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The Nature of Coke-Forming Centres on Alumina Catalyst for Oxidative Dehydrogenation of Ethylbenzene
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THE NATURE OF COKE-FORMING CENTRES ON ALUMINA CATALYST FOR OXIDATIVE DEHYDROGENATION OF ETHY LBENZENE R . FIEDOROW, W . PRZYSTAJKO and M. SOPA I n s t y t u t Chemii UAM, Grunwaldzka 6. 60-780 Poznan, Poland and I . G . DALLA LANA Department of Chemical E n g i n e e r i n g , U n i v e r s i t y of A l b e r t a , Edmonton, A l b e r t a , Canada T6G 2G6 SUMMARY: Coke formed on alumina i s c a t a l y t i c a l l y a c t i v e . By poisoning a c i d c e n t e r s through NaOH doping ( 0 . 1 t o 1 . 0 w t % ) , C / H r a t i o s of cokes were d e c r e a s e d . Cokes w i t h C / H < 1 shored less c a t a l y t i c a c t i v i t y f o r s t y r e n e f o r m a t i o n . From ESR measurements, a c t i v e s i t e s f o r s t y r e n e f o r m a t i o n a r e , most l i k e l y , paramagnetic centres. INTRODUCTION: The a c t i v i t y of alumina f o r o x i d a t i v e dehydrogenation I n t e r a c t i o n of alumina w i t h hydroof e t h y l b e n z e n e i s n o n - t y p i c a l . Instead carbons occurs v i a coke-forming c e n t e r s on t h e s u r f a c e ' ) . of t h e u s u a l d e a c t i v a t i o n of alumina by c o k e , t h e c a t a l y t i c a c t i v i t y r i s e s w i t h coke coverage and then s t a b i l i z e s . This study i s d i r e c t e d a t e x p l a i n i n g t h e g e n e s i s of t h e cokes and d e t e r m i n i n g some of t h e i r p r o p e r t i e s . EXPERIMENTAL: P r e c i p i t a t e d aluminum hydroxides were d r i e d and t h e n c a l c i n e d a t 800°C f o r 6 h b e f o r e being doped w i t h NaOH. D e t a i l s concerning t h e measurement o f c a t a l y t i c a c t i v i t y f o r o x i d a t i v e dehydrogenation of e t h y l b e n z e n e i n a f l o w r e a c t o r , and t h e a n a l y s i s of p r o d u c t s were r e p o r t e d 2 ) . Non-oxidative dehydrogenation of ethylbenzene d i d n o t occur w i t h i n t h e r a n g e of NaOH-doped aluminas t e s t e d . C a t a l y s t samples were c a r e f u l l y s t r i p p e d of ethylbenzene and l i q u i d p r o d u c t s and s t o r e d i n s e a l e d ampoules u n t i l s u b j e c t e d t o e l e m e n t a l a n a l y s i s o r ESR measurements. The a c i d - s t r e n g t h of t h e doped aluminas w a s e s t i m a t e d by measuring t h e i r a c t i v i t y f o r s e v e r a l a c i d - c a t a l y z e d model r e a c t i o n s i n a p u l s e r e a c t o r : s k e l e t a l i s o m e r i z a t i o n of cyclohexene ( I ) , i s o m e r i z a t i o n of n-butene ( 1 1 ) , and decomposition of 2-propanol (111). S u r f a c e a r e a s from H2 ads o r p t i o n i n d i c a t e d minor e f f e c t of NaOH-doping upon t h e s u r f a c e a r e a of alumina.
