Characterization and Catalyst Development - American Chemical

Chapter 7

Attrition-Resistant Porous Particles Produced by Spray Drying

Downloaded by UNIV OF PITTSBURGH on February 12, 2015 | http://pubs.acs.org Publication Date: October 3, 1989 | doi: 10.1021/bk-1989-0411.ch007

Horacio E . Bergna Chemicals and Pigments Department, Experimental Station, E . I. du Pont de Nemours and Company, Wilmington, DE 19880-0262

Attrition resistance can be conferred to porous grains in the micron size range by f i l l i n g the pores on the periphery of the grains with sub-colloidal or very small colloidal particles capable of coalescing or sintering to form a hard egg shell. This approach requires only a fraction of the hard phase volume required to form an attrition resistant continuous framework or skeleton within the grain pores. When slurries made of mixtures of micron sized particles and discrete small nanoparticles are spray dried, the nanoparticles not strongly adsorbed on the surface of the larger particles migrate to the periphery of the spray droplets where they coalesce forming an egg shell. Special character ization techniques such as electron probe micro analysis (EPMA) combined with scanning electron microscopy, X-ray and electron diffraction, and surface area measurements played a crucial role in showing that the egg shell zone is made of the micron size particles with the inter-particle voids f i l l e d by the coalesced nanoparticles. I n d u s t r i a l powders made o f g r a i n s i n t h e m i c r o n s i z e range a r e o f t e n u s e d i n p r o c e s s e s t h a t r e q u i r e h i g h a t t r i t i o n r e s i s t a n c e . A good example o f such powders a r e c a t a l y s t s f o r f l u i d b e d p r o c e s s e s which a r e g e n e r a l l y made o f c a . 45 t o c a . 150 o r 200|im p o r o u s g r a i n s h e r e b y r e f e r r e d t o as "porous m i c r o g r a i n s " o r , i f t h e y a r e s p h e r o i d a l , "porous m i c r o s p h e r e s " (PMS). A c o n v e n t i o n a l a p p r o a c h t o impart a t t r i t i o n r e s i s t a n c e t o a c a t a l y s t g r a i n i s t o embed s m a l l p a r t i c l e s o f t h e a c t i v e c a t a l y s t i n a c o n t i n u o u s framework o r s k e l e t o n made o f a h a r d and r e l a t i v e l y i n e r t m a t e r i a l . I n t h i s case, t h e p e r c e n t a g e o f h a r d m a t e r i a l s r e q u i r e d t o impart s u f f i c i e n t a t t r i t i o n r e s i s t a n c e t o t h e c a t a l y s t c o m p o s i t e p a r t i c l e c a n be as h i g h as 50% and t h e r e -

0097-6156/89/0411-0055$06.00/0 o 1989 American Chemical Society

In Characterization and Catalyst Development; Bradley, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.


