Molecular Sieves

54 Catalytic and Physicochemical Characterization of Extracted H-Mordenite

Downloaded by GEORGE MASON UNIV on July 6, 2014 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0121.ch054

DILEEP K. THAKUR and SOL W. WELLER Department of Chemical Engineering, State University of New York at Buffalo, Buffalo, Ν. Y. 14214

Samples of H-mordenite were extracted withHClto remove Al and residual Na or were exchanged withNH NO to remove only Na. The pore volume was essentially unchanged by ex traction, but the acidity measured by NH chemisorption de creased roughly linearly with Al content. The effective dif fusivity of all extracted samples was appreciably higher than tha of the starting H-mordenite. Apparent first-order rate constan for hexane cracking in a pulsed microreactor decreased with in creasing number of pulses. Of the models evaluated for de activation, the most satisfactory was an exponential function in terms of the accumulated hexane actually converted. The "initial activity" decreased at least linearly with decreasing Al content, and it increased significantly as Νa content was lowered at constant Al content. 4

3

3

^ J ^ h e o r i g i n a l r e p o r t b y B a r r e r a n d M a k k i (1) t h a t a l u m i n u m i n a h i g h silica zeolite, c l i n o p t i l o l i t e , c o u l d be e x t r a c t e d w i t h m i n e r a l a c i d t o give a ' ' s i l i c a p s e u d o m o r p h " has g i v e n rise to considerable research o n a c i d e x t r a c t e d m o r d e n i t e (2-6). H y d r o g e n m o r d e n i t e is useful as a n adsorbent a n d a c a t a l y s t , a n d i t s properties for some purposes are i m p r o v e d b y p a r t i a l e x t r a c t i o n of t h e a l u m i n u m . F u r t h e r , t h e a b i l i t y t o v a r y a l u m i n u m c o n t e n t w h i l e m a i n t a i n i n g c r y s t a l l i n i t y offers t h e o p p o r t u n i t y t o l e a r n m o r e a b o u t the n a t u r e of the a c t i v e sites i n m o r d e n i t e . A

A n earlier r e p o r t f r o m t h i s l a b o r a t o r y (7) n o t e d t h a t i n a series of m i l d l y e x t r a c t e d mordenites, the hexane c r a c k i n g a c t i v i t y i n a c o n t i n u o u s flow test w e n t t h r o u g h a m a r k e d m a x i m u m w i t h increasing s e v e r i t y of ex t r a c t i o n , w h i l e the z-butane to η-butane r a t i o c o n t i n u o u s l y increased. T h e a c t i v i t y a n d p r o d u c t d i s t r i b u t i o n were measured after 10 m i n o n s t r e a m . Since c a t a l y s t d e a c t i v a t i o n was r a p i d , i t was n o t possible t o 596 In Molecular Sieves; Meier, W., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.

54.

597

Extracted H-Mordenite

THAKUR AND WELLER

e x t r a p o l a t e these d a t a a c c u r a t e l y t o zero t i m e o n s t r e a m , i.e., t o e x t r a c t e d b u t u n c o k e d samples. M o r e r e c e n t l y (8), another series of H - m o r d e n i t e s , a c i d - e x t r a c t e d t o a greater degree, was e x a m i n e d . the m a j o r results w e r e :

F o r these samples after d r y i n g a t 1 1 0 ° C ,

(a) there was no evidence of " h y d r o x y l n e s t s "

stable a b o v e 100°C, a n d (b) N H

3

c h e m i s o r p t i o n a t 2 5 0 ° C a n d 11 t o r r

r o u g h l y corresponded t o a s t o i c h i o m e t r i c r a t i o ( 1 : 1 , ± 2 5 % ) w i t h t h e t o t a l a m o u n t of a l u m i n u m r e m a i n i n g i n the l a t t i c e . T h e present p a p e r is a n extension of p r e v i o u s w o r k (8) i n t h e f o l l o w i n g ways.

(1) S a m p l e s were e x a m i n e d i n w h i c h r e s i d u a l s o d i u m , b u t

not


a l u m i n u m , was r e m o v e d b y exchange w i t h a m m o n i u m n i t r a t e f o l l o w e d b y calcination. reactor.

