Phosphotungstic acid grafted zeolite imidazolate framework as an effective heterogeneous nanocatalyst for the one-pot solvent-free synthesis of 3,4-dihydropyrimidinones

Article abstract of DOI:10.1002/aoc.4959

Phosphotungstic acid (H₃PW₁₂O₄₀, PTA) supported on ZIF-9(NH₂) was synthesized for the first time and performed as an effective and environmental friendly catalyst in the one-pot three component Biginelli condens

Full text of DOI:10.1002/aoc.4959

Received: 30 November 2018
DOI: 10.1002/aoc.4959
Revised: 29 March 2019
Accepted: 30 March 2019
F U L L P A P E R
Phosphotungstic acid grafted zeolite imidazolate
framework as an effective heterogeneous nanocatalyst
for the one‐pot solvent‐free synthesis of 3,4‐
dihydropyrimidinones
1
,2
3
1
1
Reza Tayebee *
Mehdi Baghayeri
| Mojtaba Fattahi Abdizadeh | Nasrin Erfaninia | Amirhassan Amiri |
1
4
1
2
| Roya Mohammadzadeh Kakhki | Behrooz Maleki
| Effat Esmaili
1
Department of Chemistry, Hakim
Sabzevari University, Sabzevar
Phosphotungstic acid (H PW O , PTA) supported on ZIF‐9(NH ) was synthe-
3
12 40
2
9
2
6179‐76487, Iran
sized for the first time and performed as an effective and environmental
friendly catalyst in the one‐pot three component Biginelli condensation of dif-
ferent substituted benzaldehydes with ethyl acetoacetate and urea to afford the
corresponding 3,4‐dihydropyrimidin‐2‐(1H)‐ones under solvent‐free condi-
tions. ZIF‐9(NH ) and the prepared nanocatalyst PTA@ZIF‐9(NH ) were char-
Department of Chemistry, Payame Noor
University (PNU), Tehran 19395‐4697,
Iran
3
Cellular and Molecular Research Center,
Sabzevar University of Medical Sciences,
Sabzevar, Iran
2
2
acterized by XRD, FESEM, TEM, EDX, BET, AAS, TGA, UV–Vis, and FT‐IR.
After reaction, the nanocatalyst can be easily separated from the reaction mix-
ture by centrifuge and the recovered catalyst can be reused for at least five
times with a 14% reduction in yield after the fifth run. This study showed that
4
Department of Chemistry, Faculty of
Sciences, University of Gonabad,
Gonabad, Iran
Correspondence
R. Tayebee, Department of Chemistry,
Hakim Sabzevari, University, Sabzevar
ZIF‐9(NH ) can be utilized as a promising support for PTA and developed a
2
highly active, stable and reusable heterogeneous catalyst under easy reaction
9
6179‐76487, Iran.
condition in the multi‐component organic synthesis.
Email: rtayebee@hsu.ac.ir
M. Fattahi Abdizadeh, Cellular and
Molecular Research Center, Sabzevar
University of Medical Sciences, Sabzevar,
Iran.
KEYWORDS
3
2
,4‐Dihydropyrimidin‐2‐(1H)‐ones, heterogeneous, MOF, PTA@ ZIF‐9(NH ), solvent free
Email: fattahim@medsab.ac.ir
1
| INTRODUCTION
Application of MOFs in catalysis is limited due to their
inert microstructures; however, further investigations to
assess application of these materials in catalysis may be
interesting. Zeolite imidazolate frameworks (ZIFs), as a
new subclass of MOFs, involve highly desirable properties
of both zeolites and conventional MOFs and are recently
considered as profound highly crystalline materials with
adequate porosity and easy tunability of the pore
Metal–organic frameworks (MOFs) consist of metallic
nodes bonded by organic linkers, are currently received
significant attention because of their versatile proper-
[
1–3]
[4,5]
ties.
They have been extensively used in catalysis,
[6]
[7]
[8]
gas storage, separation, ion exchange and in drug
[9]
delivery. Compared to various microporous and meso-
porous inorganic materials, MOFs showed several advan-
tages such as structural diversity, flexibility and
geometrical control by functionalization with different
size/shape catalysts or catalyst supports by several research
[12,13]
groups.
