Catalytic Ceramic Membranes and Membrane Reactors - American

Chapter 19

Catalytic Ceramic Membranes and Membrane Reactors 1

2

1

2

M. A. Anderson, F. Tiscareño-Lechuga , Q. Xu , and C. G. Hill, Jr. 1

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Downloaded by UNIV OF ARIZONA on April 7, 2017 | http://pubs.acs.org Publication Date: September 21, 1990 | doi: 10.1021/bk-1990-0437.ch019

Water Chemistry Program and Department of Chemical Engineering, University of Wisconsin, Madison,WI53706

Ceramic membranes represent a comparatively new class of materials which can be prepared from a variety of organometallic or inorganic precursors using sol-gel synthesis routes. The physico-chemical properties of these membranes depend on both the specific compounds used as precursors and the experimental protocol used in their preparation. In this paper, we discuss the key variables controlling the preparation of these membranes as well as the physico-chemical properties which make them good candidates for use as in catalytic membrane reactors. Finally, by way of a kinetic model for a specific reaction (the dehydrogenation of diethylbenzene to styrene), we illustrate the limitations as well as advantages of employing these membranes in catalytic reactors. W h i l e c e r a m i c membranes have been h e r a l d e d as a new t y p e o f m a t e r i a l i n r e c e n t y e a r s , t h e i r h i s t o r y d a t e s back t o t h e 1940*s. Membranes f a b r i c a t e d f r o m m e t a l o x i d e s were o r i g i n a l l y c r e a t e d t o s e p a r a t e g a s e s i n u r a n i u m i s o t o p e e n r i c h m e n t p r o c e s s e s . The r e c e n t r e b i r t h o f i n t e r e s t i n c e r a m i c membranes c a n be a t t r i b u t e d p r i m a r i l y t o renewed i n t e r e s t i n s o l - g e l p r o c e s s i n g o f c e r a m i c p r e c u r s o r s and t o improved a n a l y t i c a l t e c h n i q u e s which p r o v i d e i n s i g h t i n t o t h e m o l e c u l a r e v e n t s which c o n t r o l h y d r o l y s i s and p o l y m e r i z a t i o n r e a c t i o n s i n t h e s e systems. The improved u n d e r s t a n d i n g o f t h e s e r e a c t i o n s has l e d t o t h e u s e o f an e v e r b r o a d e n i n g a r r a y o f o r g a n o m e t a l l i c and i n o r g a n i c p r e c u r s o r s i n t h e p r e p a r a t i o n o f t h e s e m a t e r i a l s . The r e s u l t a n t m a t e r i a l s possess d i f f e r e n t physico-chemical p r o p e r t i e s . Hence t h e r e i s a c o n c o m i t a n t i n c r e a s e i n t h e range o f p o s s i b l e a p p l i c a t i o n s f o r these m a t e r i a l s . While t h e o r i g i n a l a p p l i c a t i o n s o f c e r a m i c membranes i n v o l v e d g a s phase s e p a r a t i o n s , we a r e now u s i n g t h e s e membranes f o r u l t r a f i l t r a t i o n and as c a t a l y s t s , p h o t o c a t a l y s t s , s o l a r c e l l s , and s e n s o r s . T h i s i n c r e a s i n g range o f a p p l i c a t i o n s i s f u e l i n g f u r t h e r fundamental s t u d i e s w h i c h i n c r e a s e t h e a r r a y o f p r e c u r s o r s , and i n c r e a s e o u r u n d e r s t a n d i n g o f s o l - g e l reactions. A major new a p p l i c a t i o n o f c e r a m i c membranes i s i n t h e a r e a o f u l t r a f i l t r a t i o n . Ceramic membranes c a n o u t p e r f o r m o r g a n i c p o l y m e r

0097-6156/90/0437-0198$06.00/0 © 1990 American Chemical Society Baker and Murrell; Novel Materials in Heterogeneous Catalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1990.


