Catalyzed Claisen-Schmidt reaction by protonated aluminate mesoporous silica nanomaterial focused on the (E)-chalcone synthesis as a biologically active compound

Article abstract of DOI:10.1039/c5ra23435b

The mesoporous silica structure (MSN) was synthesized using the sol-gel method followed by aluminum grafting and protonation and was then denoted as HAlMSN (Si/Al = 18.9). N₂ physisorption confirmed the mesoporous structure with a pore diameter of 3.38 nm. ²⁷Al NMR showed the presence of framework and extra-framework aluminum structures, which led to the formation of strong Lewis and Br?nsted acidic sites. HAlMSN catalyzed the synthesis of (E)-chalcones through the Claisen-Schmidt reaction. Chalcone derivatives have been applied as biologically active compounds with anti-cancer, anti-inflammatory and diuretic pharmacological activities. The products were obtained via reactions on the protonic acid sites of HAlMSN. The significant advantages of this reaction are high yield, easy work up, short reaction time and also compatibility with various organic solvents. The products were obtained in an excellent conversion of 97% at 298 K. The results show that the electron donating substituents exhibit higher conversion in comparison to electron withdrawing substituents. The stability of the catalyst was investigated by reusing it five times for (E)-chalcone production and there was only a slight decrease in its catalytic activity. The highest product of (E)-chalcone was observed with a 1:2 molar ratio of benzaldehyde/acetophenone. A comparative study in chalcone synthesis using the heterogeneous catalysts demonstrated that HAlMSN has a significantly high activity at low temperature.

Full text of DOI:10.1039/c5ra23435b

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PAPER
Catalyzed Claisen–Schmidt reaction by protonated
aluminate mesoporous silica nanomaterial focused
on the (E)-chalcone synthesis as a biologically
active compound
Cite this: RSC Adv., 2016, 6, 11023
ab
c
a
Mohammad Reza Sazegar,* Shaya Mahmoudian, Ali Mahmoudi,
bd
e
f
e
Sugeng Triwahyono, Aishah Abdul Jalil, Rino R. Mukti, Nur H. Nazirah Kamarudin
c
and Monir K. Ghoreishi
The mesoporous silica structure (MSN) was synthesized using the sol–gel method followed by aluminum
grafting and protonation and was then denoted as HAlMSN (Si/Al ¼ 18.9). N physisorption confirmed
2
27
the mesoporous structure with a pore diameter of 3.38 nm. Al NMR showed the presence of
framework and extra-framework aluminum structures, which led to the formation of strong Lewis and
Brønsted acidic sites. HAlMSN catalyzed the synthesis of (E)-chalcones through the Claisen–Schmidt
reaction. Chalcone derivatives have been applied as biologically active compounds with anti-cancer,
anti-inflammatory and diuretic pharmacological activities. The products were obtained via reactions on
the protonic acid sites of HAlMSN. The significant advantages of this reaction are high yield, easy work
up, short reaction time and also compatibility with various organic solvents. The products were obtained
in an excellent conversion of 97% at 298 K. The results show that the electron donating substituents
exhibit higher conversion in comparison to electron withdrawing substituents. The stability of the
catalyst was investigated by reusing it five times for (E)-chalcone production and there was only a slight
decrease in its catalytic activity. The highest product of (E)-chalcone was observed with a 1 : 2 molar
Received 6th November 2015
Accepted 5th January 2016
DOI: 10.1039/c5ra23435b
ratio of benzaldehyde/acetophenone.
A comparative study in chalcone synthesis using the
www.rsc.org/advances
heterogeneous catalysts demonstrated that HAlMSN has a significantly high activity at low temperature.
acid sites plays a major role in their catalytic activities and
Introduction
since the pure MSN does not have Lewis and Brønsted acid
3
The hierarchically mesoporous siliceous nanostructures (MSN) sites in its structure, it cannot be used as a strong acid catalyst.
with specic properties such as high internal surface areas In order to improve its acid sites and subsequently modify its
2
ꢀ1
(
500–1000 m g ), large uniform pore dimensions (2–50 nm) catalytic activity, incorporation of some metals such as
4 1 5
and large pore volumes have received signicant attention aluminum, nickel and platinum into the MSN framework
from researchers. Their properties make them suitable for has been reported. These metals are effective for creating
various applications in the different elds of science: e.g. drug Brønsted acid sites in the structures.
1
3,6,7
However, this method
delivery, environmental renery, petroleum science and is limited to making Brønsted acid sites of moderate strength.
