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Related surface and heat of immersion characterization of some alumina samples II: Water-soaked activated alumina
Surface Technology, 18 (1983) 359 - 370
359
R E L A T E D S UR F A C E AND H E A T OF IMMERSION C H A R A C T E R I Z A T I O N OF SOME ALUMINA SAMPLES II: WATER-SOAKED A C T I V A T E D ALUMINA
A. M. KHALIL* Department of Chemistry, Faculty of Science, Ain Shams University, Abbassia, Cairo (Egypt)
(Received September 10, 1982)
Summary Soaked alumina was prepared by leaving RhSne Poulenc activated alumina in d o u b l y distilled water for 24 h. The activated alumina was then air dried. Both processes were carried out at r o o m temperature. The soaked alumina was thermally d e h y d r a t e d in vacuo at various temperatures in the range 20 - 500 °C. Water losses (in grams o f water per 100 g o f the sample) for all the heat-treated samples were determined; it was shown that the soaked alumina samples exhibit higher water losses than those determined for the parent activated alumina samples. This was considered to be ample evidence for chemisorption a n d / o r strong upt ake of water molecules at the surface and in the pore system o f the bulk alumina. Nitrogen adsorption measurements were carried out at liquid nitrogen t e m p e r a t u r e for all the samples. All the isotherms were o f t y p e II according to the classification of E m m e t t and Brunauer and were characterized by the occurrence o f hysteresis loops. The areas o f the hysteresis loops initially increased up to 200 °C; decreases in the areas of the hysteresis loops were then n o ted up to 400 °C. The increase in area of t he hysteresis loops below 200 °C is attr ib u ted to the increased loss of physisorbed water. The decreased rate o f water loss {water loss f r om the surface a n d / o r structural water loss) corresponds to the observed decrease in area of the hysteresis loops up to 400 °C. Analysis suggested that the surface areas are mostly located in wide pores. The presence of some narrow pores (or micropores) was det ect ed mainly at temperatures above 300 °C. Heat o f immersion measurements were carried o u t using water as the immersion liquid for all the samples investigated. Despite the observed increase in water loss and nitrogen surface area for t he soaked samples, the parent activated and soaked alumina samples exhibit almost the same integral heats o f immersion up to 300 °C. At higher temperatures, the soaked alumina *Present address: Department of Chemistry, Junior College of Teachers, P.O. Box 4341, Riyadh, Saudi Arabia. 0376-4583/83/0000-0000/$03.00
© Elsevier Sequoia/Printed in The Netherlands
360 exhibited higher heats of immersion in water, which is directly related to the high extent of dehydration (or to the dehydroxylation of the surface h y d r o x y l groups) and the exposure of both aluminium ions (A13÷) and oxygen anions ( 0 2 - ) to the surface, which enhance the formation of excess hydrous alumina and/or aluminium hydroxide. On calculation of the heats of immersion in water per unit area, it is found that activated alumina has higher heats of immersion up to 300 °C, whereas the soaked alumina preheated above 300 °C has higher heats of immersion because of the higher concentration of surface h y d r o x y l groups as well as the increased A13+and 0 2 - ion c o n t e n t of these soaked samples.
1. Introduction Measurements of the heats of immersion of solids in liquids permit the investigation of a number of properties of the solid surface. The thermodynamics of the immersion process developed by Harkins [1] have been revised by Eley [2] from the viewpoint of adhesion. Both the integral and differential heats of adsorption of vapours on solids can be determined from the heats of immersion. Furthermore, Chessick and coworkers [3, 4] have shown that the variation in the heat of immersion of a solid in a series of n-butyl derivatives of differing dipole m o m e n t can be used to estimate an average value for the electrostatic field strength at the solid surface. The parent activated alumina contains centres of both acidic and basic natures [5 - 8] as well as some centres with oxidation and reduction properties [9]; this activated alumina was recently presented and discussed by Khalil [10]. In a continuation of the previous investigation [10], activated alumina was water soaked for 24 h at room temperature and air dried for prolonged periods of time. Previous calorimetric results on various outgassed aluminas [11, 12] had indicated the presence of both reversible physisorption and irreversible chemisorption at the surface region. An investigation of the surface properties of the soaked alumina by both nitrogen adsorption and heat of immersion techniques is presented in this paper. The results are directly compared with those obtained for activated alumina [10]; they confirm the rapid chemisorption of water molecules at the surface and/or the strong uptake of water in the pore system. The present results are discussed in relation to those obtained for activated alumina.
2. Experimental details Activated alumina was left in contact with doubly distilled water for 24 h, and the soaked alumina was air dried for a prolonged period of time
361 b e f o r e use. B o t h t h e h y d r a t i o n a n d t h e d r y i n g processes w e r e carried o u t u n d e r identical p h y s i c a l c o n d i t i o n s { r o o m t e m p e r a t u r e , 20 °C). S o a k e d a l u m i n a was t h e r m a l l y d e h y d r a t e d i n v a c u o at t e m p e r a t u r e s o f 20 °C, 100 °C, 2 0 0 °C, 3 0 0 °C, 4 0 0 °C a n d 500 °C a n d t h e resulting s a m p l e s w e r e d e s i g n a t e d as S A ( 2 0 ) , S A ( 1 0 0 ) , S A ( 2 0 0 ) , S A ( 3 0 0 ) , S A ( 4 0 0 ) a n d S A ( 5 0 0 ) r e s p e c t i v e l y ; t h e h e a t i n g t e m p e r a t u r e s are i n d i c a t e d in p a r e n t h e s e s . H e a t i n g was carried o u t using a small electrical t u b u l a r f u r n a c e (200 W) c o u p l e d w i t h a S h i n k o t h e r m o c o n t r o l l e r ( m o d e l DIC-P) w i t h a r a n g e u p t o 6 0 0 °C; this t h e r m o c o n t r o l l e r h a d b e e n originally c a l i b r a t e d w i t h a c h r o m e l alumel thermocouple. N i t r o g e n a d s o r p t i o n m e a s u r e m e n t s w e r e carried o u t using a c o n v e n t i o n a l v o l u m e t r i c a p p a r a t u s f o r gas a d s o r p t i o n [ 1 3 ] . Highly p u r i f i e d n i t r o g e n ( N o r s k H y d r o ) was used as a d s o r b a t e w i t h o u t f u r t h e r p u r i f i c a t i o n . W a t e r losses (in g r a m s o f w a t e r p e r 100 g o f t h e s a m p l e ) w e r e e s t i m a t e d using a M c B a i n - B a k e r t y p e o f silica spring b a l a n c e w h i c h h a d a sensitivity o f 35 c m g-1. T h e values o f t h e w a t e r loss, t h e B r u n a u e r - E m m e t t - T e l l e r ( B E T ) C c o n s t a n t , t h e m o n o l a y e r c a p a c i t y Vm, t h e surface area SBETN2 (m 2 g - l ) a n d t h e t o t a l p o r e v o l u m e Vp (ml g - l ) are all listed in T a b l e 1, c o l u m n s 2, 3, 4, 5 and 6 r e s p e c t i v e l y . Using t h e c a l o r i m e t r i c m e t h o d [ 1 5 ] , t h e integral h e a t s Hi w (cal g - l ) o f i m m e r s i o n in w a t e r w e r e d e t e r m i n e d at 35 + 0 . 0 5 °C; t h e t e m p e r a t u r e o f t h e i m m e r s i o n liquid ( w a t e r ) was initially k e p t c o n s t a n t to w i t h i n +0.002 °C. A m p o u l e s , each c o n t a i n i n g a s a m p l e t h e mass o f w h i c h h a d b e e n d e t e r m i n e d a c c u r a t e l y ( a b o u t 0 . 2 0 g), w e r e c a r e f u l l y e v a c u a t e d f o r 3 h at t h e r e q u i r e d t e m p e r a t u r e , sealed o f f u n d e r v a c u u m a n d t h e n t r a n s f e r r e d t o t h e c a l o r i m eter. F o r e a c h h e a t t r e a t m e n t o f a s a m p l e , w a t e r losses a p p e a r t o be a l m o s t constant within 2 - 3 h except for the samples evacuated at room temperat u r e , and t h e r e s p e c t i v e s a m p l e s w e r e t h e r e f o r e degassed f o r p r o l o n g e d periods of time before the heat of immersion measurements were made. T h e o b s e r v e d c h a n g e in t e m p e r a t u r e p r o d u c e d o n i m m e r s i o n was c o r r e c t e d f o r r a d i a t i o n losses [ 1 5 ] , as well as f o r t h e h e a t o f capillary e n d TABLE 1 Some surface characteristics of nitrogen adsorption on soaked activated alumina Sample
Water loss x 102
BET C constant
1
(g H20 g-I) 2 3
SA(20) SA(100) SA(200) SA(300) SA(400) SA(500)
6.95 9.65 11.34 11.45 11.49 11.68
16 20 19 19 20 21
17 21 20 20 21 22
Vm
SBETN2 Vp (m 2 g-l) (ml g-l)
St a (m 2 g-l)
r ()k)
r-1 ()k-l)
4
5
6
7
8
9
99.01 108.70 114.94 112.99 111.86 108.93
431.3 473.5 500.7 492.2 487.3 475.5
0.661 0.770 0.676 0.653 0.692 0.669
430 472 490 482 498 480
30.7 32.5 27.0 26.5 28.4 28.2
0.0326 0.0308 0.0307 0.0377 0.0352 0.0355
aReference standard [ 14 ]; BET C ~ 20 - 30 was used in V l - t analysis.
362 TABLE 2 Heats of immersion of activated alumina soaked in water
Sample
Water loss
1
(g n 2 0 g - l ) 2
SBET N2 (m: g-l) 3
Hi w (cal g - l ) 4
hi w (erg cm -2) 5
SA(20) SA(100) SA(200) SA(300) SA(400) SA(500 )
6.95 9.65 11.34 11.45 11.49 11.68
431.3 473.5 500.7 492.2 487.3 474.5
3.20 3.36 3.68 6.65 7.75 10.12
31.0 29.7 30.7 56.5 66.5 89.2
×
102
breaking [16]. The calculated Hi values reported in Table 2 represent the average of at least two independent determinations which were reproducible almost to within 3% for the low temperature samples but to a considerably better degree of accuracy for samples dehydrated at high temperatures. Doubly distilled water was used for the heat of immersion measurements.
3. Results and discussion Figure 1 shows the variation in water loss (in grams of water per 100 g of the sample} as a function of the heat treatment temperature for activated alumina (curve a) and soaked alumina {curve b}. Both the parent activated alumina and the water-soaked alumina exhibited a marked continuous increase in water loss with increase in the dehydration temperature up to 500 °C. Activated alumina shows a marked abrupt increase in water loss in the range 200 - 300 °C, thus revealing a change in the nature of the water loss below 200 °C from that above 300 °C {Fig. 1, curve a). However, the soaked alumina shows an early continuous increase in water loss up to 200 °C; the water loss then increases more gradually with further rise in temperature up to 500 °C (Fig. 1, curve b). The soaked samples exhibited higher water losses
12
•
b
•
.~ 10
N 8
~o
0
100
200
300
400
500
Temperature,C
Fig. 1. Variations in the water loss for the parent activated alumina (e, curve a) and the soaked alumina (&, curve b) as functions o f the heat treatment temperature.
363
and this is taken as good evidence for strong irreversible uptake of water molecules on the surface as well as in the pore system of the alumina.
3.1. Specific surface areas from nitrogen adsorption The surface areas SBETN2 (m 2 g-l) were calculated by applying the BET equation to the nitrogen adsorption isotherms in the range P/Po = 0.05 - 0.35 and by adopting the value of 16.2 A: for the cross-sectional area of the nitrogen molecule [17]. All the adsorption isotherms are invariably of type II according to the classification of Emmett and Brunauer [18] and exhibit hysteresis phenomena. The closure points of the hysteresis loops are shifted downwards to low relative pressures when the temperature is increased to 200 °C; at higher temperatures (up to 400 °C) the closure points are shifted to higher relative pressures. The increase in the areas of the hysteresis loops when the temperature is increased to 200 °C, together with the downward shifts of the closure points of the hysteresis loops, is considered to be ample evidence for a greater loss of physisorbed water, not only from the surface but also from the pore system, i.e. from the bulk alumina (Fig. 2, curves a - c). The shifts in the closure points of the hysteresis loops, below 200 °C, to lower P/P0 values indicate that not only may molecular water be lost from mesopores
~,50 400 350 300 250 200 150 '~
m
100
0 50 0 50 0
5O 0
-
-
5O
J
0 50 0 0.1
O.
0.3
0.4
0.5
06
07
0.8
0.9
1.0
Fig. 2. Adsorption-desorption isotherms of nitrogen on soaked alumina samples (o, adsorption; o, desorption): curve a, SA(20); curve b, SA(100); curve c, SA(200); curve d, SA(300); curve e, SA(400); curve f, SA(500).
364 (or wide pores) b u t also some strongly b o u n d w a t e r m a y be lost f r o m m i c r o p o r e s (or n a r r o w pores) (Fig. 1, curve b). On d e h y d r a t i o n o f t h e alumina in t h e t e m p e r a t u r e range 200 - 400 °C t h e decrease in t h e areas o f t h e hysteresis loops, t o g e t h e r with the shifts to higher relative pressures o f t h e i r closure points, in the a d s o r p t i o n isotherms s h o w n in Fig. 2, curves d and e, is responsible for the decreased rate o f w a t e r loss n o t e d at these high t e m p e r a t u r e s (Fig. 1, curve b). As s h o w n f o r sample S A ( 5 0 0 ) (Fig. 2, curve f), the increase in the area o f the hysteresis l o o p was a c c o m p a n i e d b y a d o w n w a r d shift in the closure p o i n t o f t h e hysteresis l o o p t o P / P o = 0.25, which indicates the loss o f some c o n s t i t u t i o n a l w a t e r f r o m the alumina n e t w o r k . T h e high t e m p e r a t u r e t r e a t m e n t m a y be responsible f o r a change in the crystalline order, and this is a c c o m p a n i e d b y the c r e a t i o n o f some m i c r o p o r e s a n d / o r voids in the m o r p h o l o g y o f t h e h e a t - t r e a t e d alumina. T h e average p o r e radius was calculated using the relation
r(A)=
2Vp SBET
X 10 4
N~-
w h e r e Vp (ml g-l) is t h e t o t a l p o r e v o l u m e and SBETN2 (m 2 g-l) is the nitrogen B E T surface area; values o f r (A) are listed in Table 1, c o l u m n 8. Figure 3 shows t h e variation in B E T surface area and the reciprocal r -1 (A-1) o f t h e average p o r e radius as f u n c t i o n s o f the p r e t r e a t m e n t temp e r a t u r e . T h e h a r m o n i c change revealed in the t w o p a r a m e t e r s SBETs'~ (m 2 g-l) and r -1 (A-1) indicates t h a t the surface area should increase w h e n the average p o r e radius decreases and vice versa. On the basis o f an ideal m o d e l o f s m o o t h , u n i f o r m and n o n - p o r o u s spheroidal particles, L e a n i n s o n e t al. [19] c a m e to the c o n c l u s i o n t h a t t h e surface area should be increased b y a decrease in t h e particle size and vice versa.
~ 0 . 0 5
m 500 ~E
~4oo
0.04
300 i
100
_
b
i
i
200
300
400
,.
i o.o 500
o Ternperot ure, C
Fig. 3. Variations in the nitrogen surface a r e a SBET N2 ( m 2 g-l) (©) and reciprocal (A-1) of the average pore radius (e) as functions of the heat treatment temperature.
r- 1
T h e p a r e n t activated alumina does n o t seem to possess a similar harm o n i c relation, which m i g h t be c o n s i d e r e d an i n d i c a t i o n o f s o m e degree o f t h e r m a l stability f o r activated alumina. Thus, t h e original desiccant n a t u r e o f t h e activated a l u m i n a m a y be r e s t o r e d m e r e l y b y degassing f o r 2 - 3 h at a b o u t 200 - 250 °C. Soaking o f t h e activated alumina with w a t e r m a y be
365
responsible for the increased h y d r o x y l a t i o n [20, 21] of the surface, together with the creation of hydrous alumina and/or aluminium hydroxide [22]. The initial increase in BET surface area associated with the conversion of hydrous alumina into pseudoboehmite [23] is believed to result from the formation of micropores within the primary particles, whereas a slower development of mesopores appears to be caused by a process of aggregationcementation. At a later stage in the dehydration (above 200 °C) bayerite is formed by a recrystallization process, and above this temperature a gradual decrease in surface area was observed up to 500 °C (Fig. 5, curve c).
3.2. Pore structure analysis The porous character of the thermally dehydrated soaked alumina samples was qualitatively detected by the Vl-t m e t h o d [24] and will be discussed briefly. As shown in Fig. 4, the Vl-t plots show initial straight-line portions representing the magnitude of St (m 2 g-l). Then at t ~ 3 - 4 A a small upward deviation develops and continues up to t ~ 6 A. Thereafter, increasing slopes were noted up to t ~ 8 - 9 A. The decreased intergranular capillary condensation continued up to pressures n o t far from the saturation pressure. The finite upward deviations noted above t ~ 3 - 4 A are mainly attributed to partial compensation effects [25] that occur in the adsorption of nitrogen both in micropores (and/or narrow pores) and in wide pores (and/or mesopores).
2
4
6
8
10
12
14
16
18
20
22
2,;
26
28
t(~)
Fig. 4. V1- t p l o t s for various s o a k e d a l u m i n a s a m p l e s preheated at different curves a - f d e n o t e t h e s a m e s a m p l e s as t h o s e in Fig. 2.
temperatures:
366 T h e increase in slope n o t e d at t ~> 6 A is d u e to m u l t i l a y e r f o r m a t i o n in m e s o p o r e s , w h e r e a s t h e d e c r e a s e d slopes n o t e d at t/> 8 - 9 A are related to capillary c o n d e n s a t i o n e f f e c t s at t h e s e high relative pressures. T h e St values listed in T a b l e 1, c o l u m n 7, s h o w fairly g o o d a g r e e m e n t w i t h t h e c o r r e s p o n d i n g B E T surface area values SBETN2 listed in T a b l e 1, c o l u m n 5. This indicates a p r o p e r c h o i c e o f t h e r e f e r e n c e d a t a [14] f o r each analysis. T h e B E T C c o n s t a n t f o r t h e s a m p l e s u n d e r investigation a n d t h a t f o r t h e r e f e r e n c e s t a n d a r d [ 1 4 ] ( B E T C ~ 20 - 30) s h o u l d be c o m p a r a b l e , and t h e best c o n d i t i o n is to h a v e a l m o s t identical B E T C values. This indicates t h a t t h e heats o f a d s o r p t i o n f o r b o t h t h e s a m p l e s a n d t h e r e f e r e n c e s t a n d a r d [14] are identical. It was p r e v i o u s l y s t a t e d b y B r u n a u e r [ 2 6 ] t h a t t h e m a g n i t u d e o f t h e B E T C c o n s t a n t is c o n s i d e r e d t o be an a d e q u a t e m e a s u r e o f t h e h e a t o f a d s o r p t i o n f o r analysis. T h e surface areas l o c a t e d in wide p o r e s (or m e s o p o r e s ) w e r e a n a l y s e d b y t h e c o r r e c t e d model-less m e t h o d [ 2 7 ] using a c o m p u t e r p r o g r a m [28] w r i t t e n in F O R T R A N IV f o r t h e IBM 1 1 3 0 c o m p u t e r . T h e analysis is based o n d e s o r p t i o n b r a n c h e s o f t h e i s o t h e r m s and was c o n t i n u e d d o w n w a r d s to a relative pressure P/Po o f 0.25. In t h e c a l c u l a t i o n o f t h e c u m u l a t i v e values Scum a n d Vcum and to avoid t h e p r o b l e m s a s s o c i a t e d w i t h c o m p e n s a t i o n e f f e c t s [ 2 5 ] , t h e analysis was s t o p p e d at relative pressures P/Po c o r r e s p o n d i n g t o t h e closure p o i n t s o f t h e h y s t e r e s i s l o o p s in t h e a d s o r p t i o n i s o t h e r m s . T h e a d o p t i o n o f t h e cylindrical p o r e m o d e l ( i n d i c a t e d b y the s u p e r s c r i p t cp) led to c u m u l a t i v e values Scum Cp a n d Vcumcn larger t h a n t h e e x p e r i m e n t a l l y e s t i m a t e d a d s o r p t i o n values SBETN2 a n d Yp t (ml g 1). T h e r e f o r e t h e parallel-plate m o d e l ( i n d i c a t e d b y t h e s u p e r s c r i p t p p ) was a s s u m e d since smaller c u m u l a t i v e values Scum ~v a n d Vcu~ pp w e r e o b t a i n e d ; these c u m u l a t i v e values are listed in T a b l e 3, c o l u m n s 6 a n d 7. T h e f r a c t i o n ~w o f t h e area l o c a t e d in w i d e p o r e s was c a l c u l a t e d b y dividing t h e c u m u l a t i v e s u r f a c e areas ( l o c a t e d in wide p o r e s a n d / o r m e s o p o r e s ) b y t h e n i t r o g e n B E T s u r f a c e areas; these values are listed in T a b l e 3, c o l u m n 8: TABLE 3 Cumulative values located in wide pores for the water-soaked alumina samples BET C
SBETN2
Vpt
St
2
(ml g-t) 4
(m: g 1) 5
(m: g-l)
1
(m 2 g-l) 3
SA(20) SA(100) SA(200) SA(300) SA(400) SA(500)
16-17 20-21 19 -20 19-20 20-21 21 -22
431.3 473.5 500.7 492.7 487.2 475.5
0.661 0.771 0.676 0.653 0.692 0.669
430 472 490 482 498 480
Sample
constant
Scumpp
Vcttmpp
~w
6
(ml g-l) 7
8
316.3 389.9 414.7 418.1 413.0 393.8
0.562 0.676 0.659 0.587 0.633 0.588
0.733 0.823 0.828 0.850 0.848 0.828
367 OLw = S c u m P P / S B E T N2
As shown in Table 3, column 8, the aw values for the water-soaked alumina samples show little variation and tend to be greater over the temperature range studied (20 - 500 °C) compared with the corresponding values for the activated alumina samples. This might be considered ample evidence for the effect of soaking in water on the porosity characteristics of the alumina samples. Therefore the soaked samples exhibit the characteristic features shown in Fig. 3, whereas the parent activated alumina does not seem to show the same behaviour.
3.3. Heats o f immersion in water In Part I of this work [10] in which activated alumina was used as a starting material and water, methanol and carbon tetrachloride were used as wetting liquids, the heats of immersion were in the following order: water methanol > carbon tetrachloride. The system, activated alumina plus water, was fully discussed with respect to the detected porosity and physicotextural changes which accompanied the thermal dehydration process. The porous character includes the effects of the location for adsorption on the wetting liquid molecules, whereas the physical structure of the sample reflects the loss of water either from the pore system or from the surface and the consequent exposure of excess aluminium ions (A13+) and oxygen anions (02-) to the surface. The effects of these parameters were considerably reduced for immersion in methanol and carbon tetrachloride. Therefore water was used as the immersion liquid in this work, despite the fact t h a t water is irreversibly chemisorbed at the activated alumina surface [20, 21]. Despite the questionable choice of water as the immersion liquid for carrying out these heat of immersion measurements [29], it is still a suitable choice for soaked alumina and for alumina impregnated with some salts. This may be attributed to the fact that these samples were left for 24 h in contact with doubly distilled water, and this appeared to be a sufficient period of time for complete hydration of the oxide (e.g. to form hydrous alumina and/or hydrated aluminium hydroxide [ 22]). Therefore the interactions involved in these heat of immersion measurements will not be stronger than those involved in physical adsorption, i.e. in hydrogen bonding or simple hydration of the surface A13÷ and 0 2 - ions exposed as a result of the high temperature treatments. Figure 5 shows the variation in the integral heats Hi w (cal g-l) of immersion in water and the variation in nitrogen BET surface area as functions of the heat treatment temperature. Below 200 °C, an initial increase in the heats of immersion follows the increase in BET surface area (Fig. 5, curves b and c). On thermal dehydration above 200 °C, the heats of immersion increased abruptly at 300 °C; at higher temperatures the heats of immersion increased more gradually. This increase in the heats of immersion above 200 °C is accompanied by a general decrease in the BET surface area up to 500 °C.
368 12 --"
10
-6 O
8 6
100
~ .00
8o 0.9
6O
Z~n ~
0.8
~oo
2 0 0
n 100
i 200
a 30{)
Temperature ,°C
i t00
0 500
~-
20 0
"
"~ 1 O0
200
C 300
0.7 '.00
500
Temperat ure,°C
Fig. 5. Variations in the integral heats of immersion in water for the parent activated alumina (o, curve a) and the soaked alumina (A, curve b) and variation in the specific nitrogen surface area for the soaked alumina (a, curve c), all as functions of the heat treatment temperature. Fig. 6. Variations in the heats hi w (erg cm -2} of immersion per unit area for the parent activated alumina (©, curve a) and the soaked alumina (A, curve b) and variation in the fraction ~w of the area located in wide pores (0, curve c}, all as functions of the heat treatment temperature. B e l o w 2 0 0 °C, t h e s m o o t h increase in t h e h e a t o f i m m e r s i o n (Fig. 5, c u r v e b) and t h e increase in t h e B E T surface a r e a {Fig. 5, c u r v e c) are b o t h r e l a t e d t o a g r e a t e r loss o f p h y s i s o r b e d w a t e r u p t o 2 0 0 °C. As s h o w n in Fig. 5, curves a and b, t h e h e a t s o f i m m e r s i o n f o r b o t h activated, a l u m i n a a n d s o a k e d a l u m i n a c o i n c i d e f o r t e m p e r a t u r e s up to 3 0 0 °C. On h e a t i n g at higher t e m p e r a t u r e s {above 3 0 0 °C), the s o a k e d a l u m i n a has higher h e a t o f i m m e r s i o n values u p to 500 °C. This m a y be a t t r i b u t e d to an increase in t h e c o n c e n t r a t i o n o f s u r f a c e a l u m i n i u m ions {exposed as a result o f t h e high t e m p e r a t u r e d e h y d r a t i o n ) {Fig. 5, curves a and b) a n d t o a less significant c o n t r i b u t i o n to t h e h e a t o f i m m e r s i o n d u e to t h e c h a n g e in p o r o s i t y c h a r a c t e r i s t i c s o f t h e s a m p l e s S A ( 4 0 0 ) a n d S A ( 5 0 0 ) {Fig. 2, curves e a n d f). T h e heats hi w (erg c m -2) o f i m m e r s i o n were c a l c u l a t e d p e r unit o f n i t r o g e n B E T surface area. T h e values are listed in T a b l e 2, c o l u m n 5, a n d are g r a p h i c a l l y r e p r e s e n t e d as f u n c t i o n s o f t h e h e a t t r e a t m e n t t e m p e r a t u r e in Fig. 6. F r o m t h e s e results, t h e f o l l o w i n g m a y be c o n c l u d e d . (1) In t h e t e m p e r a t u r e r a n g e 20 - 2 0 0 °C, t h e heats o f i m m e r s i o n f o r b o t h a c t i v a t e d a l u m i n a {Fig. 6, curve a) and s o a k e d a l u m i n a {Fig. 6, curve b) are a l m o s t c o n s t a n t ; this is a t t r i b u t e d t o t h e r e m o v a l o f p h y s i s o r b e d water. (2} On h e a t t r e a t m e n t b e l o w 3 0 0 °C, t h e heats o f i m m e r s i o n in w a t e r are higher f o r t h e a c t i v a t e d a l u m i n a . This m a y be a t t r i b u t e d t o t h e p r e s e n c e o f various c e n t r e s o f b o t h acidic a n d basic n a t u r e s [6 - 8] a n d o t h e r c e n t r e s w h i c h have o x i d a t i o n a n d r e d u c t i o n p r o p e r t i e s [ 9 ] ; t h e integral e f f e c t is a d d e d t o t h e high p o l a r i t y o f t h e a c t i v a t e d a l u m i n a surface, w h i c h p r e d e t e r m i n e s its usefulness as a d r y i n g agent. In c o n t r a s t , t h e s o a k e d a l u m i n a a p p e a r s t o b e m o s t l y h y d r o x y l a t e d (cf. Fig. 1, c u r v e b). {3) On t h e r m a l d e h y d r a t i o n ( a n d / o r d e h y d r o x y l a t i o n } a b o v e 3 0 0 °C t h e values o f t h e heats o f i m m e r s i o n f o r t h e s o a k e d a l u m i n a are higher t h a n
369
t h o s e f o r t h e p a r e n t a c t i v a t e d a l u m i n a (Fig. 6, curves a a n d b). A t t h e s e high t e m p e r a t u r e s , t h e m e a s u r a b l e a l u m i n i u m ion c o n c e n t r a t i o n e x p o s e d t o t h e surface results in an increase in t h e heats o f i m m e r s i o n f o r t h e s o a k e d samples compared with those for the parent activated alumina. (4) D e s p i t e t h e less significant c h a n g e in t h e f r a c t i o n (~w o f w i d e p o r e s o v e r t h e entire t e m p e r a t u r e r a n g e studied, m o s t o f t h e p o r e s t r u c t u r e is c o n s i d e r e d to be accessible to w e t t i n g liquid m o l e c u l e s . As p r e v i o u s l y r e p o r t e d [ 10, 3 0 ] , t h e h e a t s o f i m m e r s i o n in w a t e r increase as t h e f r a c t i o n o f t h e area l o c a t e d in wide p o r e s increases. T h u s it has b e e n f o u n d in this investigation t h a t t h e p a r e n t a c t i v a t e d a l u m i n a and t h e s o a k e d a l u m i n a s h o w s o m e d i f f e r e n c e s in w a t e r loss beh a v i o u r a n d in t h e i r specific s u r f a c e areas a n d h a v e w i d e l y d i f f e r e n t h e a t s o f i m m e r s i o n . T h e s e d i f f e r e n c e s are a l m o s t c o n s t a n t a n d small b e l o w 2 0 0 °C {Fig. 6), m e a s u r a b l y higher at 3 0 0 °C a n d fully reversed at higher t e m p e r a tures. This indicates t h a t t h e p a r e n t a c t i v a t e d a l u m i n a is t h e m o r e r e a c t i v e b e l o w 3 0 0 °C, w h e r e a s t h e s o a k e d a l u m i n a s a m p l e s h e a t e d at 4 0 0 a n d 500 °C are highly a c t i v a t e d in t e r m s o f t h e h e a t s hi w (erg c m -2) o f i m m e r s i o n in water. T h e e f f e c t o f i m p r e g n a t i o n w i t h d i f f e r e n t m e t a l chlorides (e.g. CaCl2) o n t h e a c t i v a t e d a l u m i n a s u r f a c e c a n n o t be fully p r e s e n t e d w i t h o u t a s t u d y o f t h e e f f e c t o f b u l k a n d surface h y d r a t i o n f o r t h e p a r e n t a c t i v a t e d a l u m i n a , a n d this e f f e c t has b e e n t h e s u b j e c t o f t h e p r e s e n t investigation.
References 1 W. D. Harkins, The Physical Chemistry o f Surface Films, Reinhold, New York, 1952. 2 D. D. Eley (ed.), Adhesion, Oxford University Press, London, 1961, p. 39. 3 J. J. Chessick, A. C. Zettlemoyer, F. H. Healey and G. J. Young, Can. J. Chem., 33 (1955) 251. 4 J.J. Chessick, J. Phys. Chem., 66 (1962) 762. 5 W. C. Haag and H. Pines, J. Am. Chem. Soc., 82 (1960) 2471. 6 S. Malinowski and S. Sczepanska, J. Catal., 2 (1963) 310. 7 S. Malinowski, S. Sczepanska, A. Bielanski and I. Sloczynski, J. Catal., 4 (1965) 324. 8 G. M. Schwab and J. Karl, Proc. 3rd Int. Congr. on Catalysis, Amsterdam, 1964, North-Holland, Amsterdam, 1965, p. 433. 9 B. D. Flockhart, J. A. N. Scott and R. C. Pink, Trans. Faraday Soc., 62 (1966) 730. 10 A. M. Khalil, Surf. Technol., 18 (1983) 263 - 273. 11 G. Della Gatta, B. Fubini and G. Venturello, Thermochimie, Centre National de la Recherche Scientifique, Paris, 1972, p. 565. 12 G. Della Gatta, B. Fubini and G. Venturello, J. Chem. Phys., 70 (1973) 64. 13 D. L. Kantro, S. Brunauer and L. E. Copeland, in A. Flood (ed.), The Solid-Gas Interface, Dekker, New York, 1967, Chap. 12. 14 R. Sh. Mikhai], N. M. Guindy and S. Hanafi, Egypt J. Chem., Spec. Issue Tourky, (1973) 55 - 63. 15 R. I. Razouk, J. Phys. Chem., 45 (1941) 179. 16 A. M. Khalil, M.Sc. Thesis, Ain Shams University, Cairo, 1970. 17 W. D. Harkins and G. Jura, J. Am. Chem. Soc., 66 (1944) 1362. 18 P. H. Emmett and S. Brunauer, J. Am. Chem. Soc., 59 (1937) 1553. 19 R. N. T. Leaninson, Jr. Josephus and H. P. Ehrlinger, IL State Geol. Surv., Circ., (1969) 44412.
370 20 F. S. Baker, C. Phillips and K. S. W. Sing, Proc. Syrup. on Oxide-Electrolyte Interfaces, Electrochemical Society, Princeton, NJ, 1973, p. 65. 21 J. D. Carruthers, D. A. Payne, K. S. W. Sing and L. J. Stryker, J. Colloid Interface Sci., 36 (1971) 205. 22 K. S. W. Sing, in D. H. Everett (ed.), Colloid Science, Vol. II, Specialist Periodical Reports, Chemical Society., London, 1975, Chap. 1, p. 40. 23 J. H. de Boer, in M. Lafitte (ed.), Thermochimie, Colloq. Int. CNRS 201 (1972) 407. 24 B. C. Lippens, B. G. Linsen and J. H. de Boer, J. Catal., 3 (1964) 32. B. C. Lippens and J. H. de Boer, J. Catal., 4 (1965) 319. 25 G. C. Bye and K. S. W. Sing, Chem. Ind. (London), (1967) 1139. 26 S. Brunauer, in D. H. Everett and R. H. Ottewill (eds.), Surface Area Determination, Butterworths, London, 1970, p. 63. 27 S. Brunauer, R. Sh. Mikhail and E. E. Bodor, J. Colloid Interface Sci., 29 (1969) 485. 28 R. Sh. Mikhail, S. A. Selim and A. Goned, EgyptJ. Chem., 18 (5) {1975) 957. 29 S. J. Gregg and K. S. W. Sing, Adsorption, Surface Area and Porosity, Academic Press, London, 1967, Chap. 7, p. 304. 30 A. M. Khalil, J. Colloid Interface Sci., 66 (3) (1978) 509 - 514.
359
R E L A T E D S UR F A C E AND H E A T OF IMMERSION C H A R A C T E R I Z A T I O N OF SOME ALUMINA SAMPLES II: WATER-SOAKED A C T I V A T E D ALUMINA
A. M. KHALIL* Department of Chemistry, Faculty of Science, Ain Shams University, Abbassia, Cairo (Egypt)
(Received September 10, 1982)
Summary Soaked alumina was prepared by leaving RhSne Poulenc activated alumina in d o u b l y distilled water for 24 h. The activated alumina was then air dried. Both processes were carried out at r o o m temperature. The soaked alumina was thermally d e h y d r a t e d in vacuo at various temperatures in the range 20 - 500 °C. Water losses (in grams o f water per 100 g o f the sample) for all the heat-treated samples were determined; it was shown that the soaked alumina samples exhibit higher water losses than those determined for the parent activated alumina samples. This was considered to be ample evidence for chemisorption a n d / o r strong upt ake of water molecules at the surface and in the pore system o f the bulk alumina. Nitrogen adsorption measurements were carried out at liquid nitrogen t e m p e r a t u r e for all the samples. All the isotherms were o f t y p e II according to the classification of E m m e t t and Brunauer and were characterized by the occurrence o f hysteresis loops. The areas o f the hysteresis loops initially increased up to 200 °C; decreases in the areas of the hysteresis loops were then n o ted up to 400 °C. The increase in area of t he hysteresis loops below 200 °C is attr ib u ted to the increased loss of physisorbed water. The decreased rate o f water loss {water loss f r om the surface a n d / o r structural water loss) corresponds to the observed decrease in area of the hysteresis loops up to 400 °C. Analysis suggested that the surface areas are mostly located in wide pores. The presence of some narrow pores (or micropores) was det ect ed mainly at temperatures above 300 °C. Heat o f immersion measurements were carried o u t using water as the immersion liquid for all the samples investigated. Despite the observed increase in water loss and nitrogen surface area for t he soaked samples, the parent activated and soaked alumina samples exhibit almost the same integral heats o f immersion up to 300 °C. At higher temperatures, the soaked alumina *Present address: Department of Chemistry, Junior College of Teachers, P.O. Box 4341, Riyadh, Saudi Arabia. 0376-4583/83/0000-0000/$03.00
© Elsevier Sequoia/Printed in The Netherlands
360 exhibited higher heats of immersion in water, which is directly related to the high extent of dehydration (or to the dehydroxylation of the surface h y d r o x y l groups) and the exposure of both aluminium ions (A13÷) and oxygen anions ( 0 2 - ) to the surface, which enhance the formation of excess hydrous alumina and/or aluminium hydroxide. On calculation of the heats of immersion in water per unit area, it is found that activated alumina has higher heats of immersion up to 300 °C, whereas the soaked alumina preheated above 300 °C has higher heats of immersion because of the higher concentration of surface h y d r o x y l groups as well as the increased A13+and 0 2 - ion c o n t e n t of these soaked samples.
1. Introduction Measurements of the heats of immersion of solids in liquids permit the investigation of a number of properties of the solid surface. The thermodynamics of the immersion process developed by Harkins [1] have been revised by Eley [2] from the viewpoint of adhesion. Both the integral and differential heats of adsorption of vapours on solids can be determined from the heats of immersion. Furthermore, Chessick and coworkers [3, 4] have shown that the variation in the heat of immersion of a solid in a series of n-butyl derivatives of differing dipole m o m e n t can be used to estimate an average value for the electrostatic field strength at the solid surface. The parent activated alumina contains centres of both acidic and basic natures [5 - 8] as well as some centres with oxidation and reduction properties [9]; this activated alumina was recently presented and discussed by Khalil [10]. In a continuation of the previous investigation [10], activated alumina was water soaked for 24 h at room temperature and air dried for prolonged periods of time. Previous calorimetric results on various outgassed aluminas [11, 12] had indicated the presence of both reversible physisorption and irreversible chemisorption at the surface region. An investigation of the surface properties of the soaked alumina by both nitrogen adsorption and heat of immersion techniques is presented in this paper. The results are directly compared with those obtained for activated alumina [10]; they confirm the rapid chemisorption of water molecules at the surface and/or the strong uptake of water in the pore system. The present results are discussed in relation to those obtained for activated alumina.
2. Experimental details Activated alumina was left in contact with doubly distilled water for 24 h, and the soaked alumina was air dried for a prolonged period of time
361 b e f o r e use. B o t h t h e h y d r a t i o n a n d t h e d r y i n g processes w e r e carried o u t u n d e r identical p h y s i c a l c o n d i t i o n s { r o o m t e m p e r a t u r e , 20 °C). S o a k e d a l u m i n a was t h e r m a l l y d e h y d r a t e d i n v a c u o at t e m p e r a t u r e s o f 20 °C, 100 °C, 2 0 0 °C, 3 0 0 °C, 4 0 0 °C a n d 500 °C a n d t h e resulting s a m p l e s w e r e d e s i g n a t e d as S A ( 2 0 ) , S A ( 1 0 0 ) , S A ( 2 0 0 ) , S A ( 3 0 0 ) , S A ( 4 0 0 ) a n d S A ( 5 0 0 ) r e s p e c t i v e l y ; t h e h e a t i n g t e m p e r a t u r e s are i n d i c a t e d in p a r e n t h e s e s . H e a t i n g was carried o u t using a small electrical t u b u l a r f u r n a c e (200 W) c o u p l e d w i t h a S h i n k o t h e r m o c o n t r o l l e r ( m o d e l DIC-P) w i t h a r a n g e u p t o 6 0 0 °C; this t h e r m o c o n t r o l l e r h a d b e e n originally c a l i b r a t e d w i t h a c h r o m e l alumel thermocouple. N i t r o g e n a d s o r p t i o n m e a s u r e m e n t s w e r e carried o u t using a c o n v e n t i o n a l v o l u m e t r i c a p p a r a t u s f o r gas a d s o r p t i o n [ 1 3 ] . Highly p u r i f i e d n i t r o g e n ( N o r s k H y d r o ) was used as a d s o r b a t e w i t h o u t f u r t h e r p u r i f i c a t i o n . W a t e r losses (in g r a m s o f w a t e r p e r 100 g o f t h e s a m p l e ) w e r e e s t i m a t e d using a M c B a i n - B a k e r t y p e o f silica spring b a l a n c e w h i c h h a d a sensitivity o f 35 c m g-1. T h e values o f t h e w a t e r loss, t h e B r u n a u e r - E m m e t t - T e l l e r ( B E T ) C c o n s t a n t , t h e m o n o l a y e r c a p a c i t y Vm, t h e surface area SBETN2 (m 2 g - l ) a n d t h e t o t a l p o r e v o l u m e Vp (ml g - l ) are all listed in T a b l e 1, c o l u m n s 2, 3, 4, 5 and 6 r e s p e c t i v e l y . Using t h e c a l o r i m e t r i c m e t h o d [ 1 5 ] , t h e integral h e a t s Hi w (cal g - l ) o f i m m e r s i o n in w a t e r w e r e d e t e r m i n e d at 35 + 0 . 0 5 °C; t h e t e m p e r a t u r e o f t h e i m m e r s i o n liquid ( w a t e r ) was initially k e p t c o n s t a n t to w i t h i n +0.002 °C. A m p o u l e s , each c o n t a i n i n g a s a m p l e t h e mass o f w h i c h h a d b e e n d e t e r m i n e d a c c u r a t e l y ( a b o u t 0 . 2 0 g), w e r e c a r e f u l l y e v a c u a t e d f o r 3 h at t h e r e q u i r e d t e m p e r a t u r e , sealed o f f u n d e r v a c u u m a n d t h e n t r a n s f e r r e d t o t h e c a l o r i m eter. F o r e a c h h e a t t r e a t m e n t o f a s a m p l e , w a t e r losses a p p e a r t o be a l m o s t constant within 2 - 3 h except for the samples evacuated at room temperat u r e , and t h e r e s p e c t i v e s a m p l e s w e r e t h e r e f o r e degassed f o r p r o l o n g e d periods of time before the heat of immersion measurements were made. T h e o b s e r v e d c h a n g e in t e m p e r a t u r e p r o d u c e d o n i m m e r s i o n was c o r r e c t e d f o r r a d i a t i o n losses [ 1 5 ] , as well as f o r t h e h e a t o f capillary e n d TABLE 1 Some surface characteristics of nitrogen adsorption on soaked activated alumina Sample
Water loss x 102
BET C constant
1
(g H20 g-I) 2 3
SA(20) SA(100) SA(200) SA(300) SA(400) SA(500)
6.95 9.65 11.34 11.45 11.49 11.68
16 20 19 19 20 21
17 21 20 20 21 22
Vm
SBETN2 Vp (m 2 g-l) (ml g-l)
St a (m 2 g-l)
r ()k)
r-1 ()k-l)
4
5
6
7
8
9
99.01 108.70 114.94 112.99 111.86 108.93
431.3 473.5 500.7 492.2 487.3 475.5
0.661 0.770 0.676 0.653 0.692 0.669
430 472 490 482 498 480
30.7 32.5 27.0 26.5 28.4 28.2
0.0326 0.0308 0.0307 0.0377 0.0352 0.0355
aReference standard [ 14 ]; BET C ~ 20 - 30 was used in V l - t analysis.