ZIFs are extended three‐dimensional struc-
tures constructed from tetrahedral metal atoms (e.g., Zn,
[10,11]
organic linkers.
[12]
Co, Cu) bridged by imidazolate linkers
producing
*
Electronic supplementary information (ESI) available.
nanosized pores formed by four, six, eight, and twelve
Appl Organometal Chem. 2019;e4959.
wileyonlinelibrary.com/journal/aoc
© 2019 John Wiley & Sons, Ltd.
1 of 10
https://doi.org/10.1002/aoc.4959

2
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TAYEBEE ET AL.
membered rings ZnN , CoN , or CuN tetrahedral clus-
All synthesized dihydropyrimidinones were characterized
by comparison of their spectral and physical data with
those reported in the literature. Progress of the reactions
4
4
4
[13]
ters.
A number of applications have been explored for
[
14]
[38]
ZIFs as catalysts or catalyst supports in hydrogenation,
esterification, transesterification, hydrogenolysis,
reduction reactions, as sensors, and in many organic
[15]
[16]
[17]
was monitored by TLC. Infrared spectra were recorded on
[18]
[19]
8700 Shimadzu Fourier Transform spectrophotometer
[19–23]
1
13
transformations.
Among various ZIFs, the subgroup
using KBr windows. H and C NMR spectra were
of ZIF‐9 also showed interesting catalytic and photocata-
recorded on a Bruker AVANCE 300‐MHz instrument
using TMS as internal reference. Data for H NMR are
[24]
1
lytic activities in the hydrogen generation,
oxidation
[25]
[26]
reactions,
synthesis.
separation purposes
and in organic
reported as follows: chemical shift (δ) and multiplicity (s:
singlet, d: doublet, t: triplet, q: quartet, m: multiplet, qt:
quintuple, dq: doublet of quartets, br: broad). X‐ray diffrac-
tion patterns were obtained on a STOE diffractometer with
Cu Kα radiation. Electron microscopy was performed on a
Phillips XL‐30 scanning electron microscope (SEM).
Transmission Electron Microscopy was studied by TEM‐
Philips EM 208S operating at 100 kV. The BET surface
areas and pore size distributions were acquired from N2
adsorption–desorption isotherms at 77 K on a Belsorp‐mini
II (BEL Japan, Inc.). Thermal stability analysis was per-
formed with thermogravimetric analysis (TGA, Q600
TA). Melting points were recorded with a Bumstead elec-
trothermal type 9200 melting point apparatus.
Ultraviolet–visible spectra were obtained on a Photonix
UV–Vis Array spectrophotometer, Model Ar 2015.
[27]
Since, unsupported heteropolyacids are very soluble in
polar solvents and generally induce their catalytic activi-
ties under homogeneous conditions, different supports
can be employed to build up super acidic heterogeneous
catalysts applicable in polar organic solvents. Indeed,
our previous efforts revealed that the catalytic activity
and acidity of the supported heteropolyacids depend
mainly on the loading and nature of the carrier. However,
leaching of the supported heteropolyacid, as a main
drawback, reflects low interaction of this strong bronsted
[28–32]
acid with the support material.
In this work, we wish to report direct grafting of
highly dispersed phosphotungstic acid into the cage of
ZIF‐9(NH ). Then, the prepared nanocatalyst PTA@ZIF‐
2
9
(NH ) was applied for the Biginelli condensation of
2
different substituted benzaldehydes with ethyl
acetoacetate and urea to achieve the corresponding
2.2 | Preparation of ZIF‐9(NH2)
3
,4‐dihydropyrimidin‐2‐(1H)‐ones
(Scheme
1).
Co (bIm) ·(DMF)(H O) abbreviated ZIF‐9(NH ) was pre-
2
2
2
Dihydropyrimidinones have received charismatic assess-
ment due to their wide range of pharmacological and
therapeutic properties, such as antiviral, anti‐tumor, anti-
pared according to the reported procedure by Yaghi with
[39]
a
slight modification.
A
solid mixture of Co
(
NO ) ·6H O (2.10
g,
7.21 mmol) and 2‐
[33–37]
3 2
2
bacterial and anti‐inflammatory.