(2) T h e c r a c k i n g of n-hexane was s t u d i e d i n a p u l s e d m i c r o -

(3) T h e r e l a t i v e effective d i f f u s i v i t y was e s t i m a t e d b y a gas c h r o -

m a t o g r a p h i c t e c h n i q u e i n v o l v i n g b r o a d e n i n g of a N

2

pulse i n a H e c a r r i e r .

Experimental E x t r a c t i o n w i t h H C L A l u m i n u m was e x t r a c t e d f r o m t h e o r i g i n a l H m o r d e n i t e ( N o r t o n C o . " H - Z e o l o n , " L o t N o . T A - 4 ) w i t h aqueous H C 1 a t 100° C . D e t a i l s of t h e procedure are g i v e n i n R e f . 8. Exchange with N H N 0 . 24 G r a m s of g r o u n d ( 4 5 - 6 0 mesh) c a t a l y s t were exchanged i n a flask w i t h 800 m l of 0.2M N H N 0 at 100°C for 6 h r . T w o s a m p l e s were exchanged, one (sample 4) i n a single exchange e x p e r i m e n t , a n d t h e o t h e r (sample 5) i n a f o u r f o l d exchange i n w h i c h t h e s a m p l e was f i l t e r e d a n d w a s h e d w i t h d i s t i l l e d w a t e r b e t w e e n successive exchange t r e a t m e n t s w i t h N H N 0 s o l u t i o n . B o t h samples were g i v e n a final w a t e r w a s h , d r i e d o v e r n i g h t at 110°C, c a l c i n e d i n a i r for 8 h r at 5 2 5 ° C , a n d c o o l e d i n a desiccator. X - R a y D i f f r a c t i o n . A l l d i f f r a c t i o n p a t t e r n s were t a k e n o n p o w d e r e d samples w i t h a G e n e r a l E l e c t r i c X R D - 6 diffractometer. A copper t a r g e t a n d n i c k e l filter were u s e d . Acidity by A m m o n i a Chemisorption. Relative catalyst acidity was m e a s u r e d b y t h e q u a n t i t y of a m m o n i a c h e m i s o r b e d at t h e a r b i t r a r y c o n d i t i o n s of 2 5 0 ° C , 11.2 t o r r . A n I s o r p t a a n a l y z e r ( E n g e l h a r d m o d e l 3 A - 2 ) w a s u s e d ; t h e procedure w a s t h a t d e s c r i b e d i n R e f . 8. T o t a l P o r e V o l u m e . T h e t o t a l pore v o l u m e was a r b i t r a r i l y t a k e n as t h e v o l u m e of N sorbed at —195° at a r e l a t i v e pressure of 0.25 (8). E f f e c t i v e D i f f u s i v i t y . R e l a t i v e v a l u e s of t h e effective d i f f u s i v i t y were d e t e r m i n e d b y a gas c h r o m a t o g r a p h i c t e c h n i q u e i n w h i c h t h e b r o a d e n i n g of a N p u l s e i n H e c a r r i e r i s m e a s u r e d as a f u n c t i o n of c a r r i e r gas v e l o c i t y (9-13). A l t h o u g h there is debate o n t h e p r o p e r m e t h o d of a n a l y z i n g s u c h d a t a o n p e a k b r o a d e n i n g , t h e " p l a t e t h e o r y " of V a n D e e m t e r et al. (14) w a s considered adequate for o u r purposes. I n o u r m e a s u r e m e n t s e a c h c a t a l y s t s a m p l e w a s p a c k e d i n t o a stainless steel t u b e h a v i n g a 0 . 2 5 - i n c h o d (0.18-inch i d ) a n d 50 c m l o n g ; t h e s a m p l e w a s h e l d i n place b y b o r o s i l i c a t e glass w o o l p l u g s a t b o t h ends. S a m p l e s were p r e d r i e d a t 5 0 0 ° C in situ i n flowing 0 for 10 h r a n d flowing H e for a n a d d i t i o n a l 15 h r . T h e y were cooled to 25° C i n flowing H e , a n d t h e n tested at 25° C . F u r t h e r d e t a i l s are g i v e n i n R e f . 15. 4

3

4

4

3

3

2

2

2

In Molecular Sieves; Meier, W., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.

