Strongly proton exchanged montmorillonite K10 (H+-Mont) as a solid acid catalyst for highly efficient and environmental benign synthesis of biscoumarins via tandem Knoevenagel–Michael reaction

Article abstract of DOI:10.1016/j.poly.2019.04.034

In this study, synthesis of micro/meso porous acid-activated montmorillonite K10 (H⁺-Mont) was carried out by the activation of Na⁺-montmorillonite with HCl (4 M) at the controlled conditions. The prepared H⁺-Mont clay was

Full text of DOI:10.1016/j.poly.2019.04.034

Accepted Manuscript
Strongly proton exchanged montmorillonite K10 (H+-Mont) as a solid acid cat-
alyst for highly efficient and environmental benign synthesis of biscoumarins
via tandem Knoevenagel-Michael reaction
Behzad Zeynizadeh, Soleiman Rahmani, Siamand Ilkhanizadeh
PII:
S0277-5387(19)30281-5
https://doi.org/10.1016/j.poly.2019.04.034
POLY 13894
DOI:
Reference:
Polyhedron
To appear in:
Received Date:
Revised Date:
Accepted Date:
15 February 2019
19 April 2019
22 April 2019
Please cite this article as: B. Zeynizadeh, S. Rahmani, S. Ilkhanizadeh, Strongly proton exchanged montmorillonite
K10 (H+-Mont) as a solid acid catalyst for highly efficient and environmental benign synthesis of biscoumarins via
tandem Knoevenagel-Michael reaction, Polyhedron (2019), doi: https://doi.org/10.1016/j.poly.2019.04.034
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Strongly proton exchanged montmorillonite K10 (H+-Mont) as a solid acid catalyst for highly efficient and
environmental benign synthesis of biscoumarins via tandem Knoevenagel-Michael reaction
Behzad Zeynizadeh*, Soleiman Rahmani and Siamand Ilkhanizadeh
Faculty of Chemistry, Urmia University, Urmia 5756151818, Iran
E-mail: bzeynizadeh@gmail.com
Abstract: In this study, synthesis of micro/meso porous acid-activated montmorillonite K10 (H+-Mont) was carried out
by the activation of Na+-montmorillonite with HCl (4 M) at the controlled conditions. The prepared H+-Mont clay was
then characterized using FT-IR, SEM, EDX, XRD and BET analyses. SEM and BET surface analyses represented that by
the acid-activation, adjacent layers of montmorillonite K10 (Mont K10) were exfoliated to tiny segments leading to
raise of surface area and total pore volume in H+-Mont system rather than the alone Mont K10. Catalytic activity of
the prepared H+-Mont clay was further studied as a potent solid acid catalyst for synthesis of biscoumarin materials
via tandem Knoevenagel-Michael reaction of 4-hydroxycoumarin with aromatic aldehydes. All reactions were carried
out in a mixture of H2O-EtOH (1:1) at room temperature to afford the products in high to excellent yields within 25─50
min.
Keywords: Knoevenagel-Michael; Biscoumarin; H+-Mont; 4-Hydroxycoumarin; Montmorillonite K10
1. Introduction
Catalysts owing to their indisputable role in gaining goals of green chemistry have always been a subject of
more interest. Highly atom economy reactions, minimizing hazardous chemicals and eliminating the need
for harsh reaction conditions are some of the precious benefits which are only accessible by designing and
selecting the proper catalysts in chemical processes [1]. In this area, applying solid heterogeneous catalysts
which are inexpensive, non-toxic, easily separable and recoverable from the reaction mixture is a key
approach [2‒5]. In recent years a variety of heterogeneous catalytic systems such as zeolite based catalysts
[6‒9], magnetically recyclable nanocatalysts [10, 11], natural biopolymers [12‒15] and clays [16‒19] have
been successfully used in organic synthesis.
Clays are abundant, readily available and eco-friendly mineral solids that made up from layered silicates.
Clay minerals commonly occur in two basic building blocks including tetrahedral and octahedral sheets. As
well, depending to the degree of octahedral sites occupancy, they are also consisted from dioctahedral or
trioctahedral sheets [16‒19]. Montmorillonite K10 (Mont K10) has a dioctahedral layer structure. Clay
minerals generally has a molecular formula of Mx(Al2-xMgx)(Si4)O10(OH)2·nH2O [20] in which M is
exchangeable ion in the interlayers. Cation-exchange ability in interlayer of montmorillonite makes it a
tunable acid catalyst. Based on this idea, some Brønsted and Lewis acidic catalysts could be prepared by
replacing H+, Ti4+, Cu+2 and various other transition metals in the interlayers [21‒24]. Although,
montmorillonite K10 inherently has acidic properties, however, replacing the cations of interlayers with
proton increases its Brønsted acidity, specific surface area and catalytic activity even more [16‒19].
Montmorillonite K10, because of its eco-friendly nature, low cost, cation-exchange and swelling abilities
has become one of the most intensively focused catalytic materials in the realm of clays and heterogeneous
catalysts [20]. So, it has been frequently used for catalysis of several transformations [25‒29].
Coumarins are one of outstanding family of natural products occurring in various plant species [30‒32]. In
this context, biscoumarins are also well known and have been found some pharmaceutical properties
including anticoagulant [33], antibacterial [34], antifungal [34] and anti-inflammatory [35]. In addition,
some new biscoumarin derivatives exhibited antitumor [36], anti HIV [37], urease inhibitor [38], anti-
microbial [39], radical scavengers and chain-breaking antioxidant activities [40].
In line with the outlined strategies, herein, we wish to report the synthesis of proton exchanged
montmorillonite K10 (H+-Mont) as a potent solid acid catalyst for highly efficient and environmental benign