2
chemical reactions. In these types of catalysts, the presence of The pure MCM-41 is a mesoporous silica material that is widely
used in various applications as a catalyst. Introduction of
aluminum into MCM-41 increases its catalytic activity due to
a
Dept. of Chemistry, Fac. of Science, North Tehran Branch, Islamic Azad University, the generation of Lewis and Brønsted acid sites in the struc-
8
9
Tehran, Iran. E-mail: m_r_sazegar@yahoo.com; Tel: +98-2122262564
b
ture, which results in its being used as a solid acid catalyst. In
Dept. of Chemistry, Fac. of Science, Universiti Teknologi Malaysia, 81310 UTM Johor
addition, some transition metals like Pt, Zr, Ni, Ir and Zn are
Bahru, Johor, Malaysia
4,9–13
able to improve the activity of the nanomaterials.
The
c
Dept. of Polymer, Fac. of Engineering, Kashan Branch, Islamic Azad University,
mesoporous silica nanoparticles are capable of catalyzing the
Kashan, Iran
d
9
synthesis of some chemical compounds, especially in the
Ibnu Sina Institute for Fundamental Science Studies, Fac. of Science, Universiti
4,12
pharmaceutical industries and medicinal chemistry. There-
Teknologi Malaysia, 81310 U TM Johor Bahru, Johor, Malaysia
e
Institute Hydrogen Economy, Dept. of Chemical Eng., Fac. of Chemical Eng., Universiti fore, Al-graed MCM-41 can be used for the synthesis of
Teknologi Malaysia, 81310 UTM Johor Bahru, Johor, Malaysia
(E)-chalcone derivatives, which have been used in the phar-
maceutical science industries.
f
14
Division of Inorganic and Physical Chemistry, Fac. of Mathematics and Natural
Sciences, Institut Teknologi Bandung, JlGanesha No 10, Bandung, 40132 Indonesia
This journal is © The Royal Society of Chemistry 2016
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The (E)-chalcone family ((E)-1,3-diaryl-2-propenone) is
In the present study, we synthesized protonated aluminate
a subgroup of the avonoids, which has been displayed as mesoporous silica nanoparticles (HAlMSN) as a solid acid
biologically active compounds. These materials could act as catalyst and used it in order to improve the conditions of the
anti-cancer, anti-mitotic, anti-inammatory agents and cause Claisen–Schmidt condensation reaction. (E)-Chalcone deriva-
14–16
tyrosinase inhibition, and nitric oxide inhibition.
In addi- tives as a biologically active family were selected due to their
tion, chalcone is a starting material for the synthesis of several importance in the pharmaceutical science. Initially, MSN was
biologically active heterocycles such as benzothiazepines, pyr- prepared through the sol–gel process, then, HAlMSN solid
17
azolines, and avones. Therefore, this family has been utilized catalyst was prepared through aluminum loading on MSN by
by researchers to design new derivatives or to obtain improve- the post-synthesis method followed by protonation. FTIR
ment of the yields of these products. Attempts are being made spectroscopy of adsorbed pyridine was used to investigate the
in order to nd new pathways for synthesizing these acid strength of this catalyst. Physical properties of the samples
2
7
compounds in easier and faster ways.
were determined by nitrogen physisorption, XRD, FTIR and Al
The Claisen–Schmidt condensation reaction is the best solid state NMR. The mechanism of (E)-chalcone synthesis in
method for synthesizing chalcone derivatives by using strong the Claisen–Schmidt condensation over the HAlMSN solid acid
bases such as sodium and potassium hydroxide, or acids such catalyst under solvent-free conditions was studied in detail. The
18
as dry HCl, and aluminumchloride. Some of the organome- advantages of using the HAlMSN catalyst, such as high activity,
2,19
tallic catalysts such as Zn(bpy)(OAc) have also been reported short time and low temperature in this reaction, were studied in
as catalysts for this reaction. detail and a comparison of its activity with the commercial and
Moreover, chalcone can be synthesized by using the Suzuki the different heterogeneous catalysts was carried out.
20
reaction. Although there are several pathways to prepare the
chalcones, various disadvantages have been reported for these
methods such as long reaction times (14 h to 5 days), high
Experimental
temperature, expensive catalysts and reagents, production of Catalyst synthesis
poisonous substances, use of hazardous solvents, production of
The synthesis of pure MSN followed the procedure of the
several by-products and the neutralization process. Therefore,
several studies were carried out to replace the conventional and
commercial catalysts with heterogeneous materials. These
catalysts led to reactions with mild conditions and more
advantages such as saving time and energy and environmental
friendliness. Mesoporous silica nanomaterials such as MCM-41
and SBA-15 have been introduced as suitable catalysts for the
3
previous
report
and is
described
below.
