Relationship Between Structure and Catalytic Performance of

Chapter 4

Relationship Between Structure and Catalytic Performance of Dealuminated Y Zeolites R. L . Cotterman, D . A. Hickson, and M. P. Shatlock

Downloaded by UNIV OF SYDNEY on October 9, 2014 | http://pubs.acs.org Publication Date: October 3, 1989 | doi: 10.1021/bk-1989-0411.ch004

W. R. Grace and Company, Washington Research Center, Columbia, M D 21044

Dealuminated Y z e o l i t e s which have been prepared by hydrothermal and chemical treatments show differences in c a t a l y t i c performance when tested fresh; however, these differences disappear after the z e o l i t e s have been steamed. The c a t a l y t i c behavior of fresh and steamed z e o l i t e s is d i r e c t l y related to z e o l i t e s t r u c t u r a l and chemical c h a r a c t e r i s t i c s . Such c h a r a c t e r i s t i c s determine the strength and density of acid s i t e s for c a t a l y t i c cracking. Dealuminated z e o l i t e s were characterized using X-ray d i f f r a c t i o n , porosimetry, s o l i d - s t a t e NMR and elemental analysis. Hexadecane cracking was used as a probe reaction to determine c a t a l y t i c properties. Cracking a c t i v i t y was found to be proportional to t o t a l aluminum content i n the z e o l i t e . Product s e l e c t i v i t y was dependent on unit cell s i z e , presence of extraframework alumina and s p a t i a l d i s t r i b u t i o n of active s i t e s . The results from this study elucidate the role that z e o l i t e structure plays i n determining c a t a l y t i c performance.

Dealuminated Y z e o l i t e s are w i d e l y used commercially i n f l u i d c r a c k i n g c a t a l y s t s t o produce h i g h - o c t a n e g a s o l i n e - r a n g e p r o d u c t s ( l ) . V a r i o u s methods o f p r e p a r a t i o n o f d e a l u m i n a t e d z e o l i t e s can l e a d t o m a t e r i a l s w i t h a range o f d i f f e r e n t s t r u c t u r a l and c h e m i c a l c h a r a c t e r i s t i c s ( 2 ) . These c h a r a c t e r i s t i c s p l a y a key r o l e i n d e t e r m i n i n g a c t i v i t y and s e l e c t i v i t y d u r i n g c a t a l y t i c c r a c k i n g . S t r u c t u r a l f e a t u r e s such as u n i t c e l l s i z e , p r e s e n c e o f e x t r a f r a m e w o r k m a t e r i a l , p o r e - s i z e d i s t r i b u t i o n , and a c t i v e s i t e s p a t i a l d i s t r i b u t i o n a f f e c t z e o l i t e a c i d i t y and d i f f u s i o n a l p r o p e r t i e s which, i n t u r n , i n f l u e n c e hydrocarbon r e a c t i v i t y . The o b j e c t i v e o f t h i s work i s t o i d e n t i f y r e l a t i o n s h i p s between s t r u c t u r e and c a t a l y t i c performance i n t h e s p e c i f i c case o f h y d r o c a r b o n c r a c k i n g over d e a l u m i n a t e d Y z e o l i t e s . Dealuminated z e o l i t e s a r e p r e p a r e d u s i n g c h e m i c a l and h y d r o t h e r m a l methods and the e f f e c t o f d e a l u m i n a t i o n method on s t r u c t u r e - p e r f o r m a n c e 0097-6156/89/0411-0024$06.00/0 o 1989 American Chemical Society

In Characterization and Catalyst Development; Bradley, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.


4.

COTTERMAN ET AL.