Although, a num-
aminobenzimidazole (0.690 g, 5.08 mmol) were dissolved
in 180 ml of N, N′‐dimethylformamide (DMF). Then, the
mixture was transferred to a tightly capped Teflon vial
ber of protocols have been introduced for the synthesis
of 3,4‐dihydropyrimidin‐2(1H)‐ones; however, consider-
ing operational drawbacks such as needing strongly
acidic condition, high temperature, long reaction time,
and stoichiometric amounts of expensive reagents neces-
sitate the discovery of new synthetic methodologies.
−1
and heated at a rate of 5 °Cmin to 140 °C and held at
this temperature for 48 hr. After removal of the mother
liquor, chloroform (20 ml) was added to the vial. The
obtained pale pink polyhedral crystals were collected
from the upper layer, washed with DMF (10 ml × 3),
and dried in air for 2 hr.
2
2
| EXPERIMENTAL
.1 | Materials and methods
2.3 | Preparation of PTA@ZIF‐9(NH ) and
2
studying grafting of PTA into ZIF‐9(NH )
2
Cobalt nitrate hexahydrate (99%), 2‐aminobenzimidazole
98%) and phosphotungstic acid were purchased from
Merck and used as received without further purification.
(
The wet impregnation of phosphotungstic acid into ZIF‐
9(NH2) was performed by dissolving 25 mg of
SCHEME 1 General scheme for the
three component condensation of urea,
aldehyde, and ethyl acetoacetate

TAYEBEE ET AL.
3 of 10
phosphotungstic acid (PTA) in 40 ml methanol. Then,
(data are not shown). Very similar XRD diffractions
proved that the ZIF structure was unchanged after 4 hr
reflux in methanol.
5
0 mg of ZIF‐9(NH ) was added to the PTA solution
2
and the mixture was refluxed for 4 hr. Thereafter, the
solid was separated and washed with 4 × 8 ml of metha-
nol to resolve the unreacted PTA from the surface of ZIF‐
2
.4 | General procedure for the
preparation of 3, 4‐dihdropyrimidin‐2(1H)‐
9
(NH ). Finally, the purple crystals of PTA@ZIF‐9(NH )
2
2
were separated and dried at 70 °C for 15 hr (Scheme 2).
Figure 1 displays UV–Vis spectral changes with time after
ones
addition of 0.5 g ZIF‐9(NH ) to 20 ml methanol solution
of PTA (0.1 g) under reflux. This study showed that
2
A solution of aldehyde (1 mmol), ethyl acetoacetate
(
1.2 mmol), and urea (1.5 mmol) was heated to 110 °C
~
94% of PTA was grafted into ZIF‐9(NH ) after 2.5 hr
2
under solvent‐free condition in the presence of the
and 18.8 wt% PTA@ZIF‐9(NH ) was attained. This find-
2
desired amount of PTA@ZIF‐9(NH ) nanocatalyst. Com-
2
ing was also confirmed by ICP based on the tungsten con-
pletion of the reaction was monitored by TLC. After com-
pletion, the reaction mixture was cooled to room
temperature, dissolved in minimum ethanol and filtered
to separate the catalyst. Then, the crude product was
poured into crushed ice with thorough stirring for
tent. To ensure no significant leaching of PTA from ZIF‐
9
(NH ), a dried fresh sample of the impregnated
2
PTA@ZIF‐9(NH ) was refluxed in methanol for 4 hr.
2
Then, the solid catalyst was filtered and the filtrate was
tested for existence of PTA by UV–Vis. A weak absorption
band around 268 nm was developed; which confirmed
small leaching of PTA into the methanol solution. To
confirm PTA@ZIF‐9(NH2) framework is stable even after
5
min. The desired dihydropyrimidinone was further
purified by crystallization in ethanol. Spectral data of
some selected compounds are obtained as the supporting
material.
4
hr reflux in methanol, XRD of the target ZIF‐9(NH2)
after reflux was compared with the newly prepared one
3
| RESULTS AND DISCUSSION
In this work, the ZIF‐9(NH ) was synthesized using
2
cobalt
(II)
nitrate
hexahydrate
and
2‐
aminobenzimidazole by a solvothermal method in DMF.
The ZIF‐9(NH ) was then characterized using different
2
techniques as described below.
