Molecularly Engineered, High-Performance Adsorbent - ACS

C h a p t e r 30

Perspectives in Molecular Sieve Science Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SAN DIEGO on 03/15/16. For personal use only.

Molecularly Engineered, High-Performance Adsorbent Self-Bound Low-Silica

X Zeolite

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C. G. Coe, S. M. Kuznicki , R. Srinivasan, and R. J. Jenkins Corporate Science and Technology Center, Air Products and Chemicals, Inc., Allentown, PA 18195 We prepared highly crystalline, low-silica X zeolite (LSX) in a self-bound form directly from porous, preformed metakaolin pellets. In the appropriate cation form, these materials should have Improved adsorption properties for weakly interacting adsorbates such as nitrogen. The composition of the synthesis medium, substrate porosity, aging period, and crystallization conditions affected both the total amount of zeolite produced and the relative amount of X and A phases formed. Materials were characterized by elemental analysis, X-ray diffraction, scanning electron microscopy, N adsorption, and S i MAS NMR. Pellets converted in situ under optimum conditions contained >95% X zeolite having a Si/Al ratio equal to 1.0. These properties, combined with a large median pore diameter and acceptable crush strength, make this material a superior adsorbent. As predicted, a calcium low-silica X zeolite, properly activated, possesses higher N capacity (1.37 moles/g at 30°C, 1 atm) and N /O selectivity (11.5 from air at 30°C) than any other adsorbent. 2

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S y n t h e t i c f a u j a s l t e s have s u b s t a n t i a l u t i l i t y 1n a wide v a r i e t y of a d s o r p t i v e , c a t a l y t i c , and Ion exchange p r o c e s s e s . T y p i c a l l y , Y z e o l i t e s a r e used as c a t a l y s t s whereas X z e o l i t e s , h a v i n g c o m p a r a t i v e l y more c a t i o n s and a h i g h e r framework c h a r g e , have been used f o r a d s o r p t i v e and 1on exchange a p p l i c a t i o n s . The t y p i c a l X z e o l i t e p o s s e s s e s a framework S1/A1 r a t i o of about 1.25. However, the alum1num~r1ch end member of the f a u j a s i t e f a m i l y , 1n a c c o r d a n c e w i t h L o w e n s t e l n ' s r u l e , has a r a t i o of 1.0, and c o n s e q u e n t l y the maximum p o s s i b l e number of

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© 1988 American Chemical Society


