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Ap pl i ed Ca ta lys i s A: Genera l , 9 4 ( 1 9 9 3 ) 9 1 - 1 0 6 9 1E l s e v i e r S c i e n c e P u b l i s h e r s B . V ., A m s t e r d a m

A P C A T A 2 4 0 0

C a t a ly t ic c o n v e r s i o n o f m e t h a n o l i n t o l i g h t a l k e n e s o nm o r d e n i t e l i k e z e o l i t e sA.J . M a r c h i a n d G F F r o m e n tL a b o r a t o r i u m v o o r P e t r o c h e m i s c h e T e c h n i e k , R i j k s u n i v e r s i t e i t , K r i j g s l a a n 2 81 , B - 9 0 0 0 G h e n tB e l g i u m )( R e c e i v e d 2 4 S e p t e m b e r 1 9 9 2 )

b s t r a c t

T h e p a p e r d e a l s w i t h t h e e f f e c t o f th e n u m b e r a n d s t r e n g t h o f a c id s i t e s a n d w i t h f ee d c o m p o s i t io nd u r i n g m e t h a n o l c o n v e r si o n in t o h y d r o c a r b o n s o n m o r d e n i t e a n d d e a l u m i n a t e d m o r d e n i t e . T h e m o r -d e n i t e s a m p l e u s e d i n t h i s w o r k p o s s e ss e s p ri n c i p a l ly a l u m i n i u m - h y d r o x y l g r o u p s t h a t a b s o r b a t 3 6 5 0a n d 3 6 1 0 c m 1. T h e y c a n i n t e r a c t s tr o n g l y w i t h p r o d u c t m o l e c u l e s . A ra p i d d e a c t i v a t i o n b y co k e w a so b s e rv e d . T h e d e a l u m i n a t e d m o r d e n i t e s e x h i b i t p r i n c i p a ll y a l u m i n i u m h y d r o x y l g r o up s t h a t a b s o r b a t3 5 30 c m 1 T h e i r i n t e r a c t i o n w i t h t h e p r o d u c t m o l e c ul e s i s l es s p r o n o u n c e d t h a n t h a t o f t h e h i g h f re -q u e n c y a l u m i n i u m - h y d r o x y l g r o u p s. T h i s a n d t h e r e d u c t i o n o f t h e n u m b e r o f s tr o n g a c i d s it e s l ea d t o al o w e r d e a c t i v a t i o n b y c o k e a n d a h i g h e r s e l e c t i v i t y t o l i g h t a l k e n e s. D e c r e a s i n g t h e p a r t i a l p r e s s u r e o fm e t h a n o l a n d t h e a d d i t i o n o f w a t e r t o t h e f e e d l e a d t o h i g h e r y i e l d s o f l ig h t a l k e n e s, p r i n c i p a l l y o fp r o p e n e , a n d l e s s c o k e d e p o s i ti o n . W a t e r c o m p e t e s w i t h a l k e n e s a n d a r o m a t i c s f o r t h e L e w i s a n d B r o n s t e da c i d s it e s . T h e c o k e d e p o s i t i o n s u p p r e s s e s t h e c o n v e r s i o n o f a l k e n e s i n t o a l k e n e s a n d a r o m a t i c s s o t h a tt h e s e l e c t i v i ty to p r o p e n e , b u t e n e a n d C ~+ a l k e n e s i s i n c r e a s e d .Keywords: a c id i ty ; c a t a l y s t c h a r a c t e r i z a t i o n ( D R I F T ) ; d e a c t i v a ti o n ; l ig h t a lk e n e s ; m e t h a n o l c o n v e r -s i o n ; m o r d en i t e ; z eo l i t e s .

I N T R O D U C T I O N

Th e c a t a ly t i c c o n v e r s io n o f m e t h a n o l t o l o w e r a lk e n e s i s a p r o c e s s t h a t a t -t r a c t s g r o w in g in t e r e s t. R e c e n t ly F r o m e n t e t a l. [ 1 ] r e v i e w e d t h e a c tiv i tys t a b i l i ty a n d se l e c t iv i t y o f sm a l l m e d iu m a n d lar g e p o r e m o le c u la r s i e v e s u se da s c a t a ly s t s f o r th i s r e a c t io n . M o s t o f t h e l i te r a t u r e d i s c u s se s t h e u se o f s ta n -d a rd [ 2 - 1 0 ] o r m o d i f i e d [ 1 1 - 1 3 ] H Z S M - 5 z e o li te . Z e o l it e H Z S M - 5 h a s b e e nw id e ly s t u d ie d b e c a u se i t d e a c t iv a t e s r a t h e r s lo w ly . Th i s i s a t t r ib u t e d t o t h ep a r t ic u la r c h a n n e l s t r u c t u r e o f t h i s z e o l i te . Th e c o n c e n t r a t io n a n d s t r e n g t h o fa c id s i te s i s a l so a n im p o r t a n t f a c t o r in b o t h t h e d u r a b le a c t iv i t y a n d t h e s e l e c -

Correspondence to . D r . G .F . F r o m e n t , L a b o r a t o r i u m v o o r P e t r o c h e m i s c h e T e c h n i e k , R i j k s u n i v -e r s i t e i t , K r i j g s l a a n 2 8 1 , B - 9 0 0 0 G h e n t , B e l g i u m . T e l . ( + 3 2 -9 1 ) 6 4 4 5 1 6 , f a x . ( + 3 2 -9 1 ) 6 4 4 9 9 9 , e -m a i l L P T ~ A U T O C R T L . R U G . A C . B E .

0 9 2 6 - 8 6 0 X / 9 3 / 0 6 . 0 0 1 9 9 3 E l s e v i e r S c i e n c e P u b l i s h e r s B . V . A ll r i g h t s r e s er v e d .

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9 A.J. Marchi and G.F. Froment/Appl. Catal. A 94 1993) 91-106t i v i ty o f t h e c a t a l y s t, h o w e v e r . T w o w a y s o f m o d i f y i n g t h e a c i d i ty o f z e o li te sa r e d e a l u m i n a t i o n a n d c a t io n e x c h a n g e . D e a l u m i n a t e d [ 1 4 -1 9 ] a n d i o n -e x -c h a n g e d z e o li te s [ 20 ] w e r e s h o w n to p o s s e s s h i g h e r s t a b i li t y a n d s e l e c ti v i tyt h a n t h e p a r e n t z e o li te s .

T h e w o r k p r e s e n t e d h e r e is a c o n t i n u a t i o n o f t h e w o r k o n m e t h a n o l c o n ve r -s io n in t o l ow e r a l k e n e s b y D e h e r t o g a n d F r o m e n t [ 21 ] w h o u s e d H Z S M - 5c a t a ly s t s a n d b y M a r c h i a n d F r o m e n t [2 2 ] w h o u s e d S A P O - 3 4 a n d S A P O - 1 1 .I t s a i m i s t o i n v e s t i g a t e t h e e f f e c t o f t h e f e e d c o m p o s i t i o n a n d o f t h e c o n c e n -t r a t i o n a n d s t r e n g t h o f a c i d si te s o n a l k e n e f o r m a t i o n , o l ig o m e r i z a ti o n , a r o -m a t i z a t i o n , a l k y l a t i o n a n d c o k e f o r m a t i o n w i t h m o r d e n i t e - l i k e z e o li te s .EXPERIMENTAL

T h r e e d i f f e re n t m o r d e n i t e s a m p l e s f ro m Z e o c a t ( Z M - 2 10 , Z M - 7 60 a n d Z M -9 8 0 ) w i t h f r a m e w o r k s i l i c o n - t o - a l u m i n i u m a t o m i c r a t io s o f 10 , 6 0 a n d 8 0 w e r eu s e d i n t h i s w o r k .

D R I F T ( D i ff u se R e f l e c ta n c e I n f r a r e d F o u r i e r T r a n s f o r m ) s p e c t r a o f t h ef r e sh s a m p l e s w e r e r e c o r d ed o n a B r u k e r I F 5 4 8 i n s t r u m e n t e q u i p p e d w i t h ad i f fu s e r e f l e c t a n c e ce ll ( S p e c t r a - T e c h ) . T h e s a m p l e s w e r e a c t i v a t e d a t 7 2 3 Ku n d e r v a c u u m ( 1 0 - 6 m b a r ) o v e r n i g h t b e f o re re c o r d i n g t h e s p e c tr a . P o t a s s i u mb r o m i d e w a s u s e d a s a r e f e r e n c e .

