Sulfonated PEG-intercalated montmorillonite [(Mt/PEG)-SO3H] as efficient and ecofriendly nanocatalyst for synthesis of α,α′-bis(substituted benzylidene)cycloalkanones

Article abstract of DOI:10.1007/s11164-017-2983-7

(Montmorillonite/PEG)-SO₃H nanocomposite was successfully prepared for the first time and introduced as a solid acid nanocatalyst. Initially, polyethylene glycol (PEG) polymeric chains were intercalated into interlayer spaces of montmorillonite. The resulting Mt/PEG nanocomposite with good mechanical and thermal stability was chosen as a useful clay mineral/polymer support for further modification with chlorosulfonic acid. Structural characterization of (Mt/PEG)-SO₃H was carried out using X-ray diffraction (XRD) analysis, Brunauer–Emmett–Teller (BET) measurements, Barrett–Joyner–Halenda (BJH) analysis, scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and Fourier-transform infrared (FT-IR) spectroscopy. The results showed that PEG chains were intercalated into the clay mineral layers and that the Mt/PEG nanocomposite was successfully sulfonated. (Mt/PEG)-SO₃H nanocomposite exhibited high specific surface area and good stability up to around 150?°C, showing excellent potential for application as a recyclable nanocatalyst. (Mt/PEG)-SO₃H was used as an efficient and ecofriendly solid acid nanocatalyst for preparation of α,α′-bis(substituted benzylidene)cycloalkanones under solvent-free conditions, leading to many interesting findings. The excellent conversion values confirm that the catalyst has strong and sufficient acidic sites, which are responsible for its catalytic performance. The reaction under mild conditions (room temperature) with excellent yield, catalyst recyclability (up to ten times), and simple work-up procedure represent useful advantages of (Mt/PEG)-SO₃H for catalysis. Moreover, the reaction could be scaled up to 10 and 15?mmol scales.

Full text of DOI:10.1007/s11164-017-2983-7

Res Chem Intermed
DOI 10.1007/s11164-017-2983-7
Sulfonated PEG-intercalated montmorillonite [(Mt/
PEG)-SO3H] as efficient and ecofriendly nanocatalyst
for synthesis of a,a0-bis(substituted benzylidene)
cycloalkanones
Seyedeh Zahra Dalil Heirati1 Farhad Shirini1
•
•
Abdollah Fallah Shojaei1
Received: 9 February 2017 / Accepted: 8 May 2017
Ó Springer Science+Business Media Dordrecht 2017
Abstract (Montmorillonite/PEG)-SO3H nanocomposite was successfully prepared
for the first time and introduced as a solid acid nanocatalyst. Initially, polyethylene
glycol (PEG) polymeric chains were intercalated into interlayer spaces of mont-
morillonite. The resulting Mt/PEG nanocomposite with good mechanical and
thermal stability was chosen as a useful clay mineral/polymer support for further
modification with chlorosulfonic acid. Structural characterization of (Mt/PEG)-
SO3H was carried out using X-ray diffraction (XRD) analysis, Brunauer–Emmett–
Teller (BET) measurements, Barrett–Joyner–Halenda (BJH) analysis, scanning
electron microscopy (SEM), thermogravimetric analysis (TGA), and Fourier-
transform infrared (FT-IR) spectroscopy. The results showed that PEG chains were
intercalated into the clay mineral layers and that the Mt/PEG nanocomposite was
successfully sulfonated. (Mt/PEG)-SO3H nanocomposite exhibited high specific
surface area and good stability up to around 150 °C, showing excellent potential for
application as a recyclable nanocatalyst. (Mt/PEG)-SO3H was used as an efficient
and ecofriendly solid acid nanocatalyst for preparation of a,a0-bis(substituted ben-
zylidene)cycloalkanones under solvent-free conditions, leading to many interesting
findings. The excellent conversion values confirm that the catalyst has strong and
sufficient acidic sites, which are responsible for its catalytic performance. The
reaction under mild conditions (room temperature) with excellent yield, catalyst
recyclability (up to ten times), and simple work-up procedure represent useful
advantages of (Mt/PEG)-SO3H for catalysis. Moreover, the reaction could be scaled
up to 10 and 15 mmol scales.
&
&
1
Farhad Shirini
shirini@guilan.ac.ir
Abdollah Fallah Shojaei
a-fallah@guilan.ac.ir
Department of Chemistry, College of Science, University of Guilan, Rasht 41335, Islamic
Republic of Iran
123

