Liquid Membranes - American Chemical Society

Chapter 2

Chemical Aspects of Facilitated Transport Through Liquid Membranes Carl A. Koval and Zelideth E. Reyes

Downloaded by PENNSYLVANIA STATE UNIV on August 19, 2013 | http://pubs.acs.org Publication Date: July 24, 1987 | doi: 10.1021/bk-1987-0347.ch002

Department of Chemistry and Biochemistry, University of Colorado, Boulder, C O 80309

Reversible complexation reactions can be utilized to facilitate the transport of molecules from the gas phase across liquid membranes resulting in a selective separation. The effectiveness of the transport can be related to key physical properties of the system. Results for several systems are compared to the predictions of mathematical models. Advantages and difficulties associated with the use of ion-exchange membranes are discussed. Several areas for future research are suggested.

F a c i l i t a t e d t r a n s p o r t (FT) t h r o u g h l i q u i d membranes i s a phenomenon t h a t a l l o w s t h e f l u x o f a p a r t i c u l a r m o l e c u l e i n t h e gas phase (permeate) t h r o u g h t h e membrane t o be enhanced. T h i s enhanced f l u x i s due t o a r e v e r s i b l e r e a c t i o n between t h e permeate and a c h e m i c a l c a r r i e r , which has been i n c o r p o r a t e d i n t h e membrane, t o form a c a r r i e r - p e r m e a t e complex. I n c o n t r a s t w i t h s e p a r a t i o n s based on r e t e n t i o n chromatography, FT r e q u i r e s m o b i l i t y o f t h e complex s o t h a t i t can d i f f u s e i n r e s p o n s e t o t h e permeate c o n c e n t r a t i o n g r a d i e n t a c r o s s t h e membrane. The s t u d y o f FT phenomena has widespread r e l a v a n c e t o areas o t h e r than gas s e p a r a t i o n s . The c o m p l e x a t i o n c h e m i s t r y i t s e l f c a n be a p p l i e d t o a n a l y t i c a l t e c h n i q u e s such as l i q u i d - l i q u i d e x t r a c t i o n and chromatography (]_) o r t o i n d u s t r i a l s e p a r a t i o n s such as e x t r a c t i v e and a z e o t r o p i c d i s t i l l a t i o n s and s t r i p p i n g p r o c e s s e s ( 2 ) . T r a n s p o r t s t u d i e s u t i l i z i n g s y n t h e t i c membranes a r e i m p o r t a n t f o r u n d e r s t a n d i n g b i o l o g i c a l membrane p r o c e s s e s ( 3 K There a r e p o t e n t i a l a p p l i c a t i o n s based on FT f o r i n v i v o drug d e l i v e r y and maintenance o f s e l f - c o n t a i n e d environments. FT t h r o u g h l i q u i d membranes i s p a r t i c u l a r l y a t t r a c t i v e f o r i n d u s t r i a l s e p a r a t i o n s because t h e energy r e q u i r e m e n t s a r e low y e t i t i s p o s s i b l e t o a c h i e v e h i g h f l u x e s and s e l e c t i v i t y ( 4 ) . FT membrane systems have a l r e a d y been used i n t h e t r e a t m e n t o f heavy m e t a l s , such as chromium i n h y d r o m e t a l l u r g i c a l o r e p r o c e s s i n g , and i n t h e e x t r a c t i o n o f oxygen from a i r ( 5 ) . 0097-6156/87/0347-0028S06.00/0 © 1987 American Chemical Society

In Liquid Membranes; Noble, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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KOVAL A N D REYES

