42 High Temperature Properties of Lanthanum Y Zeolites
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D. BALLIVET, P. PICHAT, and D. BARTHOMEUF Institut de Recherches sur la Catalyse 39, Boulevard du 11 Novembre 1918, 69100, Villeurbanne, France and Université Claude Bernard, Lyon I, France Lanthanum Y zeolites containing a few sodium ions were studied to specify the influence of La ions and pretreatment temperature on the catalytic and acidic properties. When the La ion content is increased from 3.7 to 13.2 per unit cell, only slight changes occur in isooctane cracking as well as in the amount of pyridinium ions chemisorbed (IR study). Compared with an H Y , the structural stability of the La zeolites is enhanced even in the case of a low exchange level (3.7 La /unit cell). The catalytic activity of the La zeolites is not affected by a 9 0 0 ° - 9 2 0 ° C calcination followed by rehydration whereas H Y activity begins to decrease after the same pretreatment at only 700°C. A l l high-temperature-pretreated catalysts which are active have strong Brönsted and Lewis acid sites. 3+
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The excellent catalytic activity of lanthanum exchanged faujasite zeolites in reactions involving carbonium ions has been reported previously (1—10). Studies deal with isomerization (o-xylene (1), l-methyl-2-ethylbenzene (2)), alkylation (ethylene-benzene (3), propylene-benzene (4), propylene-toluene (6)), and cracking reactions (η-butane (δ), n-hexane, η-heptane, ethylbenzene (6), cumene (7, 8, 10)). The catalytic activity of LaY zeolites is equivalent to that of H Y zeolites (δ, 7). The stability of activity for LaY was studied after thermal treatment up to 750°C. However, discrepancies arise in the determination of the optimal tempera tures of pretreatment. For the same kind of reaction (alkylation), the activity increases (4), remains constant (δ), or decreases (3) with increasing temperatures. These results may be attributed to experimental condi tions (δ) and to differences in the nature of the active sites involved. Other factors, such as the introduction of cations (11) and rehydration treatments (δ), may influence the catalytic activity. Water vapor effects are easily 469 In Molecular Sieves; Meier, W., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.
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s h o w n b y v a r i a t i o n s i n t h e intensities of t h e O H I R bands. B a n d s a r o u n d 3640 a n d 3520 c m a l m o s t disappear after h e a t i n g t h e samples at ca. 700° C (1, 6, 12, 13) b u t are restored after r e h y d r a t i o n a t r o o m t e m p e r a t u r e (5, 13). T h e i n f r a r e d s p e c t r u m of a d s o r b e d p y r i d i n e shows t h a t Brônsted a n d L e w i s a c i d i t y are b o t h present i n L a Y c a t a l y s t s p r e t r e a t e d a b o v e ca. 450° C (1, 14)> T h e r a t i o of t h e t w o t y p e s of a c i d i c sites depends o n p r e - 1
t r e a t m e n t t e m p e r a t u r e (1), a n d t h e n u m b e r of a c i d sites depends o n t h e extent of exchange a n d p r e t r e a t m e n t t e m p e r a t u r e (16). T h e L a Y zeolite a c i d i t y is stronger t h a n t h a t of H Y o r C a H Y zeolites (16). T h e t h e r m a l s t a b i l i t y of N H Y zeolite i n w h i c h a m m o n i u m ions h a v e been exchanged a t v a r i o u s levels w i t h L a ions was s t u d i e d . T h e c a t a l y t i c a c t i v i t y of these L a zeolites i n isooctane c r a c k i n g was m e a s u r e d as a f u n c t i o n of p r e t r e a t m e n t t e m p e r a t u r e , a n d a n I R s t u d y of t h e c h e m i s o r p t i o n of p y r i d i n e w a s u s e d t o d e t e r m i n e t h e n u m b e r s of Brônsted a n d L e w i s sites. T h e s t r u c t u r a l d a m a g e r e s u l t i n g f r o m h i g h t e m p e r a t u r e c a l c i n a t i o n was e x a m i n e d q u a l i t a t i v e l y . 4
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Experimental Materials. A n a m m o n i u m Y - t y p e zeolite w a s p r e p a r e d as p r e v i o u s l y described (16) f r o m a c o m m e r c i a l N a Y faujàsite t y p e ( U n i o n C a r b i d e , S i / A l = 2.4). T h e a m o u n t of exchange i s 8 5 % . F r o m t h i s s t a r t i n g m a t e r i a l ( N H N a Y ) , c o n v e n t i o n a l exchange w i t h L a C l solutions p r o v i d e d c a t a l y s t s w i t h different l a n t h a n u m i o n contents. L a ions were also introduce/d i n a s l i g h t l y d e a l u m i n a t e d m a t e r i a l p r e p a r e d a c c o r d i n g t o B e a u m o n t (17). H y d r o g e n forms of t h e a m m o n i u m zeolites were o b t a i n e d b y h e a t i n g at 3 8 0 ° C f o r 15 h o u r s i n a s t r e a m of d r y a i r . F u r t h e r h e a t i n g f r o m 550° u p t o 1000°C u n d e r t h e same c o n d i t i o n s p r o v i d e d samples f o r s t u d y i n g t h e r m a l s t a b i l i t y . S u c h t r e a t m e n t a v o i d e d t h e f o r m a t i o n of " u l t r a s t a b l e " zeolite. T h e c h e m i c a l compositions of t h e samples are l i s t e d i n T a b l e I . 4
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Table I.
Catalysts Na-8.7 La-1 La-3.7 La-7 La-13.2 D.Na-5.4 D.La-4 c
a 6 c
c
Composition of Samples
Na
La
(Na + La)
8.7 6.6 5.6 7 1 5.4 4.0
0 1 3.7 7 13.2 0 4.0
8.7 9.6 16.7 28 40.6 5.4 16
CO
(°C)
