Comparative analysis of textural properties of SBA-15 prepared from rice husk and tetraethylorthosilicate
VIETNAM JOURNAL OF CHEMISTRY
VOL. 49(5) 551-555
OCTOBER 2011
COMPARATIVE ANALYSIS OF TEXTURAL PROPERTIES OF SBA-15
PREPARED FROM RICE HUSK AND TETRAETHYLORTHOSILICATE
Thanh Ho Van
Thua Thien Hue college of Education
Received 10 March 2011
Abstract
SBA-15 materials were synthesized using rice husk. In addition, SBA-15 was synthesized by using
tetraethylorthosilicate as a reference material. The obtained materials were characterized by XRD, TEM and
adsorption/desorption nitrogen method. Though less ordered hexagonal mesoporous SBA-15 materials could be
obtained from rice husk they exhibit high surface area and large pore size in comparison with one from
tetraethylorthosilicate. Morphology of SBA-15 prepared from rice husk consists of curve-rod like silicas while that of
SBA-15 from tetraethylorthosilicate consists of rod-like silicas. The difference in mesoporous tubular structure and
textural properties of synthesized SBA-15 could be assigned to inherent impurities of metal ions in rice husk.
1. INTRODUCTION
5.16 HCl: 162 H2O. P123 was dissolved in distilled
water until a clear solution was obtained before an
Rice husk is a by-product from rice mill that was addition of HCl and silica solution to form
used as an energy source in many industries such as precipitation. Finally, the resulting solid product was
biomass power plant and rice mill. Rice husk is rich filtered, washed with distilled water, dried at 373 K
in silica and can be an economically valuable raw for 24 hours and then calcined at 773 K for 10 hours.
material for production of natural silica [1, 2]. In The obtained SBA-15 materials were denoted as
Vietnam, a large amount of rice husk is produced RH-SBA-15. In addition, SBA-15 was synthesized
and practically used later in agriculture as low-value by using TEOS as a reference material by
material.
procedures described by Stucky et al [3] and denoted
Highly ordered large-pore mesoporous silica as TEOS-SBA-15.
SBA-15, which has considerably thicker pore walls
The silica content of dried rice husk was
than MCM-41 was recently synthesized in acid analyzed by gravimetric method.
media using amphiphilic triblock copolymer as the
Nitrogen adsorption/desorption isotherms of
structure-directing agent and various silica sources: calcined samples were obtained using Omnisorp-100
sodium silicate, tetraethylorthosilicate (TEOS) [3]. sorptometer at 77K, after degassing at 200oC and 10-
5
Thus silica source from rice husk would be
mmHg for at least 4 hours. The specific surface
alternative for synthesis of SBA-15. This benefit is areas (SBET) were calculated by the standard BET
to enhance the rice husk value. method for adsorption data in a relative pressure
In this paper, SBA-15 was prepared by using range from 0.05 to 0.3 [5] The pore-size distribution
silica source from rice husk. The textural properties (PSD) was determined by the Barrett-Joyner-
were then compared with those of SBA-15 Halenda (BJH) method from desorption isotherm
synthesized from tetraethylorthosilicate (TEOS).
data. Primary mesopore diameter, dp, was
determined from the maximum of a PSD curve. t-
plot method has been applied to quantitatively
determine the mesopore surface area (denoted as
2. EXPERIMENTAL
Raw materials used in this paper were rice husk SBJH). The t is a function of relative pressure
collected from Thua Thien Hue province. Silica expressed as Eq. 1 [6].
1
solution was extracted from rice husk under
2
refluxing in NaOH solution. Obtained filtrate was
used as silica source. The SBA-15 sample was
synthesized by using triblock copolymer (P123) as
surfactant and rice husk as silica source [4]. The
molar ratios of reactant were 1SiO2: 0.0167P123:
13.99
t =
(1)
P
0.034 − log
P
o
551
VJC, Vol. 49(5), 2011
Ho Van Thanh
Usually, when micropores are present the t-plot micropore volumes were calculated from the
will exhibit a positive intercept from which the difference between total pore volume Vt and total
micropore volume is calculated. Using the slop(s) of mesopore volume
linear part of the t-plot in the range 0.45 < t < 1.0
The mesoporous phases of Si-SBA-15 were
nm, the mesopore surface area SBJH can be calculated monitored by powder low-angle X-ray diffraction
by Eq. 2, considering mesoporous surface as (XRD), recorded on 8D Advance (Bruker, Germany)
external one of material.
with CuKα radiation in the range of 2θ from 0.5 to
10o with a scan step size of 0.01o and a scan step
time of 0.04s. The length of the hexagonal “unit
cell” ao was calculated using the formula
SBJH = s ×15.47
(2)
2d100
The constant 15.47 represents the conversation
of the gas volume to liquid volume. The micropore
surface area can be therefore calculated as the
difference between the total surface area SBET and the
ao =
[6]. Pore wall thickness, tw, was assessed
3
by subtracting dp from ao. The morphology was
studied by TEM (JEM microscopy-1010).
mesopore surface area SBJH
.
The total pore volume Vt was determined from
the amount of nitrogen absorbed at 77 K at the
relative pressure of 0.99. At such a pressure the main
channels of the sample are assumed to be completed
filled with nitrogen. The total mesopore volume was
obtained by integrating the PDS curves from pore
size of about 2.0 nm to the upper limit of 50 nm. The
3. RESULTS AND DISCUSSION
The silica content of rice husk was analyzed by
gravity method. This amount of SiO2 was obtained
up to considerable values. The amount of impurities
was also found such as CaO, MgO, MnO2 etc. The
results of element analysis were listed on table 1.