synthesis of biscoumarin materials via tandem Knoevenagel-Michael reaction of aromatic aldehydes with 4-
hydroxycoumarin in a mixture of EtOH-H2O (1:1) at room temperature (Figure 1).
R
CHO
OH
O
OH
O
OH
O
H+-Mont
H2O-EtOH (1:1)
+
2
O
O
O
R
1
2
3 (a-l)
Figure 1. Synthesis of biscoumarins 3(a-l) catalyzed by H+-Mont clay
2. Experimental
2.1. Materials and methods
All chemicals were purchased from chemical companies with the best quality and they were used without
1
further purification. H, 13C NMR and FT-IR spectra were recorded on 300 MHz Bruker Avance and Thermo
Nicolet Nexus 670 spectrometers. X-ray diffraction (XRD) measurements were carried out on a X'PertPro
Panalytical, Holland diffractometer in 40 kV and 30 mA with a CuKα radiation (λ=1.5418 Å). Signal data
were recorded in 2θ = 10°‒80° with a step interval of 0.05°. Morphology and size distribution of
nanoparticles were examined by scanning electron microscopy (SEM) using FESEM-TESCAN MIRA3
instrument. The chemical composition of the prepared nanostructured clays was determined by energy-
dispersive X-ray (EDX) spectroscopy. The specific surface area and total pore volume of the samples were
measured by Brunauer-Emmett-Teller technique on Belsorp-Max instrument (Japan). The pore size
distributions were derived by using Barrett-Joyner-Halenda method. Melting points were recorded on
Electrothermal 9100 melting point apparatus and uncorrected. Yields refer to isolated pure products. TLC
was applied for monitoring of the reactions over silica gel 60 F254 aluminum sheet. Montmorillonite K10
was purchased from Sigma-Aldrich company with art No. 69866 (pH  3‒4, surface area: 250 m2/g). All
products are known and were characterized by comparison of their physical and spectral data with those of
authentic samples.
2.2. Preparation of homoionic Na+-exchanged montmorillonite (Na+-Mont)
In a beaker (250 mL) containing distilled water (200 mL), montmorillonite K10 (5 g) was added and the
mixture was stirred vigorously at room temperature for 20 h. The mixture was then allowed to settle and
the aqueous phase was decanted. To the obtained solid residue, an aqueous solution of NaCl (2 M, 200 mL)
was added and the mixture was continued to stirring for 2 h at room temperature. The aqueous phase was
decanted and the solid residue was again charged with an aqueous solution of NaCl (2 M, 200 mL). After
stirring for 2 h at room temperature, the aqueous phase was decanted. The procedure was repeated for
additional two times. Finally, the solid residue was washed frequently with distilled water until the
conductivity of the liquid filtrate reaches to the conductivity of distilled water. The solid residue was dried
at 50 °C under air atmosphere to afford homoionic Na+-exchanged montmorillonite (Na+-Mont) [41].
2.3. Preparation of acid-activated montmorillonite [H+-Mont]
To a round-bottom flask (250 mL) containing an aqueous solution of HCl (4M, 100 mL), Na+-Mont (5 g) was
added. The mixture was stirred under reflux conditions for 2 h. After cooling, the aqueous phase was
decanted and the residue was washed frequently with distilled water. When the liquid filtrate was free of