Cetyl-
trimethylammonium bromide (CTAB, 1.17 g) was dissolved in
a solution containing double distilled water (180 g) and 1,2-
propanediol (30 mL) in an aqueous ammonia solution (7.2 mL,
25%). Aer vigorous stirring for approximately 30 min at 323 K,
tetraethylorthosilicate (TEOS, 1.43 mL) and 3-aminopropyl
triethoxysilane (APTES, 0.263 mL) were added to the mixture.
The resulting mixture was stirred for an additional 2 h at 323 K
and allowed to rest for 20 h at the same temperature. The gel
composition used in the synthesis of MSN was 1 TEOS : 0.17
APTES : 0.5 CTAB : 13.5 NH : 85.2 PD : 1042 H O. The sample
3 2
was collected by centrifugation at 20 000 rpm for 30 min and
washed with deionized water and absolute ethanol three times.
21,22
Claisen–Schmidt condensation reaction,
which can be
carried out with or without presence of the solvent. Recently, the
Claisen–Schmidt reactions carried out under solvent-free
conditions by using mesoporous silica materials were re-
ported as a green chemistry alternative in order to reduce the
environmental pollution and bring down the handling costs
21
due to the simplication of the work up technique.
The surfactant was removed by heating MSN (1 g) in an NH
4 3
NO
22
In a recent study, Romanelli et al. synthesized amino-
propylated nanosilica by a sol–gel process using tetraethyl
orthosilicate (TEOS) as a source of Si, followed by functionali-
zation with different amounts of 3-aminopropyltriethoxysilane
(0.3 g) and ethanol (40 mL) solution at 333 K. The surfactant-
free product was collected by centrifugation and dried at 383
K overnight prior to calcination in air at 823 K for 3 h. The acidic
sites of the sample were prepared by aluminum-graing
(APTES) under toluene reux. These modied nanoparticles
(
1
calculated Si/Al ¼ 20) on the template-free MSN at 353 K for
0 h, followed by centrifugation and drying at 383 K overnight,
prior to calcination in air at 823 K for 3 h. Sodium aluminate
Sigma-Adrich) was used as a precursor of aluminum. The
catalyzed the synthesis of chalcones through the Claisen–
Schmidt reaction under solvent-free conditions with high
selectivity. These results show that the activity of the catalyst
(
23
was increased over amino modied silica. Wang et al. studied
the Claisen–Schmidt condensation between benzaldehyde and
acetophenone, and subsequently, intramolecular over the
aminopropyl-functionalized SBA-15 with well-ordered hexag-
onal structure. This catalyst was synthesized by the one-pot
condensation of TEOS and APTES using amphiphilic block
copolymer as the template under acidic conditions. The results
exhibited a good activity and high selectivity for chalcone
synthesis over this catalyst in the solvent-free conditions, while
aluminum-graed MSN was denoted AlMSN. Whereas, the
protonated AlMSN (HAlMSN) was prepared by protonation of
AlMSN (1 g) using an aqueous solution of NH NO (2.5 g in
4
3
5
0 mL of double distilled water) at 333 K for 16 h, followed by
removal of solution, drying at 383 K overnight and calcination at
23 K for 3 h in air.
8
Catalyst characterization
the use of organic solvents decreased the catalytic activity and The crystallinity of the catalysts was measured with a Bruker
selectivity.
Advance D8 X-ray powder diffractometer with Cu Ka (l ¼ 1.5418
11024 | RSC Adv., 2016, 6, 11023–11031
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A˚ ) radiation as the diffracted monochromatic beam at 40 kV and
The conversion of ketone to chalcone (Xchalcone%), the yield
40 mA. Nitrogen physisorption analysis was conducted on of chalcone (Ychalcone%) and the selectivity of chalcone
a Quantachrome Autosorb-1 at 77 K. Before the measurement, (Schalcone%) were calculated according to eqn (1)–(3),
the sample was evacuated at 573 K for 3 h. Elemental analysis of respectively:
the catalyst was carried out with an Agilent 4100 Microwave
Plasma-Atomic Emission Spectrometer. Al MAS NMR spectra
were recorded on a Bruker Avance 400 MHz spectrometer at
½
KETꢁ ꢀ ½KETꢁ
2
7
i
f
X
chalconeð%Þ ¼
ꢂ 100
(1)
(2)
(3)
½
KETꢁ
f
104.3 MHz with 0.3 s recycle delays, and spun at 7 kHz to
ny
Y
chalconeð%Þ ¼ nc ꢂ 100
determine the chemical status of aluminum in the silicate
framework of the catalysts.