T e x t u r a l p r o p e r t i e s w e r e d e t e r m i n e d b y N 2 s o r p t i o n a t 7 7 K i n a M i c r o m -e r i t i c s A S A P 2 0 0 0 s o r p t o m e t e r .T h e c a t a ly t ic c o n v e r s i o n o f m e t h a n o l t o h y d r o c a r b o n s w a s c a r r ie d o u t i n a

f ix e d - b ed t u b u l a r ( 20 m m I .D . ) t i t a n i u m r e a ct o r. I n a ll t h e e x p e r i m e n t s , th ec a t a l y st w a s d i l u t e d w i t h i n e r t m a t e r i a l a n d t h e r e a c t o r w a s i m m e r s e d in a s a ltb a t h t o e n s u r e a u n i f o r m t e m p e r a t u r e a l o n g th e c a t a ly s t b e d . T h e d i l u t i o nr a t io , d e f i n e d a s t h e r a t io o f t h e w e i g h t o f i n e r t m a t e r i a l t o t h e w e i g h t o f ca t -a l y s t , WI/Wc w a s 1 0. T h e e x p e r i m e n t s w e r e c a r r i e d o u t a t 7 53 K . T h e s p a c et i m e , W/F o w a s v a r i e d b e t w e e n 1 0 a n d 3 2 g c a t. h / m o l m e t h a n o l . A d e t a i le dd e s c r i p t io n o f t h e e q u i p m e n t u s e d c a n b e f o u n d i n a p r e v i o u s p a p e r [ 2 3] .T h e p r o d u c t s w e r e a n a ly z e d in a C a r le g a s c h r o m a t o g r a p h p r o v i d e d w i th :( i) o n e R S L ( p o l y d i m e t h y l s i l o x a n e ) 1 60 c a p i l la r y c o l u m n ( 5 / ~ m f il m , 0 .3 2m m I . D . ) f o r a n a l y z i n g C 1- C 4 c o m p o u n d s , C H 3 O H , ( C H 3 ) 2 0 , C ~+ a n d a r o -m a t i c s , u s i n g a f l a m e i o n d e t e c t o r ( F I D ) ; ( ii ) a 1 0 S o r b i t o l o n a C h r o m o s o r bT c o l u m n ( l e n g t h : 1 f t . ) f o r r e s o l v i n g w a t e r , a 3 .1 C a r b o w a x 1 5 40 o n P o r a s i lC ( l e n g t h : 1 0 f t .) a n d a 2 .7 B I S 2 , 2 - e t h o x y e t h y l ( E E ) A o n C h r o m o s o r bP A W ( l e n g t h : 1 7 f t. ) c o l u m n s i n s e r ie s i n o r d e r to r e s o l v e a n d i d e n t i f y a ll t h ec o m p o u n d s a n d i s o m e r s o f t h e C 3 -C 4 f r a c ti o n , a H a y e s e p Q c o l u m n ( le n g t h : 7f t. ) t o r e s o l v e a n d i d e n t i f y e t h a n e , e t h e n e a n d c a r b o n d i o x i d e a n d a m o l e c u l a rs ie v e 13 X ( l e n g th : 9 f t .) t o r e s o lv e a n d i d e n t i f y n i t r o g e n ( i n t e r n a l s t a n d a r d ) ,m e t h a n e a n d c a r b o n m o n o x i d e , u s in g a t h e r m a l c o n d u c t i v it y d e t e c to r ( T C D )w i t h h e l i u m a s c a r r i e r g as ; a n d ( ii i) a h y d r o g e n t r a n s f e r s y s t e m ( H T S ) t r a n s -

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A . J . M a r ( h i a n d (L F . F r o m e n t / A p p l . C a t a l. A 94 ( 1 9 93 ) 9 1 1 0 6 93f e r r i n g t h e h y d r o g e n f r o m t h e h e l i u m c a r r i e r g as i n to t h e n i t r o g e n c a r r i e r ga ss t r e a m , i n o r d e r t o d e t e c t i t b y m e a n s o f a s e c o n d T C D .

T h e i n s t a n t a n e o u s c a r b o n b a la n c e c lo s u re w a s b e t t e r t h a n 9 5 .R E S U L T S A N D D I S C U S S I O N

I n f l u e n c e o f th e s i l i c o n t o a l u m i n i u m r a ti oE x p e r i m e n t s w i t h m o r d e n i t e s a m p l e s o f d if f e r e n t S i/ A 1 r a ti o s w e r e c a r r ie d

o u t a t 7 53 K a n d a t m o s p h e r i c p r e s s u r e . T h e f e e d w a s a s o l u ti o n o f m e t h a n o l( 3 0 w t . - ) a n d w a t e r ( 7 0 w t . - ) .

T h e i n i ti a l t o t a l c o n v e r s i o n o f m e t h a n o l w a s 1 00 i n a ll c a s e s f o r t h e w h o l er a n g e o f W / F o u s e d i n t h i s w o r k ( F ig . 1 ). T h e i n it ia l c o n v e r s i o n o f m e t h a n o lt o h y d r o c a r b o n s i n c r e a s e d a s t h e s i l i c o n - t o - a l u m i n i u m a t o m i c r a ti o d e c r e a se d .T h i s i n c r ea s e w a s a c c o m p a n i e d b y a n i m p o r t a n t r is e in t h e y i e ld o f m e t h a n e( T a b l e s 1 a n d 2 ) .

F o r t h e c a se o f t h e s a m p l e w i t h a f r a m e w o r k s i l ic o n - t o - al u m i n i u m a t o m i cr a t io o f 10, t h e c o n v e r s i o n o f m e t h a n o l d r o p p e d v e r y f a s t f r o m 1 0 0 t o v a l u e sb e l o w 2 0 ( F ig . 1, c u r v e a ) . I n a s i m i l a r w a y , t h e c o n v e r s i o n t o h y d r o c a r b o n sf e ll f r o m a b o u t 5 5 t o a l m o s t z er o , ( T a b l e 1 ) . T h e i n i t ia l y ie l d o f m e t h a n e w a sh i g h e r t h a n 1 0 . T h e h y d r o c a r b o n f r a c t i o n m a i n l y c o n s i s t e d o f C 1-C 4 a l k a n e sa n d C ,~-C 4 a l k e n e s ( T a b l e 1 ). T h e m a i n a l k e n e w a s e t h e n e , w h e r e a s t h e m a i na l k a n e s w e r e m e t h a n e , e t h a n e a n d p r o p a n e . O n l y sm a l l a m o u n t s o f i - b u ta n ea n d i - b u t e n e a n d t r a c e s o f a r o m a t i c s a n d C5+ n o n - a r o m a t i c c o m p o u n d s w e r ed e t e c t e d i n t h e e f f l u e n t . T h e r a t io s o f l i g ht a l k e n e y i e l d s w e r e C J C 3 = 2 . 5- 2 .6a n d C ~ /(C :~ + C 4 ) - - 1 . 8 - 2 . 0 a t 1 0 0 c o n v e r s i o n . T h e r a t i o s o f t h e y i e l d s o f t o t a l

loo

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M m ol M e O H / g c a t )F i g . 1. M e t h a n o l c o n v e r s i o n o n m o r d e n i t e - l i k e z e o l i t e s a t 7 5 3 K a n d 1 .0 4 b a r t o t a l p r e s s u r e v e r s u st h e t o t a l a m o u n t o f m e t h a n o l f e d p e r g r a m o f c a t al y s t. W / F o 1 0 - 3 2 g c a t. h / m o l m e t h a n o l . F e e d :m e t h a n o l w a t e r 3 0 : 7 0 w t .- m i x t u r e . ( a ) S i / A l = 1 0, ( b ) S i / A l = 6 0 ; ( c ) S i / A l = 8 0 .

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94 A.J. Marchi and G.F. Froment/Appl. Catal. A 94 1993) 91- I06T A B L E 1Conversion of methanol in to hyd rocarbons on m ordeni teSi/A l, 10; T, 753 K; P, 1.04 bar; W / F o, 18 .3 g ca t. h / tool methanol ; feed: wate r -meth anol 70:30w t . -M a 0.0064 0.0091 0.016 0.064 0.078t b 0.12 0.17 0,30 1.17 1.42XMeOH c 100 100 45.9 21.5 15.9XM d 55.5 57.9 14.0 0.8 0.6Yields e

CH 4 12.3 13.4 4.0 0.4 0.3C2H4 9.7 12.0 3.5 0.0 0.0C2H6 7.3 7.6 0.5 0.0 0.0C3H6 3.9 4.6 0.9 0.0 0.0C3H s 10.0 6.1 0.7 0.0 0.0C4 Hs 1.5 1.4 0.3 0.0 0.0n-C4H lo 0.7 0.5 0.1 0.0 0.0i-C4H,0 0.9 0.5 0.0 0.0 0.0CsH o 0.0 0.2 0.0 0.0 0.0Aromatics Traces Traces 0.0 0.0 0.0CH30H 0.0 0.0 54.1 78.5 84.1C2H60 0.0 0.0 17.4 14.8 10.9

Hydrocarbon distribution lCH 4 26.6 29.0 39.9 87.5 85.3C~-C ~ 32.7 38.8 47.1 12.5 14.7C2+ alk an es 40.7 31.8 13.1 0.0 0.0C5 alkenes 0.0 0.4 0.0 0.0 0.0aM, mol methanol fed /g ca ta lys t .b t, t ime on s t ream (h) .CXMeOH m ethan ol conversion.~XMH, tool hydrocarbo ns produced /100 mo l metha nol fed.eYield, g product/1 00 g meth anol fed./Hydrocarbon d is t r ibu t ion , g hyd rocarbon/100 g to ta l hydrocarbons .l i g h t h y d r o c a r b o n s w e r e C f f C 3 = 1 . 2 - 1 .8 a n d C f f ( C 3 + C 4 ) = 1 . 0 -1 . 2 . T h e s e r a -t i o s a r e in a g r e e m e n t w i t h t h o s e f o u n d f o r s m a l l - p o r e z e o l i t e s [ 2 2 ,2 4 , 2 5 ].

T h e D R I F T s p e c t r u m o f t h e f r e s h s a m p l e s h o w e d a n a b s o r p t i o n b a n d w i t ha m a x i m u m a t 3 6 50 c m - 1 a n d a s h o u l d e r a t 3 61 0 c m - ' ( F ig . 2 ) . Z M - 2 1 0 z e o l i t eh a s a f r a m e w o r k S i / A 1 r a t i o o f 10 . H o w e v e r , c o m p l e t e a n a l y s i s i n d i c a t e s t h a tt h e o v e r a l l S i /A 1 r a t i o i s 5 .5 ( T a b l e 3 ) . T h i s m a y b e e x p l a i n e d b y th e p r e s e n c eo f e x t r a - f r a m e w o r k a l u m i n a . T h u s , t h e b a n d a t 3 6 50 c m - ' m a y b e a t t r i b u t e dt o e x t r a - f r a m e w o r k m a t e r i a l [ 2 6 ] , w h e r e a s t h e s h o u l d e r a t 3 6 1 0 c m - 1 m a y b ed u e t o f r a m e w o r k a l u m i n i u m - h y d r o x y l g r o u p s t r e t c h e s [ 22 ,2 7 ]. T h e s e h y -d r o x y l g r o u p s p o s s e s s a h i g h a c id s t r e n g t h a n d c a n i n t e r a c t w i t h r e a g e n t a n dp r o d u c t m o l e c u l e s [ 2 2 ,2 7 ] . A s m a l l a b s o r p t i o n b a n d a t a r o u n d 3 7 35 c m - ' w a s

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A.J. Marchi atTd (;.F. Froment/Appl. Catal A 94 (1993) 91-106 9 5T A B L E '2C o n v e r s i o n o f m e t h a n M i n t o h y d r o c a r b o n s o n d e a l u m i n a t e d m o r d e n i t e s a m p l e s "T , 7 5 3 K : P , 1 . 0 4 b a r : W/I, ~, 2 1 .5 g c a l h / m o l m e t h a n o l f ed ; f e e d: w a t e r m e t h a n o l 7 0 : : { 0 w t ."~ I l l ix l u r eS i / A 1 r a t i o 41) 8 0M 0 . 0 1 5 0 . 0 3 2 0 , 0 4 2 0 . 0 5 2 0 . 1 5 4 0 . 1 1 ) 0 . 2 8 0 0 . 5 1 0[ ().;1 ;~ 0 .7 0 0 ,9 0 1 .10 3 .4 0 2 .17 4 .0 0 10 .92XMeO 1 0 0 1 1)0 7 4 .5 4 9 .3 3 2 .9 1 0 0 9 4 .8 5 1 .4~ MH :~5.t) 47 .1 :{5.4 20.1:; 2.1) 30 .7 27 .0 1 1.5YieldsC H ~ 2 .7 9 .5 8 .2 5 .4 0 .9 0 .8 1 .1 1 ).7

C~H~ 7 .3 ) .5 6 .2 2 .9 0 .1 2 .4 1 .2 1).3Q H , ; 1 2 .1 l :L4 1 0 .0 4 .5 0 .1 2 1 .5 2 1 .2 7 .1C:~H~ 4.t) 1.7 0.6 0.5 0.0 1).9 0.0 1).0C 4 H s 5 .1 4 . 2 5 . 5 2 . 6 0 . 0 9 . 4 9 . 3 5 . 1n-C ,~H ~,~ 1.2 0.4 0.2 0.1 0.0 //.3 0.1 1).0i -C ~H I~ 5 .4 1.:~ 0 .2 0 .1 0 .0 2 .8 0 .6 0 .0C~,+ 0.7 1.4 1.4 1.1 0.1) 2.2 3.6 0.0A r o m a t i c s 2 . 4 0 . 7 0 . 2 0 .1 0 . 0 2 .; / 0 . 6 0 .1CH :~()H (1.() 0 .0 2 :L5 50 .7 47 .1 0 .1) 13 .2 48 .4C2 H~ O 0.11 0 .0 :{.1 7 .2 21 .4 0 .0 1 .2 10 .0Hydrocarbon distributi,r7C H 4 4 . 1 2 1 . 2 2 4 . 9 3 0 . 2 7 4 . 4 1 .9 2 . 8 4 . 0C ~ ( '. 4 5 4 . 9 6 4 . 8 4 4 . 2 5 6 . 3 1 9 . 9 7 5 . 4 8 0 . 9 7 5 . 2C s + a l k a n e s ' .~1 .5 8 . 9 ' .L8 5.1 5.7 11.3 :L4 1.2A r o m a t i c s 5 . 9 1 .5 0 . 6 1 . 0 0 . 0 5 . 2 1 . 6 0 . 4C ~ , a l k e n e s 1 .4 : L 6 4 . 5 7 . 4 0 . 0 4 . 0 1 1 .2 1 9 .2

F o r a n e x p l a n a t i o n o[" l e r m s s e e T a b l e 1.

a l s o o b s e r v e d . T h i s p e a k is n o r m a l l y a s s i g n e d t o s i l ic o n - h y d r o x y l g r o u p s [ 2 6 ]T h e s e g r o u p s d o n o t i n te r a c t w i t h r e a g e n t a n d p r o d u c t m o l e c u l e s [ 2 2 , 2 6 , 2 7 ] .I t w a s p o s s i b l e t o f e e d a l ar g er a m o u n t o f m e t h a n o l p e r g r a m o f c a t a l y s t a t