Structure and Performance of Dealuminated Y Zeolites

r e l a t i o n s h i p s i s i n v e s t i g a t e d . Z e o l i t e s are e v a l u a t e d a f t e r c a l c i n a t i o n and a f t e r s e v e r e steaming t o s i m u l a t e the p r o p e r t i e s o f f r e s h and e q u i l i b r i u m FCC c a t a l y s t s . H y d r o t h e r m a l d e a l u m i n a t i o n i s c a r r i e d out by t r e a t i n g an ammonium-exchanged form o f the z e o l i t e i n the p r e s e n c e o f m i l d s t e a m ( 3 ) . The r e s u l t i n g z e o l i t e s are c a l l e d " u l t r a s t a b l e Y" (USY) z e o l i t e s due t o t h e i r enhanced t h e r m a l and h y d r o t h e r m a l s t a b i l i t y . D u r i n g the steam t r e a t m e n t , aluminum i s e x p e l l e d from the framework i n t o z e o l i t e c a g e s ( 4 ) . T h i s aluminum t h e n r e l o c a t e s p r e f e r e n t i a l l y t o the o u t e r p o r t i o n s o f the c r y s t a l g i v i n g r i s e t o an a l u m i n u m - r i c h s u r f a c e ( 5 ) . Framework v a c a n c i e s c r e a t e d by aluminum e x p u l s i o n a r e b e l i e v e d t o be a n n e a l e d by s i l i c o n m i g r a t i o n from o t h e r framework l o c a t i o n s ( 6 ) . This s i l i c o n m i g r a t i o n subsequently r e s u l t s i n c o l l a p s e o f p o r t i o n s o f the z e o l i t e framework. Hence, h y d r o t h e r m a l d e a l u m i n a t i o n g e n e r a t e s a secondary pore system ( m e s o p o r o s i t y ) w i t h i n the z e o l i t e and w i t h an o v e r a l l l o s s o f c r y s t a l l i n i t y . Evidence f o r the p r e s e n c e o f mesopores i s p r o v i d e d i n d i r e c t l y by s o r p t i o n ( 7 ) measurements and d i r e c t l y by e l e c t r o n m i c r o s c o p y s t u d i e s ( 8 ) . Chemical d e a l u m i n a t i o n i n v o l v e s r e a c t i o n o f the z e o l i t e framework w i t h any one o f a v a r i e t y o f r e a g e n t s ( 2 ) . I n t h i s work, z e o l i t e s were r e a c t e d w i t h ammonium h e x a f l u o r o s i l i c a t e i n aqueous s o l u t i o n ( 9 12) t o p r e p a r e d e a l u m i n a t e d p r o d u c t s . Aluminum was e x t r a c t e d from the z e o l i t e framework and removed from the c r y s t a l as a s o l u b l e f l u o r o a l u m i n a t e complex; the r e s u l t i n g l a t t i c e v a c a n c i e s a r e b e l i e v e d to be f i l l e d by s i l i c o n i n s o l u t i o n . C o m p o s i t i o n p r o f i l e s o f c h e m i c a l l y d e a l u m i n a t e d z e o l i t e s (AFS) a r e homogeneous and i n d i c a t e the e n t i r e c r y s t a l i s a c c e s s i b l e f o r d e a l u m i n a t i o n ( 1 3 ) . S o r p t i o n d a t a i n d i c a t e t h a t AFS z e o l i t e s do n o t p o s s e s s a secondary pore system a l t h o u g h pore b l o c k a g e may o c c u r due t o o c c l u s i o n o f fluoroaluminate species(13). The s t r u c t u r a l f e a t u r e s o f d e a l u m i n a t e d z e o l i t e samples were c h a r a c t e r i z e d u s i n g X-ray powder d i f f r a c t i o n , p o r o s i m e t r y and s o l i d s t a t e NMR measurements. Hexadecane c r a c k i n g was used as a probe r e a c t i o n t o i n v e s t i g a t e c a t a l y t i c p r o p e r t i e s o f pure z e o l i t e s . Experimental Two s e t s o f c h e m i c a l l y and h y d r o t h e r m a l l y d e a l u m i n a t e d z e o l i t e s were p r e p a r e d from s e p a r a t e s o u r c e s o f p a r t i a l l y ammonium-exchanged Y zeolite. The f i r s t s e t o f samples, d e s i g n a t e d USY-1 and AFS-1, were p r e p a r e d from D a v i s o n ammonium-exchanged Y z e o l i t e ( Y - l ) . The second s e t o f samples, d e s i g n a t e d USY-2 and AFS-2, were p r e p a r e d from L i n d e LZ-Y62 z e o l i t e ( Y - 2 ) . T y p i c a l p h y s i c a l and c h e m i c a l p r o p e r t i e s o f the two s t a r t i n g m a t e r i a l s a r e compared i n T a b l e I ; the p r i m a r y d i f f e r e n c e between t h e s e m a t e r i a l s i s the e x t e n t o f soda removal by ammonium exchange. The h y d r o t h e r m a l l y d e a l u m i n a t e d z e o l i t e s (USY-1 and USY-2) used i n t h i s s t u d y were p l a n t - g r a d e m a t e r i a l s and were used w i t h o u t f u r t h e r exchange. The c h e m i c a l l y d e a l u m i n a t e d z e o l i t e s (AFS-1 and AFS-2) were p r e p a r e d i n our l a b o r a t o r y f o l l o w i n g the method o f B r e c k and S k e e l s ( 1 2 ) . The AFS samples were washed t h o r o u g h l y u n t i l r e s i d u a l f l u o r i d e was no l o n g e r d e t e c t e d i n the f i l t r a t e ; AFS-1 r e c e i v e d a d d i t i o n a l warm-water washes t o reduce f u r t h e r f l u o r i d e l e v e l s i n the s o l i d . AFS samples were p r e p a r e d so as t o have the same e x t e n t o f framework d e a l u m i n a t i o n as the USY samples.


IS

26

CHARACTERIZATION AND CATALYST D E V E L O P M E N T

Table I .

