1. Jfuas No.2 December 2013
Characterization of Silica Catalysts Prepared by Sol-Gel
Method Using Different Starting Materials
Adam Hassan Elhaj
Department of Chemistry, Faculty of Education, University of Al Fashir, Al Fashir, Sudan
E-mail: Adamhassan43@gmail.com
14. 2013 ,-./( – *+ () - #$% ! –
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Silica catalysts were prepared by Sol-gel method using different
starting materials such as sodium silicate, tetrachorosilane and
tetraehtoxysilane. The preparation method involved the synthesis of
gels from precursors, drying them and calcined at 550 co. phase
identification of the obtained products were characterized by X-Ray
diffraction(X R D). Surface area and porosity of the prepared silica
catalysts were determined by nitrogen adsorption technique. The
results of X R D analysis showed that silica catalysts prepared from
tetraethoxysilane and tetrachlorosilane had amorphous nature and high
degree of purity, but sodium chloride was observed as impurities in
the case of sodium silicate. It was also found that the prepared silica
catalysts were mesoporous with high specific surface area as indicated
by nitrogen adsorption-desorption data. Tetraethoxysilane is the best
precursor for the preparation of silica catalysts by sol-gel method
because the physical and chemical properties of the final solid can be
controlled from the beginning and throughout the synthesis.
Keywords: sol-gel, silica catalysts, tetraethoxysilane, sodium silicate,
starting materials.
15. Sol-gel material such as silica was motivated by purity of starting
materials(Dislish 1986) and low processing temperatures
(Youldas1979).For the sake of historical justice, it should be noted
that the sol-gel method dates not from the end of the 20th century but
from the synthesis of silica gel by precipitation from sodium silicate
16. Jfuas No.2 December 2013
solution with acids (PakhomovBuyanov 2005), for example
hydrochloride acid(Baba et al 2003):
Na SiO 2HCl [SiO H O] 2NaCl 2 3 2 2 + ¾¾® × +
Then, the gels were dried and calcined to give pure silica. Also silica
gel can be synthesized of tetrachlarosilane (SiCl4). This chloride is
known to fume strongly in air, and the resulting gels are heavily
contaminated with chlorine ions(Turevskaya et al 2000). The sol-gel
method received a new impetus when tetramethoxysilane and
tetraethoxysilane (TEOS) were employed as starting
chemical(PakhomovBuyanov 2000). The sol-gel oxide synthesis is
most often based on the hydrolysis of alkoxide precursor(Turevskaya
et al 2000), as in the case of silica(ChruscieiSlusarski 2003):
2 5 4 2 4 2 5 ( ) + 4 ¾¾¾® ( ) + 4
Si OC H H O Catalyst Si OH C H OH
Si OH SiO H O 4 2 2 ( ) ¾¾® + 2
The stages of sol-gel process using tetraethoxysilane precursor are
presented in the following scheme (Khimich 2004) :–
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19. º Si −OEt + H O¾¾® º SiOH + EtOH 2
Si OH Si OH SiOSi H O 2 º − + º − ¾¾® º +
º Si −OEt+ º SiOH ¾¾® º Si −O − Si + EtOH !
Tetraethoxysilane hydrolyzes in water to form silanol and ethanol
(Eq.1). The silanol groups with elimination of water (Eq.2), or silanol
react with tetraethoxysilane with elimination of ethanol (Eq.3).
Further hydrolysis and condensation reaction forms a silicon oxide
network. The catalytic activity of silica is influenced by the conditions
at which it is prepared such as calcinations temperature and the source
from which silica is obtained. Therefore, the aim of the present work
was to study the effect of starting materials on the catalytic properties
(e.g. porosity, surface area) and purity of silica catalysts prepared by
sol – gel method using different precursors such as tetrachlorosilane,
sodium silicate and tetraethoxysilane at 550 oC calcination
temperature. The ultimate silica catalyst products were analyzed by X-Ray
diffraction (XRD) and nitrogen adsorption – desorption method.
22. Three silica catalysts composed of silica catalyst-1, silica
catalyst-2, silica catalyst-3 were prepared using sodium silicate
tetrachlorosilane and tetraethoxysilane respectively. Preparation
method of each silica catalysts can be illustrated as the following: -
2.2.1 preparation of silica catalyst-1 from sodium silicate by sol-gel
method.
200 ml of sodium silicate solution of 43% concentration was
placed in 1 liter beaker and then followed by addition of 200 ml
ethylene glycol and 400 ml of distilled water. After the reaction
mixture was shacked well and titrated by addition of dilute (1:1) HCl
drop by drop until the solution become acidic then, the resulting
product of the reaction mixture was dried at 100Co over night. The
obtained product was ground and heated at 550 Co for hours. The final
product was analyzed by XRD and N2 adsorption technique. For
results see figures (2), (6), (8) and Table (1).
2.2.2 Preparation of silica catalyst-2 form tetrachlorosilane by sol-gel
method
1 ml of tetrachlorosilane and I ml of hydrochloric acid (PH=1)
were placed in test tube. 8.3 ml of absolute ethanol was used as
solvent. Also 1 ml of double distilled water was added. The reaction
mixture began to form gel. After that test tube was warmed to take out
finger of the gel. The obtained gel was heated at different
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temperatures. These temperatures involved 65 Co for 3 – 4 hours, 95
Co for 2 – 3 hours and 120 Co for 3 hours. After that the dried gel was
heated at 180 Co for 2hours. Then, the resulting product was calcined
at 550 Co in porcelain curricible. The obtained product was analyzed
by XRD and N2 adsorption technique. For results see figures (3), (7),
(9) and table (1).