Cl‒ (testing with AgNO3), the solid material on filter paper was collected and dried at 50 °C to afford acid-
activated montmorillonite (H+-Mont) [41].
2.4. A general procedure for synthesis of biscoumarins using H+-Mont clay
In a round-bottom flask (15 mL) containing a mixture of H2O-EtOH (1:1 mL), 4-hydroxycoumarin (2 mmol),
aromatic aldehyde (1 mmol) and H+-Mont (0.02 g) were added. The mixture was stirred magnetically at
room temperature for an appropriate time mentioned in Table 4. After completion of reaction (monitored
by TLC), the clay catalyst was removed from the reaction mixture. The filtrate was evaporated and the solid
material was recrystallized from hot ethanol to obtain the pure biscoumarins 3(a-l) (Table 4, 85-95% yields).
2.5. Determining cation-exchange capacity (CEC) of a clay mineral
Measuring cation-exchange capacity (CEC) of a clay mineral was carried according to the reported
procedure [41]. To do this, a clay mineral (such as Mont K10) (0.5 g) was dispersed in a standard alcoholic
solution of CaCl2 (10 mL, 0.05 M) and the prepared mixture was stirred at room temperature for 24 h. The
suspension was filtered, washed with EtOH (20 mL) and dried at room temperature for 12 h. The Ca2+-
exchanged clay mineral was then transferred to a volumetric flask (250 mL). By adding distilled water, the
volume of flask was reached to standard limit (250 mL). The amount of Ca2+ was titrated with a standard
solution of EDTA. The difference in Ca2+concentration, before and after cation-exchange, affords CEC
(meq·g‒1) amount of a clay mineral.
Spectra data for the prepared biscoumarin materials:
3,3'-(Phenylmethylene)-bis(4-hydroxy-2H-chromen-2-one) (3a)
1
FT-IR (KBr, ʋ cm-1): 3400, 3067, 2737, 1659, 1609, 1564, 1336, 1089, 755; H NMR (300 MHz, CDCl3): δ
(ppm) 6.11 (s, 1H, CH), 7.25-8.07 (m, 13H, ArH), 11.32 (s, 1H, OH), 11.55 (s, 1H, OH); 13C NMR (75 MHz,
CDCl3): δ (ppm) 36.16, 103.88, 105.62, 116.42, 116.65, 124.39, 124.91, 126.89, 128.77, 132.88, 135.17,
152.27, 164.60, 165.02.
3,3'-(4-Chlorophenylmethylene)-bis(4-hydroxy-2H-chromen-2-one) (3b)
FT-IR (KBr, ʋ cm-1): 3428, 3073, 2730, 1664, 1614, 1565, 1494, 1344, 1094, 770; 1H NMR (300 MHz, CDCl3): δ
(ppm) 6.04 (s, 1H, CH), 7.15-8.06 (m, 12H, ArH), 11.32 (s, 1H, OH), 11.55 (s, 1H, OH); 13C NMR (75 MHz,
CDCl3): δ (ppm) 35.7, 103.69, 105.24, 116.34, 124.41, 125.01, 127.97, 128.77, 132.77, 133.04, 133.83,
152.26, 164.53, 166.02.
3,3'-(2-Chlorophenylmethylene)-bis(4-hydroxy-2H-chromen-2-one) (3c)
1
FT-IR (KBr, ʋ cm-1): 3069, 2720, 1649, 1556, 1498, 1444, 1340, 1097, 760; H NMR (300 MHz, CDCl3): δ
(ppm) 6.16 (s, 1H, CH), 7.27-8.04 (m, 12H, ArH), 10.93 (s, 1H, OH), 11.64 (s, 1H, OH); 13C NMR (75 MHz,
CDCl3): δ (ppm) 35.72, 116.59, 124.42, 124.87, 126.76, 128.59, 129.24, 130.81, 132.83, 133.49, 162.33.
3,3'-(2,4-Dichlorophenylmethylene)-bis(4-hydroxy-2H-chromen-2-one) (3d)
FT-IR (KBr, ʋ cm-1): 3068, 2719, 2594, 1649, 1565, 1497, 1097, 759; 1H NMR (300 MHz, CDCl3): δ (ppm) 6.12
(s, 1H, CH), 7.24-8.02 (m, 11H, ArH), 11.67 (s, 2H, OH); 13C NMR (75 MHz, CDCl3): δ (ppm) 35.46, 116.51,
124.48, 124.87, 126.9, 130.33, 132.92, 134.1, 152.01, 164.22, 166.8.
3,3'-(4-Bromophenylmethylene)-bis(4-hydroxy-2H-chromen-2-one) (3e)
FT-IR (KBr, ʋ cm-1): 3428, 3068, 2930, 1663, 1612, 1563, 1490, 1445, 1342, 1315, 1200, 1096, 1022, 767; 1H
NMR (300 MHz, CDCl3): δ (ppm) 6.02 (s, 1H, CH), 7.09-8.06 (m, 12H, ArH), 11.32 (s, 1H, OH), 11.54 (s, 1H,
OH); 13C NMR (75 MHz, CDCl3): δ (ppm) 35.86, 103.64, 105.18, 116.36, 116.66, 120.87, 124.4, 124.97, 128.3,
131.69, 134.4, 152.28, 162.34, 164.63, 166.02.
3,3'-(4-Methoxybenzylidene)-bis(4-hydroxy-2H-chromen-2-one) (3f)