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f o r e i t may a f f e c t t h e a c t i v i t y and/or t h e s e l e c t i v i t y o f t h e catalyst. S a t i s f a c t o r y a t t r i t i o n r e s i s t a n c e c a n be c o n f e r r e d t o t h e p o r o u s g r a i n s o f t h e a c t i v e c a t a l y s t w i t h a much s m a l l e r amount o f t h e h a r d , m o s t l y i n e r t m a t e r i a l (around 10 p e r c e n t weight) s i m p l y by d i s t r i b u t i n g t h e h a r d phase i n a t h i n l a y e r around t h e p e r i p h e r a l zone o f t h e c a t a l y s t porous g r a i n s ( 1 ) . I t i s reported i n t h i s p a p e r t h a t when aqueous s l u r r i e s made o f a m i x t u r e o f m i c r o n - s i z e p a r t i c l e s ( f o r example 0.2 t o 2|im s i z e ) and d i s c r e t e s u b - c o l l o i d a l o r v e r y s m a l l c o l l o i d a l p a r t i c l e s ( p r e f e r a b l y l e s s t h a n c a . 5nm diameter) are spray d r i e d , the s u b - c o l l o i d a l or small c o l l o i d a l p a r t i c l e s m i g r a t e w i t h t h e e v a p o r a t i n g water t o t h e p e r i p h e r y o f t h e droplets. As a consequence, t h e s p r a y d r i e d p r o d u c t i s c o n s t i t u t e d by p o r o u s m i c r o s p h e r e s (PMS) made o f t h e m i c r o n - s i z e p a r t i c l e s w i t h a narrow p e r i p h e r a l l a y e r made m o s t l y o f t h e c o a l e s c e d s u b - c o l l o i d a l o r v e r y s m a l l c o l l o i d a l p a r t i c l e s embedding t h e o u t e r zone o f t h e micron-size p a r t i c l e s . I t i s essential to retain the discreteness of t h e s u b - c o l l o i d a l o r small c o l l o i d a l p a r t i c l e s i n t h e s l u r r y p r i o r t o s p r a y d r y i n g so t h a t t h e s e p a r t i c l e s c a n m i g r a t e e a s i l y by d i f f u s i o n o r c a p i l l a r y f l o w i n between t h e l a r g e r c a t a l y s t p a r t i icles. Aggregated c l u s t e r s o f s u b - c o l l o i d a l or c o l l o i d a l p a r t i c l e s do n o t m i g r a t e e a s i l y a r o u n d t h e m i c r o n s i z e d p a r t i c l e s t o t h e p e r i p h e r y o f t h e d r o p l e t , b u t t e n d t o remain d i s t r i b u t e d a t random within the r e s i d u a l microsphere, since they c o n s t i t u t e a very small f r a c t i o n o f t h e volume o f t h e s o l i d s ; i n t h i s c a s e , t h e y a r e n o t very e f f e c t i v e i n c o n t r i b u t i n g t o the mechanical strength of the r e s u l t a n t PMS. On t h e b a s i s o f t h e s e f i n d i n g s , by j u d i c i o u s s e l e c t i o n o f a system o f d i s c r e t e s u b - c o l l o i d a l o r v e r y s m a l l c o l l o i d a l p a r t i c l e s o f a h a r d m a t e r i a l , a t t r i t i o n r e s i s t a n t c a t a l y s t powders can be made w i t h o n l y a s m a l l amount o f t h e h a r d phase by t h e s p r a y d r y i n g method. F i g u r e 1 (2) i l l u s t r a t e s b o t h t h e c o n v e n t i o n a l a p p r o a c h and my n o v e l c o n c e p t o f a t t r i t i o n r e s i s t a n t p o r o u s m i c r o s p h e r e s . I n t h e c o n v e n t i o n a l a p p r o a c h , c a . 50% c o l l o i d a l s i l i c a (22 nm d i a m e t e r ) form a c o n t i n u o u s s u p p o r t i n g framework f o r t h e l a r g e r c a t a l y s t p a r ticles. In my n o v e l approach, t h e p o l y s i l i c i c a c i d p a r t i c l e s ( c a . 2-3 nm d i a m e t e r ) a r e embedded w i t h t h e much l a r g e r c a t a l y s t p a r t i c l e s i n a narrow p e r i p h e r a l range o f t h e PMS. In b o t h c a s e s , s i l i c a c o n s t i t u t e s t h e h a r d , f a i r l y i n e r t phase c o n f e r r i n g a t t r i t i o n r e s i s t a n c e t o t h e PMS.

MATERIALS AND METHODS V a n a d y l p h o s p h a t e s (VPO) and m u l t i p l e component molybdate (MCM) a r e good examples o f c a t a l y s t s , and a l p h a a l u m i n a , amorphous s i l i c a and a l u m i n o - s i l i c a t e s a r e good examples o f c a t a l y s t s u p p o r t s t h a t c a n be f a b r i c a t e d i n t h e form o f 45 t o 150 |Jm d i a m e t e r s p r a y d r i e d p o r o u s s p h e r e s w i t h a t t r i t i o n r e s i s t a n c e improved by a r e l a t i v e l y t h i n p e r i p h e r a l l a y e r r i c h i n amorphous s i l i c a , amorphous a l u m i n a , o r phosphorus o x i d e s . The h a r d phase component o r p r e c u r s o r i s s e l e c t e d i n e a c h c a s e so t h a t i t w i l l n o t i n t e r f e r e w i t h t h e c a t a l y t i c performance o f the c a t a l y s t . As an example, t h e p r e c u r s o r t o t h e v a n a d y l phosphate c a t a l y s t was p r e p a r e d f o l l o w i n g a method (1) b a s e d on r e f e r e n c e (3). The MCM


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was p r e p a r e d b y p r e c i p i t a t i o n o f c o b a l t and n i c k e l n i t r a t e aqueous s o l u t i o n s w i t h ammonium molybdate, f o l l o w e d b y a d d i t i o n s o f i r o n n i t r a t e aqueous s o l u t i o n , b i s m u t h n i t r a t e i n d i l u t e n i t r i c a c i d , magnesium and p o t a s s i u m n i t r a t e i n water, a n d p h o s p h o r i c a c i d ( 1 ) . The s l u r r y was c o n c e n t r a t e d t o 40% s o l i d s and c e n t r i f u g e d . The s u p e r n a t a n t was d i s c a r d e d and t h e cake o f MCM was u s e d t o p r e p a r e a MCM-10% Si02 aqueous s l u r r y f o r s p r a y d r y i n g . In t h e c a s e o f t h e VPO c a t a l y s t f o r t h e butane o x i d a t i o n p r o c e s s a n d t h e MCM c a t a l y s t f o r t h e a c r y l o n i t r i l e p r o c e s s , t h e p r e f e r r e d p r e c u r s o r o f t h e p e r i p h e r a l h a r d phase i s p o l y s i l i c i c a c i d (PSA). The term " p o l y s i l i c i c a c i d " i s g e n e r a l l y r e s e r v e d f o r t h o s e " s i l i c i c a c i d s t h a t have been formed and p a r t i a l l y p o l y m e r i z e d i n t h e pH range 1-4 a n d c o n s i s t o f u l t i m a t e s i l i c a p a r t i c l e s g e n e r a l l y s m a l l e r t h a n 3-4 nm d i a m e t e r " ( 4 ) . S m a l l , d i s c r e t e p a r t i c l e s o f c o l l o i d a l s i l i c a a l s o migrate t o the p e r i p h e r y o f t h e d r o p l e t , but t h e y do n o t c o a l e s c e as e x t e n s i v e l y as PSA i n d r y i n g . The l a r g e r t h e p a r t i c l e s i z e , t h e lower t h e m e c h a n i c a l s t r e n g t h o f t h e coalesced dry product. In o u r c a s e a 6% Si02 s o l u t i o n o f p o l y s i l i c i c a c i d was p r e p a r e d by d e i o n i z i n g t o pH 3 a d i l u t e s o l u t i o n o f f i l t e r e d Du Pont JM g r a d e sodium s i l i c a t e w i t h Dowex HCR-W2-H r e s i n , a s t r o n g l y a c i d i c n u c l e a r s u l f o n i c a c i d c a t i o n exchanger s u p p l i e d by Dow C h e m i c a l Company. The p r e f e r r e d p r o c e d u r e t o f a b r i c a t e a t t r i t i o n r e s i s t a n t PMS c o n s i s t s s i m p l y i n s p r a y d r y i n g aqueous s l u r r i e s made o f comminuted p a r t i c l e s o f VPO o r MCM a r o u n d 0.5-2|im s i z e and a s m a l l amount o f PSA. I n a t y p i c a l s l u r r y , s i l i c a i s o n l y 10% o f t h e VPO o r MCM. We s p r a y d r i e d t h e s l u r r i e s i n a Bowen E n g i n e e r i n g Co. N o . l C e r a m i c Type Spray D r y e r . The s p r a y d r i e d p r o d u c t s were s c r e e n e d t o o b t a i n a c t i v e a n d s e l e c t i v e a t t r i t i o n r e s i s t a n t powders made o f 45 t o 150|Jm d i a m e t e r porous m i c r o s p h e r e s o f t h e c a t a l y s t w i t h a t h i n s i l i c a - r i c h peripheral layer. A t t r i t i o n r e s i s t a n c e was measured on c a t a l y s t powders w i t h a n d w i t h o u t PSA. The a t t r i t i o n r e s i s t a n c e method measured a t t r i t i o n a t a h i g h and c o n s t a n t a i r j e t v e l o c i t y . The f l u i d i z e d samples were subject t o a t t r i t i o n c o n d i t i o n s f o r a s p e c i f i e d length o f time. R e s u l t s a r e g i v e n i n c a t a l y s t weight l o s s p e r c e n t p e r hour v e r s u s e l a p s e d time i n h o u r s . The d i s t r i b u t i o n o f t h e h a r d e n i n g phase was i n v e s t i g a t e d u s i n g e l e c t r o n probe m i c r o a n a l y s i s t e c h n i q u e s (EPMA) and s c a n n i n g e l e c t r o n m i c r o s c o p y (SEM) b e f o r e and a f t e r l e a c h i n g t h e c a t a l y s t s w i t h a c i d s . EPMA was done w i t h a JEOL m i c r o p r o b e u s i n g a T r a c o r N o r t h e r n X - r a y system. X-ray d i f f r a c t i o n a n d e l e c t r o n d i f f r a c t i o n a n a l y s i s were u s e d t o i d e n t i f y t h e phases o f t h e c a t a l y s t s . M i c r o e l e c t r o p h o r e t i c measurements o f t h e VPO aqueous s l u r r y w i t h a n d w i t h o u t PSA added were made a t v a r i o u s pH v a l u e s t o i n v e s t i g a t e t h e p o s s i b l e change o f c h a r a c t e r o f t h e VPO s u r f a c e b y c h e m i s o r p t i o n . However, no s i g n i f i c a n t changes were o b s e r v e d i n t h e z e t a p o t e n t i a l s o r t h e i s o e l e c t r i c p o i n t o f t h e VPO a f t e r a d d i t i o n o f PSA, s u g g e s t i n g t h a t t h e PSA remains f r e e i n t h e aqueous phase w i t h o u t b e i n g a d s o r b e d on t h e VPO surface.


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RESULTS AND DISCUSSION F i g u r e 2a i l l u s t r a t e s t h e improvement i n a t t r i t i o n r e s i s t a n c e o f a VPO c a t a l y s t by t h e a d d i t i o n o f o n l y 10% s i l i c a as PSA. Both samples of VPO, one w i t h no PSA added and t h e o t h e r w i t h 10% PSA, were t e s t e d as c a t a l y s t s i n t h e butane o x i d a t i o n p r o c e s s t o make m a l e i c a n h y d r i d e and showed no d i f f e r e n c e i n a c t i v i t y o r s e l e c t i v i t y . Both f l u i d b e d and r e c i r c u l a t i n g s o l i d s r e a c t o r s were u s e d f o r t h e t e s t s of c a t a l y t i c p e r f o r m a n c e (1) ( 2 ) . F i g u r e 2b shows t h a t MCM c a t a l y s t made w i t h o n l y 10% s i l i c a as PSA a c h i e v e s p r a c t i c a l l y t h e same a t t r i t i o n r e s i s t a n c e as t h e MCM c a t a l y s t made w i t h about 50% c o l l o i d a l s i l i c a w i t h p a r t i c l e d i a m e t e r 22 nm. The d i f f e r e n c e between t h e two approaches t o d e v e l o p a t t r i t i o n r e s i s t a n t PMS as d e s c r i b e d i n t h e i n t r o d u c t i o n a r e i l l u s t r a t e d i n F i g . 3 which i n c l u d e s s c a n n i n g e l e c t r o n m i c r o g r a p h s o f t h e p u r e s i l i c a r e s i d u e o b t a i n e d by l e a c h i n g w i t h a c i d s t h r e e d i f f e r e n t catalysts. The f i r s t one was o r i g i n a l l y t h e MCM c a t a l y s t c o n s t i t u t e d by about 50% o f m e t a l o x i d e a c t i v e c a t a l y s t p a r t i c l e s embedd i n g a c o n t i n u o u s s i l i c a framework o r s k e l e t o n ( 3 - a ) . In t h i s c a s e , a f t e r l e a c h i n g o u t t h e m e t a l o x i d e s w i t h a c i d s , t h e s i l i c a framework r e t a i n s i t s morphology and i t i s almost as a t t r i t i o n r e s i s t a n t as the o r i g i n a l u n t r e a t e d MCM/silica c a t a l y s t (3-b). M i c r o g r a p h (3-d) shows what o r i g i n a l l y was t h e VPO c a t a l y s t made o f v a n a d y l phosphate w i t h o n l y 10% o f 14nm c o l l o i d a l s i l i c a c o n c e n t r a t e d i n t h e p e r i p h e r y o f t h e v a n a d y l phosphate s p h e r e s (3-c). I n t h i s c a s e , l e a c h i n g w i t h a c i d s l e a v e s t h e s i l i c a egg s h e l l w i t h o u t s u p p o r t p r o d u c i n g a c o l l a p s e o f t h e s t r u c t u r e . The s i l i c a r e s i d u e o b t a i n e d by l e a c h i n g w i t h a c i d s t h e VPO-10% PSA c a t a l y s t i s made o f fragments w i t h t h e shape o f t h e p e r i p h e r a l l a y e r o r i g i n a l l y made o f VPO embedded i n s i l i c a ( 3 - e , f ) . The s m a l l c a v i t i e s seen i n t h e f r a c t u r e d edge o f t h e egg s h e l l a r e s i t e s w i t h t h e s i z e and shape o f t h e v a n a d y l phosphate p a r t i c l e s t h a t o c c u p i e d them. The n i t r o g e n s p e c i f i c s u r f a c e a r e a o f t h e s i l i c a r e s i d u e o b t a i n e d b y a c i d l e a c h i n g o u t t h e VPO p a r t i c l e s i s between 300 and 500 m /g as opposed t o about 20-30 m /g f o r t h e o r i g i n a l VPO-10% Si02 c a t a l y s t . S i n c e t h e s u r f a c e a r e a o f t h e VPO w i t h o u t PSA i s a l s o about 20-30 m /g, t h e r e s u l t s suggest t h a t t h e VPO and t h e PSA a r e t i g h t l y embedded i n t h e p e r i p h e r y o f t h e m i c r o s p h e r e . The s p e c i f i c s u r f a c e a r e a o f t h e s i l i c a r e s i d u e (300-500 m /g) shows how e x t e n s i v e l y t h e s i l i c a has c o a l e s c e d from t h e o r i g i n a l 1000-1200 m /g o f t h e PSA d i s p e r s i o n u s e d f o r t h e f a b r i c a t i o n o f t h e c a t a l y s t . M i c r o g r a p h s o b t a i n e d by EPMA a r e shown i n F i g u r e 4. The samp l e i n t h i s c a s e was VPO w i t h 10% amorphous s i l i c a as t h e h a r d phase. T h i s c o m p o s i t i o n was p r e p a r e d by s p r a y d r y i n g an aqueous s l u r r y w i t h about 40% s o l i d s made o f l-2|im p a r t i c l e s o f t h e VPO c a t a l y s t p r e c u r s o r and p o l y s i l i c i c a c i d . The b a c k - s c a t t e r e d image shows a l l e l e m e n t s p r e s e n t i n t h e porous m i c r o s p h e r e . The X - r a y image o f s i l i c o n c l e a r l y shows t h i s element c o n c e n t r a t e d e x c l u s i v e l y on t h e p e r i p h e r y o f t h e m i c r o s p h e r e . Independent X - r a y d i f f r a c t i o n and e l e c t r o n d i f f r a c t i o n a n a l y s i s o f t h e p e r i p h e r a l l a y e r o f t h e m i c r o s p h e r e s showed t h a t t h e s i l i c o n i s p r e s e n t as amorphous s i l i c a . 2

2

2

2

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POROUS MICROSPHERES 45H50^.m Continuous supporting framework of attrition resistant component

Peripheral layer of attrition resistant component

F i g u r e 1. Diagrams i l l u s t r a t i n g two d i f f e r e n t c o n c e p t s t o c o n f e r a t t r i t i o n r e s i s t a n c e t o p o r o u s m i c r o s p h e r e s (PMS). (Reproduced w i t h p e r m i s s i o n from Ref. 2. C o p y r i g h t 1987 E l s e v i e r .)

(a) Vanadyl Phosphate (VPO)

TIME IN HOURS

b.Multicomponent Molybdate Acrylonitrile Catalyst (MCM)

TIME IN HOURS

F i g u r e 2. A t t r i t i o n r e s i s t a n c e o f c a t a l y s t powders w i t h and w i t h o u t p o l y s i l i c i c a c i d (PSA).


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Figure 3. Scanning electron micrographs of catalyst microspheres before and after leaching with acids, (a) Commercial M C M powder made with ca. 50% colloidal silica 22 nm particle size, (b) Same powder after acid leaching the active metal oxide components. Continued on

next page.


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7.

Figure 3. Continued, (c) V P O catalyst powder made with 10% colloidal silica 14 nm particle size, (d) Same powder after acid leaching the V P O . Continued on next page.


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Figure 3. Continued, (e) and (f) Residual silica egg shell after acid leaching a VPO-10% PSA catalyst.



F i g u r e 4. Micrographs of the c r o s s s e c t i o n of a vanadyl p h o s p h a t e p o r o u s m i c r o s p h e r e w i t h 10% amorphous s i l i c a o b t a i n e d by e l e c t r o n probe m i c r o a n a l y s i s (EPMA). Left: b a c k s c a t t e r e d e l e c t r o n image showing average a t o m i c number a c r o s s t h e specimen. Right: X - r a y image showing s i l i c o n distribution.