Facilitated Transport Through Liquid Membranes

29


Our o b j e c t i v e has been t o d e v e l o p a procedure through which fundamental p r o p e r t i e s o f r e v e r s i b l e c o m p l e x a t i o n r e a c t i o n systems can be used t o p r e d i c t and t o o p t i m i z e FT o f g a s e s . T h i s procedure i n c l u d e s t h e s e l e c t i o n o f c a r r i e r s , t h e measurement o f t h e r e l e v a n t p h y s i c a l p r o p e r t i e s (RPP) and f l u x e s , t h e use o f an o p t i m i z a t i o n model t o i d e n t i f y f a c t o r s t h a t l i m i t t h e t r a n s p o r t , and t h e m o d i f i c a t i o n o f t h e c a r r i e r t o improve t h e f a c i l i t a t i o n . M a t h e m a t i c a l Model The c o m p l e x a t i o n and d i f f u s i o n p r o c e s s e s i n v o l v e d i n FT a r e shown i n F i g u r e 1. The p h y s i c a l c o n s t a n t s o f t h e system t h a t a r e r e l e v a n t t o FT a r e t h e c o m p l e x a t i o n e q u i l i b r i u m and r a t e c o n s t a n t s , K ( e q ) , k ( f ) and k ( r ) , t h e d i f f u s i o n c o e f f i c i e n t s f o r t h e permeate and complex, D(A) and D(AC), i n i t i a l c o n c e n t r a t i o n s o f a l l s p e c i e s i n the membrane s o l u t i o n phase, and t h e membrane t h i c k n e s s ( L ) . The i n t e r p l a y o f these parameters i n d e t e r m i n i n g t h e f l u x o f permeate through the membrane i s a c o m p l i c a t e d t r a n s p o r t problem. S e v e r a l m a t h e m a t i c a l models t h a t attempt t o d e s c r i b e FT have been r e p o r t e d (4_). One such model developed by Noble and coworkers (_6,_7) combines t h e p h y s i c a l c o n s t a n t s mentioned above i n t o t h r e e d i m e n s i o n l e s s parameters: Κ - K(eq)[A] ε - D(AC)/k(r)L

(1) 2

α - ID(AC)/D(A)}{([C]+[AC])/[A]}

(2) (3)

The parameters Κ, ε and α can be used t o c a l c u l a t e a f a c i l i t a t i o n f a c t o r , F, w h i c h i s d e f i n e d a s : f l u x w i t h c a r r i e r i n t h e membrane F

(4) f l u x w i t h no c a r r i e r i n t h e membrane

The e q u i l i b r i u m f a c t o r Κ i s a measure o f the magnitude o f c o m p l e x a t i o n a t a g i v e n permeate c o n c e n t r a t i o n . The i n v e r s e k i n e t i c f a c t o r ε i s t h e r a t i o o f r e a c t i o n time t o d i f f u s i o n t i m e . A s m a l l value o f ε corresponds t o a d i f f u s i o n - l i m i t e d t r a n s p o r t , w h i l e a l a r g e ε c o r r e s p o n d s t o a k i n e t i c a l l y - l i m i t e d t r a n s p o r t . The c o n c e n t r a t i o n / m o b i l i t y f a c t o r , a, compares t h e c o n c e n t r a t i o n and d i f f u s i o n c o e f f i c i e n t o f t h e complex t o t h a t o f t h e permeate. For v a l u e s o f Κ, ε and α t h a t a r e l i k e l y t o be a c h i e v e d i n r e a l membrane systems, F ranges from u n i t y , w h i c h i m p l i e s no f a c i l i t a t i o n , t o v a l u e s g r e a t e r than twenty! I n r e f e r e n c e 7, a s e r i e s o f w o r k i n g c u r v e s were p r e s e n t e d t h a t d e p i c t t h e r e l a t i o n s h i p s between t h e s e f o u r parameters and p r e d i c t o p t i m a l v a l u e s o f Κ, ε and a, i . e . v a l u e s which y i e l d t h e h i g h e s t v a l u e s o f F. One i n t e r e s t i n g f e a t u r e o f p r e d i c t i o n s i n r e f e r e n c e 7 i s t h a t F r e a c h e s o p t i m a l v a l u e s f o r K«= 1-100. Large v a l u e s o f F r e q u i r e a compromise between uptake o f permeate a t t h e f e e d s i d e and t h e r e l e a s e o f permeate a t t h e sweep s i d e o f t h e membrane. For Κ > 100, t h e c o n c e n t r a t i o n o f f r e e permeate i n t h e membrane becomes s m a l l which i n h i b i t s t h e r e l e a s e o f permeate a t t h e sweep


