H6GeMo10V2O40·16H 2O nanoparticles prepared by hydrothermal method: A new and reusable heteropoly acid catalyst for highly efficient acetylation of alcohols and phenols under solvent-free conditions

Article abstract of DOI:10.1590/S0103-50532011000700018

A new Keggin-type heteropoly acid, namely decamolybdodivanadogermanic acid (H₆GeMo₁₀V₂O₄₀·16H ₂O), with nanosized particles (5-8 nm), has been synthesized by a hydrothermal method and characterized by elemental analysis, thermogravimetric analysis (TGA), powder X-ray diffractometry (XRD), Fourier-transform infrared spectroscopy (FTIR), UV-Visible spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and potentiometric titration. H₆GeMo₁₀V₂O₄₀·16H ₂O revealed high catalytic activity for acetylation of various alcohols and phenols with acetic anhydride at room temperature (298 ± 2 K) and under solvent-free conditions. The catalyst can be easily recovered and used repeatedly for five cycles with a slight loss of activity. The catalytic activity of H₆GeMo₁₀V₂O₄₀· 16H₂O was higher than that of other Keggin-type heteropoly acids, such as H₃PW₁₂O₄₀, H₃PMo ₁₂O₄₀ and H₄SiW₁₂O₄₀.

Full text of DOI:10.1590/S0103-50532011000700018

J. Braz. Chem. Soc., Vol. 22, No. 7, 1323-1332, 2011.
Printed in Brazil - ©2011 Sociedade Brasileira de Química
0103 - 5053 $6.00+0.00
Article
A
•
H6GeMo10V2O40 16H2O Nanoparticles Prepared by Hydrothermal Method:
A New and Reusable Heteropoly Acid Catalyst for Highly Efficient Acetylation of
Alcohols and Phenols under Solvent-Free Conditions
Saeid Farhadi,*,a Reza Zareisahamiehb and Masoumeh Zaidia
aDepartment of Chemistry and bFaculty of Science,
Lorestan University, Khoramabad 68135-465, Iran
•
Um novo ácido heteropolimetálico do tipo Keggin, H6GeMo10V2O40 16H2O, foi sintetizado na
forma de nanopartículas (5-8 nm) pelo método hidrotérmico e caracterizado por análise elementar,
análise termogravimétrica (TGA), difratometria de raios X de pó (XRD), espectroscopia de
infravermelho com transformada de Fourier (FTIR), espectroscopia UV-Visível, microscopia
eletrônica de varredura (SEM), microscopia eletrônica de transmissão (TEM) e titulação
•
potenciométrica. O ácido H6GeMo10V2O40 16H2O revelou alta atividade catalítica na acetilação de
diferentes alcoóis e fenóis com anidrido acético à temperatura ambiente (298 2 K) e na ausência
de solventes. O catalisador pode ser facilmente recuperado e usado repetidamente por 5 ciclos com
•
pequena perda de sua atividade.A atividade catalítica do H6GeMo10V2O40 16H2O foi maior do que
a de outros ácidos heteroatômicos do tipo Keggin como H3PW12O40, H3PMo12O40 e H4SiW12O40.
A new Keggin-type heteropoly acid, namely decamolybdodivanadogermanic acid
•
(
H6GeMo10V2O40 16H2O), with nanosized particles (5-8 nm), has been synthesized by a
hydrothermal method and characterized by elemental analysis, thermogravimetric analysis (TGA),
powder X-ray diffractometry (XRD), Fourier-transform infrared spectroscopy (FTIR), UV-Visible
spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and
•
potentiometric titration. H6GeMo10V2O40 16H2O revealed high catalytic activity for acetylation of
various alcohols and phenols with acetic anhydride at room temperature (298 2 K) and under
solvent-free conditions. The catalyst can be easily recovered and used repeatedly for five cycles
•
with a slight loss of activity. The catalytic activity of H6GeMo10V2O40 16H2O was higher than that
of other Keggin-type heteropoly acids, such as H3PW12O40, H3PMo12O40 and H4SiW12O40.
Keywords: heteropoly acid, nanoparticles, acetylation, alcohols, solvent-free conditions
Introduction
reagents or catalysts and the use of explosive, moisture-
sensitive or expensive catalysts.
