12-Tungstophosphoric acid supported on nano silica from rice husk ash as an efficient catalyst for direct benzylation of 1,3-dicarbonyl compounds in solvent-free condition

Article abstract of DOI:10.1016/j.molcata.2011.10.008

Nano silica has been prepared from rice husk, which is an agricultural waste, with high surface area and in amorphous form. 12-Tungstophosphoric acid, H₃PW₁₂O₄₀ (PW) has been supported on this silica to produce nano silica supported PW (NPW/SiO₂) as a nano catalyst. Characterization of this catalyst has been investigated by TEM, FTIR, and ICP, also, acidic properties have been studied by potentiometric titration method. 40% NPW/SiO₂ has been used as a high effective catalyst for benzylation of 1,3-dicarbonyl compounds with benzylic alcohols. The present methodology offers a clean, practical, simple, mild, environmentally friendly, green and time-saving method under solvent free condition. Also, the catalyst is a good candidate for large-scale in direct benzylation of 1,3-dicarbonyl compounds.

Full text of DOI:10.1016/j.molcata.2011.10.008

Journal of Molecular Catalysis A: Chemical 351 (2011) 204–209
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
12-Tungstophosphoric acid supported on nano silica from rice husk ash as an
efficient catalyst for direct benzylation of 1,3-dicarbonyl compounds in
solvent-free condition
Ezzat Rafieea,b,∗, Maryam Khodayaria, Shabnam Shahebrahimia, Mohammad Joshaghania,b
a Department of Chemistry, Faculty of Science, Razi University, Kermanshah, Iran
b Kermanshah Oil Refining Company, Kermanshah, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 17 June 2011
Received in revised form 3 October 2011
Accepted 8 October 2011
Available online 14 October 2011
Nano silica has been prepared from rice husk, which is an agricultural waste, with high surface area and
in amorphous form. 12-Tungstophosphoric acid, H3PW12O40 (PW) has been supported on this silica to
produce nano silica supported PW (NPW/SiO2) as a nano catalyst. Characterization of this catalyst has
been investigated by TEM, FTIR, and ICP, also, acidic properties have been studied by potentiometric titra-
tion method. 40% NPW/SiO2 has been used as a high effective catalyst for benzylation of 1,3-dicarbonyl
compounds with benzylic alcohols. The present methodology offers a clean, practical, simple, mild, envi-
ronmentally friendly, green and time-saving method under solvent free condition. Also, the catalyst is a
good candidate for large-scale in direct benzylation of 1,3-dicarbonyl compounds.
Keywords:
Nano particle
Solvent-free
Benzylation
© 2011 Elsevier B.V. All rights reserved.
Supported heteropoly acid
Nucleophilic substitution
1. Introduction
the one most often used [12]. The support can either play only a
mechanical role or modify the catalytic properties by dispersion of
The recent emergence of nano catalysis has received a lot of
attention because it opens new perspectives for the mild cataly-
sis of important reactions with lower environmental impact. The
development of an ultra high strength concrete was made possible
by the application of densified system containing homogeneously
arranged ultra fine particles [1–3]. Heteropoly acids (HPAs) such
as PW have been extensively studied as acid catalysts for many
reactions and found industrial applications in several processes
[4–8]. Heterogeneous HPAs are advantageous over conventional
homogeneous acid catalysts as they can be easily recovered from
the reaction mixture thereby eliminating the need for separation
through distillation or extraction. Supported HPA catalysts are
important for applications due to environmental and economic
considerations. They also have excellent activity and present a
greater number of surface acid sites than their bulk components
and have been pointed lately as versatile green catalysts for a vari-
ety of organic reactions [9–11]. Acidic or neutral substances, such
as SiO2, active carbon, and acidic ion-exchange resins are all suit-
able supports, but SiO2, which is relatively inert towards HPAs, is
acidic sites of HPAs as catalysts. This interaction takes place during