S. Z. Dalil Heirati et al.
Keywords Poly(ethylene glycol) Á Montmorillonite Á Clay mineral/polymer
nanocomposite Á Nanocatalyst Á a,a0-Bis(substituted benzylidene)cycloalkanones
Introduction
Nowadays, green chemistry principles encourage scientists to use solid acids as
highly efficient, easily recyclable, ecocompatible catalysts for organic reactions,
representing green alternatives to toxic and unrecyclable homogeneous acid
catalysts [1, 2]. Hence, one of the latest challenges to all organic chemists is to
design and develop new solid acid catalysts with higher efficiency in lower amounts
as well as good recyclability for easy separation from reaction media [3, 4]. The
efficiency of a catalyst mainly depends on the thin surface layer between the catalyst
and reactants. To obtain high efficiency, it is necessary to have a large and easily
accessible specific surface area. This can be easily achieved using nanoscale
catalysts. In fact, in a nanocatalyst, the nanoparticle size leads to very high surface-
to-volume ratio that results in increased catalytic activity even in lower amounts
[5, 6]. On the other hand, when designing and building a solid acid catalyst, its
recyclability is an important factor that must be considered. Catalysts with good
recyclability can be easily recovered from the reaction medium for reuse in
subsequent cycles. Recyclability is closely dependent on the mechanical and
thermal stability of a catalyst, which can be enhanced by supporting a homogeneous
catalyst on more stable materials such as clays [7]. On this basis, clay minerals such
as montmorillonite (Mt) having nanostructure have been extensively used as
supports for preparation of many highly efficient and recyclable catalysts [8–11].
Additionally, these clay minerals have been used for preparation of an interesting
class of hybrid materials named clay mineral/polymer nanocomposites (CPN) [12].
Because these nanocomposites offer the advantages of both high specific surface
area and good thermal stability, such materials have been used as catalysts for a
variety of biologically important organic transformations [13–15]. The successes
obtained in catalytic applications of these materials provide an interesting area of
research for the design of new solid acid catalysts using other types of clay mineral/
polymer nanocomposites. In this regard, montmorillonite/poly(ethylene glycol) (Mt/
PEG) hybrids having nanoscale structure and good thermal stability have been
confirmed to be useful for such applications in many studies [16–19]; catalysts
prepared using this material show high efficiency due to the nanoscale structure as
well as easy recyclability due to the presence of stable clay mineral in their
structure.
Bearing the above-mentioned features in mind, Mt/PEG nanocomposite was
chosen as support in this work, and subjected to sulfonation using chlorosulfonic
acid to obtain (Mt/PEG)-SO3H nanocomposite. Its activity was then examined as a
new and inexpensive solid acid nanocatalyst for synthesis of structurally diverse
a,a0-bis(substituted benzylidene)cycloalkanones under solvent-free condition.
a,a0-Bis(substituted benzylidene)cycloalkanones are known to have interesting
biological activities such as human immunodeficiency virus (HIV)-1 integrase
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Sulfonated PEG-intercalated montmorillonite [(Mt/PEG)-SO3H]…
inhibition [20], arginine methyltransferase inhibition [21], and cytotoxicity [22, 23]
as well as antiinflammatory [24], antimutagenic, antitumor [25], antimicrobial [26],
and antioxidant effects [27]. Moreover, they can be used as useful precursors for
synthesis of potentially bioactive pyrimidine derivatives [28], organic materials for
nonlinear optical applications [29], liquid-crystalline polymers [30], perfumes [31],
and materials for use in photolithography [32]. Thus, interest in synthesis of a,a0-
bis(substituted benzylidene)cycloalkanones from organic and medicinal chemists is
increasing. Generally, cross-aldol condensation of cyclic ketones with aromatic
aldehydes is the synthetic approach used to obtain these compounds, catalyzed by a
strong base or an acid. However, this method suffers from side-reactions such as
self-condensation of ketones and aldehydes, resulting in low yield of the
corresponding product [33]. A variety of methodologies have been reported in
literature as modifications to this synthetic approach, using catalysts such as
NH4H2PO4/SiO2 [34], [2aemim][BF4] [35], PEG-OSO3H [36], nanotitania-sup-
ported sulfonic acid (n-TSA) [37], [N2222][EtNHC3SO3] [38], nano-TiO2/HOAc
[39], FeCl3Á6H2O [40], molecular iodine [41], AlCl3 [42], and solid NaOH [43].
Although most of these catalytic methodologies offer their own advantages,
separation and recycling usually remain rather difficult. Also, use of excess amount
of catalyst, long reaction time, unsatisfactory product yield, catalyst leaching, and
use of toxic organic solvent are other possible disadvantages related to these
methods, limiting their application on industrial scale. To overcome such
drawbacks, development of more efficient and greener catalysts for promotion of
this reaction is still in demand.
Experimental
Materials and methods
All chemicals and materials were obtained from Fluka, Merck, and Aldrich
chemical companies. The montmorillonite sample was obtained from Southern Clay
Products Inc. (Gonzales, USA). All yields refer to isolated pure products. Products
were characterized by their physical constants, comparison with authentic samples,
and IR and nuclear magnetic resonance (NMR) spectroscopy. Substrate purity and
reaction monitoring were performed using thin-layer chromatography (TLC) on
silica gel SIL G/UV 254 plates. FT-IR spectra were measured using a VERTEX 70
spectrophotometer (Bruker, Germany) from KBr salt disks, and frequencies are
expressed as wavenumbers in units of cm-1. Samples were dried at 100 °C in an
oven before recording the spectrum. 1H NMR (400 MHz) and 13C NMR (100 MHz)
spectra were obtained using a Bruker AVANCE-400 spectrometer in dimethylsul-
foxide (DMSO) using tetramethylsilane (TMS) as internal reference (d in ppm).
Thermogravimetric analysis (TGA) was performed under nitrogen gas using STA
PT-1000 apparatus. Experiments were run between 25 and 600 °C at heating rate of
10 °C min-1. Powder XRD patterns of samples were recorded using a Philips PW
˚
1840 diffractometer with Cu Ka radiation (a = 1.54 A). The patterns were collected
in the 2h angle range from 0° to 10° in continuous scan mode. BET surface area was
123

