Synthesis of chalcones catalyzed by aminopropylated silica sol-gel under solvent-free conditions

Article abstract of DOI:10.1016/j.molcata.2011.03.004

Aminopropylated nanosilica was prepared by a simple sol-gel process from tetraethyl orthosilicate (TEOS) and then it was functionalized with different amounts of 3-aminopropyltriethoxysilane (APS) under toluene reflux. The prepared solids were characterized by means of XRD, FT-IR, TGA-DTA, SEM-EDS, and TEM. Their textural properties were determined by adsorption-desorption isotherms of N₂ at 77 K. It was proved that the amount of APS used in the preparation process had a great influence on the physicochemical properties of the hybrid organic-inorganic solid materials. The materials were used as catalyst in the Claisen-Schmidt preparation of chalcones for the reaction of substituted acetophenones and benzaldehydes under solvent-free conditions. The result showed that the presence of a high amount of aminopropyl groups was important for a very good performance of the catalyst in the substrate conversion. Also, the influence of different groups in the aromatic ring of acetophenones and benzaldehydes was investigated. In all cases, coproduct formation was not observed; the catalysts were recovered and can be recycled twice without appreciable loss of reactivity.

Full text of DOI:10.1016/j.molcata.2011.03.004

Journal of Molecular Catalysis A: Chemical 340 (2011) 24–32
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Synthesis of chalcones catalyzed by aminopropylated silica sol–gel under
solvent-free conditions
Gustavo Romanellia,b, Gustavo Pasqualeb, Ángel Sathicqa, Horacio Thomasa,
Juan Autinob, Patricia Vázqueza,∗
a Centro de Investigación y Desarrollo en Ciencias Aplicadas “Dr. Jorge J. Ronco” (CINDECA), CONICET-CCT La Plata, Universidad Nacional de La Plata (UNLP),
47 N◦ 257, B1900AJK La Plata, Argentina
b Cátedra de Química Orgánica, Facultad de Ciencias Agrarias y Forestales, Universidad Nacional de La Plata (UNLP), 60 y 119, B1904AAN La Plata, Buenos Aires, Argentina
a r t i c l e i n f o
a b s t r a c t
Article history:
Aminopropylated nanosilica was prepared by a simple sol–gel process from tetraethyl orthosilicate
(TEOS) and then it was functionalized with different amounts of 3-aminopropyltriethoxysilane (APS)
under toluene reflux. The prepared solids were characterized by means of XRD, FT-IR, TGA–DTA,
SEM–EDS, and TEM. Their textural properties were determined by adsorption–desorption isotherms of
Received 11 November 2010
Received in revised form 4 March 2011
Accepted 8 March 2011
Available online 13 March 2011
N
2 at 77 K. It was proved that the amount of APS used in the preparation process had a great influence on
the physicochemical properties of the hybrid organic–inorganic solid materials. The materials were used
as catalyst in the Claisen–Schmidt preparation of chalcones for the reaction of substituted acetophenones
and benzaldehydes under solvent-free conditions. The result showed that the presence of a high amount
of aminopropyl groups was important for a very good performance of the catalyst in the substrate con-
version. Also, the influence of different groups in the aromatic ring of acetophenones and benzaldehydes
was investigated. In all cases, coproduct formation was not observed; the catalysts were recovered and
can be recycled twice without appreciable loss of reactivity.
Keywords:
Silica sol–gel
3-Aminopropyltriethoxysilane
Claisen–Schmidt condensation
Chalcone synthesis
Solvent-free
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Chalcone is commonly synthesized via the Claisen–Schmidt
aldol condensation between acetophenone and benzaldehyde. This
Chalcones are a family of aromatic ketones with two aro-
matic groups bridged by an enone linkage (Ar–COCH CH–Arꢀ).
They have attracted increasing attention due to numerous phar-
macological applications. Chalcones are the main precursors for
the biosynthesis of flavonoids, which are frequent components
of the human diet [1]. For example, Licochalcone A isolated from
the roots of Glycyrrhiza inflata (licorice) has in vitro and in vivo
antimalarial [2] and antileishmanial activity [3], and 3-methoxy-4-
hydroxyloncocarpin isolated from the roots of Lonchocarpus utilis
inhibits NADH:ubiquinone oxidoreductase activity (Fig. 1) [4].
Compounds with the backbone of chalcones, which possess
various biological activities such as antimicrobial [5], anti-
inflammatory [6], antiplatelet [7], antimalarial [8], anticancer [9],
antileishmanial [10], antioxidant [11], antifungal [12], and inhi-
bition of leukotriene B4 [13], have been reported. Also some
chalcones exhibit allelopathic activity [14], and many chalcones
are used as agrochemicals, therefore chlorochalcones are a potent
insect antifeedant [15,16].
reaction is catalyzed by bases and acids under homogeneous
conditions. Wang et al. [17] used aminopropylated silica con-
taining mesopores prepared by a simple sol–gel process with
tetraethyl orthosilicate and aminopropyltriethoxysilane under
strong acidic condition. The materials were used as catalysts in
the Claisen–Schmidt condensation in the liquid phase. The results
showed that the presence of an appropriate amount of amino-
propyl groups and narrowly distributed mesopores was important
for good performance of the catalysts.
The condensation reaction in basic medium is usually carried out
in the presence of sodium hydroxide, potassium hydroxide or alkali
alcoholate [18,19], and the acid-catalyzed methodologies include
the use of dry HCl [20], Lewis acid as TiCl4 [21], p-toluenesulfonic
acid [22], and more recently BF3–Et2O [23]. There are many draw-
backs under homogeneous conditions such as catalysis recovery
and waste-disposal problems.
