Sonochemical synthesis of silica and silica sulfuric acid nanoparticles from rice husk ash: A new and recyclable catalyst for the acetylation of alcohols and phenols under heterogeneous conditions

Article abstract of DOI:10.2174/138620712803519743

Silica nanoparticles were synthesized from rice husk ash at room temperature by sonochemical method. The feeding rate of percipiteting agent and time of sonication were investigated. The nanostructure of the synthesized powder was realized by the FE-SEM photomicrograph, FT-IR spectroscopy, XRD and XRF analyses. These analytical observations have revealed that the nano-sized amorphous silica particles are formed and they are spheroidal in shape. The average particle size of the silica powders is found to be around 50 nm. The as-synthesized silica nanoparticles were subsequently modified with chlorosulfonic acid and prepared silica sulfuric acid nanoparticles, which were employed as an efficient catalyst for the acylation of alcohols and phenols with acetic anhydride in excellent yields under solvent-free conditions at room temperature. This reported method is simple, mild, and environmentally viable and catalyst can be simply recovered and reused over 9 times without any significant loss of its catalytic activity.

Full text of DOI:10.2174/138620712803519743

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Combinatorial Chemistry & High Throughput Screening, 2012, 15, 705-712
705
Sonochemical Synthesis of Silica and Silica Sulfuric Acid Nanoparticles
from Rice Husk Ash: A New and Recyclable Catalyst for the Acetylation of
Alcohols and Phenols Under Heterogeneous Conditions
*
Masoud Salavati-Niasari and Jaber Javidi
Institute of Nano Science and Nano Technology, University of Kashan, Kashan, P.O. Box 87317-51167, I.R. Iran
Abstract: Silica nanoparticles were synthesized from rice husk ash at room temperature by sonochemical method. The
feeding rate of percipiteting agent and time of sonication were investigated. The nanostructure of the synthesized powder
was realized by the FE-SEM photomicrograph, FT-IR spectroscopy, XRD and XRF analyses. These analytical
observations have revealed that the nano-sized amorphous silica particles are formed and they are spheroidal in shape. The
average particle size of the silica powders is found to be around 50 nm. The as-synthesized silica nanoparticles were
subsequently modified with chlorosulfonic acid and prepared silica sulfuric acid nanoparticles, which were employed as
an efficient catalyst for the acylation of alcohols and phenols with acetic anhydride in excellent yields under solvent-free
conditions at room temperature. This reported method is simple, mild, and environmentally viable and catalyst can be
simply recovered and reused over 9 times without any significant loss of its catalytic activity.
Keywords: Acetylation, rice husk ash, silica sulfuric acid, silica, sonochemical.
1
. INTRODUCTION
supports [11-14]. In this work, we modified the silica
nanoparticles with chlorosulfonic acid and prepared silica
sulfuric acid nanoparticles. Then this modified silica was
employed for the acetylation of alcohols and phenols as
heterogeneous catalyst. Acetylation of alcohols and phenols
are common transformations in organic chemistry and play
an important role in the synthesis of organic molecules such
as natural products [15]. Acetylation of alcohols and phenols
is usually carried out by using acid anhydrides or acyl
chlorides in the presence of various catalysts such as
Generally, approximately 600 million tones of rice paddy
are produced each year. On average 20% of the rice paddy is
husk, giving an annual total production of 120 million tones
[
1]. In the majority of rice producing countries much of the
husk produced from the processing of rice is either burnt or
dumped as a waste. The ash is 92 to 95% silica, highly
porous and light weight, with a very high external surface
area [2]. The silica in the ash undergoes structural
transformations depending on the conditions (time,
temperature and etc) of combustion. At 550 C–800
amorphous ash is formed and at temperatures greater than
this, crystalline ash is obtained [3].
_
_
o
o
Cu(OTf)
[
[
2
[16], I
19], cobalt(II) salen [20], phosphomolybdic acid [21],
TMBSA][HSO ] ionic liquid [22] and etc. Most of the
2 3 3 2 4 2
[17], La(NO ) 6H O [18], CuSO 5H O
C
4
reported methodologies suffer from various disadvantages,
such as highly toxic, use of excess acetylating agent, use of
moisture sensitive catalysts and expensive, harsh reaction
conditions, poor yields of the desired products and utilization
of halogenated volatile organic solvents. Therefore, there is a
need to develop a mild, efficient, and environmentally
benign acylation reaction.