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RESULTS AND DISCUSSION:
Cokes c o n t a i n i n g h i g h e r hydrogen c o n t e n t s ( i n c r e a s i n g l y s o , t h e g r e a t e r t h e e x t e n t of NaOH doping) were l e s s a c t i v e f o r o x i d a t i v e dehydrogenation a t t h e expense of i n c r e a s e d formation of carbon o x i d e s . With NaOH-doping from 0 t o 1.0% by w t , t h e % by w t of coke i n t h e c a t a l y s t ranged from 1 6 . 1 t o 2 . 1 and t h e C / H r a t i o s from 2.06 t o 0 . 5 2 . Agreement t h a t polynaphthoq u i n o n e - l i k e coke may be involved i s suggested by t h e e m p i r i c a l formulae f o r cokes formed on t h e 0 and 0.2 NaOH aluminas, C6.4H3.10 and C5.8H3.80, r e s p e c t i v e l y . Model r e a c t i o n s , I and 11, b o t h of which r e q u i r e s t r o n g a c i d c e n t e r s 4 ' 5, , a r e r a p i d l y poisoned by NaOtI-doping . The d i r e c t c o r r e l a t i o n between 111 and s t y r e n e f o r m a t i o n , d e c r e a s i n g a c t i v i t y w i t h i n c r e a s i n g NaOH c o n t e n t , confirms t h a t o n l y moderate t o weak a c i d c e n t e r s g e n e r a t e a c t i v e coke. ESR measurements,used t o s t u d y t h e c a t a l y t i c r o l e of t h e coke i t s e l f , i n d i c a t e d a c o r r e l a t i o n between u n p a i r e d e l e c t r o n s and coke c o n t e n t . Such paramagnetic c e n t e r s a r e probably formed from r u p t u r e of C-C bonds i n t h e coke. These r e s u l t s s u g g e s t t h a t t h e range of s u r f a c e a c i d i t y of p r a c t i c a l s i g n i f i c a n c e i n t h e p r e p a r a t i o n of s u i t a b l e alumina c a t a l y s t s i s n o t i n f l u e n c e d by e i t h e r v e r y s t r o n g o r v e r y weak a c i d c e n t e r s , S t r o n g e r a c i d c e n t e r s produce coke o f u n s a t u r a t e d c h a r a c t er whereas weaker a c i d c e n t e r s form coke r i c h e r i n hydrogen. REFERENCES 1. T . G . Alkhazov and A . E . L i s o v s k i i , K i n e t . K a t a l . , 1 7 , 434 (1976). 2 . R. Fiedorow, W. Kania, K . Nowinska, M . Sopa and MrWojciechowska, B u l l . Acad. Polon. S c i . , S e r . S c i . Chim., 27, 641 (1978). 3 . Y . Iwasawa, H . Nobe and S . Ogasawara, J . C a t a l . 3 c 444 (1973). 4. H . P i n e s and W . O . Haag, J . h e r . Chem. S O C . , 8 2 , 7 4 7 1 (1960). 5 . Y . Amenomiya, J . C a t a l . , 46, 326 (1977).
THE NATURE OF COKE-FORMING CENTRES ON ALUMINA CATALYST FOR OXIDATIVE DEHYDROGENATION OF ETHY LBENZENE R . FIEDOROW, W . PRZYSTAJKO and M. SOPA I n s t y t u t Chemii UAM, Grunwaldzka 6. 60-780 Poznan, Poland and I . G . DALLA LANA Department of Chemical E n g i n e e r i n g , U n i v e r s i t y of A l b e r t a , Edmonton, A l b e r t a , Canada T6G 2G6 SUMMARY: Coke formed on alumina i s c a t a l y t i c a l l y a c t i v e . By poisoning a c i d c e n t e r s through NaOH doping ( 0 . 1 t o 1 . 0 w t % ) , C / H r a t i o s of cokes were d e c r e a s e d . Cokes w i t h C / H < 1 shored less c a t a l y t i c a c t i v i t y f o r s t y r e n e f o r m a t i o n . From ESR measurements, a c t i v e s i t e s f o r s t y r e n e f o r m a t i o n a r e , most l i k e l y , paramagnetic centres. INTRODUCTION: The a c t i v i t y of alumina f o r o x i d a t i v e dehydrogenation I n t e r a c t i o n of alumina w i t h hydroof e t h y l b e n z e n e i s n o n - t y p i c a l . Instead carbons occurs v i a coke-forming c e n t e r s on t h e s u r f a c e ' ) . of t h e u s u a l d e a c t i v a t i o n of alumina by c o k e , t h e c a t a l y t i c a c t i v i t y r i s e s w i t h coke coverage and then s t a b i l i z e s . This study i s d i r e c t e d a t e x p l a i n i n g t h e g e n e s i s of t h e cokes and d e t e r m i n i n g some of t h e i r p r o p e r t i e s . EXPERIMENTAL: P r e c i p i t a t e d aluminum hydroxides were d r i e d and t h e n c a l c i n e d a t 800°C f o r 6 h b e f o r e being doped w i t h NaOH. D e t a i l s concerning t h e measurement o f c a t a l y t i c a c t i v i t y f o r o x i d a t i v e dehydrogenation of e t h y l b e n z e n e i n a f l o w r e a c t o r , and t h e a n a l y s i s of p r o d u c t s were r e p o r t e d 2 ) . Non-oxidative dehydrogenation of ethylbenzene d i d n o t occur w i t h i n t h e r a n g e of NaOH-doped aluminas t e s t e d . C a t a l y s t samples were c a r e f u l l y s t r i p p e d of ethylbenzene and l i q u i d p r o d u c t s and s t o r e d i n s e a l e d ampoules u n t i l s u b j e c t e d t o e l e m e n t a l a n a l y s i s o r ESR measurements. The a c i d - s t r e n g t h of t h e doped aluminas w a s e s t i m a t e d by measuring t h e i r a c t i v i t y f o r s e v e r a l a c i d - c a t a l y z e d model r e a c t i o n s i n a p u l s e r e a c t o r : s k e l e t a l i s o m e r i z a t i o n of cyclohexene ( I ) , i s o m e r i z a t i o n of n-butene ( 1 1 ) , and decomposition of 2-propanol (111). S u r f a c e a r e a s from H2 ads o r p t i o n i n d i c a t e d minor e f f e c t of NaOH-doping upon t h e s u r f a c e a r e a of alumina.