n
y
Fourier Transform Infra-Red (FTIR) measurements were
carried out using an Agilent Cary 640 FTIR Spectrometer. The
S
chalconeð%Þ ¼ n
ꢂ 100
f
i
ꢀ n
catalyst was prepared as a self-supported wafer and activated where [KET] and [KET] are the concentrations of ketone at the
i
f
ꢀ1
under H
2
stream (FH2 ¼ 100 mL min ) at 623 K for 3 h, followed initial and nal reaction times, respectively; n and n are the
y
c
24
by vacuum conditions at 623 K for 1 h. To determine the number of moles of the yielded and calculated (E)-chalcones,
catalyst acidity, the activated samples were exposed to 2 Torr of respectively; and n and n are the number of moles of ketone at
i
f
pyridine at 423 K for 30 min, followed by evacuation at 473 K for the initial and nal reaction times, respectively.
h to remove the physisorbed pyridine. All spectra were
1
recorded at room temperature. In order to compare the surface
coverage of the adsorbed species between different wafer
thicknesses, all spectra were normalized using the overtone and
combination vibrations of the MSN lattice between 2200 and
Reusability testing
The reusability of the HAlMSN catalyst for the (E)-chalcone
production (%) at 298 K was tested aer it was used ve times.
27
ꢀ
1
The catalyst was washed with hot toluene (3 ꢂ 10 mL) and dried
under vacuum at 343 K for 4 h. The weight of the catalyst, its
catalytic activity and the product were measured aer each
reaction. Aer reusability testing, the catalyst was subjected to
XRD and FTIR in order to study its properties.
1
cm
300 cm aer activation, particularly the lattice peaks at 1855
ꢀ
1
25
.
The bulk Si/Al ratio of HAlMSN was determined by using
a Bruker S4 Explorer X-ray Fluorescence (XRF) Spectrometer,
using Rh as the anode target material, operated at 20 mA and 50
kV. The analysis showed that the Si/Al ratio of the HAlMSN
framework was 18.9.
Results and discussion
Catalyst characterization
The concentration of pyridine adsorbed on Brønsted and
26
Lewis acid sites was determined based on the report of Emeis.
The number of Brønsted and Lewis acid sites was calculated Fig. 1 shows the XRD patterns of MSN and HAlMSN. These
ꢃ
ꢃ
ꢃ
using the integrated molar adsorption coefficient values. The catalysts have presented three peaks at 2.30 , 4.20 and 4.70 ,
ꢀ
1
resulting values were 3
1
¼ 1.67 cm mmol for the band at corresponding to 100, 110 and 200, respectively. The sharp
1
545
ꢀ1
ꢃ
545 cm , which is characteristic of pyridine on Brønsted acid peaks at 2.30 reveal the high order crystalline structures of the
ꢀ
1
ꢀ1
sites and 31455 ¼ 2.22 cm mmol for the 1455 cm band of catalysts and conrm the two-dimensional hexagonal (p6mm)
pyridine on Lewis acid sites.
structure with d100-spacing of approximately 3.8 nm and an
average lattice constant (a ) of 4.4 nm (Table 1).
In addition, this peak illustrates the long ordered structure
0
Catalytic activity
28
ꢃ
in MSN. The intense peak at 2.30 decreased in HAlMSN and
The catalytic activity of the HAlMSN catalyst was investigated
using the Claisen–Schmidt condensation reaction. The details
of this study are explained in the following section.
Typical experimental protocol
To a stirring mixture of aldehyde (0.02 mol), ketone (0.04 mol)
and dichloromethane (0.15 mol), HAlMSN (0.01 wt%) was
added at room temperature and the stirring was continued for
30 min. The mixture was heated at 323 K under an air atmo-
sphere for 3 h (Table 2). The samples were taken at regular
intervals and the concentrations of ketones (acetophenone
derivatives) and chalcones were identied by GC-Mass spec-
trometry (detector HP 5971) and analysed using chrompak CP
9000 GC equipped with an RTX-50 capillary column and a ame
ionization detector. The dodecane was chosen for the internal
standard.
Fig. 1 XRD patterns of MSN and HAlMSN.
