H5CoW12O40 supported on nano silica from rice husk ash: A green bifunctional catalyst for the reaction of alcohols with cyclic and acyclic 1,3-dicarbonyl compounds

Article abstract of DOI:10.1016/j.molcata.2012.03.005

Rice husk ash (RHA) is an abundant agricultural by-product. The present research work deals with the production of nano silica powders, with high surface area and in amorphous form, from RHA using optimized technique. 12-Tungestocobaltic acid, H₅CoW₁₂O₄₀ (CoW), was supported on silica from RHA to produce silica supported CoW (CoW/NSiO ₂) as a nano catalyst. The characterization data derived from FT-IR reveal that the Keggin structure of CoW remains intact in CoW/NSiO₂. TEM image showed that the catalyst had spherical shape with an average particle size of 10 nm. The acidity of the catalyst was measured by potentiometric titration with n-butylamine. To our surprise, this very strong solid acid catalyst showed an excellent distribution of acid sites, suggesting that the catalyst possesses a higher number of surface active sites compared to CoW supported on commercial silica (CoW/SiO₂), CoW and K ₅CoW₁₂O₄₀. A high catalytic activity was found over CoW/NSiO₂. Finally, CoW/NSiO₂ has been used as a highly effective catalyst for benzylation of linear 1,3-dicarbonyl compounds with benzylic alcohols and synthesis of β-keto enol ethers from cyclic 1,3-dicarbonyl compounds. The present methodology offers a practical, simple, mild, environmentally friendly, and timesaving method under solvent-free conditions.

Full text of DOI:10.1016/j.molcata.2012.03.005

Journal of Molecular Catalysis A: Chemical 358 (2012) 121–128
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
H5CoW12O40 supported on nano silica from rice husk ash: A green bifunctional
catalyst for the reaction of alcohols with cyclic and acyclic 1,3-dicarbonyl
compounds
Ezzat Rafieea,∗, Maryam Khodayaria, Masoud Kahrizia, Reza Tayebeeb
a Faculty of Chemistry, Razi University, 67149 Kermanshah, Iran
b Department of Chemistry, School of Sciences, Sabzevar Tarbiat Moallem University, Sabzevar, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 January 2012
Received in revised form 28 February 2012
Accepted 4 March 2012
Available online 13 March 2012
Rice husk ash (RHA) is an abundant agricultural by-product. The present research work deals with the
production of nano silica powders, with high surface area and in amorphous form, from RHA using
optimized technique. 12-Tungestocobaltic acid, H5CoW12O40 (CoW), was supported on silica from RHA
to produce silica supported CoW (CoW/NSiO2) as a nano catalyst. The characterization data derived from
FT-IR reveal that the Keggin structure of CoW remains intact in CoW/NSiO2. TEM image showed that
the catalyst had spherical shape with an average particle size of 10 nm. The acidity of the catalyst was
measured by potentiometric titration with n-butylamine. To our surprise, this very strong solid acid
catalyst showed an excellent distribution of acid sites, suggesting that the catalyst possesses a higher
number of surface active sites compared to CoW supported on commercial silica (CoW/SiO2), CoW and
K5CoW12O40. A high catalytic activity was found over CoW/NSiO2. Finally, CoW/NSiO2 has been used as a
highly effective catalyst for benzylation of linear 1,3-dicarbonyl compounds with benzylic alcohols and
synthesis of ␤-keto enol ethers from cyclic 1,3-dicarbonyl compounds. The present methodology offers a
practical, simple, mild, environmentally friendly, and timesaving method under solvent-free conditions.
Keywords:
Nano catalyst
12-Tungestocobaltic acid
Bifunctional catalyst
Supported catalyst
␤-Keto enol ethers
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
synthesis has now reached significant levels, not only for the pos-
sibility to perform environmentally benign synthesis, but also for
Nanotechnology is a growing and important field in the modern
science. Nano-sized catalysts continue to attract interest for differ-
ent researcher areas due to their different physical and chemical
properties when compared to bulk material. The extremely small
sized particles maximized the surface area exposed to the reactant,
allowing more reactions to occur at the same time, thus speeding
up the process. Catalysis by heteropoly acids (HPAs) and related
compounds is a field of increasing importance as nano catalysts
[1–4]. They are promising candidates for catalyst design due to
their controllable acid and redox properties and have been actu-
ally utilized in some industrial processes [5,6]. K5CoW12O40 in our
previous works showed excellent reactivity as an electron trans-
fer catalyst [7,8]. By changing this salt to acidic form (H5CoW12O40
hereafter CoW) Brönsted acidity is added to electron transfer abil-
ity of this catalyst. But there is a problem here, CoW in acidic
form is a homogeneous catalyst in most of organic processes. The
use of heterogeneous catalysts in different areas of the organic
the good yields frequently, accompanied by excellent selectivity
that can be achieved. The use of a support allowing the CoW to
be dispersed over a large surface may result in an increase of its
catalytic activity. The property as well as the catalytic behavior of
supported CoW depends on the carrier, catalyst loading and con-
ditions of pre-treatment among other variables. Diverse supports
have been tested, acidic or neutral solids such as active carbon,
SiO2 and ZrO2 being suitable as carriers [1] but SiO2, which is rela-
tively inert toward HPAs, is the one most often used. At present,
nano scale silica materials are prepared using several methods,
including vapor-phase reaction, sol–gel, and thermal decomposi-
tion technique [9–12]. However, their high cost of preparation has
limited their wide applications. The production of reactive nano
scale silica from rice husk (RH) is a simple process compared to
other conventional production techniques [13].