Several methods has been reported for extracted pure silica
from rice husk ash (RHA); these method include bubbling
fluidized bed [4], high temperature heat treatment [5], chemical
pre-and post-treatment with acid and base solutions [6], dry
vibratory grinding [7], and alkali-extracted method [8]. Witoon
et al. used rice husk ash as raw material and chitosan as a
template to prepare bimodal porous silica [9]. In this paper, we
demonstrated that silica nanoparticles could be successfully
prepared by a simple sonochemical method from rice husk ash.
The sonochemical method, comparing to the other methods
which have been used for preparing the silica nanoparticles
from RHA, is very fast and it does not need high temperatures
during the reactions, using the surfactants is not necessary for
this method and the other advantage of using ultrasound
radiation is that it yields smaller particles [10].
2
2
. MATERIALS AND METHODS
.1. Materials
Rice husk (RH) was collected from a rice mill in Shiraz,
Iran. Other materials used in our experiments were of
analytical grade and were used as received without further
purification. XRD patterns were recorded by a Rigaku D-
max C III, X-ray diffractometer using Ni filtered Cu Ka
radiation. FE-SEM images were obtained on HITACHI S-
4160. Fourier transform infrared (FT-IR) spectra were
recorded on Shimadzu Varian 4300 spectrophotometer in
KBr pellets. The NMR spectra were recorded on a Bruker
Silica nanoparticles have attracted intense interest
because of their high potential as catalysts or catalytic
*
Address correspondence to this author at the Institute of Nano Science and
3
avance DPX 250 MHz spectrometer in chloroform (CDCl )
using tetramethylsilane (TMS) as an internal reference. All
the yields were calculated from isolated products, and GC
Nano Technology, University of Kashan, Kashan, P.O. Box 87317-51167,
I.R. Iran; Tel.: +98 361 591 2383; Fax: +98 361 555 29 30;
E-mail: salavati@kashanu.ac.ir
1
875-5402/12 $58.00+.00
© 2012 Bentham Science Publishers

7
06 Combinatorial Chemistry & High Throughput Screening, 2012, Vol. 15, No. 9
Salavati-Niasari and Javidi
o
was used to establish their purities. A multiwave ultrasonic
generator (Sonicator 3000; Bandeline, MS 72, Germany),
equipped with a converter/transducer and titanium oscillator
at 0 C during 2 h. After addition was complete, the mixture
was stirred for 2 h until HCl gas evolution was stopped. The
resulting white powder was filtered and washed three times
with ethanol and distilled water. The washed powder was
dried at room temperature to obtain silica sulfuric acid
nanoparticles. A flow diagram of the above procedure for
synthesis of silica and silica sulfuric acid nanoparticles from
rice husk is shown in Fig. (1).
(
horn), 12.5 mm in diameter, operating at 20 kHz with a
maximum power output of 600 W, was used for the
ultrasonic irradiation. The ultrasonic generator automatically
adjusted the power level.
2
.2. Preparation of Silica Nanoparticles
Table 1. Experimental Condition for the Preparation of Silica
Nanoparticles
The rice husk (RH) was washed with water for several
time to remove dirt and other contaminants present in it, and
o
dried at 80 C for 12 h. The cleaned pure RH is then burnt
Time of Sonication
Feeding Rate
(ml/min)
Sample HCl (ml)
o
(min)
inside a muffle furnace and heating at 400 C for 1 hr (labeled
o
as RHA-400) and then held at 700 C for 1 hr (labeled as
1
2
3
4
5
6
7
8
4
5
10
10
10
10
15
20
30
0.4
0.5
0.7
1.0
0.7
0.7
0.7
0.7
RHA-700). The white rice husk ash (RHA) obtained was
used for silica extraction and preparation of silica
nanoparticles.
7
For extraction of silica from RHA, 5.0 g of the RHA was
10
7
added to 250 mL of 1.0 M HNO
8 h at room temperature to remove the metal oxide (CaO,
Fe , MgO, Na O, Al , MnO, P and K O) from
3
solution and stirred for about
1
O
2 3
2
O
2 3
2
O
5, TiO
2
2
7
RHA and extract the silica. The resulting white powder was
filtered and washed three times with ethanol and distilled water.