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Catalytic Ceramic Membranes & Membrane Reactors

199

membranes i n numerous r e s p e c t s . Some advantages o f c e r a m i c membranes r e l a t i v e t o t h e i r o r g a n i c polymer c o u n t e r p a r t s a r e : 1) C h e m i c a l s t a b i l i t y - Ceramic membranes a r e n o t d e g r a d e d by o r g a n i c s o l v e n t s a n d c a n w i t h s t a n d e x p o s u r e t o c h l o r i n e . Many c r y s t a l l i n e oxides a r e r e l a t i v e l y i n s o l u b l e i n a c i d i c and a l k a l i n e media; hence c e r a m i c membranes composed o f such o x i d e s s h o u l d be r e l a t i v e l y i n e r t under extreme pH c o n d i t i o n s . 2) S t a b i l i t y a t h i g h t e m p e r a t u r e - Once a c e r a m i c membrane i s f i r e d a t a g i v e n t e m p e r a t u r e , i t s p r o p e r t i e s w i l l n o t change on exposure t o lower temperatures. While t h e a p p r o p r i a t e f i r i n g t e m p e r a t u r e v a r i e s w i t h t h e t y p e o f o x i d e o r mixed o x i d e c o m p r i s i n g t h e membrane, t h e t e m p e r a t u r e l e v e l s a t which t h e s e membranes c a n be employed a r e g e n e r a l l y much h i g h e r (above 400°C) t h a n t h o s e a t which o r g a n i c membranes c a n be u s e d ( t y p i c a l l y below 100°C) . 3) S t a b i l i t y t o m i c r o b i a l d e g r a d a t i o n - C e r t a i n o r g a n i c membranes a r e q u i t e s u s c e p t i b l e t o m i c r o b i a l d e g r a d a t i o n . Ceramic membranes a r e n o t e x p e c t e d t o undergo such d e g r a d a t i o n . 4) M e c h a n i c a l s t a b i l i t y - O r g a n i c membranes compact a n d c a n undergo i n e l a s t i c d e f o r m a t i o n s under h i g h p r e s s u r e s , l e a d i n g t o l o w e r p e r m e a b i l i t i e s . Ceramic membranes s u p p o r t e d on r o b u s t m a t e r i a l s such a s s t a i n l e s s s t e e l o r s t r u c t u r a l c e r a m i c s c a n be expected t o withstand very high pressures. 5) C l e a n i n g c o n d i t i o n s - Membrane f o u l i n g i s a s e r i o u s p r o b l e m when u s i n g o r g a n i c membranes. Even i f c e r a m i c membranes s u f f e r f r o m s i m i l a r f o u l i n g problems, i t e m s 1 a n d 2 above i n d i c a t e t h a t h a r s h e r , more e f f e c t i v e c l e a n i n g t r e a t m e n t s c a n be u s e d w i t h c e r a m i c membranes t h a n w i t h o r g a n i c membranes. A l t h o u g h an e x t r e m e l y l a r g e number o f v a r i a t i o n s i n f o r m u l a t i o n c o n d i t i o n s a n d s t a r t i n g m a t e r i a l s c a n be employed t o f o r m o r g a n i c membranes, t h e s e membranes always have a c a r b o n - b a s e d backbone. C e r a m i c membranes composed o f i n o r g a n i c p r e c u r s o r s c a n be f a s h i o n e d f r o m many o f t h e elements o f t h e p e r i o d i c c h a r t . Hence t h e r e i s a wide v a r i e t y o f i n t e r e s t i n g m a t e r i a l s w i t h an e q u a l l y l a r g e v a r i e t y o f p h y s i c o - c h e m i c a l p r o p e r t i e s from which c e r a m i c membranes c a n be f a b r i c a t e d . The a f o r e m e n t i o n e d advantages l e d us t o c o n s i d e r t h e u s e o f c e r a m i c membranes i n c a t a l y t i c a p p l i c a t i o n s where t e m p e r a t u r e r a n g e s o f t e n e x c e e d s e v e r a l hundreds o f d e g r e e s a n d where h i g h p r e s s u r e s and harsh chemical environments prevent t h e use o f o r g a n i c membranes. Preparation