In this aspect, a heterogeneous catalyst is considered as an eco-
friendly alternative. Industry favors catalytic processes in view of
easy handling, simple work-up and regenerability [24]. Also green
chemistry provides a good and eco-friendly method for the organic
synthesis under solvent-free conditions.
∗
In relation to catalytic materials, the application of nanoscale
materials and structures, usually ranging from 1 to 100 nm, is an
Corresponding author. Tel.: +54 221 4210711; fax: +54 221 4210711x125.
E-mail address: vazquez@quimica.unlp.edu.ar (P. Vázquez).
1381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.03.004

G. Romanelli et al. / Journal of Molecular Catalysis A: Chemical 340 (2011) 24–32
25
O
CH3
O
H
Catalyst
O
+
R
R1
R
R1
1
2
3
Scheme 1. Chalcone synthesis by Claisen–Schmidt procedure.
nitroaldol reactions [51]. We have recently shown that amine-
functionalized silica sol–gel is a very effective catalyst for the
transesterification reaction of ␤-ketoesters in heterogeneous con-
ditions using toluene as reaction solvent [17,52,53].
Here we report our recent results on the efficient synthesis
of chalcones by the Claisen–Schmidt condensation reaction under
solvent-free conditions catalyzed by aminopropylated silica sol–gel
materials. The materials were prepared by a simple functionaliza-
tion method without the addition of any pore-directing agents.
The catalytic efficiency of the aminopropylated silica sol–gel is dis-
cussed for the Claisen–Schmidt condensation between substituted
benzaldehydes and substituted acetophenones in the absence of
solvent (Scheme 1).
2. Materials and methodology
2.1. Materials and reagents
All solvents and chemicals were commercially available and
used without further purification unless otherwise stated.
Fig. 1. Naturally occurring chalcones.
2.2. Catalysis synthesis
emerging area of nanoscience and nanotechnology. Nanomaterials
may provide solutions to technological and environmental chal-
lenges in the areas of solar energy conversion, catalysis, medicine,
and water treatment [25,26]. This increasing demand must be
accompanied by “green” synthesis methods as was mentioned
previously. In the global efforts to reduce generated hazardous
waste, green chemistry and chemical processes are progressively
integrating with modern developments in science and industry.
Implementation of these sustainable processes should adopt the
12 fundamental principles of green chemistry [27–31]. These prin-
ciples are geared to guide in minimizing the use of unsafe products
and maximizing the efficiency of chemical processes. Hence, many
synthetic routes or chemical processes should address these princi-
plesusingenvironmentallybenignsolventsandnontoxicchemicals
[27].
2.2.1. Silica preparation
Silica (SI) preparation by the sol–gel technique: all the experi-
ments were made with a final molar ratio of tetraethylorthosilicate
(TEOS), TEOS/EtOH/H2O, equal to 1:1:4, using AcOH as catalyst.
The TEOS–EtOH–AcOH sols were stirred at atmospheric pressure
at room temperature (r.t.), for 30 min. Then, the hydrolysis pro-
cess began with the slow addition of distilled water. After water
addition, gelation of the sols was carried out at r.t., and the wet
gels were then aged in the same medium until dry silica particles
were obtained. These solids were washed with distilled water, and
subsequently dried at 20 ◦C.
2.3. Functionalized silica
On the other hand, nanomaterials often show unique and con-
siderably changed physical, chemical and biological properties
compared to their macro-scaled counterparts [32].
The
catalysts
were
prepared
by
grafting
3-
aminopropyltrimethoxysilane (APS) onto the silica sol–gel
prepared previously [54,55]. One gram of SI was refluxed in dry
toluene solution (50 ml) containing the corresponding amount of
APS (see Table 1), at 110 ◦C for 5 h. The catalysts were recovered by
filtration after washing in dry toluene (20 ml), ethyl ether (20 ml)
and dichloromethane (20 ml). The catalysts were dried in an oven
at 100 ◦C and stored for later use. The nomenclature of samples is
listed in Table 1 according to the APS amount.
In this way, as a potential alternative, heterogeneous acid
or basic catalysts for chalcone synthesis, for example, alkaline-
doped carbons [33], zeolites [34], alumina [35], magnesium oxide
[36], hydrotalcites [37], natural phosphates modified with sodium
nitrate or potassium fluoride [38], potassium hydroxide impreg-
nated on silica gel [39], acid montmorillonites [40], silica–sulphuric
acid [41], zinc chloride and microwave irradiation [42], commercial
acid clays [43], and metal nanoparticles [44], have received much
attention over the last two decades. Recently, the use of acidic IL
(ionic liquid) as catalyst for chalcone synthesis has been reported
because of its benign nature as solvent and its ability to dissolve a
wide range of compounds, but the high cost, viscosity and issues
related to its separation, especially when the product is a high-
boiling liquid or a solid, limit its use [45,46].
2.4. Catalyst characterization
2.4.1. Fourier transformed infrared spectroscopy (FT-IR)
Bruker IFS 66 equipment, pellets with BrK, and a measuring
range of 400–1500 cm−1 were used to obtain the FT-IR spectra of
the solid samples.