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LIQUID MEMBRANES: THEORY AND APPLICATIONS


s i d e o f the membrane. For Κ < 1, c o m p l e x a t i o n o f the permeate becomes u n i m p o r t a n t , s i n c e t h e r e i s not a s i g n i f i c a n t c o n c e n t r a t i o n o f the complex w i t h r e s p e c t t o t h a t o f unbound permeate i n the membrane. I n g e n e r a l , the magnitude o f F i s i n v e r s e l y r e l a t e d t o the k i n e t i c f a c t o r ε. I n s p e c t i o n o f E q u a t i o n 2 r e v e a l s t h a t s m a l l v a l u e s o f ε a r e most e a s i l y a c h i e v e d by l a r g e v a l u e s o f L o r k ( r ) w h i c h l e a d t o more time f o r the c o m p l e x a t i o n r e a c t i o n . F i n a l l y , F a l w a y s i n c r e a s e s w i t h i n c r e a s i n g v a l u e s o f the c o n c e n t r a t i o n / m o b i l i t y f a c t o r a. M e t a l Ions and Complexes as C a r r i e r s I n 1960, S c h o l a n d e r d e s c r i b e d an e i g h t - f o l d i n c r e a s e i n the steady s t a t e f l u x o f 0, through aqueous s o l u t i o n s c o n t a i n i n g hemoglobin (8). S i n c e t h a t t i m e , m e t a l i o n s have been shown t o p r o v i d e FT i n a number o f systems: C u ( I ) as a c a r r i e r f o r CO ( 9 - 1 1 ) , F e ( I I ) as a c a r r i e r f o r NO (12-14), and A g ( I ) as a c a r r i e r f o r o l e f i n s (3,15). I t s h o u l d be p o s s i b l e t o improve the u t i l i t y o f metal i o n s as c a r r i e r s v i a the f o r m a t i o n o f complex i o n s . Complex i o n s d e r i v e d from m a c r o c y c l e s or o t h e r p o l y d e n t a t e a r e e s p e c i a l l y a t t r a c t i v e f o r s t u d y i n g FT. M a c r o c y c l i c complexes a r e r e l a t i v e l y s t a b l e . S i n c e the l i g a n d o c c u p i e s most o f the c o o r d i n a t i o n s i t e s on the m e t a l i o n , FT based on 1:1 r e a c t i o n between the complex and the m e t a l can be s t u d i e d w i t h i n the c o n t e x t o f s i m p l e m a t h e m a t i c a l models. F u r t h e r m o r e , m a c r o c y c l e s have been u t i l i z e d t o study the e f f e c t s o f r i n g s i z e , degree o f s a t u r a t i o n , o v e r a l l c h a r g e , l i g a n d c o n f o r m a t i o n , l i g a n d f i e l d ,and ir-bonding on the c o o r d i n a t i o n c h e m i s t r y o f m e t a l i o n s ( 1 6 ) . Thus, the b i n d i n g o f permeates by m a c r o c y c l i c complex c a r r i e r s can be v a r i e d i n a s y s t e m a t i c way a l l o w i n g f o r o p t i m i z a t i o n o f a membrane system. I n an a t t e m p t t o i l l u s t r a t e the c o n c e p t s d e s c r i b e d above, we have s t u d i e d the r e v e r s i b l e b i n d i n g o f carbon monoxide by s e v e r a l F e ( I I ) t e t r a i m i n e complexes i n b e n z o n i t r i l e . The F e ( I I ) complex d e r i v e d from the m a c r o c y c l i c l i g a n d 2 , 3 , 9 , 1 0 - t e t r a m e t h y l - 1 , 4 , 8 , 1 1 tetraaza-cyclotetradeca-1,3,8,10-tetraene (TIM) was f i r s t prepared by B a l d w i n e t a l . ( 1 7 ) . They demonstrated t h a t the n i t r i l e s o l v e n t l i g a n d s c o u l d be r e p l a c e d by o t h e r l i g a n d s such as i m i d a z o l e and CO. Carbon monoxide r e p l a c e s o n l y one o f the a x i a l n i t r i l e l i g a n d s a t ambient p r e s s s u r e s , as i l l u s t r a t e d i n F i g u r e 2 ( f o r F e ( T I M ) , R « R « CH and R - H). Due t o a r e l a t i v e l y p o s i t i v e F e ( I I I , I I ) r e d u c t i o n p o t e n t i a l , Fe(TIM) i s s t a b l e i n o x y g e n - s a t u r a t e d b e n z o n i t r i l e (BN) s o l u t i o n s f o r weeks and i n the s o l i d s t a t e f o r months. The s t a b i l i t y o f t h i s complex towards o x i d a t i o n i n b e n z o n i t r i l e t o g e t h e r w i t h the known 1:1 complexation r e a c t i o n w i t h CO suggested the p o s s i b i l i t y o f c o n s t r u c t i n g a membrane system f o r the s e p a r a t i o n o f CO from 0 . By f o l l o w i n g e l e c t r o n i c s p e c t r a l changes o f Fe(TIM) i n BN upon r e a c t i o n w i t h CO as a f u n c t i o n o f time the r a t e s and e q u i l i b r i u m c o n s t a n t s f o r the c o m p l e x a t i o n r e a c t i o n were d e t e r m i n e d ( 1 8 ) . The d i f f u s i o n c o e f f i c i e n t s f o r the c a r r i e r and the CO-complex as w e l l as the e q u i l i b r i u m c o n s t a n t were measured u s i n g c y c l i c voltammetry and r o t a t i n g d i s k v o l t a m m e t r y . These p h y s i c a l c o n s t a n t s were i n c o r p o r a t e d i n the o p t i m i z a t i o n model which p r e d i c t e d a f a c i l i t a t i o n f a c t o r F- 1.12. T h i s d a t a i s summarized i n T a b l e I . The f l u x e s o f 0 and CO through a membrane o f f i l t e r paper x