RH is an abundantly available waste material in rice producing
countries where there is a need for its disposal or utilization. It
would be of benefit to the environment to recycle the waste to pro-
duce eco-materials having a high-end value. The white ash obtained
from its controlled combustion or calcination at relatively mod-
erate temperatures produces a main residue containing silica in
∗
Corresponding author. Tel.: +98 831 427 4559; fax: +98 831 427 4559.
E-mail addresses: e.rafiei@razi.ac.ir, ezzat rafiee@yahoo.com (E. Rafiee).
1381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2012.03.005

122
E. Rafiee et al. / Journal of Molecular Catalysis A: Chemical 358 (2012) 121–128
Table 1
Comparison of catalytic activity of HPC catalysts in benzylation of ethyl acetoacetate.
Entry
Catalyst
Time (min)
60
60
3
10
10
5
Yield (%)a
Conversion (%)
1
2
3
4
5
6
–
5
10b
95
95
94
89
6
11
95
96
94
91
H5CoW12O40/NSiO2
H5CoW12O40/NSiO2
K5CoW12O40
H5CoW12O40
H5CoW12O40/SiO2
a
Isolated yield; reaction conditions: benzyl alcohol (1 mmol), ethylacetoacetate
(1 mmol), catalyst (0.4 g), 80 ◦C under solvent-free conditions.
b
Reaction proceed at room temperature.
the form of an amorphous phase [14,15,9,16,17]. The small particle
diameter and high surface area of the ultra-fine silica powders give
rise to many technological applications [9,18] especially as support
to heterogenization of catalyst [19].
Supporting the CoW on silica nano particles with high surface
area improves the catalytic performance in various organic reac-
tions and also they can exhibit quite different characteristics from
the bulk CoW and K5CoW12O40
.
The construction of carbon–carbon bonds is a fundamental task
in organic synthesis. Alkylation of 1,3-dicarbonyl compounds is
a useful transformation involving C C bond formation. However,
because of the poor leaving ability of the hydroxyl group, excess
amount of Brönsted or Lewis acids is required for this kind of trans-
formation [20–23]. Recently, many chemists have focused their
attention on the acid-catalyzed alkylation processes that are con-
sidered to be the most promising approaches in this field. Various
Lewis acids such as InCl3 [24,25], InBr3 [26], FeCl3 [27–31], M(OTf)3
[32,33], Lewis acidic ruthenium complex [34], as well as montmo-
rillonite [35], p-toluenesulfonic acid [36] have been demonstrated
to facilitate the dehydrative substitution of allylic and benzylic alco-
hols with 1,3-dicarbonyl compounds. Unfortunately many of these
methods for the benzylation reaction prepared so far has major
or minor disadvantage such as long reaction times, low yield of
the products, tedious work up procedures and use of the toxic sol-
vents. In this study we report efficient CoW/NSiO2 for benzylation
of a wide variety of 1,3-dicarbonyl compounds under solvent-free
conditions. Activity of mentioned catalyst has been compared with
Fig. 2. Potentiometric titration curves of catalysts.
2. Experimental
2.1. General remarks
All organic materials were purchased commercially from Fluka
Chemical Corp. and Merck & Co., Inc. and were used without further
purification. Aerosil 300 silica from Degussa was used. FT-IR spectra
were recorded with KBr pellets using a Shimadzu 470 spectropho-
tometer. NMR spectra were recorded on a Bruker Avance 200 MHz
NMR spectrometer with CDCl3 as solvent and TMS as internal stan-
dard. CHN compositions were measured by Hekatech elemental
analysis model Euro EA 3000. Transmission electron microscopy
(TEM) examination was performed with a TEM microscope Philips
CM 120 kV Netherland.
reactivity of CoW supported on commercial silica and K5CoW12O40
.
