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ELSEVIER Applied Catalysis A: General 154 (1997) 221-240

PPLIEDC T LYSISA GENERAL

Dealumination of HZSM-5 zeolites: Effect ofsteaming on acidity and aromatization activity

Antonio de Lucas*, P. Canizares, A. Durhn, A. CarreroDepart ment of Chemical Engineering Facult y of Chemistr y Uni versit y of Casti l la-L a M ancha.

Ciudad Real SpainReceived 22 July 1996; received in revised form 16 October 1996; accepted 16 October 1996

Abstract

Conversion of acetone/n-butanol mixtures (1 2 wt/wt) to hydrocarbons over HZSM-5 zeolitespreviously dealuminated by (a) steaming and (b) steaming with subsequent HCl leaching has beenstudied at different conditions. Steamed-treated catalysts largely enhance their deactivationbehavior. Al species are dislodged during the dealumination process, thereby reducing the strengthand density of the acid sites of the parent catalyst. Catalysts dealuminated by means of steam-HCltreatment were found to have the best aromatization stability (the loss in residual aromatizationactivity decreases from 70% in the parent to only 12% in treated catalysts after 5 h of reaction).This treatment produces a decrease in both acid strength as measured by tert-butylaminetemperature programmed desorption (from Td=280C to Td =230(Z) and acid density (from 0.23 to0.13 mmol TBA g ). Medium/strong acid sites ratio close to unity and Brijnstednewis acid sitesratio lower than unity are characteristic in catalysts with the best deactivation behavior.Dealumination treatments generate a secondary pore structure that can contribute to enhancearomatization stability by reducing diffusional limitations.Keywords: Steaming; HCl; Aromatization stability: HZSM-5; Acid sites

1 Introduction

Since the introduction of ZSM-5 by Mobil Oil [I], extensive studies have beenreported and this zeolite has gained increasing importance as a high-potentialcatalyst in a number of commercially important processes [2,3].

* Corresponding author. E-mail: [email protected].

0926-860x/97/ 17.00 0 1997 Elsevier Science B.V. All rights reservedPIZ SO926-860X(96)00367-5


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A . d e L u c a s e t a L / A p p l i e d C a t a ly s is A : G e n e r a l 1 5 4 1 9 9 7 ) 2 2 1 - 2 4 0

Z S M -5 zeo l i t e sh o w s h i g h e r r e s i s t an ce t o d eac t i v a t i o n b y co k e d ep o s i t i o nt h an o t h e r co m m erc i a l z eo l i t e s t y p es , w h i ch h a s b een r e l a t ed t o t h e ab sen ceo f l a rg e cav i t i e s i n t h e p o re s t ru c t u re an d t o i t s l o w co n cen t r a t i o n o f a c i d s i t e s[4,5].

T h e f o r m a t i o n o f c o k e i n h y d r o c a r b o n s r e a c t i o n s o v e r a c i d z e o l i t e c a t a l y s t si n h i b i t f u r th e r r e ac t i o n e i t h e r b y co m p e t i t i v e r em o v a l o f a c i d s i te s o r b y b l o c k i n gacces s o f r e ac t an t m o l ec u l e s to t h e ac i d s i te s . T h e n u m b er an d n a t u re o f t h e ac i ds i te s c l e a r l y i n f l u en ces co k e d ep o s i t i o n an d i t s e f f ec t s.

T h e co n t ro l o f t h e l o ca t i o n o f co k e fo rm a t i o n h a s so m e i n d u s t r ia l in t e r e st . W h i l ei t i s u n l i k e l y t h a t co k e fo rm a t i o n can b e en t i r e l y e l i m i n a t ed i n a l l r e ac t i o n s o fi n t e r e s t , i t ap p ea r s p o s s i b l e t o m ak e so m e ch an g es t o i m p ro v e t h e b eh av i o r .T h e r m a l a n d h y d r o t h e r m a l p r e t r e a t m e n t s o f z e o l it e s a r e u s u a l l y e m p l o y e d t om o d i fy t h e i r a c i d i ty an d ca t a l y ti c a c t i v i ty [6 -9 ] .

T h e i n f lu e n c e o f w a t e r v a p o r t r e a t m e n t a t h i g h t e m p e r a t u r e s o n t h e p r o p e r t ie s o fzeo l i t e i s p a r t i cu l a r ly i n t e r e s t i n g w h i l e p rep a r i n g ca t a l y s ts fo r i n d u s t r i a l ap p l i c a -t ions wi th h ig h ac t iv ity , se l ec t iv i ty and s t ab i li t y . I t is kno w n tha t s t eam ing l eads tod ea l u m i n a t i o n o f t h e zeo l i te s t ru c t u re [ 1 0 ] an d h en ce , t o an i n c rea se i n t h e s t ab i l it yo f t h e s am p l e . L ag o e t a l . [ 1 1] r ep o r t ed t h e g en e ra t i o n o f c a t a l y t ic s i t e s w i t hsp ec i f ic e f f i c ien cy b y m i l d s t e am i n g . D ep en d i n g o n t h e w a t e r v ap o r p re s su re , th et r e a t m e n t t e m p e r a t u r e a n d ti m e , a l u m i n i u m c o u l d b e r e m o v e d f r o m t h e f r a m e w o r kb u t n o t f ro m t h e c ry s t a l l i t e ( so ca l l ed n o n f ram ew o rk a l u m i n i u m o r ex t r a l a t t i c ea l u m i n i u m ) .