3.1 | Structural characterization of the
hybrid nanomaterial
2
SCHEME 2 Schematic for the preparation of PTA@ZIF‐9(NH )
catalyst. A part of the schematic is reproduced with permission
[23,64]
3.1.1 | FT‐IR spectroscopy
FT‐IR spectroscopy was carried out to confirm grafting of
PTA into ZIF‐9(NH ) and formation of the final material
2
PTA@ZIF‐9(NH ). FT‐IR spectra of bare ZIF‐9(NH ) and
2
2
PTA@ZIF‐9(NH ) are shown in Figure 2. A broad and
2
−
1
strong band between 2400–3400 cm is due to benzimid-
[
40,41]
−1
azole.
The band at 747 cm in the spectral region is
associated with the out‐of‐plane bending of imidazole
−
1
ring, while peaks in the region of 900–1350 cm are
assigned to in‐plane bending. The peaks at 1581–1645
−
1
and 3400 cm
would be due to the bending and
stretching vibrations of N–H in imidazole ring, respec-
−
1
tively. The intense band at 1456 cm is associated with
the entire ring stretching. FT‐IR spectrum of PTA@ZIF‐
9
(NH ) clearly confirmed that the most intensive peaks
2
FIGURE 1 UV–Vis spectral changes during loading of PTA onto
ZIF‐9(NH
belong to ZIF‐9(NH ) and the structure retained upon
PTA impregnation. Moreover, the FT‐IR spectrum of
2
2
)

4
of 10
TAYEBEE ET AL.
FIGURE 2 FT‐IR spectra of (a) PTA, (b) ZIF‐9(NH
ZIF‐9(NH
2
), (c) PTA@
2
)
PTA@ZIF‐9(NH ) exhibited the principal stretching
2
modes of the Keggin PTA units at 1029, 952, 877 and
FIGURE 3 XRD patterns for ZIF‐9(NH
2
) (a) and PTA@ZIF‐
−
1
[27]
7
61 cm , respectively. These bands correspond to the
9
(NH ) (b). Inset shows XRD of the reported ZIF‐9
2
ν(P‐O ), terminal ν(W‐O ), corner‐sharing ν(W‐O ) and
a
t
b
edge‐sharing[ ν(W‐Oc) band vibrations of PTA,
42,43]
respectively.
intensity of some peaks were changed after the PTA
impregnation. Broadening of some diffractions would be
due to homogeneous dispersion of PTA on the surface
of ZIF. However, a slight shift in some peak positions
would be explained based on the dispersion of PTA over
the surface of ZIF‐9 and considerable interaction of PTA
with the surface of ZIF‐9 which may induce partial
3.1.2 | X‐ ray diffraction and EDX
In the structure of ZIF‐9(NH ), four nitrogen atoms of
2
four different benzimidazolate (bIm) ligands are coordi-
[
45]
nated to the cobalt (II) ion and each bIm ligand bridges
changes in the crystal structure of ZIF‐9.
Observation
2+
two Co ions and formed a sodalite topology frame-
of no diffraction peak for PTA in Figure 3b confirmed
[44]
−1
[48]
work.
The cobalt loading of 4.13 mmol.g was found
that the PTA was highly dispersed on ZIF‐9(NH2).
by elemental analysis with AAS for the ZIF‐9(NH ) which
is close to the amount obtained for ZIF‐9. According to
these structural information, XRD patterns of bare ZIF‐
The energy dispersive X‐ray (EDX) analysis of ZIF‐
2
[27]
9(NH ) showed the main elements of Co, N, and C
2
(Figure S1).
9
(NH2) and PTA@ZIF‐9(NH2) were investigated
Figure 3). XRD patterns of the synthesized ZIF‐9(NH2)
and PTA@ZIF‐9(NH ) were in good agreement with the
(
2
3.1.3 | TEM and SEM studies
[27,45–47]
reported XRD of ZIF‐9.
The two sharp peaks at
2
θ < 10° in the XRD of bare ZIF‐9(NH ), demonstrated
Morphologies of the synthesized ZIF's were also studied.
