FTIR Study of the BrÃ¶nsted Acid Sites in Dealuminated HY Zeolites

Chapter

7

FT-IR Study of the Brönsted Acid Sites in Dealuminated H Y Zeolites Using Specific Probe Molecules 1

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A. Janin , J . C. Lavalley , A. Macedo , and F. Raatz

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Downloaded by UNIV LAVAL on October 9, 2014 | http://pubs.acs.org Publication Date: May 27, 1988 | doi: 10.1021/bk-1988-0368.ch007

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Laboratoire de Spectrochimie, Groupe Catalyse et Spectrochimie, Unité Associée No. 414, I.S.M.Ra, Universitéde Caen, 14032 Caen, Cedex, France Institut Français du Pétrole, Boîte Postale 311, 92506 Rueil-Malmaison, Cedex, France 2

The Brönsted acid sites of HY zeolites dealuminated either by conventional treatment (steaming + acid leaching) or isomorphous substitution (fluorosilicate) have been characterized at each step of the preparation procedures through IR spectroscopy of probe molecules with various basic strengths (pyridine, CH, H S). In contrast to pyridine, which cannot easily discriminate between OH groups with similar acid strength, CH and H S lead to a much more specific interaction. Whatever the dealumination procedures, the structural high-frequency OH is l i k e l y to be the most acidic and i t s acid strength smoothly increases with the framework dealumination l e v e l . Some of the extra-framework Al-OH groups present in conventional HY treated at low temperature indeed possess a pronounced acidic character since they can interact with CH or H S. Finally two components at 3738 and 3743 cm appear to contribute to the Si-OH at 3740 cm ; they correspond respectively to SiOH groups attached to the framework and to an extra-framework silicon-containing phase. 2

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I t i s w e l l known t h a t t h e number, t y p e a n d s t r e n g t h o f the Bronsted a c i d s i t e s which a r e p r e s e n t i n m o d i f i e d H zeolites do n o t o n l y depend on t h e S i / A l ratio ofthe framework ( 1 , 2 ) . The n a t u r e a n d t h e amount o f n o n framework s p e c i e s s u c h a s r e s i d u a l a l k a l i n e c a t i o n s , I V

Correspondence should be addressed to this author. 0097-6156/88/0368-0117$06.00/0 © 1988 American Chemical Society

In Perspectives in Molecular Sieve Science; Flank, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.


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c a t i o n i c o r n e u t r a l A l s p e c i e s a l s o p l a y an important r o l e on t h e a c i d i t y o f t h e s o l i d s ( 3 , 4 ) . To c h a r a c t e r i z e t h e Bronsted acidity of modified z e o l i t e s two methods c a n m a i n l y b e u s e d : IR s p e c t r o s c o p y (4-7) a n d MAS-NMR (8, 9) w i t h o r w i t h o u t t h e u s e o f p r o b e molecules. Concerning more specifically modified HY z e o l i t e s , t h e s e two t e c h n i q u e s h a v e l e d t o a r a t h e r c l e a r d e s c r i p t i o n o f t h e h y d r o x y l s i n t e r m s o f framework a n d non-framework species ( 1 0 ) . Moreover very useful i n f o r m a t i o n s on t h e a c i d s t r e n g t h o f t h e h y d r o x y l s h a v e been o b t a i n e d through p y r i d i n e a d s o r p t i o n f o l l o w e d e i t h e r by IR (5) o r MAS NMR (8) . P y r i d i n e c a n d i s c r i m i n a t e between OH g r o u p s w i t h v e r y d i f f e r e n t a c i d s t r e n g t h s s u c h as f o r example framework A1-0H a n d S i - O H groups o f d e f e c t s ( 1 1 ) . However, s i n c e i t i s t o o b a s i c , OH g r o u p s w i t h a s l i g h t l y d i f f e r e n t a c i d s t r e n g t h cannot be e a s i l y d i s t i n g u i s h e d ( 1 2 ) . I n t h e c o u r s e o f a more g e n e r a l s t u d y devoted t o the a c i d i t y of modified zeolites (4, 12), a d s o r p t i o n o f b a s i c probe molecules w i t h v a r i o u s b a s i c strength (alcohols, ethers, sulfides, thiols, hydrocarbons...) f o l l o w e d b y FT-IR s p e c t r o s c o p y h a s b e e n i n v e s t i g a t e d . O f t h e s e p r o b e s , H^S a n d C H (adsorbed a t a b o u t 210 K t o a v o i d p o l y m e r i z a t i o n ) a p p e a r e d t o b e more a p p r o p r i a t e (12) . I n t h i s work, u s i n g H S a n d C H a s p r o b e s , we a i m t o c h a r a c t e r i z e t h e s t r o n g e s t B r o n s t e d a c i d s i t e s o f HY solids, dealuminated either by c o n v e n t i o n a l treatment (steaming followed by a c i d leaching) o r isomorphous substitution (by f l u o r o s i l i c a t e ) a n d t o compare t h e r e s u l t s with those obtained with p y r i d i n e . 2