7
o
The washed powder was dried at 80 C for 3 h under vacuum
7
without sonication
2
and labeled as SiO extracted. For further purification of silica,
5
.0 g of the SiO extracted mixed whit 2.0 g of the NaOH and
2
o
2.4. General Procedure for Acetylation of Alcohols and
Phenols with Acetic Anhydride
heated at 600 C for 1h. The resulting powder dissolved in 50.0
ml distilled water and the solution was filtered and the residue
was washed with 20 ml boiling water. The filter solution is
sodium silicate. For preparation of silica nanoparticles, While
To a mixture of alcohol or phenol (1.00 mmol) and acetic
anhydride (1.20 mmol), silica solfonic acid (0.015 mmol)
was added and the mixture was stirred at room temperature.
The progress of the reaction was monitored by TLC or GC.
1
0 ml of sodium silicate solution (1.0 M) was irradiated with
high-intensity ultrasound simultaneously, a solution of HCl (1.0
M) was added dropwise to the Na SiO solution. The resulting
2
3
When the reaction was complete, 10.0 mL CH
added and the mixture filtered. The recovered catalyst was
washed with CH Cl (3ꢀ5 mL). The organic layer was
washed with 10% solution of saturated NaHCO (2ꢀ5 mL)
and 10 mL H O and dried over Na SO . If needed, further
2
Cl was
2
white powder was filtered and washed three times with ethanol
and distilled water. The washed powder was dried at 80 C for 3
h under vacuum. Dried powder was calcined in air at 750 C for
o
2
2
o
3
2
h. The time of sonication and volume of HCl solution were
changed for more investigation. The feeding rate for sample no.
to 4 is indicated by divided of volume of HCl per time of
2
2
4
purification was performed by column chromatography on
silica gel afforded the pure acetate.
1
sonicaton. For example: the feeding rate of sample no.1 is 4 ml
per 10 min or 0.40 (ml/min). Table 1, shows the conditions of
reactions in detail.
3
. RESULTS AND DISCUSSION
Quantitative chemical analyses of RH, RHA-400, RHA-
7
2
00 and SiO -extracted were accomplished by X-ray
2
.3. Preparation of Silica Sulfuric Acid Nanoparticles
fluorescence and shown in Table 2. This analysis showed
that inorganic content of rice ash has a high amount of silica
To a mixture of silica nanoparticles (2 g) in chloroform
(
15 mL), chlorosulfonic acid (1.65 mL) was added dropwise
(
98.30 %) and a few impurities. With increase in temperature
1
. 400oc
. 700oc
HNO3
Rice husk
Rice husk ash
SiO extracted
2
2
NaOH
6
00oc
50oc
HCl
))))))
7
SiO nano
Si(OH)4
Na SiO3
2
2
)
ClSO H
3
rt, 4h
SiO2 OSO H
3
Fig. (1). A flow diagram for synthesis of silica and silica sulfuric acid nanoparticles from rice husk.

Silica and Silica Sulfuric Acid Nanoparticles
Combinatorial Chemistry & High Throughput Screening, 2012, Vol. 15, No. 9 707
the relative contents of other elements increased in general.
[3]. X-ray diffraction pattern of silica nanoparticles is shown
Small divergences of this rule are probably caused by
inherent uncertainty of measurements. The percentage of
loss on ignition decreased after heat treatment. These results
are in agreement with previous reports [23-25] and
differences in composition are due to geographical factors,
year of harvest, sample preparation and analysis methods.
in Fig. (2c). It can be seen that broad peaks at 2Theta = 22.14
degrees confirming the formation of amorphous silica.
