Preyssler catalyst: An efficient catalyst for esterification of cinnamic acids with phenols and imidoalcohols

Article abstract of DOI:10.1016/j.apcata.2009.11.037

In the present work a series of eco-friendly Preyssler solid acids and their salts H₁₄[NaP₅W₂₉MoO₁₁₀] (PW), H₁₄[NaP₅W₃₀O₁₁₀] (PWMo), K_12.5Na_1.5[NaP₅W₃₀O₁₁₀]·15H₂O (PWK), K_12.5Na_1.5[NaP₅W₂₉MoO₁₁₀]·15H₂O (PWMoK) and 10% silica-supported H₁₄[NaP₅W₂₉MoO₁₁₀] (PWSiO₂), and H₁₄[NaP₅W₃₀O₁₁₀] (PWMoSiO₂) have been used as catalysts for the direct esterification of cinnamic acids with phenols or 2-(N-phthalimidoethanol). Effects of the reaction conditions were studied, including temperature, reaction time, and type and amount of catalyst. These solids were characterized by several techniques, such as diffuse reflectance spectroscopy, Fourier transformed infrared spectroscopy, optical and scanning electron microcopies, and X-ray diffraction, among others. The most adequate catalyst for performing the title reaction was H₁₄[NaP₅W₃₀O₁₁₀] (PWMo). The catalyst was applied for the synthesis of various substituted cinnamates, giving very good yields.

Full text of DOI:10.1016/j.apcata.2009.11.037

Applied Catalysis A: General 374 (2010) 110–119
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Preyssler catalyst: An efficient catalyst for esterification of cinnamic acids
with phenols and imidoalcohols
a,b
b
a
Diego M. Ruiz a, , Gustavo P. Romanelli , Patricia G. Vazquez , Juan C. Autino
*
´
a
´
´
´
Catedra de Quımica Organica, Facultad de Ciencias Agrarias y Forestales, UNLP, Calles 60 y 119, (B1904AAN) La Plata, Argentina
b Centro de Investigacio´n y Desarrollo en Ciencias Aplicadas ‘‘Dr. Jorge J. Ronco’’ (CINDECA), Departamento de Quı´mica, Facultad de Ciencias Exactas,
UNLP-CCT-CONICET, Calle 47 N8 257, (B1900AJK) La Plata, Argentina
A R T I C L E I N F O
A B S T R A C T
Article history:
In the present work a series of eco-friendly Preyssler solid acids and their salts H14[NaP5W29MoO110
]
Received 3 September 2009
Received in revised form 21 November 2009
Accepted 25 November 2009
Available online 1 December 2009
(PW), H14[NaP5W30O110
]
(PWMo), K12.5Na1.5[NaP5W30O110]Á15H2O (PWK), K12.5Na1.5[NaP5W29-
(PWSiO2), and
MoO110]Á15H2O (PWMoK) and 10% silica-supported H14[NaP5W29MoO110
]
H14[NaP5W30O110] (PWMoSiO2) have been used as catalysts for the direct esterification of cinnamic
acids with phenols or 2-(N-phthalimidoethanol). Effects of the reaction conditions were studied,
including temperature, reaction time, and type and amount of catalyst. These solids were characterized
by several techniques, such as diffuse reflectance spectroscopy, Fourier transformed infrared
spectroscopy, optical and scanning electron microcopies, and X-ray diffraction, among others.
The most adequate catalyst for performing the title reaction was H14[NaP5W30O110] (PWMo). The
catalyst was applied for the synthesis of various substituted cinnamates, giving very good yields.
ß 2009 Elsevier B.V. All rights reserved.
Keywords:
Acid catalysis
Cinnamates
Esterification
Heteropolyacid
Preyssler heteropolyacids
1. Introduction
The Preyssler heteropolyanion consists of a cyclic assembly of
five PW6O22 units, each derived from the Keggin anion
3À
Catalysis by heteropolyacids (HPAs) and related compounds is a
field of increasing importance worldwide. Numerous develop-
ments are being carried out in basic research as well as in fine
chemistry processes [1]. HPAs possess, on the one hand, a very
strong acidity and, on the other hand, appropriate redox
properties, which can be changed by varying the chemical
composition of the heteropolyanion. The reactions catalyzed by
both heterogeneous and homogeneous systems have been
reviewed by many researchers [2–10].
[PW12O40
]
by the removal of two sets of three corner-sharing
WO6 octahedra. A sodium ion is located within the polyanion on
the five-fold axis and 1.25 A above the pseudo mirror plane that
˚
contains the five phosphorus atoms. This heteropolyanion with
14 acidic protons is an efficient superacid solid catalyst that can
be used both in homogeneous and heterogeneous phases.