Characterization of Starting Materials

C o m p o s i t i o n , wt% A1 0 Si0 Na 0 Bulk S i 0 / A l 0 3 Degree o f Na Exchange, % XRD U n i t C e l l , A N S u r f a c e A r e a , m^/g T o t a l BET M i c r o p o r e (< 20 A) N Pore Volume, c c / g Total M i c r o p o r e (< 20 A) Mesopore (20-200 A) 2

3

2

2

2

2

Y-l

Y-2

23.63 69.69 3.94 5.0 73 24.71

23.93 70.06 2.94 5.0 80 24.70

947 929

993 975

0.482 0.356 0.033

0.473 0.376 0.025


2

2

D e a l u m i n a t e d z e o l i t e samples were c a l c i n e d i n a i r a t 540°C f o r t h r e e h o u r s p r i o r t o c a t a l y t i c t e s t i n g . A p o r t i o n o f each sample was f u r t h e r m o d i f i e d u s i n g h i g h - t e m p e r a t u r e steam. Z e o l i t e samples were p l a c e d i n a f i x e d - b e d q u a r t z tube; 95% steam was p a s s e d t h r o u g h the bed a t 750°C and a t m o s p h e r i c p r e s s u r e f o r 4 h o u r s . B u l k e l e m e n t a l a n a l y s e s were d e t e r m i n e d u s i n g X - r a y f l u o r e s c e n c e ( S i , A l ) , i n d u c t i v e l y c o u p l e d plasma (Al,Na) and i o n chromatography (F) t e c h n i q u e s . The u n i t c e l l c o n s t a n t was d e t e r m i n e d by X - r a y d i f f r a c t i o n w i t h a Siemens D-500 powder d i f f r a c t o m e t e r f o l l o w i n g the method o u t l i n e d i n ASTM D3942-80. N i t r o g e n s o r p t i o n d a t a were o b t a i n e d u s i n g a Quantachrome A u t o s o r b 6 p o r o s i m e t e r o p e r a t e d a t l i q u i d n i t r o g e n temperature. P o r e - s i z e d i s t r i b u t i o n s were c a l c u l a t e d u s i n g t h e d e s o r p t i o n b r a n c h o f the i s o t h e r m ; m i c r o p o r e c o n t r i b u t i o n s were d e t e r m i n e d by T - p l o t a n a l y s i s . S t a r t i n g m a t e r i a l s and d e a l u m i n a t e d p r o d u c t s were c h a r a c t e r i z e d u s i n g 2 ^ S i i 2 7 i MAS-NMR. ^ S i p t r a were o b t a i n e d on a B r u k e r MSL-200 s p e c t r o m e t e r a t 39.7 MHz u s i n g 30° p u l s e s and 5 second r e p e t i t i o n r a t e s . S p i n n i n g r a t e s were t y p i c a l l y 4.5 kHz and 5000-10000 scans were o b t a i n e d p e r sample. ^ A l s p e c t r a were r u n on a B r u k e r AM-400 i n s t r u m e n t a t 104.3 MHz w i t h 10° p u l s e s and 0.1 second r e p e t i t i o n r a t e s . S p i n n i n g r a t e s were t y p i c a l l y 5.0 kHz and 2000-10000 scans were o b t a i n e d p e r sample. Hexadecane c r a c k i n g r e a c t i o n s were c a r r i e d o u t o v e r pure z e o l i t e s i n an a t m o s p h e r i c f i x e d - b e d r e a c t o r a t 500°C Z e o l i t e s were s i z e d t o 40/80 mesh and mixed w i t h g l a s s beads p r i o r t o t e s t i n g . The c a t a l y s t bed was purged w i t h d r y n i t r o g e n a t 500°C f o r 30 minutes p r i o r t o r e a c t i o n and 10 minutes f o l l o w i n g r e a c t i o n . Hexadecane f e e d was i n j e c t e d v i a a s y r i n g e pump a t a c o n s t a n t r a t e o f 27.8 g/hr. Space v e l o c i t i e s were v a r i e d by c h a n g i n g c a t a l y s t l o a d i n g . S e p a r a t e gas and l i q u i d p r o d u c t s were c o l l e c t e d o v e r a t h r e e minute r u n and a n a l y z e d by gas chromatography. A p p r o x i m a t e l y 350 components i n the l i q u i d p r o d u c t were s e p a r a t e d and i d e n t i f i e d u s i n g a 50m x 0.2mm m e t h y l s i l i c o n e c a p i l l a r y column w i t h 0.5/i.m f i l m t h i c k n e s s . Spent c a t a l y s t s were removed from t h e r e a c t o r and c a l c i n e d i n a i r f o r 3 h o u r s a t 540°C t o determine coke y i e l d . Coke amount i s r e p o r t e d as the d i f f e r e n c e i n c a t a l y s t w e i g h t b e f o r e and a f t e r c a l c i n a t i o n . a n c

A

S

e c


4.

COTTERMANETAL.