Another clean alternative is the use of amino- or diamino-
modified porous materials over different supports, for example,
mesoporous silica, SBA-15 materials and zeolites. These materials
were used for the synthesis of flavanones via chalcones [47,48],
Knoevenagel condensation [49], Michael additions [50], and the
2.4.2. Electron microscopies
The scanning electron microscopy (SEM) analysis was carried
out with a scanning electron microscope Philips 515, with energy
dispersive analyzer (EDAX-Phoenix). For the transmission electron

26
G. Romanelli et al. / Journal of Molecular Catalysis A: Chemical 340 (2011) 24–32
Table 1
Nomenclature of synthesized silica and APS perceptual amount on silica surface.
Entry
Nomenclature of
samples
APS (ml)
First dried (100 ◦C) in N2
atmosphere before APS
contact (g)
Second dried (100 ◦C) in N2
atmosphere after APS
contact (g)
% APS on
surface silica
1
2
3
4
5
6
7
8
SI
0
SI-A
SI-B
SI-C
SI-D
SI-E
SI-F
SI-G
0.25
0.5
0.75
1.0
1.5
2.0
3.0
0.1533
0.1659
0.1596
0.1685
0.1675
0.1679
0.1601
0.1746
0.1989
0.2195
0.3616
0.4400
0.9255
1.8574
8.52
6.6
7.98
19.31
18.16
37.88
56.57
microscopy (TEM and EDX) study, a JEOL transmission electron
microscope, JEM-2010 model, was used.
ment. The capillary column was a 30 m Chromopack CP Sil 8 CB,
whose diameter was 0.32 mm. The identification of products was
performed with GC–MS (detector HP 5971). The conversion is
expressed in terms of amount of chalcone in wt.%.
2.4.3. XRD analyses
Power XRD patterns were recorded on the same samples that
had been analyzed by FT-IR. The equipment used was a Phillips
PW-1732 with built-in recorder, using Cu K␣ radiation, nickel filter,
20 mA and 40 kV in the high voltage source, and scanning angle
between 5 and 60◦ of 2ꢀ at a scanning rate of 2◦ per min.
2.5.2. Typical procedure for chalcone synthesis
Chalcone synthesis was performed in a sealed tube under
solvent-free conditions. A mixture of the corresponding aldehyde
(1.3 mmol), acetophenone (1 mmol) and catalyst (100 mg) was
warmed at 140 ◦C for 4 h. The reaction mixture was diluted with
hot toluene (10 ml), the catalyst was filtered off, and then the solu-
tion was washed, dried with anhydrous Na2SO4, the solvent was
evaporated, and the residue purified by column chromatography
(silica gel, ethyl acetate:hexanes) to afford pure chalcones. All the
yields were calculated from isolated products. 13C NMR and 1H
NMR spectra were recorded at room temperature on Bruker Avance
DPX-400 spectrometers using TMS as internal standard. The cata-
lyst was recycled by filtration followed by washing with toluene
(2 × 5 ml), dried under vacuum and then reused. The spectral data
of isolated compounds are listed below.
2.4.4. Potentiometric titration
A 0.05 ml portion of n-butylamine (0.1 N), in acetonitrile, was
added to a known mass of solid (between 0.1 and 0.05 g) using ace-
tonitrile as solvent, and stirred for 3 h. Later, the suspension was
titrated with the same base at 0.05 ml/min. The electrode poten-
tial variation was measured with an Instrumentalia S.R.L. digital
pH meter, using a double junction electrode. The acidic properties
of the samples measured by this technique enable the evaluation
of the number of acid sites and their acid strength. In order to inter-
pret the results, it is suggested that the initial electrode potential
(E) indicates the maximum acid strength of the surface sites, and
the values (meq/g solid), where the plateau is reached, indicate
the total number of acid sites. The acid strength of surface sites
can be assigned according to the following ranges: very strong site,
E > 100 mV, strong site, 0 < E < 100 mV; weak site, −100 < E < 0 mV,
and very weak site, E < −100 mV.
Chalcone: 1H NMR (CDCl3, 400 MHz): ı 8.10 (2H, dd, J: 8.2, J: 1.8,
H-2ꢀ, H-6ꢀ ), 7.94 (1H, d, J: 16, H-3), 7.59–7.52 (6H, m, H-2, H-3ꢀ, H-4ꢀ,
H-5ꢀ, H-2ꢀꢀ, H-6ꢀꢀ), 7.43–7.41 (3H, H-4ꢀꢀ, H-5ꢀꢀ, H-6ꢀꢀ).
13C NMR (CDCl3, 100 MHz): 190.4 (CO), 144.9 (C-3), 138.5 (C-1ꢀ),
134.7 (C-1ꢀꢀ), 132.9 (C-4ꢀ), 130.7 (C-3ꢀ, C-5ꢀ)*, 128.7 (C-3ꢀꢀ, C-5ꢀꢀ)*,
128.6 (C-2ꢀꢀ, C-6ꢀꢀ)*, 128.5(C-2ꢀ, C-6ꢀ)*, 128.4(C4ꢀꢀ), 122.5(C2). They
can be interchanged.
2.4.5. Textural properties
4-Methylchalcone: 1H NMR (CDCl3, 400 MHz): ı = 8.03 (2H, dd,
J: 8, 1.8, H-2ꢀ, H-6ꢀ), 7.86 (d, 1H, J: 16, H-3), 7.62–7.45 (m, 6H, H-2,
H-3ꢀ, H-4ꢀ, H-5ꢀ, H-2ꢀꢀ, H-6ꢀꢀ), 7.10 (d, 2H, J: 8, H-3ꢀꢀ, H-5ꢀꢀ), 2.34 (CH3).