M o r e o v e r , K u e h l [ 1 2] s u g g e s t e d t h at a l u m i n i u m r e m o v e d f r o m t h e f r a m e w o r ko c c u p i e d c a t io n i c p o s i t io n s w i t h a n o c t a h e d r al c o o r d i n a t i o n a n d e x h i b i te d L e w i sac i d p ro p e r t i e s . B y 2 7A 1 n . m . r , sp ec t ro sco p y t w o d i f f e r en t s t a te s o f n o n f r am ew o rkh av e b een d e t ec t ed [1 3 , 1 4 ] : o c t ah ed ra l l y an d t e t r ah ed ra l l y co o rd i n a t ed sp ec i e s .H o w e v e r , f e w a t t e m p t s h a v e b e e n m a d e i n o r d e r t o c o r r e l a t e t h e p r e s e n c e o fn o n f r a m e w o r k a l u m i n i u m w i t h a c t iv i ty a n d s t ab i li ty o f z e o li te s .

In t h e co n v e r s i o n o f a l co h o l s o r l o w er o l e f i n s [1 5 , 1 6 ] an d n -b u t an o l / a ce t o n em i x t u re s [ 1 7 ,1 8 ] t o h y d ro ca rb o n s , a ro m a t i c h y d ro ca rb o n s m ay r ep re sen t m o re t h an2 5 -3 0 o f t h e t o t al h y d ro ca rb o n y i e l d w i t h a h i g h se l ec ti v i ty t o w ard s t h efo rm a t i o n o f C 7 an d C s a ro m a t i c s . B o t h t h e ca t a l y t i c a c t i v i t y an d p ro d u c tse l ec t iv i t y i n t h e ab o v e m en t i o n ed r eac t io n s a r e s t ro n g l y a f fec t ed b y p o i so n i n go f s t ro n g e r a c i d s i te s o f t h e zeo l i t e w i t h co k e .

In a p rev i o u s w o rk [ 1 9 ], t h e o n e - s t ep c o n v e r s i o n o f a ce t o n e / n -b u t an o l m i x t u re s( a t c o m p o s i t i o n s e x p e c t e d f r o m f e r m e n t a t i o n p ro c e s s e s ) t o h y d r o c a r b o n s , m a i n l ya ro m a t i c s , u s i n g H Z S M -5 b ased ca t a l y s t s h av e b een s t u d i ed . A r ap i d ca t a l y s td e a c t iv a t io n w a s o b s e r v e d a n d a r o m a t i c h y d r o c a r b o n s y i e l d w a s s t r o n g ly a f f e c te dw i t h t i m e o n s t r e am .

T h e a i m o f t h is p a p e r i s t o i m p r o v e c a t a ly s ts d e a c t iv a t io n b e h a v i o r b y m o d -i f i c a t i o n o f H Z S M - 5 b y b o t h s t e a m i n g a n d s t e a m i n g w i t h s u b s e q u e n t H C 1l each i n g . T h e e f f ec t o f t h e se p re t r ea t m en t s o n ac i d i t y an d ex t en t o f a ro m a t i za t i o nreac t ions i s a l so s tud ied .


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A . d e L u c a s e t a l . / A p p l i e d C a t a l ys i s A : G e n e r al 1 5 4 1 9 9 7 ) 2 2 1 - 2 4 0 3

2 Exper imenta l2 1 Catalyst preparation

N a Z S M - 5 ( S i / A l = 4 5 ) w a s s y n t h e s i z e d a c c o r d i n g t o t h e m e t h o d d e s c r ib e de l s e w h e r e [ 2 0 ] u s in g e th a n o l a s t e m p la t e . X - r a y d i f f r a c t i o n a s d e t e r m in e d u s in ga Cuo t r a d i a ti o n a n d a n i c k e l f il t e r b y m e a n s o f a P h i l ip s P W /1 7 0 0 d i f f r a c to m e te rc o n f i r m e d t h a t t h e m a te r i a l s w e r e ZS M - 5 a n d p r e s e n t e d 1 0 0 c r y s t a l li n i t y .

T h e p a r e n t c a t a ly s t ( H Z S M 5 - P ) w a s o b t a i n e d a f t er b in d i n g a n d a c t iv a t in g t h eN a ZS M - 5 a c c o r d in g t o D u rf i n [ 1 9] . N a ZS M - 5 , a s s y n th e s i z e d , w a s f ir s t ly b o u n dw i t h s o d i u m m o n t m o r i l l o n i te (3 0 w t ) to g i v e t h e c at a l y st m e c h a n i c a l r e s is t a n ce .Th e n , i t w a s c a l c in a t e d a t 5 5 0 C f o r 5 h a n d e x c h a n g e d w i th H C1 (0 .6 N ) a t 2 5 C .F in a l l y t h e c a t a ly s t w a s a c t i v a t e d at 5 5 0 C f o r 1 4 h . Th i s m e th o d w a s s e l e c t e ds in c e t h e c a t a ly s t p r e p a r e d i n t h i s w a y p r e s e n t e d t h e b e s t d e a c t i v a t i o n b e h a v io r .2 2 Apparatus and procedure