598

MOLECULAR SIEVES

Particle Density. P a r t i c l e density was measured b y a H g displacement m e t h o d at a t m o s p h e r i c pressure. T h e s i m p l e e q u i p m e n t h a s b e e n d e s c r i b e d (13). B u l k D e n s i t y . T h i s was measured b y packing the catalyst i n a straight, stainless steel t u b e 25 c m l o n g b y 0 . 2 5 - i n c h o d i n a d r y b o x ; t h e c o l u m n was vibrated during loading. V o i d F r a c t i o n i n t h e p a c k e d b e d w a s c a l c u l a t e d as 1 — ( b u l k d e n s i t y ) / (particle density). P a r t i c l e P o r o s i t y w as c a l c u l a t e d as (pore v o l u m e ) X ( p a r t i c l e d e n s i t y ) . C a t a l y t i c A c t i v i t y a n d S e l e c t i v i t y . T h e c r a c k i n g of n-hexane w a s s t u d i e d b y a p u l s e d m i c r o c a t a l y t i c - c h r o m a t o g r a p h i c t e c h n i q u e (16). Duplicate r u n s were m a d e o n each sample. A 5 - g r a m m i x t u r e (2.5 m l ) of glass m i c r o b e a d s a n d c a t a l y s t w a s used i n each r u n . T h e q u a n t i t y of c a t a l y s t w a s v a r i e d for different c a t a l y s t s t o o b t a i n c o n v e n i e n t l y m e a s u r a b l e levels of c o n v e r s i o n at t h e fixed test t e m p e r a t u r e of 350° C . O f the t o t a l 5-gram charge, t h e q u a n t i t y of c a t a l y s t v a r i e d f r o m 0.043 g r a m for t h e m o s t a c t i v e s a m p l e t o 0.28 g r a m for t h e least a c t i v e . C o m p e n s a t i o n for t h e v a r i a b l e a m o u n t of c a t a l y s t w a s m a d e b y c o m p u t i n g a n a p p a r e n t first-order r a t e c o n s t a n t , fc, defined as


T

fc(gram-i) = (1/W) In [1/(1 -

X)]

w h e r e W = c a t a l y s t w e i g h t (grams) a n d X = f r a c t i o n a l c o n v e r s i o n . A c o n s t a n t H e c a r r i e r flow r a t e of 31 m l / m i n was m a i n t a i n e d for a l l r u n s . Successive 5-μ l i t e r samples of n-hexane were i n j e c t e d w i t h a m i c r o s y r i n g e ; a k v a l u e w a s c o m p u t e d for each i n j e c t e d pulse. R e a c t o r effluent was a n a l y z e d w i t h a 6-foot c h r o m a t o g r a p h i c c o l u m n , 0 . 2 5 - i n c h o d p a c k e d w i t h U c o n oil on 60-80 mesh G C - 2 2 Super Support. T h e column was operated i s o t h e r m a l l y a t 3 5 ° C , t h e t h e r m a l c o n d u c t i v i t y detector at 7 5 ° C . Prior t o test, each c a t a l y s t s a m p l e was h e a t e d in situ i n flowing 0 a t 500° C (as i n t h e N H c h e m i s o r p t i o n e x p e r i m e n t s ) , a n d cooled t o 3 5 0 ° C i n flowing He. A t o t a l of 10 i n j e c t i o n s of n-hexane was t h e n m a d e for each r u n . I n t h e analyses, b r a n c h e d a n d n o r m a l C p r o d u c t s are l u m p e d as " u n c o n v e r t e d . " T h e " c o n v e r s i o n s " r e p o r t e d b e l o w represent t o t a l c r a c k e d p r o d u c t s ( i n c l u d i n g coke) other t h a n hexanes. O n l y s a t u r a t e d h y d r o c a r b o n s were observed i n t h e r e a c t o r effluent. C h e m i c a l A n a l y s e s . A n a l y s e s of t h e o r i g i n a l , a c i d - e x t r a c t e d , a n d N H r N 0 - e x c h a n g e d samples were k i n d l y d e t e r m i n e d b y t h e N o r t o n C o . 2