2.2. Preparation of nano silica from RH
Previously K5CoW12O40 has been used as a reusable catalyst in
some organic transformation [7,8]. But to the best of our knowledge
this is the first time that acidic form of CoW has been supported,
characterized and used as catalyst.
RH was washed thoroughly with water to remove the soluble
particles and dust or other contaminants present in them whereby
the heavy impurities like sand are also removed. It was then dried
in an air oven at about 110 ◦C for 24 h. The dried RH was refluxed
with an acidic solution (0.1 M HCl) for nearly 90 min with fre-
quent stirring. It was cooled and kept intact for about 20 h, then
decanted, thoroughly washed with warm distilled water until the
rinse became free from acid. The wet solid was subsequently dried
in an oven at 110 ◦C for 24 h. The obtained white powder was
burned inside a programmable furnace (Model Nobertherm con-
troller B 170) at 700 ◦C with rate 10 ◦C/min and 2 h as soaking time.
We designated this as RHA.
20 g of RHA sample was stirred in 160 mL of 2.5 M sodium
hydroxide solution. The solution was heated in a covered beaker
for 3 h with constant stirring. It was filtered and the residue was
washed with 40 mL boiling distilled water. The obtained viscous,
transparent and colorless solution was allowed to cool down to
room temperature. Then H2SO4 (5.0 M) was added under constant
stirring at controlled conditions until pH = 2 and then added NH4OH
until pH = 8.5, and allowed to room temperature for 3 h. Nano silica
was prepared by refluxing of extracted silica with HCl (6.0 M) for 4 h
and then washed repeatedly using deionized water to make it acid
free. It was then dissolved in 2.5 M of sodium hydroxide stirring.
H2SO4 was added until pH = 8. The precipitate silica was washed
Fig. 1. Comparison of catalytic activity of different catalysts in benzylation of ethy-
lacetoacetate.

E. Rafiee et al. / Journal of Molecular Catalysis A: Chemical 358 (2012) 121–128
123
O
O
O
O
OH
Catalyst ( 0.4 g)
H3C
H3C
OCH2CH3
OCH2CH3
Solvent-free, 80 °C
Ph
1b
2c
4bc
Scheme 1. Model reaction.
repeatedly with warm deionized water to make it alkali free, dried
at 50 ◦C for 48 h in oven.
the cobalt (II) complex was oxidized to the cobalt (III) complex
by potassium persulfate. The crystals of K5CoW12O40·20 H2O were
dried. After this synthesis, CoW was prepared from its potassium
salt in acidic conditions and extracted from ether [7].
2.3. Preparation of CoW
CoW was synthesized from its potassium salt via acidic extrac-
tion method. The synthesis of K5CoW12O40·3H2O started with
the preparation of sodium tungstodicobalt(II)ate from cobaltous
acetate (2.5 g, 0.01 mol) and sodium tungstate (19.8 g, 0.06 mol) in
acetic acid and water. The sodium salt was then converted to the
potassium salt by treatment with potassium chloride (13 g). Finally,
2.4. Preparation of CoW/NSiO2
For the preparation of 40 wt.% CoW/NSiO2 catalyst, 3.0 g of nano
silica was dispersed in 30 mL water by sonication. Solution of CoW
(2 g in 10 mL water) was added and stirred overnight at room
°
OH
OH
R2
R1
R1
O
R3
R1
R2
°
CoW/NSiO2 CoW/NSiO2
O
R4
R2
R1
R2
+ OH °
H2O
OH
O
O
O
R4
R4
R3
R3
4
(a) Electron transfer pathway
O
O
OH2
OH
R2
OH2
R2
R1
CoW/NSiO2
H2O
R1
CoW/NSiO2
R2
R1
R2
R1
R1
o
Pathway 3
Pathway 2
C
-
OH
H
2
W
O
R2
/
N
S
O
O
i
O
R3
R4
2
R2
R1
R2
R2
R1
..
HO
O
O
O
6
OH2
R2
R1
R1
-
R4
H
2
O
O
O
..
OH
OH
O
R3
O
H+
R2
R1
R3
R4
O
OH O
R4
R1
H+
O
O
R3
R1
R4
R2
R3
R2
R2
R1
4
5
Main product
(b) Brönsted acid pathway
Scheme 2. Proposed mechanisms for catalytic addition of diketones to alcohols.

124
E. Rafiee et al. / Journal of Molecular Catalysis A: Chemical 358 (2012) 121–128
Table 2
Benzylation of different 1,3-dicarbonyl compOoundOs catalyzed by 40% CoW/NSiO .