Structure andPerformance of Dealuminated YZeolites


Presentation of Results C a t a l y s t C h a r a c t e r i z a t i o n . The c o m p o s i t i o n s o f d e a l u m i n a t e d p r o d u c t s are r e p o r t e d i n T a b l e I I on a v o l a t i l e - f r e e b a s i s . The two USY samples have s i m i l a r b u l k c o m p o s i t i o n s ; however, b o t h samples have h i g h e r s i l i c a - a l u m i n a r a t i o s t h a n the s t a r t i n g Y z e o l i t e s ( T a b l e I ) i n d i c a t i n g some a l u m i n a was l o s t d u r i n g p r e p a r a t i o n . The two AFS samples have s i m i l a r b u l k c o m p o s i t i o n s and lower aluminum c o n t e n t s t h a n the USY samples. The e x t r a wash g i v e n t o AFS-1 r e s u l t s i n a l o w e r f l u o r i d e l e v e l t h a n t h a t o f AFS-2. F l u o r i d e s remain a f t e r c a l c i n a t i o n b u t drop below 0.01% a f t e r steaming. Soda l e v e l s i n the AFS m a t e r i a l s a r e s l i g h t l y h i g h e r t h a n t h o s e i n USY m a t e r i a l s i n s p i t e o f s e v e r a l exchanges and e x t e n s i v e washing and i n d i c a t e s t h a t sodium remains bound i n the d e a l u m i n a t e d z e o l i t e . Both s e t s o f AFS and USY samples have s i m i l a r u n i t c e l l d i m e n s i o n s . T a b l e I I . C h a r a c t e r i z a t i o n o f Dealuminated

C o m p o s i t i o n , wt% 2°3 Si0 Na 0 F Bulk S i 0 / A l 0 XRD U n i t C e l l . A A l

2

2

2

2

3

Zeolites

USY-1

USY-2

AFS-1

AFS-2

21.,37 77.,77 0.,86

10

E o

o• * * • • * t

NO

USY-1 USY-2 AFS-1 AFS-2

CO 0) c 0) JZ +-> JZ a to Z 20

30

40

50

Olefins. wt%

F i g u r e 5. PONA s e l e c t i v i t i e s i n g a s o l i n e f r a c t i o n from hexadecane c r a c k i n g over USY and AFS z e o l i t e s .


34

CHARACTERIZATION AND

CATALYST D E V E L O P M E N T

L i g h t gas c o m p o s i t i o n s a r e shown i n T a b l e V I I I f o r hexadecane c r a c k i n g a t c o n s t a n t 50% c o n v e r s i o n . R e s u l t s a r e r e p o r t e d as i s o - t o - n o r m a l and o l e f i n - t o - p a r a f f i n r a t i o s f o r C4 p r o d u c t s . C a l c i n e d AFS and USY z e o l i t e s show s i g n i f i c a n t d i f f e r e n c e s i n b o t h s e l e c t i v i t y r a t i o s whereas steamed z e o l i t e s show s i m i l a r l i g h t gas s e l e c t i v i t i e s . However, steamed s e l e c t i v i t i e s a r e d r a m a t i c a l l y d i f f e r e n t from t h o s e o f c a l c i n e d z e o l i t e s . These r e s u l t s a r e i n good q u a l i t a t i v e agreement w i t h r e s u l t s o b t a i n e d f o r AFS and USY z e o l i t e s by g a s - o i l c r a c k i n g ( 1 7 ) .


Table V I I I .

Hexadecane C r a c k i n g R e s u l t s f o r Dealuminated L i g h t Gas C o m p o s i t i o n a t 50% C o n v e r s i o n

Zeolites.