Preyssler catalyst is a green catalyst with respect to safety,
noncorrosive properties, quantity of waste separability, and
stability [17].
The reactions in which they can be used, from dehydration,
cyclization or esterification up to amine oxidation or olefin
epoxidation, may find wide applications in fine chemical produc-
tion, such as fragrances, pharmaceuticals and food [11,12].
Although there are many structural types of HPAs, the majority
of catalytic applications use the most common Keggin type [13],
especially for acid catalysts, owing to its availability and chemical
stability. Other catalysts such as Wells-Dawson and Preyssler
heteropolyacids have begun to be used. [14–16].
Recent applications related to the catalytic activity of the
Preyssler heteropolyoxometalates in organic synthesis include
esterification of n-butanol with acetic acid [18], alkylation of
phenol with 1-octene [19], synthesis of isoxazoles [20], coumarins
[13], 2,4,6-triarylpyridines [21], 2-amino-4H-chromenes [22], and
1,5-benzodiazepine [23], and oxidation reactions such as alcohols
[24], primary aromatic amines [25] and thiols using air as oxidant
[26], and alkene epoxidation [27].
On the other hand,
a,b-unsaturated esters possess various
pharmacological activities including antioxidant, antimicrobial
and anticancer activities [28], besides being useful in other
synthetic applications. There are numerous methods for the
preparation of
a,b-unsaturated esters; however, most of the
´
´
´
* Corresponding author at: Catedra de Quımica Organica, Departamento de
Ciencias Exactas, Facultad de Ciencias Agrarias y Forestales UNLP, Calle 60 y 119 S/
N, (B1904AAN) La Plata, Argentina.
reported procedures require strong acids such as sulfuric acid,
hydrochloride acid and toxic chemicals such as dimethyl sulfate,
methyl iodide or diazomethane, which are environmentally
E-mail address: dimruiz@agro.unlp.edu.ar (D.M. Ruiz).
0926-860X/$ – see front matter ß 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2009.11.037

D.M. Ruiz et al. / Applied Catalysis A: General 374 (2010) 110–119
111
hazardous and hence unacceptable [29]. Particularly, cinnamates
are enormously important organic compounds due to their
application in a wide range of industrial products such as
plasticizers, graphics, lubricants, flavors, perfumes and cosmetics
[29]. Cinnamates are also antioxidant and flavoring agents in
diverse plant species. A bioactive imidoalkyl cinnamate, 2-(N-
experiment 56 g (0.169 mol) Na2WO4Á2H2O and 2 g (0.008 mol)
Na2MoO4Á2H2O were dissolved in 70 ml water and mixed at 60 8C
for 30 min. The solution was cooled to room temperature and
45 ml concentrated phosphoric acid was added. The resulting
yellow solution was refluxed for 18 h. The color turned to dark
green at the end of reaction. The solution was brought to room
temperature, diluted with 30 ml water and then 20 g potassium
chloride was added with stirring. The mixture was stirred for
30 min and then heated up to dryness, and a greenish solid was
obtained. This raw product was dissolved in 70 ml warm water and
upon cooling to room temperature yellow crystals formed, which
were collected and recrystallized from boiling water (yield, 18%).
The Mo-substituted heteropolyanion was converted to its corre-
sponding acid H14[NaP5W29MoO110] (PWMo), by passing an
aqueous solution of PWMoK through a Dowex-50W-x8 ion-
exchange column.
phthalimido) ethyl cinnamate, is a potent inhibitor of the 17-b-
hydroxysteroid dehydrogenase enzyme related to diseases such as
breast cancer, Alzheimer disease and benign prostatic hyperplasia
[30]. Aryl cinnamates have been used as intermediates for diverse
heterocyclic compounds, such as chromones, pyrazoles and
furanones [31]. Some substituted aryl cinnamates are plant-
growth inhibitors [32] and have antifungal [30] activity.
Only a few procedures have been used to prepare aryl
cinnamates, including the most used method via cinnamoyl
chloride [33], the use of DCC [34] or BOP [30] as coupling agents
to perform the direct reaction between cinnamic acid and phenol,
or from phosphorane and benzaldehyde [35]. Most recently our
research group described a green procedure to prepare these
compounds for the direct esterification of cinnamic acids with
2.1.2. Supported catalysts
Silica gel-supported Preyssler acids were prepared by wet
impregnation of Grace silica (SiO2) (Grade 59, specific area:
250 m2/g), with an acetone solution of PW or PWMo synthesized
heteropolyacids, respectively. Then, two supported catalysts,
PWSiO2, and PWMoSiO2, containing 10 wt% of active phase, were
prepared. After impregnation, samples were dried at room
temperature in vacuum desiccators for 8 h.
phenols or 2-(N-phthalimido)ethanol using
a Wells-Dawson
heteropolyacid bulk or supported on silica [36]. Under environ-
mentally friendly conditions, other cinnamates have been obtained
from cinnamaldehydes using a heterogeneous catalyst and DDQ
[37], and direct esterification using water-tolerant ZrOCl2Á8H2O
and HfOCl2Á8H2O as catalysts [29].
