H6P2W18O62/Nanoclinoptilolite as an efficient nanohybrid catalyst in the cyclotrimerization of aryl methyl ketones under solvent-free conditions

Article abstract of DOI:10.1039/c5ra01344e

A new type of nanohybrid material H₆P₂W₁₈O₆₂/nanoclinoptilolite was fabricated and performed as an efficient and reusable catalyst in the mild and one-pot condensation of different acetophenones. The operational simplicity, easy work-up, cost-effective, and solvent-free nature of the present methodology were accompanied with good to excellent yields of the desired 1,3,5-triarylbenzenes from a wide range of alkyl, aryl, and cyclic ketones. The nanocatalyst was prepared via immobilization of Wells-Dawson heteropolyacid H₆P₂W₁₈O₆₂ (HPA) on the surface of nanoclinoptilolite (NCP). The nanohybrid material was easily recovered and reused successfully at least seven times without significant loss of catalytic activity. XRD, SEM, UV-Vis, MS-ICP, DTA, and FT-IR studies confirmed that the heteropolyacid is well dispersed on the surface of NCP. This protocol developed is a safe and convenient alternate method for the synthesis of 1,3,5-triarylbenzenes utilizing an eco-friendly and a highly reusable natural nanocatalyst. Furthermore, water was the only by-product, which made the present methodology environmentally benign.

Full text of DOI:10.1039/c5ra01344e

RSC Advances
View Article Online
View Journal | View Issue
PAPER
H P W O /Nanoclinoptilolite as an efficient
6
2
18 62
nanohybrid catalyst in the cyclotrimerization of aryl
methyl ketones under solvent-free conditions
Cite this: RSC Adv., 2015, 5, 21206
R. Tayebee* and M. Jarrahi
6 2 18
A new type of nanohybrid material H P W O62/nanoclinoptilolite was fabricated and performed as an
efficient and reusable catalyst in the mild and one-pot condensation of different acetophenones. The
operational simplicity, easy work-up, cost-effective, and solvent-free nature of the present
methodology were accompanied with good to excellent yields of the desired 1,3,5-triarylbenzenes
from a wide range of alkyl, aryl, and cyclic ketones. The nanocatalyst was prepared via immobilization
6 2 18
of Wells–Dawson heteropolyacid H P W O62 (HPA) on the surface of nanoclinoptilolite (NCP). The
nanohybrid material was easily recovered and reused successfully at least seven times without
significant loss of catalytic activity. XRD, SEM, UV-Vis, MS-ICP, DTA, and FT-IR studies confirmed that
the heteropolyacid is well dispersed on the surface of NCP. This protocol developed is a safe and
convenient alternate method for the synthesis of 1,3,5-triarylbenzenes utilizing an eco-friendly and a
highly reusable natural nanocatalyst. Furthermore, water was the only by-product, which made the
present methodology environmentally benign.
Received 23rd January 2015
Accepted 17th February 2015
DOI: 10.1039/c5ra01344e
www.rsc.org/advances
tedious workup, long reaction times, moisture sensitivity,
specialized handling, non-recyclability of the catalyst, and using
1
. Introduction
Screening environmentally benign methods for chemical stoichiometric amounts of expensive and toxic catalysts.
syntheses is an important challenging and necessary subject to
Heteropolyacids constitute an extensive class of inorganic
avoid the adverse consequences of usual chemical industries. metal-oxo clusters comprising early transition metals in their
Green chemical processes comprise application of sustainable highest oxidation state which show a great deal of applications
and clean ways, using cost effective catalysts, employing water in catalysis, material sciences, pharmaceuticals, and biology.
as solvent or exploitation of solvent-free systems. The present Wells–Dawson heteropolyacid with the general formula of
n+
(16ꢀ2n)ꢀ (16ꢀ2n)+
report introduces a new green route for the preparation of [(X )
1
2
M
18
O
62
]
H
is an important subclass of het-
n+
,3,5-triarylbenzenes. These compounds are important poly- eropolyacids. In this structure, X represents a central atom
cyclic aromatic hydrocarbons which have a uniform amorphous such as, phosphorous(v), arsenic(V), sulfur(VI), uorine; sur-
phase due to their molecular shapes and have exhibited rounded by a cage of M addenda atoms, such as tung-
potential applications for fabricating electrode and electrolu- sten(VI),molybdenum(VI) or a mixture of elements, each of them
1
minescent devices, organic light emitting diodes, nano- composing of MO
6
(M-oxygen) octahedral units. One of the
2
materials, photovoltaic resisting materials, and conducting most important disadvantages of this heteropolyacid lies in
polymers. Moreover, 1,3,5-triarylbenzenes are known as high solubility in polar media which causes separation prob-
3
important intermediates in the synthesis of pharmaceuticals, lems. To overcome this limitation, it can be supported on
4
5
fullerene fragments, synthetic dendrimers, and various different weak-acidic or non-basic carriers or immobilized on
6
conjugated polyaromatics. These important compounds have positively charged nanoparticles through covalent or electro-
been made by condensation of aryl methyl ketones in various static attractions. Up to now, several supports such as silica,
acidic media via a number of synthetic procedures. Common active carbon, MCM-41, SBA-15, have been utilized to het-
Brønsted acids, para-toluene sulfonic acid, Bi(OTf) , FeCl , and erogenize heteropolyacids via immobilization. NCP as porous
8
9
10
11
3
3
Amberlyst-15 are widely handled for the preparation of triaryl- material could be used as a good candidate to accommodate
7
benzenes. Although these methods have proved to be useful, and deposit heteropolyacids and produce effective catalysts in
however, there are some limitations, including low yields, organic synthesis by consideration of either Lewis or Brønsted
12
acidic characteristics of them.