13C NMR (CDCl3, 100 MHz): ı = 190.5 (CO), 144.9 (C-3), 138.6 (C-
1ꢀ), 138.4(C4ꢀꢀ), 132.9 (C-4ꢀ), 131.7 (C-1ꢀꢀ), 130.9 (C-3ꢀ, C-5ꢀ), 128.9
(C-3ꢀꢀ, C-5ꢀꢀ), 128.6 (C-2ꢀꢀ, C-6ꢀꢀ)*, 128.5(C-2ꢀ, C-6ꢀ)*, 122.5(C2), 21.4
(CH3). They can be interchanged.
The specific surface areas (SBET) were determined by the
nitrogen adsorption/desorption technique using Micromeritics
Accusorb 2100E equipment (Table 1).
2.4.6. Thermal analysis
The TG–DTA measurements of the solid samples were
performed in a Shimadzu DT 50 thermal analyzer. The thermo-
gravimetry and differential thermal analysis experiments were
performed under argon and nitrogen, respectively, using 20–25 mg
samples and a heating rate of 10 ◦C/min. Quartz cells were used as
sample holders with Al2O3 as reference. The studied temperature
range was 25–700 ◦C.
4-Chlorochalcone: 1H NMR (CDCl3, 400 MHz): ı = 8.1 (2H, dd, J:
8, 1.8, H-2ꢀ, H-6ꢀ), 7.85 (d, 1H, J: 16, H-3), 7.73–7.59 (m, 6H, H-2,
H-3ꢀ, H-4ꢀ, H-5ꢀ, H-2ꢀꢀ, H-6ꢀꢀ), 7.35 (d, 2H, J: 8, H-3ꢀꢀ, H-5ꢀꢀ).
13C NMR (CDCl3, 100 MHz): ı = 190.2 (CO), 143.2 (C-3), 138.0 (C-
1ꢀ), 136.4 (C4ꢀꢀ), 133.4 (C-4ꢀ), 132.8 (C-1ꢀꢀ), 129.5 (C-3ꢀ, C-5ꢀ)*, 129.2
(C-3ꢀꢀ, C-5ꢀꢀ)*, 128.6 (C-2ꢀꢀ, C-6ꢀꢀ)*, 128.5 (C-2ꢀ, C-6ꢀ)*, 122.5 (C2). They
can be interchanged.
2.5. Catalytic reaction procedure
4-Methoxychalcone: 1H NMR (CDCl3, 400 MHz): ı = 8.0 (2H, dd,
J: 8, 1.9, H-2ꢀ, H-6ꢀ), 7.71 (d, 1H, J: 16, H-3), 7.3–7.7 (m, 6H, H-2,
H-3ꢀ, H-4ꢀ, H-5ꢀ, H-2ꢀꢀ, H-6ꢀꢀ), 6.93 (d, 2H, J: 8, H-3ꢀꢀ, H-5ꢀꢀ), 3.85 (s,
3H, –OCH3).
2.5.1. Chalcone preparation study
The catalytic chalcone synthesis was performed in a sealed
tube immersed in a thermostated bath with magnetic stirrer under
solvent-free conditions. A mixture of the corresponding aldehyde
(1.3 mmol), acetophenone (1 mmol) and catalyst (100 mg) was
warmed at the corresponding temperature. Aliquots of the reac-
tion mixture were withdrawn at intervals of time. Each sample
was approximately diluted with 1 ml of ethanol. Conversions were
obtained with the analysis performed with a Varian GC 3400 instru-
13C NMR (CDCl3, 100 MHz): ı = 190.4 (CO), 161.5 (C-4ꢀꢀ), 144.5
(C-3), 138.5 (C-1ꢀ), 132.4 (C-4ꢀ), 130.4 (C-2ꢀꢀ, C-6ꢀꢀ), 128.5 (C-3ꢀ, C-
5ꢀ)*, 128.4 (C-2ꢀ, C-6ꢀ)*, 127.7 (C-1ꢀꢀ), 119.5 (C-2), 114.7 (C-3ꢀꢀ,C-5ꢀꢀ),
55.2 (–OCH3). They can be interchanged.
4-Hydroxychalcone: 1H NMR (DMSO d-6, 400 MHz): ı = 10.1 (sa,
1H, OH), 8.11 (2H, dd, J: 8, 1.8, H-2ꢀ, H-6ꢀ), 7.81–7.57 (m, 7H, H-2,
H-3 H-3ꢀ, H-4ꢀ, H-5ꢀ, H-2ꢀꢀ, H-6ꢀꢀ), 6.88 (d, 2H, J: 8, H-3ꢀꢀ, H-5ꢀꢀ).

G. Romanelli et al. / Journal of Molecular Catalysis A: Chemical 340 (2011) 24–32
27
Table 2
Textural properties of functionalized nano-silicas.
