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Adsorption of nitrogen, water and certain organic vapours on fully and partially hydroxylated silica gels with different porosity characters
Surface Technology, 12 (1981) 269 - 275
269
A D S O R P T I O N OF NITROGEN, W A T E R A N D CERTAIN ORGANIC V A P O U R S ON FULLY AND PARTIALLY H Y D R O X Y L A T E D SILICA GELS WITH D I F F E R E N T POROSITY CHARACTERS T. M. EL-AKKAD, A. AMIN and S. NASHED Department of Chemistry, Faculty of Science and Faculty of Education, Ain Shams University, Cairo (Egypt)
(Received April 11, 1980)
Summary The adsorption of several adsorbate molecules, namely nitrogen, water, carbon tetrachloride, benzene and methanol, on fully and partially hydroxylated silica samples was investigated. The samples tested had different porosity characters, i.e. they were microporous (Gasil 200), mesoporous (Gasil 35) and non-porous (TK800), in order to elucidate the effect of the porosity of the gel on specific and non-specific adsorbate-adsorbent interactions. Nitrogen adsorption, which is assumed to involve non-specific interactions, is influenced to a certain extent by the specific interaction between the nitrogen quadrupole and the surface hydroxyl. The smaller polar water molecules can penetrate into pores which are too narrow to accommodate larger adsorbate molecules. The marked reduction of the water uptake upon dehydroxylation of the silicas substantiates the important role of the specific interaction in the adsorption of water on silica surfaces. The carbon tetrachloride molecule appears to be a more sensitive probe than the nitrogen molecule for detecting microporosity, its only drawback being its larger size. Benzene adsorption depends mainly on the interaction between the mobile x bonding and surface hydroxyls. Finally, non-specific interactions appear to have a marked effect on the adsorption of polar methanol molecules on silica surfaces which involve essentially specific interactions.
1. I n t r o d u c t i o n A good deal of experimental evidence supports the validity [1] of the area obtained from nitrogen adsorption on oxides. Much less information is available on the adsorption of organic vapour. However, recent studies [2, 3] have revealed significant differences in specificity between various physical adsorptions. The pore structure of the silica adsorbent and the surface 0376-4883/81/0000-0000/$02.50
© Elsevier Sequoia/Printed in The Netherlands
270 g e o m e t r y a r o u n d individual a d s o r p t i o n sites are g e n e r a l l y a s s u m e d to be i m p o r t a n t in d e t e r m i n i n g the r e l a t i v e a d s o r p t i o n of different a d s o r b a t e s . In this c o n t e x t c o m p a r a t i v e studies were c a r r i e d out on the i n t e r a c t i o n of different silica gels of v a r i o u s porosities w i t h selected a d s o r b a t e molecules. Such i n v e s t i g a t i o n s c a n e l u c i d a t e the specific and non-specific a s p e c t s of physical a d s o r p t i o n . 2. E x p e r i m e n t a l Gasil 200, Gasil 35 and TK800 silica gels supplied by D e g u s s a F o r s c h u n g C h e m i c were used as a d s o r b e n t s . The a d s o r p t i o n of n i t r o g e n at - 1 9 5 C and of o r g a n i c v a p o u r s at 35 C was d e t e r m i n e d v o l u m e t r i c a l l y u s i n g c o n v e n t i o n a l a p p a r a t u s [4]. The a d s o r p t i o n of w a t e r v a p o u r at 35 C was d e t e r m i n e d g r a v i m e t r i c a l l y by using a M c B a i n - B a k r q u a r t z s p r i n g b a l a n c e of s e n s i t i v i t y a b o u t 25 cm g ~. The e l o n g a t i o n s of the b a l a n c e p r o d u c e d by a d s o r p t i o n were followed by u s i n g a c a t h e t o m e t e r which read to 0.01 mm. 3. R e s u l t s a n d d i s c u s s i o n The n i t r o g e n i s o t h e r m s for Gasil 200, Gasil 35 and TK8O0 before and a f t e r t h e r m a l t r e a t m e n t at 400 C, w h i c h are g r a p h i c a l l y i l l u s t r a t e d in Fig. 1, were a n a l y s e d by the ~ m e t h o d [5]. The p o r o u s c h a r a c t e r s of the s a m p l e s as s h o w n by the as plots (Fig. 2) are s u m m a r i z e d in T a b l e 1. The S,ET x: a r e a s w e r e o b t a i n e d in the c u s t o m a r y w a y by a d o p t i n g 16.2 A as the crosss e c t i o n a l a r e a of the n i t r o g e n molecule. The Ss ~': a r e a s were c a l c u l a t e d from the initial slopes of the a~ plots by u s i n g the e q u a t i o n S, N~ = 2.89 V/:~ The n o r m a l i z i n g f a