In connection with our research project on the eco-friendly
synthesis of organic compounds related to selective pesticides, we
report the results obtained for the direct esterification between
cinnamic acids (1) and phenols (2b) or 2-(N-phthalimido)ethanol
(2a)(Scheme1).AseriesofsolidacidsorsaltswithPreysslerstructure,
such as H14[NaP5W29MoO110] (PW), H14[NaP5W30O110] (PWMo),
2.2. Catalyst characterization
2.2.1. Textural analysis
The specific surface area (SBET) of supports and catalysts was
determined by nitrogen adsorption/desorption techniques using
Micromeritics Accusorb 2100E equipment.
K
12.5Na1.5[NaP5W30O110]Á15H2O
MoO110]Á15H2O (PWMoK) and 10% silica-supported H14[NaP5W29-
MoO110 (PWSiO2), and H14[NaP5W30O110 (PWMoSiO2), were
(PWK),
K12.5Na1.5[NaP5W29-
2.2.2. Diffuse reflectance spectroscopy (DRS)
]
]
The solid samples were studied in the range 200–600 nm, using
UV–vis Varian Super Scan 3 equipment, fitted with a diffuse
reflectance chamber with an inner surface of BaSO4. Samples were
compacted in a Teflon sample holder to obtain a sample thickness
of 2 mm.
employed as eco-friendly and reusable catalysts in bulk and silica-
supported form; in both cases heterogeneous catalysis takes place.
2. Materials and methodology
2.1. Catalyst preparation
2.1.1. Bulk catalysts
2.2.3. Fourier transformed infrared spectroscopy (FT-IR)
Bruker IFS 66 equipment, pellets with KBr, and a measuring
range of 400–1500 cmÀ1 were used to obtain the FT-IR spectra of
the solid samples.
The Preyssler salt,
K
12.5Na1.5[NaP5W30O110]Á15H2O (PWK),
was prepared from Na2WO4Á2H2O according to a previously
2.2.4. X-ray diffraction (XRD)
reported method [38] and converted to the corresponding acid
XRD patterns of representative solid samples were recorded by
means of a Philips PW-1732 device with built-in recorder, using
H
14[NaP5W29MoO110] (PW), by passing it through a Dowex-50W-
x8 ion-exchange column. For the preparation of the Mo-
substituted Preyssler salt
12.5Na1.5[NaP5W29MoO110]Á15H2O
(PWMoK), procedure similar to the preparation of PWK
was adapted and described in the literature [39]. In a typical
the following conditions: Cu K
40 kV in the high voltage source; scanning angle (2
and scanning rate, 18 minÀ1
a
radiation; Ni filter; 30 mA and
K
u) from 58 to 508
a
.
Scheme 1.

112
D.M. Ruiz et al. / Applied Catalysis A: General 374 (2010) 110–119
2.2.5. SEM analysis
2.5.2. 2-Phthalimidoethyl 3,4,5-trimethoxycinnamate
The catalysts were characterized with a Philips 505 scanning
electron microscope (SEM) using an accelerating voltage of 15 eV.
The samples were fixed as powder on a graphitized band and
coated with graphite.
Colorless solid, mp 117–118 8C (Lit. [40] 118–120 8C); 1H NMR
(400 MHz, CDCl3),
d (ppm): 3.01 (2H, t, J = 7.3 Hz), 3.54 (9H, br s),
4.51 (2H, t, J = 7.3 Hz), 6.64 (1H, d, J = 16 Hz), 7.26–7.29 (2H, m),
7.43–7.81 (4H, m), 7.60 (2H, m), 7.90 (1H, d, J = 16 Hz); 13C NMR
(100 MHz, CDCl3),
d (ppm): 37.4, 56.4, 67.6, 105.6, 116.9, 123.4,
2.2.6. Acidity measurements
130.0, 132.2, 132.3, 134.0, 145.7, 153.6, 167.0, 168.4.
An amount of 0.05 ml of 0.1N n-butylamine (Carlo Erba) in
acetonitrile (J.T. Baker) was added to a known mass of solid
(0.05 g for bulk and 0.4 g for supported solids) using acetonitrile
as solvent, and stirred for 3 h. Later, the suspension was titrated
with the same base at 0.05 ml/min. The electrode potential
variation was measured with an Instrumentalia S.R.L. digital
pHmeter, using an LL Solvotrode LiCl–ethanol 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 interpret 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, À100 < E < 0 mV, and very weak site,
E < À100 mV.