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a
Table 1 Physical properties of MSN and HAlMSN
d
(nm)
100
a
0
S
V
p
W
(nm)
t
2
ꢀ1
3
ꢀ1
Sample
(nm)
(m g
)
(cm g
)
(nm)
MSN
HAlMSN
3.84
3.76
4.43
4.34
995
639
0.84
0.54
3.38
3.38
1.06
0.96
a
d
100, d-value of reection 100; a
0
, pore center distance is equal to
pffiffiffi
2
ꢀ1
d100 ꢂ 2= 3; S, BET surface area (m
g ) obtained from N
2
1
ꢀ
p
adsorption; V , total pore volume (mL g ); W, pore size (nm)
obtained from the BJH method; t, pore-wall thickness is equal to a
0
ꢀ W.
Table 2 Synthesis of (E)-chalcones (3a–h) using the HAlMSN solid
acid catalyst
a
Fig. 2 (A) Nitrogen sorption isotherm and (B) pore size distributions
for MSN and HAlMSN; (C) Al MAS NMR spectrum of HAlMSN.
27
Time Conv. Select. E-isomer Yield
0
Entry
R
R
(h)
(%)
(%)
(%)
Ref.
Fig. 2 shows the nitrogen sorption isotherms and the pore
size distribution of MSN and HAlMSN. Fig. 2A shows that the
isotherm representing MSN can be classied as type IV with
a type H4 hysteresis loop, which can be attributed to the mes-
oporous structures. The MSN isotherm illustrates the properties
of an intense inection of the capillary condensation within
a
1
H
H
H
H
4-Cl
2-OH
3
3
3
3
3
3
3
3
3
3
97
94
96
96
98
97
98
97
96
95
94
87
90
95
94
91
90
88
91
92
91
82
38
24
b
2
c
3
86.5 24
d
4
4-OMe 4-OMe
4-OH
H
4-OMe
H
4-OH
4-Me
91
92
88
88
37
37
24
39
e
5
4-OCH
4-OCH
H
2,6-Cl
H
3
f
6
3
i
7
k
uniform pores at P/P
MSN of small pore diameter and volume. At P/P
increase in adsorbed nitrogen illustrates an important external
surface area contribution. At relative pressure of P/P less than
0
¼ 0.2–0.4, which indicates the presence of
8
85.4 37
87.4 39
87.4 37
30
g
¼ 0.8–1.0 the
9
0
h
1
0
3-Cl
a
0
a–h are the various (E)-chalcones with the various substitutions of R
and R .
0
0.2, gradual and rounded transitions showed the presence of
a small number of non-slit-like micropores, whereas the pore
size distribution conrmed the presence of a narrow peak at
a pore diameter of 2.5–2.8 nm, Fig. 2B. In HAlMSN, the inec-
ꢃ
shied to 2.25 , which indicated the loading of Al into the tion of the isotherm at P/P0 ¼ 0.8–1.0 increased gradually,
HAlMSN framework and led to an increase in the interplanar indicating the presence of extra-framework Al that contributed
spacing of MSN. This shiing of the intense peak is probably to the external surface area, and the inection at P/P0 < 0.2
due to the presence of a residual sodium atom that interacted decreased due to the presence of Al species inside the micro-
7
with the Si atom through the O atom. A decrease in the porous MSN.
intensity of the peak in HAlMSN indicates a less uniform crystal
structure compared to MSN.
In agreement with our results, On et al. immobilized Al into
an MCM-41 mesoporous framework with the Si/Al of 33. The
31
Table 1 shows the physical characteristics of the MSN and X-ray diffraction result showed that the ordered hexagonal
HAlMSN catalysts. The surface areas of MSN and HAlMSN are structure of the parent MCM-41 did not change and exhibited
2
ꢀ1
9
95 and 639 m g , respectively. The change in the BET specic a d-spacing of ca. 40 and 41 A˚ for MCM-41 and Al-MCM-41,
surface area is probably due to the alteration in the pore size respectively. Both samples represented the narrow pore size
distributions and the change in the two dimensional hexagonal distributions of around 28 A˚ with the surface areas of 1050 and
2
ꢀ1
structure of MSN. The graing of MSN with sodium aluminate 815 m g for MCM-41 and Al-MCM41, respectively.
2
7
has plugged MSN at a pore diameter of around 2–5 nm (Fig. 2).
Fig. 2C shows the peaks of the Al MAS NMR spectrum of
However, the protonation of AlMSN removed this plugging the HAlMSN solid catalyst. Here, no peak was observed for MSN,
from MSN, therefore, their pore sizes remained in a constant while the peaks at approximately 53 and 0 ppm were observed
value. This result is also in good agreement with the alteration for HAlMSN, which are responsible for the octahedral and
32
of total pore volume and pore-wall thickness of MSN obtained tetrahedral Al atoms, respectively. The peaks of the 6-coordi-
from the BJH method analysis (Table 1).
nate structure (extra-framework) illustrate that a small part of
The plugging of MSN decreased the total pore volume from the aluminum was dislodged into the aluminosilicate frame-
3
ꢀ1
33,34
0
.84 to 0.54 cm g , whereas, the pore-wall thickness shrank work.