Sample
BET surface area
(m2/g)
t-Plot micropore
BET adsorption average
pore width (A)
BJH adsorption average
pore width (A)
t-Plot micropore
area (m2/g)
volume (m3/g)
SI
459
209
106
45
54
46
5
88
232
123
59
78
40
3
67.3
106.7
133.3
172.3
140.2
170.7
9.4
67.3
106.7
122.3
142.5
140.2
178.7
9.4
0.037
−0.015
−0.011
−0.008
−0.014
0.001
SI-A
SI-B
SI-C
SI-D
SI-E
SI-F
SI-G
0.001
0.001
5
2
8.6
7.3
13C NMR (DMSO d-6, 100 MHz): ı = 188.8 (CO), 160.5 (C-4ꢀꢀ),
144.6 (C-3), 138.0 (C-1ꢀ), 132.6 (C-4ꢀ), 130.8 (C-2ꢀꢀ, C-6ꢀꢀ), 128.6 (C-3ꢀ,
C-5ꢀ)*, 128.5 (C-2ꢀ, C-6ꢀ)*, 125.7 (C-1ꢀꢀ), 118.5 (C-2), 115.8 (C-3ꢀꢀ,C-
5ꢀꢀ). They can be interchanged.
structure (Tables 1 and 2, respectively) [61]. Regarding the textu-
ral properties, the surface area of bulk and supported acid samples
determined from N2 adsorption–desorption isotherms using BET
and BJH methods, together with the t-plot micropore area, BET
adsorption average pore width, BJH adsorption average pore width
and the t-plot micropore volume, are shown in Table 2, as was
mentioned previously. In all cases the values are close to the experi-
mental error of the methods used. Although the same samples show
negative adsorption data, attributed to the agglomeration of APS
and the possible highly hygroscopic nature of silica, there could
be electronic delocalization that repulses the nitrogen molecules,
generating gas desorption, and very different SBET results without
any relation between the APS amount and surface area. In addition,
according to the surface microvalues calculated using the t-plot
method (Table 2), the total surface area comes from a micro and
mesoporous structure depending on the APS amount.
4ꢀ-Methylchalcone: 1H NMR (CDCl3, 400 MHz): 7.95 (2H, d, J: 8,
H-2ꢀ, H-6ꢀ), 7.81 (1H, d, J: 16, H-3), 7.55–7.40 (m, 3H, H-2, H-2ꢀꢀ),
6.99–6.81 (m, 5H, H-3ꢀ, H-5ꢀ, H-3ꢀꢀ, H-5ꢀꢀ, H-4ꢀꢀ).
13C NMR (CDCl3, 100 MHz): ı = 189.8 (CO), 144.5 (C-3), 143.6 (C-
1ꢀꢀ), 135.6 (C-4ꢀ), 135.0 (C-1ꢀ), 130.3 (C-2ꢀꢀ, C-6ꢀꢀ), 129.3 (C-2ꢀ, C-6ꢀ)*,
128.9 (C-3ꢀ, C-5ꢀ)*, 128.6 (C-3ꢀꢀ, C-5ꢀꢀ)*, 128.4 (C-4ꢀꢀ)*, 122.4 (C-2),
21.6 (–CH3). They can be interchanged.
4ꢀ-Chlorochalcone: 1H NMR (CDCl3, 400 MHz): ı = 7.85 (1H, d, J:
16, H-3), 7.70 (d, J: 8, H-2ꢀ, H-6ꢀ) 7.65–7.49 (m, 5H, H-2, H-2, H-2ꢀꢀ,
H-6ꢀꢀ, H-3ꢀ, H-5ꢀ), 7.15–7.28 (m, 3H, H-3ꢀꢀ, H4ꢀꢀ, H-5ꢀꢀ).
13C NMR (CDCl3, 100 MHz): ı = 189.1 (CO), 145.5 (C-3), 139.2 (C-
1ꢀ), 136.4 (C-4ꢀ), 135.1 (C-1ꢀꢀ), 134.8 (C-4ꢀꢀ), 130.8 (C-3ꢀ, C-5ꢀ), 129.5
(C-3ꢀꢀ, C-5ꢀꢀ)*, 129.7 (C-2ꢀ, C-6ꢀ)*, 128.5 (C-2ꢀꢀ, C6ꢀꢀ), 121.8 (C-2). They
can be interchanged.
According to the results of textural properties, XRD patterns of
the samples do not present a crystalline structure (not shown here).
This may be due to the presence of high APS agglomeration without
interaction with the support [54,55]. The sample with out APS does
not show crystalline structure.
3. Results and discussion
In relation to the acidic properties, the curves obtained for
sample leaching, by means of potentiometric titration with n-
butylamine, are not shown in this paper [53]. At this point, it is
interesting to comment that the development of a potentiomet-
ric method was thought of taking into account that acid and basic
functions of a solid are not properly defined thermodynamically.
Moreover, an indicator test such as the Hammet’s method, though
considered as a reference technique, is difficult to apply to cer-
tain solids. The method used here is based on the observation that
the potential difference is mainly determined by the acidic envi-
ronment around the electrode membrane. The measured electrode
potential is an indicator of the acidic properties of dispersed solid
particles. The different samples were titrated with n-butylamine
in order to compare their acidities. Although all samples exhibited
weak sites, SI showed higher acidity than SI-A (200 mV) to SI-G
(−200 mV). It is evident from this result that the grafting process
leads to amine groups of APS attached to the surface of SI acidic
sites.
3.1. Silica characterization
The interface involving silica surface plays an important role in
the adsorption process. The surface characteristics of the adsor-
bate determine the nature of the bonding between adsorbate and
adsorbent. Different techniques are in use to characterize the sur-
face of the adsorbents. To investigate the behavior of porous silica
coated with siloxane polymers, Gilpin et al. [56] used the nitro-
gen adsorption technique for the surface area and porosity of silica.
On thermal treatment the hydrophilic surface gradually changes to
hydrophobic surface by irreversible elimination of a pair of adjacent
hydroxyls [57,58].