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Experimental S t a r t i n g f r o m a l o w s o d i u m (% Na < 0.17 wt) NH Y ( S i / A l = 2.5) zeolite, dealuminated solids have been prepared either by i ) hydrothermal treatment (HT) a t v a r i o u s temperatures (773 t o 112 3 K) f o l l o w e d b y a l e a c h i n g a t 353 K i n 1.5 N o r 3 N HC1 s o l u t i o n (HTA) , o r b y i i ) isomorphous d e a l u m i n a t i o n (ID) i n ( N H ) S i F solutions b u f f e r e d w i t h CH COONH a t 348 K ( 1 3 ) . I n f r a r e d s p e c t r a have b e e n r e c o r d e d on a NICOLET MX1 Instrument (Caen) o r on a DIGILAB FTS-15E one ( I F P ) . S e l f s u p p o r t i n g p r e s s e d d i s c s ( c a . 5 mg cm ) h a v e b e e n a c t i v a t e d b y h e a t i n g u n d e r vacuum a t 723 K f o r 12 h o u r s . P y r i d i n e ( 1 0 Pa a t e q u i l i b r i u m ) was i n t r o d u c e d a t room temperature, then immediately evacuated a t t h e same temperature a n d a t 423 K t o e l i m i n a t e physisorbed species. A n a l y s i s o f t h e spectra then obtained allows f o r t h e d e t e r m i n a t i o n o f OH b a n d s i n s e n s i t i v e t o p y r i d i n e . Subtraction of spectra after pyridine desorption from those obtained before p y r i d i n e adsorption evidences w e l l b a n d s due t o a c i d i c OH g r o u p s . I n t h e c a s e o f H^S a n d C H , a p a r t i a l p r e s s u r e o f 3 10 Pa h a s b e e n i n t r o d u c e d o n t h e a c t i v a t e d s o l i d s i n t h e 4

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7. JANIN ET AL.

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Study of the Bronsted Acid Sites

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IR c e l l a t room t e m p e r a t u r e ( H S ) o r i n t h e c e l l frozen at 2 1 0 K by a methanol c i r c u l a t i o n ( C H ) . The s p e c i e s f o r m e d were r e v e r s i b l e . T h e i r s p e c t r a were r e c o r d e d u n d e r such c o n d i t i o n s . Those s p e c t r a r e p o r t e d i n t h e f i g u r e s were o b t a i n e d a f t e r s u b t r a c t i o n o f t h e g a s p h a s e s p e c t r a . A l l t h e s p e c t r a have b e e n n o r m a l i z e d t o t h e same amount o f s o l i d ( 1 0 mg). 2

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Results The evaluation a t each step o f t h e dealumination procedures, o f the physicochemical characteristics of the s o l i d s t o g e t h e r w i t h t h e i r a c i d i c p r o p e r t i e s f o l l o w e d by IR s p e c t r a o f t h e OH, TPD o f N H a n d m i c r o c a l o r i m e t r y o f adsorbed NH , h a s a l r e a d y been d e s c r i b e d ( 4 ) . However, f o r t h e s a k e o f c l a r i t y , we r e p o r t i n T a b l e I t h e main physicochemical c h a r a c t e r i s t i c s o f t h e s o l i d s . We w i l l first summarize briefly the results related to the i n f r a r e d s t u d y o f t h e OH g r o u p s .