2

3

3

2

2


2.

KOVAL AND REYES


SELECTIVE COMPLEXATION REACTION


A + (permeate)

I +

C (carrier)

ι

AC (complex)

I I

t o G C

-

LIQUID MEMBRANE L

F i g u r e 1. C r o s s - s e c t i o n a l view o f p r o c e s s e s i n a l i q u i d membrane t h a t r e s u l t i n f a c i l i t a t e d t r a n s p o r t o f s p e c i e s A. The f l u x o f s p e c i e s Β i s s o l e l y due t o d i f f u s i o n .

F i g u r e 2. S t r u c t u r a l drawing d e p i c t i n g t h e r e v e r s i b l e 1:1 c o m p l e x a t i o n r e a c t i o n o f F e ( I I ) m a c r o c y c l i c complex i o n s w i t h carbon monoxide.


31



32

impregnated w i t h Fe(TIM)/BN were measured u t i l i z i n g a f l o w c e l l and gas chromatography ( 1 9 ) . The e x p e r i m e n t a l F- 1.14 ± .09 compared f a v o r a b l y w i t h the p r e d i c t e d v a l u e . D e s p i t e the s m a l l F t h a t was o b t a i n e d , the e x c e l l e n t agreement between the e x p e r i m e n t a l r e s u l t s and the model encouraged the use o f the o p t i m i z a t i o n model t o determine which p r o p e r t i e s of the system were l i m i t i n g the FT. A l t h o u g h s u b s t i t u t i o n k i n e t i c s , f o r the l o w - s p i n F e ( I I ) c e n t e r a r e r e l a t i v e l y s l o w , the l i m i t i n g f a c t o r was a, i . e . the s o l u b i l i t y o f the c a r r i e r . R e c e n t l y , we have attempted t o i n c r e a s e F f o r t h i s system by i n v e s t i g a t i n g d e r i v a t i v e s o f Fe(TIM) ( 2 0 ) . These d e r i v a t i v e s a r e : Fe(Me„TIM) w i t h R = R - R - CH and F e ( M e P h T I M ) w i t h R - CH , R =» C H and R - H. As shown i n Table I , these Fe(TIM) a n a l o g s a r e more s o l u b l e i n b e n z o n i t r i l e . U n f o r t u n a t e l y the c o m p l e x a t i o n e q u i l i b r i u m c o n s t a n t s a r e s m a l l e r f o r the d e r i v a t i v e s , which r e s u l t s i n poorer v a l u e s o f Κ under the c o n d i t i o n s o f the t r a n s p o r t e x p e r i m e n t s . The c o m p l e x a t i o n k i n e t i c s f o r Fe(Me,JIM) are c o n s i d e r a b l y f a s t e r than those observed f o r Fe(TIM). The v a l u e o f k ( r ) f o r F e ( M e P h T I M ) i s a l s o g r e a t e r than the v a l u e f o r Fe(TIM) w h i l e the v a l u e o f k ( f ) i s about the same. The d i f f u s i o n c o e f f i c i e n t s f o r a l l t h r e e c a r r i e r s a r e about the same as e x p e c t e d . Based on the v a l u e s o f Κ, ε and α t h a t can be c a l c u l a t e d from the RPP i n T a b l e I , Fe(Me„TIM) s h o u l d be a much b e t t e r c a r r i e r than Fe(TIM); however, r e s u l t s o f t r a n s p o r t experiments showed o n l y a s l i g h t i n c r e a s e i n F. We b e l i e v e t h a t t h i s d i s c r e p a n c y i s due t o the v a l u e o f D(AC) used i n the c a l c u l a t i o n s . The v a l u e s of D(AC) r e p o r t e d i n T a b l e I were measured a t c o n c e n t r a t i o n s r a n g i n g from 2*-5 mM, which a r e much s m a l l e r than those used i n the t r a n s p o r t e x p e r i m e n t s . R e c e n t l y , we have found t h a t lower v a l u e s o f D(AC) a r e o b t a i n e d at h i g h e r c o n c e n t r a t i o n s , presumably due t o the f o r m a t i o n o f d i m e r s or o l i g o m e r s . Based on the RPP's, F e ( M e P h T I M ) s h o u l d be a comparable c a r r i e r to Fe(TIM) y e t the v a l u e of F observed i s much l a r g e r . I t i s p o s s i b l e t h a t the v a l u e o f F(obsd) i s erroneous due t o a l e a k i n the membrane and we a r e a t t e m p t i n g t o r e p e a t t h i s experiment. l