Herein, the capability of a new nanohybrid material
HPA/NCP is assessed in the catalytic cyclotrimerization of
Department of Chemistry, School of Sciences, Hakim Sabzevari University, Sabzevar,
9
4
6179-76487, Iran. E-mail: Rtayebee@hsu.ac.ir; Fax: +98-51-44410300; Tel: +98-51-
4410310
1,3,5-triarylbenzenes under solvent-free conditions (Scheme 1).
21206 | RSC Adv., 2015, 5, 21206–21214
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
diphosphooctadecatungstic acid
prepared according to the literature method.
H
6
P
2
W
18
1
O
62$24H
2
O
was
4
2.2. Preparation of the NCP zeolite
The nanosized CP was obtained by means of a planetary ball
mill in the dry state. In most of the experiments a clinoptilolite
(
CP) powder with the average particles size of 70 mm was applied
Scheme
acetophenones.
1 General formulation for the cyclotrimerization of
as the starting material. Planetary ball milling was performed in
dry conditions with a period of 60 min, 10 balls of 20 mm per
30 g of powder and a milling speed of 350 rpm. The experiments
were performed in a 250 ml stainless steel jar with a protective
jacket of zirconium oxide. Zirconium oxide balls were utilized
for dry milling.
2
. Experimental
15
2.1. Materials and methods
All reagents and starting materials were commercially available
and were used as received. Natural clinoptilolite-rich tuffs were
obtained from Sabzevar region in the north-east of Iran. All
solutions were prepared in double-distilled deionized water.
Ball milling of the natural CP zeolite was carried out by means
of a laboratory fast mill (Model 20436, Sanatceram, Iran) and a
planetary ball mill (PM100; Retsch Corporation). Progress of the
reactions was monitored by TLC on silica gel polygram SIL G/UV
2
.3. Surface modication of NCP with the Wells–Dawson
H P W O $24H O
6
2
18 62
2
ꢁ
For this purpose, ve grams of the NCP (calcined at 650 C for
4
NH NO (1 M). Aer shaking at 75 C for 6 h in a thermostatic
bath, the sample was separated by ltration, rinsed with
deionized water, dried at 110 C for 8 h, and calcined at 540 C
for 4 h. The obtained sample was again processed by a second
acid treatment as mentioned above. To prepare the surface
6 2 18 2
modied NCP, 0.1 g of H P W O62$24H O was dissolved in
40 ml of H
h) was placed in contact with 100 ml of an aqueous solution of
ꢁ
4
3
ꢁ
ꢁ
254 plates. Melting points were recorded on a Bamstead elec-
trothermal type 9200 melting point apparatus. Differential
thermal analysis was carried out on a Netzch Germany, E404
ꢁ
ꢀ1
2
O; then, 0.5 g of NCP was dispersed in the solution
analyzer instrument in air at a heating rate of 10 C min
.
ꢁ
and the mixture was shaken at 70 C for 4 h in a thermostatic
bath. Finally, the sample was separated by ltration, rinsed with
deionized water, and dried at 80 C overnight.
Scanning electron microscope (SEM) micrographs were taken
using a KYKY-EM3200 microscope (acceleration voltage 26 kV).
Fourier transform infrared (FT-IR) spectra were recorded on a
ꢁ
8
700 Shimadzu Fourier-Transform spectrophotometer in the
ꢀ
1
2.4. General procedure for the conversion of acetophenone
into 1,3,5-triphenylbenzene
region of 400 to 4000 cm using KBr pellets. Ultraviolet-visible
spectra were recorded on a Photonix UV-Vis Array spectropho-
tometer, Model Ar 2015, Iran. X-ray powder diffraction (XRD) In a typical reaction, acetophenone (1 mmol) and the surface
analysis were performed on a XPert MPD diffractometer with Cu modied NCP (15 mg) were added to a small test tube and the
ꢁ
K
a
radiation (l ¼ 1.5406) at 40 keV and 30 mA with a scanning reaction mixture was stirred for 2.5 h at 100 C. Aer completion
ꢁ
ꢀ1
ꢁ
ꢁ
1
13
rate of 3 min in the 2q range from 5 to 80 . H and C NMR of reaction, as indicated by TLC, the reaction mass was cooled to
spectra were recorded on a Bruker AVANCE 300 MHz instru- 25 C; then, hot ethanol was added to the reaction mixture and
ꢁ
ment using TMS as internal reference. Tungsten content in the the mixture was stirred for 5 min. The insoluble catalyst was
nanohybrid catalyst was determined using Inductivity Coupled isolated via simple ltration. The ltrate containing water as the
Plasma-Mass Spectrometry (ICP-MS) conducted on a Perkin only by-product of the cyclotrimerization, was concentrated
Elmer, Elan 6000 DRC ICP-MS spectrometer. All products were
identied by comparison of their spectral and physical
13
data with those previously reported.
Wells–Dawson
Fig. 1 Tetrahedral model of clinoptilolite, indicating 10-membered A
and 8-membered B channels that are bridged with 8-membered C
channels (projection along c axis) (a) and parallel channel C formed
18
with the ellipsoidal 8-membered rings (b).
6 2 18
Fig. 2 XRD patterns of NCP (a) and H P W O62/NCP (b).
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 21206–21214 | 21207

View Article Online
RSC Advances
Paper
zeolites contain open tetrahedral cages generating a system of
channels which size is determined by the content of silicon.
These cages are formed with a network of eight-member
(
channel B) and ten-member (channel A) rings, located in ab
plane, and eight-member rings (channel C), located in bc plane
Fig. 1).
CP, as the most abundant natural zeolite, shows different
(
shape and size selectivity, catalytic properties, ion exchange
abilities, odor adsorbent behaviors, and molecular sieves
19
properties. Chemical and thermal stability of CP along with its
pore system diameter are the most important parameters in
studying conjunction of the HPA into the nanozeolite surface.