2.5.3. 2-Phthalimidoethyl 4-methylcinnamate
Colorless solid, mp 117–118 8C (Lit. no data); 1H NMR
(400 MHz, CDCl3),
4.50 (2H, t, J = 7), 6.64 (1H, d, J = 16 Hz), 7.26–7.29 (2H, m), 7.50–
7.89 (4H, m), 8.04 (2H, d, J = 16 Hz); 13C NMR (100 MHz, CDCl3),
d (ppm): 1.24 (3H, s), 3.01 (2H, t, J = 7 Hz),
d
(ppm): 22.0, 37.2, 67.7, 116.2, 125.2, 128.4, 129.1, 129.5, 130.6,
132.2, 133.8, 148.0, 164.3, 168.3.
2.5.4. 2-Phthalimidoethyl 3-chlorocinnamate
Oil (Lit. no data); 1H RMN (400 MHz, CDCl3),
d (ppm): 3.01 (2H,
t, J = 7 Hz), 4.50 (2H, t, J = 7), 6.64 (1H, d, J = 16 Hz), 7.13–7.30 (4H,
m), 7.59–7.61 (2H, m), 7.68–7.98 (4H, m), 8.04 (1H, d, J = 16 Hz);
13C RMN (100 MHz, CDCl3),
d (ppm): 37.2, 67.7, 116.2, 124.9, 126.2,
127.4, 128.3, 129.1, 129.5, 129.8, 130.6, 131.9, 132.2, 135.8, 148.0,
164.3, 168.3.
2.5.5. Phenyl cinnamate
2.3. General procedures and statements
Colorless solid, mp 75–75.5 8C (Lit. [30] 75–76 8C); 1H NMR
(400 MHz, CDCl3),
J = 7.5 Hz), 7.25 (1H, t, J = 7.3 Hz), 7.38–7.46 (5H, m), 7.56–7.62
(2H, m), 7.88 (1H, d, J = 16 Hz); 13C NMR (100 MHz, CDCl3),
d
d (ppm): 6.64 (1H, d, J = 16 Hz), 7.18 (2H, d,
All the reactions were monitored by TLC on precoated silica
gel plates (254
m
m). Flash column chromatography was
performed with 230–400 mesh silica gel. All the yields were
calculated from crystallized products. All the products were
identified by comparison of physical data (mp, TLC, NMR) with
those reported or with those of authentic samples prepared by
the respective conventional methods using sulfuric acid as
catalyst. Melting points of the compounds were determined in
sealed capillary tubes and are uncorrected. 13C NMR and 1H
NMR spectra were recorded at room temperature on Bruker AC-
250 and Bruker Advance DPX-400 spectrometers using TMS as
internal standard. Entries and target compounds have the same
number.
(ppm): 117.4, 121.6, 125.8, 128.3, 129.0, 129.4, 130.7, 134.2, 146.6,
150.8, 165.4.
2.5.6. 4-Methylphenyl cinnamate
Colorless solid, mp 96–97 8C (Lit. [41] 97–97.5 8C); 1H NMR
(400 MHz, CDCl3),
d (ppm): 2.36 (3H, s), 6.62 (1H, d, J = 16 Hz),
7.04–7.06 (2H, m), 7.19–7.21 (2H, d, J = 8.1 Hz), 7.41–7.42 (3H, m),
7.57–7.59 (2H, m), 7.86 (1H, d, J = 16 Hz); 13C NMR (100 MHz,
CDCl3),
d (ppm): 20.9, 117.4, 121.3, 128.3, 129.0, 130.0, 130.6,
134.2, 135.4, 146.4, 148.6, 165.6.
2.5.7. 4-Methoxyphenyl cinnamate
2.4. Catalytic test
Colorless solid, mp 99–101 8C (Lit. [42] 100–102 8C); 1H NMR
(400 MHz, CDCl3),
6.81–6.87 (2H, m), 6.99–7.05 (2H, m), 7.32–7.38 (3H, m), 7.48–7.54
(2H, m), 7.78 (1H, d, J = 16 Hz); 13C NMR (100 MHz, CDCl3),
(ppm): 55.6, 114.5, 117.4, 122.4, 128.3, 129.0, 130.6, 134.2, 144.3,
146.4, 157.3, 165.8.