S. Z. Dalil Heirati et al.
measured using a BELSORP-mini apparatus (BEL Japan, Inc.). Prior to N2-physical
adsorption measurements, the samples were degassed at 105 °C for 120 min in
nitrogen atmosphere. The specific surface area (SBET) of as-prepared catalyst was
determined from N2 adsorption–desorption isotherms at 77 K. Scanning electron
microscopy (SEM) images of the samples were obtained on a LEO 1430VP
microscope. Prior to SEM analysis, all samples were coated with a thin gold layer to
make the surface conductive. Melting points were recorded on a Bu¨chi B-545
apparatus in open capillary tubes.
Preparation of PEG-intercalated montmorillonite [Mt/PEG]
PEG (Mn 6000, 1.0 g) was dispersed in ethanol (25 mL) and stirred in an oil bath
(60 °C) for 30 min. Then sodium montmorillonite (1.0 g) was slowly added, and the
reaction mixture was stirred for 3 h at the mentioned temperature. After reaction
completion, the mixture was cooled to room temperature, filtered and washed with
ethanol to remove excess poly(ethylene glycol). The result was air-dried at room
temperature for 24 h to obtain montmorillonite/PEG hybrid (Mt/PEG, 1.4 g).
Sulfonation of Mt/PEG nanocomposite [(Mt/PEG)-SO3H]
Mt/PEG nanocomposite was sulfonated according to Zolfigol’s method [44]: At
0 °C, chlorosulfonic acid (0.6 mL, 9 mmol) was added dropwise to solution of Mt/
PEG nanocomposite (1.0 g) in dry dichloromethane (10 mL), and the resulting
solution was stirred for 5 h at room temperature then filtered. The filtrate was
washed with ethanol (20 mL) and air-dried at room temperature for 24 h to afford
(Mt/PEG)-SO3H as pale-yellow powder (1.5 g).
Determination of acidity value
The acidity value of the prepared nanocomposite was estimated by backtitration
using HCl (0.337 N) [13, 14]: 10 mL of NaOH (0.130 N) was added to 0.05 g
nanocomposite and stirred for 30 min. The nanocomposite was separated and
washed with deionized water. The excess amount of NaOH was titrated with HCl
(0.337 N) in presence of phenolphthalein as indicator. The acidity value was
estimated to be 5.25 mmol g-1 for (Mt/PEG)-SO3H nanocomposite.
General procedure for synthesis of a,a0-bis(substituted benzylidene)
cycloalkanones
Mixture of aldehyde (2 mmol), cycloalkanone (1 mmol), and (Mt/PEG)-SO3H
(15 mg) was stirred at room temperature for an appropriate time. After
completion of the reaction as monitored by thin-layer chromatography (TLC)
with n-hexane:ethyl acetate (3:1), the reaction mixture was diluted with ethanol
(5 mL) and the catalyst filtered off. After evaporation of the solvent from the
filtrate, the resulting crystal product was collected. The product was found to be
pure, with no need for column chromatography or further purification. Most of
123

Sulfonated PEG-intercalated montmorillonite [(Mt/PEG)-SO3H]…
the products obtained by this procedure are known literature compounds, and
their structures were determined by comparison of their physical constants, IR,
1H NMR, and 13C NMR spectroscopy data with those reported in literature.
Spectral data for new compounds
2,5-Bis(4-hydroxybenzylidene)cyclopentanone (3k) IR (KBr) vmax: 3169, 2967,
1
1670, 1597, 1452, 831, 601 cm-1; H NMR (400 MHz, DMSO-d6): d (ppm): 9.87
(s, 2H), 7.37 (m, 2H), 7.26 (m, 4H), 6.58 (m, 4H), 3.02 (s, 4H); 13C NMR
(100 MHz, DMSO-d6): d (ppm): 191.43, 159.60, 145.73, 142.32, 131.56, 129.12,
116.51, 27.75.
2,6-Bis(4-hydroxybenzylidene)cyclohexanone (3v) IR (KBr) vmax: 3200, 2948,
1
1659, 1576, 1487, 830, 604 cm-1; H NMR (400 MHz, DMSO-d6): d (ppm): 9.87
(s, 2H), 7.35 (m, 2H), 7.21 (m, 4H), 6.59 (m, 4H), 2.81 (m, 4H), 1.46 (m, 2H); 13C
NMR (100 MHz, DMSO-d6): d (ppm): 190.82, 159.60, 138.27, 136.88, 132.44,
128.26, 116.51, 30.15, 23.85.
Results and discussion
Very recently, we successfully prepared Mt-SO3H/PEG nanocomposite via
intercalation of PEG polymeric chains into interlayer spaces of sulfonated
montmorillonite. The obtained nanocomposite was used as efficient and recyclable
heterogeneous nanocatalyst for silylation of alcohols and phenols and deprotection
of the obtained silyl ethers [45], obtaining high yield with low catalyst loading.
Additionally, Mt-SO3H/PEG nanocatalyst was successfully used for highly efficient
synthesis of a variety of dihydropyrimidinone derivatives [46]. The acidity value for
this nanocatalyst was estimated to be 4.43 mmol g-1
.
In continuation of that work and aiming to prepare a catalyst with higher acidity
value and catalytic activity, two other modified nanocomposites of Mt/PEG were
prepared and related acidity values estimated. First, Mt/PEG-SO3H nanocomposite
was prepared via intercalation of sulfonated PEG into interlayer spaces of
montmorillonite (Mt). Its acidity value was estimated to be 2.31 mmol g-1
,
unfortunately lower than that of Mt-SO3H/PEG (4.43 mmol g-1) [45]. In fact, when
PEG-SO3H is intercalated into interlayer spaces of clay mineral, the bonded
sulfonyl groups are confined inside the clay mineral. In this case, fewer sulfonyl
groups are available for acidity determination, leading to this decreased acidity
value for Mt/PEG-SO3H. However, the sulfonyl groups in Mt-SO3H/PEG are
randomly anchored to both outer and inner surfaces of the clay mineral, so access to
outer sulfonyl groups of Mt-SO3H/PEG is easy, increasing its acidity value to
4.43 mmol g-1. This could be the reason why Mt/PEG-SO3H showed lower acidity
compared with Mt-SO3H/PEG. Because of this unsatisfactory result, Mt/PEG-SO3H
was abandoned. In other work, PEG polymeric chains were firstly intercalated into
interlayer spaces of montmorillonite (Mt) to obtain Mt/PEG nanocomposite, then
123