2.2.3 preparation of silica catalyst-3 from tetraethoxysilane by sol-gel
method.
6 ml of tetraethoxysilane and 1 ml of hydrochloric acid (PH1)
were placed in test tube. 1 ml of absolute ethanol was used as solvent.
Also 1 ml of double distilled water was added. The mixture began to
form gel. After 144 hours there was a finger of the gel. The obtained
gel was heated at different temperature degrees. These temperature
degrees involve 65Cofor 3-4 hours, 95oC for 2-3 hours and 120 oC for
3 hours. After that the dried gel was heated at 180 Co for 2 hours.
Then, the resulting product was calcined at 550 oC in porcelain
curricible. This ultimate product was analyzed by XDR and N2
adsorption technique. For results see figures (4), (8), (10) and Table
(1).
24. The specific surface area of the solid was determined at 77 K by
nitrogen adsorption technique using quant chrome instruments (Quant
chrome NOVA Autometal gas sorption system). Initially the sample
was degassed at 473 K for 3 hrs or at 373 K for 18 hrs in vaccum and
the adsorption – desorption isotherms were obtained at 77 K. The
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25. Jfuas No.2 December 2013
phase identication was powder method. Such measurements were
held within the angle range between 4 and 70 0 by using CuK
radiation. AD 8 advance X-Ray Diffractometer from Bruker analytical
X-Ray system was used.
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26. Specific surface areas from silica catalyst - 1 (348.56 m2g-1)
silica catalyst -2 ( 321.69 m2g-1) and silica catalyst -3 (261.91 m2g-1)
are high in three cases. For results see fig(2). fig.(3), fig(4).
31. Jfuas No.2 December 2013
Nitrogen adsorption – desorption isotherms for the prepared silica
catalysts have been presented by fig(5), fig(6) and fig(7) the values
need to fill pores at STP are 0.32, 0.38, 0.34 cm3/g. the values of pore
volume(Lin et al 2003) have been calculated by equation 4 :
p a V = 15.45×10−14 ×V #
Where Va is the volume (atstp) of gas adsorped at N2 partial
pressure of p, Vp , is total pore volume of nitrogen in the form of
liquid necessary to fill all pores.
Also pore radius(DorchehAbbasi 2008) (Aravind et al 2010) for
these prepared silica catalysts have been calculated using equation (5)
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p
V
BEt
S
r
2
¢ = $
Where r is the mean pore radius, SBEt is the surface area, Vp is as
defined in equation (4). The calculated values of Vp, SBEt and r for
silica catalyst 1, 2, 3 have been summarized in Table (8). The different
values of surface area and pore volume due to the differences in the
nature of starting materials.
51. Results present strong influence of starting materials on the
catalytic properties of silica catalysts obtained by sol-gel method. It
was also found that the purity of the prepared silica catalysts depend
mainly on the nature of precursors. Sol- gel precursor is mainly
tetraethoxysilane which can be obtained in a high degree of purity,
whereas as sodium silicate is very difficult to purify. Tetrachlorosilane
can’t be used as sol-gel precursor, because its hydrolysis generates
HCl and the presence of this byproduct environmentally unacceptable.
# $
52. 1. Aravind, P.R., Shaiesh, P., Soraru., G. D., Warrier, K.
G.K., (2010), J Sol-Gel Sci Technol, 54: 105 – 117.
2. Baba, T.,Kawanami,Y.,Yuasa,H.,Yoshida,S., (2003), J.
Catalysis letters , vol.91,Nos 1- 2, pp.31 – 34.
3. Chrusciei, J., Slusarski, (2003), Material Science, vol.12,
No.4, pp.461 – 469.
4. Dislish, H., (1986), Journal of Non-crystalline solid, vo1.80,
115-121.
5. Dorcheh, A.S., Abbasi, M.H, (2008), Journal of Material
Processing Technology, 199, 10 – 26.
6. Estella,J., Echeverria , J.C., Laguna M., Garrido J.J.,
(2007), Journal of Non-Crystalline solids 353, 286 – 294.
53. 2013 ,-./( – *+ () - #$% ! –
7. Khimich, J. (2004), Glass physics and Chemistry, vol.30,
54
No.5,pp. 430 – 442.
8. Lin C, al. Muhtaseb, S.A, pitter J.A., (2003), Journal of Sol-
Gel Sci. Tech., Vol.28. pp133 – 141.
9. Ni, H., Simonsick J.R.,William J.,Skaja,Allen
D.,Williams,Jonathan P.,Soucek,Mark D., (2000), J.
progress in organic coatings, Vol.38,pp. 97 – 110.
10. Pakhomov, N.A, Buyanov, R.A, (2005), kinetics and
Catalysis, vol. 46, No.5, pp.669 – 683.
11. Turevskaya ,E.P. Yanovskaya, M.I. and Turova, (2000), N
.ya Inorganic Materials, Vol.36, No. 3, pp 26.
12. Youldas, B.E. (1979), Journal of Material science, vo1.14,
1843 – 1842.