Re a c t i o n s w e r e c a r r i e d o u t i n a p lu g c o n t i n u o u s f l o w r e a c to r c o n s i s t i n g o f a3 6 c m s t a in l e s s s te e l tu b e ( 1 i n c h d i a m e te r ) s u r r o u n d e d b y t h r e e h e a t i n g z o n e s , o n eo f t h e m a c t i n g a s p r e h e a t e r . T h e t e m p e r a t u r e s o f e a c h z o n e w e r e c o n t r o l l e din d e p e n d e n t l y w i th in - 4 -1 C b y i n d iv id u a l c o n t r o l l e r s. A c o ld t r a p ( 0 C) d o w nt h e r e a c t o r w a s u s e d t o c o l l e c t w a t e r a n d g a s o l i n e -r a n g e p r o d u ct s . T h e r e m a i n i n gp r o d u c t s p a s s b y t h e b a c k - p r e s s u r e r e g u l a t o r a n d w e r e a n a l y z e d b y a n o n - l i n ec h r o m a t o g r a p h ( H e w l e t t - P a c k a r d S e r ie s I I ) f it te d w i t h a P o r a p l o t c o l u m n a t t a c h e dto a t h e r m ic c o n d u c t i v i t y d e t ec to r . Th e l i q u id p r o d u c t ( c o l l e c t e d in t h e c o ld t r a p )w a s p e r i o d i c a l l y d i s c h a r g e d a n d a n a ly z e d a f t e r s e p a r a t in g t h e a q u e o u s f r a c t i o n b ym e a n s o f g a s c h r o m a t o g r a p h y a n d u s in g a P O N A c o l u m n c o n n e c t e d to a f la m eio n i c d e t e cto r . Re a c t i o n w a s c a r r i e d o u t u s in g t h e f o l lo w in g c o n d i t i o n s : T = 4 0 0 C ,W H S V - I = 0 . 5 h g c a t g~eeldh - 1 ) a n d P = 1 atm . U n d e r t h e s e c o n d i t i o n s , c o n v e r s io nw a s a lw a y s 1 0 0 .

S t e a m - t r e a t e d c a t a ly s t s w e r e p r e p a r e d a s f o l l o w s : s a m p le s w e r e p l a c e d i n t h er e a c t o r a n d h e a te d u p t o t h e p r e t r e a t m e n t t e m p e r a t u r e ( 4 5 0 - 5 0 0 - 5 5 0 C ) i n a fl o wo f N 2 . W a t e r w a s a d d e d b y m e a n s o f a p is t o n p u m p , k e e p i n g W H S V - ~ ( h ) i n 0 . 3 -0 .8 d u r i n g a t r e a t m e n t t im e o f 4 - 1 2 h . C a t a ly s t s w e r e q u e n c h e d t o r o o m t e m -p e r a tu r e a n d s a m p le s w e r e t a k e n i n o r d e r t o o b t a in c h a r a c t e r i z a t i o n p a r a m e te r sb e f o r e r e a c t i o n . N o m e n c l a t u r e u s e d i n t h e c a t a l y s t s i s s t a t e d a s H Z S M 5 - T e m -p e r a t u r e - W H S V - 1 ( e . g. H Z S M 5 - 5 5 0 - 0 5 i s a c a ta l y s t s t e a m e d a t 55 0 C a n dW H S V - 1 o f 0. 5 h d u r i n g th e s t e a m i n g p r e t re a t m e n t ) .

T h e s t e a m - H C 1 t r e a te d z e o li t es w e r e o b t a i n e d u s in g H Z S M - 5 s t e a m e d a t 5 5 0 Cw i t h W H S V - ~ = 0 . 5 - 1 -1cat gwaterh f o r 4 h, f o l l o w e d b y l e a c h in g i n H C1 ( 0 .6 - 1 .2 N )a t 8 0 - 1 0 0 C f o r 1 2 h u s in g 1 5 m l a c id s o l u t i o n / g z e o l i t e ; f i n a l l y s a m p le s w e r ew a s h e d w i t h w a t er . F i n a l c at a l y st s w e r e n a m e d f r o m H Z S M 5 ( A 1) t o H Z S M 5 ( A 4 )a c c o r d in g t o i n c r e a s in g o r d e r i n t h e s e v e r i t y d u r in g t h e H C1 t r e a tm e n t .


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4 A . d e L u c a s e t a l . / A p p l i e d C a t a ly s is A : G e n e r a l 1 5 4 1 9 9 7 ) 2 2 1 - 2 4 0

2.3 . Cata lys t chara c ter i za t ionD e a l u m i n a t e d s a m p l e s w e r e c h a r a c t e r i z e d b y X - r a y p o w d e r d i f fr a c t io n i n o r d e r

to c o n f i r m th e r e s e r v a t i o n o f t h e c r y s t a l l i n i t y o f t h e z e o l i t e a f t e r t r e a tm e n t s .S p e c i f i c s u r f a c e m e a s u r e m e n t s w e r e m a d e u s i n g B E T m e t h o d .

To t a l s u r f a c e a c id i t y w a s d e t e r m in e d b y a d s o r p t i o n a n d l a t e r d e s o r p t i o n o f te r t -b u t y l a m i n e ( T B A ) i n a P e r k i n - E l m e r T G A - 7 t h e r m o g r a v i m e t r i c a n a l y z e r a c c o r d -i n g t o r e f e r e n c e [ 2 1] . T h e p e a k c o r r e s p o n d i n g t o t h e c h e m i c a l l y d e s o r b e d T B A i sr e l a t e d t o t h e a c id s i te s a n d i s q u a n t i f i e d in t e r m s o f m m o l T B A ] g c a t a l y s t T h ete m p e r a tu r e a t w h ic h t h i s p e a k a p p e a r s i s r e l a t e d t o t h e s t r e n g th o f t h e a c id s i te s .A l t h o u g h t - b u t y l a m i n e m a y p a r t i a l l y r e a c t t o d e s o r b a s a m m o n i a a n d a l k e n ep r o d u c t s a t a b o u t 4 0 0 C [ 2 2 ,2 3 ] , it w a s c o m p le t e ly d e s o r b e d a t 3 0 0 C u n d e r o u re x p e r im e n ta l c o n d i t i o n s . D e s o r p t i o n t e m p e r a tu r e s a r e s t r o n g ly a f f e c t e d b y e x p e r i -m e n t a l c o n d i t io n s a n d t y p e o f m a t e r i a l ( H Z S M - 5 b o u n d w i t h 3 5 w t o f s o d i u mm o n tm o r i l l o n i t e i n t h is s t u d y ) [ 22 ] .