the catalyst preparation and is strongly dependent on the support
characteristics, the preparation method and some of the operat-
ing variables. Also, the support can modify the properties of the
precursor deposited on the surface by favoring growth of certain
structures or by inducing different types of interaction. Rice husks
(RHs) are the natural sheaths that are formed on rice grains during
their growth and removed as waste during the processing of rice
in the mills. Having no commercial value itself, and normally, RH
usually ends up being burned openly, thus causing environmental
pollution and disposal problems. Recently, efforts are being made
not only to overcome the pollution but also to find value addition
to these wastes by using them as secondary source materials. Dry
husk contains 70–85% organic matter and the inorganic remain-
der consists of silica. The rice husk ash (RHA) containing over 90%
silica by mass with a small proportion of metallic elements is con-
sidered to be the most economical source of silica. At present, nano
scale silica materials are prepared using several methods, including
vapor-phase reaction, sol–gel, and thermal decomposition tech-
nique [13–15]. However, their high cost of preparation has limited
their wide applications. Very fine particle size silica powder with
very high purity and surface area can be prepared under controlled
conditions [16–24]. The large amount of silica freely obtained from
RHA provides an abundant and cheap alternative source of silica
∗
Corresponding author at: Faculty of Chemistry, Razi University, Kermanshah,
Iran. Tel.: +98 831 4274559; fax: +98 831 4274559.
E-mail addresses: ezzat rafiee@yahoo.com, e.rafiei@razi.ac.ir (E. Rafiee).
1381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.10.008

E. Rafiee et al. / Journal of Molecular Catalysis A: Chemical 351 (2011) 204–209
205
which is applicable particularly as a support for heterogeneous
catalysis. If the HPAs are supported on the silica nano particles,
it can exhibit quite different characteristics from the bulk HPAs.
Nowadays, supported PW are introduced to heterogeneous cat-
alytic researches [25–28]. Among these fine chemical reactions,
alkylation of 1,3-dicarbonyl compounds is a useful transforma-
tion involving C–C bond formation. In principle, direct nucleophilic
substitution of the hydroxy group in alcohols with nucleophiles
generally requires preactivation of the alcohol functionality
because of its poor leaving ability [29,30]. Recently, acid cat-
alysts such as BF3–OEt2, InCl3, Bi(OTf)3, Yb(OTf)3, FeCl3 and
H-montmorillonite have been employed to perform nucleophilic
substitution of benzylic alcohols with active methylene compounds
[31–36]. However, most of these methods have problems including
drastic reaction conditions, long reaction times and use of expen-
sive, toxic and moisture sensitive reagents. Hence, the development
of new catalysts with more efficiency is of interest. Solvent-free
reactions have been paid more and more attentions recently, often
providing clean, efficient, and high-yielding organic processes in
modern synthetic chemistry. The present work, deals with the
preparation of nano silica supported PW (NPW/SiO2). Then, reac-
tivity of the produced catalyst was investigated in benzylation of a
wide variety of 1,3-dicarbonyl compounds under solvent-free con-
dition. To the best of our knowledge this is the first attempt on using
of silica from RH as a support for PW.
with 40 mL boiling distilled water. The obtained viscous, transpar-
ent and colourless solution was allowed to cool down to room
temperature. Then, 2.5 M H2SO4 was added under constant stir-
ring at controlled condition until pH = 2 and pH readjusted to 8.5
by NH4OH addition. It was allowed to cool down to room temper-
ature for 3 h. Silica was prepared by refluxing of extracted silica
with 6.0 M HCl for 4 h and then washed repeatedly using deionised
water to make it acid free. It was then dissolved in 2.5 M sodium
hydroxide stirring. H2SO4 was added until pH 8. The precipitate sil-
ica was washed repeatedly with warm deionised water to make it
alkali free, dried at 50 ◦C for 48 h in the oven.