Most of the Al species remained in the 4-coordinate
from 1.06 to 0.96 nm due to the widening of the pore size of MSN. structure (framework), which is promising for the catalytic
These revealed that Al and residual Na were located inside and reactions. Chen et al. reported the synthesis of Al-MCM-41 by
outside of the pores in HAlMSN. Decreases in the d100-spacing the loading method, in the various Si/Al ratios and showed two
33
value and unit cell parameter indicate the immobilization of Al types of Al structure in the MCM-41 frameworks.
in the framework and the changing of the HAlMSN structure.
29
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The concentrations of Brønsted and Lewis acid sites for
ꢀ1
HAlMSN were calculated to be 96 mmol g for the Brønsted
ꢀ
1
3
sites and 189 mmol g for Lewis acid sites.
These values were measured in terms of amounts of the
pyridine ion for the Brønsted sites with corresponding band at
ꢀ
1
1
1
545 cm , and for Lewis acid sites with corresponding band at
ꢀ
1
26
455 cm
.
The morphology of the HAlMSN was observed using FESEM
and TEM images. Fig. 4 shows the HAlMSN as a uniform
spherical particle with particle size of 70–120 nm. The 2D
hexagonal mesostructure was conrmed by TEM images. These
images clearly show well-ordered pores with parallel and
cylindrical channels and honeycomb structures, which indicate
38
a 2D hexagonal p6mm mesostructure for HAlMSN. These
results are in accordance with those measured from the low-
Fig. 3 FTIR spectra of MSN and HAlMSN (A) in the region of 3800–
ꢀ
1
ꢀ
1
3
500 cm , (B) in the region of 2200–1300 cm , (C) pyridine
adsorbed FTIR.
2
angle XRD patterns and N sorption analysis.
Catalytic activity
The FTIR and pyridine adsorbed FTIR spectra of MSN and
HAlMSN are shown in Fig. 3. The pure MSN catalyst did not
show the Brønsted acid sites due to its electrically neutral
properties. The non-acidic silanol groups of MSN observed in
Chalcones are usually synthesized by using the Claisen–
Schmidt condensation reaction under basic or acidic condi-
tions in the presence of a polar solvent. This reaction involves
a difficult purication process as the condensation reaction
ꢀ1
the range of 3800–3500 cm are shown in Fig. 3A. The sharp
39,40
oen leads to a complex mixture.
HAlMSN catalysed the
ꢀ1
band at 3740 cm is attributed to the non-acidic terminal
(E)-chalcones (3a–h) synthesis, which was carried out with
35
silanol groups (^SiOH) placed on the external surface of MSN.
acetophenone (1) and benzaldehydes derivatives (2) under
solvent-free conditions at low temperatures, with excellent yield
and selectivity, without the generation of any side product and
in a short time (Scheme 1). In these reactions the (Z) isomers
were synthesized as by-products in small amounts. These one-
pot reactions occurred by focusing on the optimization of the
reaction conditions in terms of the amounts of products,
temperature and reaction time.
To compare the results of the reaction in the presence and
absence of the catalyst, the reaction was also carried out
through the traditional method without catalyst, at 323 K. No
improvement in the reaction yield was observed. The use of
Introduction of Al into the mesoporous silica decreased H-
bonds of the terminal hydroxyl groups, and the structural
ꢀ
1
ꢀ1
defects. The intense band at 3740 cm shied to 3745 cm
aer protonation, indicating the production of geminal silanol
12
groups due to the dislodging of Al from the framework. There
are no signicant differences in the band intensities in the
ꢀ
1
region of 3740 cm for MSN and HAlMSN, which illustrates
that they have a comparable primary particle size and have no
signicant differences in the d100-spacing or pore diameters.
Fig. 3B shows the vibrational lattice stretching frequency of the
ꢀ
1
mesoporous silica catalysts in the region of 2200–1300 cm
.
ꢀ
1
The bands observed at 1855 and 1640 cm probably corre-
spond to Si–O–Si bonds. Aluminum loading and protonation
ꢀ
1
intensied the band at 1640 cm , which may correspond to an
increase in the concentration of Si atoms connected to Al or H
atoms through O atoms.
