Eng4

Jfuas No.2 December 2013

37

Characterization of Silica Catalysts Prepared by Sol-GelMethod Using Different Starting Materials

Adam Hassan Elhaj

Department of Chemistry, Faculty of Education, University of Al Fashir, Al Fashir, Sudan

E-mail: [email protected]

الصول بطريقة المحضرة السيليكا حفازات جل–توصيفمختلفة إبتدائية مواد باستخدام

Wא��

א�� א �� (Silica)א�� –��(Sol – Gel)

�� !�� ، א��$��# �� %& ��'( +�'*א(�� &��א$ ��',*א#�� %�+ !��K'א� ��. /0�'1א��2 3��4 5�6(Gels)��7$א�& /�&

8� ��א ��$ *��9! א:�� א��2 �9�� ;��< =��> @��550�0Kא?�'*א(�� =��>א��9 �ABCא $�� א 8��Dא E1א��9א(XRD)KFא�� & ��*�1 G �0�

H��I��9א� א&'��Jא� ��9�1 ��א א�� E1��9א� ��&��Kא�KE)��'L M��$ N�� @ !�� ',*א#�� 8��Dא א�� א �O P��! א��9 �A��BCא $�� /�& ��! ��$ P��!� �� 5�Q ��A�R. Sא �� %�+ !��

T��9א�K#��$א�� *�� /& �א�� ;U V �� א��$��# �� א�',*א# *9!�E1א��9א WX��7 Y Z)א��[�K��\ /��& *�� ]��O�O H��I��9א� א&'��Jא� ��9�1 E)��'L ��

��'& ��&��&� ��! F�� & Sא א�� אK ��%�+ !�� ^'A��א�� WX�;� א�� א 5�6'� �&*,'�Kא א?�'*א(�� א�Kא$ ��_O /& ��

א�R*א�� /��& `�R��a /��b א�9;��( E1��9�� א��0(�� )��J��א� cא��dא �CT��9>O�א�'5�6 ��0!K

الفارش م2013ديسمرب–الثا*العدد-التطبيقيةالعلوممجلة–جامعة

38

��

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.��

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


39

solution with acids (Pakhomov&Buyanov 2005), for example

hydrochloride acid(Baba et al 2003):

NaClOHSiOHClSiONa 2][2 2232 +× →+

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(Pakhomov&Buyanov 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(Chrusciei&Slusarski 2003):

OHHCOHSiOHHOCSi Catalyst5242452 4)(4)( + →+

OHSiOOHSi 224 2)( + →

The stages of sol-gel process using tetraethoxysilane precursor are

presented in the following scheme (Khimich 2004) :–


40

��

��

reac�ons (Ni et al 2000)

EtOHSiOHOHOEtSi +≡ →+−≡ 2 ��

OHSiOSiOHSiOHSi 2+≡ →−≡+−≡ � �

EtOHSiOSiSiOHOEtSi +−−≡ →≡+−≡ �!�

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.


41

��

2.1 chemicalsAll chemicals used were of analytical grade type.2.2 prepara�on of silica catalysts:

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


42

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).2.3 Instrumenta�on:

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


43

phase identication was powder method. Such measurements were

held within the angle range between 4 and 70 0 by using CuKα

radiation. AD 8 advance X-Ray Diffractometer from Bruker analytical

X-Ray system was used.

��

��

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).

��

�� !


44

�� "� ��

�� !

�� #� �� " ��

�� !


45

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 :

ap VV ××= −141045.15 """ �#�

Where Va is the volume (atstp) of gas adsorped at N2 partialpressure of p, Vp , is total pore volume of nitrogen in the form ofliquid necessary to fill all pores.

Also pore radius(Dorcheh&Abbasi 2008) (Aravind et al 2010) forthese prepared silica catalysts have been calculated using equation (5)

BEt

p

S

Vr

2=′ """��$�

Where ŕ is the mean pore radius, SBEt is the surface area, Vp is asdefined in equation (4). The calculated values of Vp, SBEt and r forsilica catalyst 1, 2, 3 have been summarized in Table (8). The differentvalues of surface area and pore volume due to the differences in thenature of starting materials.


46

Fig. (5) :Nitrogen adsorp�on � �� $�� $� ��

��$�� %� �� &&'�

! �� $� ��

�� %� �$� �� $��$��

�� $� �� $��


47

�� $�� $� ��

� ��$�sed by calcina�on at 550�

! �� $� �� %�

�� (�� $�� $��


48

�� )� �� $�� $� �� "

synthesed by calcina�on at 550�

! �� $� �� %�

�� (�� $�� $�*��


49

Table (1): Effect of precursor on textural properties silica catalystsobtained by sol-gel method at 550C0 calcination temperature.

Precursor Adsorbent SBEt m2/gVp

cm3/gr Ao

Sodium silicate Silica catalyst -1 348.56 0.320 22.780Tetrachlorosilane Silica catalyst – 2 321.69 0.383 23.823Tetraethoxysilane Silica catalyst – 3 261.91 0.383 25.959

3.2 phases Iden�fica�on

%��&�� '()(�*� �� +�,��

�� - .� � &��

�� . � .�� / �� #�

(

characteris�c of amporphous silica(Estella et al 2007) . The

�� .� �� .�� +,0 ��

�� &� �� &��

�� &� �� &� ��

�� &� � ��&��


50

��

��

� � ��1

synthesedby

calcin

a�onat550

��

��

��


51

��!��Raypowderdiffrac�

onofsilica

catalyst

2synthesedby

calcin

a�onat550oCforthedrie

dgel

��

� � ��"��

��

��

��

��


52

��#$��Raypowderdiffrac�

onofthesilica

catalyst

3synthesedbycalcin

a�onat550oCforthedrie

d

�� "�� %��


53

!� " ��

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.

#� ��$��

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.


54

7. Khimich, J. (2004), Glass physics and Chemistry, vol.30,

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.

Eng4

Education

Transcript of Eng4