3

6

3

Results and Discussion C h e m i c a l A n a l y s i s . T a b l e I shows the c h e m i c a l c o m p o s i t i o n of the samples selected for d e t a i l e d s t u d y . T h e g r a y - w h i t e color of t h e o r i g i n a l H-mordenite was unchanged b y acid extraction; interestingly, the N H N 0 - e x c h a n g e d samples were c r e a m colored. 4

3

I n the a c i d e x t r a c t i o n , greater r e m o v a l of a l u m i n u m c o u l d h a v e been a c h i e v e d i f s m a l l e r c a t a l y s t particles a n d r e p e t i t i v e extractions w i t h H C 1 h a d b e e n e m p l o y e d . T h e progressively smaller loss o n i g n i t i o n i n samples 1, 2, a n d 3 constitutes n e g a t i v e evidence for the " h y d r o x y l n e s t " h y p o t h esis (8).


54.

Table I.

599


THAKUR AND WELLER

Chemical Composition of Various Catalyst

Samples

Sample No. 1

3 Origin

Original HHC1, QN, Mordenite 8 hr Analysis, wt%

Si0

2

AI2O3

Fe 0 Ti0 Na 0 L.O.I.* Residual N H + wt ratio mole ratio 2

3

2


2

HC1, SN, 24 hr

78.55 9.08 0.27. 0.40 0.77 10.65

85.39 3.64 0.09 0.36 0.11 9.66

88.43 2.31 0.09 0.35 0.16 8.56

8.65 14.7

23.5 39.9

38.3 65.1

4

Si0 :Al 0 , 2

a b e

2

3

N H N 0 , NH4NO3, single fourfold 4

3

86.02 9.59 0.23 0.46 0.21 3.21 0.16 8.97 15.3

81.86 9.06 0.13 0.42 0.18 7.65 0.32 9.03 15.4

Single exchange with 0.2ΛΓ NH4NO3, calcined. Fourfold exchange with 0.2M NH4NO3, calcined. Loss on ignition.

T h e N H N 0 exchange h a d t h e desired effect of l o w e r i n g t h e s o d i u m content w h i l e l e a v i n g t h e a l u m i n u m content essentially u n c h a n g e d . The s o d i u m content was o n l y s l i g h t l y l o w e r for the sample exchanged four t i m e s ( N o . 5) t h a n for t h a t exchanged o n l y once ( N o . 4). I n a d d i t i o n , s a m p l e 5 showed a higher r e s i d u a l N H content after final c a l c i n a t i o n a t 5 2 5 ° C . 4

3

4

+

I t m a y be expected t h a t b o t h H C 1 e x t r a c t i o n a n d N H N 0 exchange m i g h t increase t h e effective d i f f u s i v i t y b y r e m o v i n g r e s i d u a l exchangeable cations, n o t a b l y s o d i u m , t h a t p a r t i a l l y b l o c k t h e c r y s t a l l i n e channels. Since s o d i u m t y p i c a l l y poisons c r a c k i n g sites, i t s r e m o v a l s h o u l d also h a v e a beneficial c h e m i c a l effect. 4