2
R'
O O
H3C
H3C
2
'
R
O
R
R
R
4
6
CoW/NSiO2 (0.4 g)
O
O
ROH
1
°
Solvent-free, 80 C
3
R
R
O
R
O
O
1a: R= (Ph)2CH
1b: R= PhCH2
1c: R= 4-Cl-PhCH2
2a: R'= CH3
2b: R'= OCH3
2c: R'= OCH2CH3
6
5
Entry
Alcohol
Diketone
Product
Time (min)
Yield (%)a
Conversion (%)
O O
H C
3
CH
3
Ph
Ph
1
1a
2a
3
92
95
O
O
H3C
OCH3
Ph Ph
O O
2
3
1a
1a
2b
2c
5
5
84
73
94
96
H3C
Ph
O O
OCH2CH3
Ph
H C
3
CH
3
Ph
4
1b
2a
2
91
95
O
O
O
O
H3C
H3C
H3C
OCH3
Ph
O
5
6
1b
1b
2b
2c
3
3
86
95
96
95
OCH2CH3
OCH2CH3
Ph
O
Cl
7
8
1c
1a
1b
2c
3
5
3
3
91
70
75
95
93
94
Ph
Ph
O
O
O
Ph
O
9
3
a
Isolated yield.

E. Rafiee et al. / Journal of Molecular Catalysis A: Chemical 358 (2012) 121–128
125
Table 3
O
O
+
O
OR
CoW/NSiO2
ROH
Synthesis of ␤-keto enol ethers in the presence of CoW/NSiO2.a.
Entry
Alcohol
Amount of the catalyst (g)
Time (min)
Yield (%)b
1
2
3
4
5
6
7
MeOH
EtOH
EtOH
0.1
10
45
20
10
15
15
15
94
0.02
0.05
0.1
0.1
0.1
86 (95)c
88
EtOH
92
93
94
85
Propanol
Butanol
Hexanol
0.1
OH
8
9
0.1
0.1
20
20
73
76
OH
OH
10
12
0.1
0.1
2 (h)
15
15
84
OH
OH
MeO
O2N
13
14
0.1
15
92
OH
0.1
0.1
25
15
81
90
OH
15
a
Reaction conditions: dimedone (1 mmol), alcohol (3 mL), at 80 ◦C.
Isolated yield.
b
c
Reaction proceeded at 60 ◦C.
temperature. Finally, the solvent was removed by using rotary
evaporator, dried and calcinated at 150 ◦C for 2 h.
[37–40]. Analytical data for 5a as a new compound are presented
below:
5-Benzhydryloxy-3,3-dimethyl-cyclohex-2-enone (5a): 1H
NMR (200 MHz, CDCl3) ı: 6.8–7.21 (m, 10H), 5.95 (s, 1H), 4.82 (s,
1H), 2.81 (s, 2H), 1.84 (s, 2H), 1.05 (s, 6H); 13C NMR (100 MHz, CDCl3)
ı: 192.1, 170.5, 142.1, 127.8, 127.5, 126.4, 125.5, 86.1, 77.9, 54.2,
43.7, 26.2, 15.4; Anal. calcd for C21H22O2: C 82.32, H 7.24; found:
C 82.34, H 7.22; HRMs calcd for C21H22O2: M, 306.4036; found:
306.40.
2.5. General procedure for benzylation of the 1,3-dicarbonyl
compound
The solid acid catalyst (0.4 g), was added to a mixture of alco-
hol (1 mmol) and 1,3-dicarbonyl compound (1 mmol) at 80 ◦C. The
reaction mixture was crushed for the short period of time. The
progress of the reaction was monitored by thin-layer chromatog-
raphy (TLC). After completion of the reaction, the mixture was
washed with acetonitrile, and then the catalyst was removed by a
repeated centrifugation (4000–6000 rpm, 30 min) and decantated.
Catalyst was collected and treated with 1,2-dichloroethane (DCE)
for removing coke followed by vacuum drying and calcinations at
150 ◦C, 1 h for reusing. The filtrate was concentrated and the prod-
uct was purified by column chromatography on silica-gel using
EtOAc/hexane (1:3) as eluent. All the products were identified by
comparing their spectral data with those of the authentic samples
2.6. General procedure for synthesis of the ˇ-keto enol ethers
A mixture of dimedone (0.14 g, 1.0 mmol) and alcohol (4 mL)
was stirred well in the presence of the catalyst (0.4 g) at 80 ◦C. After
complete consumption of the starting materials as indicated by
TLC, the reaction was filtered. The excess alcohol in the filtrate was
removed by rotary evaporation and the crude purified by column
chromatography over silica-gel (ethyl acetate/hexane, 1:4).

126
E. Rafiee et al. / Journal of Molecular Catalysis A: Chemical 358 (2012) 121–128
Fig. 3. Effect of various amount of CoW/NSiO2 in model reaction.