Acetylation is an efficient route for protecting the
OH functional group of alcohols and phenols during
oxidation, peptide coupling and glycosidation reactions.1
This important transformation is typically performed
using acetic anhydride and/or acetyl chloride in the
presence of either basic or acidic catalysts. Numerous
methodologies are available for this transformation,2-17
but most of them are homogeneous and the catalysts
are non-recoverable. In addition, they have one or more
disadvantages, such as prolonged reaction times, low
yields, harsh conditions, use of harmful organic solvents,
tedious work-up procedures, the requirement for excess
The use of heterogeneous catalysts seems one of the
most promising solutions to avoid the above-mentioned
problems.18 Heterogeneous catalysts offer several advantages
over homogeneous systems with respect to easy recovery,
recycling and minimization of undesired toxic wastes.
In this framework, several heterogeneous systems,
including transition metal oxides,19 montmorillonites,20
HClO4/SiO2,21 H2SO4/SiO2,22 AlPW12O40,23 zeolites,24
HBF4/SiO2,25 MoO3/Al2O3,26 NaHSO4/SiO2,27 sulfated
zirconia,28 (NH4)2.5H0.5PW12O40,29 silica-bonded cobalt(II)
salen,30 silica-bonded N-propyl sulfamic and S-propyl
sulfuric acids,31 have been applied as heterogeneous catalysts
for the acetylation of alcohols and phenols. However, each
of these methods has advantages and limitations.
*e-mail: sfarhad2001@yahoo.com

•
1324
H6GeMo10V2O40 16H2O Nanoparticles Prepared by Hydrothermal Method
J. Braz. Chem. Soc.
•
Heterogeneous acid catalysis by heteropoly acids
of sodium molybdate dihydrate (Na2MoO4 2H2O, 6.05 g,
25 mmol) under heating. After cooling the resulting
solution to room temperature, its pH was adjusted to 2.5
with concentrated sulfuric acid to give a red solution.
Solution (B): 100 mL of sodium molybdate dihydrate
(HPAs) has attracted much interest because of its potential
for great economic rewards and green benefits.32 Unlike
metal oxides and zeolites, HPAs possess very strong
Brønsted acidity and their acid sites are more uniform
,
•
and easier to control than those in other solid acid
catalysts. These properties make them suitable solid
heterogeneous catalysts for organic transformations.
In continuation of our interest in exploring green
heterogeneous catalysts for organic reactions,33 we report
herein on the hydrothermally-assisted preparation of a new,
(Na2MoO4 2H2O, 54.45 g, 225 mmol) was mixed with
67.6 mL of the alkaline germanate stock solution
([Ge] = 0.37 mol L-1). The pH of the mixture was adjusted
to 6 with sulfuric acid.
Solution (B) was added dropwise to solution (A)
with vigorous stirring while the pH was kept at 2.5.
The final mixture was transferred into an autoclave
and heated at 423 K for 2 h. After cooling the mixture
to room temperature, it was extracted with 500 mL
of diethyl ether. The evaporation in air resulted in the
•
nanosized V-substituted HPA (H6GeMo10V2O40 16H2O)
and its application as an efficient and recyclable
heterogeneous catalyst for the acetylation of alcohols
and phenols with acetic anhydride under solvent-free
conditions. To the best of our knowledge, this is the first
report regarding the synthesis of HPA nanoparticles by
the hydrothermal method and their catalytic application
for the acetylation reaction.
•
H6GeMo10V2O40 16H2O as a crystalline, orange-red
solid product (40.31 g, 78%). Elemental analysis found:
Ge, 3.49; Mo, 46.44; V, 4.89; H2O, 14.12%. Calc. for
•
H6GeMo10V2O40 16H2O: Ge, 3.51; Mo, 46.39; V, 4.93;
H2O, 13.93%. FTIR (KBr) nmax/cm-1: 3500 (O-H),
1620 (O-H), 955 (M–Od,), 870 (M–Ob–M), 810 (Ge–Oa),
760 (M–Oc–M) and 590 (O–Ge–O), (M = Mo and V; Oa,
inner oxygen; Ob, corner-shared oxygen; Oc, edge-shared
oxygen; Od, terminal oxygen).34 UV spectrum (λmax/nm,
CH3CN): 205 (Od→M CT); 258.5 (Ob/c→M CT).
Experimental
Materials and methods
Tungsten, vanadium and germanium contents were
analyzed by ICP spectrometric method. The water content
was determined by thermogravimetric analysis using
a STA 449C thermal analyzer. The X-ray diffraction
patterns were measured on a Bruker D8 Advance X-ray
diffractometer with Cu Kα (λ = 1.5418 Å) radiation. A
digital automatic potentiometric titrator apparatus (Model:
VPT-MG, India) was used for the titration of HPA. The
UV-Vis spectra were recorded on a Shimadzu UV-Vis
recording spectrophotometer accessory in the 190-400 nm
range with a resolution of 2 nm. Infrared spectra were
recorded on a Shimadzu FTIR 8400 spectrometer in
the 4000-450 cm-1 range with a resolution of 4 cm-1
(compressed pellets of 1 mg of catalysts and 100 mg
KBr). The morphology of the catalyst was determined
by scanning electron microscopy (SEM, Philips XL-30)
and transmission electron microscopy (TEM, LEO-906E).