FT-IR (KBr, ʋ cm-1): 3064, 2944, 2842, 2727, 1666, 1612, 1562, 1507, 1447, 1343, 1309, 1094, 768; 1H NMR
(300 MHz, CDCl3): δ (ppm) 3.80 (s, 3H, OMe), 6.05 (s, 1H, CH), 6.84-8.05 (m, 12H, ArH), 11.34 (s, 1H, OH),
11.52 (s, 1H, OH); 13C NMR (75 MHz, CDCl3): δ (ppm) 33.37, 110.93, 116.5, 116.7, 120.4, 123.5, 124.3, 124.7,
128.2, 128.4, 132.44, 152.13, 157.56, 163.7, 165.2.
3,3'-(2-Methoxyphenylmethylene)-bis(4-hydroxy-2H-chromen-2-one) (3g)
FT-IR (KBr, ʋ cm-1): 3412, 3069, 2981, 2724, 1654, 1620, 1550, 1494, 1448, 1337, 1305, 1094, 758; 1H NMR
(300 MHz, CDCl3): δ (ppm) 3.5 (s, 3H, OMe), 6.09 (s, 1H, CH), 6.85-8.04 (m, 12H, ArH), 11.22 (bs, 2H, OH); 13C
NMR (75 MHz, CDCl3): δ (ppm) 35.48, 104.18, 105.74, 113.98, 116.62, 116.89, 124.36, 124.86, 126.90,
127.61, 132.83, 152.25, 152.47, 164.51, 165.69, 169.25.
3,3'-(p-Tolylmethylene)-bis(4-hydroxy-2H-chromen-2-one) (3h)
FT-IR (KBr, ʋ cm-1): 3100, 2992, 2919, 1665, 1614, 1563, 1502, 1440, 1094, 765; 1H NMR (300 MHz, CDCl3): δ
(ppm) 2.34 (s, 3H, CH3), 6.07 (s, 1H, CH), 7.12-8.04 (m, 12H, ArH), 11.34 (s, 1H, OH), 11.48 (s, 1H, OH); 13C
NMR (75 MHz, CDCl3): δ (ppm) 20.97, 35.87, 116.61, 116.93, 124.37, 124.84, 126.36, 129.33, 132.77,
136.46, 152.47, 162.34, 164.5, 165.67.
3,3'-(4-Hydroxyphenylmethylene)-bis(4-hydroxy-2H-chromen-2-one) (3i)
FT-IR (KBr, ʋ cm-1): 3361, 3072, 2731, 1660, 1614, 1565, 1509, 1443, 1096, 762; 1H NMR (300 MHz, CDCl3): δ
(ppm) 6.02 (s, 1H, CH), 6.77-8.02 (m, 12H, ArH), 11.48 (bs, 2H, OH); 13C NMR (75 MHz, CDCl3): δ (ppm)
35.49, 104.16, 115.64, 116.61, 124.37, 126.78, 127.75, 152.41, 154.66, 162.32.
3,3'-(4-Nitrophenylmethylene)-bis(4-hydroxy-2H-chromen-2-one) (3j)
FT-IR (KBr, ʋ cm-1): 3427, 3075, 2935, 1652, 1562, 1343, 1095, 762; 1H NMR (300 MHz, CDCl3): δ (ppm) 6.14
(s, 1H, CH), 7.27-8.17 (m, 12H, ArH), 11.39 (s, 1H, OH), 11.58 (s, 1H, OH); 13C NMR (75 MHz, CDCl3): δ (ppm)
36.18, 103.22, 104.62, 116.26, 116.73, 121.76, 122.12, 124.5, 125.17, 129.59, 132.7, 133.33, 137.9, 148.75,
152.35, 162.34, 166.59, 166.9.
3,3'-(2-Nitrophenyl)methylene)-bis(4-hydroxy-2H-chromen-2-one) (3k)
1
FT-IR (KBr, ʋ cm-1): 3433, 3076, 2722, 1652, 1618, 1561, 1532, 1450, 1352, 1309, 1099, 761; H NMR (300
MHz, CDCl3): δ (ppm) 6.64 (s, 1H, CH), 7.27-8.01 (m, 12H, ArH), 11.55 (bs, 2H, OH); 13C NMR (75 MHz, CDCl3):
δ (ppm) 33.86, 103.71, 116.66, 124.57, 125.03, 128.19, 129.49, 131.9, 133.2, 149.76.
3,3'-(4-Pyridylmethylene)-bis(4-hydroxy-2H-chromen-2-one) (3l)
FT-IR (KBr, ʋ cm-1): 3447, 3075, 2880, 1663, 1618, 1543, 1500, 1407, 1102, 757; 1H NMR (300 MHz, DMSO-
d6): δ (ppm) 6.47 (s, 1H, CH), 7.21-8.68 (m, 12H, ArH and pyridy-H). It is notable that, due deuterium
exchange, hydrogen of two OH groups at region of δ = 11.5 ppm was not observed. 13C NMR (75 MHz,
DMSO-d6): δ (ppm) 38.26, 101.88, 116.20, 119.82, 123.65, 124.67, 125.67, 132.04, 141.35, 153.15, 164.53,
165.34, 168.59.
3. Results and discussion
3.1 Synthesis and characterizations of micro/meso porous acid-activated montmorillonite K10 (H+-Mont)
The study was primarily started by the preparation of H+-Mont clay nanocatalyst in a three-step procedure
(Figure 2): i) preparation of swelled (hydrated) montmorillonite via vigorous stirring of montmorillonite K10
in distilled water. In similar to other clay minerals, montmorillonite K10 can absorb water in its interlayer
spaces. Through the water adsorption, the layers move apart from together giving the swelled
montmorillonite. The swelling expands the surface area of montmorillonite and exposes the cations of
interlayers to contribute in Brønsted and Lewis acid properties of the clay mineral [20], ii) preparation of
homoionic Na+-exchanged montmorillonite by an adequate stirring of the swelled montmorillonite in an