3. Results and discussion
3.1. Catalytic reaction
As a starting point for optimization of the reaction conditions,
the reaction of benzyl alcohol (1 mmol) and ethylacetoacetate
(1 mmol) under solvent-free conditions was chosen as a model
reaction (Scheme 1). Control experiment showed that the sub-
strates did not react in the absence of the catalyst (Table 1, entry
1). Only 10% yield of the corresponding product was obtained in
the presence of CoW/NSiO2 at room temperature (Table 1, entry
2). Whereas by increasing the temperature to 80 ◦C, a significant
improvement was observed and yield of the product was increased
to 95% after 5 min (Table 1, entry 3). Thus 80 ◦C was chosen as
reaction temperature in further investigations.
CoW/NSiO2 is a bifunctional catalyst with Brönsted acidity and
electron transfer properties. It can catalyze the reaction in two
different ways: Brönsted acid and electron transfer mechanisms
(Scheme 2(a) and pathway 1 of (b)). In order to investigate which
way is the main mechanism, the model reaction was carried out
in the presence of K5CoW12O40 as heterogeneous catalyst with
only electron transfer property [7] and the obtained result was
compared to CoW/NSiO2 (Table 1, entries 3 and 4). The reaction
proceeds well in the presence of K5CoW12O40 via electron-transfer
mechanism and excellent yield of the product was obtained. But,
the reaction time was increased in comparison with CoW/NSiO2
(Fig. 1). Thus, it seems that high catalytic activity of CoW/NSiO2
catalyst was mainly related to its Brönsted acidity. To test this
hypothesis, a radical scavenger (acrylonitrile) was added to the
model reaction in the presence of K5CoW12O40 as catalyst after
3 min (when 50% of the corresponding product was obtained). After
continuation of the reaction until 10 min, yield of the product was
55% and it seems that the reaction was stopped by addition of rad-
ical scavenger. This test was checked in the model reaction with
CoW/NSiO2 as catalyst to know which way was the main mech-
anism of the catalyst, Brönsted acidity, electron transfer ability or
both of them. By addition of the radical scavenger the reaction com-
pleted after 6 min. It means that the benzylation reaction probably
proceeded via both pathways. Possibly, Brönsted acidity was more
active than electron transfer pathway. More investigation about the
mechanism of the catalyst is under investigation in our laboratory.
In addition to bifunctionality, the catalytic behavior of
CoW/NSiO2 presumably depends on the carrier. In order to inves-
tigate the effect of nano silica (SBET = 623 m2/g [13]) on catalytic
activity, the model reaction was carried out in the presence of bulk
Fig. 4. FT-IR of the (a) pure CoW, (b) nano silica and (c) CoW/NSiO2.
CoW and CoW/SiO2 (CoW support on Aerosil silica (SBET = 300 m2/g)
as a commercial support) catalysts (Table 1, entries 5 and 6). The
results show that the catalytic activity of CoW/NSiO2 was higher
than the activities of bulk CoW and CoW/SiO2.
Considering that the above-mentioned catalytic reaction is
mainly acid-catalyzed organic reaction, the acidity of the cata-
lysts plays an important role in this transformation. The nature
of the supports apparently affected the acidity of the supported
CoW, which was probed by a potentiometric titration with an
organic base [41]. The acidic environment around the electrode
membrane mainly determines this method, based on the measured
potential difference. The measured electrode potential is an indi-
cator of the acidic properties of the dispersed solid particles. An
aliphatic amine, as n-butylamine, with a basic dissociation con-
stant of approximately 10−6, allows a potentiometric titration of
strong acids. The titration curves obtained for CoW, CoW/SiO2 and
CoW/NSiO2 are presented in Fig. 2. It is considered that the ini-
tial electrode potential (Ei) indicates the maximum strength of the
acid sites and that the value from which the plateau is reached
(mmol amine/g catalyst) indicates the total number of acid sites
that are present in the titrated solid (n value). The acidic strength
of the acid sites can be classified according to the following ranges:
Ei > 100 mV (very strong acid sites), 0 < Ei < 100 mV (strong acid
sites), −100 < Ei < 0 mV (weak acid sites) and Ei < −100 mV (very
weak acid sites) [41].
All kind of catalysts presented very strong acid sites, Ei > 100 mV.
Immobilization of CoW to silica or nano silica was accompanied by

E. Rafiee et al. / Journal of Molecular Catalysis A: Chemical 358 (2012) 121–128
127
material with ethyl acetoacetate in optimum reaction conditions.
After 15 min corresponding product 6 was obtained in 94% yield.
Thus, it seems there is another pathway for benzylation of the
1,3-dicarbonyl compounds (Scheme 2(b)).