The number and strength of the catalyst acid sites was
evaluated using pyridine adsorptions monitored by IR spec-
troscopy. IR spectra of the adsorbed pyridine on MSN and
ꢀ
1
HAlMSN in the region of 1600–1400 cm are shown in Fig. 3C.
ꢀ
1
The presence of absorbance bands at 1455 and 1545 cm are
indicative of the Lewis and Brønsted acid sites, respectively, Fig. 4 FESEM (A) and TEM (B) images of HAlMSN.
ꢀ
1
while the absorbance band at around 1495 cm is attributed to
35,36
the combination of the Lewis and Brønsted acid sites.
The pure MSN possesses only Lewis acid sites at 1455 cm
ꢀ
1
,
which belong to the electron pair acceptor sites of silanol
groups. Incorporation of Al increased Lewis acidity and
protonation of Al-graed MSN generated strong Brønsted acid
ꢀ
1
sites at 1545 cm , which probably correspond to the forma-
tion of acidic hydroxyl groups (OH groups that bind Si or Al
in the framework) and/or bridging hydroxyl (Si(OH)Al)
3
6,37
groups.
Scheme 1 Reagents and conditions for the synthesis of (E)-chalcones.
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at 343 K for 4 h. The catalyst was reused ve times without
a serious loss in the catalyst weight, catalytic activity and
product selectivity during each recycling process.
A decrease in the conversion of the substrate to (E)-chalcone
was not noticeable (there was a reduction from 97 to 94%),
which indicates the good reusability of this catalyst under these
41
reaction conditions.
The XRD results (not shown) of HAlMSN before and aer the
Claisen–Schmidt reactions indicate that there was no signi-
cant change aer the reaction. However, the presence of two
Scheme 2 Proposed mechanism of the (E)-chalcone synthesis over
HAlMSN.
ꢀ
1
weak bands observed at 2860 and 2930 cm in the IR spectra,
correspond to the C–H stretching vibrations, which may be
related to the plugging of the unreacted acetophenone and/or
chalcone molecules remaining on the pores. Removal of the
plug by washing with toluene reduced these two bands at 2860
high polarity solvents such as light alcohols gave a small yield,
23
which was probably due to the solvent effect.
The HAlMSN solid acid catalyzed the synthesis of (E)-chal-
cone derivatives. The selective production of (E)-1,3-diphenyl-
ꢀ1
and 2930 cm to the initial values. Fig. 6 shows the FTIR
spectra of the fresh HAlMSN, aer the Claisen–Schmidt reac-
tion at 473 K for 3 h and aer washing with toluene at the end of
the nal step.
2
-propenone derivatives was smooth and high yields were
obtained. The proposed mechanism described the formation
of an enolate intermediate through the bond formation
between carbonyl groups and oxygen atoms of Al–O bands in
HAlMSN (Scheme 2).
Table 3 shows the comparison study of the (E)-chalcone
41–45
synthesis over several types of heterogeneous catalysts.
The
This mechanism suggests that the bond formation occurs by
3
+
results indicate higher activity of HAlMSN in (E)-chalcone
synthesis, in comparison with the other catalysts such as
traditional catalysts, pure MSN, ZSM-5, SBA-15, MCM-41 and
MgO. This is probably due to the presence of stronger Lewis and
Brønsted acid sites in HAlMSN. The larger pore size and surface
area of this catalyst also provide enough space for reaction of
the reactants with the active sites inside the catalyst wall. The
results in Table 3 show that HAlMSN exhibits more activity in
comparison with the mentioned catalysts, based on shorter
reaction time (3 h), lower temperature and higher yield of (E)-
chalcone.
the reaction of Al ion species with the oxygen of the aryl
ketone carbonyl group, in order to generate the enolate inter-
mediate by the abstraction of a proton from the a-carbon of the
aryl ketone. The reaction proceeds through the further coordi-
nation with the aryl aldehyde, increasing the electrophilicity of
the aldehyde carbonyl group, thus making it susceptible to
intramolecular nucleophilic attack by the enolate ion. A further
dehydration process leads to the resultant products (3a–h). The
catalyst was recovered and can be recycled ve times without
appreciable loss of reactivity.
In Fig. 7A the conversion of the reaction with the different
substitutes (3) versus reaction time are presented. The results
show that for the (E)-chalcones with electron donating
Reuse of the catalyst
In order to investigate the reusable properties of the HAlMSN
catalyst, a recycling experiment was conducted and the result is
shown in Fig. 5. Aer reaction, the catalyst was recycled by
washing with hot toluene (3 ꢂ 10 mL) and drying under vacuum
Fig. 6 FTIR spectra of (a) fresh, (b) used and (c) washed HAlMSN in
Fig. 5 Stability of HAlMSN in the Claisen–Schmidt reaction at 298 K, the C–H stretching regions. Chalcone production was done at 473 K
after it was reused five times. for 3 h.