aqueous solution of NaCl. In this sate and through the ion-exchange ability, many cations of interlayers in
montmorillonite are exchanged with Na+ ions. Diffusion of Na+ ions brings high capacity towards the clay
mineral to exchange Na+ ions with various guest species (transition metals ions/nanoparticles, complexes
and H+ ions) and this is leading to the encapsulation of guest species in interlamellar spaces of the clay
mineral. Moreover, by altering the cations of interlayers, the acidity (Brønsted and Lewis) of the clay
mineral could be changed for different purposes, and iii) preparation of acid-activated montmorillonite by
stirring of homoionic Na+-exchanged montmorillonite in an aqueous solution of HCl.
The concentration of exchangeable cations in a clay mineral is called cation-exchange capacity (CEC) and
measured as milli-equivalent per 100 g of the dried clay [19]. The measuring resulted that CEC for Mont K10
is 85 meq·g–1 and for Na+-Mont is 110 meq·g–1. This is showing that by replacing Na+ ions instead of other
cations in interlamellar spaces, the cation exchange capacity of H+-Mont system was raised.
OH
OH OH
OH OH
O
OH
OH OH
O
OH
OH
O
O
O
O
i) Swelling
ii) NaCl
Na+ ions
OH
OH
OH
OH
OH
OH
OH
OH
O
O
O
O
exchangeable cationic
Na+, K+, Ca2+, Mg2+
and water
Na+-Mont clay
Swelled Montmoriilonite
iii) acid-activation
H+ ions
HO
HO
O
HO
OH
OH
O
O
OH OH
OH
OH
O
O
OH
O
OH
OH
O
O
H+-Mont clay
Figure 2. A three-step procedure for preparation of H+-Mont clay
3.2. FT-IR analysis
Fourier transform infrared spectroscopy as a primarily tool was utilized for structural elucidation of
montmorillonite K10 and acid-activated montmorillonite (Figure 3). The illustrated FT-IR spectrum of Mont
K10 shows the absorption bands at 3620 and 1635 cm–1 due stretching and bending vibrations of OH groups
bonded to Si, Al and other existed metals. A broad band near 3430 cm–1 is attributed to H–O–H stretching
vibrations of adsorbed water on the surface of montmorillonite, a strong absorption band at 1054 cm–1
derived from stretching vibration of Si–O bonds in tetrahedral sites, a small absorption band at 795 cm–1
attributed to the presence of amorphous silica, and the bands at 522 and 464 cm–1 due bending vibration of
Si–O–Al and Si–O–Si bonds. It is notable that although FT-IR spectrum of H+-Mont clay shows the same
absorption-pattern of montmorillonite K10, however, through the acid-activation, the position of wide
stretching vibration band of Si–O (1054 cm–1) in Mont K10 was shifted to 1080 cm–1 showing the successful
ion-exchange processing of H+-Mont system.

Figure 3. FT-IR spectra of Mont K10 and H+-Mont clay
3.3. SEM and EDX analysis
SEM images and EDX spectra of Mont K10 and acid-activated Mont were illustrated in Figure 4. Comparison
of the images (Figure 4a and c) shows that through the acid-activation, the adjacent sheets of Mont K10 are
exfoliated to tiny segments. This phenomenon is interpreted by major elimination of Al3+ and Fe2+/3+
contents from octahedral sites while the activation of montmorillonite with HCl was carried out. As well,
through the elimination of Al3+ and Fe2+/3+ ions from interlayers, many new pores on the surface and
internal layers of montmorillonite are produced leading to raise of surface area in H+-Mont system [42]. It
was also reported that the rate of metal ions-elimination in a clay mineral depends to the concentration of
HCl, temperature and processing time of the activation [43]. The acid-activation at low HCl-concentration
seems to cause little damage to the silicate layer affording the platelet-structure of clay-Mont system
remains intact. However, the prolonged processing time in acid-activation may result complete
delamination leading to the collapse of layers in the clay mineral [44].

Figure 4. a) SEM image of Mont K10, b) EDX spectrum of Mont 10, c) SEM image of H+-Mont and d) EDX
spectrum of H+-Mont
3.4. XRD analysis
More investigation towards phase purity and structural elucidation of the clay composite systems was
carried out by plotting the broad angel XRD spectrum of the prepared samples (Figure 5). As seen in Figure
5a, montmorillonite K10 possesses a number of sharp peaks corresponding to various impurities (quartz,
cristobalite and feldspar) along with hkl and two dimensional hk reflections. Figure 5b also represents that
the intensity of absorption peaks in Na+-exchanged montmorillonite was significantly increased. This