3

A n o p p o s i n g effect is possible u n d e r t h e severe c o n d i t i o n s of a single e x t r a c t i o n w i t h H C 1 : some of t h e a l u m i n u m r e m o v e d f r o m t h e c r y s t a l s t r u c t u r e m a y n o t be t r a n s p o r t e d out of t h e c a t a l y s t p a r t i c l e . T h e r e s u l t i n g " a m o r p h o u s a l u m i n a " (after subsequent calcining) r e m a i n i n g i n t h e p a r t i c l e w o u l d cause some r e d u c t i o n i n effective d i f f u s i v i t y . S u c h a m o r phous a l u m i n a has been suggested b y others (17,18). X - R a y D i f f r a c t i o n P a t t e r n s . T a b l e I I gives i n t e r p l a n a r spacings for t h e first 12 p r o m i n e n t lines observed i n the d i f f r a c t i o n p a t t e r n s for samples 1-5 as w e l l as those r e p o r t e d b y D o m i n e a n d Q u o b e x (19) for s y n t h e t i c mordenite. A few p o i n t s s h o u l d be noted. (1) I n agreement w i t h earlier i n v e s t i g a t i o n s (20), n e i t h e r a c i d e x t r a c t i o n n o r exchange caused a n y m a r k e d shift i n l i n e positions or a p p a r e n t crystallinity. (2) T h e most significant change was s u b t l e : i n s y n t h e t i c m o r d e n i t e (19) a n d samples 1, 4, a n d 5, t h e sharpest p e a k o c c u r r e d a t 3.47-3.49 A [indexed as (022)], a n d i n t h e a c i d - e x t r a c t e d samples 2 a n d 3, t h e sharpest peak was 3.37-3.38 A [indexed as (600) ].


600

MOLECULAR SIEVES

Table II.


Synthetic mordenite"

Interplanar Spacings of Various Samples Sample No.

1

13.53 (m) 10.24 9.06 (s) 6.57 (s) 5.80 (w) 4.52 (w) 4.15 4.00 (s) 3.84 3.47 (vs) 3.39 3.22

13.60 (w) 10.16 (w) 9.17 (s) 6.61 (m) 5.83 (w) 4.54 (m) 4.27 (w) 4.00 (s) 3.85 (w) 3.49 (vs) 3.38 (s) 3.23 (s)

2

8

4

δ

13.60 (w) 10.16 (w) 9.17 (s) 6.58 (m) 5.83 (w) 4.54 (m) 4.29 (w) 3.99 (s) 3.82 (w) 3.48 (s) 3.38 (vs) 3.23 (s)

13.77 (w) 10.16 (w) 9.12 (s) 6.56 (m) 5.80 (w) 4.51 (m) 4.27 (w) 3.99 (s) 3.82 (w) 3.47 (s) 3.37 (vs) 3.22 (s)

13.80 (w) 10.28 (w) 9.21 (s) 6.61 (m) 5.85 (w) 4.54 (m) 4.27 (w) 4.00 (s) 3.85 (w) 3.48 (vs) 3.38 (s) 3.23 (s)

13.80 (w) 10.28 (w) 9.17 (s) 6.61 (m) 5.83 (w) 4.54 (m) 4.27 (w) 4.00 (s) 3.85 (w) 3.49 (vs) 3.38 (s) 3.23 (s)

Domine and Quobex (19).

a

(3) T h e d i f f r a c t i o n p a t t e r n s of samples 1 a n d 3 were t h e same after d r y i n g a t 500° C (air, 7 h r ) as after d r y i n g a t 1 1 0 ° C . T h i s m a y be t a k e n as a d d i t i o n a l evidence a g a i n s t t h e existence of a p p r e c i a b l e a m o u n t s of " h y d r o x y l n e s t s " i n s a m p l e 3 after 1 1 0 ° C d r y i n g , since one w o u l d expect s u c h h y d r o x y l nests t o be u n s t a b l e a t 500° C . A c i d i t y . T h e mole r a t i o of c h e m i s o r b e d N H t o t o t a l a l u m i n u m , w h i c h w a s 1 =t 0.25 for t h e o r i g i n a l a n d a c i d - e x t r a c t e d samples (8), w a s s l i g h t l y lower for samples 4 a n d 5 ( T a b l e III). T h i s is p r e s u m a b l y b e cause of t h e r e s i d u a l N H + i n these samples. W i t h sample 5, for e x a m p l e , i f t h e r e s i d u a l N H + is a d d e d t o t h e c h e m i s o r b e d N H , t h e mole r a t i o o f t h e total ( N H + N H ) r e l a t i v e t o t h e t o t a l a l u m i n u m content becomes 0.74. 3

4

4

4

+

3

3

Table III.