Calcined Zeolites Steamed Z e o l i t e s

USY-1 2.65 1.81

Isobutane/n-Butane R a t i o USY-2 AFS-1 AFS-2 2.57 2.82 2.83 1.93 1.89 1.95

Calcined Zeolites Steamed Z e o l i t e s

USY-1 0.83 1.84

Butenes/Butanes R a t i o USY-2 AFS-1 AFS-2 0.78 0.54 0.57 1.67 1.74 1.61

Discussion of Results Catalyst Structural Characteristics. S t r u c t u r a l f e a t u r e s o f AFS and USY m a t e r i a l s have been c h a r a c t e r i z e d i n t h i s work i n terms o f u n i t c e l l s i z e , presence o f extraframework m a t e r i a l , a c t i v e - s i t e d i s t r i b u t i o n s , and p o r e - s i z e d i s t r i b u t i o n s . These f e a t u r e s a r e s i m i l a r f o r b o t h s e t s o f USY and AFS samples w h i c h i n d i c a t e s t h a t s t r u c t u r a l c h a r a c t e r i s t i c s a r e n o t r e l a t e d t o the s o u r c e o f Y zeolite. XRD measurements show t h a t c a l c i n e d AFS and USY z e o l i t e s have comparable u n i t c e l l s i z e s . Upon steaming, the u n i t c e l l s i z e s f o r b o t h AFS and USY reduce t o i d e n t i c a l v a l u e s . Hence, framework s i l i c a - a l u m i n a r a t i o s e q u i l i b r a t e t o comparable l e v e l s independent o f t h e method by w h i c h the z e o l i t e s were o r i g i n a l l y d e a l u m i n a t e d . As s y n t h e s i z e d , b o t h USY and AFS m a t e r i a l s show d i s t i n c t d i f f e r e n c e s by S i - N M R i n the d i s t r i b u t i o n o f framework s i l i c o n . The framework s i l i c o n d i s t r i b u t i o n depends on the mechanism o f aluminum removal and s i l i c o n replacement d u r i n g p r e p a r a t i o n . These d i f f e r e n c e s remain a f t e r c a l c i n a t i o n b u t d i s a p p e a r upon steaming. Severe steaming r e s u l t s i n l o s s o f a l a r g e p o r t i o n o f framework aluminum such t h a t o n l y the s t r o n g e s t - b o u n d aluminum s p e c i e s remain i n the framework. C o n s e q u e n t l y , steamed AFS and USY z e o l i t e s have s i m i l a r framework s i l i c o n d i s t r i b u t i o n s . A s - s y n t h e s i z e d AFS z e o l i t e s do n o t c o n t a i n e x t r a f r a m e w o r k aluminum as e v i d e n c e d by A l NMR. A s - s y n t h e s i z e d USY z e o l i t e s c o n t a i n a p p r e c i a b l e amounts o f extraframework m a t e r i a l as seen by comparing framework and b u l k s i l i c a - a l u m i n a r a t i o s and by examining A 1 s p e c t r a . Upon c a l c i n a t i o n b o t h AFS and USY m a t e r i a l s c o n t a i n e x t r a f r a m e w o r k aluminum. The amount o f extraframework aluminum i n b o t h AFS and USY m a t e r i a l s i n c r e a s e s on steaming. P o r o s i m e t r y d a t a c o n f i r m t h e presence o f mesopores i n c a l c i n e d 29

2 7

2 7


4.

COTTERMANETAL.