2.3. Preparation of NPW/SiO2 and PW/SiO2
For the preparation of 40 wt.% NPW/SiO2 catalyst, firstly, solu-
tion of 3.0 g silica was sonicated for 2 h. Then 2.0 g of PW was
dissolved in 40 mL of water and added to solution of silica. The
mixture was stirred overnight at room temperature. The solvent
was removed by filtration and catalyst was calcinated at 150 ◦C for
2 h.
PW/SiO2, was prepared by impregnating Aerosil 300 silica (SBET
,
300 m2/g) with an aqueous solution of PW as describe above. The
mixture was stirred overnight at room temperature, followed by
filtration and drying at 150 ◦C for 2 h.
2.4. Acidity measurement
2. Experimental
For the potentiometric titration, 0.05 g of solid was suspended
in acetonitrile (90 mL) and stirred for 3 h. The suspension was
titrated with a 0.05 mol/L solution of n-butylamine in acetonitrile.
The potential variation was measured with a Hanna 302 pH meter
using a double junction electrode.
2.1. General remarks
All organic materials were purchased commercially from Fluka
and Merck companies that were used without further purification.
PW from Aldrich and Aerosil 300 silica from Degussa was used.
The raw material was supplied from a rice mill of the north of
Iran. FTIR spectra were recorded with KBr pellets using a WQF-
510 FTIR Rayleigh. NMR spectra were recorded on a Bruker Avance
200 MHz NMR spectrometer with CDCl3 as the solvent and TMS
as the internal standard. CHN compositions were measured by
Hekatech elemental analysis model Euro EA 3000. Tungsten con-
tent of the catalyst was measured by inductively coupled plasma
(ICP atomic emission spectroscopy) on a Spectro Ciros CCD spec-
trometer. Surface area was calculated from the linear part of the
BET plot. Low temperature nitrogen adsorption experiments were
performed using a Quantachrome instrument, model Nova 2000,
USA system for measuring surface area. Transmission electron
microscopy (TEM) examination was performed with a TEM micro-
scope Philips CM 120 KV Netherland.
2.5. General procedure for benzylation of 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 stirred for the short period of time
listed in Table 1. Progress of the reaction was monitored by TLC.
After completion of the reaction, the mixture was washed with
acetonitrile then the catalyst was removed by a repeated cen-
trifugation (4000–6000 rpm, 30 min) and decantation. The catalyst
was washed with acetonitrile again, followed by calcination at
150 ◦C for 2 h for reusing. The filtrate was concentrated and prod-
uct was purified by column chromatography on silica-gel using
EtOAc/hexane as eluent. All products were identified by comparing
their spectral data with those of the authentic samples except for
new compounds (3cd, 3ad). Analytical data for these compounds
are presented below:
2.2. Preparation of silica from rice husk
3-(3-Methoxybenzyl)pentane-2,4-dione (compound 3ad): Pale
yellow, M.p. 84–85 ◦C, 1H NMR (200 MHz, CDCl3) ı 6.41–7.15 (m,
4H), 3.71 (s, 3H), 3.00–3.55 (m, 3H), 2.06 (s, 3H); 13C NMR (100 MHz,
CDCl3) ı 202.1, 157.8, 140.5, 130.9, 118.1, 109.4, 102.6, 68.7, 54.8,
25.9, 19.7; Anal. Calcd for C13H16O3: C, 70.89; H, 7.32; found: C,
70.86; H, 7.35; HRMs calcd for C13H16O3: M, 220.26434; found:
220.2643.
Ethyl 2-(3-methoxybenzyl)-3-oxobutanoate (compound 3cd):
Pale yellow, M.p. 65–67 ◦C, 1H NMR (200 MHz, CDCl3) ı 6.51–7.15
(m, 4H), 4.01 (q, 2H, J = 7.1 Hz), 3.75 (s, 3H), 3.08–3.65 (m, 3H), 2.06
(s, 3H), 1.27 (t, 3H, J = 7.1 Hz); 13C NMR (100 MHz, CDCl3) ı 203.1,
169.1, 158.2, 139.4, 130.2, 1180.9, 110.3, 108.5, 58.9, 57.1, 53.6, 30.9,
20.4, 110.3; Anal. Calcd for C14H18O4: C, 67.18, H, 7.25; found: C,
67.20; H, 7.23; HRMs calcd for C14H18O4: M, 250.29032; found:
250.2903.
