Influence of Cations on the Thermal Stability of Modified Y Zeolites

22 Influence of Cations on the Thermal Stability of Modified Y Zeolites H. BREMER, W. MÖRKE, R. SCHÖDEL, and F. VOGT

Downloaded by UNIV OF AUCKLAND on May 3, 2015 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0121.ch022

Department of Process Chemistry, Technical College for Chemistry "Carl Schorlemmer" Leuna-Merseburg, Merseburg, German Democratic Republic

The thermal stabilities of cation exchanged Y zeolites as reveale by DTA exhibit variable behavior depending the nature of the cation and degree of exchange. This behavior is explained by IR and ESR (X- and Q-band) spectroscopic results. With regard to thermal stabilities three groups of ion-exchanged zeo can be distinguished experimentally: (1) minimal stability at 20-40% exchange (Mg , Ca , Co , Ni , Zn ), (2) continuously increasing stability with increasing degree of exchan (Ce , H ), and (3) continuously decreasing stability with increasing degree of exchange (Cu ). These different thermal stabilities arise from specific interactions between the cation the zeolite framework. 2+

3+

2+

2+

2+

2+

+

2+

C y n t h e t i c a n d n a t u r a l zeolites are b e c o m i n g i n c r e a s i n g l y i m p o r t a n t as ^ catalysts, carriers of catalysts, a n d adsorbents. Zeolites are especially s u i t e d to these purposes because t h e i r properties c a n be m o d i f i e d b y c a t i o n exchange. T h e l i t e r a t u r e describes several studies w h i c h s h o w c h a r a c t e r istic changes i n p h y s i c o c h e m i c a l properties r e s u l t i n g f r o m c a t i o n exchange—• e.g., c a t a l y t i c a c t i v i t y (1,2), a c i d i c properties (3), a d s o r p t i o n b e h a v i o r (4) s t r u c t u r e of s o l i d (5,6), a n d t h e r m a l s t a b i l i t y (7,8). }

T h e c r y s t a l l i n e s t r u c t u r e of m o d i f i e d zeolites determine a n u m b e r of properties w h i c h are specific a n d f a v o r a b l e for c a t a l y t i c reactions. The complete or p a r t i a l loss of c r y s t a l l i n e s t r u c t u r e d u r i n g c a t a l y t i c reactions or regeneration is i n m o s t cases a c c o m p a n i e d b y decreased c a t a l y t i c a c t i v i t y . T h e r m a l s t a b i l i t y or s t r u c t u r a l s t a b i l i t y characteristics are therefore s u i t able for e v a l u a t i n g s u c h catalysts or s u p p o r t e d c a t a l y s t s . F e w s y s t e m a t i c i n v e s t i g a t i o n s of t h e b e h a v i o r of t h e t h e r m a l s t a b i l i t y of c a t i o n exchanged Y zeolites as a f u n c t i o n of m o d u l ( S i 0 / A 1 0 m o l e r a t i o ) , c a t i o n t y p e , degree of exchange, a n d a c t i v a t i o n c o n d i t i o n s h a v e been p u b lished. T h i s w o r k uses the results of I R a n d E S R spectroscopy to e x p l a i n the b e h a v i o r of t h e r m a l s t a b i l i t y of modified Y zeolites. 2

2

3

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

250

MOLECULAR SIEVES

Experimental Materials. T h e zeolites s t u d i e d are s u m m a r i z e d i n T a b l e I . u n i t cells of the s t a r t i n g m a t e r i a l s c o n t a i n , i n the d e h y d r a t e d f o r m : N a X (x = 2.5):86(H .i7Nao.83) 86 A 1 0 " 106 S i 0 N a Y (x = 4 . 3 ) : 6 1 ( H 09Na .9i) 61 A 1 0 ~ 131 S i 0 N a Y (x = 5 . 2 ) : 5 3 . 5 ( H . i i N a o . 8 9 ) 5 3 . 5 A 1 0 ~ 1 3 8 . 5 S i 0 +

0

0

0

2

+


Starting Material

2

2

2

+

0

The

2

2

Table I. Zeolites Studied Modul (x) Modified Matenal

NaX NaY NaY

2.5 4.3 5.2

CeNaX CeNaY CeNaY, CaNaY, NiNaY, ZnNaY,

MgNaY, CoNaY, CuNaY, HNaY

Conditions of Exchange. T o prepare t h e m o d i f i e d samples ( T a b l e I) i o n exchange was done at 70° C w i t h O . l i V n i t r a t e solutions of t h e m e t a l ( a m m o n i u m ) ions. T h e degree of exchange was d e t e r m i n e d b y a n a l y z i n g t h e s o l i d for the a m o u n t of s o d i u m a n d exchanged m e t a l ions r e m a i n i n g . Pretreatment of Samples. F o r s t a n d a r d i z a t i o n a l l samples used i n I R a n d D T A studies were p r e t r e a t e d i n a i r at 4 5 0 ° C , followed b y a special a c t i v a t i o n procedure (described below). S a m p l e s for E S R studies were h e a t e d for 10 h o u r s i n a i r at different t e m p e r a t u r e s (see R e s u l t s ) a n d for 10 h o u r s u n d e r v a c u u m ( 1 0 ~ t o r r ) a t t h e same t e m p e r a t u r e s . Experimental Technique. T h e I R spectra of d e h y d r a t e d samples were recorded b y U R 10 spectrometer ( V E B C a r l Zeiss J e n a ) . T o o b t a i n spectra for d e h y d r a t e d zeolites, samples were a c t i v a t e d for 10 h o u r s i n a i r at 570° C , cooled to r o o m t e m p e r a t u r e i n the presence of P O i , a n d g r o u n d w i t h N u j o l . T h e a c c u r a c y of t h e b a n d m a x i m u m d e t e r m i n a t i o n of the D o ring b a n d was ± 1 . 5 c m . I R c h a r a c t e r i z a t i o n of t h e zeolites after C O a d s o r p t i o n was done i n a cell w i t h N a C l w i n d o w s as described b y D u n k e n a n d coworkers (9). T h e samples were heated at 550° C for 3 hours u n d e r v a c u u m . A f t e r cooling u n d e r v a c u u m to r o o m t e m p e r a t u r e , C O was a d sorbed (pco = 450 t o r r ) , a n d t h e s p e c t r a were recorded. D T A studies were done w i t h a D T A a p p a r a t u s (Netzsch-Geràtebau, G m b H , Selb) i n a n a r g o n atmosphere w i t h h e a t i n g a t 1 0 ° / m i n . ESR signals were t a k e n i n t h e X - b a n d w i t h a n E R 9 spectrometer ( V E B C a r l Zeiss Jena) a n d i n t h e Q - b a n d w i t h a 3 5 - G H z E S R - X Q spectrometer (Akademie der Wissenschaften der D D R , B e r l i n ) . 4

4

0

- 1

Results T o determine t h e t h e r m a l s t a b i l i t y of t h e zeolites as a f u n c t i o n of t h e m o d u l , c a t i o n t y p e , a n d degree of exchange (a) we used the p o s i t i o n of t h e exothermic peak i n the D T A diagram to indicate lattice breakdown. The results are g i v e n i n F i g u r e 1. T h r e e zeolite groups are d i s t i n g u i s h e d . I n t h e first g r o u p ( M g + - , C a - , C o - , N i - a n d Z n + - e x c h a n g e d Y zeolites 2

2 +

2 +

2 +

2

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

22.

251

Thermal Stability of Y Zeolites

B R E M E R E T AL.

(χ = 5,2)) t h e t h e r m a l s t a b i l i t y a t first decreases w i t h i n c r e a s i n g degree of exchange; t h e n i t increases, b e g i n n i n g w i t h 2 0 - 4 0 % degree of exchange. I n t h e second g r o u p ( C e - a n d u n e x c h a n g e d zeolites), t h e t h e r m a l s t a b i l i t y increases w i t h a. F o r C u - e x c h a n g e d zeolites t h e r m a l s t a b i l i t y decreases w i t h increasing a. F i g u r e l c shows t h a t for C e N a X a n d C e N a Y zeolites t h e t h e r m a l s t a b i l i t y increases w i t h t h e m o d u l . T h i s i s i n accor dance w i t h t h e l i t e r a t u r e (10). 3 +

2 +

(b)


940.

^

>

900. 860. 820 20

40

60 *(%)

80

20

100

(c)

X

40

C^NiNaY

60 «(%)

CuNaY 80

100

CeNaY (x-52)

^

920

CeNaY

—

CeNaY (x-43)

A

900 . /

880

CeNaX (x-2,5)

860. 20 Figure 1.

40

60 *(%)

80

100 -

Thermal stability (°C) of cation-exchanged zeolites as a function of the degree of exchange (a) (a,b) and modul (c)

T h e I R spectra of d e h y d r a t e d zeolites show t h e f o l l o w i n g changes as c o m p a r e d w i t h t h e s p e c t r u m of t h e N a Y zeolite. T h e f r e q u e n c y of t h e D 6 - r i n g b a n d a t 570-600 c m (11) changes w i t h increasing degree of exchange i n different w a y s ( F i g u r e 2) : -

1

(a) M g + - , C o - , N i + - a n d Z n + - m o d i f i e d zeolites show a significant shift t o h i g h e r w a v e n u m b e r s at l o w v a l u e s of a. (b) W i t h a r i s i n g v a l u e s of a we o b s e r v e d , for C a - e x c h a n g e d zeolites, a slight shift t o s m a l l e r w a v e n u m b e r s a n d f o r C e ^ c o n t a i n i n g zeolites a stronger shift. (c) C u - m o d i f i e d zeolites show, w i t h i n c r e a s i n g a , a g r a d u a l shift t o higher w a v e n u m b e r s . 2

2 +

2

2

2 +

3

2 +


252

MOLECULAR SIEVES

595.

(a)

Z^ ^

CoNoY MgNaY

.

ΖηΝαΥ

0

— 585 'ε υ

13*575 c OJ

(b)

585.


575

\CeNaY

595. te)

^^•NiNaY — C u N a Y —

585.

'

7 PaMaY

575. 20

40

60

80 odVo)

100 -

Figure 2. Frequency of the Ώβ-ring band vs. degree of exchange for ion-ex changed zeolites I R c h a r a c t e r i z a t i o n of t h e zeolite samples after C O - a d s o r p t i o n shows t h a t for C a - a n d Z n - e x c h a n g e d samples c a t i o n - C O i n t e r a c t i o n ( b a n d a t 2200 c m - ) is i n d i c a t e d o n l y a t h i g h values of a ( F i g u r e 3). F o r M g (highest a = 6 1 % ) a n d C e - (highest a = 7 3 % ) c o n t a i n i n g zeolites t h i s c h a r a c t e r i s t i c b a n d does n o t appear. I n agreement w i t h A n g e l l a n d Schaffer (12), f o r C o - a n d N i - e x c h a n g e d zeolites t h i s b a n d a t 2200 c m " appears a t l o w values of a. I t s i n t e n s i t y rises s u d d e n l y a t a = 4 0 - 5 0 % . W i t h C u - m o d i f i e d zeolites a s t r o n g i n t e r a c t i o n w i t h C O occurs a t l o w degrees of exchange. 2 +

2+

1

2 +

3 +

2 +

2 +

1

2 +

F i g u r e 4 shows l o w field c o m p o n e n t s of t y p i c a l E S R s p e c t r a of d e h y d r a t e d C u N a Y samples of different C u concentrations. U p to a = 1 9 % t h e spectra h a v e c h a r a c t e r i s t i c forms p e r m i t t i n g a n assignment of C u cations t o t w o different positions (position 1 = gj\ p o s i t i o n 2 = g^) (13, 14,15,16). I n t h e spectra of t h e samples w i t h higher a, the s i g n a l a p p e a r i n g a t g = 2.16 ( m a r k e d b y arrows, C u - p o s i t i o n 3) p r e v a i l s o v e r those c h a r a c t e r i z i n g positions 1 a n d 2 (13, 15). Q - b a n d measurements of a C u N a Y sample ( F i g u r e 5 shows t h e l o w field p a r t ) s h o w t h a t t h e f o r m a t i o n of a n e w C u - O - p h a s e (position 3, m a r k e d b y arrows) begins a t 300° C . I n a d d i t i o n t h e five H F S lines i n d i c a t e t h e presence of t h e t w o C u posi tions c i t e d above. T o d i s t i n g u i s h t h e copper p o s i t i o n , E S R spectra of p a r t i a l l y r e h y d r a t e d (22 ° C , 6 5 % r e l a t i v e h u m i d i t y ) a n d r e d u c e d ( H , C O ) C u N a Y samples were recorded. I n F i g u r e 6 i n t e n s i t y ratios (/2//1) of 2 +

2 +

2 +

2 +

2


22.

253


BREMER ET AL.

peaks 1 a n d 2 ( F i g u r e 4) are p l o t t e d vs. r e h y d r a t i o n t i m e .

The rapid de

crease i n t h e i n t e n s i t y r a t i o indicates t h a t d u r i n g r e h y d r a t i o n , copper ions absorb w a t e r a n d change f r o m p o s i t i o n 2 t o p o s i t i o n 1. Dtscusston T h e t h e r m a l s t a b i l i t y of m o d i f i e d zeolites depends n o t o n l y o n t h e m o d u l ( F i g u r e l c ) b u t also o n c a t i o n t y p e a n d the degree of exchange (a) ( F i g u r e l a - c ) . F o r M g + - , C o - , N i - , a n d Z n - e x c h a n g e d zeolites, w h i c h s h o w l i t t l e o r n o c a t i o n - C O i n t e r a c t i o n a t s m a l l values of a (i e., preferred o c c u p a t i o n of positions S i resp. S ^ ) , there is a r e l a t i v e l y s t r o n g m i n i m u m i n t h e r m a l s t a b i l i t y a t a = 2 0 - 4 0 % . T h e same cations cause significant l a t t i c e d i s t o r t i o n i n t h e hexagonal p r i s m a t l o w exchange d e grees after d e h y d r a t i o n , w h i c h is d e m o n s t r a t e d b y t h e shift of t h e D 6 - r i n g b a n d t o higher w a v e n u m b e r s ( F i g u r e 2 ) . T h e increase i n t h e t h e r m a l s t a b i l i t y a g a i n a t higher degrees of exchange is e x p l a i n e d as follows. Mul t i v a l e n t cations M e and M e i n t h e S i p o s i t i o n reduce t h e d i a m e t e r of t h e s i x - m e m b e r e d r i n g i n t h e hexagonal p r i s m because of s t r o n g p o l a r i z a -


2

2 +

2 +

2 +

2 +

3 +

NaY (0) NiNaY (10)

NaY (0)

(2D

(58) (68)

CuNaY (19)

(76)

ZnNaY

CaNcY (8) (18)

Λ 2100 ?200

2100 22003

(61) (81)

2100 2200

Frequency (cm*) — * Figure S. IR spectra of CO adsorbed on ion-ex changed zeolites. Numbers are degree of exchange



254

MOLECULAR SIEVES

41

29 19 10(5

V f

2 g;

_

Figure 4. ESR spectrum (low field components) of CuNaY zeolites dehydrated at 300° C and taken at room temperature in the X-band as a function of degree of exchange (numbers on the left)

t i o n (17). T h i s l a t t i c e d i s t o r t i o n s h o u l d result i n a decrease i n t h e r m a l s t a b i l i t y . T h e f r a m e w o r k distortions s h o u l d be p a r t l y compensated, a n d therefore t h e t h e r m a l s t a b i l i t y s h o u l d rise at h i g h a values at w h i c h the S u a n d S u ' , positions are occupied. ( C a t i o n - C O i n t e r a c t i o n is increased, F i g u r e 3.) I n c o n t r a s t t o h i g h l y exchanged M g , C o , N i + , a n d Z n zeolites, whose t h e r m a l s t a b i l i t y is l o w e r t h a n t h a t of N a Y , t h e s t a b i l i t y of Ca zeolites a t h i g h a v a l u e s exceeds t h a t of N a Y . I n t h i s case t h e m i n i m u m t h e r m a l s t a b i l i t y is s h o w n o n l y w e a k l y a l t h o u g h C a - i o n s p r e f e r a b l y o c c u p y S i positions as i n d i c a t e d b y t h e i n a c c e s s i b i l i t y t o C O molecules ( F i g u r e 3). I n g o o d agreement w i t h the b e h a v i o r of C a N a Y zeolites w i t h respect t o t h e r m a l s t a b i l i t y is t h e s m a l l frequency shift of the D 6 - r i n g b a n d w i t h i n c r e a s i n g a, i n d i c a t i n g a slight d i s t o r t i o n i n the hexagonal p r i s m . T h i s result is c o n t r a r y t o t h e d a t a of B e n n e t t a n d S m i t h (18), w h o f o u n d u n d e r t h e i r e x p e r i m e n t a l c o n d i t i o n s considerable d i s t o r t i o n of t h e h e x a g onal p r i s m i n the Ca-faujasite structure. W e explain the increasing therm a l s t a b i l i t y of H N a Y zeolites, especially a t a > 4 0 % , b y c o n d i t i o n s d u r i n g p r e t r e a t m e n t a n d D T A i n v e s t i g a t i o n s , b y w h i c h t h e m o d u l is increased l e a d i n g t o m o r e stable faujasites. M a h e r a n d co-workers (19) p r o v e d b y c r y s t a l s t r u c t u r e i n v e s t i g a t i o n s t h a t o n t h e r m a l t r e a t m e n t of N a N H Y zeolites, A l is r e m o v e d f r o m the anionic zeolite f r a m e w o r k , a n d occupies c a t i o n positions as different species. I n o u r samples we c o u l d p r o v e q u a l i 2 +

2 +

2

2

2 +

2 +

4


+

22.

255


BREMER ET AL.

t a t i v e l y t h e existence of A l ions after exchange w i t h A g ions. Consequently, i n these samples H A l N a Y zeolites are f o r m e d . The x-ray i n v e s t i g a t i o n s a n d t h e I R spectra of h i g h l y exchanged H N a Y zeolites (20) also i n d i c a t e a n increase of t h e m o d u l . T h e l a t t i c e constants decrease w i t h i n c r e a s i n g a w h i l e t h e I R b a n d of t h e s y m m e t r i c v a l e n c e v i b r a t i o n is shifted b y 25 c m t o higher w a v e n u m b e r s . T h e rise i n t h e r m a l s t a b i l i t y w i t h increasing C e c o n t e n t ( F i g u r e l b ) is e x p l a i n e d b y t h e existence of a sodalite u n i t c o m p l e x as p r o p o s e d b y O l s o n a n d c o - w o r k e r s (21). As a consequence of t h e f o r m a t i o n of s u c h a c o m p l e x the p o l a r i z i n g effect of Ce ions o n t h e l a t t i c e o x y g e n s h o u l d decrease a n d t h u s t h e l a t t i c e d i s t o r t i o n also. T h e f r e q u e n c y decrease of the D 6 - r i n g b a n d a t h i g h a agrees w e l l w i t h t h i s result ( F i g u r e 2b). 3 +

+

- 1

3 +


3 +

Figure 6. ESR spectrum (low field components) of CuNaY zeolite (a = 29%) in the Q-band taken at room temperature as a function of dehydration temperature (numbers on the right are pretreatment temperatures)


256

MOLECULAR SIEVES

C o m p r e h e n s i v e statements o n t h e n a t u r e a n d extent of t h e i n t e r a c t i o n between m e t a l cations a n d a n i o n i c zeolite f r a m e w o r k are possible f r o m ESR spectroscopic results o n t h e C u N a Y zeolites. T h e s e studies show t h a t i n a l l samples (a > 19%) copper ions m a y be present i n three d i s t i n guishable c a t i o n positions of t h e zeolite, where w e differentiate t w o i s o l a t e d Cu positions (position 1 = Su a n d p o s i t i o n 2 ^ Su/ a n d SiO a n d a C u O phase (cluster) i n t h e large cages ( F i g u r e s 4, 5). B e c a u s e t h e cluster phase c a n be p r o v e d a t l o w degrees of exchange (a > 19%) w e t a k e i n t o account a n e n r i c h m e n t of C u ions i n t h e large cage a n d a n e x h a u s t i o n of C u ions i n t h e sodalite u n i t i n i s o l a t e d positions (Si/, Su/). I n good agreement w i t h t h a t r e s u l t is t h e i n t e r a c t i o n w i t h C O w h i c h begins at l o w values of a ( F i g u r e 3) a n d t h e s m a l l shift of t h e D 6 - r i n g b a n d f r e q u e n c y ( F i g u r e 2c). 2 +

2 +


2 +

time (h) Figure 6.

Influence of rehydration time on intensity ratio

W i t h r i s i n g values of a , we find a n increase of t h e ESR s i g n a l i n t e n s i t y for t h e c l u s t e r phase ( F i g u r e 4) a n d a n increase i n t h e i n t e n s i t y r a t i o I2/I1 (22). T h i s m a y be e x p l a i n e d b y t h e fact t h a t t h e c o n c e n t r a t i o n of C u ions i n t h e 2 p o s i t i o n (Si/, Su/) rises. T h e l o c a l i z a t i o n of C u cations i n t h e h e x a g o n a l p r i s m o b s e r v e d b y I m e l i k a n d co-workers (28) c a n n o t be c o n f i r m e d b y us, p r o b a b l y because of different p r e t r e a t m e n t c o n d i t i o n s . I n s e r t i o n of C u + cations i n p o s i t i o n 2 (in our o p i n i o n p r e f e r a b l y Si/) causes a d i s t o r t i o n i n t h e h e x a g o n a l p r i s m a n d t h u s affects t h e t h e r m a l s t a b i l i t y , decreasing w i t h i n c r e a s i n g a ( F i g u r e l b ) . T h u s , there is a freq u e n c y shift of t h e D - r i n g b a n d t o h i g h e r w a v e n u m b e r s ( F i g u r e 2c). On t h e whole, the t h e r m a l s t a b i l i t i e s of the exchanged Y zeolites differ f r o m c a t i o n t o c a t i o n b u t c a n be e x p l a i n e d . 2 +

2 +

2

6


22.

BREMER ET AL.

257


Literature Cited 1. Minachev, Ch. M., Kinetika i Kataliz (1970) 11, 413. 2. Venuto, P. B., Landis, P. S., Advan. Catalysis Related Subj. (1968) 18,

259.


3. Ward, J. W., ADVAN. CHEM. SER. (1971) 101, 380. 4. Kiselev, Α. V., ADVAN. CHEM. SER. (1971) 102, 37. 5. Smith, J. V., ADVAN. CHEM. SER. (1971) 101, 171.

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295.

RECEIVED January 3, 1973.


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