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Table

(HT) Solids

HY

HY HY

1 Q

2 Q

3 Q

HY

4 Q

HY

5 Q

(HTA) Solids

I - Physicochemical c h a r a c t e r i s t i c s o f t h e (HT), (HTA) a n d (ID) s o l i d s

(HT) T° (K)

% Crystallinity

Unit c e l l parameter (nm)

x v

Si/Al total

Si/Al (NMR)

773

70

2.443

2.37

823 873

75 75

2.440 2.438

2.37 2.37

973

75

2.434

2.47

15

800 ppm). F o r i n s t a n c e , i f we f o c u s on t h e HT s e r i e s , t h e N a / A l r a t i o increases to about 50 % when t h e u n i t c e l l parameter decreases to 2.428 nm. T h i s means t h a t p o i s o n i n g o f framework s i t e s and, more s p e c i f i c a l l y , o f t h e l e s s a c c e s s i b l e o n e s , i . e . l . f . OH g r o u p s , i s l i k e l y f o r HT o r HTA s o l i d s w i t h an u n i t c e l l p a r a m e t e r b e l o w a b o u t 2.430 nm. To v e r i f y t h i s important a s p e c t , HT and HTA solids with a very low s o d i u m c o n t e n t (< 300 ppm) have been p r e p a r e d . P r o v i d i n g t h a t t h e s e new s o l i d s have a u n i t c e l l p a r a m e t e r l o w e r than about 2.430 nm (HY ) , the l . f . OH begins to interact with C H ( f i g u r e 4B) . On t h e o t h e r hand, C^H d o e s n o t i n t e r a c t w i t h t h e l . f . band o f t h e ID s o l i d s w h i c h a r e a l s o c h a r a c t e r i z e d by a v e r y low s o d i u m c o n t e n t (< 3 00 ppm). Therefore the interaction, which is evidenced between C H and the l . f . band of highly dealuminated HT o r HTA s o l i d s w i t h a v e r y low sodium c o n t e n t , i s l i k e l y t o r e s u l t from t h e i n c r e a s e o f t h e a c i d s t r e n g t h o f t h e l . f . OH w i t h t h e d e a l u m i n a t i o n l e v e l o f t h e framework. Taking i n t o account a l l these r e s u l t s , i t appears t h a t framework d e a l u m i n a t i o n l e a d s t o an i n c r e a s e o f t h e a c i d s t r e n g t h o f t h e h . f . a s w e l l a s o f t h e l . f . OH groups.


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E x t r a - f r a m e w o r k Al-OH g r o u p s . Acidic and non-acidic extra-framework Al-OH groups are evidenced in solids steamed a t m o d e r a t e t e m p e r a t u r e s , i . e . b e l o w a b o u t 973 K. Such groups a r e not d e t e c t e d i n s o l i d s t r e a t e d a t h i g h e r temperatures s i n c e under these c o n d i t i o n s they are l i k e l y to dehydroxylate. The mild acid l e a c h i n g does not e l i m i n a t e a l l t h e e x t r a - f r a m e w o r k OH b a n d s b u t l e a d s t o a severe decrease of the non-acidic c o n t r i b u t i o n . The first q u e s t i o n t o be answered concerns the i n t e r p r e t a t i o n o f t h e s e Al-OH b a n d s . C l e a r l y , a s shown by independent experiments (4), extra-framework alumina and/or s i l i c a - a l u m i n a s p e c i e s are p r e s e n t i n dealuminated m a t e r i a l s . Indeed, i n t e r p r e t a t i o n i n terms of aluminal i k e s p e c i e s has a l r e a d y been p r o p o s e d i n t h e l i t e r a t u r e for the n o n - a c i d i c components ( 1 1 ) . However, to our knowledge, t h e f r e q u e n c y o f t h e v a r i o u s extra-framework A l - O H d o e s n o t e x a c t l y c o r r e s p o n d t o any A l - O H f r e q u e n c y a l r e a d y known f o r a l u m i n a , s i l i c a o r amorphous s i l i c a -