2
that a highly crystalline material was obtained. Moreover,
SEM and TEM micrographs of ZIF‐9(NH ) and SEM pic-
2
the crystal structure of PTA@ZIF‐9(NH ) seems to
ture of PTA@ZIF‐9(NH ) are described in Figure 4,
2
2
remain unchanged during PTA grafting, while the
respectively. ZIF‐9 showed uniform nanoparticles of

TAYEBEE ET AL.
5 of 10
FIGURE 4 Electron microscopy of ZIF‐9(NH
2
) and PTA@ ZIF‐9(NH
2
). (a) SEM and (b) TEM micrographs of ZIF‐9(NH
2
), (c) SEM image
2
of PTA@ ZIF‐9(NH )
about 100 nm in size; but, PTA@ZIF‐9(NH ) indicated a
150–220 °C is due to the removal of the adsorbed water
and organic solvents; whereas the main weight loss at
>550 °C corresponded to the combustion of the organic
parts and destruction of the heteropolyacid moiety.
2
[49]
disordered structure.
However, previous reports on
ZIF‐9 recommended that the pores are complex and
[
27]
may involve both micropore and mesopore arrays.
Moreover, grafting of PTA into the cage of ZIF may
induce some demolition of ZIF‐9 crystal structure and
[45]
particles aggregation as shown in Figure 4b.
3.2 | Catalytic performance of PTA@ZIF‐
9
(NH )
2
3
.1.4 | BET and TGA analysis of ZIF‐9
3.2.1 | Studying effect of the catalyst
amount on the condensation reaction
(NH )
2
The nitrogen adsorption–desorption isotherms of ZIF‐
(NH ) sample measured at 77 K is shown in Figure S2.
The catalytic efficiency of PTA@ZIF‐9(NH ) heteroge-
neous catalytic system was assessed for the synthesis of
2
9
2
The type IV isotherm for ZIF‐9(NH ) along with a hyster-
ethyl‐6‐methyl‐2‐oxo‐4‐phenyl‐1,2,3,4‐
2
esis loop at P/P = 0.85–1 with an abrupt increase of gas
tetrahydropyrimidine‐5‐carboxylate with various catalyst
amounts (Figure 5). First, a model reaction was planned
in the absence of catalyst. The reaction of benzaldehyde,
ethyl acetoacetate and urea afforded 25% of the desired
dihydropyrimidinone in the absence of catalyst after
30 min; whereas, 10 mg of catalyst afforded 65% of con-
version at the same time. This finding revealed that the
heterogeneous catalyst exhibited high catalytic activity
in the desired transformation. Findings showed that
30 mg of catalyst was enough to endorse the reaction to
85% yield at a short time of 30 min. Higher amount of
0
adsorption at the mentioned relative pressures denoted
[
50]
the mesopores and macropores of the material.
The
specific surface area of ZIF‐9(NH ) calculated by the
2
2
−1 [51,52]
BET method was 407 m .g .
The BET surface area
was substantially decreased upon the inclusion of PTA
2
−1
species (342 m .g ), which may indicate the presence
of guest components supported on the ZIF‐9(NH ) frame-
2
work. Furthermore, TGA analysis of ZIF‐9(NH ) con-
2
firmed an intended trend which was previously
[27]
observed for ZIF‐9 (Figure S3).
The weight loss at

6
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TAYEBEE ET AL.
3
.2.3 | Effect of reaction temperature on
the condensation reaction
To enhance the yield% and accomplishing the best reac-
tion conditions, the impact of temperature was contem-
plated on the condensation reaction of ethyl
acetoacetate with benzaldehyde and urea in the presence
of ZIF‐9(NH2), H PW O
and PTA@ZIF‐9(NH2)
3
12 40
(
Figure 6). As is anticipated, yield% was enhanced with
temperature for all the examined catalysts and 32, 35,
and 85% yields were obtained for ZIF‐9(NH2),
H PW O
and PTA@ZIF‐9(NH ) at 110 °C after
3
12 40
2
3
0 min, respectively. Clearly, the principal catalyst
FIGURE
condensation of benzaldehyde (1 mmol), ethyl acetoacetate
1.2 mmol), and urea (1.5 mmol) at 110 °C after 30 min
5
Effect of PTA@ZIF‐9(NH
2
)
amount on the
PTA@ZIF‐9(NH ) acquired the best catalytic activity
among the three inspected ingredients. Although, better
2
(
yield (95%) was attained at 130 °C for PTA@ZIF‐
9
(NH ); however, considering the experimental require-
2
catalyst, >30 mg, resulted in diminishing yield to 80% at
the same time. This behavior would be explained consid-
ering the fact that increasing the number of active sites
with enhancing catalyst amount, results decreasing con-
centration of reactants at the active sites, which leads to
decline in yield%. Moreover, masses of catalyst >30 mg
actually led to diffusion limited condition and no
improvement in the yield% was attained.
ments under scale‐up conditions, the lower temperature
10 °C seems to be more suitable and accessible for large
1
scale points of view. Consequently, the reaction tempera-
ture 110 °C was kept for all runs.
3
3
.2.4 | Synthesis of different
,4‐dihydropyrimidin‐2(1H)‐ones
Under the optimized reaction condition, generality of the
method was assessed by using different substituted aro-
matic aldehydes including electron withdrawing/
releasing substituents on the phenyl ring in the presence
3
.2.2 | Catalytic efficacies of the
ingredients
of 10 mg PTA@ ZIF‐9(NH ) (Table 2). In accordance with
past reports, aromatic aldehydes bearing electron releas-
ing groups behaved better than those involving electron
2
Condensation of benzaldehyde, ethyl acetoacetate and
urea obtained only 12% of the desired product in the
absence of catalyst after 30 min; whereas, 30 mg of
H PW O provided 35% yield after the same time
withdrawing ones. According to the reaction pathways
3
12 40
[53]
introduced by several research groups,
the first step
(
Table 1). Interestingly, ZIF‐9(NH ) was effective and
2
3
0 mg of it led to 32% yield after 30 min. The best yield
was obtained with main catalyst PTA@ZIF‐9(NH ) and
2
8
5% yield attained after 30 min. Ionic grafting of HPA
onto ZIF‐9(NH ) increased the dispersion of the catalyti-
2
cally active Brønsted acid sites throughout the network
of ZIF‐9(NH ) and hence the best catalytic activity was
2
observed.
TABLE 1 Catalytic activity of the ingredients in the preparation
of 3,4‐dihydropyrimidinones
Catalyst
Mass (mg) Reaction time (min) Yield%
‐
‐
30
30
30
30
30
12
35
32
85
H
3
PW12
O
40
FIGURE 6 Effect of reaction temperature on the condensation of
benzaldehyde (1 mmol), ethyl acetoacetate (1.2 mmol), and urea
ZIF‐9(NH
2
)
30
(
1.5 mmol) in the presence of 0.03 g of PTA@ZIF‐9(NH
0 min
2
) after
PTA@ZIF‐9(NH
2
) 30
3

TAYEBEE ET AL.
7 of 10
TABLE 2 Three‐component solventless condensation of ethyl
acetoacetate, urea and different aldehydes catalyzed by PTA@ZIF‐
of the mechanism involves formation of an acylimine
intermediate by the reaction of the aromatic aldehyde
with urea. Then, the iminium ion intermediate interacts
with ethyl acetoacetate to produce an open chain ureide
that subsequently cyclizes to the dihydropyrimidinone.
It seems that electron releasing group on the phenyl ring
of aromatic aldehyde, facilitates formation of this inter-
mediate. Therefore, higher yields are obtained with aro-
matic aldehydes bearing electron releasing groups on
phenyl rings.
9
2
(NH )
Entry Aldehyde
product
Yield (%) m.p.(°C)
1
85
201–203
2
48
203–204
3
9
.2.5 | Comparing excellence of PTA@ZIF‐
(NH ) with some reported catalysts
2
3
4
31
64
227–228
208–210
Predominance of the present protocol was compared over
some reported ones by observing the obtained yields
(
Table 3). The three component condensation of urea,
acetoacetate, and benzaldehyde to prepare ethyl‐6‐
methyl‐4‐(phenyl)‐2‐oxo‐1,2,3,4‐tetrahydropyrimidine‐5‐
carboxylate was selected as a model reaction and the
comparison was in terms of mol% of catalysts, reaction
time, and percentage yield. Although, some of the
reported additives led to marginally higher conversions,
however, they required longer reaction times and higher
mol% of catalyst. The present methodology used low
5
6
7
58
49
91
219–221
203–205
209–211
TABLE 3 Comparison of the reactivity of PTA@ ZIF‐9(NH
2
)
with some other catalysts used for the synthesis of ethyl‐6‐methyl‐4‐
(
phenyl)‐2‐oxo‐1,2,3,4‐tetrahydropyrimidine‐5‐carboxylate
Catalyst Time Yield Reaction
Catalyst
amount (h)
(%)
cond.