Employing
cyclic
1,3-dicarbonyls,
such
as
5,5-
dimethylcyclohexane-1,3-dione (dimedone), using the same
reaction conditions, produced corresponding 2-benzylic-1,3-
dicarbonyl compounds in very low yield (<10%), and an unknown
compounds were generated as main products. This compound
was isolated by column chromatography and characterized by
FTIR, NMR, and CHN techniques. The results indicated that the
unknown compound is the corresponding ␤-keto enol ethers (5a
and 5b). Due to the wide range applications of ␤-keto enol ethers as
synthons in several key intermediate compounds [42,43], various
alcohols were treated with dimedone as a cyclic 1,3-dicarbonyl
compound (Table 3). Toward these studies, effect of amount of
the catalyst and reaction temperature was investigated (Table 3,
entries 2–4). The present conversion with primary alcohols pro-
ceeded rapidly to form the products in high to excellent yields. This
protocol was then applied to secondary, vinyl or benzyl alcohols,
which are structurally different. Reactions with these type alcohols
took longer reaction times. Even, 4-nitrobenzyl alcohol responded
t
well with longer duration and good yield. To our surprise, butyl
alcohol as substrate afforded 16% of corresponding product after
1 h (Table 3, entry 10), as previous approaches gave negative
results for similar reactions [44,45]. More amount of the catalyst
(0.12 mg) was not improved reaction time or yield of this reaction.
It should be mentioned that no acetal adducts were detected, even
in the case of linear 1,3-diketones for which it has been reported
[46].
A possible mechanism of the reaction in the presence of
CoW/NSiO2 as a Brönsted acid catalyst is proposed (Scheme 2(b)).
The benzylation of linear 1,3-dicarbonyl compounds possibly pro-
ceeded by pathway 1. By action of CoW/NSiO2 the alcohol was
protonated to generate a stable benzyl cation after dehydration.
This carbocation could be quickly combined with the employed
1,3-dicarbonyl compound to produce, after the release of H+, the
final alkylated product.
Production of the symmetrical ether as the side reaction
explains by pathway 2. In the presence of CoW/NSiO2, the alco-
hol is protonated to give oxonium ion. Subsequent dehydration of
oxonium ion results carbocation intermediate. Binding of cation
with the same alcohol, followed by the release of H+, generated the
symmetrical ether.
The suggested mechanism to explain the reaction between
cyclic 1,3-dicarbonyl compounds with alcohols is shown in path-
way 3. Similar to other routes, by action of the catalyst a stable
benzyl cation was produced from initial alcohol after dehydration.
Under the present conditions, due to the resonance of oxygen with
adjacent ␲-bonding electrons, cyclic 1,3-dicarbonyl compound as a
nucleophile combined with the stable carbocation to produce, after
the release H+, ␤-keto enol ethers.
Fig. 5. TEM image of the CoW/NSiO2 (a) and the diagram of particle size distribution
of the CoW/NSiO2 particles (b).
a gradual increase in total number of surface acidic sites. But “n”
value in the case of CoW/NSiO2 was about 2 and more than other
ones due to its higher surface area and higher dispersion of acidic
protons.
As a result, it seems that higher activity of CoW/NSiO2 catalyst
is related to surface area enhancement and higher dispersion of the
acidic protons of nano catalyst and possibly also bifunctionality of
this catalyst.
For establishing the best reaction conditions, the model reaction
was carried out in the presence of different amounts of CoW/NSiO2
catalyst. Improvement in time of the reaction was observed as the
catalyst quantity increased from 0.1 to 0.4 g. Further increasing in
the catalyst quantity showed no improvement in the yield or reac-
tion time (Fig. 3). Thus, 0.4 g of CoW/NSiO2 was the suitable choice
for catalyst loading.
Encouraged by these results, we turned our attention to various
cyclic and linear 1,3-dicarbonyls and benzylic alcohols (Table 2).
Reaction of the linear 1,3-dicarbonyls such as acetylacetone with
different types of benzyl alcohols proceeded smoothly provid-
ing 73–95% yields (entries 1–7). However, symmetrical ether 6
from self-condensation of alcohol was observed in some cases. To
investigate that this ether (6) is as an intermediate to produce prod-
uct 4 in different pathway, dibenzyl ether was used as starting
3.2. Characterization of the catalyst
3.2.1. FT-IR of CoW/NSiO2
Fig. 4 shows the FT-IR spectra of the pure CoW, nano silica and
CoW/NSiO2. The FT-IR spectrum of the pure CoW indicates that typ-
ical bands for CoW, belong to Co O, 1105 cm−1; W = Ot (terminal
oxygen), 943 cm−1; W Oc W (corner sharing oxygen), 887 cm−1
and W Oe W (edge sharing oxygen), 779 cm−1. These character-
istic peaks of Keggin structure were observed in FT-IR spectrum of
CoW/NSiO2, although some bands were overlapped with those of
the support. FT-IR of the supported CoW indicates that the primary
Keggin structure is preserved after supporting CoW on nano silica.