A n im p o r t a n t i n f o r m a t io n c a n b e o b t a in e d a b o u t t h e a c id i t y s t r e n g th d i st r i b u ti o no f t h e z e o l it e b y s t u d y i n g c h a n g e s i n t h e v i b r at io n s o f p y r i d i n e p r e v i o u s l y a d s o r b e din t h e r e g io n o f 1 6 0 0 c m - 1 t o 1 4 0 0 c m - 1 [ 24 ] . Th u s , i n f r a r e d s p e c t r a a t d i f f e r e n ts a t u ra t io n t e m p e r a t u r e s o f p y r i d i n e w e r e r e c o r d e d u s i n g a P e r k i n E l m e r 1 6 PC - F T -I R . P r i o r to a d s o r p t i o n , t h e s a m p le s w e r e p r e s s e d i n to t h in w a f e r s , e v a c u a t e d u n d e rv a c u u m a t 4 0 0 C f o r 1 6 h , c o o l e d t o r o o m t e m p e r a tu r e a n d s a tu r a t e d w i th p y r id in e .

E x t e r n a l s u r f a c e a c i d it y o f c a ta l y s ts w a s m e a s u r e d a s t h e m o n o l a y e r m e t h y l e n eb lu e ( M B) a d s o r p t i o n c a p a c i t y [ 2 5 ]. Th r e e s t o c k s o lu t i o n s o f m e th y l e n e b lu e( c o n c e n t r a ti o n r a n g e 2 . 7 6 1 0 - 4 1 . 2 3 1 0 - 3 m o l d m - 3 ) w e r e p r e p a re d b y w e i g h t.S a m p le s o f t h e z e o l i t e ( ca . 1 0 0 m g ) w e r e e q u i l i b r a t e d w i th a q u e o u s s o lu t i o n s o f t h ed y e f o r 2 4 h a t r o o m t e m p e r a tu r e . Th e s u p e r n a t a n t s o lu t i o n w a s d i l u t e d a n d it sa b s o r b a n c e a t 6 6 4 n m w a s r e c o r d e d b y a s p e c t r o p h o t o m e t e r ( P e rk i n E l m e rL a m b d a 3 B ). M o n o l a y e r a d so r p t io n w a s e s t i m a t e d f r o m L a n g m u i r i s o th e r m p lo t .

A 1 c o n t e n t o f c at a l y st s w a s d e t e r m i n e d b y m e a n s o f a T h e r m o J a r re l A s h A t o m i cA b s o r p t i o n S p e c t r o p h o t o m e t r y at 3 09 .3 n m a n d u s i n g a N 2 0 - C z H 2 f la m e .

T h e c o k e c o n t e n t w a s d e t e r m i n e d b y c o m b u s t i o n i n a th e r m o g r a v i m e t r i ca n a l y z e r u s i n g a s t r e am o f 2 0 c m 3 o f o x y g e n a n d i n c r e a si n g t e m p e r a t u r e f r o m1 5 0 t o 7 0 0 C a t 1 0 C m in - 1 . T h e w e ig h t l o s s b e tw e e n 3 5 0 a n d 7 0 0 C w a sa t t r ibu ted to coke .

3 R e s u l t s a n d d i s c u s s io n

3 .1 . R ea c t i o n w i t h t h e p a ren t ca ta l y st H Z S M 5 -PRe a c t i o n w a s c a r r i e d o u t u s in g t h e f o l l o w in g c o n d i t i o n s : T= 4 0 0 C ,W H S V - I = 0 . 5 h -1 -1gcat greedh ) and P = 1 a tm . U n d e r t h e s e c o n d i ti o n s , c o n v e r s io nw a s a lw a y s 1 0 0 , b u t p r o d u c t d is t r i b u ti o n s u f f e r e d im p o r t a n t c h a n g e s w i th t im e


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A. d e u ca s e t al . /A pp l ie d Cata lys is A: Genera l 154 1997) 221-2 40 225Table 1Hydrocarbons distribution in the catalytic conversion of n-butanol/acetone mixtures on HZSM5-P. (T=400 C;WHSV-~=0.5 h; P=I atm; Conversion=100 )Time on stream (h) 0.5 1.5 2.5 3.5 4.5Hydrocarbons ( wt)Methane 0.1 0.1 - - -Etha ne 0.6 0.5 0.3 - -Prop ane 15.1 15.7 12.1 9.6 6.7/-Butane 11.6 11.5 10.4 8.4 6.1n-Butane 8.5 7.5 7.3 7.1 6.6Total paraffins 35.9 35.3 30.2 24.9 19.5

Ethylene 1.3 1.3 1.9 2.3 2.1Propy lene 3.8 3.7 5.1 6.7 9.81-Butene 3.6 3.7 5.0 6.9 9,5t-2-Butene 0.3 0.5 1.4 1.6 2,3c-2-Butene 0.2 0.4 1.2 1.5 2,2Total ole fins 9.2 9.6 14.6 19.0 26.1

C~ paraff ins 7.0 6.5 6.3 4.5 3.7C5 ole fins 0.7 3.6 5.6 7.3 10.1C 6 C 9 non-a romati c 5.6 9.8 14.7 21.9 25.1C5-C9 non -aromatic 13.3 19.9 26.6 33.7 38.9