362 TABLE 2 Heats of immersion of activated alumina soaked in water
Sample
Water loss
1
(g n 2 0 g - l ) 2
SBET N2 (m: g-l) 3
Hi w (cal g - l ) 4
hi w (erg cm -2) 5
SA(20) SA(100) SA(200) SA(300) SA(400) SA(500 )
6.95 9.65 11.34 11.45 11.49 11.68
431.3 473.5 500.7 492.2 487.3 474.5
3.20 3.36 3.68 6.65 7.75 10.12
31.0 29.7 30.7 56.5 66.5 89.2
×
102
breaking [16]. The calculated Hi values reported in Table 2 represent the average of at least two independent determinations which were reproducible almost to within 3% for the low temperature samples but to a considerably better degree of accuracy for samples dehydrated at high temperatures. Doubly distilled water was used for the heat of immersion measurements.
3. Results and discussion Figure 1 shows the variation in water loss (in grams of water per 100 g of the sample} as a function of the heat treatment temperature for activated alumina (curve a) and soaked alumina {curve b}. Both the parent activated alumina and the water-soaked alumina exhibited a marked continuous increase in water loss with increase in the dehydration temperature up to 500 °C. Activated alumina shows a marked abrupt increase in water loss in the range 200 - 300 °C, thus revealing a change in the nature of the water loss below 200 °C from that above 300 °C {Fig. 1, curve a). However, the soaked alumina shows an early continuous increase in water loss up to 200 °C; the water loss then increases more gradually with further rise in temperature up to 500 °C (Fig. 1, curve b). The soaked samples exhibited higher water losses
12
•
b
•
.~ 10
N 8
~o
0
100
200
300
400
500
Temperature,C
Fig. 1. Variations in the water loss for the parent activated alumina (e, curve a) and the soaked alumina (&, curve b) as functions o f the heat treatment temperature.
363
and this is taken as good evidence for strong irreversible uptake of water molecules on the surface as well as in the pore system of the alumina.
3.1. Specific surface areas from nitrogen adsorption The surface areas SBETN2 (m 2 g-l) were calculated by applying the BET equation to the nitrogen adsorption isotherms in the range P/Po = 0.05 - 0.35 and by adopting the value of 16.2 A: for the cross-sectional area of the nitrogen molecule [17]. All the adsorption isotherms are invariably of type II according to the classification of Emmett and Brunauer [18] and exhibit hysteresis phenomena. The closure points of the hysteresis loops are shifted downwards to low relative pressures when the temperature is increased to 200 °C; at higher temperatures (up to 400 °C) the closure points are shifted to higher relative pressures. The increase in the areas of the hysteresis loops when the temperature is increased to 200 °C, together with the downward shifts of the closure points of the hysteresis loops, is considered to be ample evidence for a greater loss of physisorbed water, not only from the surface but also from the pore system, i.e. from the bulk alumina (Fig. 2, curves a - c). The shifts in the closure points of the hysteresis loops, below 200 °C, to lower P/P0 values indicate that not only may molecular water be lost from mesopores
~,50 400 350 300 250 200 150 '~
m
100
0 50 0 50 0
5O 0
-
-
5O
J
0 50 0 0.1
O.
0.3
0.4
0.5
06
07
0.8
0.9
1.0
Fig. 2. Adsorption-desorption isotherms of nitrogen on soaked alumina samples (o, adsorption; o, desorption): curve a, SA(20); curve b, SA(100); curve c, SA(200); curve d, SA(300); curve e, SA(400); curve f, SA(500).
364 (or wide pores) b u t also some strongly b o u n d w a t e r m a y be lost f r o m m i c r o p o r e s (or n a r r o w pores) (Fig. 1, curve b). On d e h y d r a t i o n o f t h e alumina in t h e t e m p e r a t u r e range 200 - 400 °C t h e decrease in t h e areas o f t h e hysteresis loops, t o g e t h e r with the shifts to higher relative pressures o f t h e i r closure points, in the a d s o r p t i o n isotherms s h o w n in Fig. 2, curves d and e, is responsible for the decreased rate o f w a t e r loss n o t e d at these high t e m p e r a t u r e s (Fig. 1, curve b). As s h o w n f o r sample S A ( 5 0 0 ) (Fig. 2, curve f), the increase in the area o f the hysteresis l o o p was a c c o m p a n i e d b y a d o w n w a r d shift in the closure p o i n t o f t h e hysteresis l o o p t o P / P o = 0.25, which indicates the loss o f some c o n s t i t u t i o n a l w a t e r f r o m the alumina n e t w o r k . T h e high t e m p e r a t u r e t r e a t m e n t m a y be responsible f o r a change in the crystalline order, and this is a c c o m p a n i e d b y the c r e a t i o n o f some m i c r o p o r e s a n d / o r voids in the m o r p h o l o g y o f t h e h e a t - t r e a t e d alumina. T h e average p o r e radius was calculated using the relation
r(A)=
2Vp SBET
X 10 4
N~-
w h e r e Vp (ml g-l) is t h e t o t a l p o r e v o l u m e and SBETN2 (m 2 g-l) is the nitrogen B E T surface area; values o f r (A) are listed in Table 1, c o l u m n 8. Figure 3 shows t h e variation in B E T surface area and the reciprocal r -1 (A-1) o f t h e average p o r e radius as f u n c t i o n s o f the p r e t r e a t m e n t temp e r a t u r e . T h e h a r m o n i c change revealed in the t w o p a r a m e t e r s SBETs'~ (m 2 g-l) and r -1 (A-1) indicates t h a t the surface area should increase w h e n the average p o r e radius decreases and vice versa. On the basis o f an ideal m o d e l o f s m o o t h , u n i f o r m and n o n - p o r o u s spheroidal particles, L e a n i n s o n e t al. [19] c a m e to the c o n c l u s i o n t h a t t h e surface area should be increased b y a decrease in t h e particle size and vice versa.
~ 0 . 0 5
m 500 ~E
~4oo
0.04
300 i
100
_
b
i
i
200
300
400
,.
i o.o 500
o Ternperot ure, C
Fig. 3. Variations in the nitrogen surface a r e a SBET N2 ( m 2 g-l) (©) and reciprocal (A-1) of the average pore radius (e) as functions of the heat treatment temperature.
r- 1
T h e p a r e n t activated alumina does n o t seem to possess a similar harm o n i c relation, which m i g h t be c o n s i d e r e d an i n d i c a t i o n o f s o m e degree o f t h e r m a l stability f o r activated alumina. Thus, t h e original desiccant n a t u r e o f t h e activated a l u m i n a m a y be r e s t o r e d m e r e l y b y degassing f o r 2 - 3 h at a b o u t 200 - 250 °C. Soaking o f t h e activated alumina with w a t e r m a y be
365
responsible for the increased h y d r o x y l a t i o n [20, 21] of the surface, together with the creation of hydrous alumina and/or aluminium hydroxide [22]. The initial increase in BET surface area associated with the conversion of hydrous alumina into pseudoboehmite [23] is believed to result from the formation of micropores within the primary particles, whereas a slower development of mesopores appears to be caused by a process of aggregationcementation. At a later stage in the dehydration (above 200 °C) bayerite is formed by a recrystallization process, and above this temperature a gradual decrease in surface area was observed up to 500 °C (Fig. 5, curve c).
3.2. Pore structure analysis The porous character of the thermally dehydrated soaked alumina samples was qualitatively detected by the Vl-t m e t h o d [24] and will be discussed briefly. As shown in Fig. 4, the Vl-t plots show initial straight-line portions representing the magnitude of St (m 2 g-l). Then at t ~ 3 - 4 A a small upward deviation develops and continues up to t ~ 6 A. Thereafter, increasing slopes were noted up to t ~ 8 - 9 A. The decreased intergranular capillary condensation continued up to pressures n o t far from the saturation pressure. The finite upward deviations noted above t ~ 3 - 4 A are mainly attributed to partial compensation effects [25] that occur in the adsorption of nitrogen both in micropores (and/or narrow pores) and in wide pores (and/or mesopores).
2
4
6
8
10
12
14
16
18
20
22
2,;
26
28
t(~)
Fig. 4. V1- t p l o t s for various s o a k e d a l u m i n a s a m p l e s preheated at different curves a - f d e n o t e t h e s a m e s a m p l e s as t h o s e in Fig. 2.
temperatures:
366 T h e increase in slope n o t e d at t ~> 6 A is d u e to m u l t i l a y e r f o r m a t i o n in m e s o p o r e s , w h e r e a s t h e d e c r e a s e d slopes n o t e d at t/> 8 - 9 A are related to capillary c o n d e n s a t i o n e f f e c t s at t h e s e high relative pressures. T h e St values listed in T a b l e 1, c o l u m n 7, s h o w fairly g o o d a g r e e m e n t w i t h t h e c o r r e s p o n d i n g B E T surface area values SBETN2 listed in T a b l e 1, c o l u m n 5. This indicates a p r o p e r c h o i c e o f t h e r e f e r e n c e d a t a [14] f o r each analysis. T h e B E T C c o n s t a n t f o r t h e s a m p l e s u n d e r investigation a n d t h a t f o r t h e r e f e r e n c e s t a n d a r d [ 1 4 ] ( B E T C ~ 20 - 30) s h o u l d be c o m p a r a b l e , and t h e best c o n d i t i o n is to h a v e a l m o s t identical B E T C values. This indicates t h a t t h e heats o f a d s o r p t i o n f o r b o t h t h e s a m p l e s a n d t h e r e f e r e n c e s t a n d a r d [14] are identical. It was p r e v i o u s l y s t a t e d b y B r u n a u e r [ 2 6 ] t h a t t h e m a g n i t u d e o f t h e B E T C c o n s t a n t is c o n s i d e r e d t o be an a d e q u a t e m e a s u r e o f t h e h e a t o f a d s o r p t i o n f o r analysis. T h e surface areas l o c a t e d in wide p o r e s (or m e s o p o r e s ) w e r e a n a l y s e d b y t h e c o r r e c t e d model-less m e t h o d [ 2 7 ] using a c o m p u t e r p r o g r a m [28] w r i t t e n in F O R T R A N IV f o r t h e IBM 1 1 3 0 c o m p u t e r . T h e analysis is based o n d e s o r p t i o n b r a n c h e s o f t h e i s o t h e r m s and was c o n t i n u e d d o w n w a r d s to a relative pressure P/Po o f 0.25. In t h e c a l c u l a t i o n o f t h e c u m u l a t i v e values Scum a n d Vcum and to avoid t h e p r o b l e m s a s s o c i a t e d w i t h c o m p e n s a t i o n e f f e c t s [ 2 5 ] , t h e analysis was s t o p p e d at relative pressures P/Po c o r r e s p o n d i n g t o t h e closure p o i n t s o f t h e h y s t e r e s i s l o o p s in t h e a d s o r p t i o n i s o t h e r m s . T h e a d o p t i o n o f t h e cylindrical p o r e m o d e l ( i n d i c a t e d b y the s u p e r s c r i p t cp) led to c u m u l a t i v e values Scum Cp a n d Vcumcn larger t h a n t h e e x p e r i m e n t a l l y e s t i m a t e d a d s o r p t i o n values SBETN2 a n d Yp t (ml g 1). T h e r e f o r e t h e parallel-plate m o d e l ( i n d i c a t e d b y t h e s u p e r s c r i p t p p ) was a s s u m e d since smaller c u m u l a t i v e values Scum ~v a n d Vcu~ pp w e r e o b t a i n e d ; these c u m u l a t i v e values are listed in T a b l e 3, c o l u m n s 6 a n d 7. T h e f r a c t i o n ~w o f t h e area l o c a t e d in w i d e p o r e s was c a l c u l a t e d b y dividing t h e c u m u l a t i v e s u r f a c e areas ( l o c a t e d in wide p o r e s a n d / o r m e s o p o r e s ) b y t h e n i t r o g e n B E T s u r f a c e areas; these values are listed in T a b l e 3, c o l u m n 8: TABLE 3 Cumulative values located in wide pores for the water-soaked alumina samples BET C
SBETN2
Vpt
St
2
(ml g-t) 4
(m: g 1) 5
(m: g-l)
1
(m 2 g-l) 3
SA(20) SA(100) SA(200) SA(300) SA(400) SA(500)
16-17 20-21 19 -20 19-20 20-21 21 -22
431.3 473.5 500.7 492.7 487.2 475.5
0.661 0.771 0.676 0.653 0.692 0.669
430 472 490 482 498 480
Sample
constant
Scumpp
Vcttmpp
~w
6
(ml g-l) 7
8
316.3 389.9 414.7 418.1 413.0 393.8
0.562 0.676 0.659 0.587 0.633 0.588
0.733 0.823 0.828 0.850 0.848 0.828
367 OLw = S c u m P P / S B E T N2
As shown in Table 3, column 8, the aw values for the water-soaked alumina samples show little variation and tend to be greater over the temperature range studied (20 - 500 °C) compared with the corresponding values for the activated alumina samples. This might be considered ample evidence for the effect of soaking in water on the porosity characteristics of the alumina samples. Therefore the soaked samples exhibit the characteristic features shown in Fig. 3, whereas the parent activated alumina does not seem to show the same behaviour.
3.3. Heats o f immersion in water In Part I of this work [10] in which activated alumina was used as a starting material and water, methanol and carbon tetrachloride were used as wetting liquids, the heats of immersion were in the following order: water methanol > carbon tetrachloride. The system, activated alumina plus water, was fully discussed with respect to the detected porosity and physicotextural changes which accompanied the thermal dehydration process. The porous character includes the effects of the location for adsorption on the wetting liquid molecules, whereas the physical structure of the sample reflects the loss of water either from the pore system or from the surface and the consequent exposure of excess aluminium ions (A13+) and oxygen anions (02-) to the surface. The effects of these parameters were considerably reduced for immersion in methanol and carbon tetrachloride. Therefore water was used as the immersion liquid in this work, despite the fact t h a t water is irreversibly chemisorbed at the activated alumina surface [20, 21]. Despite the questionable choice of water as the immersion liquid for carrying out these heat of immersion measurements [29], it is still a suitable choice for soaked alumina and for alumina impregnated with some salts. This may be attributed to the fact that these samples were left for 24 h in contact with doubly distilled water, and this appeared to be a sufficient period of time for complete hydration of the oxide (e.g. to form hydrous alumina and/or hydrated aluminium hydroxide [ 22]). Therefore the interactions involved in these heat of immersion measurements will not be stronger than those involved in physical adsorption, i.e. in hydrogen bonding or simple hydration of the surface A13÷ and 0 2 - ions exposed as a result of the high temperature treatments. Figure 5 shows the variation in the integral heats Hi w (cal g-l) of immersion in water and the variation in nitrogen BET surface area as functions of the heat treatment temperature. Below 200 °C, an initial increase in the heats of immersion follows the increase in BET surface area (Fig. 5, curves b and c). On thermal dehydration above 200 °C, the heats of immersion increased abruptly at 300 °C; at higher temperatures the heats of immersion increased more gradually. This increase in the heats of immersion above 200 °C is accompanied by a general decrease in the BET surface area up to 500 °C.
368 12 --"
10
-6 O
8 6
100
~ .00
8o 0.9
6O
Z~n ~
0.8
~oo
2 0 0
n 100
i 200
a 30{)
Temperature ,°C
i t00
0 500
~-
20 0
"
"~ 1 O0
200
C 300
0.7 '.00
500
Temperat ure,°C
Fig. 5. Variations in the integral heats of immersion in water for the parent activated alumina (o, curve a) and the soaked alumina (A, curve b) and variation in the specific nitrogen surface area for the soaked alumina (a, curve c), all as functions of the heat treatment temperature. Fig. 6. Variations in the heats hi w (erg cm -2} of immersion per unit area for the parent activated alumina (©, curve a) and the soaked alumina (A, curve b) and variation in the fraction ~w of the area located in wide pores (0, curve c}, all as functions of the heat treatment temperature. B e l o w 2 0 0 °C, t h e s m o o t h increase in t h e h e a t o f i m m e r s i o n (Fig. 5, c u r v e b) and t h e increase in t h e B E T surface a r e a {Fig. 5, c u r v e c) are b o t h r e l a t e d t o a g r e a t e r loss o f p h y s i s o r b e d w a t e r u p t o 2 0 0 °C. As s h o w n in Fig. 5, curves a and b, t h e h e a t s o f i m m e r s i o n f o r b o t h activated, a l u m i n a a n d s o a k e d a l u m i n a c o i n c i d e f o r t e m p e r a t u r e s up to 3 0 0 °C. On h e a t i n g at higher t e m p e r a t u r e s {above 3 0 0 °C), the s o a k e d a l u m i n a has higher h e a t o f i m m e r s i o n values u p to 500 °C. This m a y be a t t r i b u t e d to an increase in t h e c o n c e n t r a t i o n o f s u r f a c e a l u m i n i u m ions {exposed as a result o f t h e high t e m p e r a t u r e d e h y d r a t i o n ) {Fig. 5, curves a and b) a n d t o a less significant c o n t r i b u t i o n to t h e h e a t o f i m m e r s i o n d u e to t h e c h a n g e in p o r o s i t y c h a r a c t e r i s t i c s o f t h e s a m p l e s S A ( 4 0 0 ) a n d S A ( 5 0 0 ) {Fig. 2, curves e a n d f). T h e heats hi w (erg c m -2) o f i m m e r s i o n were c a l c u l a t e d p e r unit o f n i t r o g e n B E T surface area. T h e values are listed in T a b l e 2, c o l u m n 5, a n d are g r a p h i c a l l y r e p r e s e n t e d as f u n c t i o n s o f t h e h e a t t r e a t m e n t t e m p e r a t u r e in Fig. 6. F r o m t h e s e results, t h e f o l l o w i n g m a y be c o n c l u d e d . (1) In t h e t e m p e r a t u r e r a n g e 20 - 2 0 0 °C, t h e heats o f i m m e r s i o n f o r b o t h a c t i v a t e d a l u m i n a {Fig. 6, curve a) and s o a k e d a l u m i n a {Fig. 6, curve b) are a l m o s t c o n s t a n t ; this is a t t r i b u t e d t o t h e r e m o v a l o f p h y s i s o r b e d water. (2} On h e a t t r e a t m e n t b e l o w 3 0 0 °C, t h e heats o f i m m e r s i o n in w a t e r are higher f o r t h e a c t i v a t e d a l u m i n a . This m a y be a t t r i b u t e d t o t h e p r e s e n c e o f various c e n t r e s o f b o t h acidic a n d basic n a t u r e s [6 - 8] a n d o t h e r c e n t r e s w h i c h have o x i d a t i o n a n d r e d u c t i o n p r o p e r t i e s [ 9 ] ; t h e integral e f f e c t is a d d e d t o t h e high p o l a r i t y o f t h e a c t i v a t e d a l u m i n a surface, w h i c h p r e d e t e r m i n e s its usefulness as a d r y i n g agent. In c o n t r a s t , t h e s o a k e d a l u m i n a a p p e a r s t o b e m o s t l y h y d r o x y l a t e d (cf. Fig. 1, c u r v e b). {3) On t h e r m a l d e h y d r a t i o n ( a n d / o r d e h y d r o x y l a t i o n } a b o v e 3 0 0 °C t h e values o f t h e heats o f i m m e r s i o n f o r t h e s o a k e d a l u m i n a are higher t h a n
369
t h o s e f o r t h e p a r e n t a c t i v a t e d a l u m i n a (Fig. 6, curves a a n d b). A t t h e s e high t e m p e r a t u r e s , t h e m e a s u r a b l e a l u m i n i u m ion c o n c e n t r a t i o n e x p o s e d t o t h e surface results in an increase in t h e heats o f i m m e r s i o n f o r t h e s o a k e d samples compared with those for the parent activated alumina. (4) D e s p i t e t h e less significant c h a n g e in t h e f r a c t i o n (~w o f w i d e p o r e s o v e r t h e entire t e m p e r a t u r e r a n g e studied, m o s t o f t h e p o r e s t r u c t u r e is c o n s i d e r e d to be accessible to w e t t i n g liquid m o l e c u l e s . As p r e v i o u s l y r e p o r t e d [ 10, 3 0 ] , t h e h e a t s o f i m m e r s i o n in w a t e r increase as t h e f r a c t i o n o f t h e area l o c a t e d in wide p o r e s increases. T h u s it has b e e n f o u n d in this investigation t h a t t h e p a r e n t a c t i v a t e d a l u m i n a and t h e s o a k e d a l u m i n a s h o w s o m e d i f f e r e n c e s in w a t e r loss beh a v i o u r a n d in t h e i r specific s u r f a c e areas a n d h a v e w i d e l y d i f f e r e n t h e a t s o f i m m e r s i o n . T h e s e d i f f e r e n c e s are a l m o s t c o n s t a n t a n d small b e l o w 2 0 0 °C {Fig. 6), m e a s u r a b l y higher at 3 0 0 °C a n d fully reversed at higher t e m p e r a tures. This indicates t h a t t h e p a r e n t a c t i v a t e d a l u m i n a is t h e m o r e r e a c t i v e b e l o w 3 0 0 °C, w h e r e a s t h e s o a k e d a l u m i n a s a m p l e s h e a t e d at 4 0 0 a n d 500 °C are highly a c t i v a t e d in t e r m s o f t h e h e a t s hi w (erg c m -2) o f i m m e r s i o n in water. T h e e f f e c t o f i m p r e g n a t i o n w i t h d i f f e r e n t m e t a l chlorides (e.g. CaCl2) o n t h e a c t i v a t e d a l u m i n a s u r f a c e c a n n o t be fully p r e s e n t e d w i t h o u t a s t u d y o f t h e e f f e c t o f b u l k a n d surface h y d r a t i o n f o r t h e p a r e n t a c t i v a t e d a l u m i n a , a n d this e f f e c t has b e e n t h e s u b j e c t o f t h e p r e s e n t investigation.
References 1 W. D. Harkins, The Physical Chemistry o f Surface Films, Reinhold, New York, 1952. 2 D. D. Eley (ed.), Adhesion, Oxford University Press, London, 1961, p. 39. 3 J. J. Chessick, A. C. Zettlemoyer, F. H. Healey and G. J. Young, Can. J. Chem., 33 (1955) 251. 4 J.J. Chessick, J. Phys. Chem., 66 (1962) 762. 5 W. C. Haag and H. Pines, J. Am. Chem. Soc., 82 (1960) 2471. 6 S. Malinowski and S. Sczepanska, J. Catal., 2 (1963) 310. 7 S. Malinowski, S. Sczepanska, A. Bielanski and I. Sloczynski, J. Catal., 4 (1965) 324. 8 G. M. Schwab and J. Karl, Proc. 3rd Int. Congr. on Catalysis, Amsterdam, 1964, North-Holland, Amsterdam, 1965, p. 433. 9 B. D. Flockhart, J. A. N. Scott and R. C. Pink, Trans. Faraday Soc., 62 (1966) 730. 10 A. M. Khalil, Surf. Technol., 18 (1983) 263 - 273. 11 G. Della Gatta, B. Fubini and G. Venturello, Thermochimie, Centre National de la Recherche Scientifique, Paris, 1972, p. 565. 12 G. Della Gatta, B. Fubini and G. Venturello, J. Chem. Phys., 70 (1973) 64. 13 D. L. Kantro, S. Brunauer and L. E. Copeland, in A. Flood (ed.), The Solid-Gas Interface, Dekker, New York, 1967, Chap. 12. 14 R. Sh. Mikhai], N. M. Guindy and S. Hanafi, Egypt J. Chem., Spec. Issue Tourky, (1973) 55 - 63. 15 R. I. Razouk, J. Phys. Chem., 45 (1941) 179. 16 A. M. Khalil, M.Sc. Thesis, Ain Shams University, Cairo, 1970. 17 W. D. Harkins and G. Jura, J. Am. Chem. Soc., 66 (1944) 1362. 18 P. H. Emmett and S. Brunauer, J. Am. Chem. Soc., 59 (1937) 1553. 19 R. N. T. Leaninson, Jr. Josephus and H. P. Ehrlinger, IL State Geol. Surv., Circ., (1969) 44412.
370 20 F. S. Baker, C. Phillips and K. S. W. Sing, Proc. Syrup. on Oxide-Electrolyte Interfaces, Electrochemical Society, Princeton, NJ, 1973, p. 65. 21 J. D. Carruthers, D. A. Payne, K. S. W. Sing and L. J. Stryker, J. Colloid Interface Sci., 36 (1971) 205. 22 K. S. W. Sing, in D. H. Everett (ed.), Colloid Science, Vol. II, Specialist Periodical Reports, Chemical Society., London, 1975, Chap. 1, p. 40. 23 J. H. de Boer, in M. Lafitte (ed.), Thermochimie, Colloq. Int. CNRS 201 (1972) 407. 24 B. C. Lippens, B. G. Linsen and J. H. de Boer, J. Catal., 3 (1964) 32. B. C. Lippens and J. H. de Boer, J. Catal., 4 (1965) 319. 25 G. C. Bye and K. S. W. Sing, Chem. Ind. (London), (1967) 1139. 26 S. Brunauer, in D. H. Everett and R. H. Ottewill (eds.), Surface Area Determination, Butterworths, London, 1970, p. 63. 27 S. Brunauer, R. Sh. Mikhail and E. E. Bodor, J. Colloid Interface Sci., 29 (1969) 485. 28 R. Sh. Mikhail, S. A. Selim and A. Goned, EgyptJ. Chem., 18 (5) {1975) 957. 29 S. J. Gregg and K. S. W. Sing, Adsorption, Surface Area and Porosity, Academic Press, London, 1967, Chap. 7, p. 304. 30 A. M. Khalil, J. Colloid Interface Sci., 66 (3) (1978) 509 - 514.