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CONCLUSIONS A t t r i t i o n r e s i s t a n c e c a n be c o n f e r r e d t o porous g r a i n s i n t h e m i c r o n s i z e range by f i l l i n g t h e p o r e s on t h e p e r i p h e r y o f t h e g r a i n s w i t h s u b - c o l l o i d a l o r very s m a l l c o l l o i d a l p a r t i c l e s capable of s i n t e r i n g t o form a h a r d egg s h e l l . T h i s approach r e q u i r e s o n l y a f r a c t i o n o f t h e h a r d phase volume r e q u i r e d t o form an a t t r i t i o n r e s i s t a n t cont i n u o u s framework o r s k e l e t o n w i t h i n t h e g r a i n p o r e s . When s l u r r i e s made o f m i x t u r e s o f m i c r o n s i z e d p a r t i c l e s and d i s c r e t e s m a l l nanop a r t i c l e s n o t s i g n i f i c a n t l y adsorbed on t h e m i c r o n s i z e d p a r t i c l e surface a r e spray d r i e d , t h e n a n o p a r t i c l e s migrate t o the p e r i p h e r y of t h e s p r a y d r o p l e t s where t h e y c o a l e s c e f o r m i n g an egg s h e l l . The egg s h e l l zone i s made o f t h e m i c r o n s i z e p a r t i c l e s w i t h t h e i n t e r p a r t i c l e v o i d s f i l l e d by t h e c o a l e s c e d n a n o p a r t i c l e s . F o r t h i s r e a son, s p r a y d r y i n g i s a p r e f e r r e d method t o make a t t r i t i o n r e s i s t a n t p o r o u s m i c r o s p h e r e s w i t h a r e l a t i v e l y s m a l l amount (5-10 wt. %) o f p a r t i c l e s o f a h a r d phase. ACKNOWLEDGMENTS I am g r a t e f u l t o my c o l l e a g u e s a t t h e Du Pont E x p e r i m e n t a l S t a t i o n l i s t e d below f o r t h e i r h e l p and s u p p o r t t o a p p l y t h e n o v e l c o n c e p t s d e s c r i b e d i n t h i s r e p o r t t o produce v i a b l e c a t a l y s t s . B r i a n S. Malone and Rashmi M. C o n t r a c t o r conducted t h e a t t r i t i o n r e s i s t a n c e and a c t i v i t y / s e l e c t i v i t y measurements o f t h e VPO-PSA c a t a l y s t s i n a p p a r a t u s e s o f t h e i r own d e s i g n . W i l l i a m J . L i n n t e s t e d c a t a l y t i c performance o f t h e MCM-PSA p r o d u c t s i n t h e a c r y l o n i t r i l e p r o c e s s . EPMA m i c r o g r a p h s were o b t a i n e d by Joseph W. Brennan. SEM p i c t u r e s were t a k e n b y M i c h a e l L. Van K a v e l a a r . The l a t e Gunther T e u f e r c o n d u c t e d t h e X-ray d i f f r a c t i o n and e l e c t r o n d i f f r a c t i o n a n a l y s i s . E. C a r r o l l made t h e BET measurements o f s u r f a c e a r e a and R. E. Johnson and G. Hughes t h e e l e c t r o k i n e t i c measurements. O r a l R. Van B u s k i r k d i r e c t e d most o f t h e s p r a y d r y i n g e x p e r i m e n t s . H. J . McQueston, W. B. Hambleton, J r . , R. E. P o l i n i , J . Watson, P. W. J o h n s t o n , J r . , and D. N i c k e r s o n a s s i s t e d i n t h e e x p e r i m e n t a l work. I am s p e c i a l l y g r a t e f u l t o Uma Chowdhry, A r t h u r W. S l e i g h t , and J . P e t e r J e s s o n f o r i n t e l l i g e n t l y s u g g e s t i n g and e n t h u s i a s t i c a l l y s u p p o r t i n g o u r work i n t h i s s p e c i f i c a r e a o f t e c h n o l o g y .

REFERENCES 1. Bergna, Horacio E. U. S. Patents 4 677 084, 1987 and 4 769 477, 1988. 2. Contractor, R.M., Bergna, H . E . , et a l . Catalysis Today, 1987, 1, 54. 3. Bither, T. U. S. Patent 4 371 703, 1987. 4. Iler, R.K. The Chemistry of Silica; John Wiley and Sons: New York, 1979; p 287. RECEIVED January26,1989


Characterization and Catalyst Development - American Chemical

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