c t o r 2.89 was o b t a i n e d by c a l i b r a t i o n a g a i n s t the SLEETx~' of Fransil. A c o m p a r i s o n of S~~~ and SBETx~' a r e a s (Table 1) shows t h a t the S~~= a r e a of Gasil 200 is a p p r e c i a b l y h i g h e r t h a n the c o r r e s p o n d i n g SBETN: area. This m a y be due to the p r e s e n c e of the m i c r o p o r e s w h i c h d i s t o r t the s h a p e of the i s o t h e r m in the B r u n a u e r - E m m e t t Teller (BET) range, leading to i n c o r r e c t e v a l u a t i o n of the s u r f a c e a r e a by the B E T e q u a t i o n [5, 6]. However, good a g r e e m e n t b e t w e e n S, x~ a n d S~E,,,N~ is found for m e s o p o r o u s Gasil 35 and n o n - p o r o u s TK800. The n i t r o g e n a d s o r p t i o n - d e s o r p t i o n i s o t h e r m s at - 1 9 5 C on fully and p a r t i a l l y h y d r o x y l a t e d TK8O0 a n d Gasil 35 (Fig. 1) show a slight d e c r e a s e in the a m o u n t a d s o r b e d a f t e r t h e r m a l t r e a t m e n t at 400 C a n d this e x t e n d s to h i g h e r P/Po values. H o w e v e r , for Gasil 200 a d e c r e a s e in the a m o u n t a d s o r b e d was d e t e c t e d for v a l u e s of P/Po up to a b o u t 0.2 followed by a m a r k e d i n c r e a s e at h i g h r e l a t i v e p r e s s u r e values. T h e c h a n g e in the struct u r e of the n i t r o g e n m o n o l a y e r s and m u l t i l a y e r s [7- 9] m a y be r e l a t e d to the c h a n g e in the c h e m i c a l n a t u r e of the s u r f a c e a f t e r d e h y d r o x y l a t i o n at
271
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Fig. 1. Adsorption-desorption isotherms for nitrogen on fully and partially hydroxylated silica samples: 0, fully hydroxylated silica; 0, partially hydroxylated silica. Fig. 2. rs plots of nitrogen isotherms on fully and partially hydroxylated 0, fully hydroxylated silica; 0, partially hydroxylated silica.
silica samples:
C. In other words nitrogen quadrupole-surface dipole interactions appear to play an important role during nitrogen adsorption which cannot be considered to be mainly non-specific. Adsorption isotherms at 35 -C of non-polar carbon tetrachloride molecules on fully and partially hydroxylated samples for each of the three silica gels tested are illustrated in Fig. 3. The coincidence of the isotherms for the fully and partially hydroxylated silica gels gives good evidence that this adsorbate provides a more sensitive probe [lo, 111 than nitrogen for the detection of the porosity character rather than the chemistry of the gel. The drawback of carbon tetrachloride is its large size which prevents its molecules from penetrating narrow pores, as shown in Table 1. This explains the marked difference between SBETNz and SBETcc’*,particularly for microporous Gasil 200. The limited amount of experimental evidence available to date [I2 - 141 suggests that carbon tetrachloride provides a more sensitive probe than nitrogen for the detection of microporosity. 400
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Fig. 3. Adsorption desorption isotherms for carbon tetrachloride on fully and partially hydroxylated silica samples: O, fully hydroxylated Gasil 200; A, fully hydroxylated Gasil 35; D, fully hydroxylated TK800; O, partially hydroxylated Gasil 200; A, partially hydroxylated Gasil 35; B, partially hydroxylated TK800. Fig. 4. Adsorption desorption isotherms for benzene on fully and partially hydroxylated silica samples (symbols as-ffor Fig. 3). The adsorption of the aromatic molecule benzene on various silica gels (Fig. 4) provides an interesting example of the correlation of the chemistry of a surface with its geometry. Thus, because of the non-uniformity of its electron distribution, the benzene molecule can interact with hydroxyl groups via rc bonding [15]. It is clear from Table 1 and Fig. 4 that SBETCoH~is greater than SBETCC~', and in the case of benzene this may be due to nonspecific forces acting side by side with specific forces. Figure 5 shows the a d s o r p t i o n ~ l e s o r p t i o n isotherms of m e t h a n o l on fully and partially hydroxylated surfaces, and it can be seen that after d e h y d r o x y l a t i o n the surfaces have a higher meth~inol uptake. The partially hydroxylated silicas have a composite surface since on thermal treatment the surface hydroxyls dehydrate to give siloxane bridges. After partial d e h y d r o x y l a t i o n the reaction of methanol is not restricted to S i - - O H groups but also occurs with the strained siloxane bridges [16].
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Fig. 5. Adsorption desorption isotherms for methanol on fully and partially hydroxylated silica samples (symbols as for Fig. 3). Fig. 6. Adsorption desorption isotherms for water vapour on fully and partially hydroxylated silica samples (symbols as for Fig. 3).
The increased adsorption of alcohols on silicas that have been partially dehydrated above 400 C is accompanied by the appearance of a hysteresis effect despite the absence of marked textural changes. This effect has been observed previously by Gani [17]. The increase can be interpreted on the basis of the adsorption of m e t h a n o l on partially dehydroxylated silica via three main processes: (i) the interaction of methanol with hydroxyl groups ; (ii) m e t h o x y l a t i o n of dehydrated groups; (iii) the participation of dispersion fields. The adsorption isotherms of water vapour on fully and partially d e h y d r o x y l a t e d silica samples are s h o w n in Fig. 6. The drastic reduction in water uptake at P/Po ~ 0.35 indicates that water adsorption is mainly due to the specific dipole dipole interaction. Micropore filling does not appear to influence the water adsorption to the extent that has been found in the case
275
o f n i t r o g e n [17, 18] a n d o t h e r a d s o r b a t e m o l e c u l e s . T h i s e x p l a i n s w h y t h e SBETH2° v a l u e is l o w e r t h a n t h e v a l u e s c a l c u l a t e d f r o m t h e a d s o r p t i o n o f o t h e r a d s o r b a t e s . T h e e x t r e m e s p e c i f i c i t y o f w a t e r a d s o r p t i o n c a n be u s e d only to provide information about the chemistry of the surface rather than i t s e x t e n t o r t e x t u r e . T h e u p w a r d d e v i a t i o n a t h i g h P / P o t h a t c a n be s e e n in F i g . 6 for t h e w a t e r i s o t h e r m c a n be a t t r i b u t e d t o a s e c o n d a r y e f f e c t o f porosity.
References 1 S. Brunauer, P. H. Emmett and E. Teller, J. Am. Chem. Soc., 60 (1938) 309. 2 A.V. Kiselev, Discuss. Faraday Soc., 40 (1965) 205; Colloid Interface Sci., 98 (1968) 430. 3 R.M. Barrer, J. Colloid Interface Sci., 21 (1966) 415. 4 R.I. Razouk and A. S. Salem, J. Phys. Chem., 52 (1948) 1908. 5 K . S . W . Sing, D. H. Everett and R. H. Ottemill, Surface Area Determination, Butterworths, London, 1970, p. 25. 6 B.C. Lippens and J. H. de Boer, J. Catal., 4 (1965) 319. 7 J . D . Carruthers, D. A. Payne, K. S. W. Sing and L. J. Strylev, J. Colloid Interface Sci., 36 (1971) 205. 8 C. Pierce, J. Phys. Chem., 72 (1968) 3673. 9 A.C. Zettlemayer, J. Colloid Interface Sci., 28 (1968) 343. 10 A.V. Kiselev, Rev. Gen. Caoutch. Plast., 41 (1964) 377. 11 A.V. Kiselev, Zh. Fiz. Khim., 38 (1964) 2753. 12 B.G. Aristov, I. Yu. Babkin, V. Ma. Davydov and A. V. Kiselev, Zh. Fiz. Khim., 37 (1963) 2372. 13 S . J . Gregg and K. S. W. Sing, J. Phys. Chem., 66 (1951) 597. 14 T . M . El-Akkad, J. Chem. Tech. Biotechnol., 29 (1979) 413. 15 V.M. Bermudez, J. Phys. Chem., 75 (1971) 3249. 16 L.D. Balaakova and A. V. Kiselev, Zh. Fiz. Khim., 33 (1959) 1534. 17 B.P. Gani, J. Indian Chem. Soc., 27 (1950) 577. 18 M.M. Zgorov, K. G. Krasil'nikov and V. F. Kiselev, Zh. Fiz. Khim., 32 (1958) 55.
269
A D S O R P T I O N OF NITROGEN, W A T E R A N D CERTAIN ORGANIC V A P O U R S ON FULLY AND PARTIALLY H Y D R O X Y L A T E D SILICA GELS WITH D I F F E R E N T POROSITY CHARACTERS T. M. EL-AKKAD, A. AMIN and S. NASHED Department of Chemistry, Faculty of Science and Faculty of Education, Ain Shams University, Cairo (Egypt)
(Received April 11, 1980)
Summary The adsorption of several adsorbate molecules, namely nitrogen, water, carbon tetrachloride, benzene and methanol, on fully and partially hydroxylated silica samples was investigated. The samples tested had different porosity characters, i.e. they were microporous (Gasil 200), mesoporous (Gasil 35) and non-porous (TK800), in order to elucidate the effect of the porosity of the gel on specific and non-specific adsorbate-adsorbent interactions. Nitrogen adsorption, which is assumed to involve non-specific interactions, is influenced to a certain extent by the specific interaction between the nitrogen quadrupole and the surface hydroxyl. The smaller polar water molecules can penetrate into pores which are too narrow to accommodate larger adsorbate molecules. The marked reduction of the water uptake upon dehydroxylation of the silicas substantiates the important role of the specific interaction in the adsorption of water on silica surfaces. The carbon tetrachloride molecule appears to be a more sensitive probe than the nitrogen molecule for detecting microporosity, its only drawback being its larger size. Benzene adsorption depends mainly on the interaction between the mobile x bonding and surface hydroxyls. Finally, non-specific interactions appear to have a marked effect on the adsorption of polar methanol molecules on silica surfaces which involve essentially specific interactions.
1. I n t r o d u c t i o n A good deal of experimental evidence supports the validity [1] of the area obtained from nitrogen adsorption on oxides. Much less information is available on the adsorption of organic vapour. However, recent studies [2, 3] have revealed significant differences in specificity between various physical adsorptions. The pore structure of the silica adsorbent and the surface 0376-4883/81/0000-0000/$02.50
© Elsevier Sequoia/Printed in The Netherlands
270 g e o m e t r y a r o u n d individual a d s o r p t i o n sites are g e n e r a l l y a s s u m e d to be i m p o r t a n t in d e t e r m i n i n g the r e l a t i v e a d s o r p t i o n of different a d s o r b a t e s . In this c o n t e x t c o m p a r a t i v e studies were c a r r i e d out on the i n t e r a c t i o n of different silica gels of v a r i o u s porosities w i t h selected a d s o r b a t e molecules. Such i n v e s t i g a t i o n s c a n e l u c i d a t e the specific and non-specific a s p e c t s of physical a d s o r p t i o n . 2. E x p e r i m e n t a l Gasil 200, Gasil 35 and TK800 silica gels supplied by D e g u s s a F o r s c h u n g C h e m i c were used as a d s o r b e n t s . The a d s o r p t i o n of n i t r o g e n at - 1 9 5 C and of o r g a n i c v a p o u r s at 35 C was d e t e r m i n e d v o l u m e t r i c a l l y u s i n g c o n v e n t i o n a l a p p a r a t u s [4]. The a d s o r p t i o n of w a t e r v a p o u r at 35 C was d e t e r m i n e d g r a v i m e t r i c a l l y by using a M c B a i n - B a k r q u a r t z s p r i n g b a l a n c e of s e n s i t i v i t y a b o u t 25 cm g ~. The e l o n g a t i o n s of the b a l a n c e p r o d u c e d by a d s o r p t i o n were followed by u s i n g a c a t h e t o m e t e r which read to 0.01 mm. 3. R e s u l t s a n d d i s c u s s i o n The n i t r o g e n i s o t h e r m s for Gasil 200, Gasil 35 and TK8O0 before and a f t e r t h e r m a l t r e a t m e n t at 400 C, w h i c h are g r a p h i c a l l y i l l u s t r a t e d in Fig. 1, were a n a l y s e d by the ~ m e t h o d [5]. The p o r o u s c h a r a c t e r s of the s a m p l e s as s h o w n by the as plots (Fig. 2) are s u m m a r i z e d in T a b l e 1. The S,ET x: a r e a s w e r e o b t a i n e d in the c u s t o m a r y w a y by a d o p t i n g 16.2 A as the crosss e c t i o n a l a r e a of the n i t r o g e n molecule. The Ss ~': a r e a s were c a l c u l a t e d from the initial slopes of the a~ plots by u s i n g the e q u a t i o n S, N~ = 2.89 V/:~ The n o r m a l i z i n g f a c t o r 2.89 was o b t a i n e d by c a l i b r a t i o n a g a i n s t the SLEETx~' of Fransil. A c o m p a r i s o n of S~~~ and SBETx~' a r e a s (Table 1) shows t h a t the S~~= a r e a of Gasil 200 is a p p r e c i a b l y h i g h e r t h a n the c o r r e s p o n d i n g SBETN: area. This m a y be due to the p r e s e n c e of the m i c r o p o r e s w h i c h d i s t o r t the s h a p e of the i s o t h e r m in the B r u n a u e r - E m m e t t Teller (BET) range, leading to i n c o r r e c t e v a l u a t i o n of the s u r f a c e a r e a by the B E T e q u a t i o n [5, 6]. However, good a g r e e m e n t b e t w e e n S, x~ a n d S~E,,,N~ is found for m e s o p o r o u s Gasil 35 and n o n - p o r o u s TK800. The n i t r o g e n a d s o r p t i o n - d e s o r p t i o n i s o t h e r m s at - 1 9 5 C on fully and p a r t i a l l y h y d r o x y l a t e d TK8O0 a n d Gasil 35 (Fig. 1) show a slight d e c r e a s e in the a m o u n t a d s o r b e d a f t e r t h e r m a l t r e a t m e n t at 400 C a n d this e x t e n d s to h i g h e r P/Po values. H o w e v e r , for Gasil 200 a d e c r e a s e in the a m o u n t a d s o r b e d was d e t e c t e d for v a l u e s of P/Po up to a b o u t 0.2 followed by a m a r k e d i n c r e a s e at h i g h r e l a t i v e p r e s s u r e values. T h e c h a n g e in the struct u r e of the n i t r o g e n m o n o l a y e r s and m u l t i l a y e r s [7- 9] m a y be r e l a t e d to the c h a n g e in the c h e m i c a l n a t u r e of the s u r f a c e a f t e r d e h y d r o x y l a t i o n at
271
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.
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Fig. 1. Adsorption-desorption isotherms for nitrogen on fully and partially hydroxylated silica samples: 0, fully hydroxylated silica; 0, partially hydroxylated silica. Fig. 2. rs plots of nitrogen isotherms on fully and partially hydroxylated 0, fully hydroxylated silica; 0, partially hydroxylated silica.
silica samples:
C. In other words nitrogen quadrupole-surface dipole interactions appear to play an important role during nitrogen adsorption which cannot be considered to be mainly non-specific. Adsorption isotherms at 35 -C of non-polar carbon tetrachloride molecules on fully and partially hydroxylated samples for each of the three silica gels tested are illustrated in Fig. 3. The coincidence of the isotherms for the fully and partially hydroxylated silica gels gives good evidence that this adsorbate provides a more sensitive probe [lo, 111 than nitrogen for the detection of the porosity character rather than the chemistry of the gel. The drawback of carbon tetrachloride is its large size which prevents its molecules from penetrating narrow pores, as shown in Table 1. This explains the marked difference between SBETNz and SBETcc’*,particularly for microporous Gasil 200. The limited amount of experimental evidence available to date [I2 - 141 suggests that carbon tetrachloride provides a more sensitive probe than nitrogen for the detection of microporosity. 400
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Fig. 3. Adsorption desorption isotherms for carbon tetrachloride on fully and partially hydroxylated silica samples: O, fully hydroxylated Gasil 200; A, fully hydroxylated Gasil 35; D, fully hydroxylated TK800; O, partially hydroxylated Gasil 200; A, partially hydroxylated Gasil 35; B, partially hydroxylated TK800. Fig. 4. Adsorption desorption isotherms for benzene on fully and partially hydroxylated silica samples (symbols as-ffor Fig. 3). The adsorption of the aromatic molecule benzene on various silica gels (Fig. 4) provides an interesting example of the correlation of the chemistry of a surface with its geometry. Thus, because of the non-uniformity of its electron distribution, the benzene molecule can interact with hydroxyl groups via rc bonding [15]. It is clear from Table 1 and Fig. 4 that SBETCoH~is greater than SBETCC~', and in the case of benzene this may be due to nonspecific forces acting side by side with specific forces. Figure 5 shows the a d s o r p t i o n ~ l e s o r p t i o n isotherms of m e t h a n o l on fully and partially hydroxylated surfaces, and it can be seen that after d e h y d r o x y l a t i o n the surfaces have a higher meth~inol uptake. The partially hydroxylated silicas have a composite surface since on thermal treatment the surface hydroxyls dehydrate to give siloxane bridges. After partial d e h y d r o x y l a t i o n the reaction of methanol is not restricted to S i - - O H groups but also occurs with the strained siloxane bridges [16].
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Fig. 5. Adsorption desorption isotherms for methanol on fully and partially hydroxylated silica samples (symbols as for Fig. 3). Fig. 6. Adsorption desorption isotherms for water vapour on fully and partially hydroxylated silica samples (symbols as for Fig. 3).
The increased adsorption of alcohols on silicas that have been partially dehydrated above 400 C is accompanied by the appearance of a hysteresis effect despite the absence of marked textural changes. This effect has been observed previously by Gani [17]. The increase can be interpreted on the basis of the adsorption of m e t h a n o l on partially dehydroxylated silica via three main processes: (i) the interaction of methanol with hydroxyl groups ; (ii) m e t h o x y l a t i o n of dehydrated groups; (iii) the participation of dispersion fields. The adsorption isotherms of water vapour on fully and partially d e h y d r o x y l a t e d silica samples are s h o w n in Fig. 6. The drastic reduction in water uptake at P/Po ~ 0.35 indicates that water adsorption is mainly due to the specific dipole dipole interaction. Micropore filling does not appear to influence the water adsorption to the extent that has been found in the case
275
o f n i t r o g e n [17, 18] a n d o t h e r a d s o r b a t e m o l e c u l e s . T h i s e x p l a i n s w h y t h e SBETH2° v a l u e is l o w e r t h a n t h e v a l u e s c a l c u l a t e d f r o m t h e a d s o r p t i o n o f o t h e r a d s o r b a t e s . T h e e x t r e m e s p e c i f i c i t y o f w a t e r a d s o r p t i o n c a n be u s e d only to provide information about the chemistry of the surface rather than i t s e x t e n t o r t e x t u r e . T h e u p w a r d d e v i a t i o n a t h i g h P / P o t h a t c a n be s e e n in F i g . 6 for t h e w a t e r i s o t h e r m c a n be a t t r i b u t e d t o a s e c o n d a r y e f f e c t o f porosity.
References 1 S. Brunauer, P. H. Emmett and E. Teller, J. Am. Chem. Soc., 60 (1938) 309. 2 A.V. Kiselev, Discuss. Faraday Soc., 40 (1965) 205; Colloid Interface Sci., 98 (1968) 430. 3 R.M. Barrer, J. Colloid Interface Sci., 21 (1966) 415. 4 R.I. Razouk and A. S. Salem, J. Phys. Chem., 52 (1948) 1908. 5 K . S . W . Sing, D. H. Everett and R. H. Ottemill, Surface Area Determination, Butterworths, London, 1970, p. 25. 6 B.C. Lippens and J. H. de Boer, J. Catal., 4 (1965) 319. 7 J . D . Carruthers, D. A. Payne, K. S. W. Sing and L. J. Strylev, J. Colloid Interface Sci., 36 (1971) 205. 8 C. Pierce, J. Phys. Chem., 72 (1968) 3673. 9 A.C. Zettlemayer, J. Colloid Interface Sci., 28 (1968) 343. 10 A.V. Kiselev, Rev. Gen. Caoutch. Plast., 41 (1964) 377. 11 A.V. Kiselev, Zh. Fiz. Khim., 38 (1964) 2753. 12 B.G. Aristov, I. Yu. Babkin, V. Ma. Davydov and A. V. Kiselev, Zh. Fiz. Khim., 37 (1963) 2372. 13 S . J . Gregg and K. S. W. Sing, J. Phys. Chem., 66 (1951) 597. 14 T . M . El-Akkad, J. Chem. Tech. Biotechnol., 29 (1979) 413. 15 V.M. Bermudez, J. Phys. Chem., 75 (1971) 3249. 16 L.D. Balaakova and A. V. Kiselev, Zh. Fiz. Khim., 33 (1959) 1534. 17 B.P. Gani, J. Indian Chem. Soc., 27 (1950) 577. 18 M.M. Zgorov, K. G. Krasil'nikov and V. F. Kiselev, Zh. Fiz. Khim., 32 (1958) 55.