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exchangeable c a t i o n s . In p r e v i o u s work we have shown t h a t h i g h l y exchanged CaX (S1/A1 = 1.23), w i t h most of the c a l c i u m 1n the d e h y d r a t e d / d e h y d r o x y l a t e d s t a t e , p o s s e s s e s N2 c a p a c i t i e s and N2/O2 s e l e c t l v 1 t i e s t h a t a r e much l a r g e r than t y p i c a l l y reported.(1) S i n c e the s t r e n g t h of c a t l o n / s o r b a t e Interactions Is r e l a t i v e l y Independent of the number of c a t i o n s p r e s e n t ( 2 ) , the a d s o r p t i o n c a p a c i t y f o r weakly I n t e r a c t i n g a d s o r b a t e s such as n i t r o g e n should be d i r e c t l y r e l a t e d to the number of sorbate-accesslble cations a v a i l a b l e . Improving the dynamic c a p a c i t y 1s a prime c o n s i d e r a t i o n and goal f o r a l l s e p a r a t i o n and a d s o r p t i o n a p p l i c a t i o n s . ( 3 ) Thus, we f e l t 1t would be w o r t h w h i l e t o p r e p a r e an X z e o l i t e having the maximum aluminum content. T h i s z e o l i t e Is r e f e r r e d to as " r e d u c e d s i l i c a X" or " l o w ~ s 1 H c a X" ( L S X ) . ( 4 ) In a d d i t i o n , 1n a c c o r d a n c e w i t h L o w e n s t e l n ' s r u l e the LSX would have an o r d e r e d , u n i f o r m d i s t r i b u t i o n of aluminum atoms l e a d i n g to a more e n e r g e t i c a l l y homogeneous s u r f a c e . The u t i l i t y of an a d s o r b e n t 1n a g i v e n p r o c e s s 1s a f f e c t e d not o n l y by I t s chemical c o m p o s i t i o n , but a l s o by I t s p h y s i c a l form. For most commercial a p p l i c a t i o n s , z e o l i t e s must be p e l l e t l z e d t o produce a p a r t i c l e h a v i n g 1) s u f f i c i e n t s i z e to e l i m i n a t e massive p r e s s u r e drops through a packed bed, 2) adequate c r u s h s t r e n g t h and a t t r i t i o n r e s i s t a n c e , and 3) enough m a c r o p o r o s l t y to m i n i m i z e d i f f u s i o n r e s i s t a n c e . G e n e r a l l y 1n o r d e r to f o r m u l a t e a p e l l e t l z e d m a t e r i a l the a c t i v e z e o l l t l c component must be d i l u t e d w i t h an I n o r g a n i c b i n d e r and then e x t r u d e d or formed Into beads. Adding a b i n d e r , however, d i l u t e s the a c t i v e z e o l i t e phase, l o w e r i n g Its c a p a c i t y . Thus our goal was to produce a s e l f - b o u n d LSX p e l l e t , e l i m i n a t i n g the need f o r a b i n d e r . Some I n f o r m a t i o n on s e l f - b o u n d z e o l i t e s has been published.(5) Howell and Acara (6) have c o n v e r t e d preformed a g g r e g a t e s of r e a c t i v e k a o l i n - t y p e c l a y , under a p p r o p r i a t e c o n d i t i o n s , d i r e c t l y t o a z e o l l t l c phase. Metakaolln having a S1/A1 r a t i o of 1.0 was e a s i l y c o n v e r t e d to z e o l i t e A. Also, Breck (5) d i s c u s s e s the f a b r i c a t i o n of pure s e l f - b o u n d A z e o l i t e p a r t i c l e s s e v e r a l Inches 1n s i z e by c o n v e r t i n g m e t a k a o l l n 1n situ. S i n c e LSX and A z e o l i t e have the same o v e r a l l c h e m i c a l c o m p o s i t i o n and both a r e c o n s t r u c t e d by l i n k i n g s o d a l l t e c a g e s , we p o s t u l a t e d t h a t preformed m e t a k a o l l n c o u l d be c o n v e r t e d d i r e c t l y to LSX. S e v e r a l r e p o r t s show t h a t k a o l i n - t y p e c l a y s can be t r a n s f o r m e d to X- or Y - t y p e z e o l i t e s having S1/A1 r a t i o s e x c e e d i n g 1.2 by e i t h e r adding s i l i c a to or removing alumina from the m e t a k a o l l n . ( 7 - 9 ) In each c a s e , w i t h o u t a d d i t i o n of s i l i c a or d e l e t i o n of a l u m i n a , o n l y A z e o l i t e 1s formed. Only s c a t t e r e d r e p o r t s a r e a v a i l a b l e on the p r e p a r a t i o n of LSX even 1n an unbound f o r m . Simply e x t r a p o l a t i n g the s t a n d a r d s y n t h e s i s r e a g e n t s and c o n d i t i o n s used to produce t y p i c a l X z e o l i t e w i l l not y i e l d a S1/A1 r a t i o of l e s s than 1.2. Kuhl and S h e r r y have p u b l i s h e d the most comprehensive I n f o r m a t i o n on LSX synthesis.(10) In a B r i t i s h p a t e n t , they d e s c r i b e a p r e p a r a t i o n wherein a l u m l n o s l 1 1 c a t e g e l s In h i g h l y a l k a l i n e mixed N a V K * a r e s u b j e c t e d to a m u l t l d a y a g i n g p e r i o d at 4 0 ° C , f o l l o w e d by