situation shows that via the ion-exchange ability, Na+ ions were successful diffused to interlamellar spaces
of the clay mineral leading to dramatic raise of Na+-occupation [45, 46]. In contrast, Figure 5c shows that
the intensity of absorption peaks in H+-Mont clay system was notably reduced. This phenomenon is
attributed to exfoliation/destruction of layers in Na+-Mont system through the acid-activation of Mont K10
with HCl [47].
Figure 5. Broad angle XRD patterns of a) Mont K10, b) Na+-Mont and c) H+-Mont clays
3.5. BET analysis
In order to investigate the porosity and specific surface area of the clay systems, N2 adsorption‒desorption
isotherm of Mont K10 and H+-Mont clays were illustrated in Figure 6. The plot-analysis shows that the
shape of isotherms for the examined clay minerals is similar and according to BDDT classification is adapted
to the isotherm of type IV with H3 hysteresis loop. This type of isotherm is a characteristic of micro/meso
porous materials [48, 49]. As well, the obtained data for Mont K10 and H+-Mont clays represented that the
BET surface area and total pore volume of H+-Mont system was increased rather than the alone Mont K10
(Table 1). This is attributed to exfoliation/destruction of adjacent layers in Mont K10 to tiny segments by
the acid-activation. In addition, via the processing of montmorillonite K10 with hydrochloric acid and
elimination of inherent consisted metal ions of octahedral sites, more new pores are generated affording
the rise of pore volume and specific surface area in H+-Mont system [50, 51].

Figure 6. N2 adsorption–desorption isotherms of a) Mont K10 and b) H+-Mont clays
Table 1. Surface properties of Mont K10 and H+-Mont clays
SBET
Vm
Average pore
diameter (nm)
Vp
Total pore
Samples
(m2 g‒1)
(cm3g‒1)
(cm3g‒1)
volume (cm3g‒1)
Mont K10
H+-Mont
250.11
280.52
51.835
50.313
6.427
6.7483
0.4016
0.419
0.4298
0.5081
3.6. Synthesis of biscoumarins catalyzed by H+-Mont system
The literature review shows that synthesis of biscoumarin materials due their wide spread biological
activities has attracted the attention of many scientists. In this context, phosphotungstic acid [52], I2 [53],
ZnCl2 [54], Zn(Proline)2 [55], sodium dodecyl sulfate (SDS) [56], n-dodecylbenzene sulfonic acid (DBSA) [57],
n-Bu4NBr [58], [Et3NH][HSO4] [59], sulfated titania [60], propane-1,2,3-triyl tris(hydrogen sulfate) [61],
sulfonated rice husk ash (RHA-SO3H) [62, 63], P4VPy-CuO NPs [64], silica-supported Preyssler NPs [65], silica
sulphuric acid NPs [66], silica-bonded N-propylpiperazine sodium n-propionate [67], silica supported
sodium hydrogen sulfate and indion 190 resin [68], mesoporous FeTUD-1 [69], nano Fe3O4@ZrO2-H3PO4
[70], graphene oxide nanosheets [71], tetramethylguanidium-based ionic liquids [72], task-specific ionic
liquids [73], [MIM(CH2)4SO3H][HSO4] [74] and SO3H-functionalized ionic liquids based on benzimidazolium
cation [PSebim][OTf] [75] are some of the catalyst systems which have been successfully served for
synthesis of biscoumarins. Although, the mentioned protocols have the own merits or shortcomings,
however, introduction of new, efficient and ecologically safe methods is a subject of more interest and is
still demanded.
After the successful synthesis and characterization of acid-activated montmorillonite, we therefore
prompted to investigate the catalytic activity of the prepared H+-Mont clay system towards tandem
Knoevenagel-Michael reaction of aromatic aldehydes with 4-hydroxycoumarin under green and more
efficient conditions.
The investigation was primarily carried out by performing the condensation reaction of 4-hydroxycoumarin
(2 mmol) and 4-chlorobenzaldehyde (1 mmol) in the presence and absence of H+-Mont clay under different
reaction conditions. The results of this investigation are summarized in Table 2. Observation of the results
represented that in the absence of H+-Mont clay, the reaction did not show any efficiency (entries 1‒3).
However, by adding a small amount of H+-Mont nanocatalyst, the rate of condensation reaction was
dramatically accelerated. More examinations resulted that using 20 mg H+-Mont clay per 1 mmol of 4-
chlorobenzaldehyde in a mixture of EtOH-H2O (1:1) at room temperature was the requirements to afford