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RESULTS AND DISCUSSION:
Cokes c o n t a i n i n g h i g h e r hydrogen c o n t e n t s ( i n c r e a s i n g l y s o , t h e g r e a t e r t h e e x t e n t of NaOH doping) were l e s s a c t i v e f o r o x i d a t i v e dehydrogenation a t t h e expense of i n c r e a s e d formation of carbon o x i d e s . With NaOH-doping from 0 t o 1.0% by w t , t h e % by w t of coke i n t h e c a t a l y s t ranged from 1 6 . 1 t o 2 . 1 and t h e C / H r a t i o s from 2.06 t o 0 . 5 2 . Agreement t h a t polynaphthoq u i n o n e - l i k e coke may be involved i s suggested by t h e e m p i r i c a l formulae f o r cokes formed on t h e 0 and 0.2 NaOH aluminas, C6.4H3.10 and C5.8H3.80, r e s p e c t i v e l y . Model r e a c t i o n s , I and 11, b o t h of which r e q u i r e s t r o n g a c i d c e n t e r s 4 ' 5, , a r e r a p i d l y poisoned by NaOtI-doping . The d i r e c t c o r r e l a t i o n between 111 and s t y r e n e f o r m a t i o n , d e c r e a s i n g a c t i v i t y w i t h i n c r e a s i n g NaOH c o n t e n t , confirms t h a t o n l y moderate t o weak a c i d c e n t e r s g e n e r a t e a c t i v e coke. ESR measurements,used t o s t u d y t h e c a t a l y t i c r o l e of t h e coke i t s e l f , i n d i c a t e d a c o r r e l a t i o n between u n p a i r e d e l e c t r o n s and coke c o n t e n t . Such paramagnetic c e n t e r s a r e probably formed from r u p t u r e of C-C bonds i n t h e coke. These r e s u l t s s u g g e s t t h a t t h e range of s u r f a c e a c i d i t y of p r a c t i c a l s i g n i f i c a n c e i n t h e p r e p a r a t i o n of s u i t a b l e alumina c a t a l y s t s i s n o t i n f l u e n c e d by e i t h e r v e r y s t r o n g o r v e r y weak a c i d c e n t e r s , S t r o n g e r a c i d c e n t e r s produce coke o f u n s a t u r a t e d c h a r a c t er whereas weaker a c i d c e n t e r s form coke r i c h e r i n hydrogen. REFERENCES 1. T . G . Alkhazov and A . E . L i s o v s k i i , K i n e t . K a t a l . , 1 7 , 434 (1976). 2 . R. Fiedorow, W. Kania, K . Nowinska, M . Sopa and MrWojciechowska, B u l l . Acad. Polon. S c i . , S e r . S c i . Chim., 27, 641 (1978). 3 . Y . Iwasawa, H . Nobe and S . Ogasawara, J . C a t a l . 3 c 444 (1973). 4. H . P i n e s and W . O . Haag, J . h e r . Chem. S O C . , 8 2 , 7 4 7 1 (1960). 5 . Y . Amenomiya, J . C a t a l . , 46, 326 (1977).