2

6

S

2

3

3

2

2

x

3

3

2

2

2

2

T a b l e I . Summary o f P h y s i c a l P r o p e r t i e s and F a c i l i t a t i o n F a c t o r s f o r F e ( I I ) ( T I M ) and D e r i v a t i v e s Carrier

Solubility

(mM) Fe(TIM) 15 Fe(Me„TIM) 46 F e ( M e P h T I M ) 50 2

2

K(eq)

k(f)

k(r)

(M~ ) (M~V ) 1

420 180 130

1

0.14 0.92 0.11

1

(s" ) 3.3x10-4 5.0x10-3 9.0x10-4

F

D(AC)

(cm s

_ cale

F

obsd

)

3.1x10-6 2.5x10-6 4.8x10-6

1.12 1.86 1.28

1.14 1 .23 2.8

I n a d d i t i o n t o c a r r i e r d e r i v a t i z a t i o n , we attempted t o i n c r e a s e the s o l u b i l i t y of the F e ( I I ) c a r r i e r s through the a d d i t i o n o f c o - s o l v e n t s t o the l i q u i d membrane. A d d i t i o n o f 50% (v:v) propylene carbonate to b e n z o n i t r i l e r e s u l t e d i n s i x - to e i g h t f o l d i n c r e a s e i n s o l u b i l i t y w i t h o u t l o s s o f s t a b i l i t y towards d i o x y g e n .