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c r y s t a l l i z a t i o n at higher temperature. They c l a i m the long a g i n g p e r i o d i s c r i t i c a l t o e l i m i n a t i n g the f o r m a t i o n of A zeolite.(10) They a l s o s t a t e d t h a t many r e a c t i v e s i l i c a - a l u m i n a s o u r c e s may be used to p r e p a r e LSX, i n c l u d i n g c a l c i n e d c l a y s , but g i v e no examples. (ljO) B a r r e r has c a r e f u l l y s t u d i e d the h y d r o t h e r m a l r e a c t i o n s of m e t a k a o l i n i n both s i n g l e and mixed bases a t temperatures of 80°C and above but d i d not observe the f o r m a t i o n of L S X . ( H ) We found no r e p o r t s of the d i r e c t p r e p a r a t i o n of LSX from m e t a k a o l i n i n e i t h e r a powder or s e l f - b o u n d p e l l e t i z e d form. In a d d i t i o n , the a i r s e p a r a t i o n p r o p e r t i e s f o r any i o n form of LSX have never been r e p o r t e d . The work r e p o r t e d here d e s c r i b e s the p r e p a r a t i o n and c h a r a c t e r i z a t i o n of h i g h l y c r y s t a l l i n e s e l f - b o u n d LSX from a porous m e t a k a o l i n p r e c u r s o r and the s u p e r i o r a i r s e p a r a t i o n p r o p e r t i e s o b s e r v e d f o r the c a l c i u m f o r m . Experimental Porous m e t a k a o l i n p e l l e t s were p r e p a r e d from E n g e l h a r d ASP-400 k a o l i n u s i n g food grade c o r n s t a r c h as the pore f o r m e r . Pellets c o n t a i n i n g 30% s t a r c h gave the b e s t r e s u l t s . A f t e r 30% s t a r c h was m u l l e d i n t o the k a o l i n c l a y , the m i x t u r e was e x t r u d e d by s t a n d a r d t e c h n i q u e s to form 1/8 i n c h d i a m e t e r p e l l e t s of s t a r c h - f i l l e d kaolin clay. E x t r u s i o n was f o l l o w e d by a heat t r e a t m e n t a t 700°C s i m i l a r to t h a t d e s c r i b e d by Howell and Acara.(6) The heat t r e a t m e n t s were c a r e f u l l y c o n t r o l l e d s i n c e the p e l l e t s have a h i g h o r g a n i c f r a c t i o n . U n c o n t r o l l e d " b u r n o u t " of the o r g a n i c s may cause l o c a l i z e d hot spots i n the p e l l e t s . T h e r e f o r e , p r i o r to t r e a t m e n t a t 700°C i n a i r , the samples were heated a t a lower temperature under an i n e r t atmosphere to remove most of the o r g a n i c s . Heat t r e a t m e n t tubes loaded w i t h the d r i e d p e l l e t s were lowered i n t o 595°C ovens under a n i t r o g e n atmosphere. The p e l l e t t e m p e r a t u r e r o s e r a p i d l y t o 4 8 0 - 5 4 0 ° C ; as the o r g a n i c s burned o f f a b l a c k smoke e v o l v e d . The N2 atmosphere was m a i n t a i n e d u n t i l smoke f o r m a t i o n c e a s e d . To remove t h e b a l a n c e of the o r g a n i c s , a small f r a c t i o n (5-10%) of the n i t r o g e n was r e p l a c e d w i t h a i r . The r a t e of a i r a d d i t i o n was c o n t r o l l e d to m a i n t a i n the ΔΤ i n the treatment v e s s e l below 40°C so t h a t the p e l l e t temperature was m a i n t a i n e d below 700°C. A f t e r a l l the o r g a n i c m a t e r i a l had burned out and n i t r o g e n had been r e p l a c e d by a i r , the p e l l e t bed t e m p e r a t u r e was r a i s e d to 700°C and h e l d f o r 2 h r s . The t o t a l heat t r e a t m e n t time was t y p i c a l l y from 4 to 6 h r s . The s e l f - b o u n d LSX p e l l e t s were p r e p a r e d d i r e c t l y from the porous m e t a k a o l i n p e l l e t s . In a t y p i c a l p r e p a r a t i o n 22.2 g of the p e l l e t s was mixed w i t h 39.0 g of NaOH, 18.2 g of Κ0Η and 234 cc of H2O so t h a t S i 0 / A l 0 3 = 2.0, ( N a 0 + K 0)/Si0 = 3.25, N a 0 / ( N a 0 + K2O) = 0 . 7 5 , and H 0 / ( N a 0 + K2O) 20. Aging a n d / o r c r y s t a l l i z a t i o n temperatures and times were v a r i e d t o a s c e r t a i n o p t i m a l conditions. The b e s t r e s u l t s were o b t a i n e d when the above m i x t u r e was a l l o w e d t o r e a c t w i t h c o n s t a n t c i r c u l a t i o n a t about 50°C f o r a p e r i o d of 10 d a y s . 2

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One pound l o t s of s e l e c t e d a d s o r b e n t s were p r e p a r e d by a d i r e c t s c a l e u p of the above p r o c e d u r e , f o r c o n v e r s i o n t o o t h e r Ion forms and a p p l i c a t i o n s t u d i e s . The s e l f - b o u n d LSX 1s r e a d i l y c o n v e r t e d to o t h e r 1on forms u s i n g s t a n d a r d Ion exchange t e c h n i q u e s . The c a l c i u m form was prepared In most cases d i r e c t l y from the mixed Na/K a d s o r b e n t s . Four 2 - h r . exchanges, w i t h at l e a s t a f o u r f o l d excess of 1 M CaCl2 a t temperatures between 90 and 100°C, g e n e r a l l y produced samples having >98% of t h e i r exchange c a p a c i t y 1n the c a l c i u m form. The amounts of X and A z e o l i t e s p r e s e n t 1n the a d s o r b e n t s were determined by X - r a y d i f f r a c t i o n . The sodium forms were used In a l l c a s e s . The method I n v o l v e d adding a-Al2Û3 to the sample as an I n t e r n a l standard and then e s t a b l i s h i n g the r a t i o s of the I n t e g r a t e d peak areas f o r s e l e c t e d z e o l i t e l i n e s In the sample of I n t e r e s t to those of pure z e o l i t e s 1n the sodium form. T o t a l m i c r o p o r e a c c e s s i b i l i t y was determined by oxygen a d s o r p t i o n a t - 1 9 6 ° C and 20 t o r r on a vacuum m i c r o b a l a n c e a f t e r a c t i v a t i o n of the z e o l i t e a t 1 t o 2°C per minute to a f i n a l temperature of 4 0 0 ° C . S 1 NMR s p e c t r a were o b t a i n e d a t ambient t e m p e r a t u r e u s i n g the Bruker CXP-200 FT-NMR (4.77 1) s p e c t r o m e t e r equipped w i t h a magic a n g l e s p i n n i n g m u l t l n u c l e a r p r o b e . The D e l r l n r o t o r was spun at about 3 kHz, and the p u l s e r e p e t i t i o n time was 10 s e c . The chemical s h i f t s c a l e was c a l i b r a t e d u s i n g h e x a m e t h y l d l s i l o x a n e as an e x t e r n a l r e f e r e n c e (6 = 6.83 ppm). 29

Crush s t r e n g t h s were measured u s i n g a C h a t H l o n a p p a r a t u s . Values r e p o r t e d were the average of 20 d e t e r m i n a t i o n s . Pore s i z e d i s t r i b u t i o n s were measured by mercury I n t r u s i o n u s i n g a M l c r o m e r l t l c s Autopore 9220. A c o n t a c t a n g l e of 130° was assumed f o r a l l measurements. The e q u i l i b r i u m c a p a c i t i e s of n i t r o g e n and oxygen from a i r and the a d s o r b e n t c a p a c i t i e s f o r pure n i t r o g e n were determined f o r an 8-Inch bed of each adsorbent c o n t a i n e d In a 1-Inch I.D. by 24 In. s t a i n l e s s s t e e l t u b e . The packed bed was p l a c e d In a t h r e e - z o n e tube f u r n a c e . The a d s o r b e n t s were a c t i v a t e d under a f l o w of dry n i t r o g e n and c o n t r o l l e d temperature ramp, In a manner we p r e v i o u s l y found l e a v e s a h i g h p r o p o r t i o n of the z e o l i t e c a t i o n s 1n a d e h y d r o x y l a t e d state.(1_2) To a s c e r t a i n the n i t r o g e n c a p a c i t y and n i t r o g e n s e l e c t i v i t y of each a d s o r b e n t at 30°C and 1 atm p r e s s u r e , we passed a stream of dry C Û 2 - f r e e a i r through the a c t i v a t e d a d s o r b e n t bed a t 30°C f o r s u f f i c i e n t time to ensure e q u i l i b r i u m ( a p p r o x i m a t e l y 12 hrs). To a n a l y z e the adsorbed p r o d u c t , the a d s o r b e n t bed was t h e r m a l l y desorbed at about 400°C Into a gas b u r e t t e c o l l e c t i o n chamber a t a t m o s p h e r i c p r e s s u r e . V o i d volumes In the a d s o r b e n t beds were determined by h e l i u m d i s p l a c e m e n t . Measurements of the d i s p l a c e d volume 1n the gas c o l l e c t o r , c o u p l e d w i t h v o i d volume measurements, y i e l d e d the amount of n i t r o g e n p l u s oxygen adsorbed. The c o m p o s i t i o n s of the adsorbed gases were determined by gas chromatography. The s e l e c t i v i t y (a) of the n i t r o g e n / o x y g e n s e p a r a t i o n Is c a l c u l a t e d from:


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[CL]

adsorbed

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gas phase

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N i t r o g e n c a p a c i t i e s f o r pure gas were determined i n a s i m i l a r manner. F o l l o w i n g a c t i v a t i o n , the adsorbent beds were a l l o w e d to e q u i l i b r a t e i n a stream of d r y n i t r o g e n a t 3 0 ° C . Thermal d e s o r p t i o n c o u p l e d w i t h v o i d volume measurements by h e l i u m d i s p l a c e m e n t i n t o a gas b u r e t a t a t m o s p h e r i c p r e s s u r e y i e l d e d the amount of n i t r o g e n a d s o r b e d . R e s u l t s and D i s c u s s i o n Synthesis. E x t r a p o l a t i o n of standard s y n t h e s i s t e c h n i q u e s f o r X z e o l i t e w i l l not produce s y n t h e t i c f a u j a s i t e s having a S1/A1 below 1.2. Kuhl and S h e r r y ' s methods f o r p r e p a r i n g LSX vary from usual X - t y p e syntheses i n s e v e r a l ways.(10) They s y n t h e s i z e d LSX i n a mixed base (NaOH * KOH) system, whereas t y p i c a l X-synthes1s i s conducted i n pure aqueous NaOH u s i n g a SIO2/AI2O3 ^ 3 . 0 . However, the c h e m i c a l c o m p o s i t i o n of u n m o d i f i e d m e t a k a o l i n f i x e s t h e SIO2/AI2O3 a t 2.0 f o r these reactant compositions. Thus w i t h o u t e l e v a t i n g t h i s r a t i o , the m i x t u r e w i l l t y p i c a l l y be c o n v e r t e d t o z e o l i t e A. The r a t i o of t o t a l base t o s i l i c a i s h i g h , (Na 0 + K 0 ) / S 1 0 ~ 3.25 compared t o Na2Û/S102 ~ 1.0-1.5 i n a more t y p i c a l X zeolite synthesis. The water c o n t e n t of the system i s s i m u l t a n e o u s l y low, H2Û/(Na2Û + K2O) = 20, compared t o H20/Na2Û - 4 0 f o r X z e o l i t e s . We presumed t h a t the i n c r e a s e d a l k a l i n i t y of the system e l e v a t e s the A1/S1 r a t i o i n both the s o l u t i o n and c r y s t a l l i n e z e o l i t e p r o d u c t . This presumption i s supported by B a r r e r ' s o b s e r v a t i o n t h a t a g e n e r a l c o r r e l a t i o n e x i s t s between a l k a l i n i t y and the framework S1/A1 r a t i o of the z e o l i t e product.(1_3) 2

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Kuhl and S h e r r y a l s o c l a i m t h a t p r e c r y s t a l l i z a t i o n a g i n g c o n d i t i o n s of t h e system a r e c r u c i a l . Due t o t h e low SIO2/AI2O3 (-1.0) employed immediate c r y s t a l l i z a t i o n r e s u l t e d i n the f o r m a t i o n of p r e d o m i n a n t l y z e o l i t e A. The a g i n g p r o c e s s i s r e p o r t e d t o be h i g h l y s e n s i t i v e to both time and temperature. These workers suggested t h a t n u c l e a t i o n c e n t e r s l e a d i n g t o f a u j a s i t e form more r a p i d l y than those f o r A z e o l i t e at temperatures below 50°C. The h i g h l y r e a c t i v e m e t a k a o l i n used i n our work i s formed d u r i n g the 700°C heat t r e a t m e n t which i s a l s o needed t o remove the o r g a n i c pore former i n t h e p e l l e t i z e d a d s o r b e n t s . M e t a k a o l i n i s b e l i e v e d t o e x i s t as a d e f e c t phase where s i l i c a l a y e r s of the o r i g i n a l c l a y a r e i n c l o s e p r o x i m i t y t o A10* t e t r a h e d r a l u n i t s coming from the o r i g i n a l o c t a h e d r a l l a y e r . ( 1 4 ) F i g u r e 1 shows the o v e r a l l scheme f o r the p r e p a r a t i o n of a s e l f - b o u n d LSX a d s o r b e n t , from m e t a k a o l i n p e l l e t s which a f t e r c a l c i u m exchange have improved performance as a d s o r b e n t s f o r a i r separation. We have made s e l f - b o u n d LSX by c o n t a c t i n g the porous m e t a k a o l i n p e l l e t w i t h a m i x t u r e of sodium and p o t a s s i u m h y d r o x i d e s a t r e l a t i v e l y low temperatures and p r e s s u r e s . Depending on the c o n d i t i o n s , these aggregates a r e e i t h e r c o n v e r t e d t o a m i x t u r e of A and LSX or t r a n s f o r m e d e n t i r e l y t o

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Molecularly Engineered, High-Petformance

Adsorbent 483

an e s s e n t i a l l y pure LSX. The f o r m a t i o n of LSX does not r e q u i r e any s i l i c a s o u r c e i n a d d i t i o n to the m e t a k a o l l n . The time r e q u i r e d f o r a g i n g a n d / o r c r y s t a l l i z a t i o n of LSX and the p o r o s i t y of the m e t a k a o l l n p e l l e t vary g r e a t l y depending on the c o n d i t i o n s employed ( v i d e i n f r a ) . Both N a and K* a r e n e c e s s a r y f o r the f o r m a t i o n of LSX from m e t a k a o l l n . Similar p r e p a r a t i o n s l a c k i n g K produced p r e d o m i n a n t l y h y d r o x y s o d a l i t e , and removing the Na* produced a phase i d e n t i f i e d by XRD as KF. As s y n t h e s i z e d , the s e l f - b o u n d LSX p e l l e t s a r e i n the mixed i o n form c o n t a i n i n g about 75% N a and 25% K*. Other c a t i o n forms are r e a d i l y p r e p a r e d u s i n g s t a n d a r d i o n exchange p r o c e d u r e s . The p e l l e t s m a i n t a i n t h e i r m a c r o s c o p i c i n t e g r i t y d u r i n g t h e i r c o n v e r s i o n to z e o l i t e . f