Structure and Performance of Dealuminated YZeolites


USY z e o l i t e s ; t h i s m e s o p o r o s i t y i n c r e a s e s on steaming. C a l c i n e d and steamed AFS z e o l i t e s show some m e s o p o r o s i t y ; however, the amounts remain l o w e r t h a n those i n USY z e o l i t e s . C a t a l y s t Performance R e l a t i o n s h i p s . Hexadecane c r a c k i n g a c t i v i t y o f AFS and USY z e o l i t e s , when c o r r e c t e d f o r d e a c t i v a t i o n e f f e c t s , shows l i t t l e o r no dependence on framework c o m p o s i t i o n . R a t h e r , as shown i n F i g u r e 6, a c t i v i t y appears t o be a f u n c t i o n o f t o t a l aluminum c o n t e n t independent o f the method o f d e a l u m i n a t i o n . T h i s r e s u l t i m p l i e s t h a t hexadecane c r a c k i n g o c c u r s over b o t h framework and e x t r a f r a m e w o r k a c i d s i t e s and t h a t i t i s the t o t a l number o f such s i t e s w h i c h determines c a t a l y t i c a c t i v i t y . Hence, e x t r a f r a m e w o r k m a t e r i a l i n the USY samples makes a s i g n i f i c a n t c o n t r i b u t i o n t o c a t a l y s t a c t i v i t y as r e p o r t e d by o t h e r s ( 1 8 . 1 9 ) . Coke s e l e c t i v i t y d i r e c t l y i n f l u e n c e s the r a t e o f c a t a l y s t d e a c t i v a t i o n as seen by comparing coke s e l e c t i v i t i e s i n T a b l e s VI and V I I w i t h o b s e r v e d r a t e c o n s t a n t s i n T a b l e V. Our d a t a i n d i c a t e c a l c i n e d AFS z e o l i t e s show h i g h e r coke s e l e c t i v i t i e s t h a n USY z e o l i t e s when compared a t s i m i l a r u n i t c e l l s i z e s . T h i s r e s u l t s u g g e s t s t h a t d i s t r i b u t i o n o f framework a c i d s i t e s ( a s r e f l e c t e d by the d i s t r i b u t i o n o f framework s i l i c o n ) has a s t r o n g impact on coke selectivity. I n a d d i t i o n , coke s e l e c t i v i t y has been shown t o c o r r e l a t e w i t h the d e n s i t y o f s t r o n g a c i d s i t e s i n the framework(20). Our d a t a c o n f i r m t h i s and show t h a t steaming d e c r e a s e s the d e n s i t y o f such s i t e s w h i c h , i n t u r n , l e a d s t o d e c r e a s e d coke s e l e c t i v i t i e s . G a s o l i n e s e l e c t i v i t y i s i n f l u e n c e d by b o t h the method o f d e a l u m i n a t i o n and steam t r e a t m e n t and, hence, depends on framework a c i d s i t e s and on presence o f extraframework m a t e r i a l . Both framework and extraframework s i t e s c o n t r i b u t e t o the o v e r a l l z e o l i t e acidity. Framework A l ( I V ) a c i d s i t e s a r e a s s o c i a t e d w i t h B r o n s t e d a c i d i t y whereas extraframework A l ( V I ) a c i d s i t e s are a s s o c i a t e d w i t h Lewis a c i d i t y ( 2 1 ) . C a l c i n e d AFS samples a r e r e p o r t e d (22) t o c o n t a i n g r e a t e r B r o n s t e d a c i d i t y t h a n USY samples from i n f r a r e d c h a r a c t e r i z a t i o n and t o have s t r o n g e r a c i d i t y as measured by ammonia d e s o r p t i o n . As a r e s u l t , the s t r o n g e r a c i d i t y o f c a l c i n e d AFS samples c r a c k s hexadecane t o lower m o l e c u l a r w e i g h t p r o d u c t s t h a n USY. A f t e r steaming, the a c i d i t i e s o f b o t h AFS and USY a r e reduced t o s i m i l a r l e v e l s and l e a d t o s i m i l a r g a s o l i n e s e l e c t i v i t i e s which a r e i n c r e a s e d r e l a t i v e t o the c a l c i n e d z e o l i t e s . L i g h t gas s e l e c t i v i t i e s from hexadecane c r a c k i n g ( T a b l e V I I I ) c o n f i r m the B r o n s t e d / L e w i s a c i d c h a r a c t e r o f AFS and USY z e o l i t e s . H i g h i s o - t o - n o r m a l r a t i o s and low o l e f i n - t o - p a r a f f i n r a t i o s c o r r e l a t e w i t h i n c r e a s e d B r o n s t e d a c i d c h a r a c t e r ( 2 3 ) . C a l c i n e d AFS has more B r o n s t e d a c i d i t y t h a n c a l c i n e d USY z e o l i t e s ; a f t e r steaming, b o t h AFS and USY have s t r o n g Lewis a c i d c h a r a c t e r . D i f f e r e n c e s between AFS and USY jamples are s u b t l e r e l a t i v e t o the d i f f e r e n c e s seen between steamed and c a l c i n e d samples. Hence, e f f e c t s due t o s t e a m i n g o v e r r i d e e f f e c t s due t o o r i g i n a l method o f d e a l u m i n a t i o n . G a s o l i n e c o m p o s i t i o n from hexadecane c r a c k i n g over c a l c i n e d and steamed AFS and USY z e o l i t e s can be r e p r e s e n t e d by g e n e r a l c o r r e l a t i o n s as shown i n F i g u r e 5. As t h e s e c o r r e l a t i o n s a r e unique t o z e o l i t e Y, they i n d i c a t e t h a t the Y z e o l i t e framework t o p o l o g y p l a y s an i m p o r t a n t r o l e i n the mechanism o f p r o d u c t f o r m a t i o n . The method o f d e a l u m i n a t i o n and subsequent steam t r e a t m e n t l e a d t o v a r i o u s PONA c o m p o s i t i o n s ; however, these c o m p o s i t i o n s r e s u l t from a