biscoumarin 3b in 95% isolated yield (Table 2, entry 5), while the reaction was carried out with 100%
conversion.
Table 2. Optimization experiments for synthesis of biscoumarin 3b catalyzed by H+-Mont claya
Entry
H+-Mont (mg)
Solvent (2 mL)
H2O
EtOH-H2O (1:1)
EtOH-H2O (1:1)
H2O
EtOH-H2O (1:1)
EtOH-H2O (1:1)
EtOH-H2O (1:1)
CH3CN
Condition
r.t.
r.t.
reflux
r.t.
Time (min)
360
360
120
120
25
Yield (%)b
1
2
3
4
5
6
7
8
9
‒
‒
‒
20
20
30
10
20
20
10
10
20
65
95
90
70
34
20
r.t.
r.t.
25
r.t.
25
r.t.
r.t.
180
180
CH2Cl2
a The condensation reactions were carried out with 4-hydroxycoumarin (2 mmol) and 4-chlorobenzaldehyde (1 mmol).
b Isolated yield.
In continuation, the capability of nano/meso porous H+-Mont clay was highlighted by a comparison for
synthesis of biscoumarin 3b catalyzed by H+-Mont system versus the parent montmorillonite K10 and other
metal ions (Fe3+, Al3+ and Na+) exchanged montmorillonites at the optimized reaction conditions (Figure 7
and Table 3). The obtained results represented that the condensation reaction in the presence of Mont K10
and other metal ions exchanged Monts was only progressed up to 75% within 120 min. This is showing that
although Mont K10 is naturally acidic clay, however, it exhibited less reactivity towards synthesis of
biscoumarin 3b. In contrast, replacing of protons instead of other metal ions in interlayers of Mont K10
strongly enhances the Brønsted acidity of H+-Mont system leading to the perfect efficiency of the titled
reaction [76]. Moreover, generation of numerous nano/meso pores on the surface as well as distancing of
interlayers in the clay mineral (this is led to dramatic raise of specific surface area) are the reasons which
H+-Mont clay performs the condensation reaction more easily and with the perfect efficiently.
Cl
H+-Mont, 20 mg, r.t.
H2O-EtOH, 25 min, 95%
Cl
OH
O
OH
O
OH
O
+
2
O
O
O
Mont K10, 20 mg, r.t.
CHO
H2O-EtOH, 120 min, 45%
1
2
3b
Figure 7. Comparison of the catalytic activity of Mont K10 and H+-Mont clays in synthesis of biscoumarin 3b
Table 3. Synthesis of biscoumarin 3b catalyzed by Mont claysa
Entry
Catalyst
Mont K10
Al3+-Mont
Na+-Mont
Fe3+-Mont
H+-Mont
Time (min)
120
Yield (%)b
1
2
3
5
4
45
60
50
75
95
120
120
120
25
a The condensation reactions were carried out using 4-hydroxycoumarin (2 mmol), 4-chlorobenzaldehyde (1 mmol) and 20
mg of the clay catalyst in a mixture of EtOH-H2O (1:1 mL). b Isolated yield.
At the next, generality and versatility of this synthetic method was more investigated by synthesis of
structurally diverse biscoumarin materials through the condensation reaction of 4-hydroxycoumarin and
substituted aromatic aldehydes at the optimized reaction conditions. The results of this investigation are

summarized in Table 4. As seen, all reactions were carried out successfully within 25‒50 min at room
temperature to afford biscoumarins in high to excellent yields.
Table 4. One-pot synthesis of biscoumarins catalyzed by H+-Mont claya
R
CHO
OH
O
OH
O
OH
H+-Mont (20 mg), r.t.
EtOH-H2O (1:1)
2
+
O
O
O
O
R
M.p.
Entry
Ar
Ph
4-ClC6H4
2-ClC6H4
(2,4-Cl2)C6H3
4-BrC6H4
4-MeOC6H4
2-MeOC6H4
4-MeC6H4
4-OHC6H4
4-NO2C6H4
2-NO2C6H4
4-Pyridyl
Product
Time (min)
Yield (%)b
Found
Reported
1
2
3
4
5
6
7
8
9
3a
3b
3c
3d
3e
3f
3g
3h
3i
25
25
40
40
20
35
35
30
40
30
50
30
93
95
87
92
94
90
90
92
85
90
90
88
230‒232
256‒257
203‒205
203‒206
267‒268
247‒249
235‒238
268‒269
222‒224
232‒234
202‒204
253‒255
228‒230 [55]
257‒259 [63]
205‒207 [63]
200‒202 [52]
269‒271 [63]
249‒251 [63]
236‒237 [38]
269‒270 [74]
222‒225 [55]
233‒235 [63]
203‒204 [60]
252‒254 [55]
10
11
12
3j
3k
3l
a All reactions were carried out with the molar ratio of 1:2 (aromatic aldehyde/4-hydroxycoumarin) in the presence of 20
mg of H+-Mont clay at room temperature. b Isolated yield.
Although the exact mechanism of this synthetic protocol is not clear, however, a proposed mechanism for
synthesis of biscoumarin materials catalyzed by H+-Mont clay involves: i) the activation of carbonyl group of
aromatic aldehyde with H+-Mont clay as a Brønsted acid-catalyst, ii) the Knoevenagel condensation of 4-
hydroxycoumarin with the activated aldehyde affords intermediate I, iii) the Michael addition of second
molecule of 4-hydroxycoumarin with intermediate I produces intermediate II, and iv) a simple enolization
of intermediate II gives the final biscoumarin product (Figure 8).