Samples for TEM were prepared by deposition of HPA
in EtOH solution onto a standard Cu grid covered with a
holey carbon film.
Catalytic tests
The acetylation of alcohols and phenols with acetic
anhydride as the acetylating agent was carried out at room
temperature (298 2 K) in a glass thermostated vessel
equipped with a stirrer. The reactor was charged with a
mixture of alcohol or phenol (20 mmol), acetic anhydride
•
(20 mmol) and H6GeMo10V2O40 16H2O (0.15 g, 0.36 mol%
based on the alcoholic or phenolic substrate). The progress
of the reaction was monitored by thin layer chromatography
(TLC) and/or gas chromatography-mass spectrometry
(GC-MS). The catalyst was separated by addition of ethyl
acetate (2 × 20 mL) to the reaction mixture. The filtrate was
washed with 15 mL of a 10% aqueous solution of NaHCO3
and dried under Na2SO4. The solvent was removed under
reduced pressure to give the product. The results are shown
in Table 3. All products were identified by comparing their
physical and spectral data (mp, TLC, FTIR, 1H NMR and
GC-MS) with known samples or literature data.
•
Preparation of the H6GeMo10V2O40 16H2O catalyst
Recycling tests
•
The H6GeMo10V2O40 16H2O catalyst was synthesized
via application of (A) and (B) solutions as follows.
Solution (A): 50 mL of sodium metavanadate dihydrate
After each run, the catalyst was separated by adding
ethyl acetate to the reaction mixture. The recovered
catalyst was dried and then reused (Table 5).
•
(NaVO3 2H2O, 15.6 g, 100 mmol) was mixed with 20 mL

Vol. 22, No. 7, 2011
Farhadi et al.
1325
Results and Discussion
The X-ray powder diffraction of the obtained HPA is shown
in Figure 3. The diffraction peaks at 2θ (in degree) = 8-11,
18-22, 24-30 and 33-40 are characteristic of HPA anions
with Keggin structure.36 The average particle size was
calculated by X-ray diffraction line broadening using the
Debye-Scherrer equation:37 d = (0.89 λ)/(h1/2 cosθ), where
d represents the grain size; λ is the wavelength of the
X-ray (Cu Kα, 0.15418 nm); θ is the diffraction angle of
the peak and h1/2 stands for the full-width at half-height of
the peaks. The particle sizes calculated using the intense
peaks at 2θ (in degree) = 8, 21, 25.5and 35 were 7.6, 7.4,
8.2 and 8 nm, respectively. These values are in accordance
with TEM observation (vide infra).
Characterization of H6GeMo10V2O40
Initially, nanosized HPA catalyst was prepared by a
hydrothermal-assisted method and characterized by various
techniques including elemental analysis, TGA, FTIR, XRD,
UV-Vis spectroscopy and potentiometric titration. The
thermogravimetric analysis of HPA is shown in Figure 1. The
firstweightlossat323-383Kwasattributedtotheevaporation
of physically adsorbed water. The second weight loss step
occurredinthetemperaturerangeof423-653K,corresponding
tothelossofchemicallyadsorbedwatermolecules.Thepercent
of weight loss of this step was about 14%, which is consistent
with the theoretical weight loss (13.93%) calculated for the
formation of the HPA molecule with 16 molecules of water.
The weight loss observed at 693-973 K can be attributed to
the decomposition of Keggin anions.
Figure 3. The powder XRD pattern of the HPA catalyst.
In the Keggin structure, intense absorption bands at
about 200 and 260 nm are caused by charge-transfer of
the terminal oxygen and bridge-oxygen to metal atoms,
respectively. As we can see in Figure 4, the UV-Vis
spectrum of the catalyst shows two characteristic bands at
205 and 258.5 nm.34 Combined with FTIR spectra and XRD
diffratogram, this result confirms that the HPA possesses
Keggin structure.
Figure 1. The TGA curve of the HPA catalyst.
The FTIR spectrum of HPA is shown in Figure 2. The
characteristic bands of HPA with Keggin structure appeared
in the range of 500-1000 cm-1.34 Also, the bands related
to the stretching and bending vibrations of the crystalline
water molecules are observed around 3500 and 1620 cm-1,
respectively.
Figure 4. The UV-Vis spectrum of the HPA catalyst.
