Journal of Minerals and Materials Characterization and Engineering, 2012, 11, 976-981
Published Online October 2012 (
Sol-Gel-Derived Porous Silica: Economic Synthesis and
Enobong R. Essien1*, Oluyemi A. Olaniyi1, Luqman A. Adams2, Rafiu O. Shaibu2
1Department of Chemical Sciences, College of Natural Sciences, Bells University of Technology, Ota, Nigeria
2Department of Chemistry, Faculty of Science, University of Lagos, Lagos, Nigeria
Received August 8, 2012; revised September 12, 2012; accepted September 20, 2012
Porous silica was synthesized via the sol-gel process using clay obtained locally from Ijero-Ekiti in Ekiti State, Nigeria
and compared with silica synthesized under similar conditions from sodium metasilicate (Na2SiO3) obtained comer-
cially. The clay was initially refluxed with sodium hydroxide (NaOH) for 2 hours to extract SiO2 to form Na
which was subsequently hydrolyzed to form a gel. The gel obtained was washed with deionized water to get rid of im-
purities, dried and calcined at 800˚C for 3 hours. The obtained silica powders were characterized using atomic absorp-
tion spectrophotometer, Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD) and scanning electron
microscopy (SEM). Results showed that the vibrational modes and diffraction patterns of the silica derived from com-
mercial Na2SiO3 and that prepared from clay were similar containing pure amorphous SiO2. The morphology of the
commercially obtained silica showed better arrangement of particles and exhibited slightly lesser porosity (62.4%)
compared to that derived from clay which had a porosity of 65.5%. The result indicates that clay has a potential for use
as an environmentally safe and economic starting material for preparing porous silica instead of high quality precursors.
Keywords: Clay; Sodium Metasilicate; Economic; Porous Silica; Environmentally Safe
1. Introduction
Porous silica is used in many fields including biotech-
nology, biomedical sciences, selective separation and ca-
talysis [1,2]. Unlike conventional glass, which is a vis-
cous fluid that results from the fusion of SiO2 at high
temperature, sol-gel-derived silica is obtained at mild
conditions [3] compatible with the stability of most bio-
active compounds. The sol-gel process is capable of ge-
nerating materials with controlled surface properties and
pore structures between 1 - 500 nm [4-7]. The matrix of
the silica thus formed can be exploited for immobili-
zation of biomolecules or cations, inorganic membranes
and support for catalysts [6-9].
Silica polymer networks may be obtained by hy-
drolysis and condensation reactions of silica precursors
such as tetraethyl orthosilicate (TEOS). By controlling
synthesis conditions carefully, the sol morphology can be
directed towards weakly branched polymeric systems or
to particulate systems [7]. Important process parameters
include water content, the solvent, the catalyst used and
its concentration and type of alkoxide used [4,10,11].
Several methods, most of them based on TEOS as
starting material have been used to prepare porous silica.
The most popular is the sol-gel synthesis proposed by
Stöber et al. [12] which is based on hydrolysis and con-
densation of highly reactive silicon alkoxide precursor at
low temperature. Consequently, silica particles with vari-
ous characteristics were prepared by ammonia-catalyzed
reactions of tetraethyl orthosilicate (TEOS) with water in
low molecular weight alcohol [12,13]. Due to high cost,
applicability of other precursors has been investigated.
Thus, different types of silica sources have been used,
such as sodium silicate [14,15] and even environmentally
safe and renewable resources, like rice husk ash [16,17],
rice hull ash [18] and rice straw ash [19] have also been
In continuation of the search for a cheap and environ-
mentally safe silica source, the work herein investigated
the synthesis of porous silica from clay and sodium
metasilicate under similar conditions for the purpose of
comparing their physical and morphological characteri-
2. Materials and Methods
2.1. Materials
The chemicals used were sodium metasilicate (Na2SiO3),
(Sigma-Aldrich) with composition SiO2 24.9 and Na2O
20.9 wt% respectively, clay from Ijero-Ekiti in Nigeria
*Corresponding author.
Copyright © 2012 SciRes. JMMCE
having composition (as previously analyzed by Olasupo
and Omotoyinbo [20]) as shown in Table 1 and sulphuric
acid (H2SO4), (Fluka, Germany).
2.2. Preparation of Porous Silica from Sodium
Metasilicate (SS)
Sodium metasilicate (5.0 g) was stirred in distilled water
(10.0 ml) to give a clear solution. Thereafter, 10.0 ml of
2 M H2SO4 was added dropwise to the mixture under
magnetic stirring at room temperature for about 2 hours
to form a gel. Deionized water was added successively to
the gel to wash and remove sodium sulphate (Na2SO4).
