of the 4H-pyrans is depicted in
Scheme 2. Initially, catalyst activates the carbonyl group of the aromatic aldehyde
and CaO as a base accepts a proton from malononitrile. The Knoevenagel con-
densation occurred to form the arylidenemalononitrile (I). Subsequently, diketone
tautomerized in the presence of CaO, and then nucleophilic addition reaction with
arylidenemalononitrile provides the Michael adduct (II). The Michael adduct (II)
tautomerizes by CaO to create intermediate (III) which cyclizes and then tautomer-
izes to provide the 4H-pyran.
Catalyst reusability and stability
Also, the catalyst reusability was investigated in the model reaction. For this pur-
pose, after the reaction was completed, the catalyst was separated by fltration,
washed several times with warm ethanol, dried and reused in the next run. The
recycled catalyst was used up to fve cycles in the reaction without any signifcant
loss in the activity (Table 8). The XRD patterns of the reused catalyst (Fig. 1D)
showed that the Co/Ca–Al2O3 nanocatalyst structure was maintained after fve
runs. Furthermore, the adjustment of XRD patterns of the reused catalyst and the
1 3

S. Khaledi et al.
Table 5 Preparation of tetrahydrobenzo[b]pyran derivatives catalyzed by Co/Ca–Al2O3
Entry ArCHO
Producta Time (min) Yield (%)b Mp (°C)
Found
Reported
1
2
3
4
5
6
7
8
4-NO2C6H4CHO
2a
2b
2c
2d
2e
5
10
10
10
10
15
10
5
95
90
85
90
90
80
90
92
92
90
85
82
90
185–187 179–181 [21]
193–195 198–200 [21]
240–242 247–249 [12]
203–205 203–205 [24]
210–212 203–205 [15]
211–215 208–210 [15]
225–227 229–232 [15]
209–201 214–216 [22]
212–214 209–211 [20]
228–230 225–227 [24]
204–206 198–200 [22]
175–177 182–184 [22]
219–220 222–224 [22]
4-CH3OC6H4CHO
2-HOC6H4CHO
2-CH3C6H4CHO
4-BrC6H4CHO
4-Dimethylaminobenzaldehyde 2f
C6H5CHO
4-ClC6H4CHO
3-NO2C6H4CHO
4-HOC6H4CHO
4-CH3OC6H4CHO
C6H4CH=CHCHO
Furfural
2g
2h
2i
2j
2k
2l
9
5
10
11
12
13
10
10
15
15
2m
Reaction conditions: aryl aldehyde (1 mmol), malononitrile (1 mmol), dimedone (1 mmol) and Co/Ca–
Al2O3 (0.02 g) in ethanol (3 mL) at refux conditions
aAll synthesized compounds are known in the literature
bIsolated yields
Table 6 Infuence of catalyst
Entry
Catalyst
Time (min)
Yield (%)
component
1
2
3
Al2O3
Co/Al2O3
Co/Ca–Al2O3
60
60
60
35
50
85
Reaction conditions: 4-nitrobenzaldehyde (1.0 mmol), malononi-
trile (1.0 mmol), ethyl acetoacetate (1.0 mmol), ethanol (3 mL) and
0.02 g of the catalyst at refux conditions
Table 7 Comparison of Co/Ca–Al2O3 with other recently reported catalysts
Entry
Catalyst
Conditions
Time (min)
Yield (%)
References
1
2
3
4
5
6
7
8
l-valine
H2O/40 °C
Ethanol/ r.t.
H2O/refux
Ethanol/refux
Ethanol/refux
Ethanol–H2O/refux
Methanol/refux
Ethanol/refux
45–80
1.5–5 h
30–60
78–94
63–91
74–86
84–90
84–96
77–98
55–92
83–95
[18]
[21]
[15]
[20]
[22]
[23]
[25]
This work
KF–Al2O3 (10 mol%)
AuNPs@RGO‐SH (0.05 g)
SO3H-dendrimer (0.05 g)
Sodium alginate (10 mol%)
H5BW12O40 (10 mol%)
Mg/La (0.05 g)
40–90
40–125
90–300
60–180
15–45
Co/Ca–Al2O3 (0.02 g)
1 3

Preparation and characterization of Ca‑modifed Co/Al2O3…
Scheme 2 The suggested mechanism for the synthesis of 4H-pyran derivatives using Co/Ca–Al2O3 cata-
lyst
Table 8 The reusability of the
Run
Time (min)
Yield (%)
1
15
95
2
15
92
3
20
90
4
20
90
5
20
87
catalyst Co/Ca–Al2O3 in the
model reaction
Reaction conditions: 4-nitrobenzaldehyde (1.0 mmol), malononi-
trile (1.0 mmol), ethyl acetoacetate (1.0 mmol), ethanol (3 mL) and
0.02 g of the catalyst at refux conditions
fresh catalyst indicates that there was no change in the crystal structure of the
catalyst. The SEM image of recycled catalyst is presented in Fig. 4b. The com-
parison between fresh and reused catalysts showed that the morphology of the
catalyst was maintained after fve cycles. To determine the stability of the cata-
lyst, the amount of calcium leaching from the catalyst was measured by ICP spec-
troscopy. For this purpose, the catalyst was heated and stirred in ethanol for 2 h;
then, the catalyst was removed from the solution. The calcium concentration in
ethanol solution was 0.06 ppm. The synthesis reaction of 4H-pyran was then car-
ried out in the residual ethanol solution under similar reaction conditions with the
heterogeneous catalyst, but no progress was observed in the reaction. This obser-
vation confrmed that the catalytic activity of Co/Ca–Al2O3 is closely related to
the metal oxide properties on the surface of the catalyst and is not related to the
leached CaO. Low concentrations of CaO leached out from the catalyst surface
are insufcient for catalyzing the reaction in the homogeneous phase, and the
nature of the reaction is heterogeneous.
1 3

S. Khaledi et al.
Conclusion
Co/CaO–Al2O3 hybrid nanocatalyst was prepared by the co-precipitation method.
The catalytic activity of Co/Ca–Al2O3 was investigated for the one-pot synthesis of
4H-pyrans. 4H-pyran derivatives were successfully synthesized in the presence of
Co/Ca–Al2O3 in high yields. It is found that the addition of CaO can improve the
catalytic properties of Co/Al2O3. Compared to Al2O3 and the bare Co/Al2O3, CaO
has a considerable efect on the interaction between Co oxide and Al2O3 support,
and as a result, it increases the catalytic activity of Co oxide supported on Al2O3.
The CaO leaching does not contribute to the production of 4H-pyrans, and the for-
mation of 4H-pyrans is due to the heterogeneous properties of the catalyst. The cata-
lyst can be recovered and reused without a considerable loss of its catalytic activ-
ity for up to fve reaction runs. Moreover, easy separation of the catalyst from the
reaction mixture, simple workup procedure and short reaction times are some of the
other advantages of this method.
Acknowledgements We are grateful to the research council of Shahrekord University for the support of
this research.
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Publisher’s Note Springer Nature remains neutral with regard to jurisdictional claims in published
maps and institutional afliations.
Afliations
Sakineh Khaledi1 · Mahboobe Rajabi1 · Ahmad Reza Momeni1 ·
Heshmat Allah Samimi1 · Jalal Albadi1
*
Ahmad Reza Momeni
Ahmadrmomeni@gmail.com; momeni-a@sku.ac.ir
1
Department of Chemistry, Faculty of Science, Shahrekord University, P.O. Box 115-381,
Shahrekord, Iran
1 3