128
E. Rafiee et al. / Journal of Molecular Catalysis A: Chemical 358 (2012) 121–128
Acknowledgments
The authors thank the Razi University Research Council and Ker-
manshah Oil Refining Company for support of this work.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molcata.2012.03.005
References
[1] N. Mizuno, M. Misono, Chem. Rev. 98 (1998) 199–218.
[2] I.V. Kozhevnikov, J. Mol. Catal. A: Chem. 305 (2009) 104–111.
[3] T. Okuhara, Chem. Rev. 102 (2002) 3641–3666.
[4] U. Kortz, A. Muller, J. Slageren, J. Schnack, N.S. Dalol, M. Dressel, Coord. Chem.
Rev. 253 (2009) 2315–2327.
Fig. 6. Reusability of the CoW/NSiO2 in the reaction of benzyl alcohol (1 mmol),
ethylacetoacetate (1 mmol), catalyst (0.4 g), 80 ◦C under solvent-free conditions
after 30 min.
[5] M. Misono, Chem. Commun. (2001) 1141–1152.
[6] J.S. Min, N. Mizuno, Catal. Today 71 (2001) 89–96.
[7] E. Rafiee, A. Azad, Bioorg. Med. Chem. Lett. 17 (2007) 2756–2759.
[8] E. Rafiee, Sh. Tangestaninejad, M.H. Habibi, V. Mirkhani, Bioorg. Med. Chem.
Lett. 14 (2004) 3611–3614.
[9] L. Sun, K. Gong, Ind. Eng. Chem. Res. 40 (2001) 5861–5877.
[10] M. Tomozawa, D.L. Kim, V. Lou, J. Non-Cryst. Solids 296 (2001) 102–106.
[11] P.A. Tanner, B. Yan, H. Zhang, J. Mater. Sci. 35 (2000) 4325–4328.
[12] G. Wu, J. Wang, J. Shen, T. Yang, Q. Zhang, B. Zhou, Z. Deng, F. Bin, D. Zhou, F.
Zhang, J. Non-Cryst. Solids 275 (2000) 169–174.
[13] E. Rafiee, M. Khodayari, Sh. Shahebrahimi, M. Joshaghani, J. Mol. Catal. A 351
(2011) 204–209.
[14] S. Huang, S. Jing, J. Wang, Z. Wang, Y. Jin, Powder Technol. 117 (2001)
232–238.
3.2.2. TEM of CoW/NSiO2
The particle size and shape of the CoW/NSiO2 was observed
by transmission electron microscopy (Fig. 5(a)). Shape of the
CoW/NSiO2 is spherical with an average particle size of 10 nm.
Fig. 5(b) shows the diagram of particle size distribution of these
particles.
3.3. Recyclability
[15] E.S.M. Seo, M. Andreoli, R. Chiba, J. Mater. Process. Technol. 141 (2003)
351–356.
[16] T.-H. Liou, Mater. Sci. Eng. A 364 (2004) 313–323.
[17] N. Yalc¸ in, V. Sevinc¸ , Ceram. Int. 27 (2001) 219–224.
[18] H.O. House, Modern Synthetic Research, 2nd ed., Benjamin, Menlopark, CA,
1972, p. 492.
[19] F.W. Chang, T.J. Hsiao, J.D. Shih, Ind. Eng. Chem. Res. 37 (1998) 3838–3845.
[20] G.C. Gullickson, D.E. Lewis, Aust. J. Chem. 56 (2003) 385–388.
[21] F.S. Prout, E.P.Y. Huang, R.J. Hartman, C.J. Korpied, J. Am. Chem. Soc. 76 (1954)
1911–1913.
[22] F. Bisaro, G. Prestat, M. Vitale, G. Poli, Synlett (2002) 1823–1826.
[23] J.T. Adams, B. Abramovitch, C.R. Hauser, J. Am. Chem. Soc. 65 (1943)
552–554.
[24] M. Yasuda, T. Somyo, A. Baba, Angew. Chem. Int. Ed. 45 (2006) 793–796.
[25] T. Saito, Y. Nishimoto, M. Yasuda, A. Baba, J. Org. Chem. 71 (2006) 8516–8522.
[26] P. Vicennati, P.G. Cozzi, Eur. J. Org. Chem. (2007) 2248–2253.
[27] J. Kischel, K. Mertins, D. Michalik, A. Zapf, M. Beller, Adv. Synth. Catal. 349 (2007)
865–870.