S. Z. Dalil Heirati et al.
OH
OH
O
S O
O
S O
O
OH
O
OH
O
OH
OH
OH
O
ClSO3H
OH
OH
O
Na+
O
HO
Na+
CH2Cl2, 0 °C
Na+
Na+
OH Na+
OH
O
O
Na+
OH
S
Na+
OH
Na+
O
Na+
OH
OH
S
O
O
OH
PEG chains
-HCl
O
OH
O
O
S
O
S
O
O
O
OH
OH
(Mt/PEG)-SO3H
Mt
Mt/PEG
Scheme 1 Preparation of (Mt/PEG)-SO3H
the resulting nanocomposite was subjected to sulfonation using chlorosulfonic acid
(Scheme 1).
The obtained nanocomposite is denoted as (Mt/PEG)-SO3H, and its acidity value
was determined to be 5.25 mmol g-1. Surprisingly, (Mt/PEG)-SO3H showed an
acidity value higher than those of Mt/PEG-SO3H (2.31 mmol g-1) or Mt-SO3H/
PEG (4.43 mmol g-1). This enhancement in the acidity value is due to sulfonation
of both PEG and Mt hydroxyl groups in the (Mt/PEG)-SO3H nanocomposite. In this
case, more hydroxyl groups are accessible for sulfonation, leading to the higher
acidity value for (Mt/PEG)-SO3H compared with the other two nanocomposites.
Hence, (Mt/PEG)-SO3H, because of its higher acidity value that is necessary to
perform many important reactions, was subjected to further structural and catalytic
investigation.
Characterization of (Mt/PEG)-SO3H nanocomposite
The XRD patterns for Mt, Mt/PEG, (Mt/PEG)-SO3H, and Mt-SO3H/PEG are shown
in Fig. 1. The corresponding basal spacing values (d) were calculated from Bragg’s
Mt
Mt/PEG
(Mt/PEG)-SO3H
Mt-SO3H/PEG
2
4
6
8
10
2
Fig. 1 Powder XRD patterns of Mt, Mt/PEG, (Mt/PEG)-SO3H, and Mt-SO3H/PEG
123

Sulfonated PEG-intercalated montmorillonite [(Mt/PEG)-SO3H]…
formula: nk = 2dsin h. Montmorillonite showed basal spacing of 0.98 nm at
2h = 9°, corresponding to reflection from (001) planes of the layered material. Mt/
PEG nanocomposite showed a reflection with basal spacing of 1.31 nm at
2h = 6.47°. Intercalation of PEG polymeric chains into clay mineral layers leads
to a shift of the reflection to lower 2h value, indicating expansion of the clay mineral
structure.
Additionally, the reflection related to (Mt/PEG)-SO3H nanocomposite appeared
at 2h = 4.93° with basal spacing of 1.82 nm, indicating an even greater increase in
basal spacing than for the Mt/PEG nanocomposite. This increase in the basal
spacing value could be due to sulfonation of hydroxyl groups of PEG (intercalated
into the clay mineral) as well as hydroxyl groups located on the inner surface of Mt.
In fact, on entry of sulfonic acid moieties into montmorillonite interlayer spaces,
great expansion of the clay mineral structure occurs, leading to the shift of the
reflection towards lower angle according to Bragg’s law. Comparison of the XRD
pattern for Mt-SO3H/PEG obtained in our previous work with that for (Mt/PEG)-
SO3H herein is interesting: Mt-SO3H/PEG showed a sharp reflection at 2h = 5.07°
with basal spacing of 1.77 nm [45], while (Mt/PEG)-SO3H showed a sharp
reflection at lower angle (2h = 4.93°) with greater increase in the basal spacing
(1.82 nm). This difference is due to the amount of functionalization of hydroxyl
groups available in these two nanocomposites. In the case of Mt-SO3H/PEG, only
hydroxyl groups of montmorillonite are functionalized, while in (Mt/PEG)-SO3H,
hydroxyl groups of both PEG and clay mineral are functionalized by sulfonyl
groups. Therefore, (Mt/PEG)-SO3H has more sulfonyl groups in interlayer spaces of
the clay mineral, which leads to greater expansion of the clay mineral structure. This
could also explain why (Mt/PEG)-SO3H shows higher acidity value compared with
Mt-SO3H/PEG.
The FT-IR spectra of Mt, Mt/PEG, (Mt/PEG)-SO3H, and Mt-SO3H/PEG samples
are shown in Fig. 2. In the case of Mt, the peaks at 1035, 915, and 795 cm-1 can be
collectively attributed to stretching vibration of Si–O in Si–O–Al, and bending
vibration modes of Al–Al–OH and Mg–Mg–OH groups, respectively [47, 48]. On
intercalation of PEG polymeric chains into interlayer spaces of the clay mineral,
three additional bands appeared at 2883, 1469, and 1353 cm-1, due to stretching
and bending vibrations of aliphatic C–H bond and wagging vibrations of CH2 of
PEG, respectively [17, 49].
These bands are also seen in the FT-IR spectra of (Mt/PEG)-SO3H and Mt-SO3H/
PEG, clearly indicating that PEG polymeric chains are present in the final structure
of these nanocomposites. After sulfonation of Mt/PEG, noticeable changes were
observed in the stretching region of the FTIR spectrum. The presence of a broad
band at around 1038 cm-1 in the spectrum for (Mt/PEG)-SO3H is assigned to
stretching modes of S=O bonds, overlapping with the stretching vibration of Si–O.
Additional bands at 854 and 578 cm-1 are assigned to S–OH bending and
symmetric S–O stretching vibrations of –SO3H group present in the nanocomposite
skeleton, respectively [50, 51]. All these observations confirm that the Mt/PEG
nanocomposite was successfully functionalized by sulfonic groups.
In the FT-IR spectrum of Mt, the peak at 3624 cm-1 is attributed to OH units
bonded to aluminum and/or magnesium [8]. Also, the broad peak at 3423 cm-1 is
123

S. Z. Dalil Heirati et al.
Fig. 2 FT-IR spectra of Mt, Mt/PEG, (Mt/PEG)-SO3H, and Mt-SO3H/PEG
attributed to hydroxyl stretching of hydrogen-bonded internal silanol groups and O–
H stretching of water, while the peak at 1638 cm-1 corresponds to bending mode of
O–H of water [15]. These peaks can be seen in the FTIR spectra of all samples.
123