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alumina m a t e r i a l s . T h i s renders d i f f i c u l t the reliable i n t e r p r e t a t i o n of the s p e c t r a . We t e n t a t i v e l y s u g g e s t t h a t t h e non acidic Al-OH would r e s u l t from h i g h l y d i s p e r s e d a l u m i n a - r i c h silicaa l u m i n a p h a s e s w h i l e t h e a c i d i c A l - O H w o u l d be due t o a more s i l i c a - r i c h p h a s e . T h i s a p p r o x i m a t i v e i n t e r p r e t a t i o n w o u l d e x p l a i n why t h e n o n - a c i d i c A l - O H b a n d s a s s i g n e d t o a l u m i n a - r i c h p h a s e s a r e more e a s i l y e l i m i n a t e d by the acid leaching. Silanolgroups. The s o - c a l l e d n o n - a c i d i c S i - O H band a t 374 2 cm i s more complex t h a n e x p e c t e d : i t r e s u l t s f r o m two components located at 3738 and 3 74 3 era . The 3738 cm i s predominant f o r w e l l c r y s t a l l i n e m a t e r i a l s w h i l e t h e 374 3 cm" band i s o n l y d e t e c t e d i n t h e c a s e o f n e a r l y amorphous s a m p l e s ( f i g u r e 9 ) . T h i s w o u l d mean t h a t t h e 3738 and 3743 cm" bands c o r r e s p o n d t o Si-OH a t t a c h e d t o t h e framework ( d e f e c t s s i t e s ) and t o S i - O H a t t a c h e d t o an extra-framework phase containing silicon, respectively. According to t h i s i n t e r p r e t a t i o n a small proportion of the non-framework Si-OH i s able to interact with p y r i d i n e . Does t h i s mean t h a t t h e s e Si-OH g r o u p s b e h a v e as B r o n s t e d s i t e s ? S u c h a s u r p r i s i n g l y a c i d i c b e h a v i o u r i s u n l i k e l y . A d s o r p t i o n e x p e r i m e n t s p e r f o r m e d on s i l i c a rich amorphous s i l i c a - a l u m i n a i n d i c a t e t h a t t h e Si-OH band l o c a t e d a t 3745 cm" i s p a r t l y a f f e c t e d by p y r i d i n e adsorption, e v e n a f t e r e v a c u a t i o n a t 723 K. T a k i n g i n t o account t h a t non-protonated p y r i d i n e species are detected under such c o n d i t i o n s , the p e r t u r b a t i o n of the Si-OH w o u l d r e s u l t f r o m an i n d i r e c t i n t e r a c t i o n w i t h p y r i d i n e a d s o r b e d on n e a r - b y L e w i s a c i d s i t e s . The same phenomenon i s l i k e l y to occur f o r non-framework S i - O H p r e s e n t in m o d i f i e d z e o l i t e s s i n c e t h e s o l i d s a r e known t o c o n t a i n a l a r g e amount o f e x t r a - f r a m e w o r k L e w i s s i t e s ( 4 ) . M o r e o v e r this i n t e r p r e t a t i o n would support the presence of a s i l i c a r i c h e x t r a - f r a m e w o r k p h a s e i n HT as w e l l as i n HTA solids. 1


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Conclusion Methods g e n e r a l l y u s e d t o s t u d y t h e zeolites acidity, such as titration in aprotic solvent (19), and temperature programmed desorption of basic probe m o l e c u l e s (20) reveal a heterogeneity of Bronsted acid s i t e s . The p r e s e n t r e s u l t s show t h a t IR s p e c t r o s c o p y of adsorbed probe molecules with various basic strengths gives direct informations on the respective acid s t r e n g t h s o f t h e d i f f e r e n t t y p e s o f OH g r o u p s . They a l l o w t o compare t h e i r e v o l u t i o n w i t h t h e A l / S i + A l r a t i o of HY z e o l i t e s d e a l u m i n a t e d by conventional treatment or isomorphous substitution. Figure 8 demonstrates a m o n o t o n i c i n c r e a s e i n t h e a c i d s t r e n g t h o f t h e OH g r o u p s indicated by the high-frequency band w i t h decreasing I V

I V


FT-IR

Study of the Bronsted Acid Sites


JANIN ET AL.