Results obtained from the FT-IR spectroscopy of RH,
RHA-400 and RHA-700 are shown in Fig. (3a-c)
respectively. As seen in Fig. (3a), RH contains several
absorption bands, which have been assigned to individual
structural units of carbon compounds. Heat treatment of RH
Table 2. Chemical Composition of RH, RHA-400, RHA-700
o
o
and SiO
2
-Extracted
at 400 C and 700 C caused to organic compounds were
decomposed and intensity of absorption peaks at 2870-2975
-1
-
1
-1
cm (C-H), 1200–1500 cm (CH
(
3
and CH
2
), 1627cm
Component
SiO
RH
RHA-400 RHA-700
SiO2 Extracted
-
1
C=C) and 1720 cm (C=O) was diminished. (Fig. 3d, e)
2
64.60
0.12
0.06
0.18
0.20
0.25
0.31
0.09
0.03
0.01
_
73.30
0.25
0.10
0.32
0.47
0.60
0.67
0.20
0.08
0.06
22.50
93.88
0.34
0.22
0.43
0.23
0.8
98.30
Al
2
2
O
O
3
3
_
_
_
_
_
_
_
_
_
_
Fe
CaO
Na
MgO
2
O
K
2
O
0.85
0.22
0.04
0.72
2.27
MnO
TiO
2
2 5
P O
Loss of Ignition
XRD analysis was employed to investigate the crystalline
properties of the prepared sample, and results were shown in
Fig. (2). The presence of crystalline or amorphous silica, or
both, in RHA depends on the burning temperature achieved
in the ash production [26]. In Fig. (2a) and b X-ray
diffraction patterns of RHA-400 and RHA-700 are shown
respectively. With increased in temperature, as remove
impurities the content of silica in RHA become major and
the strong broad peak of a characteristic of amorphous SiO
2
is explicit. The type of silica phases of the rice husk ash
depends on the combustion temperature. Amorphous rice
husk ash was formed when the rice husk was burnt below
o
8
00 C for 10 hours whereas crystalline rice husk ash of the
crystalline silica of ꢀ-cristobalite and tridymite were formed
o
when the combustion temperature was higher than 800 C
Fig. (3). IR pattern of RH (a), RHA-400 (b), RHA-700 (c) and
silica-extracted (d), silica nanoparticles (e) and silica sulfuric acid
nanoparticles (f).
are show FT- IR spectroscopy of SiO
2
-extracted and silica
Fig. (2). XRD pattern of RHA-400 (a), RHA-700 (b) and silica
nanoparticles (c).
nanoparticles. The absorption bands at 3300-3500, (850-900
-
1
and 1000-1300) and 900 cm corresponded to silanol OH

7
08 Combinatorial Chemistry & High Throughput Screening, 2012, Vol. 15, No. 9
Salavati-Niasari and Javidi
groups, Si–O–Si and Si–OH stretching respectively. For
silica sulfuric acid nanoparticles, the absorption peak at 1020
and 1080 cm in Fig. (3f) were attributed the symmetric and
ultrasonic cavities, caused the nanoparticle growing [10]. At
the same time, the mechanical influence produced by
ultrasonic cavities was increased with prolonging of ultrasonic
time. Thus, it could prevent the nanoparticle growing and
aggregating. The mechanical influence produced by ultrasonic
activate such as emulsification, stir, and others, could prevent
the crystal growth and aggregation. But when the ultrasonic
time was too long, shock wave with high pressure, micro-
emission fluid and “Brown phenomenon” of nano-particle
itself could make nano-particles aggregated. For investigating
the effect of sonication on the morphology and particle size of
the products, the reaction carried out in the absence of
sonication (Fig. 5e). In the absence of sonication, when HCl
solution was added the milky color of the samples suspension
-
1
asymmetric stretching modes of the O=S=O group
-
1
respectively. The S–O stretching mode lies around 571 cm .
FT-IR spectrum shows the overlap asymmetric and
2
symmetric stretching bands of SO with Si–O–Si stretching
bands in the silica functionalized alkyl-sulfuric acid. The
spectrum also shows a broad OH stretching absorption
-
1
around 3000–3600 cm .
The effect of feeding rate on the morphology and particle
size of the products was investigated. FE-SEM images of the
formed nanoparticles (Sample no. 1 to 4) are shown in Fig.
(
4). As shown, with the decrease of feeding rate, the size of
are formed, indicating that the Si(OH)
4
was formed, but the
particle size of obtained product is large and these aggregated.
nanoparticles became smaller because the rate of nucleation
is higher than the rate of growth. On the other hand at the
same time, the mechanical influence produced by ultrasonic
cavities was increased with prolonging of feeding rate. Thus,
it could prevent the nanoparticle growing and aggregating
The application of silica sulfuric acid as a mild
heterogeneous catalyst to acetylation of alcohols and phenols
with acetic anhydride at room temperature under solvent-free
conditions was investigated Scheme (1).