The number of protons in the HPA molecule and their
ionization states can be determined by potentiometric
titration. In the potentiometric titration curve (Figure 5),
the [OH]/[HPA] ratio is about 6 at the end point, which
shows that there are six protons in the HPA and that the
Figure 2. The FTIR spectrum of the HPA catalyst.
X-ray powder diffraction analysis is widely used to study
the structural features of HPA and explain their properties.35

•
1326
H6GeMo10V2O40 16H2O Nanoparticles Prepared by Hydrothermal Method
J. Braz. Chem. Soc.
electric charge of the heteropoly anion is six. This finding
confirms that the protons of the HPA are equivalent and
are dissociated in one step. This property is beneficial to
its catalytic activity.
acetylation reaction of alcohols and phenols. Initial
experiments were focused on the acetylation of benzyl
alcohol as a model substrate to check the catalytic activity
of this HPA and to optimize the reaction conditions.
Effect of the amount of catalyst
In a typical reaction, a mixture of benzyl alcohol
(20 mmol) and acetic anhydride (20 mmol) was stirred in
the presence of different amounts of the nanosized HPA
catalyst under solvent-free conditions at room temperature.
The progress of the reaction was monitored by TLC. To
optimize the catalyst requirement, the amount of catalyst
was changed from 0.12 to 0.72 mol% with respect to the
amount of benzyl alcohol. As shown in Table 1, the yield
of benzyl acetate was increased with the increase of the
amount of catalyst from 0.12 to 0.36 mol%, which is due to
the proportional increase in the number of active sites on the
surface of the catalyst nanoparticles (Table 1, entries 1-4).
However, no significant improvement in the yield was
observed with higher loads (Table 1, entries 5 and 6). The
optimum amount of catalyst in the reaction mixture was
0.36 mol% with respect to the alcohol, which resulted in
the best yield. The essential role played by the catalyst is
evident from the extremely low yield of benzyl acetate
found in the absence of the catalyst (Table 1, entry 7).
Figure 5. The potentiometric titration curve of the HPA catalyst.
Figure 6 shows the SEM and TEM images of the HPA
powder. The SEM image reveals that the powder is
composed of aggregated, extremely fine semi-spherical
particles. This observation indicates that the HPA powder
is a crystalline aggregate of nanoparticles. The TEM
image confirms that the HPAparticles have a narrow size
distribution in a range from 5 to 8 nm with the mean particle
size of about 6.5 nm. The particle size determined by
TEM is close to the particle size calculated by the Debye-
Scherrer’s formula from the XRD pattern.
Table 1. Effect of the amount of catalyst on the reaction of benzyl alcohol
with acetic anhydridea
entry
Catalyst / mol%
time / min
Yield / %
11
1
2
3
4
5
6
7
0.12
0.18
0.24
0.36
0.48
0.72
0
4
4
16
4
38
4
96
4
95
4
95
30
trace
aReaction conditions: benzyl alcohol (20 mmol), acetic anhydride
(20 mmol), without solvent, at room temperature (r.t.).
Figure 6. SEM andTEM images registered for the HPA catalyst (× 50000).
Effect of the solvent nature
In order to choose the best medium, we also tested
the effect of solvents such as acetonitrile, toluene,
dichloromethane, chloroform and tetrahydrofurane (thf) in
the reaction. According to Table 2, the yield of product was
lower in homogeneous systems (CH3CN and thf) comparing
to heterogeneous systems (toluene, dichloromethane,
Catalytic activity of H6GeMo10V2O40
In the second part of this study, the activity of
•
H6GeMo10V2O40 16H2O nanoparticles as a new
heterogeneous HPA catalyst was investigated on the

Vol. 22, No. 7, 2011
Farhadi et al.
1327
chloroform).The low yield in toluene is very likely explained
by the side reactions between toluene and the acetylating
agent. However, the best results in terms of time and yield
have been achieved without the use of any solvent. So, we
have continued the reactions under solvent-free conditions.
Generally, the reduction of the activity of catalyst in the
presence of solvent and in homogeneous systems could
be attributed to the existence of various interactions, e.g.,
solvation and/or complex formation with the reactants and/
or catalyst. When the reaction mixture is stirred without a
solvent, these interactions or interferences are eliminated,
so that the higher efficiency of the catalytic system could
be achieved, probably through increasing the number of
effective collisions between reactants and catalyst.
bond remained intact under the reaction conditions (Table 3,
entry 19).Also, various secondary alcohols were converted
with high selectivity to their corresponding acetates with
high efficiency under the same reaction conditions (Table 3,
entries 20-24). Sterically hindered tertiary alcohols such
as triphenylmethanol can also be acetylated with high
yield, but it takes longer (Table 3, entry 25). As shown
by GC-MS analysis, there was no elimination product
in the mixture. The efficacy of the catalyst can be seen
clearly in the acetylation of di-hydroxy compounds under
similar conditions (Table 3, entries 26 and 27). Cyclic
alcohols were also converted into the corresponding acetate
compounds with high efficiency (Table 3, entries 28-29).