RH was washed thoroughly with water to remove the soluble
particles, dust and other contaminants. 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 frequent stirring. It was cooled
and kept intact for about 20 h. Then it was 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 pro-
grammable furnace (Model Nobertherm controller B 170) at 700 ◦C
with rate of 10 ◦C/min and 2 h as soaking time. We designated this
as RHA.
20 g RHA sample was stirred in 160 mL of 2.5 M sodium hydrox-
ide solution. The solution was heated in a covered beaker for 3 h
with constant stirring. It was filtered and the residue was washed

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E. Rafiee et al. / Journal of Molecular Catalysis A: Chemical 351 (2011) 204–209
Table 1
O
O
O
O
NPW/SiO2 ( 0.4 g)
Solvent free, 80 °C
R'
H3C
R'-OH
H3C
R
R
3
1a: R= CH3
1b: R= OCH3
1c: R= OCH2CH3
2a: R'= (Ph)2CH
2b: R'= PhCH2
2c: R'= 2-OH-PhCH2
2a: R'= 3-OCH3-PhCH2
Benzylation of different 1,3-dicarbonyl compounds catalyzed by 40% NPW/SiO2. .
Entry
Diketone
Alcohol
Product
Time (min)
Yield (%)a
TONb
O
O
H3C
CH3
3aa
Ph
Ph
O
1
2
1a
1b
2a
2a
6
4
95
94
21
20
O
H3C
OCH3
3ba
Ph Ph
O
O
H3C
OCH2CH3
3ca
Ph
Ph
3
4
5
6
1c
1a
1b
1c
2a
2b
2b
2b
5
5 (10)
3 (10)
5 (10)
96
21
21
21
21
O
O
O
O
O
H3C
H3C
H3C
H3C
CH3
3ab
Ph
O
98 (90)
OCH3
Ph
3bb
3cb
98 (92)
97 (88)
O
OCH2CH3
Ph
O
OCH3
OH
3bc
7
1b
2c
5
95
21
O
O
3cd
H3C
OCH2CH3
OCH3
OCH3
8
9
1c
1b
1a
2d
2d
2d
2
2
2
99
97
98
22
21
21
O
O
O
H3C
H3C
3bd
OCH3
O
CH3
3ad
OCH3
10
a
Isolated yield, results in parentheses refer to large scale syntheses.
Turn over number.
b
2.6. Large scale synthesis
3. Results and discussion
Three entries in Table 1 (entries 4–6) were selected for large-
scale synthesis. The reactions of 1a, 1b and 1c (20.0 mmol) with 2b
(20.0 mmol), in the presence of 40% NPW/SiO2 (8.0 g) were done
at 80 ◦C in solvent free condition. As can be seen, 40% NPW/SiO2 is
a good candidate for large-scale and yields of the reactions were
in the range of 88–92% in larger reaction times compared to small
scale.
The average size of 40% NPW/SiO2 particles was estimated to be
about 7 nm by TEM (Fig. 1). By processing BET results, the surface
area of nano silica particles and 40% NPW/SiO2 were determined
623 and 270 m2/g, respectively. Impregnation of PW on silica sup-
port caused a remarkable decrease in the surface area, but still
showed a very high value. The reduction in the surface area of the
supported catalyst may be due to the blockage of smaller pores by

E. Rafiee et al. / Journal of Molecular Catalysis A: Chemical 351 (2011) 204–209
207
Fig. 3. Potentiometric titration curves of nano silica, 40% NPW/SiO2 and bulk PW.
Fig. 1. TEM image of 40% NPW/SiO2.