Fortunately, the prepared NCP was suitably chemically and
thermally stable in the presence of the utilized heteropolyacid.
Since, the heterogeneous porous structure of CP consists of two
6 2 18
Fig. 3 Representative SEM images of NCP (a) and H P W O62/NCP
(
b).
under reduced pressure and nally the obtained crude product types of primary (microporosity) and secondary (meso- and
was puried through re-crystallization in EtOH : H O (3 : 1) as macro-porosity), therefore, heteropolyacids with the diameter
2
˚
conrmed by an intense single spot in TLC. The pure products ꢂ10 A, could be either encapsulated in the super cages of
20
were specied based on the spectral data and determination of nanozeolite or supported in the secondary pore system.
their melting points.
3.1.1. XRD and XRF patterns. Fig. 2 shows XRD patterns of
NCP and HPA/NCP. With regards to the intensity ratio of major
ꢁ
peak observed at 2q about 22.5 , the crystalline structure of NCP
2.5. Calculation of pHpzc (point of zero charge pH)
was clearly attributed to the d004 reection. Moreover, presence
of broad lines in the XRD pattern of the prepared NCP particles
established formation of nanoparticles during the ball
milling process. The average crystallite size was calculated by
Debye–Sherrer's equation (eqn (1)), in which b denotes the
excess width line at half-maximum of the diffraction peak in
radians, q is Bragg angle in degrees, and l is the selected
wavelength. By calculation, the average crystallite size of the
prepared nanozeolite was attained ꢂ40 nm. These results
The pHpzc is a point at which the surface acidic or basic func-
tional groups no more chip in to the pH value of the solution.
Effect of surface modication with HPA on the acidity of
nanozeolite was studied by calculating pHpzc for HPA/NCP and
NCP. The pH of a series of NaCl solutions (50 ml, 0.01 M) was
adjusted to the value between 2 and 12 by addition of HCl
16
(0.1 M) or NaOH (0.1 M) solutions in closed Erlenmeyer asks.
Then, pH of the solutions dened as the initial pHs (pH ).
I
Thereaer, 0.2 g of modied HPA/NCP (or unmodied NCP)
21
approved our method for the preparation of the nanosized CP.
:98l
b cosðqÞ
was added and the nal pH (pH ) was measured aer 48 h.
F
0
Finally, the values of pHF vs. pH and also pH vs. pH were
I
I
I
L ¼
(1)
17
plotted and the intersect of the lines provided the pHpzc
.
3
. Results and discussion
X-Ray uorescence (XRF) revealed that SiO (62.68%) and
2
Al O (9.57%) are the main constituents of NCP; whereas, P O
2 5
2
3
3.1. Characterization and physicochemical properties of the
22
(
0.03%) is the least component. It has been established that
unmodied and modied NCP
the mole ratio of Si/Al would be a conventional distinction point
between zeolites, CP has the ratio of Si/Al $ 4.0. The high mole
ratio of silica/alumina (11.14) resulted in sufficient thermal and
chemical stability, hydrophilic nature, and acidic strength of
the nanozeolite to prepare acidic HPA/NCP catalyst.
CP is a natural material that belongs to the class of alumino-
silicates containing exchangeable metal cations and water
molecules in the internal crystalline cages. These types of
3
.1.2. SEM images. Morphology of the modied and
unmodied NCP was studied by scanning electron microscopy
Fig. 3). The SEM image of the nanozeolite showed the layered
(
structure of CP with layer thicknesses in nanometer range. The
crystallites of the unloaded and loaded NCP with the hetero-
polyacid showed similar patterns; thus, loading process did not
affect the crystallites structure. Although, there are some
particles with micrometer dimension in the SEM pictures and
lack of uniformity and wide range of particle dimensions, but
the major particles had nanometer dimensions ranging from
6 2 18
30 to 70 nm. Loading of H P W O62 led to enhancing particles
size of the resultant nanocatalyst. Findings obtained by XRD
and SEM approved that the selected method for the preparation
6 2 18 62
Fig. 4 Differential thermal analysis of NCP modified with H P W O .
21208 | RSC Adv., 2015, 5, 21206–21214
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
ꢀ1
peak at about 1520 cm
is assigned to the O–H bending
vibration of the adsorbed water molecules. Strong and broad
bands in the region of stretching Si–O and Al–O bands and
bending O–Si–O and O–Al–O vibrations conrmed high surface
23
area of the prepared NCP.
The structure of Wells–Dawson H
units of a central PO tetrahedron surrounded by nine WO
octahedra; therefore, four kinds of oxygen atoms appear in the FT-
IR of HPA (Fig. 6). The rst is due to P–O in which the oxygen
6 2 18
P W O62 involves two half
4
6
a
atom is connected to the tungsten atom. The second is W–O –W
b
oxygen bridges (corner-sharing oxygen bridges between different
W O groups), the third is W–O –W oxygen bridges (edge-sharing
3
13
c
oxygen bridge within W O13 groups), and the last is W–O
3 d
terminal oxygen atoms. Therefore, four characteristic bands of
ꢀ
1
H
6
P
2
W
18
ꢀ
O
62 were appeared as nas (W–O
d
, 970 cm ); nas (W–O
b
–W,
1
ꢀ1
ꢀ1 24
915 cm ); nas (W–O
c
–W, 781 cm ) and nas (P–O
a
, 1098 cm ).
3.1.5. UV-Vis spectroscopy. The UV-Vis spectrum of the
6 2 18 6 2 18 62
Fig. 5 FT-IR spectra of NCP (a), H P W O62 (b), and H P W O /
NCP (c).