The APS was used for the surface modification of hydrophilic
silica obtained via sol–gel by grafting. The functionalization process
occurs as shown in the following equation:
SiOH + H2N(CH2)3Si(OEt)3 → (Si–O)3Si(CH2)3NH2 + 3EtOH
On the other hand, near-IR spectra of water adsorbed on sil-
ica are also used for the study of the surface behavior of silica.
Silanols are proposed to be the water adsorption sites [58–66].
This model can also explain the hydrophobic character of silica
with low silanol densities. From intensity measurement of near-IR
diffuse-reflectance spectra, Klier et al. [58] claimed the existence
of SiOH–OH2 complexes with about one half of the silanols as
“BET monolayer” on Hisil 233 silica. Additional water adsorption
takes place around adsorbed water molecules rather than on free
silanols, because such clusters would be energetically more favor-
able. Yamauchi and Hondo [66] obtained more accurate IR spectra
of the silanols and the adsorbed water on a silica gel. They sug-
gested that water would settle only on part of the silanols at first
as SiOH–OH2 complexes, where the water hydroxyl would absorb
The equation is a very simplified version of the inclusion of APS
ethoxy-type groups on the silica surface. The modification of the sil-
ica surface using APS consumes three silanol groups and generates
a terminal amino group.
It is important to mention that the amount of APS in the
functionalization process and during the washing and drying of
samples had a great influence on the textural properties of the
hybrid organic–inorganic materials. The samples prepared by TEOS
hydrolysis and with different amounts of APS (Table 1) had a narrow
pore size distribution structure and a very different surface area and
pore volume depending on the APS content (Table 2), as it were
obtained in previous results [54,55,59,60]. In relation to that reac-
tion, the SBET decreases from 459 m2/g (SI) to 5 m2/g (SI-G) because
of the reaction between APS and TEOS, and steric effects due to their

28
G. Romanelli et al. / Journal of Molecular Catalysis A: Chemical 340 (2011) 24–32
Table 3
Chalcone synthesis using different synthesized silicas as catalyst.
Entry
Catalyst
Yields of chalcone (%)
1
2
3
4
5
6
7
8
9
None
SI
5
38
27
21
41
39
74
75
87
SI-A
SI-B
SI-C
SI-D
SI-E
SI-F
SI-G
aReaction conditions: acetophenone (1 mmol), benzaldehyde (1.3 mmol), catalyst
(0.100 g), solvent-free, 4 h.
APS. In Fig. 3b, TEM micrographs of the synthesized functionalized
silicas (S1-C, S1-E, and S1-G) are shown. In these micrographs it
can be observed that silicas with APS in their structure have larger
particles and agglomeration areas in comparison to SI. The agglom-
eration is independent of the quantity of APS incorporated during
the functionalization process, which may be due to the reaction of
the OH surface with the APS.
Fig. 2. FTIR of Si and SI-G samples.
at 5270 cm−1. A second water molecule absorbing at 5150 cm−1
would then settle on the previous silanol water. In accordance with
these concepts, Fig. 2 shows the main features of pure silica (SI),
and functionalized (SI-G) absorption bands. For unchanged silica,
at 3450 cm−1 a band due to the OH bonds of silanol groups exists.
Also, bands for symmetrical and asymmetrical vibration of the Si–O
are located at 810 and 1107 cm−1, respectively. A small shoulder
observed at about 960 cm−1 is assigned to the SI–OH groups. The
band at 1629 cm−1 is attributed to bending Si–O–H silica silanol
groups. Functionalized silica shows a moderately different behav-
ior from that of previous FTIR spectrum, the disappearance of the
band at 3450 cm−1 and a low broad band near 3400 cm−1, which is
related to the tension of primary amine groups. Also, bands appear
at 2926 and 1097 cm−1, corresponding to vibration groups of C–H
and Si–O–C tension bands of the ethoxy group, respectively. At
1564 cm−1 the band assigned to the deformation of the NH plane is
observed, and at 1325 cm−1 the bending band of C–N appears [67].
These results suggest a mechanism of nucleophilic substitution,
as has been mentioned in previous work [59,60]. It may be sug-
gested that the results obtained for the reaction over functionalized
SI could be because of this mechanism [52], which could be due to
the Si–O–Si group formed on the surface inducing an interaction
between silanol groups and the APS, and leads to the formation
of two residual ethoxy groups. Also, the functionalization process
did not eliminate surface silanol groups completely and afforded a
certain degree of hydrophilicity to silica, which can be explained
by the steric agent effect. This would act as an umbrella, protect-
ing the surface silanol groups from the attack of polar solvents and
therefore reduces the wettability of silica [57,59,60]. This is consis-
tent with the results obtained by the FTIR spectrum of SI-G (Si–O–C
group at 1097 cm−1).
In addition, TGA–DTA techniques to verify the presence of OH
groups on bulk and functionalized silica (SI and SI-G samples) were
applied. It is observed that at low temperatures (less than 50 ◦C),
loss of moisture results (ambient water taken by both solids). The
greatest weight losses are at over 120 ◦C and 450 ◦C, respectively.
Below 200 ◦C, the weight loss is attributable to the evaporation of
water and the volatilization of organic material as thermal decom-
position of the remains of solvents in both solids. Between 200 and
300 ◦C, weight loss is attributed to the combustion of organic com-
pounds that are part of the silica network (SI and SI-G). The weight
loss around 450 ◦C is due to the complete carbonization of organic
compounds in the functionalized sample (SI-G).