Ref.
[
57]
Si‐[SbSipim]
0.4 g
3.5
90
Reflux in
EtOH
6
[PF ]
[
58]
Silica/PPA
0.3 g
0.5
85
Reflux in
MeCN
[
[
59]
60]
Yb (PFO)
3
5 mol%
4
8
87
81
S. F., 120 °C
p‐SA/
calixarene
0.5 mol%
Reflux in
EtOH
[
[
38]
61]
HPA/Pip‐SBA‐ 0.6 g
0.3
6
90
72
100 °C
8
55
221–220
15
NH
4
H
2
PO
4
/
0.04 g
S. F., 100 °C
MCM‐41
[
[
62]
54]
Clay/HPA
0.05 mol% 1.5
87
86
S. F., 120 °C
Reaction conditions: aldehyde (1 mmol), ethyl acetoacetate (1.2 mmol), urea
1.5 mmol). Reaction temperature and time 110 °C, 30 min. 30 mg of
PTA@ZIF‐9(NH was used. Yields refer to the isolated 3,4‐
(
[PEG‐DAIL]
[Cl]
0.3 mmol
5
Toluene,
80 °C
2
)
dihydropyrimidinone and probable side products. Selectivity wasn't
considered.
[63]
ZnO@SBA‐15 0.02 g
PTA@ZIF‐ 0.01 g
(NH
2.5
0.5
96
86
EtOH, 65 °C
110 °C
‐
9
2
)
S. F refers to Solvent free.

8
of 10
TAYEBEE ET AL.
catalyst amount under solvent‐free condition and
(1.5 mmol), 0.01 g of PTA@ZIF‐9(NH ) was added and
2
required relatively short reaction time.
the reaction was started for 15 min. At this stage, the
yield of the product was 50%. At that point, hot chloro-
form was added and the catalyst was separated off and
with the filtrate, the reaction was proceed for another
3.2.6 | Reusability of PTA@ZIF‐9(NH2)
1
5 min at 100 °C. Yield% was enhanced to 58%; conse-
The ease of separation and reusability of a heterogeneous
catalyst is an important issue which must be assessed dur-
ing utilization of the catalyst. Consequently, PTA@ZIF‐
quently, no significant increase in yield% was observed.
This observation clearly affirmed the satisfactory steadi-
ness of PTA@ZIF‐9(NH ) in the condensation reaction
and no significant degradation of the catalyst occurred
during the course of the reaction. ICP Analysis of the fil-
trate after separation of the catalyst proved that the
grafted PTA was decreased from 18.8% in the fresh
2
9
(NH ) nanocatalyst was probed for the recoverability
2
and reusability in the Biginelli reaction over five consecu-
tive runs. The condensation reaction was carried out with
ethyl acetoacetate (1.2 mmol), benzaldehyde (1 mmol)
and urea (1.5 mmol) at 110 °C under solvent‐free condition
PTA@ZIF‐9(NH ) to 16.5% in the hot filtered catalyst.
2
in the presence of PTA@ ZIF‐9(NH ) (30 mg) for 30 min.
2
Therefore, 2.3% of the grafted PTA was leached from
ZIF‐9 during hot filtration test. These results were further
asserted by the reproducibility and reusability investiga-
tions of the catalyst as specified previously.
Then, the catalyst was separated from the organic phase
after the reaction and washed with abundant amounts of
ethanol to get rid of any physisorbed reagents, and dried
at room temperature for 6 hr after each run. Then, the
recovered catalyst was reused under same reaction condi-
tion to those of the first run, and it was observed that
3
.4 | Some mechanistic highlights
PTA@ZIF‐9(NH ) nanocatalyst could be recovered easily
2
and reused with a gradual decrease in activity (Figure 7).