SiO2
CaO
MgO
MnO2
Na2O
K2O
8.96%
0.0024%
0.0026%
0.0043%
0.0200%
0.0300%
It is noted that the silica source from rice husk uniform hexagonal mesoporous structure. It is noted
contains remarkable amount of transition metals that in the case of RH-SBA-15, the peak (100) is
including magnesium, calcium, magnesium etc. broader than that of TEOS-SBA-15. The dspacing of
while one from TEOS does not. Thus the textural (100) peak increases from 10.4 nm for TEOS-SBA-
properties of synthesized SBA-15 materials should 15 to 11.7 nm for RH-SBA-15. This implies the
depend on silica sources.
enlargement of the distance between two centers of
Fig. 1 shows XRD patterns of RH-SBA-15 and adjacent pores (“unit cell” ao) in hexagonal
TEOS-SBA-15. Three reflections of mesoporous arrangement. This effect may be due to the
phases were found at low angle reflection which introduction of metals as impurities in rice husk in
were indexed as (100), (110), and (200) [7]. These which the Me-O bond are longer than the Si-O bond.
sharp signals indicated the long-range orders of the
TEOS-SBA-15
RH-SBA-15
0
2
4
6
8
10
2 thetha (degree)
Fig. 1: The XRD patterns of RH-SBA-15 and TEOS-SBA-15 materials
552
VJC, Vol. 49(5), 2011
Comparative analysis of textural properties…
Figs. 2 and 3 show TEM observations of TEOS- the gathering of rod-like silica while RH-SBA-15
SBA-15 and RH-SBA-15, respectively. Both top and consists of curve-rod like silica.
side views obtained from TEM indicate the well-
The nitrogen adsorption-desorption isotherms of
ordered hexagonal mesoporous structure for all RH-SBA-15 and TEOS-SBA-15 were shown in Fig.
samples. The cross sectional patterns showed 3. Very similar type IV isothermers and large
hexagonal structure which is the distinctive feature desorption hysteresis were observed for both
of SBA-15. The pore diameters estimated from the samples, which is characteristic of mesoporous
top view were approximately 4-5 nm for both SBA- materials with cylindrical pores.
15. The highly ordered TEOS-SBA-15 consists of
a
b
50 nm
Fig. 2: TEM images of TEOS-SBA-15: side view (a) and top view (b)
a
b
Fig. 3: The TEM images of RH-SBA-15: side view (a) and top view (b)
The textural parameters of mesoporous silicas SBA-15. Even dp of RH-SBA-15 is larger than that
calculated from adsorption/desorption nigtrogen data TEOS-one its Smes is lower than that of TEOS-SBA-
are summarized in table 2. As can be seen, SBET of 15. The increasing total surface area for RH-SBA-15
RH-SBA-15 is rather higher than that of TEOS- should be related to the formation of microporosity.
553
VJC, Vol. 49(5), 2011
Ho Van Thanh
TEOS-SBA15
RH-SBA15
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/Po)
Fig. 4: Adsorption/desorption nitrogen isotherms of SBA-15 synthesized from TEOS and rice husk
Table 2: Textural properties of synthesized SBA-15 samples
Sample
SBET (m2.g-1) Smes (m2.g-1) Smic (m2.g-1)
dp (Å)
77.2
Vmes (cm3.g-1)
1.17
tw (Å)
43.4
TEOS-SBA-15
RH-SBA-15
626.0
733.7
511.9
466.4
114.1
267.3
91.3
1.43
37.7
It is clear that the polymer-ion interaction plays N0[(Mn+H+)X-]I+, where Mn+ is metal ions such as
an important role in the morphology. In fact, it is Mn2+, Mg2+, K+, etc. Thus, the proposed assembly
well-known that metal ions form crown-ether-type mechanism for these diverse mesoporous metal
complexes with polymeric PEO and PPO units [8], oxides includes PEO-metal chelating interactions in
the multivalent metal species (Mn+) can associate conjunction with electrostactics, van der Waals
preferentially with the hydrophilic PEO moieties, forces, ect., to direct mesostructure formation.
because of their different binding affinities for PEO
Futhermore, since the structure directing effect is
and PPO. It is known that, the SBA-15 materials are irrespective of the anions used in the synthesis, and
prepared by the use of nonionic surfactants as only the Cl- anion resulted in a branched, network-
organic structure directing agents in acid media via like structure, presumably the complex symmetry as
(S0H+)(X-I+) synthesis route where S0 is nonionic illustrated in Fig. 5 may also play an important role
surfactant, H+ proton, X- acid anion, and I+ in the synthesis by modification of the electrostatic
protonated silanol group.
interaction or the local curvature energy at the
interface of the inorganic silica and the surfactant
C
C
4. CONCLUSIONS
C
O
O
Synthesis of SBA-15 material from rice husk
was investigated. The obtained SBA-15 material
possesses high specific surface area, large pore
diameter. The silica from rice husk can be utilized
for synthesis of SBA-15 material instead of
commercial silica.
C
C
Men+
O
Cl
Cl
Fig. 5: Molecular model of the transition metal-PEO
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Corresponding author: Thanh Ho Van
Thua Thien Hue college of Education
123 Nguyen Hue, Phu Nhuan, Hue City
Email: thanh.cdsphue@gmail.com
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