[
10]. Sample no. 3 (Fig. 4c) has the smallest particle size and
the feeding rate equal with 0.70 ml/min is optimum
condition.
SiO2 _ OSO H (0.015 mmol)
3
ROH + Ac O
ROAc
2
Solvent-free, rt
For investigating the effect of sonication time on the
morphology of the products, the reaction carried out in 10, 15,
R=aryl,alkyl
2
0, and 30 min at the feeding rate 0.70 ml/min and FE-SEM
Scheme 1.
images of the resulted nanoparticles (Sample no. 3, 5, 6 and 7)
are shown in Fig. (5). In Fig. (5a) FE-SEM image of the
sample no. 3 (optimum condition) was shown. With the
increase of the sonication time from 10 min. to 20 min., the
particle size of the formed nanoparticles become smaller (Fig.
First, the reaction conditions for acetylation of alcohols
and phenols in acetic anhydride were optimized. The
acetylation of benzyl alcohol (1.00 mmol) with acetic
anhydride (1.20 mmol) in the presence of 0.005, 0.010,
0
.015, 0.020, 0.025 and 0.030 mmol of SiO
2 3
-OSO H nano
5
b) (sample no. 5). Further increase in the sonication time, due
catalyst at room temperature was investigated. When the
to the effect of the mechanical stress which produced by
Fig. (4). The effect of feeding rate on the morphology and particle size of (a) sample no. 1, (b) sample no. 2, (c) sample no. 3, (d) sample no.
4
.

Silica and Silica Sulfuric Acid Nanoparticles
Combinatorial Chemistry & High Throughput Screening, 2012, Vol. 15, No. 9 709
Fig. (5). The effect of sonication time on the morphology and particle size of (a) sample no. 3, (b) sample no. 5, (c) sample no. 6, (d) sample
no. 7 and (e) sample no 8.
catalyst was used in amount of 0.015 mmol, the reactions
were completed in shorter time and 97% isolated yield. In
the absence of the catalyst, at the same reaction conditions,
benzyl acetate was obtained in low yield (4%) even after a
prolonged reaction time. When the reaction was carried out
under solvent free condition, the best rate of acetylation was
observed.
It should be noted that the catalyst is reusable for nine
cycles without significant loss of its efficacy, as shown in
Table 4.
Table 4. Reuse of the Catalyst for Acetylation of Benzyl
Alcohol
Cycle
Yield
0
1
2
3
4
5
6
7
8
9
To show the generality and scope of this procedure, a
wide range of alcohols and phenols containing various
electron-donating and -withdrawing groups were treated with
acetic anhydride Table 3. The rate of acetylation is also
influenced by the steric and electronic factors associated
with substrates (Table 3, entry 20, 3-6, 12-18).
97
96
96
94
92
92
87
87
87
87
CONCLUSION
A novel method for the synthesis of silica nanoparticles
form rice husk ash by sonochemical method was developed.
The effect of some parameters such as, feeding rate of
percipiteting agent and time of sonication on the size and
morphology of the obtained products were investigated by
FE-SEM. The silica nanoparticle had a uniform spherical
structure with a diameter of 45–55 nm. The as-synthesized
silica nanoparticles were subsequently modified with
chlorosulfonic acid and prepared silica sulfuric acid
nanoparticles, were employed as an efficient catalyst for the
acylation of alcohols and phenols. We described for the first
As shown in Table 3, both primary and secondary
alcohols reacted very well and 1-phenyl-2-methyl-2-
propanol was also acetylated smoothly without any side
reactions. The applicability of this method to various
alcohols and phenols was evaluated by electron-withdrawing
(
Table 3, entry 4-6, 13-16) and electron-releasing alcohols
and phenols (Table 3, entry 3, 8-10,18) at the same
conditions. In all cases, rapid acetylation was observed in
excellent yields. Optically active substrates are efficiently
acetylated without any detrimental effect on the optical
purity (Table 3, entry 19).