O
O
O
O
OH
NPW/SiO2 ( 0.4 g)
Solvent free, 80 °C
H3C
H3C
OCH2CH3
OCH2CH3
active species. Elemental analysis from ICP showed that PW con-
tent of 40% NPW/SiO2 was 33.2 wt.% of PW to silica. Typically, the
PW content from ICP was slightly less than the amount of expected
from the preparation stoichiometry.
Ph
1c
2b
3cb
Scheme 1. Model reaction.
The FTIR spectrum of 40% NPW/SiO2 catalyst is shown in Fig. 2.
For comparative purposes, the FTIR spectrum of bulk PW is also
included. Characteristic bands of the bulk PW appeared at 1080
(P–O), 985 (W O), 890 and 814 cm−1 (W–O–W). In 40% NPW/SiO2,
the PW band placed at the 1080 cm−1 zone is masked by the Si–O–Si
absorption band. Anyway, information can still be obtained from
the less affected regions, which showed an intensity increase of
the band placed at 814 cm−1 and non-overlapped bands at 890 and
985 cm−1 that confirm the PW on support exists in Keggin structure.
The acidity measurements of the catalyst by means of poten-
tiometric titration with n-butylamine were used to estimate its
relative acid strength according to the two values (a) Ei as initial
potential and (b) number of acid sites (Fig. 3). The initial electrode
potential (Ei) indicates the maximum strength of the acid sites
and the value from which the plateau is reached (mmol amine/g
solid) indicates the total number of acid sites that are present
in the titrated solids [28]. The acidic strength of the solids can
be classified according to the following range: Ei > 100 mV (very
strong sites), 0 < Ei < 100 mV (strong sites), −100 < Ei < 0 mV (weak
sites) and Ei < −100 mV (very weak sites). According to potentio-
metric titration curves (Fig. 3), PW presented very strong acidic
sites (Ei = 650 mV). Also, the total number of acid sites increased
compared to the silica and bulk PW. With these results, it seems
that this nano catalyst is active in organic reactions. Benzylation
of 1,3-dicarbonyl compounds was employed for assessment of the
catalytic activity.
The reaction of benzyl alcohol (1 mmol) and ethylacetoacetate
(1 mmol) under solvent free condition was carried out as a model
system for studying the effects of catalyst loading and reaction
temperature on the product yield (Scheme 1). Initially, the quan-
tity of the catalyst used in this reaction was optimized (Fig. 4).
Improvement in time of the reaction was observed as the catalyst
quantity increased from 0.3 to 0.4 g. Further increase in the catalyst
quantity showed no improvement in the yield or reaction time. It
should be noted that the reaction did not proceed in the absence
Fig. 2. FTIR of: (a) bulk PW and (b) 40% NPW/SiO2.
Fig. 4. Efficiency of the catalyst loading on the yield of the model reaction.

208
E. Rafiee et al. / Journal of Molecular Catalysis A: Chemical 351 (2011) 204–209
Table 2
Investigation of the reusability of the catalyst.
Run
Time (min)/yield (%)
Without calcinations
With calcinations
1
2
3
4
5/97
30/10
–
5/97
20/91
20/87
20/85
–
to recollect and can be reused after the first run. Hence, higher
quantity of the catalyst was taken to investigate its reusability.
The model reaction was carried out by using 1 g of the catalyst and
the experiments were properly scaled up. After each run, the cat-
alyst was washed with EtOAc/hexane (1:3) as solvent and dried
as described in Section 2. As shown in Table 2, only 12% reduc-
tion in product yield was observed after four times reuse. It should
be noted that the reusability of the catalyst without calcinations
was found unfavorable. In this case, when the supernatant portion
of the NPW/SiO2 catalyzed reaction mixture was subjected to UV
spectrum, it exhibits the presence of absorption band at 265 nm
assigned to Keggin type of PW12O403−. It was due to the leaching
of PW from the support into the liquid phase. This catalyst could be
recovered and subsequently reused several times in a solventless
system just after calcinations at 150 ◦C for 2 h.