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U n f o r t u n a t e l y , the r e a c t i o n o f the F e ( I I ) ( T I M ) complex w i t h CO becomes c o m p l i c a t e d i n the mixed s o l v e n t . The r e a c t i o n d i s p l a y s p s e u d o - f i r s t - o r d e r b e h a v i o r o n l y a t l o n g times (>200 s ) and c y c l i c voltammetry r e v e a l s more than one e l e c t r o a c t i v e component, even i n the absence o f CO. I t i s p o s s i b l e t h a t propylene c a r b o n a t e d i s p l a c e s b e n z o n i t r i l e as an a x i a l l i g a n d t o F e ( I I ) and t h a t t h i s s u b s t i t u t i o n r e s u l t s i n the f o r m a t i o n o f a h i g h - s p i n complex. G e n e r a l P r o p e r t i e s o f I m m o b i l i z e d L i q u i d Membranes, Ion-Exchange Membranes and P o l y m e r i c Membranes D e s p i t e the p h y s i c a l s t r e n g t h o f f e r e d by a macroporous s u p p o r t , most i m m o b i l i z e d l i q u i d membrane (ILM) systems a r e not p r a c t i c a l f o r i n d u s t r i a l s e p a r a t i o n s because they a r e not s u f f i c i e n t l y s t a b l e . The two most i m p o r t a n t t y p e s o f i n s t a b i l i t y a r e s o l v e n t e v a p o r a t i o n and l o s s o f s o l v e n t and/or c a r r i e r from the s u p p o r t caused by a p r e s s u r e d i f f e r e n t i a l a c r o s s the membrane. These i n s t a b i l i t i e s can be c o m p l e t e l y e l i m i n a t e d by removing the s o l v e n t , i . e . , r e p l a c i n g the l i q u i d membrane w i t h a p o l y m e r i c membrane (PM). P o l y m e r i c membranes have been o f i n t e r e s t f o r i n d u s t r i a l s e p a r a t i o n s f o r some time. The main advantage o f PM's w i t h r e s p e c t t o ILM's i s s t a b i l i t y . The main d i s a d v a n t a g e s o f PM's a r e l o w s o l u b i l i t y o f the permeate and l a c k o f s e l e c t i v i t y . C a r r i e r s can be i n t r o d u c e d i n t o a PM e i t h e r v i a c o - p o l y m e r i z a t i o n o f a p p r o p r i a t e monomers o r by c h e m i c a l l y bonding the c a r r i e r t o a p p r o p r i a t e f u n c t i o n a l groups on the polymer backbone. R e c e n t l y , t h e r e have been two r e p o r t s o f f a c i l i t a t e d d i o x y g e n t r a n s p o r t u t i l i z i n g t h i s approach (21 ,22). I n each c a s e , a t r a n s i t i o n m e t a l complex i n w h i c h the m e t a l i s known t o b i n d d i o x y g e n was i n t r o d u c e d i n t o the a PM. Even though enhanced oxygen f l u x e s were r e p o r t e d , these r e s u l t s cannot be e x p l a i n e d u s i n g the framework o f f a c i l i t a t e d t r a n s p o r t a s d e f i n e d e a r l i e r i n t h i s paper. F a c i l i t a t i o n i n ILM's r e q u i r e d t h a t the c a r r i e r and complex can d i f f u s e f r e e l y w i t h i n the membrane. I n the l i m i t o f z e r o m o b i l i t y o f the c a r r i e r and complex, w h i c h i s presumably the case f o r a PM, the membrane m a t e r i a l s h o u l d r e t a r d the permeate a s i s the case f o r numerous p o l y m e r i c m a t e r i a l s c u r r e n t l y used a s c h r o m a t o g r a p h i c s u p p o r t s . T h i s e f f e c t i s i l l u s t r a t e d by the r e c e n t r e p o r t o f a m a t e r i a l prepared by the i m m o b i l i z a t i o n o f C o ( I I ) S c h i f f - b a s e complexes on a p o l y s t y r e n e / p o l y p y r i d i n e copolymer (J_). Ion-exchange membranes (IEM's) a r e p o l y m e r i c m a t e r i a l s t h a t c o n t a i n i o n i c f u n c t i o n a l i t i e s c h e m i c a l l y bound t o the polymer backbone. The bound i o n i c f u n c t i o n a l i t i e s can be a n i o n i c o r c a t i o n i c . I n o r d e r t o m a i n t a i n e l e c t r o n e u t r a l i t y w i t h i n the membrane, each bound i o n i c s i t e must be p a i r e d w i t h an i o n o f the o p p o s i t e c h a r g e . When the membrane i s s w o l l e n w i t h an a p p r o p r i a t e s o l v e n t , these Ions become m o b i l e and can be exchanged w i t h o t h e r i o n s o f l i k e charge. The i n t e r n a l s t r u c t u r e o f c e r t a i n IEM's such as N a f i o n (23) r e s u l t s i n a p r o p e r t y known a s p e r m s e l e c t i v i t y . P e r m s e l e c t i v e IEM's r e j e c t i o n s o f the same charge a s the bound i o n i c s i t e s , i . e . i n N a f i o n v i t u a l l y a l l the i o n i c c o n d u c t i v i t y t h r o u g h the membrane i s due t o the m o b i l e c a t i o n s . W i t h r e s p e c t t o FT, IEM's t h a t have been s w o l l e n w i t h s o l v e n t d i s p l a y p r o p e r t i e s t h a t a r e i n t e r m e d i a t e between ILM's and PM's. C a r r i e r s t h a t have the same charge a s the m o b i l e i o n s can be exchanged i n t o the IEM s i m p l y by s o a k i n g the IEM i n a s o l u t i o n