[28] Y. Yuan, Z. Shi, X. Feng, X. Liu, Appl. Organomet. Chem. 21 (2007) 958–964.
[29] U. Jana, S. Biswas, S. Maiti, Tetrahedron Lett. 48 (2007) 4065–4069.
[30] I. Iovel, K. Mertins, J. Kischel, A. Zapf, M. Beller, Angew. Chem. Int. Ed. 44 (2005)
3913–3917.
One of the advantages of using heterogeneous catalyst is
the possibility of their reusability and/or regeneration to reduce
wastage and production of wasteful material. The stability of the
supported catalyst has been studied by running the reaction suc-
cessively with the same catalyst in the same reaction. Fig. 6 shows
the results of this study in the model reaction in the presence of
CoW/NSiO2. Calcinations of the fresh and reused catalyst can help
to increase reactivity and reusability. Also, the treatment with 1,2-
dichloroethane (DCE) was attempted for removing coke from the
catalyst. The catalyst could, after a specified workup, be reused sev-
eral times, albeit with slightly decreasing activity. Fig. 6 shows a
slightly loss of activity after four successive runs in longer reaction
times. It could thus be drawn that CoW/NSiO2 could be a satisfied
catalyst for this reaction with high activity and good reusability.
4. Conclusion
[31] P. Salehi, N. Iranpoor, F.K. Behbahani, Tetrahedron 54 (1998) 943–948.
[32] M. Noji, Y. Konno, K. Ishii, J. Org. Chem. 72 (2007) 5161–5167.
[33] W. Huang, J. Wang, Q. Shen, X. Zhou, Tetrahedron Lett. 48 (2007) 3969–3973.
[34] P.N. Liu, Z.Y. Zhou, C.P. Lau, Chem. Eur. J. 13 (2007) 8610–8619.
[35] K. Motokura, N. Nakagiri, T. Mizugaki, K. Ebitani, K. Kaneda, J. Org. Chem. 72
(2007) 6006–6015.
The studies show that the silica support with the fine grain size,
large surface area, high degree of purity and the amorphous state is
successfully obtained through a consecutive preparation method
from RHA as an agricultural waste. CoW/NSiO2 was synthesized
by impregnation method as an efficient catalyst. According to TEM
results, the average particle size of CoW/NSiO2 was 10 nm. FT-IR of
the supported CoW indicated that the primary Keggin structure was
preservedafter supportingCoWon nano silica. The catalyticactivity
of CoW/NSiO2 was studied for direct benzylation reactions of linear
1,3-dicarbonyl compound under solvent-free conditions. Results
showed that reaction of the cyclic 1,3-dicarbonyl compounds with
alcohols produced ␤-keto enol ethers as main products. The cat-
alytic reactions proceeded with excellent yields in very short
reaction times. It seems CoW/NSiO2 is a bifunctional catalyst with
Brönsted acidity and electron transfer properties. It can catalyze
the reaction in two different ways. Also, this catalyst can be a satis-
fied catalyst for this reaction with good reusability. The present
methodology offers a practical, simple, mild, environmentally
friendly, and timesaving method under solvent-free conditions.
[36] R. Sanz, A. Martıınez, D. Miguel, J.M.A. Álvarez Gutiérrez, F. Rodrııguez, Adv.
Synth. Catal. 348 (2006) 1841–1845.
ˇ
ˇ
[37] K. Funabiki, T. Komeda, Y. Kubota, M. Matsui, Tetrahedron 65 (2009)
7457–7463.
[38] P. Thirupathi, S.S. Kim, Tetrahedron 66 (2010) 2995–3003.
[39] B. Das, K. Laxminarayana, B. Ravikanth, J. Mol. Catal. A: Chem. 271 (2007)
131–133.
[40] E. Rafiee, M. khodayari, M. Joshaghani, Can. J. Chem. 89 (2011),
doi:10.1139/V11-134.
[41] E. Rafiee, M. Joshaghani, S. Eavani, S. Rashidzadeh, Green Chem. 10 (2008)
982–989.
[42] G. Tanyeli, D. Ozdemirhan, Tetrahedron Lett. 44 (2003) 7311–7313.
[43] M.G. Constantino, V.L. Junior, L.C. da Silva Filva, G.V. da Silva, Lett. Org. Chem.
1 (2004) 360–364.
[44] P. Srihari, S.S. Mandal, J.S.S. Reddy, R. Srinivasa Rao, J.S. Yadav, Chin. Chem. Lett.
19 (2008) 767–770.
[45] M. Curini, F. Epifano, S. Genovese, Tetrahedron Lett. 47 (2006) 4697–4700, and
references cited herein.
[46] A. Clerici, N. Pastorie, O. Porta, Tetrahedron 57 (2001) 217–225.