o f Ceramic

Membranes

I n t h e f a b r i c a t i o n o f c e r a m i c membrane modules, s e v e r a l p r o c e s s i n g s t e p s must be a c c o m p l i s h e d : s o l p r e p a r a t i o n , g e l a t i o n , c o a t i n g o f supports, and f i r i n g a t e l e v a t e d temperatures. W i t h i n each s t e p , s e v e r a l i n d e p e n d e n t v a r i a b l e s c a n be u s e d t o t a i l o r t h e p r o p e r t i e s of the ultimate product f o r the s p e c i f i c a p p l i c a t i o n o f i n t e r e s t . A l t h o u g h d i s c u s s i o n o f t h e i n f l u e n c e o f t h e s e v a r i a b l e s i s beyond t h e scope o f t h e p r e s e n t paper, a c u r s o r y t r e a t m e n t o f e a c h s t e p a n d t h e most i m p o r t a n t v a r i a b l e s w i l l be g i v e n h e r e . Sol Preparation. The s y n t h e s i s o f o x i d e s o l s v i a t h e s o l - g e l p r o c e s s i n v o l v e s t h e c o n t r o l l e d h y d r o l y s i s o f m e t a l a l k o x i d e s and/or m e t a l s a l t s . However, t h e p r o p e r t i e s o f t h e r e s u l t i n g s o l , i n c l u d i n g t h e s i z e o f t h e s o l p a r t i c l e s , a r e d e t e r m i n e d by t h e i n t e r a c t i o n s o f s e v e r a l v a r i a b l e s which must be c a r e f u l l y c o n t r o l l e d . Such f a c t o r s as t h e s o l v e n t s used, t h e r a t i o s o f a l k o x i d e o r s a l t t o water and/or a l c o h o l , t h e amount o f added a c i d o r base, t h e r e a c t i o n t e m p e r a t u r e , t h e s t i r r i n g speed, and t h e r a t e a t which r e a c t a n t s a r e added t o t h e

Baker and Murrell; Novel Materials in Heterogeneous Catalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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NOVEL MATERIALS IN HETEROGENEOUS CATALYSIS

s o l v e n t must a l l be s t u d i e d i n o r d e r t o d e v e l o p a s y n t h e s i s p r o t o c o l w h i c h can p r o d u c e a s o l w i t h t h e d e s i r e d p r o p e r t i e s (1,2). The s i z e of the primary p a r t i c l e s i s determined a t t h i s stage, although p a r t i c l e s i z e s c a n change a s a g i n g o c c u r s ( 3 ) . NMR s t u d i e s have p r o v i d e d u s e f u l i n f o r m a t i o n about t h e r e a c t i o n s w h i c h o c c u r d u r i n g h y d r o l y s i s (4). I n g e n e r a l , h y d r o l y s i s a n d c o n d e n s a t i o n r e a c t i o n s can be d e s c r i b e d by e q u a t i o n s 1 a n d 2 below: OR I RO-M-OR

+

H0 2

OR I RO-M-OH

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OR

+

ROH

(1)

OR

OR I RO-M-OH I OR

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OR

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ι

(2) H0 RO-M-O-M-OR I I OR OR where M • c e n t r a l m e t a l a n d R o r g a n i c moiety The r e l a t i v e r a t e s o f t h e h y d r o l y s i s a n d c o n d e n s a t i o n r e a c t i o n s l a r g e l y d e t e r m i n e whether p o l y m e r i c o r p a r t i c u l a t e membranes a r e o b t a i n e d . I f h y d r o l y s i s r a t e s are r a p i d i n comparison t o those o f the condensation r e a c t i o n s , l a r g e r p a r t i c u l a t e s p e c i e s are obtained. On t h e o t h e r hand, i f one c a n reduce t h e r a t e o f t h e h y d r o l y s i s r e a c t i o n t o t h e p o i n t where i t i s s l o w e r t h a n t h a t o f t h e c o n d e n s a t i o n r e a c t i o n (e.g., by l o w e r i n g t h e pH, r e d u c i n g t h e amount t o H2O p r e s e n t , o r b y c h a n g i n g t h e c h e m i c a l n a t u r e o f t h e s u b s t i t u e n t g r o u p s on t h e p r e c u r s o r , one can p r o d u c e p o l y m e r i c o r very small p a r t i c u l a t e species. F o r example, we have u s e d s e v e r a l methods t o p r e p a r e s t a b l e t i t a n i a s o l s by t h e s o l - g e l t e c h n i q u e . One g e n e r a l t e c h n i q u e i s p e p t i z a t i o n , i n w h i c h h y d r o l y s i s o c c u r s w i t h o u t a d d i t i o n o f any a c i d o r b a s e . A g g r e g a t e s o f p r i m a r y p a r t i c l e s form under t h e s e c o n d i t i o n s . These a g g r e g a t e s c a n t h e n be e l e c t r o s t a t i c a l l y d i s p e r s e d by a d d i n g a c i d o r base t o i n c r e a s e t h e s u r f a c e c h a r g e on t h e s o l i d p a r t i c l e s . However, a c i d p e p t i z a t i o n o f o u r t i t a n i a s o l s a p p e a r s t o produce s m a l l e r aggregates o f primary p a r t i c l e s r a t h e r than a d i s p e r s e d s o l o f t h e p r i m a r y p a r t i c l e s t h e m s e l v e s . We have s u c c e s s f u l l y prepared suspensions o f f i n e r t i t a n i a p a r t i c l e s by h y d r o l y z i n g t i t a n i u m t e t r a - i s o p r o p o x i d e under s t r o n g l y a c i d i c c o n d i t i o n s . Kormann e t a l . have even s y n t h e s i z e d s o - c a l l e d Q p a r t i c l e s " (quantum s i z e ) o f t i t a n i a (

1

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1

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1

30

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1

40

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Reactor Length, cm F i g u r e 10. Combined dependence o f t h e c o n v e r s i o n on t h e t e m p e r a t u r e (°C) a n d on t h e p a r t i a l p r e s s u r e o f t h e hydrogen on t h e sweep s i d e (atm) .



19.

ANDERSON ET AL.

Catalytic Ceramic Membranes

& Membrane Reactors

213

1.0

0.7 H 0

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1 • 1 • 1

ι • 10 20

30

40

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Reactor Length, cm F i g u r e 11. Combined dependence o f t h e s e l e c t i v i t y t o s t y r e n e on t h e t e m p e r a t u r e (°C) a n d on t h e p a r t i a l p r e s s u r e o f t h e h y d r o g e n on t h e sweep s i d e (atm).


214

NOVEL MATERIALS IN HETEROGENEOUS CATALYSIS


s e l e c t i v i t y d e c r e a s e s . However, t h e e f f e c t o f t e m p e r a t u r e on t h e s e l e c t i v i t y f o r t h e p e r m s e l e c t i v e membrane r e a c t o r i s n o t a s d r a s t i c as t h a t f o r t h e c o n v e n t i o n a l r e a c t o r . Combining t h e r e s u l t s shown on F i g u r e s 10 a n d 11, f o r o p e r a t i o n a t 600°C , one p r e d i c t s t h a t i n t h e b e s t c a s e c a . 15% improvement i n t h e y i e l d can be a c h i e v e d when a p e r m s e l e c t i v e membrane r e a c t o r i s u s e d . We a r e c u r r e n t l y i n t h e p r o c e s s o f p r o d u c i n g a new g e n e r a t i o n o f c e r a m i c membranes which a r e i n t e n d e d t o have s m a l l e r p o r e s i z e s , h i g h e r s p e c i f i c s u r f a c e a r e a s , and improved p e r m s e l e c t i v i t i e s . These new membranes a r e b e i n g d e s i g n e d t o o p e r a t e a t t e m p e r a t u r e s n e a r 600°C. The u s e o f a p e r m s e l e c t i v e membrane f o r t h e dehydrogenation o f e t h y l b e n z e n e c a n have a g r e a t impact n o t o n l y on t h e c o n v e r s i o n a c h i e v e d but more i m p o r t a n t l y on t h e s e l e c t i v i t y o f t h e r e a c t i o n network.