Wells–Dawson heteropolyacid shows two types of ligand /
metal charge-transfer bands originating from different oxygen
atoms (Fig. 7). The highest energy absorption band below
of the nanosized CP was acceptable. It should be mentioned 250 nm (not shown) has been assigned to the O / W charge-
d
that mechanical production of zeolitic nanoparticles by means transfer band due to the terminal oxygen atoms. Other weak
of planetary ball mills may also reduce CP crystallinity.
and broad bands around 260 and 310 nm are characteristic
3
.1.3. DTA analysis. Fig. 4 shows differential thermal bands of H P W O and have been assigned to the O / W or
6
2
18 62
b
25
analysis of H P W O /HPA. A continuous weight loss was O / W charge-transfer bands of the bridge-oxygen atoms.
occurred below 280 C in the DTA analysis which was due to the
6
2
18 62
ꢁ
c
To nd the minimum time required to load the maximum
removal of the physical adsorbed and coordinated water mole- amount of H P W O on NCP, further study was outlined
6
2
18 62
ꢁ
cules. The modied NCP was stable up to 500 C; however, the through UV-Vis spectroscopy. Therefore, 0.1 g of NCP was sus-
heteropolyacid structure started to degradation above this pended in 40 ml aqueous solution of HPA with the initial
temperature.
.1.4. FT-IR spectroscopy. The FT-IR spectroscopy was 70 C for different times ranging from 1 to 90 min. By
applied to characterize the composition of H P W O /NCP comparison with the standard solutions, the heteropolyacid
concentration of 2500 ppm. Then, the suspension was heated to
ꢁ
3
6
2
18 62
(
Fig. 5). The nanozeolite showed three kinds of bands related to: concentration had declined to 800 ppm aer 90 min. This
ꢀ
1
(a) the internal broad Si–O(Si) and Si–O(Al) in the range of 1200– amount corresponds to 0.0311 mmol g of H P W O loaded
6 2 18 62
ꢀ
1
7
00 cm , (b) zeolitic water molecules in the range of 2900–3500 on NCP. This result was conrmed by the amount obtained with
ꢀ
1
ꢀ1
cm , (c) pseudo-lattice vibrations of structural units in the MS-ICP which resulted in 0.0315 mmol g of HPA.
ꢀ
1
ꢀ1
range of 500–700 cm . A broad band about 3320 cm is
assigned to the symmetric and asymmetric stretching vibration
modes of the surface silanol groups and stretching vibration
modes of hydrogen-bonded water molecules adsorbed on the
surface of nanozeolite. The Si–O–Si asymmetric stretching
vibration overlapped with the stretching vibrations of Al–O–Si
ꢀ
1
and Al–O and produced a broad band about 1070 cm . The
Fig. 7 Time dependence UV-Vis spectral changes after addition of
6 2 18
H P W O62 to NCP.
Fig. 6 Finger printing FT-IR bands of Wells–Dawson H
6
2
P W
18
O
62
.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 21206–21214 | 21209

View Article Online
RSC Advances
Paper
6 2 18 62
Table 1 The calculated and experimental parameters obtained for different initial weights of H P W O
Initial weight
of HPA (g)
Amount of adsorbed
HPA (mmol g of NCP)
ꢀ1
ꢀ1
ꢀ1
Entry
Weight of adsorbent (g)
C
e
(g l
)
Q
e
(mg g
)
1
2
3
4
5
0.02
0.05
0.10
0.20
0.30
7.7
19.4
32.9
35.9
37.5
0.5
0.5
0.5
0.5
0.5
0.070
0.190
0.700
1.790
2.950
34
85
144
157
164
3
.1.6. Equilibrium studies. Different amounts of HPA were solute molecules on the adsorbent surface. The non-linear
added to 40 ml deionized water in six separate conical asks. To expression of Langmuir model is given by eqn (4):
each ask was added the adsorbent NCP (0.5 g) with strong
q ¼ abC /(1 + bC )
(4)
shaking. Then, the asks were placed for almost 4 h with
periodic shaking. Finally, the content of each ask was centri-
fuged and the separated solution was titrated against NaOH
e
e
wherein, constants “a” and “b” are the Langmuir constants
showing the capacity and energy of adsorption, respectively,
and can be determined from the non-linear tting. Therefore,
(
0.01 N) in the presence of phenolphthalein with taking three
readings in each case. The amount of the adsorbed HPA was
calculated using eqn (2).
“a” is the maximum amount of adsorbate per unit weight of
ꢀ
1
sorbent to form a complete monolayer on the surface (mg g
)
and corresponds to the free energy or net enthalpy of the
adsorption. Moreover, “b” is a constant related to the energy of
q
e
¼ (C
0
ꢀ C
e
)V/m
(2)
C
0
and C
polyacid, respectively. m is the amount of adsorbent (g) and V is composite. The applicability of Langmuir and Freundlich
the volume (lit) of the reaction mixture (Table 1).
adsorption isotherms in the interaction of HPA with NCP is
.1.7. Adsorption isotherms. The extent of HPA adsorbed determined based on the correlation coefficient (R ). The results
e
are initial and nal concentrations of the hetero- adsorption and indicates the stability of adsorbate–adsorbent
2
3
on NCP was calculated by the adsorption isotherm. Among summarized in Table 2 and Fig. 8, indicate that the reported
several familiar isotherm equations, Freundlich and Langmuir experimental data correlates better with the Langmuir isotherm.
isotherms are applied for this study and the experimental data
3.1.8. pHpzc of the modied and unmodied NCPs. The
were tted with non-linearly. The Langmuir isotherm theory pHPZC for an inorganic material is the value at which the surface
considers monolayer coverage of the adsorbate over a homog- has a net neutral charge. It encompasses positive charge in
enous adsorbent surface and hints surface homogeneity of the solution with pH < pHPZC; whereas, in a solution with pH >
adsorbent. Whereas, the Freundlich type adsorption isotherm is PHPZC the negative charge would be developed at the surface of
an indication of the surface heterogeneity of the adsorbent. The the mineral compound. These effects could be explained by
mass of HPA adsorbed on the solid adsorbent at various deprotonation or protonation of aluminol Al–OH and silanol
26
28
concentrations was calculated from the Freundlich eqn (3).