N H chemisorbed, 10 moles/gram NH /A1, moles/gramatom Pore vol, ml/gram Bulk density, grams/ml Particle density, grams/ ml Void fraction in packed bed, v / v Particle porosity, v / v Effective diffusivity, 10 cm /sec

Physicochemical Characterization Sample No. 1

2

8

4

5

1.69

0.955

0.373

1.39

1.23

0.85 0.163 0.943

1.21 0.172 0.866

0.75 0.166 0.814

0.72 0.147 0.854

0.64 0.169 0.860

1.492

1.341

1.285

1.466

1.395

0.368 0.244

0.354 0.231

0.367 0.214

0.417 0.208

0.383 0.236

2.4

8.4

6.1

6.7

3

3

a

3

6

3

a 6

2

Measured at 250°C, 11.2 torr. Determined from N uptake at - 195°C, p/p° = 0.25. 2


10.4

54.

THAKUR AND WELLER


601

T o t a l P o r e V o l u m e . A s the d a t a i n T a b l e I I I show, t h e pore v o l u m e was e s s e n t i a l l y u n c h a n g e d b y e i t h e r a c i d e x t r a c t i o n or N H N 0 exchange. T h e single e x c e p t i o n w a s sample 4, for w h i c h t h e pore v o l u m e w a s u n e x p e c t e d l y a n d i n e x p l i c a b l y lower. 4

3

E f f e c t i v e D i f f u s i v i t y . T h e effective d i f f u s i v i t y for N / H e at 2 5 ° C was c a l c u l a t e d f r o m the slope of the s t r a i g h t - l i n e p o r t i o n o b t a i n e d i n t h e h i g h v e l o c i t y region of a " v a n D e e m t e r p l o t " [height of a n e q u i v a l e n t p l a t e vs. i n t e r s t i t i a l v e l o c i t y (14, 15)]. A b i n a r y diffusion coefficient for N - H e of 0.717 c m / s e c w a s c o m p u t e d f r o m R e f . 21, a n d t h e p a r t i t i o n coefficient was t a k e n as the r e c i p r o c a l of the p a r t i c l e p o r o s i t y ( T a b l e I I I ) o n the a s s u m p t i o n t h a t the a d s o r p t i o n of N at 25° C c a n be neglected. T h e c a l c u l a t e d diffusivities are l i s t e d i n T a b l e I I I . 2

2

2


2

A l t h o u g h there is considerable scatter i n the d i f f u s i v i t y values for the t r e a t e d samples, t h e m a j o r effect is a large increase i n d i f f u s i v i t y , r e l a t i v e t o t h e o r i g i n a l H - m o r d e n i t e , o n either a c i d e x t r a c t i o n or NH4NO3 exchange. O n t h i s basis we w o u l d conclude t h a t t h e s o d i u m r a t h e r t h a n the a l u m i n u m content appears t o be t h e factor of greatest i m p o r t a n c e . Catalytic Activity, Selectivity, and Deactivation. T h e product dist r i b u t i o n (in the C 1 - C 5 range) r e m a i n e d r e l a t i v e l y u n c h a n g e d w i t h i n c r e a s i n g n u m b e r of pulses for a n y g i v e n sample. F o r t h e o r i g i n a l H - m o r d e n i t e a n d the N H N 0 - e x c h a n g e d samples, p r o p a n e w a s t h e m a j o r p r o d u c t (45-55 mole % of C 1 - C 5 ) . P r o p a n e a n d isobutane were c o m p a r a b l e i n a m o u n t (35-40 mole % each) for t h e t w o a c i d - e x t r a c t e d samples. The z - C : n - C r a t i o was a b o u t 2 : 1 for samples 1, 4, a n d 5, a n d a b o u t 3 : 1 for samples 2 a n d 3, i n d e p e n d e n t of pulse n u m b e r . 4

4

3

4

A s i n d i c a t e d above, o v e r a l l a c t i v i t y for hexane c r a c k i n g was expressed, for each pulse, i n t e r m s of a n a p p a r e n t first-order r a t e constant, k. T h e a c t i v i t y d e c l i n e d s u b s t a n t i a l l y w i t h i n c r e a s i n g pulse n u m b e r (i.e., w i t h i n creasing t o t a l a m o u n t of n-hexane fed) for a l l samples. A l l a t t e m p t s t o find a " d e a c t i v a t i o n r a t e l a w " r e l a t i n g k t o t o t a l hexane fed f a i l e d . T h e most s a t i s f a c t o r y r e c t i f y i n g p l o t was f o u n d t o be l o g k vs. t h e c u m u l a t i v e a m o u n t of hexane a c t u a l l y c r a c k e d (designated Y) i n a n y g i v e n r u n . T h e p l o t s of l o g k vs. Y are s h o w n i n F i g u r e 1. ( T h e scale for Y is s h o w n at the t o p of F i g u r e 1 for samples 1, 4, a n d 5, a n d at the b o t t o m for samples 2 a n d 3.) T h e d e a c t i v a t i o n b e h a v i o r is w e l l fitted for samples 1, 3, a n d 4, somewhat less w e l l for sample 5, a n d p o o r l y for sample 2. T h e lines were o b t a i n e d b y least-mean-squares fitting of the d a t a . T h i s c o r r e l a t i o n corresponds t o a n e x p o n e n t i a l d e c a y m o d e l , k = kae~ . T h i s expression differs f r o m t h e c o n v e n t i o n a l e x p o n e n t i a l m o d e l often used i n continuous-flow systems (22, 23), k = k e~ , i n t h a t t h e a n a l o g t o t i m e i n a p u l s e d reactor is pulse n u m b e r or i t s e q u i v a l e n t , c u m u l a t i v e feed i n t r o d u c e d . I n our case the c o r r e l a t i n g q u a n t i t y is c u m u l a t i v e feed c o n v e r t e d , Y. I f one assumes t h a t d e a c t i v a t i o n is caused b y coke, t h e a m o u n t of w h i c h is p r o p o r t i o n a l t o hexane a c t u a l l y c o n v e r t e d , t h i s aY

0

at



602

MOLECULAR SIEVES

*

•

20

•

•

40

'

I

60

I

80

ι

ι

100

1

1

120

L

Y, ftl/g Figure 1. Exponential model for deactivation: apparentfirst-orderrate constant vs. cumulative hexane converted.

d e a c t i v a t i o n m o d e l becomes s i m i l a r to t h a t used b y L a m b r e c h t et al. (24) for the f o u l i n g of a r e f o r m i n g c a t a l y s t . T a b l e I V lists t h e v a l u e s of t h e t w o p a r a m e t e r s , k a n d a, i n the ex p o n e n t i a l decay m o d e l for each sample. T o o m u c h credence s h o u l d n o t be p l a c e d i n t h e exact m a g n i t u d e s of these values since i t is k n o w n for a n e x p o n e n t i a l m o d e l t h a t t h e covariance of the t w o p a r a m e t e r s is v e r y h i g h (26). I t is clear, nevertheless, t h a t the " i n i t i a l a c t i v i t y / ' p r e s u m a b l y m e a s u r e d b y k , decreases m a r k e d l y as a l u m i n u m is progressively e x t r a c t e d b y a c i d e x t r a c t i o n (samples 2 a n d 3) b u t increases as s o d i u m is r e m o v e d b y NH4NO3 exchange (samples 4 a n d 5). 0

0

Table IV.

k , grams" a, 10 grams/uliter 1

0

3

a

k =

P a r a m e t e r s i n D e a c t i v a t i o n Equation» Sample No. 1

2

3

4

6

40.2 5.3

7.8 10.0

2.5 7.9

62.5 5.7

84.5 8.1

koe~

aY


54.

THAKUR AND WELDER


603


Summary The major results of this study are consistent with a simple picture of mordenite catalysts. An increase in effective pore diameter, whether by extraction or exchange, will increase the rate of transport of reactant and product molecules to and from the active sites. However, aluminum ions are necessary for catalytic activity; as aluminum is progressively removed by acid extraction, the number of active sites and the initial activity de crease. Coke deposition is harmful in two ways: coke formation as the reaction proceeds will cause a decrease in effective pore diameter and effective diffusivity, and coke deposited on active sites will result in a chemi cal deactivation as well. In the sequence of catalyst samples 1, 4, and 5, there is both an in crease in effective diffusivity and a decrease in sodium content. Both factors operate in the same direction, and it is not possible to say whether the increase in "initial activity" (i.e., fc ) is caused more by an improved physical situation or by decreased chemical poisoning. In the sequence of samples 1, 2, and 3, the decreased number of active sites plays the pre dominant role; the "initial activity" drops sharply in spite of the higher diffusivity. 0

Literature Cited 1. Barrer, R. M., Makki, M. B., Can. J. Chem. (1966) 42, 1481. 2. Belen'kaya, I. M., Dubinin, M. M., Krishtofori, I. I., Izv. Akad. Nauk SSSR, Khim. (1967) 2164. 3. Kranich, W. L., Ma, Y. H., Sand, L. B., Weiss, A. H., Zwiebel, I., Int. Conf. Mol. Sieves (1970) 2, 802. 4. Dubinin, M. M., Fedorova, G. M., Plavnik, D. M., Piguzova, L. I., Prokof'eva, Ε. N., Bull. Acad. Sci. USSR, Chem. Ser., (1968) 11, 2429. 5. Piguzova, L. I., Prokof'eva, Ε. N., Dubinin, M. M., Bursian, N. R., Shavandin, Yu. Α., Kinet. Catal. (1969) 10, 252. 6. Eberly, Jr., P. E., Kimberlin, Jr., C. N., Voorhies, Jr., Α., J. Catal. (1971) 22, 419. 7. Weller, S. W., Brauer, J. M., Preprints, 62nd Annual Α. I. Ch. Ε. Meeting, Washington, D. C., Nov. 1969. 8. Thakur, D., Weller, S. W., J. Catal. (1972) 24, 543. 9. Davis, B. R., Scott, D. S., Preprint 48D, 58th Annual A.I.Ch.E. Meeting, Philadelphia, Pa., Dec. 1965. 10. Eberly, P. E., Ind. Eng. Chem., Fund. (1969) 8 (1), 25. 11. Leffler, A. J., J. Catal. (1966) 5, 22. 12. MacDonald, W. R., Meier, H. L., Habgood, H. W., Preprint, 3rd Canadian Symposium on Catalysis, Edmonton, Alberta, Oct. 1969. 13. Muchhala, M. R., M.S. Thesis, State University of New York at Buffalo, June 1970. 14. Van Deemter, J. J., Zuiderweg, F. J., Klinkenberg, Α., Chem. Eng. Sci. (1956) 5, 271. 15. Thakur, D. K., Ph.D. Dissertation, State University of New York at Buffalo, Aug. 1972.


MOLECULAR SIEVES


604

16. Kokes, R. J., Tobin, H., Emmett, P. H., J. Amer. Chem. Soc. (1955) 77, 5860. 17. Sand, L. B., Conf. Mol. Sieves, Soc. Chem. Ind., London (1967). 18. Satterfield, C. N., Frabetti, A. J., Α. I. Ch. E. J. (1967) 13, 731. 19. Domine, D., Quobex, J., Conf. Mol. Sieves, Soc. Chem. Ind., London (1967). 20. Eberly, P. E., Kimberlin, C. N., Ind. Eng. Chem., Prod. Res. Develop. (1970) 9 (3), 335. 21. Giddings, J. C., Seager, S. L., Ind. Eng. Chem., Fund. (1962) 1, 277. 22. Graven, W. M., Weller, S. W., Peters, D. L., Ind. Eng. Chem., Prod. Res. Develop. (1966) 5, 183. 23. Szépe, S., Levenspiel, Ο., Proc. IV Eur. Fed., Chem. React. Eng. (1970). 24. Lambrecht, G. C., Nussey, C., Froment, G. F., Preprints, 5th European, 2nd International Symposium on Chemical Reaction Engineering, Amsterdam, Elsevier, 1972. 25. Himmelblau, D. M., "Process Analysis by Statistical Methods," Wiley, New York, 1970. RECEIVED November 23, 1972.


Molecular Sieves

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