1 00 c o n v e r s i o n w h e n a d e a l u m i n a t e d s a m p l e w i t h a s i l ic o n - t o - a l u m i n i u ma t o m i c r a ti o o f 6 0 w a s u s e d a s c a ta l y s t , i n s t e a d o f t h e n o n - d e a l u m i n a t e d s a m -p l e ( F ig . 1, c u r v e s a a n d b ) . T h e c o n v e r s i o n o f m e t h a n o l d r o p p e d f r o m 1 0 0t o v a l u e s l o w e r t h a n 4 0 , b u t in t h i s c a se i t d i m i n i s h e d m o r e s l o w l y t h a n w i t ht h e s a m p l e h a v i n g a s i l i c o n - t o - a l u m i n i u m a t o m i c r a t io o f 1 0. A s im i l a r b e h a v -i or w a s o b s e r v e d f o r t h e c o n v e r s i o n o f m e t h a n o l t o h y d r o c a r b o n s . T h e y i e l d o fm e t h a n e w a s a l w a y s l e s s t h a n 1 0 . T h e y i e ld o f l ig h t a l k e n e s w a s h ig h e r t h a n2 0 a t 1 0 0 c o n v e r s i o n ( T a b l e 2 ) . T h e r a t i o s o f l i g h t a l k e n e y i e l d s w e r e C 2 /C ~ = 0 . 7 0 - 0 . 7 5 a n d C ~ /( C :~ + C ~ ) = 0 . 4 0 - 0 . 5 0 . T h e r a t i o s o f y i e l d s o f t o t a l l i g h th y d r o c a r b o n s w e r e C 2/C :~ = 0 . 4 5 - 0 . 5 5 a n d C 2 / ( C : ~ + C 4 ) = 0 . 3 0 - 0 . 4 0 . T h e s e v a l -

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96 A.J. Marchi and G.F. Froment /Appl. Catal. A 94 1993) 91-106

difI - -z

wozr n0,9n ~

5735 cm 1

_~ 8 0 0 5 6 0 0 3 4 0 0W VENUMBER c m - t )

Fig. 2. DRIF T spectra of m o r d e n i t e a n d dealuminated mordenite samples used in this study. a)Si /Al = 10; b) Si/A l=60; c) Si/A l=80.

TABLE3T e x t u r a l p r o p e r t i e s o f m o r d e n i t e and dealuminated mordenite samplesSample Overall Si/A1 Framework Si/A1 Vpa Sg b

atomic ratio atomic ratio cm3/g) m2/g)HM c 5 10 0.19 500DHM1 ~ 60 60 0.28 610DHM2 ~ 80 80 0.30 650a V . pore volume.bSg, specific surface area.O H M H - m o r d e n i t e zeolite.dDHM, d e a l u m i n a te d H - m o r d e n i t e zeolite.

u e s a r e c o n s id e r a b l y l o w e r t h a n i n t h e f o r m e r c a se a n d a g re e w i t h t h o s e f o u n df o r l ar g e a n d m e d i u m p o r e z e o l i t e s. I n a d d i t i o n , th e i n i t ia l y i e ld o f b r a n c h e di s o m e r s a n d a r o m a t ic s w a s m u c h h i g h e r ( T a b l e s 1 a n d 2 ) . T e t r a -, p e n t a - a n dh e x a - m e t h y l b e n z e n e s w e r e d e t e c te d i n th e e f f l u e n t in c o n si d e r a b le a m o u n t s .T h e y w e r e f o r m e d i n t h e 1 2 - m e m b e r e d r i n g c h a n n e l s .

F u r t h e r d e a l u m i n a t i o n l e d t o a n i m p o r t a n t i n c r e a s e i n t h e s t a b i l it y o f c a t a -l y s t a n d a l k e n e y i e l d , i n a g r e e m e n t w i t h r e s u l t s o b t a i n e d b y B a n d i e r a e t a l .[ 1 7 ] , w h e r e a s t h e i n it ia l c o n v e r s i o n o f m e t h a n o l t o h y d r o c a r b o n s a n d t h e y i e l do f m e t h a n e w e r e l o w e r t h a n i n t h e f o r m e r c a s e s ( F ig . I a n d T a b l e 2 ) . T h e y i e ldo f m e t h a n e w a s a lw a y s lo w e r t h a n 2 . T h e p r in c i p a l a l k e n e w a s p r o p e n e ( T a -

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A . J . M a r c h i a n d ; .F . F r ~ r n e n t / A p p l . C a t a l . A 94 I 9 9 3 ) 9 1 1 0 6 9

ble 2). The ratios of light alkene yields were CJC:~=0.10-0.30 and C2/(C3+C4) =0.05-0.25 at 100 conversion. The ratios of yields of total lighthydrocarbons were C~/C:~ = 0.07-0.25 and C2/(C:, + C4 ) = 0.05-0.15. These val-ues are lower tha n in the former cases and correspond to those of large porezeolites. The initial yields of alkanes and aromatics were also lower tha n in thecase of the sample with a sil icon-to-aluminium atomic ratio of 60 (Table 2).

The D RIFT spectra (Fig. 2) showed some remarkable differences betweenthe dea luminated samples and the non-dealuminated mordenite. The intensit yof the absorption band at 3730 cm-I increased significantly. This could beattributed to the fact that on dealuminated H-mordenite defects are generatedwith the subsequent formation of silanol groups, which can not interact withreagent molecules. The high-frequency aluminium-hydroxylgroup band shiftedto higher wavenumbers and its in tens ity diminished as the silicon-to-alumin-ium ratio was raised. This band practically disappears for the sample with asilicon-to-a luminium ratio of 80. A new absorption band at 3530 cm - 1, tha tcan be attributed to framework aluminium-hydroxyl group stretches [22,28]appeared. The intensity of this band increases as the Si/A1 ratio is raised. Theband at 3610 cm - ~was not observed, probably due to lack of sensitivity sincethe dealumination process leads to an important decrease of the number ofacid sites [17].The dealuminated samples have a higher pore volume and specific surfacearea than the sample with a Si/A1 atomic ratio of 10 (Table 3), which is inagreement with the fact that the larger pores of ZM-210 could be partiallyblocked by extra-f ramework material.By pyridine desorption, Karge and Dondur [ 29 ] detected three types of acidsites in dealuminated mordenites: one Bronsted and two Lewis centers. Am-monia desorption showed four types of acid sites: two Brons ted and two Lewiscenters. The second type of Bronsted site had a higher activation energy thanthe one detected by pyridine desorption. On the other hand, the pore structureof mordenite consists of parallel channels of 0.67 0.70 n m( 12-membered ringpores) and 0.29 0.57 nm (8-membered ring pores), interconnected via smallside pockets of 0.29 nm [30]. Lee et al. [16] suggested that in the alkylationof polynuclear aromatics, propene can diffuse through both the 12-memberedring and the 8-membered ring channels. Thus, the mordenite porous networkmay behave in a one-dimensional man ner for large molecules, e.g. aromaticsand branched isomers, and in a bi-dimensional manner for small molecules,such as light linear alkenes. On the basis of the results mentioned above, itmay be concluded tha t the second type of Bronsted acid site might be locatedin the 8-membered ring channels of mordenite. The hydroxyl groups that ab-sorb at 3530 cm-1 must be principally located in the large channels. In thenon-dealuminated mordenite, they may be poisoned by cationic aluminiumspecies from the ext ra-f ramework material. This can explain why they are notobserved in the corresponding DRIFT spectrum.