It is very interesting that in the case of an optically active
cyclic alcohol, the reaction proceeded well with the
retention of configuration (Table 3, entry 29).
Table 2. Effect of the solvent on the reaction of benzyl alcohol with acetic
a
•
anhydride catalyzed by H6GeMo10V2O40 16H2O
Among the various hydroxy groups studied, the benzylic
OH group was found to be the most reactive. As we can
see from Table 3, functional groups such as -OMe, -CHO,
-COMe, -CN and -NO2 remained unchanged under the
reaction conditions.Also, the conversion of benzyl alcohol
into benzyl acetate on a 100 mmol scale proceeded just as
well as the 20 mmol reaction (10 min, 95%).
The scope of this reaction was further extended for
the acetylation of phenols. Phenol, substituted phenols,
hydroquinone, 1-naphthol and thiophenol were
acetylated with quantitativeyields after longer reaction
times in comparison with alcohols (Table 3, entries 30-38).
The excellent activity of HPA was demonstrated by the high
yields obtained for phenols having electron-withdrawing
groups (Table 3, entries 34 and 35).
entry
Solvent
acetonitrile
time / min
Yield / %b
1
2
3
4
5
6
60
60
60
60
60
4
56
40
75
64
55
96c
toluene
dichloromethane
chloroform
tetrahydrofuran
without solvent
aReaction conditions: benzyl alcohol (20 mmol), acetic anhydride
(20 mmol), catalyst (0.36 mol%), solvent (10 mL) at room temperature;
bGC-MS yields; cisolated yields.
Effect of the reagent nature
To show the generality and scope of this procedure,
we studied the acetylation of various alcohols, such as
primary, secondary and tertiary alcohols, as well as an
allylic alcohol and diols. The results in Table 3 show that
all alcohols were selectively converted to the corresponding
acetates in quantitative yields without any evidence of the
formation of side products. The general efficiency of this
catalytic system was evident from the variety of hydroxy
compounds that were acetylated with excellent yields and
within short reaction times.
Theacetylationofphenolswasslowerthanthatofalcohols
and required longer reaction times, so that the selective
intramolecular and intermolecular acetylation of an alcoholic
OH group in the presence of a phenolic OH group could
be achieved by the appropriate choice of reaction time. For
example, the reaction of 4-hydroxybenzyl alcohol with one
equivalent of acetic anhydride gave only
4-hydroxybenzyl
acetate with 94% yield after very short reaction time
a
(5 min) at room temperature. Indeed, when an equimolar
mixture of benzyl alcohol and phenol was subjected to
The acetylation of a wide range of ring-substituted
primary benzyl alcohols having various electron-donating
and electron-withdrawing groups was investigated with
•
acetylationreactioninthepresenceofH6GeMo10V2O40 16H2O
under the present reaction conditions, the alcohol was
converted to its corresponding acetate with 96% yield after
stirringfor5minwhilethephenolremainedunchanged.These
results show the high selectivity of our method.
•
acetic anhydride over the H6GeMo10V2O40 16H2O catalyst.
These alcohols and also primary aliphatic alcohols were
efficiently converted to their corresponding acetates with
excellent yields and the nature of the substituents had
no significant effect on the reaction times and yields
(Table 3, entries 1-18). The α,β-unsaturated primary
alcohol (cinnamyl alcohol) was selectively converted to
the corresponding acetate, and the carbon-carbon double
•
The activity of H6GeMo10V2O40 16H2O as a general
catalyst was tested via acylation of benzyl alcohol with
various anhydrides under the same reaction conditions
(Table 4). In comparison with acetic anhydride, the
reactions with higher anhydrides took longer times at