d (ppm): 3.74 (3H, s), 6.55 (1H, d, J = 16 Hz),
All catalysts were dried overnight prior to use. The esterification
reaction was performed in a round bottom flask, which was
equipped with a condenser and immersed in an oil bath. A solution
of cinnamic acid 1 (1.1 mmol) and 2-phthalimidoethanol 2a or
phenol 2b (1 mmol) in toluene (4 ml) and the bulk or silica-
supported WD acid (10–2 mmol) were refluxed with stirring for
the indicated time. In both cases the catalyst was removed by
filtration and washed twice with toluene (1 ml each). The organic
solution was washed with cold 1 M NaOH (2 Â 2 ml) and H2O
(2 Â 2 ml) and then dried over anh. Na2SO4. Evaporation of the
solvent under reduced pressure and recrystallization from petro-
leum ether, or silica flash column chromatography gave the pure
cinnamate 3.
d
2.5.8. 4-Bromophenyl cinnamate
Colorless solid, mp 111–112 8C (Lit. [43] 113–115 8C); 1H NMR
(400 MHz, CDCl3),
J = 8.9 Hz), 7.25–7.38 (3H, m), 7.43 (2H, d, J = 8.8 Hz), 7.57–7.62
(2H, m), 7.86 (1H, d, J = 16 Hz); 13C RMN (100 MHz, CDCl3),
d (ppm): 6.60 (1H, d, J = 16 Hz), 7.06 (2H, d,
d
(ppm): 117.1, 119.1, 123.7, 128.6, 129.3, 131.1, 132.7, 134.3, 147.3,
150.1, 165.3.
2.5.9. 4-Nitrophenyl cinnamate
2.5. Spectral data for products
Colorless solid, mp 145–146 8C (Lit. [44] 145–146 8C); 1H NMR
(400 MHz, CDCl3),
J = 9 Hz),7.40–7.50(3H,m),7.55–7.65(2H,m),7.92(1H,d,J = 16 Hz),
8.30 (2H, d, J = 8 Hz); 13C NMR (100 MHz, CDCl3),
(ppm): 116.2,
d (ppm): 6.63 (1H, d, J = 16 Hz), 7.37 (2H, d,
2.5.1. 2-Phthalimidoethyl cinnamate
Colorless solid, mp 114–115 8C (Lit. [40] 113–115 8C); 1H NMR
(400 MHz, CDCl3): 3.01 (2H, t, J = 7.3 Hz), 4.50 (2H, t, J = 7.2 Hz),
6.37 (1H, d, J = 15.8 Hz), 7.30–7.58 (5H, m), 7.58–7.90 (4H, m),
7.59–7.61 (2H, m), 7.90 (1H, d, J = 16 Hz); 13C NMR (100 MHz,
CDCl3): 37.2, 67.7, 105.6, 117.6, 123.4, 128.4, 129.1, 130.4, 130.6,
132.2, 134.3, 145.7, 166.9, 168.3.
d
122.5, 125.2, 128.5, 129.1, 129.5, 133.8, 148.0, 155.6, 164.3.
2.5.10. 1-Naphthyl cinnamate
Colorless solid, mp 106–107 8C (Lit. [40] 105–108 8C); 1H RMN
(400 MHz, CDCl3),
d (ppm): 6.64 (1H, d, J = 16 Hz), 7.26–7.29 (2H,

D.M. Ruiz et al. / Applied Catalysis A: General 374 (2010) 110–119
113
m), 7.45–7.43 (3H, m), 7.59–7.61 (2H, m), 7.90 (1H, d, J = 16 Hz),
8.04 (2H, d, J = 8.7 Hz); 13C RMN (100 MHz, CDCl3),
(ppm): 117.0,
d
118.1, 121.3, 125.4, 126.0, 126.4, 127.0, 128.0, 128.4, 129.0, 130.8,
134.2, 134.7, 146.7, 147.0, 165.4.
3. Results and discussion
3.1. Catalyst characterization
According to the literature [39], the characteristic bands
associated with the Pope–Jeannin–Preyssler (PW) structure are
three bands due to P–O stretching at 1163, 1079 and 1022 cmÀ1
,
and two bands attributed to W–O–W at 941 and 913 cmÀ1; a band
at 757 cmÀ1 corresponding to W55O; and one band at 536 cmÀ1
due to P–O bending. Finally, the H2O bending at 1616 cmÀ1 is
observed. The PWMo-substituted acid shows similar bands with
little displacement. Previously, our investigations discussed the
vibrational spectra of other heteropolycompounds (Keggin HPA)
[45] as a function of the nature of the metal (Me) introduced,
considered as a perturbing element. The typical spectrum is
modified as follows: the introduction of a Me, other than W, in the
structure (Mo in this case) induces a decrease of the W–O
stretching frequencies and a possible splitting of the P–O band,
depending on the Me nature. This splitting can be considered as an
indirect evaluation of the strength of the Me–O interaction.
