Sol-Gel-Derived Porous Silica: Economic Synthesis and Characterization

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Journal of Minerals and Materials Characterization and Engineering, 2012, 11, 976-981

Published Online October 2012 (http://www.SciRP.org/journal/jmmce)

Sol-Gel-Derived Porous Silica: Economic Synthesis and

Characterization

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

Email: [email protected]

Received August 8, 2012; revised September 12, 2012; accepted September 20, 2012

ABSTRACT

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

2SiO3,

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

exploited.

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-

stics.

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.

E. R. ESSIEN ET AL. 977

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 cm−1. 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



 (1)

where, M is the mass of the sample measured with a

microbalance (10−5 g accuracy), and V is the volume

measured by ﬁlling 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·cm−3

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

E. R. ESSIEN ET AL.

978

Na2SiO3

H2SO4

HOSi

Na2SiO3

H2OH2SO 4

H2O

(OH)4Si8Al4O20.nH2O

16 NaOH

poly-

condensation

4Al(OH)3+(4 + n)H2O

Extraction

Sodium metasilicate

-nH2O

Polysilicic acid

Silicic acid

Clay

Na2SO4

·nH

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

cm−3 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 cm−1 and a smaller signal around

1630 cm−1 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 cm−1 [31,32] and the absorption signal at

920 cm−1 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

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

E. R. ESSIEN ET AL. 979

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).

E. R. ESSIEN ET AL.

980

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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