The applicability of this method for the large-scale operations
was investigated in the reactions of 1a, 1b and 1c (20.0 mmol) with
2b (20.0 mmol) under the optimized conditions (Table 1, entries
4–6). Corresponding products were obtained with approximately
similar yields but in longer reaction times. As can be seen, 40%
NPW/SiO2 is a good candidate for large-scale in direct benzylation
of 1,3-dicarbonyl compounds.
Fig. 5. Efficiency of support only, bulk PW, 40% PW/SiO2 and NPW/SiO2 in the model
reaction.
of NPW/SiO2. The effect of temperature was also studied. It was
observed that with increase in the reaction temperature from 25
to 80 ◦C, the yield of the product was increased from 10 to 98%
after 5 min. To understand the effect of support on catalytic activ-
ity nano silica (support only), bulk PW and 40 wt.% of PW/SiO2 (PW
support on Aerosil silica as a commercial support) were checked
as catalysts in model reaction (Fig. 5). NPW/SiO2 showed higher
catalytic activity in comparison with PW/SiO2 due to its higher
surface area and higher dispersion of acidic protons. Encouraged by
these results; we turned our attention to various 1,3-diketones and
benzylic alcohols (Table 1). The experimental results showed that
benzyl alcohols or dicarbonyl compounds both with electron with-
drawing or donating substitution performed to afford the products
without the formation of any side products, in excellent yield.
To our surprise, all reactions proceeded efficiently and completed
within 2–6 min. By action of 40% NPW/SiO2 the alcohol was pro-
tonated 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 (Scheme 2).
4. Conclusion
The silica support was successfully prepared from rice husk
as an agriculture waste in nano sized particles with high sur-
face area. PW was supported on the silica to make an effective
catalyst with very strong acidic property. An efficient methodol-
ogy for solvent-free direct benzylation reactions of 1,3-dicarbonyl
compounds catalyzed by 40% NPW/SiO2 was demonstrated. The
catalytic reactions proceeded in excellent yields in very short times
without toxic organic solvent. 40% NPW/SiO2 was found as an effi-
cient, environmentally friendly, cheap and non toxic nano catalyst.
The catalyst can be a good candidate for large-scale in direct ben-
zylation of 1,3-dicarbonyl compounds.
The stability of the active species in solution has been of concern
for solid acids, especially for supported materials. The reusability
of the catalyst was studied by employing it in a model reaction. In
the present cases, the quantity of the catalyst used was too less
OH
R2
R1
Acknowledgements
O
O
The authors thank the Razi University Research Council and Ker-
manshah Oil Refining Company for support of this work.
NPW/SiO2
R3
R4
References
OH2
R2
[1] L. Hui, X. Hui-Gang, Y. Jie, O. Jinping, Composites Part B: Eng. 35 (2003) 185–189.
[2] Y. Qing, Z. Zhang, D. Kong, R. Chen, Constr. Build. Mater. 21 (2007) 539–545.
[3] P.K. Jal, M. Sudarshan, A. Saha, P. Sabita, B.K. Mishra, Colloids Surf. 240 (2004)
173–178.
[4] S.Y. Oh, T. Yoshida, G. Kawamura, H. Muto, M. Sakai, A. Matsuda, J. Power
Sources 195 (2010) 5822–5828.
R1
..
OH
O
R3
R4
-H2O
[5] T. Okuhara, N. Mizuno, M. Misono, Adv. Catal. 41 (1996) 113–252.
[6] M. Misono, Chem. Commun. (2001) 1141–1152.
[7] E.F. Kozhevnikova, J. Quartararo, I.V. Kozhevnikov, Appl. Catal. A: Gen. 245
(2003) 69–78.
[8] J.R. Ferrell lll, M.C. Kuo, M. Herring Andrew, J. Power Sources 195 (2010) 39–45.