Si–OH groups in the framework of NCP. The pHpzc of the
modied and unmodied NCPs were analyzed to evaluate effect
(3) of HPA loading on the acidity of NCP. Higher pHpzc for the
1
/n
q ¼ KC
e
unmodied nanozeolite (pHpzc 6.6) compared with HPA/NCP
In which, K and n are Freundlich constants. Table 1 shows
calculated and experimental values of q and C obtained at
e
different HPA concentrations. K in this equation would be an
indication of the adsorption capacity and 1/n would show the
adsorption intensity. In general, as the K value increases, the
adsorption capacity of the used adsorbent for a given adsorbate
would be enhanced.
27
The Langmuir adsorption model
supposes that the
maximum adsorption corresponds to a saturated monolayer of
Table 2 Values of Langmuir and Freundlich constants and regression
coefficients
Freundlich constants
Langmuir constants
ꢀ1
ꢀ1
K (mg g
/n
R
)
8.184
0.397
0.872
a (mg g
)
200
0.00398
0.980
ꢀ
1
1
b (g
)
2
2
R
Fig. 8 The experimental and theoretical plots of HPA adsorption on NCP.
21210 | RSC Adv., 2015, 5, 21206–21214
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
Table 3 Physicochemical properties of NCP and HPA/NCP
Noteworthy, HPA/NCP containing a lower amount of HPA
(
7.7 mmol) was more effective than the situation utilizing 15 mg
Characteristic
PH in water
NCP
HPA/NCP
of the unmodied H P W O (entry 3). The HPA loading was
6
2
18 62
varied over the range 7.7–37.5 mmol HPA per g NCP. The
conversion increased with enhancing catalyst loading, which
was due to the proportional increase in the number of active
sites. However, beyond a catalyst loading of 32.9 mmol HPA per g
NCP, there was no signicant improvement in the nal yield%
and reaction time as the available active sites exceeded that
required; hence, all further experiments were carried out at
8.7
6.6
125.5
141.99
0.0268
2.353
8.1
6.0
91.597
91.32
0.0183
2.699
a
PHpzc
2
ꢀ1
ꢀ1
The BET specic surface area (m g
Micropore specic surface area (m g
Micropore volume (cm g
Average pore diameter of BJH desorption (nm)
)
2
)
3
ꢀ1
)
a
Point of zero charge.
32.9 mmol HPA per g NCP.
3
.3. Effect of catalyst amount on the cyclotrimerization
(
pHpzc 6), conrmed increment in the acidity of NCP aer
surface modication.
.1.9. Pore characteristics of NCP before and aer modi-
reaction
The catalytic efficiency of various amounts of the heterogeneous
HPA/NCP was investigated for the preparation of 1,3,5-triphe-
nylbenzene (Table 5). At rst, the nanosized HPA/NCP behaved
better than the bulk modied clinoptilolite (HPA/CP). When
3
cation with HPA. Nitrogen adsorption isotherm, BET method,
was applied to investigate changes in the specic surface area
2
ꢀ1
(
m g ) of NCP aer surface modication with HPA (Table 3).
Micropore volumes were calculated by the t-plot method. Pore
volumes were estimated from the amount of N adsorbed at
p/p
1
5 mg of the nanozeolite/HPA was employed, the desired
product was gained in 90% yield within 2 h; whereas, only 58%
yield was achieved during the same time with HPA/CP.
Increasing concentration of HPA/CP from 5 to 15 mg
enhanced yield% from 31 to 58% during the xed time of 2 h.
Noteworthy, no improvements were observed in yield% by
enhancing either the reaction temperature or prolonging time.
In order to evaluate the appropriate catalyst amount, a model
reaction was carried out by 5–20 mg of HPA/NCP. As is expected,
yield% was grown by enhancing catalyst concentration. More-
over, only a little amount of HPA/NCP exhibited high catalytic
activity in the desired transformation. Higher amounts of the
2
0
¼ 0–0.98. Barret Joyner Halenda (BJH) calculation was
employed to estimate the pore-size distribution for NCP and
HPA/NCP. Findings showed that the surface area of NCP was
decreased aer modication with HPA. Presumably, as HPA
adsorbs on the external surface of HPA and pore opening takes
place, the internal surface area of the NCP would be blocked
29
and the micropore surface area decreases.
3
.2. Optimization of the reaction conditions
The best reaction conditions were attained by studying catalyst (>20 mg) had not a signicant effect on the yield%.
parameters affecting efficacy of the catalytic system. At rst, a Therefore, 15 mg of catalyst was found to be sufficient to push
model reaction was conducted in the absence of catalyst which the reaction forward.
led to a very low yield (Table 4, entry 1). Then, effect of HPA
loading on NCP was studied. As depicted in Table 4, 94% yield 3.4. Surface modication of NCP by some heteropolyacids
was attained in the presence of 15 mg HPA/NCP; whereas, only
Since, almost all heteropolyacids have low charge density on the
surface and have no charge localization, the protons are very
mobile, thus giving raise to extremely high Brønsted acidity.
6 2 18
Nevertheless, catalytic activity of H P W O62 was compared
13% was gained with the unmodied NCP. The results clearly
approved the enforcing effect of HPA on the yield%. Effect of the
amount of HPA was studied on the catalyst efficacy. The best
yield achieved in the presence of 32.9–35.9 mmol HPA per g NCP.