In Fig. 3a, the micrographs of SI are shown. The magnification
used in both cases corresponds to 200 nm and 50 nm, respectively.
From this it can be inferred that SI has a small particle size (near
12 nm in diameter) with a spherical shape. Based on this unifor-
mity in morphology, SI was chosen to perform the grafting with
3.2. Catalytic tests
3.2.1. Catalytic activity in the presence of the silica sol–gel (SI)
support
The Claisen–Schmidt synthesis of chalcones involving the con-
densation of acetophenone and benzaldehyde is illustrated in the
reaction Scheme 1.
Before attempting a detailed catalytic work, a noncatalytic reac-
tion between acetophenone and benzaldehyde was examined, and
it was observed that under the experimental condition (140 ◦C, 4 h),
only 5% formation of chalcone was detected, indicating that from a
practical point of view the reaction is not taking place in the absence
of a catalyst (Table 3, entry 1).
The Claisen–Schmidt condensation of benzaldehydes and ace-
tophenones was reported to be catalyzed by acid or bases [18–23].
Pure silica sol–gel (SI) was first tested for the catalytic conden-
sation between benzaldehyde and acetophenone without solvent
at 140 ◦C for 4 h. The GC yield of chalcone was 38% and no other
side products were observed, indicating that the Cannizzaro reac-
tion of benzaldehyde, condensation of acetophenone or aldehyde
oxidation did not take place under their reaction conditions. The
GC–MS analysis confirms that, under our experimental conditions,
trans-chalcone is obtained selectively (MS) m/s: 208, (M+), 179,
103, 77 (100), 65, 51. The result indicated that silica sol–gel (SI)
showed poor activity to chalcone (38%, Table 2, entry 2). This
result revealed that SI acts not only as a support but also as an
acid–base bifunctional catalyst. The conversion over this solid is
due to coordinate-assisted condensation of acetophenone with
benzaldehyde for the presence of basic and acid sites on the surface
of the silica sol–gel, as shown in Scheme 2.
3.2.2. Influence of aminopropyl group loading
The catalytic activity of aminopropylated silica sol–gel prepared
with different amounts of APS is displayed in Table 3. The reaction
was studied at 140 ◦C, using a ratio of benzaldehyde:acetophenone
of 1:1.3, and an amount of catalyst of 100 mg, the reaction time was
of 4 h. The catalyst samples were called SI-A, SI-B, SI-C, SI-D, SI-E,
SI-F and SI-G, and the amounts of APS used during the functional-
ization are indicated in Table 1. From this table, it can be observed
that the catalysts SI-E, SI-F and SI-G, which contain greater amounts
of APS, show very high chalcone yields (see Table 3, entries: 7:74%;
8: 75%, and 9: 87%, respectively).

G. Romanelli et al. / Journal of Molecular Catalysis A: Chemical 340 (2011) 24–32
29
Fig. 3. TEM micrographs of (a) SI sample and (b) SI-C, SI-E and SI-G samples.
In contrast, the yields of catalysts SI-A, SI-B, SI-C and SI-D, which
3.2.3. Effect of reaction temperature and time on catalyst
performance
contain low amounts of amino groups, decreased dramatically (see
Table 3, entries: 3: 27%; 4: 21%; 5: 41%, and 6: 39%, respectively).
Since the best catalyst was SI-G, it was used to continue with our
investigation for obtaining better chalcone yields. In these cases it
can be observed that SI-A and SI-B have yielded less chalcone than
SI, which could be due to SBET and the amount of APS on the silica
surface.
The effect of temperature plays an important role in the catalytic
synthesis of chalcones. It was examined in the temperature range
between 120 and 150 ◦C, in the absence of solvent, on the SI-G cat-
alyst, and the results are illustrated in Fig. 4. Temperature increase
leads to a higher chalcone yield. The yield of chalcone for a reaction
time of 4 h at 120 ◦C is only 38%, whereas at 140 ◦C the conversion

30
G. Romanelli et al. / Journal of Molecular Catalysis A: Chemical 340 (2011) 24–32
Scheme 2. Proposal mechanism of chalcone synthesis catalyzed by silica sol–gel (SI).
is 83%, which is 3.0 times higher than at 120 ◦C. For the same time
of 4 h, an increase of only 3% is observed when the temperature is
raised from 140 to 150 ◦C. For this reason, 140 ◦C was employed as
the ideal temperature to continue with the analysis of other reac-
tion variables. However, the catalytic performance of catalyst SI-G
as a function of the reaction time in the condensation of benzalde-
hyde and acetophenone was studied at three temperatures (120,
140 and 150 ◦C) under solvent-free conditions (Fig. 4). The catalytic
activity for the three temperatures was found to increase rapidly
with the reaction time up to 4 h, and then remains almost constant.
As reported by Wang et al. [17], the constant value is probably due
to the blockage of the catalytic center by the high concentration of
chalcone.
100
SI-G
90
80
70
60
50
40
30
20
10
0
3.2.4. Effect of the amount of catalyst
The optimal reaction conditions (benzaldehyde 1 mmol, ace-
tophenone 1.3 mmol, a temperature of 140 ◦C and 4 h) were used for
the next experiment. Different amounts of the SI-G catalyst were
tested (25, 50, 100 and 300 mg). The results show that 100 mg is
an optimal quantity of catalyst to obtain the best yields (see Fig. 5,
column 3: 83%). When the experiment was carried out employing
300 mg of SI-G catalyst, a yield of 79% was obtained and a small
amount of an unidentified secondary product was detected. Per-
haps this behavior could be due to the % APS in this sample.