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9 A.J. Marchi and G.F. Froment/Appl. Catal. A 94 I993) 91-106T h e p r e s e n c e o f e x t ra - f r a m e w o r k m a t e r i a l i n t h e l ar g e r p o r e s, w h i c h r e d u c e s

t h e e f f e ct iv e p o r e s i ze o f t h e s e , a n d B r o n s t e d a c i d s i te s i n th e 8 - m e m b e r e d r in gc h a n n e l s o f n o n - d e a l u m i n a t e d m o r d e n i t e , p r o v id e s a n e x p l a n a t i o n f o r t h e r e-s u l ts o b t a i n e d i n t h e c o n v e r s i o n o f m e t h a n o l i n t o h y d r o c a r b o n s w h e n a m o r -d e n i t e s a m p l e w i t h a f r a m e w o r k s i l i c o n - t o - a l u m i n i u m r a ti o o f 1 0 w a s u s ed . T h e1 2 - m e m b e r e d ri n g c h a n n e l s m u s t b e b l o c k e d o r p a r t ia l ly b l o c k e d b y t h e e x tr a -f r a m e w o r k m a t e r i a l , l ik e l y a m o r p h o u s a l u m i n a . T h u s , t h e e f fe c t iv e p o r e s iz eo f Z M - 2 1 0 z e o l it e is lo w e r t h a n c a n b e e x p e c t e d . T h i s e x p l a i n s t h e s i m i l a r i t yi n c a t a l y ti c b e h a v i o r o f t h i s s a m p l e w i t h t h a t o f s m a l l -p o r e z e o li te s . T h e s m a l la m o u n t s o f i-C 4 a n d a r o m a t i c s d e t e c t e d in t h e e f f lu e n t c a n b e f o r m e d o n t h ea c i d s i t e s p r e s e n t o n t h e c a t a l y s t e x t e r n a l s u r f a c e a n d / o r i n s i d e t h e p a r t i a l l yb l o c k e d 1 2 - m e m b e r e d r i n g c h a n n e l s . D u e t o t h e r e d u c t i o n o f th e e f fe c ti ve p o r es iz e o f t h e s e p o r e s b y e x t r a - f r a m e w o r k a l u m i n a , t h e d i f fu s i o n o f a r o m a t i c s a n db r a n c h e d h y d r o c a r b o n s b e c o m e s d if f ic u l t, a n d t h e i r y i e ld is s m a l l.

T h e r a p i d d e a c t i v a t io n o f th i s c a t a l y s t m a y b e e x p l a i n e d b y co k e f o r m e d o nt h e e x t e r n a l s u r fa c e a n d i n s id e o f th e p a r t i a l ly b l o c k e d 1 2 - m e m b e r e d ri n gc h a n n e l s o f t h e n o n - d e a l u m i n a t e d m o r d e n i te , w h i c h c a n b lo c k t h e e n t r a n c e o ft h e o n e - d i m e n s i o n a l c h a n n e l s . D u e t o t h e s m a l l e f fe c t iv e p o r e s iz e a n d t h e o n e -d i m e n s i o n a l p o r e s t r u c t u r e , b l o c k a g e o f t h e p o r e s b y c o k e d e p o s i t i o n l e a d s toa d r a m a t i c d e c r e a se o f t h e c o n c e n t r a t i o n o f th e B r ~ n s t e d a c i d c e n t e r s a v a il ab lef o r i n t e r a c t i o n w i t h t h e r e a g e n t m o l e c u l e s a n d , t h e r e f o r e , o f t h e c a t a l y t i c a c -t i v i ty o f t h e z e o li te Z M - 2 1 0 [ 3 1 - 3 3 ] . T h e h i g h i n i ti a l y i e ld o f m e t h a n e is in -d i c a t iv e o f a r a p i d l o s s o f a c t i v i ty o f th i s m o r d e n i t e s a m p l e b y c o k e d e p o s i t i o n( T a b l e 1 ) [ 3 4 ] . I t is g e n e r a l l y a c c e p t e d t h a t a m e t h o x y g r o u p c a n b e f o r m e do n B r o n s t e d ac i d s it es . T h i s m e t h o x y s p ec ie s c o u l d t h e n b e c o n v e r t e d t om e t h a n e b y 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 [ 3 5] . T h i s s e e m s to b e f a v o re d in t h ec a s e o f Z M - 2 1 0 b e c a u s e o f t h e r a p i d d e a c t i v a t i o n b y c o k e d e p o s i t io n . T h em e t h y l o x o n i u m s pe c ie s h a s a l s o b e e n s u g g e s t e d a s a n i n t e r m e d i a t e f o r t h e i n i-t ia l C - C b o n d f o r m a t i o n [ 36 ,3 7 ]. T h u s , C 2 -C 4 a l k e n e f o r m a t i o n t h r o u g h t h i sm e c h a n i s m m i g h t b e p la u s ib l e i n t h e c a se o f Z M - 21 0 .

T h e h i g h yi e ld s o f e t h a n e a n d p r o p a n e c o u l d b e d u e t o r a p i d c o n v e r s i o n o fa l k e n e s i n t o a l k a n e s o n t h e s t r o n g a c i d si te s p r e s e n t i n Z M - 2 1 0 .T h e h i g h r a ti o o f e t h e n e t o p r o p e n e m a y b e d u e t o s t e ri c i n h i b i ti o n o f t h e

m e t h a n o l a l k y l a t i o n r e a c t io n , d i f f u s io n c o n s t r a i n t s a n d h i g h e r r e ac t iv i ty o fp r o p e n e i n 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 a n d o l ig o m e r fo r m a t i o n , w h i c h i s i na g r e e m e n t w i t h t h e p r o d u c t d i s t r ib u t i o n o b s e r v e d in t h i s w o r k ( T a b l e 1 ).

T h e d e a l u m i n a t i o n w o u l d r e m o v e th e e x t r a - f r a m e w o r k a l u m i n a [ 1 7] , le a d-i n g to a n e n l a r g e m e n t o f th e e f fe c ti v e p o r e s iz e a n d t o t h e u n c o v e r i n g o f t h eB r o n s t e d a c id s it e s lo c a t e d i n t h e 1 2 - m e m b e r e d r i n g c h a n n e l s . T h u s , m e t h a n o lc a n d i f fu s e t h r o u g h t h e m a n d b e c o n v e r t e d t o l in e a r a n d b r a n c h e d a l k e n e s a n da l k a n e s a n d a r o m a t i c s . T h e a l m o s t c o m p l e t e r e m o v a l o f t h e e x t r a - f r a m e w o r ka l u m i n a a n d t h e r e d u c t i o n o f t h e n u m b e r o f s t r o n g a c id s it e s p e r m i t s a m o r -d e n i t e t o b e o b t a i n e d w i t h a h i g h s t a b i l i t y a n d s e l e c t i v i t y f o r c o n v e r s i o n o f

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A . J . M a r ( h i a n d ~ ; .F . F r . m e r t t / A p p l . C a t a l. A 9 4 ( 1 9 9 3 ) 9 1 1 0 6

m e t h a n o l i n t o l ig h t a l k e n e s . T h e m o s t p r e d o m i n a n t B r o n s t e d - t y p e s it e s i nd e a l u m i n a t e d m o r d e n i t e a r e t h o s e d ue t o s i l i c o n - h y d r o x y l a n d a l u m i n i u m -h y d r o x y l g r o u p s t h a t c a n a b s o r b a t 3 7 3 0 a n d 3 5 3 0 c m - 1 , r e s p e c ti v e ly . T h es i la n o l g r o u p s c a n n o t i n t e r a c t w i t h r e a g e n t a n d p r o d u c t m o l e c u l e s [ 2 2 ,2 6 ,2 7 ].T h e B r o n s t e d - t y p e s i te s t h a t a b s o r b a t 3 5 3 0 c m i i n t e r a c t w i t h o x y g e n a t e sa n d h y d r o c a r b o n s , b u t th e i n t e r a c t i o n s e e m s t o b e w e a k e r th a n i n t h e c a s e o ft h e h y d r o x y l s t h a t a b s o r b a t 3 6 1 0 c m ~. I n a d d it io n , t h e n u m b e r o f a l u m i n -i u m - h y d r o x y l s i te s i s s o lo w , a s r e v e a l e d b y D R I F T , t h a t t h e y a r e v e r y i s o la t e df r o m e a c h o t h e r [ 1 4 ,1 5 , 18 ] . T h e s e t w o f e a t u r e s e x p l a in t h e h i g h e r s t a b il i ty o ft h is c a t a ly s t . T h e s m a l le r i n t e r a c t i o n b e t w e e n h y d r o c a r b o n m o l e c u l e s a n d h y -d r o x y l g r o u p s a n d t h e l o w e r p r o b a b i l i t y t h a t t w o m o l e c u l e s o f a r o m a t i c s a n d /o r a l k e n e s a d s o r b e d o n s t r o n g a c i d s i te s c a n i n t e r a c t l e a d t o a lo w e r p r o d u c t i o no f c o k e a n d, t h e r e f o r e , l e s s d e a c t i v a t io n . T h e m e s o p o r e s c r e a t e d d u r in g d e a l-u m i n a t i o n m i g h t a l s o p l a y a n i m p o r t a n t r o le o n m o r d e n i t e s t a b i li ty . T h e r e-d u c t i o n o f t h e n u m b e r o f s t r o n g e s t a c i d s i t e s a n d t h e s m a l l c o n c e n t r a t i o n o fl o w f r e q u e n c y a l u m i n i u m - h y d r o x y l g r o u p s a l so e x p la i n th e l o w e r c o n v e r s i o no f a l k e n e s i n to a l k a n e s a n d a r o m a t i c s . T h e a b s e n c e o f e x t r a - f r a m e w o r k a lu -m i n a a n d t h e r e d u c t i o n o f t h e n u m b e r o f s t r o n g a c i d s it e s p e r m i t s c a t a l y s t s t ob e o b t a i n e d w i t h h i g h d u r a b l e a c t i v i t y a n d s e l e c t i v i t y f o r l ig h t a l k e n e p r o d u c -t io n , a s in th e c a s e o f d e a l u m i n a t e d m o r d e n i t e w i t h a s i l i c o n - t o - a lu m i n i u ma t o m i c r a t i o o f 80 .

n f lu en ce o f wa te r a n d me th a n o l p a r t ia l p re ssu re a n d coke d ep o si tio nE x p e r i m e n t s w i th m i x t u r e s o f m e t h a n o l - w a t e r a n d m e t h a n o l - n i t r o g e n o f

v a r i o u s c o n c e n t r a t i o n s w e r e c a r r ie d o u t a t 75 3 K a n d 1 .0 4 b a r o n t h e s a m p l ew i t h a s i l i c o n - t o - a l u m i n i u m a t o m i c r a t i o o f 8 0 .

I t w a s o b s e r v e d t h a t t h e c a t a l y s t s t a b i li t y w a s i m p r o v e d a s th e c o n c e n t r a t i o no f m e t h a n o l i n t h e f e e d w a s l o w e r e d b y t h e a d d i t i o n o f w a t e r ( F ig . 3 ) . I n a llc a s e s , a f t e r a c e r t a i n l e n g t h o f t i m e , t h e c o n v e r s i o n d r o p p e d f r o m 1 0 0 t o af in a l v a l u e , w h i c h d e p e n d e d u p o n t h e c o n c e n t r a t i o n o f w a t e r . T h e h i g h e r t h isc o n c e n t r a t i o n , t h e l o w e r t h e c o n v e r s i o n a t t h e f i na l s t a te . T h e p r i n c i p a l r e a c -t i o n t h a t t a k e s p l a c e i n t h i s f in a l s t a t e is t h e e q u i l i b r iu m r e a c t i o n m e t h a n o l -d i m e t h y l e t h e r , w i t h w a t e r a s a p r o d u c t . A s a c o n s e q u e n c e , i n c r e a s in g t h e c o n -c e n t r a t i o n o f w a t e r i n th e f e e d s h if t s th e e q u i l ib r i u m t o l ow e r c o n v e r s i o n s o fm e t h a n o l . A s im i l a r b e h a v io r w a s o b s e r v e d fo r m e t h a n o l c o n v e r s io n o n S A P O -3 4 m o l e c u l a r s i e v e s [ 2 2 ] .

A s t h e c o n c e n t r a t i o n o f m e t h a n o l w a s lo w e r e d f r o m 1 0 0 t o 3 0 b y a d d i n gw a t e r , w h i c h m e a n s a d e c r e a s e o f t h e p a r t i a l p r e s s u r e o f m e t h a n o l f r o m 1 .0 t o0 .2 b a r , t h e m a x i m u m y i e l d o f a l k e n e s i n c r e a s e d f r o m 2 3 t o 3 5 g o f l ig h t a l k e n e sp e r 1 00 g o f m e t h a n o l f ed . T h e i n it ia l y i e l d o f a l k a n e s d e c r e a s e d f r o m a b o u t 3 0t o 1 4 g o f l ig h t a l k a n e s p e r 1 0 0 g o f m e t h a n o l f e d a n d t h e i n i ti a l y i e l d o f a r o -

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Fig . 3 . M e t h a n o l c o n v e r s io n o n d e a lu m i n a t e d m o r d e n i t e (S i / A 1 = 80 ) a t 753 K a nd 1 .04 ba r t o t a lp r e s s u r e v e r s u s t h e t o t a l a m o u n t o f m e t h a n o l f ed p er g ra m o f c a t a l y s t . W/F 10-32 g c a t . h / m o lm e t h a n o l . F e e d : ( a ) m e t h a n o l 1 00 ; ( b ) m e t h a n o l w a t e r 6 0 : 4 0 w t .- ; ( c ) m e t h a n o l - w a t e r 3 0 : 7 0wt . - .

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F i g . 4 . L i g h t a l k e n e y i e l d s o n d e a l u m i n a t e d m o r d e n i te ( S i / A I : 8 0 ) a t 75 3 K a n d 1.0 4 b a r t o t a lp r e s s u r e v e r s u s t h e t o ta l a m o u n t o f m e t h a n o l f e d p e r g r a m o f c a ta l y s t . W / F : 10-32 g c a t . h / m o lm e t h a n o l . F e e d: m e t h a n o l w a t e r 3 0 : 7 0 w t . - m i x t u r e . ( a ) P r o p e n e ; ( b ) t o t a l b u t e n e s ; ( c) e t h e n e .

m a t ic s f r o m a b o u t 1 0 t o 5 g o f a r o m a t i c s p e r 1 0 0 g o f m e t h a n o l f e d , in d ic a t in gt h a t t h e c o n v e r s io n o f a lk e n e s t o a lk a n e s a n d a r o m a t i c s i s l o w e r.

In a ll c a se s , t h e y i e ld o f p r o p e n e a n d b u t e n e s in c r e a se d w i t h t h e a m o u n t o fm e t h a n o l f e d p e r g r am o f c a t a ly s t , u n ti l a m a x i m u m w a s r e a ch e d F i g . 4 ) . T h i si n c r ea s e w a s l e s s p r o n o u n c e d w h e n t h e c o n c e n t r a t i o n o f m e t h a n o l i n t h e f e e dw a s l o w e r ed . T h e m a x i m u m y i e ld o f p r o p e n e a n d b u te n e b e c o m e s m o r e i m -p o r t a n t a s t h e w a t e r c o n c e n t r a t io n in t h e f e e d i s r a is e d F ig . 5 ) . A f t e r t h em a x im u m , t h e y i e ld o f C 3 -C 4 a lk e n e s d im in i sh e d f a s t e r a c c o r din g to t h e a m o u n to f m e t h a n o l f e d p e r g ra m o f ca t a l y s t a s t h e c o n c e n t r a t i o n o f m e t h a n o l i n t h e

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F ig . 5 . M a x i m u m y i e l d o f li g ht a l k e n e s a s a f u n c t i o n o f w a t e r c o n t e n t i n t h e t e e d a t 7 5 3 K a n d 1 .0 4b a r t o t a l p r e s s u r e . ( a ) P r o p e n e , ( b ) t o t a l b u t e n e s .