Complete removal of sodium ions was indicated by ab-
sence of white precipitate when the liquid from the last
filtration was tested with dilute lead(II) ethanoate solu-
tion. The washed gel was dried in an oven at 120˚C for 1
day, calcined at 800˚C for 3 hours in a furnace with a
heating rate of 10˚C/min and thereafter milled to form
powders and then labelled as SS.
2.3. Preparation of Porous Silica from Clay (CS)
The as-received potter’s clay was dried in the sun,
following which it was ground into powder in a ball mill
and sieved through a 100 µm to remove oversize parti-
cles. The obtained clay (10.0 g) was refluxed in 1 M
NaOH solution (200.0 ml) for 2 hours, then allowed to
cool before filtering to remove the residue which con-
tained Al(OH)3 as major impurity. The filtrate obtained
was evaporated to dryness to give as residue sodium
metasilicate, Na2SiO3 (5.0 g). The basic sodium metasi-
licate was dissolved in distilled water (10.0 ml). The
solution was hydrolyzed with 2 M H2SO4 (20.0 ml) and
subsequently treated the same way as in section 2.2
above to afford the silica powder which will henceforth
be called CS.
3. Characterization
X-ray diffraction (XRD) analysis was carried out in the
2θ range of 10˚ - 80˚ and 5˚ - 120˚ using Xpert PRO
PANalytical diffractometer employing CuKα radiation
Table 1. Mineralogical composition of Ijero-Ekiti clay.
Minerals Amount (%) Oxides Composition (%)
Kaolinite 72 SiO2 63.3
Quartz 22 Al2O3 16.5
Feldspar 4 Fe2O3 5.8
Illite 2 CaO 1.7
MgO 0.6
LoI 12.5
Others 0.8
LoI = Loss on ignition.
(0.154060 nm) source operated at 40 kV and 40 mA.
Fourier transform infrared spectroscopy (FTIR) studies
were carried out using Buck Scientific 500 Infrared spec-
trophotometer with KBr as reference in the wave number
range of 600 - 4000 cm1. Atomic absorption spectros-
copy Perkin Elmer A Analyst 200 was used to determine
residual amount of sodium ions (Na+) present in the silica
networks. Morphological characterization of the samples
regarding the particles and pore sizes distribution were
performed using a SEM (EVO/MA10) at an accelerating
voltage of 10 kV.
The bulk densities, ρb of the silica particles were mea-
sured from their weight to volume ratio using the formula
where, M is the mass of the sample measured with a
microbalance (105 g accuracy), and V is the volume
measured by lling the silica particles in a column of
known volume [21].
The porosity, P(%) was estimated using the relation-
ship [22]
1 100
 (2)
in which ρp is specific density assumed to be 2.0 g·cm3
for amorphous silica particles based on the sperical
model (a typical average density of silica prepared via
wet-synthesis conditions [23]).
4. Results and Discussion
4.1. Hydrolysis and Gelation
Acidic hydrolysis of sodium metasilicate (Na2SiO3) gives
silicic acid [24] in solution. Sodium hydroxide is reacted
with clay to initially afford sodium metasilicate in a
strongly basic medium. The water glass obtained from
the basic reaction is similarly hydrolysed to silicic acid
which undergoes condensation to disilicic acid and
further reaction to a polycondensed hydrogel [24] as
shown in Scheme 1. Thereafter, the gel obtained from
both precursor compounds were subjected to multiple
washings [25] with deionised water to free the gel of
sodium sulphate formed during hydrolysis that is trapped
in the pores of the gel network. During aging, the 3-D
framework of the silicate glass continues to grow and
becomes more rigid by contracting and expelling liquid
water present inside the pores [26]. Drying is important
as it modifies the gel characteristics and involves two
steps [27]. During the first step, the pores are emptied;
the capillary gradient induces the liquid flow along the
pore walls, towards the external surface where it evapo-
rates. At the interior, the pores are still filled with liquid,
while the air enters the most external pores, which can
cause the opacity of gel. The pressure capillary gradient
decreases gradually, the flow is increasingly slower then
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16 NaOH
4Al(OH)3+(4 + n)H2O
Sodium metasilicate
Polysilicic acid
Silicic acid
Scheme 1. Hydrolysis and polycondensation of sodium metasilica te and clay.
stops. In the second step the remaining liquid can then
leave the gel only in gas form, with vapour diffusion
towards the surface. After drying at 120˚C, the porous
gel contains a small quantity of liquid trapped in the
pores. Calcination at high temperature further removes
absorbed water trapped in the pores.