34


c o n t a i n i n g the c a r r i e r i n an i o n i c form. I t i s o f t e n p o s s i b l e t o exchange more than 90Ï o f the m o b i l e i o n s which l e a d s t o v e r y h i g h c a r r i e r concentrations (1 - 10 M). High c o n c e n t r a t i o n s o f c a r r i e r a r e n e c e s s a r y f o r l a r g e v a l u e s o f the d i m e n s i o n l e s s parameter α and l a r g e f a c i l i t a t i o n f a c t o r s . In g e n e r a l , IEM's are f a r more s t a b l e than ILM's, a t l e a s t on the t i m e s c a l e o f f l u x measurements (2-12 h ) . S i n c e the s o l v e n t i n t e r a c t s w i t h the support m a t e r i a l , i t e v a p o r a t e s much more s l o w l y . The pores i n IEM's such as N a f i o n are r e l a t i v e l y narrow (0.1 M) i n t h e l i q u i d membrane, be s t a b l e i n s o l u t i o n a s t h e c a r r i e r and complex, and r e a c t r e v e r s i b l y and s e l e c t i v e l y w i t h the permeate. The o p t i m a l v a l u e o f t h e c h e m i c a l e q u i l i b r i u m c o n s t a n t K(eq) depends on t h e c o n c e n t r a t i o n o f the permeate i n the membrane, b u t t h e v a l u e o f t h e d i m e n s i o n l e s s e q u i l i b r i u m c o n s t a n t Κ s h o u l d be c l o s e t o 10. The k i n e t i c s o f the c o m p l e x a t i o n r e a c t i o n s h o u l d be f a s t compared t o the time i t t a k e s t h e permeate and complex t o d i f f u s e a c r o s s t h e membrane. The s e l e c t i v i t y o f t h e s e p a r a t i o n w i l l depend on the l a c k o f r e a c t i v i t y o f t h e c a r r i e r w i t h o t h e r componenets o f the m a t r i x t h a t c o n t a i n s the permeate. W i t h these c o n s t r a i n t s i n mind, c h e m i s t s can b e g i n t o s e a r c h f o r c a r r i e r s w i t h d e s i r a b l e c o m p l e x a t i o n s e l e c t i v i t i e s and t o d e v e l o p s t r u c t u r e / p r o p e r t y r e l a t i o n s h i p s t h a t would l e a d t o the d e s i g n o f c a r r i e r s w i t h o p t i m a l RPP's. At t h i s p o i n t i n t i m e , an i n c r e a s e d awareness on the p a r t o f