Table 5 Effects of the size and amounts of the modified CP on the
yield of the cyclotrimerization of acetophenone
Table 4 Screening effect of H
activity of HPA/NCP
P
6 2
W
18
O62 loading on the catalytic
a
a
Yield (%)
mmol HPA
per g NCP Time (h) Yield (%) Entry
b
Entry Catalyst
Catalyst (mg)
Time (h)
HPA/CP
HPA/NCP
1
2
3
4
5
6
7
8
None
—
—
—
24
3
3
3
3
3
3
3
<3
13
81
68
79
94
96
96
1
2
3
4
5
6
7
—
5
24
2
2
1
2
—
31
40
35
58
59
59
—
51
75
59
90
94
91
NCP (nanoclinoptilolite)
HPA (H
6
P
2
W
18
O
62
)
10
15
15
15
20
HPA/NCP
HPA/NCP
HPA/NCP
HPA/NCP
HPA/NCP
7.7
19.4
32.9
35.9
37.5
3
2
a
ꢁ
Reaction conditions: acetophenone (1 mmol), at 100 C, solvent free.
a
b
Reaction conditions are described in the experimental section. 15 Pr ea sr pt iec cl te i sv e sl yi z. e of HPA/CP and HPA/NCP were ꢂ70 mm and <70 nm,
mg of each catalyst was used.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 21206–21214 | 21211

View Article Online
RSC Advances
Paper
Fig. 10 Yield % as a function of reaction temperature in the cyclo-
trimerization of acetophenone.
Fig. 9 Comparison of the catalytic activity of some heteropolyacids
loaded on NCP. HPA/NCP (including ꢂ35 mmol HPA/0.5 g NCP) was
prepared as illustrated in Section 2.3. 0.01 g of each nanohybrid
catalyst was used.
3
.6. Comparison of the catalytic efficiency of HPA/NCP with
some reported catalysts for cyclotrimerization of
acetophenone
Catalysts presented in Table 6 are Brønsted and/or Lewis acids
and facilitate activation of the oxygen carbonyl group via coor-
dination to the active site of acid catalyst. Moreover, hydrophilic
characteristics of the used catalysts would help in adsorption of
the generated water molecules and push the reaction forward.
Considering these features, superiority of the presented
heterogeneous catalyst was studied over some reported
methodologies. The reaction of acetophenone for the synthesis
of 1,3,5-triphenylbenzene was selected as a model reaction and
the comparison was in terms of mol% of the catalysts,
temperature, reaction time, and percentage yields. Obviously,
the present new catalyst was superior over the reported catalysts
considering the above variables. The present protocol utilized a
very low amount of catalyst under solvent free condition.
Obviously, the present protocol using a natural and simple
catalyst was better than most of the conventional catalysts
mentioned in Table 6.
with other indicative familiar heteropolyacids in the prepara-
tion of 1,3,5-triphenylbenzene. All the introduced hetero-
polyacids are strong acids and behaved as good catalysts in the
respective transformation (Fig. 9). However, Wells–Dawson poly
tungstic acid H P W O showed higher activity in the prepa-
ration of 1,3,5-triaryl benzene derivatives. Many factors such as
acidity of the heteropolyacid, negative charge density smeared
over oxygen atoms, structural composition and distortions, and
absorption of the substrate molecule into the bulk of hetero-
polyacid would contribute to the catalytic efficiency of the het-
eropolyacids under the reaction conditions.
6
2
18 62
3.5. Effect of reaction temperature on the cyclotrimerization
reaction catalyzed by HPA/NCP
Effect of temperature on the efficacy of the catalytic system was
studied for the preparation of 1,3,5-triphenylbenzene under the
optimized conditions (Fig. 10). The results showed that lower
temperatures disfavored the reaction and only 13% yield was
3.7. Synthesis of different 1,3,5-triarylbenzenes catalyzed by
ꢁ
HPA/NCP
achieved aer 180 min at room temperature; whereas, at 60 C
the reaction poorly proceeded and maximum conversion of 49% Encouraged by the results, the capability of the present one pot,
ꢁ
was reached aer 3 h. The best result was gained at 100 C and novel, and highly efficient methodology was assessed for the
ꢁ
caused 94% yield at the same time. The product yield at 120 C preparation of different 1,3,5-triphenylbenzene derivatives in
ꢁ
was slightly higher than that of 100 C and further elevating the presence of HPA/NCP as a green, heterogeneous, and
temperature did not improve conversion. Therefore, we kept the reusable natural catalyst under solvent-free conditions. For this
ꢁ
reaction temperature as 100 C.
purpose, a range of structurally diverse ketones such as methyl–
Table 6 Comparison of the catalytic activity of HPA/NCP with some reported catalysts towards cyclotrimerization of acetophenone
ꢁ
Entry
Catalyst
CAN
Catalyst (mol%)
Solvent
Time (h)
Temp. ( C)
Yield (%)
References
1
2
3
4
5
6
7
10
5
10
10
20
10
15 mg
Free
Free
Ethanol
Ethanol
Free
10
10
11
24
3
130
130
Reux
Reux
130
>10
38
45
55
60
62
94
7c
7c
7b
7b
6b
6b
CuCl
HCl
2
SnCl
4
PTSA
DBSA
HPA/NCP
Free
Free
6
3
150
100
This work
21212 | RSC Adv., 2015, 5, 21206–21214
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
a
Table 7 Synthesis of different 1,3,5-triarylbenzenes catalyzed by HPA/NCP
ꢁ
Entry
R
Time (h)
Yield (%)
Mp ( C)
Product
References
1
2
3
4
5
6
7
8
9
H
4-OMe
4-Br
4-NO
4-Me
4-OH
3
3
2.5
3
3
3.5
2.5
2.5
3.5
3.5
94
79
91
88
76
77
96
98
91
76
172–174
141–143
263–265
151–152
178–180
237–239
247–249
237–239
212–214
95–96
(Ph)
(4-OMePh)
(4-BrPh)
(4-NO Ph)
(4-MePh)
(4-OHPh)
(4-ClPh)
(4-FPh)
3
C
6
H
3
30
30
31
13a
30
—
30
31
13a
13a
3
C
H
6
H
3
3
C
6
3
2
2
3
C
6
H
3
3
C
6
H
3
3
6
C H
3
4-Cl
4-F
3
C
6
H
3
3
C
6
H
3
Cyclohexanone
Cyclopentanone
Dodecahydro-triphenylene
Trindane
1
0
a
ꢁ
Reaction conditions: substrate (1 mmol), 100 C, 15 mg of HPA/NCP, solvent free. The structures of the products were established from their
spectral properties and also by comparison with available literature data.
