Hunger Et Al 1997. Adsorption of Water on Zeolites of Different Types

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Jornal of Thermal Analysis , Vo l . 49 (1997) 553-565

A D S O R P T I O N O F W A T ER O N Z E O L I T ES O F D I F F E R E N TT Y P E S

B . H u n g e ? , S. M a t y s i k ~, M . H e u c h e l 1, E . G e i d e l a n d H . T o u f a rl lnst i tu te of Physical and Theoret ical C hemistry , Universi ty of Leipzig , D -04103 L eipzig ,G e r m a n y2Ins t itu te o f Phys ica l Chemist ry , Un ivers i ty o f Ham burg , D -20146 Ham burg , G ermany3C entrum voo r Opp ervlaktechem ie en Katalyse , Katholieke Universite it Leu venB-3001 L euven (Hever lee ) , Be lg iumAbstrac t

We have investigated the in teract ion o f water with Na +-io n exchanged zeoli tes of d ifferents t ruc tu res (LTA, FAU , ER I , M O R and MF I) by means o f tempera tu re -p rogram med deso rp t ion(TPD ) . The non- i so the rmal desorp t ion o f water shows , depend ing on the zeo l i te type , d i f fe ren t lystructured desorption profi les . In every ease the profi les have, however , two main ranges. Usinga regu la r iza t ion method , desorp t ion energy d is t r ibu t ion func t ions have been ca lcu la ted . Thedeso rption e nerg y dis tr ibutions betwee n 42--60 kJ mol ~, wh ich can be a t tr ibuted to a non-s pe-cif ic in terac t ion of water , show two clear ly dist inguished energ y ranges. Th e water desorptio nbehav iour o f th is range correla tes w ith the e lectronegativi ty of the zeoli tes and the average charg eof the lat tice oxygen a toms ca lcu la ted by means o f the e lec t ronega t iv i ty equa l iza t ion m ethod(EE M ). Th e part of the desorption energy distributions in the range of 60 -90 kJ tool ~ , ref lect inginteract ions of water with N a + cat ions, shows two m ore or less pronounc ed ma xim a. In agree-ment with vibrat ional spectroscopic s tudies in the far infrared region, i t may be concluded thata l l sam ples un der s tudy possess a t least two different cat ion s ites.Keywords: deso rption energy distributions, EE M calculat ions, far- infrared spectroscopy,N a+- ion exchanged zeoli tes o f d ifferent types, tem peratu re-prog ram me d desorp-t ion (TPD) , wa te r adsorp t ionIntroduct ion

K n ow l e dge o f i n t er a c ti ons o f w a t e r w i t h z e o l i te s i s o f g r e a t i n t e r e s t fo r t he i r a p -p l i c a t i on a s c a ta l y s ts a nd a ds o r be n t s . F o r t ha t the r m oa na l y t i c a l m e t hods a r e f r e -que n t l y u s e d to c ha r a c t e r i z e t he i r hyd r ophob i c p r ope r t i e s ( e .g . , [ 1 - 3 ] ) . H ow e ve r ,non - i s o t he r m a l i nve s t i ga t i ons a l l ow de t a i l e d i n f o r m a t i on w i t h r e s pe c t t o t hes t r e ng t h o f i n t e r a c t i on o f m o l e c u l e s w i t h d i f f e r e n t a ds o r p ti on s it e s, a s i t w a s s ho w nf o r o t h e r s y st e m s ( e .g . , [ 4 - 6 ] ). T h e r e f o r e , w e h a v e p e r f o r m e d te m p e r a t u r e - p r o -g r a m m e d d e s o r p t i o n ( T P D ) o f w a t er o n N a + - i o n e x c h a n g e d z e o li te s o f d i f f e r e n tt ype s i n o r de r t o c ha r a c t e r i z e the i r a d s o r p t i on p r ope r t i e s w i t h r e s pe c t to w a t e r a sp r obe m o l e c u l e i n m o r e de t a il s. E l e c t r one ga ti v i ti e s a nd pa r t ia l c ha r ge s o f t he l a t t ic eoxy ge n a t om s ha ve be e n c a l c u l a t e d w i t h t he e l e c tr one ga t i v i t y e qua l i z a t ion m e t ho d0 3 6 8 -4 4 6 6 / 9 7 / $ 5 . O09 1997 Akaddmiai Kiad6, Budapest

John Wiley & Sons LimitedChichester


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5 54 H U N G E R e t al. : Z E O L I T E S

( E E M ) i n o r d e r t o c o r r e la t e t h e T P D r e s u l ts w i t h c h e m i c a l a n d s t ru c t u r a l p r o p e r t i e so f t h e z e o l it e s . T h e r e s u l t s f o r t h e i n t e ra c t io n o f wa t e r w i t h th e N a + - c a t i o n s w i l l b ed i s c u s s e d t o g e t h e r w i t h fa r i n f ra r e d s p e c t r o s c o p i c o b s e r v a t io n s .E x p e r i m e n t a lZeolites

T h e z e o l i t e s we r e c o m m e r c i a l l y a v a i l a b l e m a t e r i a l s s u p p l i e d ' b y t h e B i t t e r f e l dAG ( Ge r m a n y ) . T h e c h a r a c t e r is t i c s a re s u m m a r i z e d i n T a b l e 1 , w h e r e b y t h e z e o -l i t e s a r e a r r a n g e d w i t h d e c r e a s i n g a l u m i n i u m c o n t e n t . T h e p o r e v o l u m e s we r e d e -t e r m i n e d b y t h e g r a v i m e t r ic u p t a k e o f n - h e x a n e a t 2 9 8 K .Temperature-programmed deso rption (TPD)

T h e t e m p e r a t u r e - p r o g r a m m e d d e s o r p t i o n ( T P D ) w a s c a r r i e d o u t i n a c o n v e n -t i o n a l f l o w d e v i c e w i t h h e l i u m a s c a r r i e r g a s ( 3 1 h - l ), F o r e v o l v e d g a s d e t e c t i o nb o t h a t h e rm a l c o n d u c t i v i t y d e t e c t o r ( T C D ) a n d a q u a d r u p o l e m a s s s p e c t r o m e t e r( L e y b o l d , T r a n s p e c t o r C I S S y s t e m ) w i t h a c a p i l la r - c o u p l in g s y s t e m w e r e u s e d . T h ez e o l i t e s w e r e e q u i l i b r a t e d w i t h wa t e r v a p o u r o v e r a s a t u r a t e d C a (NO3 )2 - s o l u t i o ni n a d e s i c c a to r . F o r e a c h e x p e r i m e n t 2 0 - 1 0 0 m g o f t h e wa t e r l o a d e d z e o l i t e we r eu s e d i n a m i x t u r e w i t h 1 g q u a r tz o f t h e s a m e g r a i n s i z e ( 0 . 2 . .. 0 . 4 r a m ) . A t fi r s ta l l s a m p l e s w e r e f l u s h e d w i t h h e l i u m a t r o o m t e m p e r a t u r e f o r a p e r i o d o f 4 0 m i n .A f t e r wa r d s t h e l i n e a r t e m p e r a t u r e p r o g r a m ( 1 0 K m i n -1 ) wa s s t a r te d . F o r a k i n e t ice v a l u a t i o n e x p e r i m e n t s we r e c a r r i e d o u t u s i n g d i f f e r e n t h e a t i n g r a t e s ( 2 . . . 2 0K r a i n - l ). I n s o m e i n v e s ti g a ti o n s th e a d s o r p t i o n wa s p e r f o r m e d b y m e a n s o f a wa t e rv a p o u r p u l s e a n d a n i s o t h e r m a l d e s o r p t i o n o f t h e wa t e r s u r p l u s.

T h e a d s o r b e d w a t e r a m o u n t s ( T a b le 1 ) w e r e d e t e r m i n e d a d d i t i o n a l l y b y a s i m u l -t a n e o u s t h e rm a l a n a l y s is a p p a ra tu s ( T G - D T A - Q M S , N E T Z S C H , S y s t e m S T A - Q M S4 0 9 / 4 0 3 ) . F o r a l l o f th e s e e x p e r im e n t s t h e s a m p l e we i g h t wa s a b o u t 4 0 m g . A h e a t -i n g r a t e o f 1 0 K m i n -1 a n d a h e l i u m f l o w o f 4 . 5 1 h -1 we r e u s e d .Far-infrared spectroscop ic investigations

S a m p l e s w e r e p r e s s e d t o f o r m s e l f -s u p p o r t i n g w a f e r s a n d d e h y d r a t e d i n a f a r - I Rc e l l w i t h p o l y e t h y l e n e w i n d o w s i n a s tr e a m o f d r y n i t ro g e n a t 6 2 3 K . A f t e r c o o l i n gm e a s u r e m e n t s o f th e f a r in f r a re d s p e c t r a we r e c a r r i e d o u t i n a n i t ro g e n a t m o s p h e r ea s we l l f o r s u p p r e s s i o n o f ro t a t io n a l l in e s o f wa t e r v a p o u r . A b s o r p t i o n s p e c t r a a tr o o m t e m p e r a t u r e we r e r e c o r d e d u s i n g a D i g i l a b F T S 1 5 E s p e c t r o m e t e r , e q u i p p e dwi t h a m e r c u r y v a p o u r s o u r c e , a 6 . 2 5 l a in m y l a r b e a m s p l i t te r a n d a l i q u i d - h e l i u m -c o o l e d s i l i c o n b o l o m e t e r ( I n f ra r e d L a b o r a t o r i e s ) a s h i g h s e n s i t iv e d e t e c to r . S p e c t r awe r e t a k e n i n th e r a n g e 4 0 0 - 2 0 c m -1 w i t h a r e s o l u t io n o f 4 c m -1 c o a d d i n g 2 5 6 s c a n st o o b t a i n a n a c c e p t a b l e s i g n a l - t o - n o i s e r a t i o . A t r i a n g l e a p o d i z a t i o n f u n c t i o n wa sa p p l i e d f o r t h e F o u r i e r t r a n sf o r m a t io n .

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C a l c u l a t i o n sCry s ta l s t ruc ture s

Cry s ta l s t r uc ture ca lcu la t ions we re ca r r ied ou t on zeo l i t e N aA (S i/A1 = 1), N aX( S i /A I = 1 ), N a Y ( S i / A I = 3 ) a n d N a - m o r d e n i te ( S i/ A I = 5 ) . T h e s p a c e g r o u p s o f t h es t r u c t u r e s we r e r e d u c e d t o F 4 3 c f o r L T A , F 2 3 f o r F A U an d P 2 1 /m f o r M O R i n o r-d e r t o a l l o w t h e i n s e rt i o n o f a l u m i n u m a t o m s o b e y i n g L 6 w e n s t e i n s ru l e . T h e r e s u l t-i n g s t r u c t u r e s we r e o p t i m i z e d u s i n g t h e DL S p a c k a g e [ 7 ] w i t h t h e f o l l o wi n g p a -r a m e t e r s : S i- -O d i s t a n c e = 0 . 1 6 1 u rn , A1 -- O d i s t a n c e = 0 . 1 7 4 n m , O- T - - O a n g l e =1 0 9 . 5 ~ T - - O - T a n g l e = 1 45 ~ T h e c a t i o n s w e r e p l a c e d i n t o th e u n i t c e l l s u s in g l i t-e r a tur e da ta [8 ] in o rde r to ( i ) maximize the in te r - ca t ion ic d i s tance and to ( i i ) maxi -m i z e t h e c o o r d i n a t i o n b y l a tt ic e o x y g e n s . T h e l o c a l p o s i t i o n o f th e c a t i o n s w e r e a d -j u s t e d d u r i n g t h e E E M c a l c u l a ti o n s a t t h e m i n i m u m o f t h e e l e c tr o s t a t ic e n e r g y .E E M c a lc u la t io n s

T h e e l e c t r o n e g a t i v i t y e q u a l i z a ti o n m e t h o d ( E E M ) a l l o ws t h e c a l c u l a t i o n o f e l e c -t ronega t iv i t i e s and a tomic charges in molecu les as we l l a s in l a rge c lus te r s and in -f i n i t e c r y s t a l s t r u c t u r e s , wh e n t h e s t r u c t u r e a n d c h e m i c a l c o m p o s i t i o n a r e k n o wn .F o r a d e t a i le d d e s c r i p t i o n o f t h e E E M a n d i ts a p p li c a t io n s [ 9 - 1 1 ] , i ts p r e d e c e s s o r s[12] and i ts r igorous de r iva t ion in the den s i ty func t iona l theo ry [13] , we r e f e r to theo r i g in a l l i te r a tu r e . F o r th e p r e s e n t c a lc u l a t io n s a c o m b i n e d M o n t e C a r l o / E E M p r o -c e d u r e [ 1 4 ] wa s a p p l i e d . T h i s p r o g r a m a l l o ws t o o p t i m i z e t h e c a t i o n p o s i t i o n swi t h i n t h e c r y s t a l s tr u c t u r e o n t h e b a s e o f t h e t o t al e l e c tr o n i c e n e r g y o f t h e s y s t e m ,i . e . , inc lud ing the e lec t ros ta t i c in te r ac t ions as we l l a s the charge dependent se l f - en-e r g y o f t h e a t o m s . L o n g r a n g e e f f e c t s we r e t a k e n in t o a c c o u n t b y a T a p e r t y p e s u m -m a t i o n d u r i n g t h e o p t im i z a t i o n . Af t e r t h e e n e r g y o f t h e s y s te m h a d r e a c h e d a m i n i -m u m , t h e c a l c u l a t io n o f t h e re l e v a n t p a r a m e t e r s w a s re d o n e a p p l y i n g a m o r e a c c u -r a t e B e r t a u t t y p e s u m m a t i o n . T h e r e s u l t s we r e , h o we v e r , i d e n t i c a l w i t h i n t h e m a r -g i n s o f t h e n u m e r i c a l e r r o r o f t h e p r o g r a m . T h e z e o l i t e la t t ic e wa s a s s u m e d t o b er i g id d u r i n g a l l co m p u t a t io n s . T h r e e d i f fe r e n t s c e n a r i o s w e r e a p p l i e d f o r t h e c h a r g et r a n s fe r b e t w e e n l a t ti c e a n d c a ti o n s : ( i ) c a t io n s w e r e n o t in c l u d e d i n to t h e E E M c a l -cu la t ions , the to ta l cha rge of the l a t t ice was se t ze ro a rb i t r a r i ly ; ( ii ) f r ee charget r ans f e r w as a l lowed be tw een zeo l i t e l a t t ice and ca t ions ; ( i i i) the charge of the ca t -i o n s w a s f i x e d t o + 1 a n d t h e t o ta l c h a r g e o f t h e l a tt i ce w a s f ix e d t o t h e c o r r e s p o n d -i n g n e g a t i v e v a l u e (qlattiee =--]Eqcation). S c e n a r i o ( i ) a ll o ws t o s e p a r a t e t h e i n f l u e n c eo f t h e z e o l i t e s tr u c t u r e p a r t i a ll y f r o m t h e e f fe c t o f th e c h e m i c a l c o m p o s i t i o n . H o w -e v e r, s i n c e i n p r a c t ic e t h e e f f e c t o f th e c o m p o s i t i o n w i l l a lwa y s b e d o m i n a t in g , t h e s eresu l t s cannot be cor r e la ted d i r ec t ly to exper imenta l r e su l t s . The scenar io ( i i ) i s thep h y s i c a l l y m o s t r e a li s ti c o n e . Un f o r t u n a t e ly , t h e d e te r m i n a t i o n o f a c c u r a t e E E M p a -r a m e t e r f o r a l k a li - m e t a ls i s st il l a n u n s o l v e d p r o b l e m , m o s t l y d u e t o t h e d i s c o n t i n u -i t y o f t h e s e p a r a m e t e r s a t a c h a r g e o f + 1 [ 1 5 ]. T h e r e f o r e , i t i s m o r e s u i t a b l e inm o s t c a s e s t o a p p l y s c e n a r i o ( ii i) a s a f a i r ly g o o d a p p r o x i m a t i o n o f th e r e a li ty . I nt h i s c a s e , E E M p a r a m e t e r s f o r t h e c a t i o n s a r e n o t r e q u i r e d . On l y t h e e l e c t r o s t a t i cpo la r iza t ion of the l a t t i ce by the ca t ions i s t aken in to accoun t in th i s way , whi le the

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charge t rans fe r pa r t i s ignored . Ab in i t io ca lcu la t ions on a lka l i ca t ion - wa te r sys -t e m s i nd i c a te , how e ve r , t ha t the c ha r g e t ra ns f e r be t w e e n t he c a t ions a nd c oo r d i na t -i ng oxy ge n a tom s i s r a t he r s m a l l [ 16 ]. T h e e l e c t rone ga t i v i ti e s c a l c u l a t e d by m e a nso f s c e na r i o ( i ii ) a nd t he p r e d i c t e d a ve ra ge c ha r ge o f the l a t t ic e ox yge n a t om s a r es u m m a r i z e d i n T a b le 2 .R e m i t s a n d d i s c u s s i o n

F or a ll z e o l i te s t he de s o r p t i on c u r ve s o f w a t e r a r e s how n i n F i g '. 1 . I n e a c h c a s e ,r e ve r s i b il i ty c ou l d be p r oo f e d by pe r f o r m i n g r e pe a t e d a ds o r p t i on / de s o r p t i on c yc l es .T he m a s s - s pe c t r o s c op i c a na l y s i s ( 1 - 120 : 18 a i nu ; CO 2: 44 a m u; N 2 : 28 a i nu ; 02 : 32a m u) s ho w e d t ha t on l y w a t e r w a s de s o r be d . Ch a r a c t e r i st i c f o r a ll z e o li te s a r e t w oc l e a r l y v i s ib l e r a nge s o f de s o r p t ion , T h e p r e dom i na n t a m oun t o f w a t e r de s o r bs upt o a b o u t 4 0 0 - 4 5 0 K . S m a l le r a m o u n t s d e s o r b b e tw e e n ab o u t 4 5 0 - 5 0 0 K a n d a b o u t7 00 K . T he a m oun t o f w a t e r de s o r b i ng i n t h i s t e m pe r a t u r e r a nge w a s s e pa r a t e df r o m d e s o r p t io n u p t o a b o u t 4 5 0 K b y c o m b i n a ti o n o f i s o th e r m a l a n d n o n - i so t h e r -m a l de s o r p t i on . F o r t h i s pu r pos e , he a t i ng w a s c a r r i e d ou t up t o the f i r s t pe a k m a x i -m u m a n d t h e n t h e te m p e r a t u re p r o g r a m w a s st o pp e d . W h e n n o f u r t h e r d e s o r p ti o nw a s obs e r ve d a t th i s t e m pe r a t u r e , t he te m pe r a t u r e p r og r a m w a s r e s t a rt e d . T h e ob -t a in e d d e s o r p t i o n c u r v e s a r e s h o w n i n F ig . 2 a n d t h e d e s o r b e d a m o u n t s a r e s u m m a -r i z e d i n T a b le 2 . A c om pa r i s on w i t h the a ds o r be d a m o un t s de t e r m i ne d a t s a t u r a t i onp r e s s u r e i n t he de s i c c a t o r (T a ble 1 ) s how s t ha t du r i ng f l u s h ing w i t h he l i um a t r oo mt e m p e r a t u r e a l r e a d y 2 5 - 4 0 % o f w a t e r d e s o rb s b e f o r e t h e T P D w a s s ta r te d . T h e r e -f o r e , t he T P D a l low s i n f o r m a t i on e s pe c ia l l y w i t h r e s pe c t t o l ow i n i ti a l c ove r a ge .T h i s s hou l d be a n a dva n t a ge i n c om pa r i s on t o i s o the r m a l a ds o r p t i on m e a s u r e m e n t s ,

i i , f300 400 500 600 700

T / KFig. 1 Desorption curves of water: 1: NaA, 2: NaX, 3: NaY, 4: Na,K-erionite, 5: Na-mor-denite, 6:NaZSM-5

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Fig . 2 D esorp t ion cu rves a f te r iso the rmal desorp tion : 1: NaA , 2 : NaX , 3 : N aY, 4 : N a ,K-e r io n i te , 5 : N a - m o r d e n it e , 6 : N a Z S M - 5

w he re th i s rang e i s re l a t ive ly d i f f icu l t to inves t iga te , e spec ia lly , becaus e of the v e rys m a l l e qu i l ib r i um p r e s s u r e s.

I n o r de r t o ob t a i n de t a i l e d i n f o r m a t i on a bou t t he de s o r p t i on p r oc e s s a f u r t he re va l ua t i on o f t he e xpe r i m e n t a l c u r ve s w a s c a r r i e d ou t . F o r t h i s pu r pos e a r a t e l a wof f i r s t o r d e r w i t h a d i s t ri bu t i on f unc t i on f ( E ) o f de s o r p t i on e n e r gy E w a s c o ns i d -e red [17] :

EmaxdO= A J" 0t (E,T) exp (-E/R T) f(E ) dE= ---~-

E m i n(1 )

w h e r e ra i s the ov era l l desorp t ion ra te , 0 i s the ov era l l deg ree of coverag e , and A thep r e - e xpo ne n t i a l f a c t o r. 01 i s t he l oc a l c ove r a ge o f s i te s c h a r a c t e r i z e d by a de s o r p t i one ne r gy E . E m ina nd E m ax a r e t he l i m i ts o f t he r a nge o f de s o r p t i on e ne r gy . T he c a l c u -l a ti o n s w e r e c a r r i e d o u t b y m e a n s o f t h e p r o g r a m I N T E G [ 1 8] , w h i c h in v o lv e s a~ regula r i za tion m eth od for so lv ing th i s in tegra l equa t ion .T h e p r e - e xp one n t i a l f a c t o r A r e qu i r e d t o so l ve E q . ( 1 ) w a s e s t im a t e d by u s i ng

t h e d e p e n d e n c e o f th e t e m p e r a t u re o f th e p e a k - m a x i m u m o n t h e h e a t i n g ra t e [ 19 ,2 0 ] . F o r N a Z S M - 5 t h e p r e- e x p o n e n ti a l fa c to r w a s d e t e r m i n e d to b e b e t w e e n 2 x 1 0 6a n d l x l 0 7 m i n -1 [6 ] . T he va l ue f o r N a X w a s be t w e e n 7x106 a nd 2x107 m i n -1. Thisr a n g e n e a r l y c o r r e s p o n d s t o t h e a c c u r a c y o f e st im a t i o n o f t h e s e p a ra m e t e r s , O n t h eo t he r ha nd t he p r e - e xpone n t i a l f a c t o r r e p r e s e n t s a n e f f e c t ive pa r a m e t e r c on t a i n i nga l s o c on t r ibu t i ons o f t he m a s s t ra ns po r t [ 21 ]. T h e r e f o r e , a de pe nd e nc e on t he s a m -p l e a m o u n t c a n n o t b e e x c l u d e d . B u t, d i f fe r e n t s a m p l e a m o u n t s w e r e n e c e s s a r y t o

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ensure, that s imi lar amounts o f water desorb , and the experimental condi t ions couldb e k ep t co mp arab le . Beca u se the shape o f the deso rp t io n en erg y d i s tr ib u t io n i s n o ts ig n i f ican t ly in f lu en ced b y the v alue o f the pre-ex po n en t ia l fac to r [1 7 ] , a co n s tan tv a lu e o f 5 x 1 0 6 ra in -1 was assu m ed fo r a ll zeo l i t es . The resu l tin g erro r i s o f the o rd ero f 7-8 % .

T h e n u m e r i ca l s o l u t io n o f E q . ( 1 ) b y m e a n s o f th e I N T E G p r o g r a m w a s c a r ri e dout wi thout any assumptions or constraints about the resul t ing d istr ibut ion func-

Lr2

I ~ . , [ ! I r , ~3 0 4 0 5 0 6 0 7 0 8 0 9 0

E / k J m o l 'F i g . 3 Desorption energy distributions: 1: N a A , 2 : N a X , 3 : N a Y

, i , i i , i ,3 0 4 0 5 0 6 0 7 0 8 0 9 0

E / k J m o l ~F i g . 4 Desorption energy distributions: 4: Na,K-erionite, 5: Na-mordenite, 6 : N a Z S M - 5

J . T h e r m a l A n a l . , 4 9 , 1 9 9 7


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HUNGER e t al.: ZEOLITES 561

t ions . Th us ne ga t ive pa r t s in the d i s t r ibu t ions a re poss ib le . T he y fa il t o posses s an yphys i c a l m e a n i ng , bu t i f t he d i s t ri bu t i on is dom i na t e d by the ne ga t i ve pa r t s , t he u s e dl oc a l m ode l doe s no t de s c r i be a pp r op r i a t e l y t he e xpe r i m e n t a l da t a . T he ne ga t i vepa r t s a r e ne g l e c t e d i n t he i n t e r p r e t a ti on o f t he d i s tr i bu t ion f unc t i ons .T h e c a l c u l a t e d de s o r p t i on e ne r gy d i s t ri bu t ion f unc t i ons a r e p r e s e n t e d i n F i g s 3a nd 4 . T h e e n e r gy r a ng e o f a bou t 42 t o 90 k J t oo l - l a g r e e s w e l l w i t h r e po r t e d he a t so f a ds o r p t i on on c o m pa r a b l e z e o l i te s f o r t he r e s pe c ti ve c ove r a ge w i t h w a t e r ( e . g . ,[ 22 - 24 ] ) . T he r e f o r e , t he d i s tr i bu t ion f unc t i ons s e e m t o be a u s e f u l . m e a s u r e t o c ha r -a c t e r i z e t he i n t e r a c ti on s t re ng t h o f w a t e r ve r y de t a il e d . A s c a n be s e e n c l e a r l y , t hed i s t r ibu t i on f un c t i ons a r e d i f f e r e n t l y s t r uc t u r e d i n de pe n de n c e on t he Ze o l it e t ype .E x c e p t f o r N a Z S M - 5 , a ll e n e r g y d is tr ib u t io n s c o n s i st o f fo u r , d i ff e r e n t s u p e ~ o s e dr a nge s . T h e t w o m a x i m a a t a bou t 47 -- 49 k J t oo l -1 a nd a bou t 5 2 - 5 5 k J t oo l - ' , a p -p e a r i n g w i t h d i f f e r e n t fr e q u e n cy , c o r r e s p o n d to w a t e r d e s o r b i n g u p t o a b o u t 4 5 0 -5 00 K . T h i s w a t e r c a n be a t t r i bu te d t o non - s pe c i f i c in t e r a c ti ons w i t h t he z e o l i t icf r a m e w or k ( l a tt i c e oxyg e n ) . S i m i la r va lue s f o r he a ts o f a ds o r p t i on ha ve be e n f oundf o r w a t e r on s o l i d s w i t h t he s a m e s t r uc t u r e bu t w i t hou t s pe c i f i c a ds o r p t i on s i t e s ,e . g . , c a t i ons ( e . g . , s i l ic a l it e [25 ] ). F o r N a , K - e r i on i t e , t he r a nge w i t h t he h i ghe s t e n -e r g y a ppe a r s on l y a s a shou l de r . T he l ow e s t e ne r g y r a nge f o r N a - m or de n i t e a ls o a p -pe a r s a s a s hou l de r . I n t he c a s e o f N a ZS M - 5 t h i s pa r t o f t he d i s tr i bu t i on f unc t i on i sa v e r y b r o a d p e a k o n l y . S u m m a r i z e d , t h e e n e r g y r a n g e b e t w e e n 4 2 - 6 0 k J t o o l -1s h o u l d i n d i ca t e t h e i n f l u e n c e o f b o t h a l u m i n i u m - c o n t e n t a n d f r a m e w o r k s t ru c t u r eon t he s t r e ng t h o f w a t e r i n te r a c t ion .

C o n s i d e r i n g t h e p e a k t e m p e r a t u r e o f t h e d e s o r p ti o n c u r v e a s a n a v e r a g e m e a s u r ef o r t he s t r e ng t h o f i n t e r a c t i on a good l i ne a r c o r r e l a t i on w i t h t he c a l c u l a t e d e l e c -t r one ga t i v i t y w a s f ound , a s c a n be s e e n i n F ig . 5 . A s i m i l a r c o r r e l a t i on o f t he pe a kt e m p e r a t u r e w a s f o u n d i n d e p e n d e n c e o n t h e m e a n c h a r g e o f l a tt ic e o x y g e n a t o m s( T a b le 2 ) , w h i c h a l s o m e a s u r e s t he ba s i c i t y o f z e o li te s .

H ow e ve r , a n e s s e n t ia l p r op e r t y o f the de s o r p t i on e n e r gy d i s tr i bu t ions i n F i g s 3a nd 4 is t he d i f f e r e n t i n t e ns it y i n t he t w o e ne r gy r a nge s at 42 - 5 0 k J t oo l- I a n d 5 0 -60 k J m o1-1 . T he i n t e r p r e ta t i on o f t he r e l a ti ve r a t io o f t he s e i n t e ns i ti e s s hou l d g i ve

4 1 0"~ 390

3 7 0

3 5 0

1

- - i t i _ , i

-4 -2 0 2 4e l e c t r o n e g a t i v i t yF i g . 5 P e a k t e m p e r a t u r e i n d e p e n d e n c e o n t h e e l e e t ro n e g a t i v i ty : 1: N a A , 2 : N a X , 3 : N a Y,5 : N a - m o r d e n i t e

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a m or e c om pr e he n s i ve p i c t u r e o f the e ne r ge t i c he te r oge n it y . F o r a l l z e o li t e s d i s ti nc tca lcu la ted charge va lues resu l t fo r oxygen a toms a t d i f fe ren t c rys ta l lographic pos i -t ions . F o r N a A a nd N a X, w he r e e ne r g i e s in t he s e c ond r a nge o f h i ghe r de s o r p t i onene rgy appe ar wi th m ore in tense f requency (curves 1 and 2 in F ig . 3 ) , l a rg e r pa r t i a lcharges predomina te . In cont ras t , l ower pa r t i a l cha rges a re more f requent for NaY.In tha t case , the f i r s t r ange of lower deso rp t ion energ ies (curve 3 in F ig . 3 ) appea rsw i t h h i gh e r i n t e nsi ty . F o r N a - m or de n i t e t he pa r ti a l oxyge n c ha r ge s a r e d i s t r i bu t e dre la t ive ly un i formly . There fore , i t may be supposed tha t a cor re la t ion ex i s t s be -t w e e n t he r e la t ive f r e que nc y o f t he t w o e ne r gy r a nge s o f t he de s o i: p ti on e ne r gy d i s -t r ibu t ions and the bas ic i ty (e l ec t ronega t iv i ty ) o f the zeo l i te s . F or tha t the d i s t r ibu-t ion fun c t ions bew een 40- -60 kJ mo1-1 were f i t ted by two Gauss ian func t ions . F igu re6 shows , tha t wi th inc reas ing e lec t ronega t iv i ty (dec reas ing bas ic i ty of f ramework)t he r e l a ti ve f r e que nc y o f t he low e r e ne r gy r a nge , de t e r m i ne d by t he pe a k a r e as o fthe Gauss -peaks , inc reases .Bo t h c o r r e l a t i ons ( F i g s 5 a nd 6 ) s how t ha t N a - m or de n i t e ha s a h i ghe r ba s i c it yt ha n i t w ou l d c o r r e s p ond t o i ts a l um i n i um c on t e n t ( s e e a ls o T a b le 2 , w h e r e t he z e o -l it e s a r e a r r a nge d w i t h de c r e a s i ng a l um i n i um c on t en t ) . T he e xc e p t i ona l s t r ongba s i c it y o f t h is z e o l i te t ype w a s a l s o s ho w n f o r py r r o l e a ds o r p t i on by m e a ns o f i n -f ra red spec t ro scopic inves t iga t ions [26] .T h e pa r t o f t he d i s t ri bu t i on f unc t i on be t w e e n 6 0 - 90 k J t oo l -1 c a n be a s s i gne d t oin te rac t ion wi th Na+-ca t ions . Typica l ly , the d i s t r ibu t ions show two ex tend ed energyr a nge s w i t h f r e que nc i e s a nd pos i t i ons c ha r a c t e r i s t i c f o r e a c h z e o l i t e t ype . I f t hede s o r p t i on e ne r gy d i s tr i bu t ions a r e c a l c u l a te d w i t h t he de s o r p t i on c u r ve s o f F i g . 2 ,bo t h e ne r gy r a nge s be c om e v i s i b l e a l s o f o r N a , K - e r i on i t e a nd N a - m or de n i t e . T hereason for the resu l t ing he te rogene i ty seems to be the d i f fe ren t loca l i za t ion of thecat ions .

~ 0 . 4 I

0.22 ~ 1 1/ 9o / i

I ~ 5I fqr1

a-4 i i i- 2 0 2 4e l e c t r one ga t i v i t y

Fig. 6 Relative frequency of the low energy range (42 -50 ld mo1-1) in dependence on theelectronegativity: 1: Na A, 2" NaX , 3: N aY , 5: Na-mordenite

I n f o r m a t i on a bou t t he s e e x t r a - f r a m e w or k c a t i on l oc a t i ons a nd t he i r oc c upa nc yare usua l ly ob ta ined by d i f f rac t ion measurements . However , fo r h igh-s i l i ca zeo l i t e s



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HUNGER t al . : ZEOLITES 563

problems arise due to the low cation concentration. So it seems appropriate to em-ploy other methods yielding additional information on cations in zeolites such assolid-state NMR [27] or vibrational spectroscopic techniques, Vibrational spectro-scopic studies in the far infrared region provide access to the normal modes of thecations with respect to the framework [28-31]. Because zeolites reveal a charac-teristic spectral pattern in the far infrared, several attempts have been made to relatethese features with vibrational modes of cations on distinct sites [30, 32]. Although

1

L J ~300 200 100wavenumber / cm'

Fig. 7 Far-infrared spectra of the dehydratedzeolites: 1: NaA, 2: NaX, 3: NaY, 4: Na,K-erionite, 5: Na-mordenite

c . )

C )

i300 2()0 100wavenumber / cm'

Fig. 8 Far-infrared spectrum of dehydrated NaZSM-5

J . T h e r m a l A n a l . , 4 9 , 1 9 9 7


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564 HUNG ER et al.: ZEOLITES

r e c e n t s tud i e s ha ve t u r ne d ou t t he ove r s i m p l i fi c a t ion o f t h is m ode l , e a c h c a t i on s i teg i ve s a d i s ti nc t s pe c tr a l pa t t e r n be l ow 25 0 c m -1 sum m i ng up t o t he obs e r ve d f a r i n -f r a r e d s pe c t r a [33 - 36 ] .

a

9

3 o 0 2 6 0 i b 0w a ve num be r / c m "

Fig . 9 Far - in f ra red spec t ra o f NaX : a : hydra ted sample , b : dehydra ted sam ple

T h e f a r i n f r a r e d s pe c t r a o f de hyd r a t e d N a + - i ons c on t a i n i ng z e o l it e s w i t h qu i t ed i f fe ren t l a t t ice s t ruc tures and d i f fe ren t S i /Al - ra t ios a re sho wn in F igs 7 and 8 . Asc a n b e s e e n , w i t h i n c r e as i n g ca t io n c o n t e n t t h e n u m b e r o f o b s e r v e d b a n d s i n c r e a s e si n t h e r e g i o n b e lo w 2 5 0 c m -1 a t t r ibu tab le to ca t ion v ibra t ions . I n ge nera l th i s ca n ber e ga r d e d t o be i nd i c a ti ve o f a n i nc r e a s ing oc c upa t i on o f d i f f e r e n t c a t i on s i te s w i t hi n c r e a s in g h e t e r o g e n e i ty . T h e o b s e r v e d s p e c tr u m o f t h e z e o l it e N a Z S M - 5 e x h i b i tt w o ba nds ne a r 18 0 a nd 145 c m -1 ( F ig . 8 ) . Re c e n t l y f r om a j o i ne d a pp l i c a t ion o f f a ri n f r a r e d a n d X - r a y a bs o r p t i on s pe c t r o s c opy t w o d i f f e r e n t s it es o f c a t ions w e r e de -du c e d f o r a s e r i e s o f M F I t yp e z e o li te s [37] . Re ga r d i ng th i s, a t l e a st tw o d i f f e r e n tc a t i on s i te s f o r s a m p l e s unde r s t udy c a n be c onc l ude d on t he ba s is o f a l l s pe c t ra .T h i s c on c l u s i on s hou l d be t r a ns f e r a b l e to t he hyd r a t e d s ta t e o f s od i um - c on t a i n i ngz e o l i te s , t oo , be c a u s e no s i gn if i c a n t c ha nge s w e r e ob t a ine d du r i ng w a t e r a ds o r p t i oni n t he f a r i n f r a r e d s pe c t ra . A s c a n be s e e n in F i g . 9 f o r N a X, a f t e r hyd r a t i on o f thes a m pl e t he ba nds be c om e l es s re s o l ve d a nd s om e s h i f ts i n pos i t i on oc c u r . N o ne wba nds w e r e , how e ve r , obs e r ve d . S i m i l a r r e s u lt s w e r e ob t a ine d by a F T I R s t udy o ft he d e hyd r a t i on o f Y z e o l it e s w i t h d i f f e r e n t m o nova l e n t c a t i ons [38 ]. E v e n i f t he a d -s o r p t i on o f w a t e r m a y i n f l ue nc e the c a t i on l oc a t ion r e s u l ti ng i n a r e d i s t r ibu t i on , i ts hou l d no t p r ov i de a de c r e a s e o f he te r oge ne i ty .

Th e autho rs grateful ly acknow ledge the partia l f inancial support of the Fond s der C hem i-schen Indus t r ie and the Deu tsche Forschungsgem einschaf t , Phys ica l Chem is t ry o f In te r faces -Grad uate C ollege. H.T. is grateful for a Hum an Capita l and Mo bil i ty grant of the Eur ope anCo m m is s io n .



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HUNGER et al . : ZEOLITES 565

R e f e r e n c e s1 M . W . A nder son and J. Klinowski, J . Chem . S oc. , Faraday Trans. I , 82 (1986) 1449.2 J. Weitkamp, P . Kleinschm it , A . Kiss and C. H . B erke, Proceed ings of the 9th Internat ionalZeo l i te Confe rence , Mo ntrea l , 1992 , Vol . I I , (Eds : R. von Ba l lmoos , J . B . Higg ins , M . M .J. Treacy) , Butterworth-Heinemann, 1993, p . 79.3 G. D ebras , A . G ourgue, J . B. N agy and G. D e Clippeleir, Zeoli tes , 5 (1985) 377.4 E. Dima and L. V. C. Rees, Zeoli tes , 7 (1987) 219.5 B. Hunger , J . Hof f inann , O. He i tzsch and M . Hunger, J . Thermal Ana l . , 36 (1 99 0) 1379 .6 B. Hunger , M . H euche l , S . Matys ik , K. Beck and W.-D. E in icke , Therm och im . A c ta ,269 /270 (1995) 599 .7 C. Baer locher , A. H epp and W. M . M eie r, DL S-76 : A Program fo r the S imula ton o f Crys ta lS t ruc tu res (ETH Zur ich , Swi tze r land 1978) .8 W. J. M or t ie r , C omp i la t ion o f Ex t ra F ramew ork S i tes in Zeo l i te s , Bu t te rw or th Sc ien t i f icL td . , Gu i ld fo rd , 1982 .9 W. J . Mort ier , S. K. Gh osh and S. Shankar, L A m. C hem . Soc . , 108 (1986) 4315.10 W. J . Mort ier , Structure and Bonding, 66 (1987) 125.11 W. J . Mort ier , K. A . Van Gene ehten and J . Gaste iger, J . A m . C hem . Soc . , 107 (1985) 829.12 R. T . Sanderson , J . Am. Chem. Soc . , 74 (1952) 272 .13 R. G. Parr , R . A . Donnelly , M . L evy and W. E. Paike, J. Chem . Phy s. , 68 (1978) 3801.14 H. Toufar , B. G. Baekelandt, G. O. A. Janssens, W. J . M ort ier and R. A . S choon heydt , J.Phys . Chem. , 99 (1995) 13876 .15 K. D. Sen, T . V. Gayatr i and H. Toufar , J . M ol. Struc. (T HE O CH EM ), 361 (1996) 1 .16 H. K istenma cher , H . Popkie and E. Clem enti , J . Ch em. Phys. , 58 (1973) 1689.17 B. Hu nger , M . yo n Szombathely , J . Ho ffm ann and P. Brauer , L The rma l Ana l . , 44 (1995)293 .18 M . y on Szom bathely , P . Bri iuer and M. Jaroniec , J . Co m put. C hem ., 13 (1992) 17.19 P. T . Daw son and Y. K. Peng , Surface Sci . , 33 (1972) 565.20 B. Hunger and J . Hof fmann , Thermoch im. Ac ta , 106 (1986) 133 .21 E. Tro nconi and P. Forzat t i , Ch em. En gng. Sci . , 42 (1987) 2779 .22 M. M . Dubin in , A . A. I s i r ik j an , G. U. Rachm atkar iev and V. V. Se rp insk i, I zv . Akad . N aukSS SR , Ser. Kh im ., 10 (1972) 1269.23 M. M . Dubin in , A . A . I s i rikyan , G. U. Rachmatkar iev and V. V. Se rp inski , I zv . Ak ad . N aukSSSR, Se t . Kh im. , 4 (1973) 934 .24 P. L . L lewellyn, N . Pei lenq, Y. Gri lle t, F . Rouq uerol and J. Rouquerol , L Th erm al An al . , 4 2( 1 9 9 4 ) 8 55 .25 F. Vign6-M aede r and A . Au roux, J . Phys. Che m ., 94 (1990) 316.26 B. L . Su and D. Barthomeuf, Appl. Catal . , 124 (1995) 81.

27 G. Enge lhardt , M . Hun ger , H. Koller and J . Weitkamp, Stud. Su rf . Sci . Catal . , 80 (1994)421 .28 I . A. B rodskii , S . P. Zhd anov and A . E . Stanevich, Opt. S peetrosc . , 30 (1971) 58.29 W. M. Bu t le r , C. L . Ange l l , W. McAl l i s te r and W. M. Risen , J . Phys . Chem. , 81 (1977)2061 .30 M. D . Baker , G. A . Oz in and J. Godber , Catal . Rev.-Sci . E ng. , 27 (1985) 591.31 C. Br~m ard and M . L e M aire , J . Phys. C hem ., 97 (1993) 9695.32 J. Godbe r , M. D. Ba ker and G. A, O zin, J . Phys. Ch em ., 93 (1989) 1409.33 K. S. Smirnov, M . Le Maire , C. Br~:mard and D. Bou geard, C hem . Ph ys. , 179 (1994) 445 .34 C. Br6mard and D. Bougeard, Adv. Mater . , 7 (1995) 10.35 K. Krause, E . Geidel , J . Kindler , H. F6rster and H. B6hlig , J . Chem. Soc. , Chem. Com-mun . , (1995) 2481 .36 K . Krause, E . Geidel, J. Kindler, H. F~rster and K. S. Smirnov, Vib r. Spectrosc., 12 (1996) 45.37 H. E semann , H. F6rs te r, E . G e ide l and K. Krause , Mic roporous Mate r . , accep ted .38 W. P. J. H . Ja cobs , J. H . M . C. van Wolput and R. A . van Santen, Zeo lites, 13 (19933 170.

d . T h e r m a l A n a l . , 4 9 , 1 9 9 7

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