The spectra of silica, PW and PWMo supported on silica are
shown in Fig. 1. As reported in the literature [11], silica exhibits
three main bands at 1100, 800 and 470 cmÀ1. The PW or PWMo
band placed in the 1100 cmÀ1 region of the PW or PWMo on SiO2
spectrum is masked by that of silica. Anyway, information can still
be obtained from the less affected regions that show the
characteristic bands that confirm the presence of the undegraded
anion. These bands are those mentioned previously: 941 and
913 cmÀ1 (attributed to W–O–W), 757 cmÀ1 (corresponding to
W55O), and finally, 536 cmÀ1 due to P–O bending.
Fig. 1. FTIR spectra of silica and supported PW and PWMo.
and 380 nm; this result discards the structures of the main
impurities from the synthesis of Preyssler acids, Wells-Dawson
H6P2W18O62 (DRS bands at 190, 250 and 300 nm) and Keggin
H3PW12O40 (DRS bands at 190 and 260 nm) acids [47]. When PWK
spectrum is observed, the second band is obtained at 340 nm, very
close to the first one, and very different from the band at 380 nm
when K is no present in the Pressley structure. PWMoK shows
absorption bands at 310 and 330 nm shifted with respect to the
characteristic bands at 220 and 260 nm of tetrahedral Mo, bands
attributed to oxygen–metal transfers at 210–230 nm, and another
one that extends up to 400 nm indicating Mo in octahedral
position. These bands are present in the Keggin structure. This
spectrum is difficult to be assigned an intact Preyssler structure,
but with other techniques it is possible to say that the majority of
the Pressley structure is present in the salt.
X-ray diffraction patterns shown in Fig. 2 are similar with
Preyssler structures, showing two main diffraction lines placed at
6.58 and 17.58 2
13.68, 25.58 and 33.78 2
u
, and some wide, low intensity signals at 8.88, 128,
. All diffractograms shown in Fig. 2 present
u
In relation to acidic properties, when measuring the strength of a
solid acid or base, it should be recognized that activity coefficients
for species on the solid are unknown. Acidity and basicity functions
for the solid are not properly defined thermodynamically. The acid
strength of a solidis defined as the abilityof the surface to convert an
adsorbed neutral base into its conjugated acid. In the case of the HPA
studied it is not easy to analyze this property, so we will try to
explain its behavior by potentiometric titration.
very low definition due to the experimental conditions used and to
their high hygroscopicity during their scanning.
The information about the substitution of ions into the
geometry of the structure is given by DRS spectra; the charge-
transfer absorption spectra of the most nonreduced polyanions
appear in the 200–500 nm region, and consist of bands attributed
to oxygen–metal transfers [46]. The DRS spectra of PW, PWK and
PWMoK are shown in Fig. 3. The PW acid presents two bands at 330
Fig. 2. XRD pattern for the acids and salts with Preyssler structure.

114
D.M. Ruiz et al. / Applied Catalysis A: General 374 (2010) 110–119
bulk HPA, but the number of acid sites is different. From 822 mV,
the potentiometric curve abruptly decreases to 50 mV for 0.3 meq
of n-butylamine/g of catalyst. This behavior could be due to the fact
that Mo presence could be inducing the redox internal reaction and
the electron ring in the Pressley structure could be stimulating it. In
this case, the acidic properties decrease.
In relation to the textural 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 1. All properties of bulk HPA are lower, in all
cases the values are close to the experimental error of the methods
used. This is in agreement with previously reported information
[48], although the bulk acids show negative adsorption data,
attributed to a partial reduction of the acids due to the highly
hygroscopic nature of the solid, causing an electronic delocaliza-
tion that repulses the nitrogen molecules, generating gas desorp-
tion. In addition, according to the surface microvalues calculated
using the t-plot method (Table 1), more than 70% of the total
surface area comes from a mesoporous structure.
Fig. 3. DRS of acids and salts with Preyssler structure.
Finally, in Fig. 5, SEM micrographs of PWMoSiO2 are shown,
together with superficial analysis (mapping) of Si, Mo and W,
respectively. The morphological aspect of all synthesized samples
is similar, and the distribution of Mo/W atoms in the solid is
homogeneous.
3.2. Catalytic test
Optimum reaction conditions were examined employing
cinnamic acids and phthalimido ethanol as test reaction sub-
strates. Initially, silica gel was used as catalyst in refluxing toluene
and no reaction was observed (Table 2, entry 1). Then Preyssler
acids and salts were tested (Table 2, entries 3–4, 6–7), and the
results are interesting to analyze. The K salts, in both cases, do not
yield any product; perhaps the intrinsic acid of Preyssler structure
salt is not strong enough to produce phtalimidoethyl cinnamate.