Benzene 2.8 1.0 0.5 0.2 0.2Toluene 14.8 9.1 5.4 3.3 2.0Ethylbenzene 2.0 2.0 1.5 0.7 0.5m-p-xylenes 13.3 12.9 11.4 8.8 5.6o-xylenes 1.2 0.7 0.6 0.4 0.4n-propylbenzene 0.3 0.2 0.3 0.2 0.2Trimethylbenzenes 5.0 5.9 6.1 5.0 3.8Clo + arom atics 2.2 3.4 2.8 3.8 2.8C 6 C 1 o aroma tic 41.6 35.2 28.6 22.4 15.5

o n s t r e a m ; a r o m a t i c h y d r o c a r b o n s a n d g a s e o u s p a r a f f i n s d e c r e a s e d , w h e r e a so l e f i n s a n d l i q u i d p a r a f f i n s i n c r e a s e d a s a c o n s e q u e n c e o f c a t a l y s t d e a c t i v a t i o nb y c o k i n g . D e t a i l e d e v o l u t i o n o f h y d r o c a r b o n s d i st r ib u t io n i s s u m m a r i z ed i nT a b l e 1

T h e a r o m a t i z a t io n a c t i v i ty o f t h e c a t a l y st i s s tr o n g l y a f f e c t e d b y t h e d e c r e a s e i nt h e n u m b e r o f a c t i v e s i t e s a v a i l a b l e w i t h t i m e o n s t r e a m . H o w e v e r , e v e n w h e nm a n y a c i d s i t e s a re b l o c k e d a f t e r 5 h ) t h e n - b u t a n o l / a c e t o n e m i x t u r e s a rec o m p l e t e l y c o n v e r t e d i n t o h y d r o c a r b o n s , s u g g e s t i n g t h a t t h e p r i m a r y r e a c t i o n si n v o l v e d i n t h e c o n v e r s i o n o f th e m i x t u r e t o lo w e r h y d r o c a r b o n s o c c u r o n w e a k e rs i te s , w h i l e t h e d e h y d r o c y c l i z a t i o n r e a c t i o n s l e a d i n g t o t h e fo r m a t i o n o f a r o m a t i cr e q u i r e s s t r o n g a c i d s i t e s , w h i c h i s c o n s i s t e n t w i t h e a r l i e r o b s e r v a t i o n s [ 1 6 ] .


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226 A . d e L u c a s e t a l . / A p p l i e d C a t a l y s i s A : G e n e r a l 1 5 4 1 9 9 7 ) 2 2 1 - 2 4 0AROM ATI C HYDROCARBONS D I STRIBUTI ON ( w t )

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Fig. 1. Aromatic hydrocarbons distribution in HZSM5-P during the conversion of n-butanol/acetone mixtures tohydrocarbons. (Reaction conditions: T=400C, WH SV l =0.5 h, P= I atm). (o benzene; O toluene; C8 AR; C9 AR; ~ Clo AR).

Th e d i s t r i b u t i o n o f a r o m a t i c p r o d u c e d i n t h e r e a c t i o n s h o w a d e c r e a s e i n t h ec o n c e n t r a t i o n o f b e n z e n e a n d t o l u e n e a n d a n i n c r e a s e in t h a t o f h i g h e r a r o m a t i c sw i th t im e o n s t r e a m , d u e t o t h e a v a i l a b i l it y o f s t r o n g s i te s ( F ig . 1 ). Th i s i s e x p e c t e dto b e d u e t o t h e d e a lk y l a t i o n o f t h e h ig h e r a r o m a t i c s o n s t r o n g e r s i t e s .

I n t h i s w o r k , t h e a r o m a t i c h y d r o c a r b o n s f r a c t i o n w i l l b e c o n s id e r e d a s ar e f e r e n c e to s t u d y t h e d e a c t i v a t i o n o f c a t a ly s t s , s i n c e t h e y a r e t h e m a in f r a c t i o nin t h e l iq u id h y d r o c a r b o n p r o d u c t a n d a r e im p o r t a n t p r e c u r s o r s o f c o k e .3 2 D ea l u m i n a t i o n b y s t ea m tr ea t m en t3 2 1 In f lu en ce o f t em p era t u re a n d WHSV 1 (gcat -1water h - 1S e v e r a l c a t a ly s t s w e r e f i r s t p r e p a r e d b y s t e a m in g t h e p a r e n t c a t a ly s t a t d i f f e r e n tc o n d i t io n s o f t e m p e r a t u r e a n d W H S V - 1 d u r i n g t h e p r e t r e a t m e n t ( d e ta i ls i nTab le 2 ) .

I n i t i a l a r o m a t i c h y d r o c a r b o n s y i e l d ( d e f i n e d a s a r o m a t i c h y d r o c a r b o n s y i e l d( w t ) a t t im e o n s t r e a m = 0 .5 h ) v e r s u s s t e a m in g t e m p e r a tu r e i s s h o w n in F ig . 2,f o r c a t a ly s t s s t e a m e d a t d i f f e r e n t W H S V - ~ r a t i o s.

A d e c r e a s e i n i n i t i a l a r o m a t i z a t i o n a c t i v i t y c a n b e o b s e r v e d w h e n t h e r e i s a nin c r e a s e i n t e m p e r a tu r e d u r in g t h e s t e a m in g p r e t r e a tm e n t . Th i s i s d u e t o t h ep r e s e n c e o f s i g n i f i ca n t a m o u n t s o f A 1 a to m s i n t h e z e o l i t e s u r f a c e a s a c o n s e q u e n c eo f t h e m ig r a t i o n o f re t i c u l a r A 1 i n t h e t e t r a h e d r a l p o s i ti o n s t o e x t e m a l z o n e s o f th ez e o l i t i c s t ru c tu r e , t h e r e f o r e o c c u p y in g c a t i o n i c p o s i t io n s . I n o th e r w o r d s , A 1 a to m sin a t e t r a h e d r a l p o s i t i o n a r e d i s p l a c e d t o a n o c t a h e d r a l o n e , [ 2 6 ] , d e s t r o y in g


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F i g. 6 . A c i d s t re n g t h d i s tr i bu t i on i n a ) H Z S M 5 - P , b ) H Z S M 5 - 5 5 0 4 ) 5 , c ) H Z S M 5 - 5 5 0 - 0 3 a n d d ) H Z S M 5 -4 5 0 - 0 3 . D L e w i s s i te s B r 6 n s t e d s i te s )

T h e h i g h d en s i t y o f w eak s i t e s i n a l l t h e s t e am ed ca t a l y s t s w o u l d ex p l a i n t h el o w er i n it i a l a ro m a t i c h y d ro ca rb o n s y i e l d fo u n d w h en u sed i n reac t i o n s i n cea ro m a t i za t i o n r eac t io n s r eq u i r e t h e p re sen c e o f s t ro n g s i te s [2 9 ] .

O n t h e o t h e r h an d t h e w eak e r s t r en g t h o f a c t i v e s it e s i n s t e am ed ca ta l y s tsco m p are d to t h e p a ren t c a t a ly s t sh o u l d im p l y a d ec rea se o f t h e s e l ec t iv i t ie s to w ard s


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l i g h t a r o m a t i c h y d r o c a r b o n s ( b e n z e n e + t o lu e n e ) i n th e f i rs t h o u r o f r e a c t i o n ,w h i c h a r e f o r m e d t h r o u g h o u t d e h y d r o c y c l i z a t i o n a n d d e a l k y l a t io n r e a c ti o n s t h a tt a k e p l a c e o n s t r o n g s it e s. E f f e c t i v e ly , a d e c r e a s e i n t h i s p a r a m e te r ( f r o m 4 2 .3 i nH Z S M 5 - P t o 3 4 .4 i n H Z S M - 5 5 0 - 0 5 ) i s o b s er v ed .

To s u m i t u p , i t c a n b e c o n c lu d e d t h a t s t e a m e d c a t a ly s t s w i th ab e t t e r a r o m a t i z a t i o n s t a b i l i t y p r e s e n t a c o n s id e r a b l e l o w e r a c id i t y t h a n p a r e n tc a t a l y s t (0 .1 3 a g a i n s t 0. 23 m m o l T B A g -1 r e s p e c ti v e l y ) a n d a m e d i u m / s t r o n gs i te s r a ti o c l o s e t o 1 , w i th t h e n u m b e r o f Le w i s s i te s h ig h e r t h a n t h a t o f B r tn s t e do n e s .3 2 2 Influence of initial acidity o f the parent catalyst

I n o r d e r t o s t u d y t h e i n f l u e n c e o f t h i s p a r a m e te r , tw o a d d i t i o n a l c a t a ly s t s w e r ep r e p a r ed . O n e o f t h e m c o n s i s t e d o f a z e o l it e Z S M - 5 i n it s s o d i u m f o r m s t e a m e d i nt h e a b o v e s e l e c t e d c o n d i t io n s ( T - 5 5 0 C , t = 4 h , W H S V - 1 - - 0 . 5 h) a n d l a te l yb o u n d a n d e x c h a n g e d w i th H C1 0 .6 N a s d e s c r i b e d a b o v e . I n t h e s e c o n d c a s e ,t h e c a t a l y s t in i t s a c i d f o r m w a s d e a l u m i n a t e d , a l t h o u g h t h e c o n c e n t r a t i o n o f H C Iu s e d i n t h e i o n - e x c h a n g e s t e p w a s i n c r e a s e d t o 1 .2 N .I n F ig . 7 , a ~ v e r s u s t im e o n s t r e a m f o r t h e tw o c a t a ly s t s s e l e c t e d a b o v e , t h ep a r e n t a n d H ZS M 5 - 5 5 0 - 0 5 , i s s h o w n . I t c a n b e n o t e d t h a t t h e c a t a ly s t d e a l -u m i n a t e d i n i ts s o d i u m f o r m h a s t h e s a m e b e h a v i o r t h a n t h e p a r e n t c a ta l y st .

I n d e e d , h y d r o l y t i c re m o v a l o f A 1 a to m s i s o n l y p o s s ib l e f r o m d e c a t i o n i z e dl a t t i c e s i t e s w h ic h a r e f o r m e d b y z e o l i t e i o n - e x c h a n g e w i th H CI :

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a n d n o t f r o m N a + c o n t a in in g s i te s o f t h e t y p e :N a -Si .....O .... A I ' - O - S i -

I t c a n b e a s s u m e d t h a t w e a k e n i n g o f t h e A 1 0 b o n d s t r en g t h t h r o u g h p ol a r iz a t io nb y t h e p r o to n f a c i l i ta t e s A 1 0 b o n d s p l i tt i n g i n t h e H - f o r m o f t h e z e o l it e s . To t ala c id i t y a n d a c id s t r e n g th Ta) r e m a i n s u n c h a n g e d w h e n d e a l u m i n a t i o n i s m a d e o nth e s o d iu m z e o l i t e ( Ta b l e 2 ) , c o n f i r m in g th e a b o v e o b s e r v a t i o n s . F u r th e r m o r e , n od i s l o d g e m e n t o f f r a m e w o r k a l u m i n i u m i s a c h i e v e d d u r in g s t e a m i n g u n d e r t h e s ec o n d i t i o n s .

M o r e o v e r , a n i n c r e a s e i n t h e c o n c e n t r a t i o n o f H C1 f r o m 0 .6 N to 1 .2 N d o e s n o tp r o d u c e a n im p r o v e m e n t i n c a t a ly s ts d e a c t i v a t i o n b e h a v io r, b u t a d e c r e a s e i n i n it i ala r o m a t i c h y d r o c a r b o n s y i e ld ( f r o m 2 8 .4 t o 2 0 .3 w t ) , p r o b a b ly d u e to a p a r t ia l l o s so f c r y s t a l l i n i t y a s d e t e c t e d b y X - r a y d i f f r a c t i o n ( Ta b l e 2 ).3 2 3 Influence o f steaming time

I n o r d e r t o s t u d y t h e e ff e c t o f th e s t e a m i n g t i m e o n c a t a l y st d e a c t i v at i o n t w oa d d i t i o n a l c a t a l y s t s w e r e p r e p a r e d b y d e a l u m i n a t i o n u n d e r m i l d c o n d i t i o n s( T - 4 5 0 C ; W H S V - 1 - - 0 . 5 h ) a n d i n c re a s i n g t h e d e a l u m i n a t i o n t i m e f ro m 4 to12 h (Tab le 2) .

I n f l u e n c e o f s t e a m i n g t i m e o n a r o m a t i c h y d r o c a r b o n s y i e l d e v o l u t i o n is s h o w n i nF ig . 8 . Th i s p a r a m e te r h a s n o s i g n i f ic a n t e f f e c t o n a r o m a t i z a t i o n s t a b i li t y in t h es tu d i e d r a n g e . W h e n t h e s t e a m in g t im e i n c r e a s e s t o 8 a n d 1 2 h , t h e c a t a ly s td e a c t i v a t i o n b e h a v io r i s o n ly s l i g h t l y b e t te r .

A l t h o u g h s t e a m i n g t i m e d o e s n o t p r o d u c e a n y s e n s i ti v e c h a n g e i n a c i d s t r e ng t h ,a p r o g r e s s iv e d e c r e a s e i n t h e n u m b e r o f a c id s i t es is r e m a r k a b l e a s s t e a m in g t im e

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i n c r e a s e s ( T a b l e 2 ) . N e v e r t h e l e s s , t h i s c h a n g e i s n o t v e r y i m p o r t a n t i n th e s t u d i e dr a n g e o f t i m e s o f p r e t r e a t m e n t a n d l e a d s t o s i m i l a r s t a b i l it y i n a ll t h e c a t a l y st s . I tw a s c o n c l u d e d t h a t s te a m i n g t e m p e r a t u r e i s th e m o s t i m p o r t a n t v a r ia b l e t o b ec o n s i d e r e d . T h e i n c r e a s i n g e f f e c t o f d e a l u m i n a t i o n w i t h r i s i n g t e m p e r a t u r e w a sa l s o o b s e r v e d b y L u t z e t a l. ( 1 9 9 4 ) w h e n s t u d y i n g t h e s t a b il i ty o f h i g h - s i l i c a Y - t y p ez e o l i t e s [ 3 0 ] .3.3. Dealumination with stea m HCl treatment

I n t h i s s e c t i o n , t h e p a r e n t c a t a l y s t w a s f i r s t l y d e a l u m i n a t e d b y s t e a m i n g u n d e rs e l ec t e d c o n di t io n s ( T - - 55 0 C ; W H S V - I = 0 . 5 h ) , t o g e th e r w i t h a s u b s e q u e n te x t r a c t i o n w i t h H C 1 s o l u t i o n s ( T a b l e 3 ) .

I n i t ia l a r o m a t i c h y d r o c a r b o n s y i e l d ( F i g. 9 a ) r e m a i n s c o n s t a n t f o r a l l th ec a t a l y s t s t e s t e d i n t h i s s e c t i o n . M o r e o v e r , t h i s t r e a t m e n t d o e s n o t p r o d u c e a n yc h a n g e i n t h e c a t a ly t i c a c t iv i t y o f d e a l u m i n a t e d c a t a ly s t s c o m p a r e d w i t h t h e p a r e n tc a t a l y s t. R e s i d u a l a r o m a t i z a t i o n a c t i v i t y ( F i g . 9 b ) s h o w t h a t t h is g r o u p o f c a t a l y s tsp r e s e n t s a l a r g e l y e n h a n c e d a r o m a t i z a t i o n s t a bi li t y. I n t h is c a s e , t h e l o s s o f i n it i ala r o m a t i c h y d r o c a r b o n s y i e l d i s a r o u n d 12 ( in H Z S M 5 ( A 2 ) ) w h e r e a s s t e a m e dc a t a ly s t i n t h e b e s t c o n d i t io n s ( H Z S M 5 - 5 5 0 - 0 5 ) a n d p a r e n t c a ta l y s t p r e s e n t a l o s so f 3 0 a n d 7 0 , r e s p e c t i v e l y , a f t e r 5 h o n s t r e a m .

R e g a r d i n g z e o l it e s t r u c tu r e , a l u m i n i u m l o ss i n d u c e d i n t h is g r o u p o f c a t al y s ts( T a b l e 3 ) n e v e r e x c e e d e d 2 0 a n d n o s i g n i f i c a n t d e s t r u c t i o n o f t h e l a t ti c e w a so b s e r v e d d u r i n g d e a l u m i n a t i o n u n d e r t h e s e c o n d i t i o n s .T a b l e 3 r ep o r t s to t a l su r f a c e a c id i ty a n d T B A d e s o r p t i o n t e m p e r a t u re . S t e a m -H C 1 tr e a t m e n t c a u s e s a d e c r e a s e i n t h e n u m b e r ( f r o m 0 . 2 3 t o 0 . 1 3 m m o l T B A g~-alt)a n d s t r e n g t h o f a c i d s i te s ( f r o m T d = 2 8 0 t o 2 3 0 C ) , F i g . 1 0 a. N e v e r t h e l e s s , t h e n e wa c i d s i te s a p p e a r i n g a t 2 3 0 C h a v e e n o u g h s t r e n g t h s o t h a t a r o m a t i z a t i o n r e a c t i o n sc a n t a k e p l a c e o v e r t h e m . T h e l e s s er n u m b e r o f s t ro n g s i t es c o n f i r m s t h e d e c r e a s ei n c o k e f o r m a t i o n a n d t h e r e f o r e a b e s t d e a c t i v a t i o n b e h a v i o r i s s e e n w h e nc o m p a r e d w i t h t h e p a r e n t c a t a l y s t .

A c i d s t r e n g th d is t r ib u t i o n i n H Z S M 5 - P a n d H Z S M 5 ( A 2 ) i s d e p i c t e d i n F i g . 1 0 b.D e a l u m i n a t i o n c a t a l y s ts p r e s e n t a l a r g e r n u m b e r o f L e w i s s i te s t h a n t h a t o fB r r n s t e d s i t e s i n a l l t h e t e m p e r a t u r e i n t e r v a l s . T h i s f a c t a l s o h a p p e n e d w h e nt h e p a r e n t c a t al y s t w a s d e a l u m i n a t e d b y s t e a m i n g u n d e r t h e b e s t c o n d i t io n s . I nc o n t r a s t t o s t e a m e d c a ta l y st s , m e d i u m a n d s t r o n g L e w i s s it e s a r e p r e d o m i n a n t i nH Z S M 5 ( A 2 ) , w h e r e t h e r at i o o f m e d i u m / s t r o n g s i te s i s v e r y c l o s e to u n i ty , a n dw e a k s i te s a r e l o w e r i n n u m b e r . T h u s , s e l e ct i v it i es t o l i g th a r o m a t i c h y d r o c a r b o n si n t h e fi rs t h o u r o f r e a c t i o n (4 7 . 7 i n H Z S M 5 ( A 2 ) ) a r e in th e s a m e o r d e r o fm a g n i t u d e t h a n t h o s e o b t a in e d w i th H Z S M 5 - P ( 4 2 .3 ) .

M o r e o v e r , s t e a m i n g p r o d u c e s a n e n r i c h m e n t o f A 1 a t th e e x t e r n a l s u r fa c e ( f r o m3 . 9 t o 4 . 6 m m o l M B g c ~ . , t h a t i s a n i n c r e a s e f r o m 1 6 . 9 t o 3 5 . 4 o f t o t a l a c i d i t y ),b u t a c i d l e a c h i n g r e m o v e s p a r t o f t h e s e a l u m i n i u m s p e c i e s p r e fe r e n t i a ll y a t t h ee x t e rn a l su r f a c e ( 1. 6 m m o l M B g - 1 i n H Z S M 5 ( A 2 ) , 1 2 .3 o f t o ta l a c i di ty ) .


18/20

2 3 8 A . d e L u c a s e t a l . / A p p l i e d C a t a ly s is A : G e n e r a l 1 5 4 1 9 9 7 ) 2 2 1 - 2 4 0

0 . 0

- o . 1z

- 0 . 2

l.-o - 0 . 3

- 0 . 4. . . . H Z S M 5 A 2 )

~ o ~ o ~ o ~ o ~ oT E M P E R A T U R E ( C )

a

4

Al a .u . ) zx a .u . )1.6 0 .8

1 .2 0 . 6

l. 4 0 . 2 1 5 0 2 0 0 2 0 0 - 4 0 0 > 4 0 01 5 0 - 2 0 0 2 0 0 - 4 0 0 > 4 0 0D e s o r p t i o n t e m p e r a t u r e * C ) D e s o r p t io n t e m p e r a t u r e * C )i i

bF i g . 10 . a ) D T G c u r v e s c o r r e s p o n d i n g to d e s o r p t i o n o f T B A i n H Z S M 5 - P a n d H Z S M 5 A 2 ) . b ) A c i d s t re n g t hd i s t r i b u t i o n i n i ) H Z S M 5 - P , i i ) H Z S M 5 A 2 ) . [ ] L e w i s s i t e s , B r 6 n s t e d s i t e s ) .

I n a d d i t io n , s p e c i f i c s u r fa c e , m e s o p o r e a r e a a n d c o k e d e p o s i t e d a f t e r 5 h i nH Z S M 5 - 5 5 0 - 0 5 , H Z S M 5 A 2 ) a n d H Z S M 5 - P c a ta l ys ts a re re p o r te d in T a bl e 4 .A f t e r d e a l u m i n a t i o n b y s t e a m i n g a n d s t e a m - H C 1 t r e a t m e n t , t h e m e s o p o r e a r e ai n c r e a s e s . T h i s f a c t i s e x p l a i n e d s i n c e a s e c o n d a r y p o r e s t r u c t u r e o v e r 2 n md i a m e t e r ) c a n b e p r o d u c e d a s a c o n s e q u e n c e o f a l u m i n u m e x t r a ct i o n i n d e a l -u m i n a t e d z e o l i t e s [ 3 1 ]. T h e f o r m a t i o n o f t h is n e w s t r u c t u re c o u l d a c t p o s i t i v e l y inc a t a l y s t s d e a c t i v a t i o n b e h a v i o r , a s i t c a n r e d u c e d i f f u s i o n a l l i m i t a t i o n s i n t h ezeo l i t i c channe l .


19/20


20/20

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