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M r e al M e O H / g c o t )F i g 6 L i g ht a l k a n e y i el d s o n d e a l u m i n a t e d m o r d e n i t e ( S i / A I = 8 0 ) a t 75 3 K a n d 1 .0 4 b a r to ta lp r e s s u r e v e r su s t h e t o t a l a m o u n t o f m e t h a n o l f e d p e r g r a m o f c a t a ly s t . W / F : 1 0 3 2 c a t . h / m o lm e t h a n o l . F e ed : m e t h a n o l - w a t e r 3 0 : 7 0 w t . - m i x t u r e . ( a ) i - B u t a n e ; ( b ) p r o p a n e ; ( c ) n - b u t a n e .

f e e d w a s r a is e d . I n a ll c a se s , t h e e t h e n e y i e l d d e c r e a s e d m o n o t o n i c a l l y w i t h t h ea m o u n t o f m e t h a n o l f ed p e r g ra m o f c a t a l y st .P r o p e n e a n d b u t e n e s a r e t h e m o s t i m p o r t a n t a l k e n es . C o n s i d e r in g t h e m a x -i m u m y i e ld s , t h e r a ti o C J C 4 i n c r e a s e d f r o m 1 .6 t o 2 .3 a s t h e c o n c e n t r a t i o n o fw a t e r i n t h e f e e d w a s r a i s e d f r o m 0 t o 7 0 w t . - ( F i g . 5 ) . T h e a l k a n e y i e l dd e c r ea s e d m o n o t o n i c a l l y t o z e ro a s t h e a m o u n t o f m e t h a n o l f e d pe r g r a m o fc a t a l y s t in c r e a s e d ( F ig . 6 ) . T h i s d e c r e a s e w a s l es s p r o n o u n c e d w h e n t h e c o n -c e n t r a t i o n o f m e t h a n o l i n t h e f e e d w a s l o w e r e d . P r o p a n e a n d i - b u t a n e w e r et h e p r in c i p a l a l k a n e s . E t h a n e w a s n o t d e t e c t e d i n t h e e f f l u e n t o f t h e r e a c to r .V e r y s m a l l a m o u n t s o f C5+ a l k a n e s c o u l d b e d e te c t e d .

T h e s a m e b e h a v i o r a s f or C 3 C 4 a l k a n e s w a s o b s e r v e d i n t h e c a se o f a r o m a t -i cs . C 5+ a l k e n e s f o l l o w e d a b e h a v i o r s i m i l a r t o t h a t o b s e r v e d i n t h e c a s e o f C .~ -C 4 a l k e n e s ( F i g . 7 ) .

I n p r e v io u s w o r k , it h a s b e e n o b s e r v e d t h a t w i t h a fe e d o f n i t r o g e n - m e t h a n o l

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F i g . 7 . A r o m a t i c a n d C .~+ a l k e n e y i e l d s o n d e a l u m i n a t e d m o r d e n i t e ( S i / A 1 = 8 0 ) a t 7 5 3 K a n d 1 .0 4b a r t o t a l p r e s s u r e v e r s u s t h e t o t a l a m o u n t o f m e t h a n o l f e d p e r g r a m o f c a ta l y s t. W / F : 1 0 - 3 2 gc a t. h / t o o l m e t h a n o l . F e e d : m e t h a n o l - w a t e r 3 0 : 7 0 w t . - m i x t u r e . ( a ) T o t a l a r o m a t i c s ; ( b ) C s+a l k e n e s .

mixtures, the evolution of the conversion of methanol with the total amountof methanol fed per gram of catalyst was close to tha t observed with a feed of100 methanol [22,38]. Thus, the higher total amount of methanol fed pergram of catalyst when water was used as diluent can not be att ributed to thepartial pressure of methanol, but to the presence of water in the feed. Theseresults suggest tha t water plays a very important role in the suppression of thesteps that lead to coke formation in the catalytic conversion of methanol onSAPO-34 [22]. For dealuminated mordenite, the influence of water on thetotal amount of methanol fed per gram of catalyst was less pronounced thanin the case of SAPO-34, probably because of a d ifferent acidity. However, itwas observed that after 0.015 mol of methanol per gram of catalyst was fed,i.e., almost at the beginning of the reaction, and a mixture of methanol-nit ro-gen with a parti al pressure of methanol of 0.2 bar was used as feed, the yield ofC2-C4 alkenes was 18.3 , while the yield of Cs+ non-aromatics was 6.0 . Whena mixture of methanol- water with the same partial pressure of methanol wasfed, the yield of C2-C4 alkenes was 21.9 and the yield of C~+ non-aromaticswas less than 3.0 , for the same amount of methanol fed per gram of catalyst.

According to the results obtained in the present work, the hydrocarbon dis-tribution changes dramatically with the methanol partial pressure. The yieldof coke precursors decreased as the concentration of methanol in the feed waslowered, thus leading to a slower deactivation by coke formation. The changein the hydrocarbon distribution may be attribu ted to both the partial pressureof methanol and, to some extent, to the interaction between water moleculesand Bronsted acid sites. The decrease of the partial pressure of methanol leadsto the suppression of the alkene conversion steps. On the other hand, it isknown that water can interact with both Lewis and Bronsted acid centers[39,40]. Je nty s et al. [39] have shown by means of IR and mass spectroscopy

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A . J . M a r c h i a n d ; .F . F r o n w r ~ t / A p p l . C a ta l . ,4 9 4 1 9 9 3 ) 9 1 1 0 6 103

t h a t w a t e r c a n b e a d s o r b e d o n B r o n s t e d - t y p e s i t e s, y i e ld i n g a h y d r o x o n i u mio n . T h u s , t h e B r o n s t e d a c i d s it e s o n w h i c h w a t e r i s a d s o r b e d a r e n o t a v a i l a b lef o r i n t e r a c t i o n w i t h t h e r e a g e n t a n d t h e p r o d u c t m o l e c u l e s o f t h e m e t h a n o lc o n v e r s i o n r e a c t i o n . T h i s i s i n a g r e e m e n t w i t h t h e r e s u l t s o f V e d r i n e e t al.[ 4 1 ] , w h o d e m o n s t r a t e d t h a t w a t e r w e a k e n s t h e a c i d s it es . T h e r e f o r e , t h e i n-t e r a c t i o n b e t w e e n a l k e n e m o l e c u l e s a n d t h e s e a c i d s it e s d i m i n i s h e s a n d s o d o e st h e c o n v e r s i o n o f l ig h t a l k e n e s i n t o o l ig o m e r s , a l k a n e s a n d a r o m a t i c s , t h e r e b yi n c r e a s i n g t h e i n i ti a l s e l e c t i v i ty t o C ~ - C 4 . I n a d d i t i o n , w a t e r c a n t r a n s f o r mL e w i s i n to B r o n s t e d a c i d c e n t e r s. T h e r e d u c t i o n o f t h e c o n c e n t r a t i o n o f L e w i sa c i d c e n t e r s b y w a t e r l e a d s to a l o w e r d e a c t i v a t io n b y c o k e , s in c e c o k e f o r m a -t i o n s e e m s t o be f a s t e r o n th i s t y p e o f s i te s [ 4 0 ]. T h e p r o p e n e y i e ld w a s m o r ea f f e c t e d t h a n t h e b u t e n e y i e l d b y th e r e d u c t i o n o f m e t h a n o l c o n c e n t r a t io n .T h i s i n d i c a t e s t h a t t h e f i r s t i n t e r a c t s m o r e s t r o n g l y w i t h t h e a c t iv e a c i d s i te sa n d m u s t b e t h e p r i n c ip a l c o m p o u n d t o b e c o n v e r t e d i n to o l ig o m e r s , a lk a n e sa n d a r o m a t i c s .

O n t h e o t h e r h a n d , a c c o r d i n g t o t h e r e s u l t s o b t a i n e d i n t h i s w o r k , in a g re e -m e n t w i th r e s u l ts r e p o r t e d b y N a c c a c h e e t a l. f o r d e a l u m i n a t e d m o r d e n i t e s[ 1 8 ] , d e a c t i v a t i o n s e e m s to t a k e p l a c e i n t w o s t a g e s. I n t h e e a r l y s t a g e o f t h er e a c t io n , t h e c o n c e n t r a t i o n o f B r o n s t e d a c i d s it e s s e e m s to b e g r a d u a l ly r e-d u c e d b y c o k e p r e c u r s o r s t h a t a r e s tr o n g l y a d s o r b e d . T h u s , t h e p r o b a b i l i t y o fi n t e r a c t io n w i t h a l k e n e s d i m i n i s h e s a n d s o d o e s th e i r c o n v e r s i o n i n t o o l ig o -m e r s , a l k a n e s a n d a r o m a t i c s . T h i s e x p l a i n s th e i n c r e a s e o f p r o p e n e , b u t e n ea n d C.~, a l k e n e y i e l d s a n d t h e d e c r e a s e o f a l k a n e a n d a r o m a t i c y i e l d s w i t h t h ea m o u n t o f m e t h a n o l f e d p e r g r a m o f c a t a l y s t i n t h e e a r l y s ta g e o f t h e r e a c t i o n .T h e e t h e n e y i e ld is n o t a f f e c t e d in t h e s a m e w a y a s t h e o t h e r a l k e n e s , in a g r ee -m e n t w i t h t h e o b s e r v a t i o n t h a t e t h a n e w a s n o t d e t e c t e d in t h e e f f lu e n t . L i g h ta n d C :,, a l k e n e s a r e c o n v e r t e d i n t o a lk a n e s , o l i g o m e r s a n d a r o m a t i c s. P r o p e n es e e m s t o be p r in c i p a l l y c o n v e r t e d i n to p r o p a n e a n d b u t e n e s i n to b u t a n e s . T h e s er e s u l ts a r e in a g r e e m e n t w i th m e c h a n i s m s p r o p o s e d in t h e l i te r a t u r e[ 2 ,1 0 ,4 2 ,4 3 1 .