35


36

CHARACTERIZATION AND

CATALYST D E V E L O P M E N T

s i n g l e mechanism f o r h y d r o c a r b o n c o n v e r s i o n . T h i s r e s u l t s u g g e s t s t h a t m o d i f y i n g Y z e o l i t e i n f l u e n c e s the r e l a t i v e r a t e o f i n d i v i d u a l s t e p s i n t h e o v e r a l l mechanism ( f o r example, h y d r o g e n - t r a n s f e r r e a c t i o n s ) through m o d i f i c a t i o n o f z e o l i t e a c i d i t y . F i g u r e 7 shows c a l c u l a t e d octane numbers from hexadecane c r a c k i n g as a f u n c t i o n o f g a s o l i n e y i e l d . C a l c i n e d and steamed z e o l i t e s a r e r e p r e s e n t e d by open and c l o s e d symbols, r e s p e c t i v e l y . The c a l c u l a t e d octane number r e f l e c t s changes i n the g a s o l i n e m o l e c u l a r w e i g h t d i s t r i b u t i o n and, t o a l e s s e r e x t e n t , c o m p o s i t i o n e f f e c t s . I n c r e a s e s i n octane f o r c a l c i n e d AFS z e o l i t e s a r e o f f s e t by decreased gasoline y i e l d s . G a s o l i n e octane number f o r steamed USY z e o l i t e s has been shown t o correlate with unit c e l l size(24). T h i s concept has been e x p l o i t e d to d e s i g n USY c a t a l y s t s f o r octane production(2J3) . I n t h i s work we demonstrate t h a t parameters o t h e r t h a n u n i t c e l l s i z e have an impact on o c t a n e ; s i m i l a r c o n c l u s i o n s have been r e p o r t e d by o t h e r s ( 2 6 ) . As shown i n F i g u r e 7, c a l c i n e d z e o l i t e s w i t h s i m i l a r u n i t c e l l s i z e s demonstrate d i f f e r e n t o c t a n e - p r o d u c i n g b e h a v i o r . Hence, s t r u c t u r a l c h a r a c t e r i s t i c s such as extraframework aluminum and s p a t i a l d i s t r i b u t i o n of a c i d s i t e s are important f a c t o r s a f f e c t i n g z e o l i t e a c i d i t y . A f t e r steaming, the octane b e h a v i o r s o f AFS and USY z e o l i t e s are s i m i l a r r e f l e c t i n g s i m i l a r i t i e s i n z e o l i t e a c i d i t i e s . Extraframework aluminum c o n t r i b u t e s t o the o b s e r v e d c a t a l y t i c b e h a v i o r i n b o t h a c t i v i t y and s e l e c t i v i t y . I t i s i n t e r e s t i n g t o note t h a t a l t h o u g h steamed USY c o n t a i n s more e x t r a f r a m e w o r k aluminum t h a n steamed AFS, b o t h AFS and USY g i v e s i m i l a r p r o d u c t s e l e c t i v i t i e s . Observed p r o d u c t s e l e c t i v i t i e s from steamed z e o l i t e s a r e i n s e n s i t i v e to t h e amount o f extraframework aluminum p r e s e n t w i t h i n t h e c o m p o s i t i o n range i n v e s t i g a t e d i n t h i s s t u d y . Through the use o f hexadecane c r a c k i n g a l o n e , we have been u n a b l e i n t h i s work t o e l u c i d a t e t h e r o l e o f m e s o p o r o s i t y i n t h e c a t a l y t i c b e h a v i o r o f c a l c i n e d o r steamed z e o l i t e s . Steamed AFS and USY z e o l i t e s show d i f f e r e n c e s i n m e s o p o r o s i t y b u t e x h i b i t s i m i l a r c a t a l y t i c performance. W h i l e m e s o p o r o s i t y may a f f e c t d i f f u s i o n i n a c t u a l FCC c a t a l y s t s , l a r g e r m o l e c u l e s t h a n hexadecane w i l l be r e q u i r e d t o determine mesopore e f f e c t s . Conclusions C h a r a c t e r i z a t i o n and c a t a l y t i c d a t a have been p r e s e n t e d f o r c h e m i c a l l y and h y d r o t h e r m a l l y d e a l u m i n a t e d Y z e o l i t e s . These d a t a show t h a t z e o l i t e s t r u c t u r a l d i f f e r e n c e s l e a d t o d i f f e r e n c e s i n c a t a l y t i c b e h a v i o r . USY and AFS z e o l i t e s show d i s t i n c t s t r u c t u r a l d i f f e r e n c e s when f r e s h l y p r e p a r e d and a f t e r c a l c i n a t i o n , however, t h e s e d i f f e r e n c e s a r e d i m i n i s h e d on steam t r e a t m e n t . As a r e s u l t , the c a t a l y t i c b e h a v i o r o f c a l c i n e d AFS and USY z e o l i t e s appears d i f f e r e n t w h i l e t h a t o f steamed z e o l i t e s i s s i m i l a r . No apparent e f f e c t s due t o s o u r c e o f Y z e o l i t e were observed. Hexadecane c r a c k i n g a c t i v i t y c o r r e l a t e s w i t h t o t a l aluminum c o n t e n t ; USY m a t e r i a l s a r e more a c t i v e t h a n AFS m a t e r i a l s b e f o r e and a f t e r steaming. Extraframework aluminum c o n t r i b u t e s t o c a t a l y t i c cracking a c t i v i t y . G a s o l i n e s e l e c t i v i t y and octane performance a r e f u n c t i o n s o f n o t o n l y u n i t c e l l s i z e b u t a l s o f a c t o r s such as e x t r a f r a m e w o r k m a t e r i a l and s p a t i a l d i s t r i b u t i o n o f a c t i v e s i t e s . These f a c t o r s influence


Structure and Performance of Dealuminated Y Zeolites

COTTERMANETAL.

2.0

c +(0 -» CO c

o O

(0


>

CD

0.5

o.o

15

17

19

21

23

25

Total Al Content. wt%

F i g u r e 6. C r a c k i n g a c t i v i t y o f d e a l u m i n a t e d z e o l i t e s as a f u n c t i o n o f t o t a l aluminum c o n t e n t .

90

82

0)

c 4-" O 0 _c o

CD c (0 o O o o

CD CO 0

22

26

Gasoline

30

34

Yield, wt%

F i g u r e 7. Octane performance o f USY and AFS z e o l i t e s from hexadecane c r a c k i n g a t 50% c o n v e r s i o n .