aryl ketones as well as cyclic ketones were condensed to furnish triarylbenzenes varied with respect to the position of the
the corresponding products in good yield. As shown in Table 7, substituents attached to acetophenone. The triple condensation
the acetophenones bearing electron-withdrawing substituents reaction led to poor yields when ortho-CH substituted aceto-
3
in the aromatic ring cyclotrimerized with higher rates and phenone was used. Although, some previous reports declared
afforded the desired products in 88–98% yields. In contrast, the no reaction with strong electron-withdrawing nitro group, the
6b,7c
acetophenones having electron-donating substituents such as present system provided 88% yield aer 3 h.
The obtained
CH and –OCH at the aromatic ring reacted sluggishly and yields were found to be reproducible within ꢃ3% variation.
3
3
produced the corresponding 1,3,5-triarylbenzene derivatives in
lower yields. In the case of acetophenones containing halogen
at para-position of the aromatic ring, the yield of the cyclo-
trimerized product was signicantly reduced, with decreasing
electronegativity of the halogen. Therefore, yields of the
3
.8. Studying reusability of HPA/NCP in the
cyclotrimerization of acetophenone
To investigate reusability of nanozeolite/HPA, it was easily
separated from the reaction mixture by centrifuge and washed
thoroughly with hot EtOH. Then, the catalyst was slowly dried in
ꢁ
air and then was activated in a vacuum oven at 70 C for 4 h.
Finally, the recycled catalyst was reused for another condensa-
tion reaction. Findings revealed the same catalytic activity such
as fresh catalyst, without any signicant loss of activity (Fig. 11).
Moreover, to ensure reproducibility of the transformation,
repeated typical experiments were carried out under identical
reaction conditions. The obtained yields were found to be
reproducible within ꢃ3% variation.
4. Conclusion
Immobilization of Wells–Dawson H P W O on the external
6
2
18 62
surface of the nanosized CP and generation of a new hetero-
geneous nanohybrid material, HPA/NCP, has been the target of
this research. Then, the prepared catalyst was fully character-
ized and applied in the fast synthesis of different 1,3,5-triaryl
Fig. 11 Reusability of HPA/NCP in the cyclotrimerization of
acetophenone.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 21206–21214 | 21213

View Article Online
RSC Advances
Paper
benzenes. This methodology offers the advantages of high yield,
short reaction times, simple work-up, and green reaction
conditions without formation of any by-products. Moreover, low
162, 380; (c) M. Akgul and A. Karabakan, Microporous
Mesoporous Mater., 2010, 131, 238; (d) M. Akgul, B. Ozyagcı
and A. Karabakan, J. Ind. Eng. Chem., 2013, 19, 240.
cost as well as promoting the target transformation with 13 (a) H. R. Safaei, M. Davoodi and M. Shekouhy, Synth.
appreciable yields are the main goals from both industrial and
economic point of view. These advantages, in general, highlight
this protocol as a useful and attractive methodology, among the
methods reported in the literature, for the rapid synthesis of
Commun., 2013, 43, 2178; (b) A. Kumar, M. Dixit,
S. P. Singh, A. Goel, R. Raghunandan and P. R. Maulik,
Tetrahedron Lett., 2009, 50, 4335; (c) X. Jing, F. Xu, Q. Zhu,
X. Ren, C. Yan, L. Wang and J. Wang, Synth. Commun.,
2005, 35, 3167.
1,3,5-triaryl benzenes. Finally, water was the only by-product in
the whole process, which added to its attractiveness.
14 G. P. Romanellia, D. M. Ruiza, H. P. Bideberripea,
J. C. Autinob, G. T. Baronetti and H. J. Thomasa, ARKIVOC,
2
007, 1, 1.
Acknowledgements
1
5 S. M. Baghbanian, N. Rezaei and H. Tashakkorian, Green
Partial nancial support from the Research Councils of Hakim
Chem., 2013, 15, 3446.
Sabzevari University is greatly appreciated.
16 P. C. C. Faria, J. J. M. Orfao and M. F. R. Pereira, Water Res.,
004, 38, 2043.
2
1
7 A. Nezamzadeh-Ejhieh and H. Zabihi-Mobarakeh, J. Ind. Eng.
Chem., 2013, 20, 1421.
References
1
2
3
M. J. Plater, M. Mckay and T. Jackson, J. Chem. Soc., Perkin 18 A. Alberti, On the crystal structure of the zeolite heulandite,
Trans. 1, 2000, 2695.
Tschermaks Mineral. Petrogr. Mitt., 1972, 18, 29.
T. Kadota, H. Kayeyama, F. Wakaya, K. Gamo and Y. Shirota, 19 (a) H. Lin, Q. Liu, Y. Dong, Y. Chen, H. Huo and S. Liu,
Chem. Lett., 2004, 33, 706.
R. Ghanbaripour, I.
J. Mater. Sci. Res., 2013, 2, 37; (b) O. Y. Saiapina,
V. M. Pyeshkova, O. O. Soldatkin, V. G. Melnik, B. A. Kurç,
A. Walcarius, S. V. Dzyadevych and N. Jaffrezic-Renault,
Mater. Sci. Eng., C, 2011, 31, 1490.