I n t h e l a t e r s t a g e s o f t h e r e a c t io n , t h e g r o w t h o f t h e c o k e b e c o m e s m o r ei m p o r t a n t , l e a d in g t o t h e b l o c k a g e o f t h e p o r e s , w h i c h r e d u c e s t h e n u m b e r o fa c i d c e n t e r s r a p i d ly , s o t h a t t h e y i e l d o f l ig h t a l k e n e s d e c r e a s e s . A t t h e e a r l ys t a g e o f t h e r e a c t i o n , p o l y - s u b s t i t u t e d a r o m a t i c s c a n b e f o r m e d a n d d i ff u s ee a s il y t h r o u g h t h e 1 2 - m e m b e r e d r in g c h a n n e l s . A s t h e r e a c t i o n p r o g r e s s e s, th ee f f e c ti v e p o r e s i ze b e c o m e s s m a l l e r a n d t h e r e b y t h e d i f fu s i o n o f t h e s e a r o m a t i cm o l e c u l e s b e c o m e s m o r e d i f fi c ul t, f a v o r in g c o k e g r o w t h.CONCLt SIONS

T h r e e t y p e s o f a c i d s i te s d u e t o a l u m i n i u m - h y d r o x y l g r o u p s h a v e b e e n d e -t e c t e d i n t h e m o r d e n i t e s a m p l e s u s e d in th i s w o r k . O n e o f t h e m a b s o r b e d a t3 6 5 0 c m - 1 w a s a t t r i b u t e d t o e x t r a - f r a m e w o r k m a t e r i a l . T h e o t h e r t w o , a t 36 1 0

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1 4 A.J. Marchi and G.F. Froment/Appl. Catal. A 94 1993) 91-106a n d 3 5 4 0 c m - 1 c o r r e sp o n d t o f r a m e w o r k a l u m i n u m - h y d r o x y l gr o up s o f t h ez e oli te . I n t h e n o n - d e a l u m i n a t e d m o r d e n i t e , t h e l o w - f r e q u e n c y a l u m i n i u m -h y d r o x y l g r o u p s s e e m s t o b e p o i s o n e d b y t h e e x t r a - f ra m e w o r k a l u m i n a . A s t h es i l ic o n - t o - a l u m i n i u m r a t io i s ra i s e d by d e a l u m i n a t i o n , th e c o n c e n t r a t i o n o fa l u m i n i u m - h y d r o x y l g r o u p s t h a t a b s o r b a t 3 6 50 a n d 3 61 0 c m - 1 d e c re a se s . T h er e m o v a l o f e x t r a- f r am e w o r k a l u m i n a o p e n s t h e 1 2 - m e m b e r e d r i n g c h a n n e l sa n d m a k e s t h e a l u m i n i u m - h y d r o x y l g r o u p s t h a t a b s o rb a t 3 5 40 c m - 1 a v ai l-a b le . T h e d e c r ea s e o f t h e h i g h - f r e q u e n c y a l u m i n i u m - h y d r o x y l g r o u p s le a d s toa c a t a l y s t w i t h h i g h d u r a b l e a c t i v i ty a n d s e l e c ti v i ty fo r l ig h t a l k e n e s. T h i s ist h e c a se f or m o r d e n i t e s a m p l e s w i th s i l i c o n - t o - a l u m i n i u m a t o m i c r a t io s o f 80o r h ig h e r . I n t h e s e s a m p l e s , th e 1 2 - m e m b e r e d ri n g c h a n n e l s a r e c o m p l e t e lyf r e e a n d l a rg e m o l e c u l e s l ik e p o l y - s u b s t i t u t e d b e n z e n e s c a n b e f o r m e d a n d d if -f u se t h r o u g h t h e p o r o u s s t r u c t u r e o f th e z e ol ite . T h e i n t e r a c t i o n o f h y d r o c a r-b o n s w i t h t h e c a t a l y st s u r f ac e i s le ss p r o n o u n c e d t h a n i n t h e c a s e o f s a m p l e sw i t h h i g h e r s i l ic o n - t o - a l u m i n i u m r at io s , d u e t o t h e l o w n u m b e r a n d t h e t y p eo f a c i d si te s t h a t a r e p r e s e n t i n t h e s e d e a l u m i n a t e d m o r d e n i te s .

T h e d u r a b l e a c t iv i ty o f t h e d e a l u m i n a t e d m o r d e n i t e s a n d t h e s e le c ti v it y t ol i g h t a l k e n e s c a n a l so b e i m p r o v e d b y d e c r e a s i n g t h e p a r t i a l p r e s s u r e o f t h em e t h a n o l a n d a d d i n g w a t e r t o t h e f e e d. T h e d e c r ea s e o f t h e p a r t i a l p r e s s u r e o fm e t h a n o l l e a d s t o a h i g h e r y i e l d o f li g h t a l k e n e s a n d a l o w e r y ie l d o f a l k a n e s ,o l ig o m e r s a n d a r o m a t i c s. W a t e r c o m p e t e s w i t h l i g h t a l k e n e s f or t h e B r ~ n s t e da n d L e w i s a c id s it e s. T h e a d s o r p t i o n o f w a t e r o n t h e s e a c i d c e n t e r s r e d u c e st h e i r s t r e n g t h a n d c o n c e n t r a t i o n a n d , t h e r e b y , t h e p r o b a b i l i ty o f t h e i r i n t e r -a c t i o n w i t h h y d r o c a r b o n s . A s a c o n s e q u e n c e , th e i n i ti a l c o n v e r s i o n o f l i g h ta l k e n e s , p r i n c i p a l l y p r o p e n e , i n t o o l i g o m e r s , a r o m a t i c s a n d c o k e d e c r e a s e s .B o t h t h e l ow e r y ie l d o f c o k e p r e c u r s o r s a n d t h e w e a k e r i n t e r a c t i o n b e t w e e nc o k e p r e c u r s o r s a n d a c i d si te s r e d u c e s t h e d e a c t i v a t i o n b y c o k e .

A t t h e e a r ly s t a g e s o f t h e r e a c ti o n , t h e g r a d u a l r e d u c t i o n o f t h e n u m b e r o fa c i d s it e s d e c r e a s e s t h e p r o b a b i l i t y o f t h e i r i n t e r a c t i o n w i t h l ig h t a l k e n e m o l -e c u le s . T h e r e b y , t h e c o n v e r s i o n o f l i g h t a l k e n e s i n t o a l k a n e s a n d a r o m a t i c sd e c r e a s e s a n d t h e s e l e c t iv i t y t o l i g h t a l k e n e s i n c r e as e s .ACKNOWLEDGMENTS

T h e a u t h o r s a r e g ra t e fu l t o t h e B e l g i a n M i n i s te r i e vo o r d e P r o g r a m m a t i ev a n h e t W e t e n s c h a p s b e l e i d f o r t h e I U A P C e n t e r o f E x c e l le n c e G r a n t . O n e o ft h e a u t h o r s ( A . J .M . ) w i s h e s t o t h a n k t o C o n s e jo N a c i o n a l d e I n v e s ti g a c io n e sC i e n t if ic a s y T ~ c n i c a s ( C O N I C E T - A r g e n t i n a ) f o r a l ea v e o f a b s en c e .

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