38

CHARACTERIZATION AND CATALYST D E V E L O P M E N T

overall zeolite acidity. S t r o n g a c i d i t y i n c a l c i n e d AFS z e o l i t e s l e a d s t o d e c r e a s e d g a s o l i n e s e l e c t i v i t i e s and h i g h e r octane p r o d u c t s t h a n c a l c i n e d USY. A f t e r steaming, however, a c i d i t y l e v e l s o f USY and AFS a r e comparable and y i e l d s i m i l a r p r o d u c t d i s t r i b u t i o n s . No e f f e c t s due t o m e s o p o r o s i t y were o b s e r v e d i n hexadecane cracking. Acknowledgments


The a u t h o r s a r e g r a t e f u l t o W.R. Grace and Co. f o r p e r m i s s i o n t o p u b l i s h t h i s work and t o L. P e t e r s , D.M. R o b e r t s and C.F. M i l l e r f o r a s s i s t a n c e w i t h e x p e r i m e n t a l measurements.

Literature Cited 1. Catalytica Advances in Fluid Catalytic Cracking-Part 1; Study No. 4186CC: Mountain View, California, 1987. 2. Scherzer, J . ACS Symposium Series No. 248; American Chemical Society: Washington, DC, 1984, p 157. 3. Maher, P.K.; McDaniel, C.V. U.S. Patent 3 293 192 assigned to W.R. Grace and Co., 1966. 4. Kerr, G.T. J. Phys. Chem. 1968, 72, 2594. 5. Dwyer, J.; Fitch, F.R.; Qin, G.; Vickerman, J . C . J. Phys. Chem. 1982, 86, 4574. 6. Maher, P.K.; Hunter, F.D.; Scherzer, J . Advances in Chemistry Series No. 101; American Chemical Society: Washington, DC, 1981; p 266. 7. Lohse, U.; Mildebrath, M. Z. Anorg. Allg. Chem. 1981, 476, 126. 8. Lynch, J.; Raatz, F . ; Dufresne, P. Zeolites 1987, 7, 333. 9. E l l i o t , C.H. U.S. Patent 3 594 331 assigned to W.R. Grace and Co., 1971. 10. E l l i o t , C.H. U.S. Patent 3 933 983 assigned to W.R. Grace and Co., 1976. 11. Skeels, G.W.; Breck, D.W. Proc. Sixth Intl. Zeolite Conf., Reno, 1984, p 87. 12. Breck, D.W.; Skeels, G.W. U.S. Patent 4 503 023 assigned to Union Carbide Corp., 1985. 13. Akporiaye, D.; Chapple, A.P.; Clark, D.M.; Dwyer, J.; E l l i o t , I.S.; Rawlence, D.J. Proc. 7th Intl. Conf. Zeolites, Tokyo, 1986, p 351. 14. Ray, G . J . ; Meyers, B . L . ; Marshall, C.L. Zeolites 1987, 7, 4. 15. Gilson, J . - P . ; Edwards, G.C.; Peters, A.W.; Rajagopalon, K.; Wormsbecher, R.F.; Roberie, T . G . ; Shatlock, M.P. J. Chem. Soc., Chem. Commun. 1987, 91. 16. Freude, D.; Hunger, M.; Pfeifer, H. Z. Phys. Chemie (NF) 1987, 152. 171. 17. Corma, A.; Herrero, E . ; Martinez, A.; Prieto, J . ACS Preprints. Div. of Pet. Chem., Sept. 1987, p 639. 18. Beyerlein, R.A.; McVicker, G.B.; Yacullo, L . N . ; Ziemiak, J.J. J. Phys. Chem. 1988, 92, 1967. 19. Corma, A.; Fornes, V.; Martinez, A.; Melo, F . ; Pallota, O. Int. Symp. on Innovation in Zeolite Matl. Science, Nieuwpoort (Belgium), 1987, p 495. 20. Rajagopalan, K.; Peters, A.W. J. Cat. 1987, 106, 410.



4.

C O T T E R M A N ET AL.

Structure and Performance ofDealuminated Y Zeolites 39

21. Freude, D.; Frohlich, T . ; Hunger, M.; Pfeifer, H.; Scheler, G. Chem. Phys. Letters 1983, 98, 263. 22. Macedo, A . ; Raatz, F . ; Boulet, R.; Janin, a.; Lavalley, J . C . Int. Symp. on Innovation in Zeolite Matl. Science, Nieuwpoort (Belgium), 1987, p 375. 23. Wojciechowski, B.W.; Corma, A. Catalytic Cracking: Catalysts, Chemistry and Kinetics; Marcel-Dekker: New York, 1986. 24. Pine, L . A . ; Maher, P . J . ; Wachter, W.A. J. Cat. 1984, 85, 466. 25. Ritter, R . E . ; Creighton, J.E.; Chin, D.S.; Roberie, T.G. Paper AM-86-45 presented at NPRA Annual Meeting, Los Angeles, March 1986. 26. Corma, A . ; Fornes, V . ; Martinez, A . ; Orchilles, A.V. ACS Symposium Series No. 368; American Chemical Society: Washington, DC, 1988; p 542. RECEIVED January 26, 1989


Relationship Between Structure and Catalytic Performance of

Recommend Documents