The efficiency of deionised water washings to free the
gel from sodium sulphate was monitored by atomic
absorption spectroscopy (AAS) analysis of the silicas
obtained after drying. The result indicated that Na+
decreased from 36.5% (on the basis of stoichiometry) [25]
to 1.6% in the SS-based silica and to 2.1% in the CS
based silica.
4.2. Bulk Density and Porosity
The silica obtained from SS and CS after calcination
were found to have bulk density of 0.752 and 0.690 g
cm3 respectively. The difference may be due to better
packing of the 3-D framework in the SS as a result of
more efficient removal of the interfering Na+ from succe-
ssive deionized water washings of the gels [28]. Their
bulk densities as expected had influence on their porosity,
as SS and CS exhibited 62.4% and 65.5% respectively.
4.3. FTIR
The FTIR spectra obtained for the SS and CS-based
silicas respectively are characterized by a broad band
centred around 3400 cm1 and a smaller signal around
1630 cm1 that corresponds to O-H absorption band
[29,30] as shown in Figures 1(a) and (b). Furthermore, a
diagnostic Si-O-Si asymmetric stretching vibration is
centred on 1132 cm1 [31,32] and the absorption signal at
920 cm1 is assigned to the stretching vibration of silanol
groups on the surface of the amorphous solid [33].
4.4. XRD
The XRD spectra of the SS and CS powders obtained are
shown in Figures 2(a) and (b) respectively. The diff-
raction patterns are similar for both samples which
appear as broad bands with reflection at 2θ = 22˚ in-
dicating that the materials are amorphous and composed
of SiO2 [34-36]. There are no additional peaks observed
in both spectra. This result indicates the absence of im-
purities in the gel networks after the deionised water
washing removal of Na2SO4 formed during the gelation
reaction (Scheme 1), the extent which was confirmed
earlier with AAS.
4.5. SEM
The particle sizes of the silicas measured by SEM are
presented in the micrographs shown in Figure 3. The
average particle size of SS was 353.95 nm, Figure 3(a)
while that of CS was 755.35 nm, Figure 3(b). The differ-
ence in particle sizes in the two samples may be the
result of agglomeration [37] which is higher in CS than
SS. This fact is more evident in Figure 4 where in the
micrograph of SS, Figure 4(a), the particles appear as
discrete with a few agglomerates, whereas the particles
of CS as seen in the micrograph shown in Figure 4(b),
appear to adhere to each other forming aggregate of par-
ticles which results in irregular arrangement when
compared to SS. It has been proposed [38] that the initial
particle size distribution, polydispersity, concentration of
particle size distribution, polydispersity, concentration of
particles, viscosity of the continuous phase, Van der
Waals forces of attraction, and hydrodynamic conditions
govern the extent of inter-particle collisions, aggregation
and the temporal evolution of the average diameter of
aggregates. The morphology of CS therefore appears to
be more porous than that of SS which agrees with the
porosity values earlier obtained. According to studies
[39], porosity arises from the assumed packing of pri-
mary particles in the agglomerates.
5. Conclusion
Porous silica has been successfully synthesized via an
Figure 1. (a) FTIR spectrum of SS-based silica; (b) FTIR spectrum of CS-based silica.
Figure 2. XRD p atterns obtained from (a) SS and (b) CS powders.
Figure 3. SEM micro graphs of samples (a) SS and (b) CS showing pa rticle sizes measur ed at different positions: Pa 1 and Pa
2 at positional angles of Pb 1 and Pb 2 respectively.
Figure 4. SEM micrographs showing the porous morphology of the sol-gel-derived silicas. It can also be seen that SS, (a)
contains better packed particles than CS, (b).
Copyright © 2012 SciRes. JMMCE
economic route using a Nigerian clay. Results obtained
from FTIR, XRD and SEM characterization showed that
the material possessed properties comparable to silica
obtained from commercial sodium metasilicate prepared
under the same conditions. It is therefore concluded that
clay which is cheap and widely available in Ekiti State,
Nigeria may be a potential starting material for commer-
cial scale preparation of porous silica.
6. Acknowledgements
The authors are grateful to the Central Research Labo-
ratory, Bells University of Technology, Ota, Nigeria for
their assistance in carrying out this project.
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