Ar
O
O
O
O
O
H+
H+
H+
H+
H+
H+
H+
OH OH
H
H+
H+
Ar
H+
H+-Mont
H+
H+
H+
O
O
H+
H
H+
H+
H+
H+
H+
H+
H+
+ H+
H
O
H+
H
H+
(II)
Ar
H+
O
HO
H+
Ar
H+
H+
O
H+
O
H2O
HO
(I)
H2O
H+
O
H+
H+
O
H+
H+
H+
H+
H+
OH
O
H+
H+
O
H
O
Ar
O
O
Figure 8. A proposed mechanism for synthesis of biscoumarins catalyzed by H+-Mont clay
The usefulness and capability of H+-Mont mesoporous clay in one-pot synthesis of biscoumarin 3b was
highlighted by a comparison of the obtained results with the previously reported catalyst systems (Table 5).
A survey shows that in terms of low reaction time, mild reaction conditions, perfect reusability of H+-Mont
clay and high yield of the product, the present work exhibits more or comparable efficiency than the
previous systems.
Table 5. Comparison of the synthesis of biscoumarin 3b with H+-Mont clay and other reported catalyst systems
Time
(min)
25
Yield
(%)
95
Entry
Catalyst
Condition
Reusability
Ref.
1
2
H+-Mont mesoporous clay
Phosphotungstic acid
H2O/r.t.
6
5
*
20
27
93
93
93
87
95
96
92
92
90
97
80
96
H2O/80 °C
[52]
3
4
Molecular iodine
H2O/100 °C
H2O/60 °C
‒
5
‒
‒
5
‒
‒
3
4
4
2
[53]
[56]
[57]
[58]
[60]
[63]
[66]
[68]
[70]
[71]
[75]
Sodium dodecyl sulfate
n-Dodecylbenzene sulfonic acid
n-Bu4NBr
150
60
5
H2O/40 °C
6
30
H2O/100 °C
H2O/80 °C
7
Sulfated titania
15
8
RHA-SO3H
25
Solvent-free/80 °C
H2O/reflux
9
Silica sulfuric acid NPs
NaHSO4/SiO2
20
10
11
12
13
30
Toluene/100 C
H2O/reflux
nano Fe3O4@ZrO2-H3PO4
Graphene oxide nanosheets
[PSebim][OTf] as IL (ionic liquid)
12
15
H2O/reflux
120
IL/70 °C
* Present work

3.7. Reusability of H+-Mont clay
For industrial and practical purposes, recyclability of the applied catalyst is a subject of more interest. This
issue was investigated through the synthesis of biscoumarin 3b in the presence of H+-Mont clay as a model
reaction. After completion of the reaction at the first run, the clay nanocatalyst was separated, washed
with ethanol and then dried for reusing at the next runs. The model reaction was again charged with the
fresh solvent, starting materials and the recovered H+-Mont clay. Figure 9 shows that the H+-Mont
nanocatalyst can be reused for six consecutive cycles without the significant loss of catalytic activity.
100
98
96
94
92
90
1
2
3
4
5
6
Run
Figure 9. Reusability of H+-Mont clay for synthesis of biscoumarin 3b
Conclusion
In this study, the mesoporous acid-activated montmorillonite (H+-Mont) was easily prepared and then
characterized using FT-IR, SEM, EDX, XRD and BET analyses. The prepared Brønsted acid nanocatalyst
showed a perfect catalytic activity towards tandem Knoevenagel-Michael reaction of various aromatic
aldehydes with 4-hydroxycoumarin. All reactions were carried out at room temperature in a mixture EtOH-
H2O (1:1) to afford biscoumarin materials within 20–50 min. H+-Mont as a heterogeneous clay catalyst was
easily separated from the reaction mixture and reused for 6 consecutive cycles without the significant loss
of catalytic activity. This synthetic protocol offers numerous advantages in terms of easy synthesis of
nano/mesoporous H+-Mont clay system, mild reaction conditions, using a mixture of H2O-EtOH as green
solvent, short reaction times, high yield of the products, simple work-up procedure and the great
reusability of the nanocatalyst.
Acknowledgement
We gratefully acknowledge the financial support of this work by Research Councils of Urmia University.
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Strongly proton exchanged montmorillonite K10 (H+-Mont) as a solid acid catalyst for highly efficient and
environmental benign synthesis of biscoumarins via tandem Knoevenagel-Michael reaction
Behzad Zeynizadeh*, Soleiman Rahmani and Siamand Ilkhanizadeh
Faculty of Chemistry, Urmia University, Urmia 5756151818, Iran
E-mail: bzeynizadeh@gmail.com
In this study, the mesoporous acid-activated montmorillonite (H+-Mont) was easily prepared and then
characterized using FT-IR, SEM, EDX, XRD and BET analyses. The prepared Brønsted acid nanocatalyst
showed a perfect catalytic activity towards tandem Knoevenagel-Michael reaction of various aromatic
aldehydes with 4-hydroxycoumarin. All reactions were carried out at room temperature in a mixture EtOH-
H2O (1:1) to afford biscoumarin materials within 20–50 min.
HO
HO
O
HO
OH
O
OH OH
OH
OH
OH
R
O
O
O
OH
R
O
OH
OH
O
O
O
O
O
O
CHO
H+-Mont Mesoporous Clay
H2O-EtOH (1:1), r.t., 25-50 min
OH
O
OH
O
OH OH
O
O
85-95 %