When PW and PWMo are used as catalysts, the yields are 62 and
66, for an initial E of 862 and 833 mV, respectively, this behavior
confirming that the strong acid sites drive the reaction. Finally,
when supported catalysts are used (Table 2, entries 5 and 8),
PWMoSiO2 is the most suitable catalyst. The reuse of this acid was
tested under the same conditions, showing a small decrease of the
yield (Table 2, entry 8b).
In order to obtain the optimal temperature, three temperatures
(20, 80 and 110 8C, Table 3) were tested, using PWMoSiO2 (1 mol%)
under toluene as solvent, for different reaction times. At 20 8C, the
reaction systems do not yield any reaction product at 7, 14 and
24 h. When the reaction temperature is increased to 80 8C, for the
same reaction times, the yield increases to 58% for 24 h. The
toluene boiling point (110 8C) was then tested under the same
reaction conditions. The increase of the yields with temperature
and reaction time was consistent (Table 3, entry 9). It is important
to say that the PWMo acid is insoluble in a reaction medium
because toluene is a nonpolar solvent. For this reason when PWMo
is supported on silica, it is not soluble in the solvent although the
temperature used was very high for the HPA. In order to evaluate
Fig. 4. Potentiometric titration curves of acids and salts with Preyssler structure.
The maximum acid strength of the acid sites for PW and PWMo
is 862 and 833 mV, respectively (Fig. 3). This E indicates very
strong acid sites for both bulk solid acids, very similar to the bulk
Keggin HPA. The values of meq base/g solid where the plateau is
reached indicate the total number of acid sites (11 for PW and 13
for PWMo). PWMoK shows weak acid sites (87 mV) and PWK gives
a very weak maximum strength (À20 mV), not shown in this work.
As regards supported acids, the results show a low decrease in
the maximum acid strength, 784 mV for PWSiO2 (from 862 mV)
and 822 mV for PWMoSiO2 (from 833 mV). The support (silica)
acid strength was tested, with an initial potential of 25 mV. When
PW is supported on silica, the number of acid sites is very similar to
bulk PW (Fig. 4). When Mo is incorporated into the structure of PW
and then supported on silica, the initial E is very close to that of
Table 1
Textural properties of catalysts.
Catalyst
BET surface
area (m2/g)
t-Plot micropore
area (m2/g)
BET adsorption average
BJH adsorption average
t-Plot micropore
volume (m3/g)
˚
˚
pore width (A)
pore width (A)
PW
0.81
1.12
0.23
0.07
93.11
95.81
295.6
0.00
0.00
0.01
0.01
PMo
223.76
164.45
169.92
PWSiO2
PWMoSiO2
239.74
227.28
28.92
27.09
166.86
172.18

D.M. Ruiz et al. / Applied Catalysis A: General 374 (2010) 110–119
115
Fig. 5. SEM micrographs of PWMoSiO2 and mapping on the same sample of Si, W and Mo.
the possible catalyst solubilization, an additional test was
performed. A PWMoSiO2 sample was refluxed in toluene for 2 h,
filtered and dried in vacuum till constant weight. The activity of the
so-treated catalyst was the same as that of the fresh catalyst (81%
in 24 h). The refluxed toluene was used as solvent for attempting
the reaction without adding the catalyst. After 24 h cinnamate was
not detected and the starting material was quantitatively
recovered.
In relation to the catalyst amount, the results (Table 4) show
1 mol% concentration as the optimal quantity (Table 4, entry 3). In
Table 2
Table 3
Esterification of cinnamic acid with 2-(N-phthalimido)ethanol using various
Preyssler heteropolyacid catalystsa.
Temperature optimization for the synthesis of 2-phthalimidoethyl cinnamatea
using PWMoSiO2 as catalyst.
Entry
Catalyst
Yields (%)
Entry
Temperature (8C)
Time (h)
Yields (%)
1
2
3
4
5
6
7
8
None
0
1
2
3
4
5
6
7
8
9
20
20
7
14
24
7
0
0
Silica (support)
PWK
0
0
20
0
PW
62
80
42
49
58
43
58
82
PWSiO2
PWMoK
PWMo
75
80
14
24
7
0
80
66
82 (76)b
110
110
110
PWMoSiO2
14
24
a
Reaction conditions: 1 mmol cinnamic acid, 1 mmol 2-(N-phthalimido)ethanol,
a
4 ml of toluene, 1 mmol% catalyst, 110 8C, stir, 24 h.
Reaction conditions: Cinnamic acid (1 mmol), 2-(N-phthalimido)ethanol
(1 mmol) and PWMoSiO2 (1 mol%) at different temperatures under toluene.
b
Reuse.

116
D.M. Ruiz et al. / Applied Catalysis A: General 374 (2010) 110–119
Table 4
For phenol esterification, the subject reaction was performed
using cinnamic acid and phenol as substrate. The test for the
optimal catalyst gives the better results using again PWMoSiO2 in a
7 h time period (Table 6, entry 5). The reuse assays give no changes
after the catalyst reuse under the same conditions (Table 6, entry
6).