+

f

We c a r r i e d out our i n i t i a l s y n t h e t i c work u s i n g f i n e l y powdered m e t a k a o l l n . Its optimum a g i n g p e r i o d was about 5 days a t 40°C; a g i n g was f o l l o w e d by c r y s t a l l i z a t i o n a t 1 0 0 ° C . These r e s u l t s were u n a n t i c i p a t e d i n l i g h t of Kuhl and S h e r r y ' s r e s u l t s for aluminosilicate gels. Kuhl and S h e r r y showing a c o n t i n u o u s r i s e i n LSX p u r i t y w i t h extended aging.(1_0) Even a f t e r the optimum a g i n g p e r i o d , we found t h a t the c o n v e r t e d m e t a k a o l l n c o n t a i n e d about 75% LSX w i t h 10% A z e o l i t e as a minor phase. Having l e a r n e d how the powdered m e t a k a o l l n r e a c t s , we f o c u s e d our a t t e n t i o n on i n s i t u c o n v e r s i o n of p e l l e t l z e d materials. We found t h a t a p o r e - f o r m e d m e t a k a o l l n was n e c e s s a r y f o r p r e p a r i n g s e l f - b o u n d LSX; attempts to c o n v e r t e x t r u d e d m e t a k a o l l n p e l l e t s made w i t h o u t a pore former were unsuccessful. A p p a r e n t l y the dense p e l l e t s do not have s u f f i c i e n t m a c r o p o r o s i t y to i n t e r a c t w i t h the s y n t h e s i s medium. When a pore former such as s t a r c h i s e x t r u d e d w i t h the k a o l i n c l a y and s u b s e q u e n t l y burned o u t , the r e s u l t i n g m e t a k a o l i n a g g r e g a t e has a median pore d i a m e t e r an o r d e r of magnitude l a r g e r than t y p i c a l l y found i n a commercial p e l l e t l z e d X or A-type adsorbent. We c a r r i e d out a g i n g s t u d i e s on both powdered and p e l l e t i z e d metakaolin. Exposing porous m e t a k a o l i n p e l l e t s to the same s y n t h e s i s c o n d i t i o n s used f o r powdered m e t a k a o l i n at 40°C produced a s i m i l a r optimum a g i n g p e r i o d (about 5 days) but a low l e v e l of z e o l i t e s . In c o n t r a s t , when aged a t 50°C p r i o r to c r y s t a l l i z a t i o n the powder c o u l d be c r y s t a l l i z e d at 100°C to pure LSX a f t e r 7 days whereas the p e l l e t s always c o n t a i n e d an A z e o l i t e i m p u r i t y (about 10%) and a maximum of 75% LSX. The r e s u l t s from a g i n g s t u d i e s at 50°C a r e p l o t t e d i n F i g u r e 2. A g i t a t i n g the a g i n g m i x t u r e had no e f f e c t on the c o n v e r s i o n of m e t a k a o l i n to LSX. However, we found t h a t c i r c u l a t i n g the a l k a l i n e s o l u t i o n d u r i n g the c r y s t a l l i z a t i o n s t e p improved the degree of c o n v e r s i o n to z e o l i t e and i n c r e a s e d the f r a c t i o n of LSX produced i n both powders and p e l l e t s . For samples aged f o r 5 days a t 40°C c i r c u l a t i n g the a l k a l i n e s o l u t i o n d u r i n g c r y s t a l l i z a t i o n a t 100°C c o m p l e t e l y c o n v e r t e d m e t a k a o l i n t o z e o l i t e h a v i n g about 80% LSX and 20% A. The b e n e f i c i a l e f f e c t of a g i t a t i o n d u r i n g c r y s t a l l i z a t i o n i n the f o r m a t i o n of LSX has not been recognized. In f a c t , p r e v i o u s workers have s t a t e d t h a t q u i e s c e n t c o n d i t i o n s a r e p r e f e r r e d d u r i n g the c r y s t a l l i z a t i o n of s y n t h e t i c f a u j a s i t e or A z e o l i t e s from m e t a k a o l i n . ( 6 )

484


KAOLIN CLAY + PORE FORMER EXTRUSION SOLID PELLET CALCINATION, 7 0 0 ° C


J

POROUS METAKAOLIN NaOH/KOH, 4 0 - 8 0 ° C SELF-BOUND LSX POST MODIFICATION

F i g u r e 1. I n s i t u p r e p a r a t i o n of a v e r s a t i l e s e l f - b o u n d LSX.

0

1

2

3

4

5

6

7

AGING PERIOD (DAYS)

• SELF-BOUND PELLET Ο FINELY POWDERED LSX ZEOLITE A ZEOLITE

e

ALL SAMPLES WERE CRYSTALLIZED FOR 8 HOURS AT 100 C WITH NO AGITATION.

F i g u r e 2. E f f e c t of 50°C aging p e r i o d on LSX f o r m a t i o n .


30. COE ET AL.

Molecularly Engineered, High-Performance Adsorbent 485

E x t e n s i v e work on the f o r m a t i o n of LSX from f i n e l y powdered m e t a k a o l i n r e s u l t e d i n the d i s c o v e r y t h a t e s s e n t i a l l y pure LSX c o u l d be p r e p a r e d i n a o n e - s t e p s y n t h e s i s w i t h o u t a g i n g u s i n g a temperature of 50°C or s l i g h t l y h i g h e r . C r y s t a l l i z a t i o n curves f o r LSX produced by c i r c u l a t i n g the a l k a l i n e m i x t u r e c o n t i n u a l l y over the m e t a k a o l i n a t 50°C a r e shown i n F i g u r e 3. In both the powdered and porous p e l l e t s , A z e o l i t e d i d not form i f the bases were c i r c u l a t e d and the temperature m a i n t a i n e d at 50°C. At t h i s t e m p e r a t u r e , c l a s s i c a l c r y s t a l l i z a t i o n b e h a v i o r was o b s e r v e d . A f t e r an i n d u c t i o n p e r i o d , the LSX c r y s t a l l i n i t y f o r m a t i o n e x h i b i t s a smooth S-shaped c u r v e . Attempts to a c c e l e r a t e LSX f o r m a t i o n by i n c r e a s i n g the r e a c t i o n temperature enhanced the f o r m a t i o n of A z e o l i t e but d i m i n i s h e d the y i e l d of LSX. However, a d d i t i o n a l s t u d i e s on powdered m e t a k a o l i n showed the f o r m a t i o n of LSX can be s i g n i f i c a n t l y a c c e l e r a t e d by c h e m i c a l means. C i r i c has shown t h a t A z e o l i t e , which has a S i / A l r a t i o of 1.0 and the same s t r u c t u r a l components as f a u j a s i t e , has a r a t e of f o r m a t i o n p r o p o r t i o n a l to [ O H * ] a t a g i v e n t e m p e r a t u r e . (1_5) T h e r e f o r e , we were i n t e r e s t e d i n examining how h y d r o x i d e c o n c e n t r a t i o n i n f l u e n c e s LSX f o r m a t i o n . By r e d u c i n g the water c o n t e n t to h a l f the amount used i n the standard r e a c t i o n m i x t u r e ( d o u b l i n g the c o n c e n t r a t i o n of h y d r o x i d e s ) , the s y n t h e s i s was shortened from s e v e r a l days to a p p r o x i m a t e l y 1 day. An a d d i t i o n a l b e n e f i t of u s i n g l e s s water was a l a r g e i n c r e a s e i n the amount of LSX produced per u n i t volume of the r e a c t o r . Attempts to a p p l y t h i s to the porous m e t a k a o l i n p e l l e t s were hampered by the lack of s u f f i c i e n t c i r c u l a t i o n of the v i s c o u s s y n t h e s i s medium. However, we f e e l t h e r e i s no c h e m i c a l reason why s i m i l a r r e s u l t s f o r the p e l l e t s should not be p o s s i b l e . 2

Characterization. Our s e l f - b o u n d p e l l e t s c o n t a i n e d d i f f e r e n t amounts of LSX and A z e o l i t e s depending on the s y n t h e t i c procedures. The amounts of A and X p r e s e n t were determined u s i n g the X - r a y d i f f r a c t i o n t e c h n i q u e s d e s c r i b e d i n the Experimental s e c t i o n . On a l i n e - b y - l i n e b a s i s the LSX phase i n these products d i s p l a y s i g n i f i c a n t l y d i f f e r e n t X-ray d i f f r a c t i o n p a t t e r n i n t e n s i t i e s compared to a s t a n d a r d X z e o l i t e ( S i / A l = 1.25). However, summation of a l l the major peaks f o r a pure LSX g i v e s a t o t a l i n t e n s i t y t h a t i s v i r t u a l l y i d e n t i c a l t o the standard X z e o l i t e . We measured the framework S i / A l r a t i o p r e s e n t i n the p e l l e t s d i r e c t l y u s i n g s o l i d - s t a t e S 1 NMR and a l s o e s t i m a t e d the r a t i o from the u n i t c e l l c o n s t a n t , an., u s i n g an e x t r a p o l a t i o n of the l i n e a r r e l a t i o n s h i p d e v e l o p e d by Breck and Flanigen.(TM6) E x t r a p o l a t i o n of t h i s c o r r e l a t i o n g i v e s a l a t t i c e c o n s t a n t of 25.02 A f o r a f a u j a s i t e h a v i n g a 1:1 S i / A l ratio. The p r e f e r r e d LSX p r o d u c t , which was c r y s t a l l i z e d a t 50°C, had a u n i t c e l l c o n s t a n t of 25.03 A, c o r r e s p o n d i n g to a framework S i / A l of 0.99. F i g u r e 4 shows the S i NMR i s a s i n g l e resonance at - 8 5 . 7 ppm, c h a r a c t e r i s t i c of the S i (4A1) group i n the z e o l i t e . ( 1 7 . ) The absence of a resonance a t -90 ppm i n d i c a t e s t h e r e i s

Molecularly Engineered, High-Performance Adsorbent - ACS

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