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Table 3 Comparison of (E)-chalcone yield over HAlMSN and the other consumed, while benzaldehyde remained in the container. The
homogeneous and heterogeneous catalysts
results conrm that the amount of the product depends on the
a
amount of acetophenone as the limiting reagent.
Conv. Yield Select. Time Temp.
In the (E)-chalcone synthesis, the turnover frequency (TOF) is
the number of moles of (E)-chalcone that a mole of HAlMSN can
produce per unit time, before becoming inactive. Here, TOF was
determined from the rate of (E)-chalcone synthesis and the
number of Brønsted acid sites. The TOF of (E)-chalcone
synthesis over HAlMSN is shown in Fig. 8. Although the TOF
increased with the reaction time, it became constant aer 180
minutes, which indicates that the highest yield of (E)-chalcone
was obtained aer 3 h.
Catalyst
Blank
(%)
(%)
(%)
(h)
(K)
Ref.
13.1 12.9 98.5
6
6
24
3
6
6
6
6
6
6
6
6
18
423
423
393
298
423
413
413
423
423
423
423
423
523
43
41
44
H
2
SO
4
98
—
97
71.2 72.7
62.2
92
NaHCO
3
—
95
68.2
—
—
HAlMSN
MSN
This study
This study
10.5 7.2
2.6
66.4 34.1
9.7 6.2
2
SiO (230–280 mesh)
6
45
45
41
41
41
41
41
42
Zn–Al(6)
SBA-15
ZSM-5
MCM-49
SBA-15-SO
Al(11)SBA-15-SO
MgO
63.9
17.7 10.9 61.6
13.7 13.7 100
50
88.7 87.3 98.4
40
3
H
41.7 83.4
The effect of the benzaldehyde to acetophenone (B/A) molar
ratio
3
H
—
—
a
The inuence of the benzaldehyde/acetophenone (B/A) molar
ratio on the conversion of acetophenone to (E)-chalcone using
HAlMSN is shown in Fig. 9. The effect of the B/A molar ratios of
The selectivity of (E)-chalcone.
1
: 1, 1 : 2, 2 : 1, 3 : 1 and 4 : 1 on the production of (E)-chalcone
substituents, there is higher conversion in the Claisen–Schmidt in Claisen–Schmidt condensation were studied. Aer the
reaction in comparison to those with electron withdrawing condensation reaction was carried out for 3 h, the (E)-chalcone
substituents. The reaction time with electron donating yield decreased from 97% to 68% from the B/A ratio of 1 : 2 to
substituents is less than that with electron withdrawing 4 : 1, respectively.
substituents. The lone pair electrons with enough activation
This result is probably due to the presence of excess benz-
energy react with the Lewis acid sites of HAlMSN and conse- aldehyde surrounding the active sites, which could have
quently, the conversion of starting material to the product a blocking-effect on improving the catalytic activity of the
occurs with higher yield and selectivity.
Claisen–Schmidt reaction. On the other hand, the excess
Fig. 7B shows the number of moles of (E)-chalcone and benzaldehyde diluted the concentration of acetophenone in the
starting materials that were produced or consumed during the media, consequently decreasing the contact probability
Claisen–Schmidt reaction. Here, the consumption of starting between acetophenone molecules and the active acid sites on
materials and production of (E)-chalcone was investigated. The the surface of HAlMSN. The results show that the highest
number of moles of benzaldehyde and acetophenone were product of (E)-chalcone synthesis was observed with a 1 : 2
reduced while the number of moles of (E)-chalcone increased. molar ratio of B/A.
In this reaction, acetophenone is the limiting reagent and as
was expected, the acetophenone was almost completely of Li et al. They synthesized chalcone via a condensation reac-
tion over the Al-SBA-15-SO H acid catalyst, under solvent-free
The obtained results are in strong agreement with the results
27
3
Fig. 7 (A) Development of the production of (E)-chalcone derivatives
during the reaction time, (B) the moles of benzaldehyde, acetophe-
none and (E)-chalcone produced during the Claisen–Schmidt reaction Fig. 8 Turnover frequency of (E)-chalcone production during the
process.
Claisen–Schmidt reaction.
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Acknowledgements
This work was supported by the University Teknologi Malaysia
under RUG Project No. 06H17 and special thanks to North
Tehran Branch, Islamic Azad University for helping in this
research.
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