•
1328
H6GeMo10V2O40 16H2O Nanoparticles Prepared by Hydrothermal Method
J. Braz. Chem. Soc.
a
•
Table 3. Results of the acetylation of alcohols and phenols with acetic anhydride catalyzed by H6GeMo10V2O40 16H2O
entry
1
Substrate
Productb
time / min
Yield / %c
96
C6H5CH2OH
C6H5CH2OAc
4
3.5
3
2
4-Pri-C6H4CH2OH
4-MeO-C6H4CH2OH
3-MeO-C6H4CH2OH
4-Br-C6H4CH2OH
2,4-(Cl)2-C6H3CH2OH
2-Cl-C6H4CH2OH
4-NO2-C6H4CH2OH
2-NO2-C6H4CH2OH
3-NO2-C6H4CH2OH
4-CF3-C6H4CH2OH
4-CN-C6H4CH2OH
3-CHO-C6H4CH2OH
3-MeCO-C6H4CH2OH
C6H5CH2CH2OH
4-Pri-C6H4CH2OAc
4-MeO-C6H4CH2OAc
3-MeOC6H4CH2OAc
4-BrC6H4CH2OAc
97
3
95
4
4
93
5
4.5
5
92
6
2,4-(Cl)2C6H3CH2OAc
2-Cl-C6H4CH2OAc
4-NO2-C6H4CH2OAc
2-NO2-C6H4CH2OAc
3-NO2-C6H4CH2OAc
4-CF3-C6H4CH2OAc
4-CN-C6H4CH2OAc
3-CHO-C6H4CH2OAc
3-MeCO-C6H4CH2OAc
C6H5CH2CH2OAc
94
7
6
92
8
6
95
9
6.5
5.5
6
90
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
94
92
6
90
7
86
7
84
4.5
5
94
CH3CH(C6H5)CH2OH
(C6H5)CH2CH2CH2OH
CH3(CH2)5CH2OH
C6H5CH=CHCH2OH
C6H5CHOHCH3
CH3CH(C6H5)CH2OAc
(C6H5)CH2CH2CH2OAc
CH3(CH2)5CH2OAc
C6H5CH=CHCH2OAc
C6H5CH(OAc)CH3
C6H5CH(OAc)CH2CH3
CH3(CH2)5CH(OAc)CH3
(C6H5)2CHOAc
94
5
90
8
88d
4.5
4.5
4
92
96
C6H5CHOHCH2CH3
CH3(CH2)5CHOHCH3
(C6H5)2CHOH
95
8
86d
5
94
C6H5C(=O)CHOHC6H5
(C6H5)3C-OH
C6H5C(=O)CH(OAc)C6H5
(C6H5)3C-OAc
6
89
14
8
86
C6H5CHOHCH2OH
CH2OHCH2OH
C6H5CH(OAc)CH2OAc
CH2(OAc)CH2OAc
cyclo-C6H11OAc
82
8
87
cyclo-C6H11OH
10
85d
29
7
86d
30
31
32
33
34
35
36
C6H5OH
C6H5OAc
4-Me-C6H4OAc
4-Cl-C6H4OAc
2,4-Cl2C6H3OAc
4-NO2-C6H4OAc
2,4-(NO2)2C6H3OAc
AcOC6H4OAc
22
15
18
24
26
30
26
90
94
92
90
88
86
84
4-Me-C6H4OH
4-Cl-C6H4OH
2,4-(Cl)2C6H3OH
4-NO2-C6H4OH
2,4-(NO2)2C6H3OH
HOC6H4OH
37
22
90
38
C6H5SH
C6H5SAc
25
88
aReaction conditions: substrate (20 mmol), acetic anhydride (one equiv. per OH), catalyst (0.36 mol%), solvent-free, room temperature; ball products were
characterized on the basis of GC-MS, IR and 1H NMR spectral data and comparison with those of authentic samples or reported data; cisolated yield on
the basis of the weight of the pure product obtained; dGC-MS yields.

Vol. 22, No. 7, 2011
Farhadi et al.
1329
a
•
Table 4. Results of the acylation of benzyl alcohol with various anhydrides catalyzed by H6GeMo10V2O40 16H2O
Anhydride
time / min
Yield / %b
Ac2O
4
(EtCO)2O
(iso-PrCO)2O
(tert-BuCO)2O
(PhCO)2Oc
10
91
13
88
20
88
28
86
96
aReaction conditions: benzyl alcohol (20 mmol), anhydride (20 mmol), catalyst (0.36 mol%) at room temperature; bisolated yield of the corresponding
acylated product; ctwo equivalents of (PhCO)2O were used.
•
room temperature. However, the reactions were completed
in 10-28 min under solvent-free conditions affording the
corresponding acylated derivatives, providing excellent
yields (86-91%). It seemed that the rate of acylation was
influenced by the steric and electronic factors of anhydrides
and followed the order Ac2O > (EtCO)2O > (iPrCO)2O >
(tBuCO)2O > (PhCO)2O. The longer times (10-20 min)
required for the reaction with (EtCO)2O, (iPrCO)2O and
(tBuCO)2O were mainly due to the steric effect of the alkyl
groups of these anhydrides. The longer reaction time and
the requirement of two equiv. of (PhCO)2O against one
equiv. ofAc2O were due to the combined effect of the steric
and electronic factors of the phenyl group in (PhCO)2O.
The phenyl group makes the carbonyl group in (PhCO)2O
less electrophilic due to the resonance effect.
present reaction catalyzed by H6GeMo10V2O40 16H2O is
heterogeneous in nature and the catalyst is stable under the
reaction conditions employed in this work
Table 5. Reusability of the catalyst for the acetylation of benzyl alcohol
with acetic anhydridea
Cycle
Yield / %
0
1
2
3
4
5
96
94
90
90
91
88
aReaction conditions: benzyl alcohol (20 mmol),Ac2O (20 mmol), without
solvent at room temperature.
Reusability and stability of the catalyst
In order to confirm the reusability and stability of
•
H6GeMo10V2O40 16H2O catalyst, it was separated from
the reaction mixture after its first use in the acetylation
of benzyl alcohol and washed with ethyl acetate. The
recovered catalyst was found to be reusable for five cycles
with a slight loss in activity (Table 5). At the same time,
the concentration of Mo in the filtrate was determined to
be less than 1-1.5% by ICP-AES (inductively coupled
plasma atomic emission spectroscopy). In one experiment,
when the catalyst was separated from the reaction mixture
2 min after the beginning of the reaction and the filtrate
was further stirred at room temperature, no extra formation
of benzyl acetate was observed via GC-MS and TLC even
after 25 min. On the other hand, the catalyst recovered in
this experiment was used for recycling and its deactivation
was not observed in the conversion of benzyl alcohol to
benzyl acetate (4 min, 94%). Furthermore, the nature of
the recovered catalyst after the fifth cycle was studied
by elemental analysis, UV-Vis spectrophotometry and
TEM. As shown in Figure 7, elemental analysis, UV-Vis
spectrum and morphology of the recovered catalyst at
the last cycle showed no observable changes compared
to the fresh catalyst. All these findings confirm that the
Figure 7. (a) UV-Vis spectrum and (b) TEM image of the recovered
HPA catalyst after the last cycle. Elemental analysis: Ge, 3.46;
Mo, 46.27; V, 4.68%.

•
1330
H6GeMo10V2O40 16H2O Nanoparticles Prepared by Hydrothermal Method
J. Braz. Chem. Soc.
Table 6. Acetylation of benzyl alcohol catalyzed by different HPAsa
•
Comparison of the activity of H6GeMo10V2O40 16H2O
with conventional Keggin-type heteropoly acids and some
reported solid catalysts
entry
HPA
time / min
Yield / %b
•
•
1
2
3
4
5
6
7
H3PMo12O40 28H2O
60
60
60
60
60
60
4
54
The activity of H6GeMo10V2O40 16H2O in the model
•
reaction was compared with several Keggin-type HPAs
with known water content including H3PMo12O40 28H2O
(Fluka, 99%),H3PW12O40 30H2O (synthesized according to
reference 34), H3PW12O40 13H2O (synthesized according
H3PW12O40 30H2O
63
•
•
H3PW12O40 13H2O
62
•
•
H3PW12O40 7H2O
62
•
•
H4SiW12O40 23H2O
52
•
to reference 34), H3PW12O40 7H2O (Merck, 99%),
•
Na3PW12O40 xH2O
trace
•
•
H4SiW12O40 23H2O (Fluka, 99%) and Na3PW12O40 xH2O
(Fluka, 99%). The results listed inTable 6 show that, in terms
of reaction time and product yield, our catalyst is superior. It
•
H6GeMo10V2O40 16H2O
92
aReaction conditions: benzyl alcohol (20 mmol), Ac2O (20 mmol), HPA
(0.36 mol %), without solvent at room temperature;bisolated yields.
•
isnoteworthytomentionthatthreetypesofH3PW12O40 nH2O
with different water contents (n = 30, 13, 7) gave similar
results, indicating that the efficiency of catalyst was not
affected by the water content. Also, we have used the Na+
to the reaction conditions (substrate:Ac2O mole ratio,
reaction time) and/or product yield, the present method is
superior. For example, the acetylation of benzyl alcohol
in the presence of most listed catalysts takes longer time
for completion.
The activity of the silver salt of the catalyst,
Ag6GeMo10V2O40, was also tested for the model reaction.