[9] I.V. Kozhevnikov, Catalysis by Polyoxometalates, 2, Wiley, Chichester, 2002.
[10] I.V. Kozhevinkov, Chem. Rev. 98 (1998) 171–198.
[11] E. Rafiee, F. Shahbazi, M. Joshaghani, F. Tork, J. Mol. Catal. A: Chem. 242 (2005)
129–134.
O
O
R4
R2
R3
R1
R2
R1
Scheme 2. Proposed mechanism for catalytic addition of diketones to alcohols.

E. Rafiee et al. / Journal of Molecular Catalysis A: Chemical 351 (2011) 204–209
209
[12] I.V. Kozhevinkov, Catal. Rev. Sci. Eng. 37 (1995) 311–352.
[13] M. Tomozawa, D.L. Kim, V. Lou, J. Non-Cryst. Solids 296 (2001) 102–106.
[14] P.A. Tanner, B. Yan, H. Zhang, J. Mater. Sci. 35 (2000) 4325–4328.
[15] 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.
[25] E. Rafiee, M. Joshaghani, F. Tork, A. Fakhri, S. Eavani, J. Mol. Catal. A: Chem. 283
(2008) 1–4.
[26] E. Rafiee, H. Mahdavi, M. Joshaghani, Mol. Divers. 15 (2011) 125–134.
[27] E. Rafiee, H. Mahdavi, S. Eavani, M. Joshaghani, F. Shiri, Appl. Catal. A: Gen. 352
(2009) 202–207.
[16] D. Li, D. Chen, X. Zhu, Bioresour. Technol. 102 (2011) 7001–7003.
[17] D. Li, X. Zhu, Mater. Lett. 65 (2011) 1528–1530.
[28] E. Rafiee, M. Joshaghani, S. Eavani, S. Rashidzadeh, Green Chem. 10 (2008)
982–989.
[18] T.H. Liou, Ch. Yang, Mater. Sci. Eng. B 176 (2011) 521–529.
[19] D. An, Y. Guo, B. Zou, Y. Zhu, Z. Wang, Biomass Bioenergy 35 (2011)
1227–1234.
[20] Y. Liu, Y. Guo, Y. Zhu, D. An, W. Gao, Z. Wang, Y. Ma, Z. Wang, J. Hazard. Mater.
186 (2010) 1314–1319.
[29] B.M. Trost, M.L. Crawley, Chem. Rev. 103 (2003) 2921–2943.
[30] S. Ma, S. Yu, Z. Peng, H. Guo, J. Org. Chem. 71 (2006) 9865–9868.
[31] F. Bisaro, G. Pretat, M. Vitale, G. Poli, Synlett (2002) 1823–1826.
[32] M. Yasuda, T. Somyo, A. Baba, Angew. Chem. 45 (2006) 793–796.
[33] M. Rueping, B.J. Nachtsheim, A. Kuenkel, Org. Lett. 9 (2007) 825–828.
[34] W. Huang, J. Wang, Q. Shen, X. Zhou, Tetrahedron Lett. 48 (2007)
3969–3973.
[21] S.P.B. Lima, R.P. Vansconcelos, O.A. Paiva, G.C. Cordeiro, M.R.M. Chaves, R.D.T.
Filho, E.M.R. Fairbairn, Quim. Nova 34 (2011) 71–78.
[22] J. Umeda, K. Kondoh, Ind. Crops Prod. 32 (2010) 539–544.
[23] D. An, Y. Guo, Y. Zhu, Z. Wang, Chem. Eng. J. 162 (2010) 509–514.
[24] N. Thuadaij, A. Nuntiya, C. Mai, J. Sci. 35 (2008) 206–211.
[35] U. Jana, S. Biswas, S. Maiti, Tetrahedron Lett. 48 (2007) 4065–4071.
[36] K. Motokura, N. Fujita, K. Mori, T. Mizugaki, K. Ebitani, K. Kaneda, Angew. Chem.
45 (2006) 2605–2609.