Acknowledgments The a u t h o r s g r a t e f u l l y acknowledge f i n a n c i a l s u p p o r t r e c e i v e d f r o m t h e U n i t e d S t a t e s Department o f Energy (DE-AS07-861D12626), (PO#AX079886-1); The N a t i o n a l S c i e n c e F o u n d a t i o n (ECE-8504276); a n d t h e U.S. E n v i r o n m e n t a l P r o t e c t i o n Agency (R-813457-01-0). One o f us (F. T.) a l s o w i s h e s t o thank CONACYT-México f o r f i n a n c i a l s u p p o r t . Legend o f Symbola A Pre-exponential f a c t o r i n rate constant E A c t i v a t i o n energy K3 E q u i l i b r i u m constant f o r r e a c t i o n 3 Pi P a r t i a l p r e s s u r e o f component i tot Total pressure H20 Steam t o o i l m o l a r r a t i o δ F r a c t i o n hydrogen removed ξ Extent o f r e a c t i o n 3 Subscripts EB Ethylbenzene H2 Hydrogen ST Styrene a

p

r

Η 2

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8.

Anderson, M. A; Gieselmann, M. J.; Xu, Q. J. Memb. Sci., 1988, 39, 243. Xu, Q.; Anderson, M. A. Mat. Res. Soc. Symp. Proc., 1989, 132, 41. Scherer, G. W. J. Non-Cryst. Solids, 1988, 100, 77. Pouxviel, J. C.; Boilot, J. P. J. Non-Cryst. Solids, 1987, 94, 374. Kormann, C.; Bahnemann, D. W.; Hoffmann, M. R. J. Phys. Chem., 1988, 92, 5196. Hackley, V. Α.; Anderson, M. A. Lanqmuir, 1989, 5, 191. Ramsay, J.D. F. Mat. Res. Soc. Symp. Proc., 1988, 121, 293. Gieselmann, M. J.; Anderson, M. A. J. Amer. Ceram. Soc., 1989, 72(6),

9.

980.

Anderson, Μ. Α.; Gieselmann, M. J.; Villegas, M. A. J. NonCryst. Solids, 1989,110,17.


19. ANDERSON ET AL. Catalytic Ceramic Membranes & Membrane Reactors 215 10. 11. 12. 13. 14.


15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26.

Leenaars, A. F. M.; Burggraaf, A. J . J . Colloid Interface S c i . , 1985, 105(1) 27. Mukherjee, S. P. Ultrastructure Processing of Ceramics, Glasess and Composites, John Wiley and Sons: New York, 1984, 178. Glaves, C. L.; Brinker, C. J.; Smith D. M.; Davis, P. J . Chem Materials, 1989, 1, 34. Uhlheron, R. J . R.; Huis in 't Veld, M. M. B. J.; Reiser, K.; Brurggraaf, A. J . J . Mat. Sci. Lett., 1989, 8(10), 1135. Huis in 't Veld, M. M. B. J.; Uhlheron, R. J . R.; Reiser, K.; Brurggraaf, A. J . Proceedings of the 1st International Conf. on Inorganic Membranes, Montpellier France, 1989. Clough, D. E.; Ramirez, W. F. AIChE Journal, 1976, 22(6), 1097105. Lee, Ε. H. Catalysis Reviews,1973, 8(2), 285-305. Sheel, J . G. P. ; Crowe, C. M. The Canadian Journal of Chemical Engineering, 1969, 47, 183-7. Wang, I.; Wu, J.-C.; Chung, C.-S. Applied Catalysis., 1985, 16, 89-101. Reid, R. C . ; Prausnitz, J . M.; Sherwood, T. K. The Properties of Gases and Liquids. McGraw-Hill: New York, 1977. Raymond, M. E. D. Hydrocarbon Process. 1975. 57(4) 139 Ito, N.; Shindo, Y.; Hakuta,T.; Yoshitome H. Int. J . Hydrogen Energy. 1984, 9(10), 835-9. Shinji, O.; Misono, M.;Yoneda, Y. Bull. Chem. Soc. Jpn., 1982, 55, 2760-4. Mohan, K. ; Govind, R. Separation Science and Technology, 1988, 23, 1715-33. NAG Library. Numerical Algorithm Group Inc. 1984. Wenner, R. R.; Dybdal, E. C. Chemical Engineering Progress.1948,44 (4), 275-86. Sheppard, C. M.; Maier, Ε. E. ; Caram, H. S.Ind. Eng. Chem. Process Des. Dev.1986, 25, 207-10.

RECEIVED May 9,

1990


Catalytic Ceramic Membranes and Membrane Reactors - American

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