Under this condition, the yield of benzyl acetate was about
10% after a long reaction time of 1 h. This test confirmed
that the catalytic activity of the HPA arises mainly from
its very strong Brønsted acidity. In contrast to polyprotic
inorganic acids, e.g., sulfuric acid and phosphoric acid,
which ionize in a stepwise manner, the ionization of the
•
salt of H3PW12O40 (Na3PW12O40 xH2O) as a catalyst, and that
wasnoteffective.ThisfindingconfirmsthatNa+ ionsandalso
H2O do not participate in the reaction.Although it is difficult
to offer an explanation for the different activities of these
HPAs, certainly there is a complex relationship between the
activity and structure of HPAs. By changing the constituent
elements of HPAs (both hetero and addenda atoms), their
acid strength and catalytic activity could be changed in a
wide range.32 Also, the transition metal cations such as V5+
have an important effect on the catalytic properties of these
compounds when they substitute molybdenum cations in the
octahedral MoO6 groups of the Keggin structure.32 Organic
reagents can coordinate with V5+-sites of the heteropoly
anion and therefore change the charge density in the organic
groups. Furthermore, the nanosized catalysts provide more
contact surface for the reactants in comparison with the
bulk catalysts. At this time, although the nano nature of the
•
six protons of H6GeMo10V2O40 16H2O was completed in
one step (Figure 5). This is probably due to a discrete ionic
structure of solid HPAs, comprising fairly mobile basic
structural units, i.e., heteropolyanion (GeMo10V2O406-) and
countercations (e.g., H+, H3O+ and H5O2+).32 This unique
structure manifests itself by exhibiting an extremely high
proton mobility and very strong Brönsted acidity. On
the other hand, heteropolyanions can stabilize cationic
organic intermediates. According to these facts, the
activation of the carbonyl group in acetic anhydride takes
place by coordination to the protons on the surface of
•
H6GeMo10V2O40 16H2O particles has been confirmed by
TEM and XRD techniques, we can not state that a nano size
effect is the responsible for the higher activity of this catalyst.
A comparison of the catalytic efficiency of
•
•
H6GeMo10V2O40 16H2O with selected, previously known
the H6GeMo10V2O40 16H2O nanoparticles. The activated
catalysts is collected in Table 7. It is clear that with respect
carbonyl group is attacked by the OH group of the substrate,
Table 7. Comparison of the results obtained for the acetylation of benzyl alcohol in the present work with those obtained by some reported solid catalysts
Alcohol:Ac2O
(mole ratio)
entry
Solid catalyst
Conditions
Catalyst
time
Yield / %
Ref.
1
2
3
4
5
6
Mont. KSF
SiO2/H2SO4
Solvent-free, r.t.
In CH2Cl2, r.t.
1:2
0.1 g
0.2 g
1 h
4 h
90
90
91
93
92
96
20
1:2
22
24
FER zeolite
Solvent-free, 75 oC
Solvent-free, r.t.
Solvent-free, r.t.
Solvent-free, r.t.
1:1.5
1:1
0.15 g
2 h
ZrO2/SO42-
0.05 g
10 min
1 h
28
(NH4)2.5HPW12O40
H6GeMo10V2O4
1:1
0.5 mol%
0.36 mol%
29
1:1
4 min
This work

Vol. 22, No. 7, 2011
Farhadi et al.
1331
which is a moderately reactive nucleophile, resulting in
the formation of the corresponding acetate and acetic acid
(Scheme 1).
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Scheme 1. Proposed catalytic cycle for the acetylation reaction over HPA.
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Conclusions
A new V-containing HPA with nanosized particles was
prepared by the hydrothermal process. The nano nature of
its particles was confirmed by XRD and TEM results. This
HPA was used as a new recyclable heterogeneous catalyst
for the acetylation of alcohols and phenols with acetic
anhydride under solvent-free conditions. Various hydroxy
groups can be efficiently converted into their corresponding
acetates with very short reaction times. The catalyst
can be used for subsequent cycles of acetylation without
observable loss of activity. For all the examples studied,
this simple method eliminates completely the necessity of
using organic solvents in the reaction.
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P.; Chem. Rev. 2004, 104, 199.
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Thakuria, H. T.; Borah, B. M.; Das, G.; J. Mol. Catal. A: Chem.
2007, 274, 1.
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22. Shirini, F.; Zolfigol, M. A.; Mohammadi, K.; Bull. Korean
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Acknowledgments
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Chem. 2001, 3, 320; Kumareswaran, R.; Pachamuthu, K.;
Vankar, Y. D.; Synlett 2000, 1652.
The financial support from the Lorestan University
Research Council for this work is greatly appreciated.
25. Chakraborti, A. K.; Gulhane, R.; Tetrahedron Lett. 2003, 44,
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Submitted: June 18, 2010
Published online: March 22, 2011