Effect of catalyst concentration for the synthesis of 2-phthalimidoethyl cinnamatea.
Entry
Catalyst concentration (mol%)
Yields (%)
1
2
3
4
0.1
0.5
1
26
55
82
83
2
The influence of water was tested by reproducing the optimal
reaction conditions for cinnamic acid esterification with phenol
a
Reaction conditions: Cinnamic acid (1 mmol) and 2-(N-phthalimido)ethanol
(1 mmol) and PWMoSiO2, toluene 110 8C, 24 h.
˚
(Table 6) in the presence of molecular sieves (3 A). The resulting
91% yield (Table 6, entry 6) shows no significant difference,
suggesting low influence of water on the reaction.
this case it is possible to observe that the yield is directly
proportional to the amount of catalyst used. The acid strength of
PWMoSiO2 is high but the number of acid sites is low, so it is
necessary to increase the amount of active phase.
With optimal conditions defined (PWMoSiO2 1 as catalyst, 24 h
in refluxing toluene), substituted phthalimidoethyl cinnamates
were prepared from substituted cinnamic acids (Table 5). The
lowering in the yields shows the effect of electron-withdrawing
groups (Table 5, entry 3), or a steric effect caused by the three
methoxy groups (Table 5, entry 4).
In this case, the solvent volume is a relevant aspect, because 5%
of subproduct 4-phenyl-3,4-dihydrocoumarin was observed when
a low volume (1 ml) was used. In fact, this is the main selective
product in the case of solvent-free reaction conditions from the
same substrates [49] (Scheme 2). The lowering of reaction time
when phenol was used replacing phthalimidoethanol can be
attributed to an interaction of the oxygen atoms of the imido group
with the acid catalyst (Scheme 3); this fact is supported by
evidence of the decrease in the yield after reuse (Table 2, entry 8),
probably due to the catalyst-imidoalcohol interaction.
Table 5
Synthesis of different 2-phthalimidoethyl cinnamates using PWMoSiO2 as catalysta.
Entry
Cinnamic acid
Product
Yields (%)
1
82
2
3
84
27
62
4
a
Reaction conditions: cinnamic acids (1 mmol) and 2-(N-phthalimido)ethanol (1 mmol). PWMoSiO2 (1 mol%) at 110 8C, toluene, 24 h stirring.
Table 6
Esterification of cinnamic acid with phenol. Reaction conditionsa optimization.
Entry
Catalyst
Time (h)
Yields (%)
1
2
3
4
5
6
None
7
7
–
Silica (support)
PWMo
–
4
39
PWMoSiO2
PWMoSiO2
PWMoSiO2
4
49
7
87
14
89 (86)b; (91)c
a
Reaction conditions: 1 mmol cinnamic acid, 1 mmol phenol, 4 ml of toluene, 1 mmol% catalyst, 110oC, stirring.
Reuse.
b
c
˚
Using molecular sieves (3 A).

D.M. Ruiz et al. / Applied Catalysis A: General 374 (2010) 110–119
117
Scheme 2.
Scheme 3.
Again, with the optimal conditions defined (PWMoSiO2
1 mmol% as catalyst, 7 h in refluxing toluene), substituted
arylcinnamates were prepared from substituted cinnamic acids
(Table 7). No changes were observed with different electronic
substitutions of phenol.
catalyst. The cycle begins with the protonation of the carboxylic
oxygen of the cinnamic acid caused by the HPA; this interaction
generates a carbocation in the carboxylic carbon atom that is
later attacked by a phenol molecule acting as nucleophile. The
instability of the generated ion promotes the proton transfer
and consequent water molecule elimination, generating a new
cation that finally deprotonates, giving the aryl cinnamate and
recovering the catalyst.
As a result of the assays performed and informed in this
research, in Fig.
6 we propose a catalytic cycle for the
esterification of cinnamic acids with phenols using PWMo as
Table 7
Synthesis of different phenyl cinnamates using PWMoSiO2 as catalysta.
Entry
1
Phenol
Products
Yields(%)
87
2
3
4
5
89
85
86
78
95
6
a
Reaction conditions: cinnamic acid (1 mmol) and phenols (1 mmol). PWMoSiO2 (1 mol%) at 110 8C, toluene, 7 h stirring.

118
D.M. Ruiz et al. / Applied Catalysis A: General 374 (2010) 110–119
Fig. 6. Proposed catalytic cycle for the aryl cinnamate Preyssler acid-mediated esterification.
´
´
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We thank Agencia Nacional de Promocion Cientıfica
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