Mohammad
poor-Baltork,
M. Moghadam, A. R. Khosropour, S. Tangestaninejad and
V. Mirkhani, J. Iran. Chem. Soc., 2012, 9, 791.
4
5
S. Zhang, Z. Xue, Y. Gao, S. Mao and Y. Wang, Tetrahedron 20 (a) N. Mansouri, N. Rikhtegar, H. Ahmad Panahi, F. Atabi
Lett., 2012, 53, 2436.
(a) X. Y. Cao, X. H. Liu, X. H. Zhou, Y. Zhang, Y. Jiang, Y. Cao,
Y. X. Cui and J. Pei, J. Org. Chem., 2004, 69, 6050; (b)
and B. Karimi Shahraki, Environ. Prot. Eng., 2013, 39, 139;
(b) Z. Olejniczak, B. Sulikowski, A. Kubacka and M. Gasior,
Top. Catal., 2000, 391, 11.
E. J. Juarez-Perez, C. Vinas, F. Teixidor, R. Santillan, 21 (a) A. W. Burton, K. Ong, T. Rea and I. Y. Chan, Microporous
N. Farfan, A. Abreu, R. Yepez and R. Nunez,
Macromolecules, 2010, 43, 150.
Mesoporous Mater., 2009, 117, 75; (b) A. Nezamzadeh-Ejhieh
and M. Shahanshahi, J. Ind. Eng. Chem., 2013, 19, 2026.
6
7
(a) B. P. Dash, R. Satapathy, E. R. Gaillard, J. A. Maguire and 22 R. Tayebee, M. Jarrahi, B. Maleki, M. Kargar Razi,
N. S. Hosmane, J. Am. Chem. Soc., 2010, 132, 6578; (b)
D. Prasad, A. Preetam and M. Nath, C. R. Chimie, 2013, 16,
Z. B. Mokhtari and S. M. Baghbanian, RSC Adv., 2015, 5,
10869.
2
52.
23 A. Nezamzadeh-Ejhieh and N. Moazzeni, J. Ind. Eng. Chem.,
2013, 19, 1433.
(a) G. P. Zhang, R. H. Qiu, X. H. Xu, H. H. Zhou, Y. F. Kuang
and S. H. Chen, Synth. Commun., 2012, 42, 858; (b) 24 R. Tayebee, F. Nehzata, E. Rezaei-Sereshta, F. Z. Mohammadi
K. Phatangare, V. Padalkar, D. Mhatre, K. Patil and and E. Raee, J. Mol. Catal. A: Chem., 2011, 351, 154.
A. Chaskar, Synth. Commun., 2009, 39, 4117; (c) Y. Zhao, 25 R. Tayebee, M. M. Amini, H. Rostamian and A. Aliakbari,
J. Li, C. Li, K. Yin, D. Ye and X. Jia, Green Chem., 2010, 12, Dalton Trans., 2014, 43, 1550.
370; (d) F. Ono, Y. Ishikura, Y. Tada, M. Endo and T. Sato, 26 E. Korngold, N. Belayev and L. Aronov, Sep. Purif. Technol.,
1
Synlett, 2008, 15, 2365; (e) Z. Li, W. H. Sun, X. Jin and
2003, 33, 179.
C. Shao, Synlett, 2001, 1947; (f) N. Iranpoor and 27 J. Febrianto, A. Kosasih, J. Sunarso, Y. Ju, N. Indraswati and
B. Zeynizaded, Synlett, 1998, 1079. S. Ismadji, J. Hazard. Mater., 2009, 162, 616.
F. Marme, G. Coudurier and J. C. V ´e drine, Microporous 28 H. Guan, E. Bestland, C. Zhu, H. Zhu, D. Albertsdottir,
8
9
Mesoporous Mater., 1998, 22, 151.
M. E. Chimienti, L. R. Pizzio and C. V. C ´a ceres, Appl. Catal.,
A, 2001, 208, 7.
J. Hutson, C. T. Simmons, M. Ginic-Markovic, X. Tao and
A. V. Ellis, J. Hazard. Mater., 2010, 183, 616.
29 (a) R. Wei, J. Wang and G. Xiao, Catal. Lett., 2009, 127, 360;
(b) Z. Fumin, W. Jun, Y. Chaoshu and R. Xiaoqian, Sci. China,
Ser. B: Chem., 2006, 49, 140; (c) B. Sulikowski and
R. Rachwalik, Appl. Catal., A, 2003, 256, 173.
1
1
0 R. Tayebee and B. Maleki, J. Chem. Sci., 2013, 125, 335.
1 (a) R. Tayebee, M. M. Amini, M. Ghadamgahia and
M. Armaghan, J. Mol. Catal. A: Chem., 2013, 366, 266; (b)
R. Tayebee, M. M. Amini, F. Nehzata, O. Sadeghi and 30 R.
M. Armaghan, J. Mol. Catal. A: Chem., 2013, 366, 140.
Ghanbaripour,
I.
Mohammadpoor-Baltork,
M. Moghadam, A. Khosropour, S. Tangestaninejad and
12 (a) M. Akgul, A. Karabakan, O. Acar and Y. Yurum,
V. Mirkhani, Polyhedron, 2012, 31, 721.
Microporous Mesoporous Mater., 2006, 94, 99; (b) 31 Q. Gao, F. Bao, X. Feng, Y. Pan, H. Wang and D. Li, ARKIVOC,
G. S. P. Soylu, Z. Ozcelik and I. Boz, Chem. Eng. J., 2010, 2013, iii, 49.
21214 | RSC Adv., 2015, 5, 21206–21214
This journal is © The Royal Society of Chemistry 2015