7
10 Combinatorial Chemistry & High Throughput Screening, 2012, Vol. 15, No. 9
Salavati-Niasari and Javidi
a
Table 3. Acetylation of Alcohols and Phenols Catalyzed by Silica Sulfuric Acid
b
Entry
Substrates
OH
Products
Time(min)
Yield (%)
OAc
1
5
97
97
96
95
94
OH
OAc
2
3
4
5
5
5
OH
OAc
MeO
MeO
OH
OAc
8
Cl
Cl
OH
OAc
10
Cl
Cl
OH
OAc
6
7
10
7
93
96
O N
O N
2
2
OH
OAc
CH3
CH3
8
9
6
8
95
95
OH
OAc
OH
OAc
1
1
0
1
7
94
95
OH
OH
OAc
OAc
10
OH
Cl
OAc
Cl
1
2
15
93
OH
OAc
1
1
1
3
4
5
15
10
10
94
93
95
Cl
Cl
OH
Br
OAc
Br
OH
OAc
Br
Br

Silica and Silica Sulfuric Acid Nanoparticles
Combinatorial Chemistry & High Throughput Screening, 2012, Vol. 15, No. 9 711
Table 3) contd…..
(
b
Entry
Substrates
Products
Time(min)
Yield (%)
OH
OAc
1
6
15
91
O N
O N
2
2
OH
OAc
c
1
7
10
10
95
95
HO
AcO
OH
OAc
1
1
8
9
H3C
H C
3
15
30
93
92
HO
AcO
CH3
CH3
CH3
CH3
2
0
OH
OAc
a
b
c
Reaction conditions: Alcohol or Phenol (1.0 mmol); Catalyst (0.015 mmol); Ac
Isolated yields.
Ac O (2.4 mmol); Catalyst (0.03 mmol); Solvent free; rt.
2
2
O (1.2 mmol); Solvent free; rt.
time a simple, clean, and efficient method for the acetylation
of alcohols and phenols under solvent free condition. The
features of this procedure are reusable catalyst, excellent
yields, and enhanced reaction rates, a very small amount of
acetic anhydride usage, retention of chirality and simplicity
of manipulation.
[6]
Dongmin, A.; Yupeng, G.; Zou, B.; Yanchao, Z.; Wang, Z. A study
on the consecutive preparation of silica powders and active carbon
from rice husk ash. Biomass Bioenergy., 2011, 35, 1227-1234.
Cordeiro, G. C.; Filho, R. D. T.; Tavares, L. M.; Fairbairn, E. M.
R.; Hempel, S. Influence of particle size and specific surface area
on the pozzolanic activity of residual rice husk ash. Cem. Concr.
Compos., 2011, 33, 529-534.
Liou, T. H.; Yang, C.C. Synthesis and surface characteristics of
nanosilica produced from alkali-extracted rice husk ash. Mater. Sci.
Engin. B., 2011, 176, 521-529.
[7]
[
[
[
8]
CONFLICT OF INTEREST
9]
Witoon, T.; Chareonpanich, M.; Limtrakul, J. Synthesis of bimodal
porous silica from rice husk ash via sol-gel process using chitosan
as template . Mater. Lett., 2008, 62, 1476-1479.
The authors confirm that this article content has no
conflict of interest.
10]
Salavati-Niasari, M.; Javidi, J.; Davar, F. Sonochemical synthesis
of Dy2(CO
3
)
3
nanoparticles, Dy(OH)
3
nanotubes and their
conversion to Dy
Ultrasonics Sonochemistry., 2010, 17, 870-877.
2
O
3
nanoparticles Ultrasonics Sonochemistry.
ACKNOWLEDGEMENT
[11]
Stamatisa, A.; Giasafakia, D.; Christoforidis b, K.C.;
Deligiannakisb, Y.; Louloudia, M. The catalytic function of SiO -
2
immobilized Mn(II)-complexes for alkene epoxidation with H O .
2 2
Journal of Molecular Catalysis A: Chemical, 2010, 319, 58-65.
Authors are grateful to the council of Iran National
Science Foundation and University of Kashan for supporting
this work by Grant No (159271/18).
[
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Ahmed, A. E.; Adam, F. Indium incorporated silica from rice husk
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Received: February 29, 2012
Revised: March 24, 2012
Accepted: June 1, 2012