Sulfonated PEG-intercalated montmorillonite [(Mt/PEG)-SO3H]…
However, in the FT-IR spectra of (Mt/PEG)-SO3H and Mt-SO3H/PEG, peaks at
around 3400 cm-1 are stronger than those of Mt and Mt/PEG. This may be due to
stretching vibrations of OH groups of the polymer and –SO3H groups, overlapped
with OH stretching vibrations of silanol groups. Surprisingly, comparison of the FT-
IR spectra of (Mt/PEG)-SO3H and Mt-SO3H/PEG reveals considerable differences
in this spectral range. The –OH stretching peak of (Mt/PEG)-SO3H is very broad
with a subsequent increase in intensity. This broadening of the –OH peak clearly
indicates greater functionalization of (Mt/PEG)-SO3H compared with Mt-SO3H/
PEG. In fact, the –OH group of –SO3H can form a hydrogen bond with OH groups
of PEG and clay mineral sheets, as well as the coordinated water molecule. Since
the clay mineral structure contains –OH groups with differing bond strengths due to
their location and coordination, it is expected that a wide range of hydrogen bonds
with different strengths can form in the sulfonated nanocomposite. These hydrogen
bonds are responsible for the broadening of the OH peak. Hence, because the –OH
peak of (Mt/PEG)-SO3H is broader than that of Mt-SO3H/PEG, it can be concluded
that (Mt/PEG)-SO3H has more sulfonyl groups. This result is clearly in agreement
with the results obtained from the XRD patterns, which revealed that (Mt/PEG)-
SO3H had more sulfonyl groups and thus higher acidity value.
The thermal stability of Mt, Mt/PEG, (Mt/PEG)-SO3H, and Mt-SO3H/PEG
samples was determined by thermogravimetric analysis (TGA), as shown in Fig. 3.
The TGA curve for Mt shows a typical two-step mass loss process. The first
weight loss from Mt, at just below 100 °C, is due to loss of water from interlayer
galleries. The second mass loss, between 100 and 600 °C, is attributed to
dehydroxylation of the silicate structure, clearly indicating that Mt has relatively
high thermal stability [8]. The complete thermal decomposition of pure PEG (Mn
6000) is a one-step process starting at 175 °C and completing at 300 °C [45].
Mt
100
95
90
85
80
75
70
Mt/PEG
(Mt/PEG)-SO3H
Mt-SO3H/PEG
0
50
100
150
200
250
300
350
400
450
500
550
600
Temprature (°C)
Fig. 3 TGA curves of Mt, Mt/PEG, (Mt/PEG)-SO3H, and Mt-SO3H/PEG
123

S. Z. Dalil Heirati et al.
However, as seen from the TGA curve for Mt/PEG, intercalation of PEG chains into
interlayer spaces of montmorillonite changed the trend of the mass loss curve for the
polymer. This thermogram shows two mass loss steps: a small mass loss between 50
and 250 °C related to loss of adsorbed water, and a mass loss between 250 and
450 °C, which can be attributed to thermal decomposition of poly(ethylene glycol)
chains [52]. The higher decomposition temperature of poly(ethylene glycol) chains
in the Mt/PEG sample (250–450 °C) in comparison with pure PEG (175–300 °C)
indicates that no polymers were adsorbed onto the surface of the Mt sheets. The
increased thermal stability of the poly(ethylene glycol) chains could be attributed to
strong interfacial forces and formation of hydrogen bonds between inner hydroxyl
groups of Mt clay mineral and intercalated poly(ethylene glycol). In the case of (Mt/
PEG)-SO3H, there are three mass loss steps. The first mass loss is observed below
150 °C and can be attributed to loss of adsorbed water. The second mass loss
between 150 and 400 °C could be due to decomposition of –SO3H groups [53],
while the third step that started from 400 °C can be attributed to thermal
decomposition of poly(ethylene glycol) [54]. According to the TGA thermograms,
the sulfonated samples showed lower decomposition temperature compared with the
Mt/PEG sample. This difference can be explained by the enhanced asymmetry of
the Mt/PEG structure due to introduction of SO3H groups. In fact, introduction of
sulfonic groups into Mt/PEG made it less regular and less stable [55]. Similarly, the
slight decrease observed in the thermal stability of (Mt/PEG)-SO3H compared with
Mt-SO3H/PEG could be explained by the greater degree of sulfonation of (Mt/
PEG)-SO3H. However, the (Mt/PEG)-SO3H sample still showed reasonable thermal
stability up to around 150 °C, making it suitable for use as a catalyst for medium-
and high-temperature operation. This level of thermal stability is clearly comparable
to those reported for other solid acid catalysts (e.g., bentonite/PS-SO3H [13] and
Fe3O4@silica sulfonic acid [56]).
SEM images of Mt, Mt/PEG, and (Mt/PEG)-SO3H samples are presented in
Fig. 4. The images of Mt (Fig. 4a, b) show that unmodified montmorillonite
contained lamellar aggregates with irregular plate-like shape and smooth and fluffy
appearance. After intercalation of poly(ethylene glycol) into interlayer spaces of
montmorillonite, the aggregates were foliated with rougher surfaces, and the
specific surface area increased (Fig. 4c, d). Sulfonation of Mt/PEG resulted in a
nanocomposite with different morphology (Fig. 4e, f). As can be seen, the particles
became rougher and the specific surface area of the (Mt/PEG)-SO3H nanocomposite
was significantly increased in comparison with the Mt/PEG nanocomposite.
Additionally, BET and BJH textural analyses were used for exact characteriza-
tion of the surface area, pore volume, and pore size of the Mt, Mt-SO3H/PEG, and
(Mt/PEG)-SO3H samples. The N2 adsorption–desorption isotherms for these
samples are shown in Fig. 5. As can be seen, all samples presented type IV
isotherms according to International Union of Pure and Applied Chemistry (IUPAC)
nomenclature, indicating that the samples were mesoporous. Furthermore, the
hysteresis loop of the samples revealed category H3, indicative of large slit-like
pores with narrow size distribution [57].
The BET surface area, pore volume, and BJH pore diameter for the samples are
summarized in Table 1. Specific surface area of 29.03 m2 g-1, large pore volume of
123

Sulfonated PEG-intercalated montmorillonite [(Mt/PEG)-SO3H]…
Fig. 4 SEM images of Mt (a, b), Mt/PEG (c, d), and (Mt/PEG)-SO3H (e, f)
0.052 cm3 g-1, and pore size of 7.18 nm were obtained from the isotherm for Mt,
indicating its potential for application as a host for organic materials. After
modification by PEG and chlorosulfonic acid, Mt-SO3H/PEG and (Mt/PEG)-SO3H
exhibited smaller specific area and pore volume compared with those of pure Mt,
which could be due to blocking of pores in Mt by PEG chains and/or sulfonyl
groups in the nanocomposite samples. However, there was a noticeable increase in
the pore diameter for Mt-SO3H/PEG and (Mt/PEG)-SO3H compared with pure Mt.
This could be due to incorporation of PEG chains and/or sulfonyl groups inside
pores of the nanocomposites. Such incorporation could result in some pressure on
the mesoporous structures (physical pressure on the wall of the channels) and
thereby increase the pore diameter.
123

S. Z. Dalil Heirati et al.
Adsorpꢀon / Desorpꢀon isotherm
30
20
10
0
a
b
c
0
0.5
1
p/p0
Fig. 5 Nitrogen adsorption–desorption isotherms for (a) Mt, (b) (Mt/PEG)-SO3H, and (c) Mt-SO3H/PEG
Table 1 BET and BJH data for the textural properties of Mt, Mt-SO3H/PEG, and (Mt/PEG)-SO3H
SBET (m2 g-1
)
Vp (cm3 g-1
b
)
Pore sizec (nm)
a
Entry
Sample
1
2
3
a
Mt
29.03
6.32
9.65
0.052
0.018
0.027
7.18
11.61
11.19
Mt-SO3H/PEG
(Mt/PEG)-SO3H
BET surface area
b
c
Total pore volume
BJH pore size diameter
It is clear that (Mt/PEG)-SO3H had larger surface area and total pore volume than
Mt-SO3H/PEG. However, its BJH pore size diameter was slightly smaller than that
of Mt-SO3H/PEG. These results suggest that (Mt/PEG)-SO3H has reasonable
surface area (9.65 m2 g-1) and more mesoporosity than Mt-SO3H/PEG, being
suitable to act as a catalyst with satisfactory results.
Catalytic activity
The results of the structural investigation of (Mt/PEG)-SO3H revealed that it has
high acidity value, good thermal stability, and greater surface area, with potential
123

Sulfonated PEG-intercalated montmorillonite [(Mt/PEG)-SO3H]…
for use as a solid acid catalyst in reactions requiring an acidic catalyst to speed up.
Also, these investigations clearly showed that (Mt/PEG)-SO3H could have even
higher catalytic performance than Mt-SO3H/PEG. To investigate this potential, (Mt/
PEG)-SO3H was used as catalyst for cross-aldol condensation between cyclopen-
tanone (or cyclohexanone) with various aromatic aldehydes.
The reaction was optimized to determined the best conditions, especially in terms
of green chemistry principles. Therefore, the reaction between benzaldehyde
(2 mmol) and cyclopentanone (1 mmol) was selected as model reaction to examine
the effects of solvent, temperature, and amount of catalyst. A summary of the
optimization reactions is provided in Table 2.
Initially, the reaction was investigated in ethanol as green and commercially
available solvent. When the reaction mixture was stirred in ethanol under reflux
condition, the product was obtained in 65% yield in 60 min (Table 2, entry 1). Also,
performing this reaction at room temperature afforded the related product in 60%
yield in 80 min (Table 2, entry 2). The reaction was then carried out under solvent-
free condition at room temperature; interestingly, the reaction completed in 25 min,
affording the desired product in 98% yield (Table 2, entry 3). This acceleration
effect can be attributed to a higher concentration of local reaction sites in case of
solid-state conditions. Therefore, solvent-free conditions were selected as the best
choice for this reaction, in good agreement with the aims of green chemistry. To
determine the optimal catalyst amount leading to high yield in shorter reaction time,
the catalyst amount was gradually decreased from 30 to 10 mg. Decreasing the
amount of nanocatalyst from 30 to 15 mg did not have an appreciable effect on
product yield or reaction time (Table 2, entries 3–6), but use of 10 mg nanocatalyst
increased the reaction time to 40 min (Table 2, entry 7). Hence, the catalyst amount
was fixed at 15 mg. Finally, using 15 mg catalyst at room temperature under
solvent-free conditions, resulting in high product yield in short reaction time, was
chosen as the best reaction condition.
To study the generality of the procedure, the reactions between a series of
aromatic aldehydes with cyclopentanone and cyclohexanone were investigated
using the optimal reaction condition (Scheme 2).
Table 2 Optimization of reaction conditions
Entry
Catalyst (mg)
Condition
Time (min)
Yield (%)a
1
2
3
4
5
6
7
30
30
30
25
20
15
10
EtOH, reflux
60
80
25
25
27
30
40
65
60
98
97
97
97
90
EtOH, r.t.
Solvent free, r.t.
Solvent free, r.t.
Solvent free, r.t.
Solvent free, r.t.
Solvent free, r.t.
Reaction conditions: benzaldehyde (2 mmol) and cyclopentanone (1 mmol)
a
Yields refer to isolated pure products
123

S. Z. Dalil Heirati et al.
O
O
(MMT/PEG)-SO3H (15mg)
Solvent-free, r.t.
Ar
Ar
+
ArCHO
2
(CH2)n
1 (n=1, 2)
(CH2)n
3(a-v)
Scheme 2 Synthesis of a,a0-bis(substituted benzylidene)cycloalkanones catalyzed by (Mt/PEG)-SO3H
As can be seen from Table 3, a variety of aromatic aldehydes with both electron-
withdrawing and electron-donating groups were used in this reaction to afford the
desired product in high yield and short reaction time. The electronic nature of the
substituent on the benzene ring of the aldehyde did not affect the reaction time or
yield. However, it seems that aldehydes with substituent at para position (Table 3,
entries 2, 4, 6–9) reacted faster than aldehydes with substituent at ortho position
(Table 3, entries 3, 10, 13, 20). Interestingly, the ring size of the cycloalkanone
affected the reaction time and yield, as cyclopentanone reacted faster than
cyclohexanone. This is because the protons of cyclopentanone are slightly more
acidic than those of cyclohexanone [42]. The outstanding advantage of this catalytic
system is that low amount of catalyst is active under solvent-free condition to
promote this reaction. In all cases, clean and pure product was obtained by a rapid
synthetic method. The products were isolated by simple filtration without use of
chromatographic separation, showing physical and spectroscopic data in good
agreement with those of authentic samples.
Because of the importance of scale-up for laboratory and industrial purposes, the
present methodology was extended for large-scale synthesis of a,a0-bis(substituted
benzylidene)cycloalkanones. In this regard, some reactions from Table 3 were
performed with 10- or 15-fold amounts of reactants and catalyst with respect to
those mentioned in ‘‘Experimental’’ section. The results are summarized in Table 4,
revealing that the reactions could be successfully performed at large scales without
significant loss of yield. The reaction on large scale required longer time, which
may be due to difficulty in stirring the reaction mixture under solvent-free
conditions.
To show another advantage of the present synthetic method, the effect of
moisture and hydrolysis on the catalyst structure was examined. For this purpose,
1 g catalyst was dissolved in water (10 mL) and kept at room temperature for a
week. After this time, water was evaporated and the obtained solid was dried under
reduced pressure for 5 h and its catalytic behavior examined in the synthesis of
entry 2 in Table 3. The results showed no difference between the activity of the
freshly synthesized catalyst versus the catalyst recovered from water, indicating that
the catalyst structure remained stable at room temperature.
A proposed mechanism for the preparation of a,a0-bis(substituted benzylidene)-
cycloalkanones catalyzed by (Mt/PEG)-SO3H is illustrated in Scheme 3.
The reaction predominantly proceeds via the enolate pathway. First, the
cycloalkanone is deprotonated to form the enolate in the presence of the solid
acid nanocatalyst. The nucleophilic enolate ion attacks the electrophilic carbonyl
123

Sulfonated PEG-intercalated montmorillonite [(Mt/PEG)-SO3H]…
Table 3 Preparation of a,a0-bis(substituted benzylidene)cycloalkanones using (PEG/Mt)-SO3H under
solvent-free condition
Entry Aldehyde
n
Product Time (min) Yield (%)a M.p. (°C) M.p. (°C) references
CHO
1
2
1
3a
3b
30
20
97
98
187–189
230–233
188–189 [39]
230–231 [39]
CHO
1
1
1
Cl
CHO
Cl
3
4
3c
30
25
95
95
158–161
254–256
160–161 [39]
254–257 [39]
CHO
3d
Br
Br
CHO
CHO
5
6
1
1
3e
3f
30
25
92
94
211–213
208–210
210–212 [35]
209–210 [39]
MeO
CHO
7
1
3g
25
95
236–239
237–239 [39]
F
CHO
CHO
8
1
1
1
1
3h
3i
30
28
35
35
92
92
90
90
239–242
228–230
165–168
250–255
239–241 [39]
229–231 [34]
168–171 [39]
Not reported
Me
O2N
9
CHO
10
11
3j
NO2
CHO
3k
HO
Cl
CHO
12
13
2
2
3l
36
25
94
96
158–161
154–156
159–161 [39]
154–156 [39]
CHO
3m
CHO
Cl
14
15
2
2
3n
3o
35
30
95
94
101–104
207–210
102–106 [39]
208–211 [39]
CHO
Br
Br
CHO
CHO
16
17
2
2
3p
3q
34
30
92
93
178–182
162–165
179–181 [35]
163–165 [39]
MeO
Me
CHO
18
2
3r
38
89
154–158
155–157 [39]
123

S. Z. Dalil Heirati et al.
Table 3 continued
Entry Aldehyde
n
Product Time (min) Yield (%)a M.p. (°C) M.p. (°C) references
CHO
19
20
21
22
2
3s
30
35
40
47
90
88
85
90
163–166
160–163
102–105
185–189
163–165 [39]
161–162 [34]
102–106 [39]
Not reported
F
CHO
2
2
2
3t
O2N
CHO
NO2
3u
3v
CHO
HO
Reaction conditions: aldehyde (2 mmol), cycloalkanone (1 mmol), solvent free, room temperature, (Mt/
PEG)-SO3H (15 mg)
Products characterized by comparison of physical constants and IR and NMR spectra with literature data
a
Yields refer to isolated pure products
Table 4 Large-scale
preparation of a,a0-
bis(substituted
Product Aldehyde Cycloalkanone Catalyst Time
(mmol)
Yield
(min) (%)a
(mmol)
(mg)
benzylidene)cycloalkanones
using (PEG/Mt)-SO3H under
solvent-free condition
3a
3f
2
20
30
2
1
10
15
1
15
150
225
15
30
34
37
25
28
33
25
30
34
97
90
87
94
89
85
96
93
88
20
30
2
10
15
1
150
225
15
3m
20
30
10
15
150
225
a
Yields refer to isolated pure
products
group of aldehyde that is activated by the solid acid nanocatalyst to form b-
hydroxyketone followed by a dehydration step. The same nucleophilic attack on the
other side of the cycloalkanone, followed by dehydration again, produces the
corresponding product. The recovered catalyst was reused to evaluate its
performance in subsequent cycles.
Recyclability
After the above-mentioned studies, the catalytic activity of the recycled nanocat-
alyst was examined. When the reaction between 4-chlorobenzaldehyde and
cyclopentanone completed, ethanol was added and the catalyst was separated by
simple filtration. The recovered catalyst was washed with ethanol, dried, and reused
in subsequent reactions. The results obtained are shown in Fig. 6. It can be seen that
123

Sulfonated PEG-intercalated montmorillonite [(Mt/PEG)-SO3H]…
HO3S
H
H
O
O
O
O
OH
O
C
H
SO3H
Ar
Ar
Ar
-H2O
(CH2)n
(CH2)n
(CH2)n
(CH2)n
SO3H
(PEG/MMT)-SO3H =
SO3H
SO3H
O
H
HO3S
H
O
O
OH
H
O
C
Ar
Ar
Ar
Ar
Ar
Ar
-H2O
(CH2)n
(CH2)n
(CH2)n
Scheme 3 A plausible mechanism for the reaction
Yields(%)
Time(min)
90
98
98
94
92
100
90
80
70
60
50
40
30
20
10
0
90
88
88
87
87
37
35
33
30
30
30
25
20
20
20
1
2
3
4
5
6
7
8
9
10
Run(s)
Fig. 6 Recycling of (Mt/PEG)-SO3H
(Mt/PEG)-SO3H showed good catalytic activity in the first and second runs, with
yield of 98%. Slight deactivation occurred between runs 3 and 6, with smooth loss
of catalytic activity observed from the seventh reuse. However, the yield was still
approximately 87% after ten runs. Based on these results, it is reasonable to
conclude that the reusability of (Mt/PEG)-SO3H is relatively high. This good
recyclability is most probably due to the covalent bonding of the sulfonic groups
(–SO3H) to the surface of the Mt/PEG nanocomposite. Meanwhile, the slight
reduction of catalytic activity of the catalyst after recycling is probably due to
blockage of active sites on the catalyst surface by either products or intermediates
involved in the reaction.
It should also be noted that the FT-IR spectrum of recovered nanocatalyst was
very similar to that of fresh catalyst. This result suggests that the catalyst remained
intact after each run.
123

S. Z. Dalil Heirati et al.
Table 5 Comparison of catalytic activity of (Mt/PEG)-SO3H with other reported catalysts
Entry Catalyst (mg)
Reaction condition
Time (min) Yield (%) References
1
2
3
4
5
6
7
8
9
NH4H2PO4/SiO2 (150 mg)
Solvent free, 110 °C
Solvent free, 80 °C
Solvent free, 90 °C
15
90
90
90
90
94
91
89
97
88
83
[34]b,c
[36]a,b,c
[37]b
[38]a,c
[39]a,d
[41]a,d,e
This work
This worke
This worke
PEG-OSO3H (180 mg)
n-TSA (32 mg)
25
[N2222][EtNHC3SO3] (1.0 g) H2O, 60 °C
360
370
270
30
Nano-TiO2/HOAc (16 mg)
Molecular iodine (76 mg)
(Mt/PEG)-SO3H (15 mg)
Mt-SO3H/PEG (15 mg)
Mt-SO3H (15 mg)
a
EtOH, reflux
CH2Cl2, r.t.
Solvent free, r.t.
Solvent free, r.t.
Solvent free, r.t.
c
30
30
Drawbacks: long reaction time, bhigh temperature, high catalyst loading, duse of solvent, low yield
e
Comparison with other catalysts
To assess the efficiency of this method in comparison with other reported methods,
the results obtained for synthesis of 2,5-dibenzylidenecyclopentanone catalyzed by
(Mt/PEG)-SO3H were compared with those obtained using some other catalysts.
The comparison is presented in Table 5.
Although NH4H2PO4/SiO2, PEG-OSO3H, and n-TSA can catalyze this reaction
under solvent-free condition, higher temperature or larger amount of catalyst are
required compared with the catalytic system presented herein (Table 5, entries 1–3).
Promotion of the presented reaction requires 150 mg of NH4H2PO4/SiO2, 180 mg of
PEG-OSO3H, and 32 mg of n-TSA, while the amount of (Mt/PEG)-SO3H required is
only 15 mg. Catalysis of the reaction by [N2222][EtNHC3SO3], nano-TiO2/HOAc, and
molecular iodine suffers from disadvantages such as use of solvent, long reaction time,
high temperature, and large amount of catalyst (Table 5, entries 4–6), whereas use of
(Mt/PEG)-SO3H as catalyst avoids the mentioned disadvantages (Table 5, entry 7).
Additionally, the catalytic activity of (Mt/PEG)-SO3H was investigated in
comparison with that of Mt-SO3H/PEG and Mt-SO3H in the same reaction condition.
When 15 mg (Mt/PEG)-SO3H was used as catalyst, the reaction completed in 30 min,
affording the desired product in 97% yield. Meanwhile, 15 mg Mt-SO3H/PEG or
Mt-SO3H in the same reaction condition led to the corresponding product in 88 and
83% yield, respectively (Table 5, entries 8, 9). These results show that the catalytic
activity of (Mt/PEG)-SO3H is greater than that of Mt-SO3H/PEG or Mt-SO3H, due
to the difference in their acidity values [4.43 and 2.41 mmol g-1 for Mt-SO3H/PEG
and Mt-SO3H, respectively, versus 5.25 mmol g-1 for (Mt/PEG)-SO3H].
Conclusions
Sulfonated montmorillonite/poly(ethylene glycol) nanocomposite [(Mt/PEG)-
SO3H] was prepared and its structure characterized by XRD analysis and FT-IR
spectroscopy. Thermogravimetric analysis (TGA) and SEM confirmed the thermal
123

Sulfonated PEG-intercalated montmorillonite [(Mt/PEG)-SO3H]…
stability and nanoscale structure of the (Mt/PEG)-SO3H nanocomposite, respec-
tively. Because of these excellent properties as well as its high acidic content, (PEG/
Mt)-SO3H could be used as a green solid acid nanocatalyst for synthesis of a,a0-
bis(substituted benzylidene)cycloalkanone derivatives under solvent-free condition
at room temperature. This synthetic method showed good catalytic activity to obtain
high yield of pure product in mild condition, and the catalyst was recyclable in up to
ten runs without significantly decreased product yield. Considering these results,
(Mt/PEG)-SO3H could open a new avenue towards efficient and ecofriendly
synthesis of other biologically active compounds in the future.
Acknowledgements We are grateful to the University of Guilan Research Council for partial support of
this work.
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