36


c h e m i s t s as t o t h e s e p a r a t i o n problems t h a t a r e i m p o r t a n t i n t h e p r i v a t e s e c t o r w o u l d be e x t r e m e l y u s e f u l . T h i s i n f o r m a t i o n c o u l d be p r o v i d e d by t h e i n d u s t r i a l a n d c h e m i c a l e n g i n e e r i n g communities. One example o f a p e r t i n a n t problem i s t h e need f o r c a r r i e r s f o r dioxygen t h a t a r e not s u s e c p t i b l e to a u t o x i d a t i o n . T h i s paper has s t r e s s e d t h e a b i l i t y t o p r e d i c t the e f f e c t i v e n e s s o f f a c i l i t a t e d t r a n s p o r t b a s e d on measurable p h y s i c a l p r o p e r t i e s o f t h e system (RPP). U n d e r s t a n d i n g which p r o p e r t i e s l i m i t the t r a n s p o r t i s e s s e n t i a l f o r improving separations i n a s y s t e m a t i c way. A l t h o u g h t h e agreement between m a t h e m a t i c a l models and e x p e r i m e n t a l d a t a i s a c c e p t a b l e f o r ILM's i n v o l v i n g s i m p l e c o m p l e x a t i o n r e a c t i o n s , t h i s methodology s h o u l d be extended t o more complex systems. Some o f t h e a r e a s r e q u i r i n g a d d i t i o n a l a t t e n t i o n include : E x t e n s i o n o f m a t h e m a t i c a l models t o i n c l u d e more c o m p l i c a t e d c o m p l e x a t i o n c h e m i s t r y . I n p a r t i c u l a r , c a r r i e r s w i t h more than one b i n d i n g s i t e might be more e f f e c t i v e than 1:1 b i n d e r s . I t s h o u l d be noted t h a t e x t r e m e l y l a r g e c a r r i e r s , such a s p r o t e i n s , may s u f f e r from low m o b i l i t y and/or low s o l u b i l i t y . Development o f a d d i t i o n a l e x p e r i m e n t a l p r o c e d u r e s f o r measuring t r a n s p o r t r a t e s . Most o f t h e i n s t r u m e n t a l d e s i g n s a v a i l a b l e a r e e l a b o r a t e and/or e x p e n s i v e . T r a n s p o r t o f permeates c o n t a i n e d w i t h i n l i q u i d phases a s w e l l a s i n t h e gas phase must be c o n s i d e r e d . Development o f membrane s u p p o r t m a t e r i a l s t h a t p r o v i d e g r e a t e r s t a b i l i t y than ILM's b u t s t i l l a l l o w t h e s y s t e m a t i c i n c o r p o r a t i o n of c a r r i e r s . Far greater understanding o f d i f f u s i o n a l processes and c h e m i c a l r e a c t i o n s w i t h i n IEM's i s e s s e n t i a l . The n e c e s s i t y f o r i n s i t u e x p e r i m e n t a l t e c h n i q u e s was d i s c u s s e d i n t h e p r e v i o u s section. U n d e r s t a n d i n g o f s o l u b i l i t i e s and t r a n s p o r t phenomena w i t h i n p o l y m e r i c membranes c o n t a i n i n g i m m o b i l i z e d c a r r i e r s . I t i n u n c l e a r why t h e s e m a t e r i a l s l e a d t o r e t e n t i o n o f permeates i n some i n s t a n c e s and enhanced f l u x e s i n o t h e r s . F i n a l l y , an e x c i t i n g a r e a i s t h e e f f e c t s o f energy i n p u t on membrane t r a n s p o r t . Even though one o f t h e most d e s i r a b l e a s p e c t s o f f a c i l i t a t e d t r a n s p o r t t h r o u g h l i q u i d membranes i s t h e low energy r e q u i r e m e n t , t h e use o f e l e c t r i c a l o r l i g h t energy has i n t e r e s t i n g consequences. I n p a r t i c u l a r , e f f e c t i v e FT f o r permeates p r e s e n t i n low c o n c e n t r a t i o n s r e q u i r e s a l a r g e v a l u e o f K ( e q ) . However, s t r o n g b i n d i n g o f t h e permeate may r e s u l t i n an u n a c c e p t a b l y s m a l l v a l u e o f k ( r ) . Photochemisty c o u l d be used t o p r o v i d e an a d d i t i o n a l mechanism f o r t h e d i s s o c i a t i o n o f the p e r m e a t e - c a r r i e r complex. P h o t o d i s s o c i a t i o n r e a c t i o n s a r e common f o r c o o r d i n a t i o n compounds. S i m i l a r l y , t h e e l e c t r o c h e m i s t r y a s s o c i a t e d w i t h t h e complex c o u l d be used t o r e l e a s e permeate on the sweep s i d e o f t h e membrane. The a b i l i t y o f most m e t a l i o n s t o b i n d permeates v a r i e s w i t h t h e o x i d a t i o n s t a t e o f the m e t a l . Use o f photo- and e l e c t r o c h e m i c a l r e a c t i o n s t o enhance FT i s d e p i c t e d i n F i g u r e 3. Even though p h o t o f a c i l i t a t i o n and e l e c t r o f a c i l i t a t i o n r e q u i r e an energy i n p u t , t h e s e p r o c e s s e s negate t h e s o l e dependence on t h e p r o p e r t i e s o f the c a r r i e r t o p r o v i d e t h e s e l e c t i v i t y , k i n e t i c s and thermodynamics t h a t a r e n e c e s s a r y f o r an e f f i c i e n t s e p a r a t i o n .



2.

KOVAL AND REYES


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ELECTROF ACILITATION ( A C - e - ^ A O - » A + C) :·«— porous electrodes — ^C n/ir\ ^AC tXAC) 4

FEED (A + B)

SWEEP (He + A)

C

cathode

anode

F i g u r e 3. C r o s s - s e c t i o n a l views d e p i c t i n g p r o c e s s e s t h a t r e s u l t i n p h o t o f a c i l i t a t i o n and e l e c t r o f a c i l i t a t i o n o f s p e c i e s A. Acknowledgments T h i s paper i s based upon r e s e a r c h s u p p o r t e d by t h e 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 under Grant No. CBT-8604518. Acknowledgement i s made t o the donors o f the P e t r o l e u m Research Fund, a d m i n i s t e r d by t h e American Chemical S o c i e t y . Z.E.R was s u p p o r t e d by a f e l l o w s h i p from the U n i v e r s i t y o f P u e r t o R i c o . The a u t h o r s thank S. Drew, R. N o b l e , T. S p o n t a r e l l i and J . Way f o r u s e f u l d i s c u s s i o n s .

Liturature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

G i l l i s , J.N.; Sievers, R.E.; Pollock, G.E. Anal. Chem. 1985, 57, 1572. King, C.J. "Trends in Industrial Separation Processes," First AIChE/CIESC Joint Meeting, Beying, China, 1982. LeBlanc, O.H., Jr.; Ward, W.J.; Matson, S.L.; