Chapter 3
and Al Magic-Angle Spinning-NMR Spectroscopic Study of Rare-Earth-Exchanged Y Zeolites 27
29
Downloaded by NORTH CAROLINA STATE UNIV on December 3, 2012 | http://pubs.acs.org Publication Date: May 27, 1988 | doi: 10.1021/bk-1988-0368.ch003
Si
P. S. Iyer, J . Scherzer, and Z. C. Mester Science and Technology Division, Unocal Corporation, P.O. Box 76, Brea, CA 92621
Lanthanum (La), cerium (Ce), praseodymium (Pr) and neodymium (Nd) exchanged Y zeolites with different rare earth content, as well as mixed lanthanum-cerium exchanged zeolites, have been prepared and characterized. Si and Al MASNMR spectra, surface area, crystallinity and unit cell size data were obtained for fresh and steamed zeolites. The stability of these zeolites toward steam has been investigated. An increase in lanthanum content from about 14 wt% La O to over 20 wt% enhances the stability of the LaY zeolite towards dealumination by steam. A similar effect was observed for NdY and PrY zeolites. The resolution loss of Si-NMR signals observed for paramagnetic ions (Nd , Pr , Ce ) and higher-valency ions (Ce ) is discussed. Upon steaming zeolites containing lanthanum-cerium mixtures, dealumination increases and stability decreases with increased cerium content of the zeolite. The differences in observed stability are discussed. XPS data show that steaming induces the migration of both aluminum and rare earths to the surface of the zeolite crystals. 29
27
2
3
29
3+
3+
3+
4+
Rare e a r t h (RE) exchanged Y z e o l i t e s a r e o f c o n s i d e r a b l e i n t e r e s t due t o t h e i r h i g h c a t a l y t i c a c t i v i t y f o r many r e a c t i o n s (1-4). Such z e o l i t e s a r e t h e major component o f many modern hydrocarbon cracking catalysts (5,6). The s t r u c t u r a l c h a r a c t e r i s t i c s o f t h e s e z e o l i t e s have been e x t e n s i v e l y i n v e s t i g a t e d . The c a t i o n i c dist r i b u t i o n i n r a r e e a r t h exchanged, n a t u r a l f a u j a s i t e , as w e l l as i n LaY, LaX and CeX z e o l i t e s , has been i n v e s t i g a t e d by x - r a y crystallography (7-10). The s t r u c t u r a l c h a r a c t e r i s t i c s o f f r e s h and h y d r o t h e r m a l l y t r e a t e d r a r e e a r t h Y z e o l i t e s have i n f a c t been described i n numerous papers (3,10-31). Using a variety of physical methods (e.g., x-ray diffraction, infrared and NMR s p e c t r o s c o p y ) , i t was shown t h a t hydrothermal t r e a t m e n t o f r a r e e a r t h Y z e o l i t e s r e s u l t s i n framework d e a l u m i n a t i o n . In a r e c e n t p u b l i c a t i o n ( 2 7 ) , i t was shown t h a t steaming o f REY z e o l i t e s not
0097-6156/88/0368-0048$06.00/0 © 1988 American Chemical Society
In Perspectives in Molecular Sieve Science; Flank, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
Downloaded by NORTH CAROLINA STATE UNIV on December 3, 2012 | http://pubs.acs.org Publication Date: May 27, 1988 | doi: 10.1021/bk-1988-0368.ch003
3. IYER ET AL.
Rare-Earth-Ezchanged
Y Zeolites
only r e s u l t s i n framework dealumination but i s accompanied by aluminum migration towards the zeolite particle surface. C o r r e l a t i o n s between s t r u c t u r a l c h a r a c t e r i s t i c s (including extra framework aluminum) and c a t a l y t i c properties of rare earth Y z e o l i t e s have been d i s c u s s e d more r e c e n t l y by Corma e t a l . (29) and Lemos e t a l . ( 3 0 ) . In t h e i r use as c a t a l y s t components, r a r e e a r t h Y z e o l i t e s a r e f r e a u e n t l y p r e p a r e d by i o n exchange w i t h commercial r a r e e a r t h salt solutions. Such commercial s a l t s a r e m i x t u r e s o f d i f f e r e n t r a r e e a r t h s , i n which the major components a r e lanthanum, c e r i u m , praseodymium and neodymium ( 5 J . These r a r e e a r t h elements t h e r e f o r e p l a y a major r o l e i n d e t e r m i n i n g the p h y s i c o - c h e m i c a l c h a r a c teristics and s t a b i l i t y of Y z e o l i t e s that a r e used i n many commercial catalysts. Most o f the p u b l i s h e d s t r u c t u r a l s t u d i e s were done e i t h e r on lanthanum o r mixed r a r e e a r t h exchanged Y z e o l i t e s . Few s t u d i e s can be found on the e f f e c t o f i n d i v i d u a l rare earths on the s t r u c t u r a l c h a r a c t e r i s t i c s a n d $ t a b i l i t y of Y z e o l i t e s . In t h i s p a p e r , Si and Al MASNMR s p e c t r o s c o p y i s used i n c o n j u n c t i o n w i t h c r y s t a l l i n i t y , s u r f a c e a r e a and u n i t c e l l size measurements to study i n d i v i d u a l r a r e e a r t h exchanged Y z e o l i t e s i n o r d e r t o d e t e r m i n e the e f f e c t o f i n d i v i d u a l r a r e e a r t h s c a t i o n s on t h e i r s t r u c t u r e and s t a b i l i t y . These methods a r e used to f u r t h e r probe r a r e e a r t h i n d u c e d s t r u c t u r a l changes t h a t o c c u r d u r i n g hydrothermal t r e a t m e n t of the z e o l i t e s . The s t u d i e s were extended t o a l s o e s t a b l i s h the e f f e c t o f d i f f e r e n t lanthanumcerium mixtures on z e o l i t e s t a b i l i t y . The data p r e s e n t e d and d i s c u s s e d a r e f o r lanthanum, c e r i u m , praseodymium and neodymium exchanged Y z e o l i t e s , as w e l l as f o r z e o l i t e s exchanged with d i f f e r e n t lanthanum-cerium m i x t u r e s . 2
Experimental Materials. The r a r e e a r t h exchanged Y z e o l i t e s were p r e p a r e d from commercial NaY zeolite (LZ-Y-52 from Union Carbide Co.) and i n d i v i d u a l r a r e e a r t h c h l o r i d e s (99.9% p u r e , from A l f a P r o d u c t s ) . The NaY z e o l i t e had a S i O ^ / A l p O - mole r a t i o o f 4.93 and a u n i t cell composition of (Na i)) \A10 ) (Si0 )-i37. Portions of water washed NaY z e o l i t e were exchanged t w i c e S n t n the c o r r e s p o n d ing r a r e earth c h l o r i d e s o l u t i o n , a c c o r d i n g to a procedure des c r i b e d i n the l i t e r a t u r e (1_0). The p a r t i a l l y rare earth exchanged z e o l i t e s (RE,NaY) were c a l c i n e d a t 540°C f o r two hours i n air. The c a l c i n e d m a t e r i a l was ammonium exchanged t o further reduce the sodium c o n t e n t o f the z e o l i t e . The f i n a l p r o d u c t was a r a r e e a r t h , ammonium Y z e o l i t e (RE,NH*Y) and c o n t a i n e d between 14 and 15 wt% r a r e e a r t h o x i d e . P a r t o f t h i s m a t e r i a l was f u r t h e r rare earth-exchanged, r e s u l t i n g in a high-rare earth Y z e o l i t e (Hi-REY). In the case o f c e r i u m exchanged z e o l i t e s , the i o n i c exchanges were c a r r i e d out under n i t r o g e n , i n o r d e r t o a v o i d the o x i d a t i o n of cerium (III) ions. P a r t o f the c e r i u m ( I I I ) exchanged z e o l i t e was c a l c i n e d under oxygen f o r 6 hours a t 540°C i n o r d e r t o c o n v e r t cerium (III) t o c e r i u m (IV) ions. Lanthanum-cerium exchanged z e o l i t e s were p r e p a r e d w i t h the f o l l o w i n g c o m p o s i t i o n : 75% L a , 25% C e ; 50% L a , 50% C e ; and 25% L a , 75% Ce. 2
2 7
5
2
5 K
o
In Perspectives in Molecular Sieve Science; Flank, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
49
PERSPECTIVES IN MOLECULAR SIEVE SCIENCE
50
To e s t a b l i s h the e f f e c t o f hydrothermal t r e a t m e n t upon the r a r e e a r t h exchanged z e o l i t e s , p o r t i o n s o f t h e s e z e o l i t e s were steamed a t d i f f e r e n t temperatures under 100% steam, and c h a r a c t e r i z e d by d i f f e r e n t p h y s i c a l methods. 29
27 Si
and
Al
NMR S p e c t r o s c o p y :
Instrumentation
and
Measurements.
Downloaded by NORTH CAROLINA STATE UNIV on December 3, 2012 | http://pubs.acs.org Publication Date: May 27, 1988 | doi: 10.1021/bk-1988-0368.ch003
29 S i MASNMR s p e c t r a were o b t a i n e d on an IBM AF270 NMR s p e c t r o m e t e r a t a f r e q u e n c y o f 53.7 MHz. Samples were spun i n 7mm alumina r o t o r s a t speeds between 2.5 and 3.5 KHz. Chemical s h i f t s were r e f e r e n c e d to e x t e r n a l TMS s e t to 0.0 ppm by sample exchange, u s i n g a s u i t a b l e secondary s t a n d a r d . For each s p e c t r u m , 2K data p o i n t s were d i g i t i z e d d u r i n g a 51 msec data a c q u i s i t i o n . The d a t a were z e r o f i l l e d t o 4K and a p o d i z a t i o n e q u i v a l e n t t o 20 t o 50 Hz of l i n e b r o a d e n i n g was a p p l i e d p r i o r to F o u r i e r t r a n s f o r m a t i o n . Typically, 200 to 2000 scans were needed to produce suitable signal to noise> levels. 70.4 mHz Al MASNMR s p e c t r a were a l s o r e c o r d e d on the same instrument. T y p i c a l scan c o n d i t i o n s i n v o l v e d 18° (1 y s e c ) p u l s e w i t h a r e c y c l e d e l a y o f 0.2 s e e s . 2K data p o i n t s were a c q u i r e d o v e r 31 msec a c q u i s i t i o n time ( c o r r e s p o n d i n g to a sweep w i d t h o f 33.3 k H z ) . The data were z e r o f i l l e d t o 4K and 50 Hz l i n e broadening a p p l i e d before F o u r i e r t r a n s f o r m a t i o n . Samples were spun i n 7 mm z i r c o n i a r o t o r s f i t t e d w i t h v e s p e l end c a p s , a t 3.0 t o 3.5 k H z ^ Chemical s h i f t s r e p o r t e d a r e r e f e r e n c e d t o e x t e r n a l aqueous A l ( H ^ 0 ) ( s e t t o 0.0 ppm). 7 A 18° f n ' p a n g l e was employed t o make the Al $ i 9 j A i n t e n s i t i e s as q u a n t i t a t i v e as p o s s i b l e . In c a l c u l a t i n g Al /Al r a t i o s , i t was assumed t h a t the s p i n n i n g sidebands (SSB) were o f equal i n t e n s i t i e s , as o b s e r v e d i n the f r e s h z e o l i t e samples. 7
6
2
n
v
O t h e r Measurements. The x - r a y d i f f r a c t i o n p a t t e r n s were r e c o r d e d and u n i t c e l l sTzes measured on a Siemens D-500 d i f f r a c t o m e t e r . The XPS data were o b t a i n e d on a E s c a l a b Mark V i n s t r u m e n t , made by V.G. R e s u l t s and D i s c u s s i o n The chemical c o m p o s i t i o n and s e l e c t e d p r o p e r t i e s o f the m a t e r i a l s p r e p a r e d a r e l i s t e d i n T a b l e 1. The r a r e e a r t h (RE) c o n t e n t o f RE,NH.Y z e o l i t e s shows t h a t about 2/3 o f the exchange s i t e s a r e o c c u p i e d by r a r e e a r t h i o n s . The r a r e e a r t h c o n t e n t o f Hi-REY z e o l i t e s shows t h a t o v e r 75% o f s i t e s a r e o c c u p i e d by r a r e e a r t h ions. In some o f the Hi-REY samples where r a r e e a r t h e x c h a n g e ^ were c a r r i e d out a f t e r c a l c i n a t i o n , the sum o f RE and Na equivalents is higher than that r e q u i r e d to compensate the framework c h a r g e . Jfiis is du£ t o f o r m a t i o n o f lower-valency hydroxyl-ions [RE0H , RE(0H) ] during calcination. Steaming r e s u l t s i n g r e a t e r u n i t c e l l s i z e s h r i n k i n g f o r RE,NH Y than f o r Hi-REY. A significant loss of crystallinity occurs during steaming o f RE,NaY z e o l i t e s , due t o t h e i r r e l a t i v e l y h i g h sodium content. +
z
4
In Perspectives in Molecular Sieve Science; Flank, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
3. IYER ET AL.
Rare-Earth-Exchanged
51
Y Zeolites
Downloaded by NORTH CAROLINA STATE UNIV on December 3, 2012 | http://pubs.acs.org Publication Date: May 27, 1988 | doi: 10.1021/bk-1988-0368.ch003
TABLE I. COMPOSITION AND SELECTED PROPERTIES OF RARE EARTH EXCHANGED Y ZEOLITES
REY Type
R E ^ , wt%
STEAMED REY 4788°C/5h) % Cryst, ( e [ SA, nT/g U.C, A (e)
Na,,0, wt% 3.9 0.3 0.6
73 73 71
630 680 650
24.64 24.58 24.70
11.7/3.8^ 103/3.5 16.5/5.4
4.0 0.6 0.4
69 54
330 660 550
24.62 24.54 24.63
La,Ce,NaY (1:1) La,Ce,NH.Y (1:1) Hi-La,CeY (1:1)
7.7/8.4 6.7/7.1 10.5/13.6
4.2 0.3 0.6
0 59 36
260 645 430
24.58 24.46 24.61
La,Ce,NaY (1:3) La,Ce,NH.Y (1:3) Hi-La,CeY (1:3)
3.9/12.2 3.5/10.9 5.3/16.6
4.0 0.6 0.6
0 56 21
180 600 280
24.59 24.43 24.59
Ce,NaY Ce,NH,Y Hi-CeY
14.2^ 15.9 22.8
4.1 0.2 0.4