0
50
100
150
200
250
300
Catalyst (mg)
Fig. 5. Effect of the catalyst amount.
was a minor loss in catalyst weight during each recycles (total loss
12%). For the SI-F catalyst, the isolated yields of chalcone were 74%,
75% and 72%, and for SI-G 83, 80% and 79%, which showed, under
these conditions, the good reusability of this catalyst.
3.2.5. Reuse of the catalyst
The reuse of catalysts is central to their utility. In order to inves-
tigate the reusable properties of the aminopropylated silica sol–gel
catalysts, recycle experiments were conducted and the results are
shown in Fig. 6. After reaction, the catalyst was recycled by washing
with hot toluene (2 × 7 ml), dried under vacuum at 50 ◦C for 5 h and
then reused. The better two catalysts, SI-F and SI-G, were used for
this test. They were reused twice, and it was observed that there
3.2.6. Reaction mechanism
The Claisen–Schmidt condensation of benzaldehydes and ace-
tophenones has been studied under acidic or basic conditions.
Wang and Cheng [48] described an alternative mechanism to
the classic basic catalyst procedure. This rationalization is carried
out for the reaction of benzaldehyde and 2ꢀ-hydroxiacetophenone
100
100
SI-G
SI-F
120ºC
90
90
80
70
60
50
40
30
20
10
0
140ºC
150ºC
80
70
60
50
40
30
20
10
0
0
2
4
6
8
1
2
3
Time (h)
Reuse
Fig. 4. Effect of reaction temperature on the catalytic performance.
Fig. 6. Reuse of SI-F and SI-G catalysts.

G. Romanelli et al. / Journal of Molecular Catalysis A: Chemical 340 (2011) 24–32
31
O
O
O
O
O
O
O
O
-H2C
H
O
+
CH3
O
O
O
O
O
Si
Si
NH3
Si
Si
NH2
O
N
H
NH2
OH
H
2
1
-H2O
O
O
O
O
O
O
O
O
Si
NH2
NH
+
O
O
O
O
O
Si
NH3
-H2C
O
Si
Si
N
C
H2
O
4
3
Scheme 3. Proposal mechanism of chalcone synthesis catalyzed by aminopropylated silica sol–gel.
assisted by aminopropylated mesoporous silica. Our experimen-
tal results for the reaction of benzaldehyde and acetophenone are
in accordance with this mechanism. The modified reaction mecha-
nism is shown in Scheme 3. Over the aminopropylated silica sol–gel
catalyst, the benzaldehyde molecules and the amino groups form
the C N imine species, which was then attracted by the anion of
acetophenone to produce the corresponding adduct. The first step
of imine formation is the attack by the nucleophilic amine on the
carbonyl. Rapid proton transference results in a carbinolamine. The
carbinolamine reacts to form imine by loss of water. The imine
intermediate is attacked by the acetophenone anion and one proton
is incorporated to form the intermediate 4. Finally the intermediate
4 forms chalcone and the catalysts are regenerated.
(140 ◦C, benzaldehyde/acetophenone ratio 1.3/1, catalyst 100 mg,
and 240 min). Results of the obtained yields are listed in Table 3. The
results showed that, in general, the reactions were clean and prod-
ucts were isolated by liquid column chromatography in pure form
without further purification (1H and 13C NMR). The reaction is very
selective and no competitive side reactions such as product decom-
position, acetophenone condensation, Cannizzaro’s reaction, etc.,
were observed (GC).
Finally, we studied the influence of the substituting groups on
the aromatic ring of acetophenones and benzaldehydes. The reac-
tions of aldehydes bearing electron-donating groups such as –Me,
–OMe and –OH gave yields similar to those of the corresponding
reaction with unsubstituted benzaldehyde (Table 4, entries 2: 83%,
3: 83%, and 4: 84%, respectively).
The presence of electron-withdrawing groups in acetophenone
such as –Cl gave a comparable rate of condensation, even when
the aldehyde contains an electron-withdrawing group such as –Cl,
presumably due to ease of formation of the enolate anion in the
first step (see Table 4, entries 5: 88% and 7: 89%). The physical
3.2.7. Preparation of substituted chalcones
To explore the general validity of the process previously
described, a series of chalcone derivatives were prepared under the
optimal conditions. The reactivity of different aryl-benzaldehydes
and methyl aryl-ketones was tested under the same conditions

32
G. Romanelli et al. / Journal of Molecular Catalysis A: Chemical 340 (2011) 24–32
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Entry
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R
R1
Yields (%)b
1
2
3
4
5
6
7
3a
3b
3c
3d
3e
3f
H
H
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–CH3
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Calculated yield based on isolated product.
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In the present work we reported the preparation, characteriza-
tion, and a catalytic study of various synthesized aminopropylated
silica sol–gel catalysts with different amino group loadings. The
materials were an efficient basic catalyst for the synthesis of
chalcones via the Claisen–Schmidt condensations. The optimal con-
version was obtained for the amino-functionalized silica sol–gel
(S-G: 3 ml APS per 1 g silica (56.57% APS)), and a decreasing amount
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silica gel provide efficient, selective, inexpensive, and eco-friendly
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and the environmentally benign nature of the reaction would allow
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catalysts to other reactions would be of further interest.
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