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PERSPECTIVES IN MOLECULAR SIEVE SCIENCE IV

Iv

Al /Si+Al . interestingly, the acid strength continues to increase when Al /Si+Al < n.l, although for such ratios NMR studies concluded that the strength of acidity remains constant (9). Our results relative to the h.f. OH groups are confirmed by those relative to the l.f. ones, since the latter also become more acidic (sensitive to ethylene) for high degrees of dealumination level. Extra-framework Al-OH groups are present in conventional HY treated at low temperatures. Most of these OH groups are non- or weakly acidic. However, under some specific preparation conditions, some of these extra-framework Al-OH groups behave as strong acid sites. The Si-OH band appears to be more complex than expected since a framework and an extra-framework component are involved.


IV

IV

Literature Cited 1. Ghosh ,A. K.; Curthoys, G. J. Chem. Soc., Faraday Trans I 1984, 80, 99. 2. Barthomeuf, D. Molecular Sieves II, ACS Symposium 1977, 40, 453. 3. Scherzer, J. ACS Symposium Series No.248, 1983, 157. 4. Macedo, A.; Raatz, F.; Boulet, R.; Janin, A.; Lavalley, J.C. Proc. Innovation in Zeolite Materials Science Nieuwpoort (Belgium), 1987. 5. Anderson, M. W. ; Klinowski, J. Zeolites 1986, 6 455. 6. Topsoe, N. Y.; Pedersen, K.; Derouane, E. G. J. Catal. 1981, 70, 41. 7. Dakta, J.; Tuznik, E. J. Catal. 1986, 102, 43. 8. Pfeifer, H.; Freude, D.; Hunger, M. Zeolites 1985, 5, 274. 9. Freude, D.; Hunger, M.; Pfeifer, H.; Schwieger, W. Chem. Phys. Letters 1986, 128, 62. 10. Lohse, U.; Löffler, E . ; Hunger, M.; Stöckner, J.; Patzelova, V. Zeolites 1987, 7, 11. 11. Kubelkova, L . ; Seidl, V.; Novakova, J.; Bednarova, S.; Jiru, P. J. Chem Soc., Faraday Trans I 1984, 80, 1367. 12. Cornac, M. Ph.D. Thesis, I.S.M.Ra, Caen, 1986. 13. Skeels, G. W.; Breck, D. W. Proc.6th Int.Zeolite Conf., Reno, 1983, p 87. 14. Akporiaye, D.; Chapple, A. P.; Clark, D. M.; Dwyer, J.; Elliott, I. S.; Rawlence, D. J. Stud. Surf. Sc. Catal. 1986, 28, 351. 15. Rouxhet, P. G.; Sempels, R. E. J. Chem. Soc., Faraday Trans I 1974, 70, 2021. 16. Jacobs, P. A.; Uytterhoeven, J. B. J . Chem. Soc., Faraday Trans I 1973, 69, 359. 17. Ward, J. W. J. Phys. Chem. 1969, 73, 2086. 18. Macedo, A.; Auroux, A.; Raatz, F.; Jacquinot, E . ; Boulet, R. In this book, pp 98-116.


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Beaumont, R.; Barthomeuf, D. J . Catal. 1972, 26, 218; J. Catal. 1972, 27, 45. Dondur, V . ; Karge, H. G. Surface S c i . 1987, 189/190. 873.


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FTIR Study of the BrÃ¶nsted Acid Sites in Dealuminated HY Zeolites

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