Comparison of the infrared spectra of PTA@ZIF‐9(NH2)
before and after condensation reaction (regenerated after
According to the literature, condensation of benzalde-
hyde, ethyl acetoacetate and urea in the presence of
PTA@ZIF‐9(NH ) was assessed under three different
2
5
8
runs) and observation of the characteristic bands around
[
54]
pathes (Figure 8).
For path A, we found that the
−
1
00–1100 cm
clearly confirmed preservation of the
Knoevenagel condensation product of benzaldehyde and
ethyl acetoacetate did not afford the final product in the
presence of urea and in the absence of catalyst; whereas,
25% of dihydropyrimidinone was attained in the presence
of PTA@ ZIF‐9(NH ) ZIF‐9(NH ) after 30 min. This find-
Keggin structure.
3.3 | Hot filtration test
2
2
In order to find weather leaching of PTA occurred during
the condensation reaction, a hot filtration test was
planned. This experiment was accomplished to assess
contribution of the leached active species into the reac-
tion medium. Therefore, to a mixture of benzaldehyde
ing showed that the nanocatalyst revealed some basic
characteristic in the reaction medium. In the case of path
C, no significant amount of the final product was
observed after 3 hr due to the low amount of the pro-
[
55,56]
duced enaminone intermediate.
This study clearly
(1 mmol), ethyl acetoacetate (1.2 mmol) and urea
confirmed the results obtained by Kadam et al on the
FIGURE 7 Yield% as a function of reusability of PTA@ZIF‐
(NH ) catalyst. Reaction temperature 100 °C. 30 mg of PTA@
ZIF‐9(NH ) was used and reaction time was 30 min
9
2
FIGURE 8 Suggested pathways for the synthesis of 3,4‐
dihydropyrimidin‐2‐(1H)‐ones catalyzed by PTA@ ZIF‐9(NH )
2
2

TAYEBEE ET AL.
9 of 10
progress of the three‐component condensation reaction
through formation of the iminium ion (path B); further-
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| CONCLUSIONS
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In conclusion, PTA supported crystalline zeolitic
imidazolate framework PTA@ ZIF‐9(NH ) was success-
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O'keeffe, O. M. Yaghi, Acc. Chem. Res. 2010, 43, 58.
2
fully synthesized and characterized using a variety of dif-
ferent techniques, including FT‐IR, TEM, FESEM and
XRD. Then, this new nanocatalyst was used as an effi-
cient heterogeneous catalyst for the preparation of a
range of 3,4‐dihydropyrimidin‐2‐(1H)‐ones. PTA@ZIF‐
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(NH ) nanocatalyst could be reused several times
2
[
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1
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that PTA@ZIF‐9(NH ) nanocatalyst can be an alternative
2
to other solid catalysts for the Bigineli condensation reac-
2
tion. Interesting properties of PTA@ZIF‐9(NH ) such as
2
[
[
18] W. L. Xin, K. K. Lu, D. Shan, Appl. Surf. Sci. 2019, 481, 313.
non‐toxic nature, easy to separate and recyclable material
offer potential advantages over conventional catalysts,
and would be suitable to the chemical industry.
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Actuators B 2019, 284, 213.
[20] D. Pan, S. Xu, Y. Miao, N. Xu, H. Wang, X. Song, L. Gao, G.
Xiao, Catal. Sci. Technol. 2019, 9, 739.
ACKNOWLEDGEMENTS
[21] C. Song, X. Li, X. Zhu, S. Liu, F. Chen, F. Liu, L. Xu, Appl.
Catal. Gen. 2016, 5, 48.
This work has been supported by the Center for Interna-
tional Scientific Studies & Collaboration (CISSC). Partial
financial/material support of Hakim Sabzevari university
and Sabzevar university of medical sciences are greatly
appreciated.
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ORCID
Reza Tayebee https://orcid.org/0000-0003-1211-1472
Mehdi Baghayeri https://orcid.org/0000-0001-7438-9010
Behrooz Maleki https://orcid.org/0000-0002-3740-7096
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Hartmann, F. J. Keil, Eur. J. Inorg. Chem. 2016, 2016, 4440.
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How to cite this article: Tayebee R, Fattahi
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acid grafted zeolite imidazolate framework as an
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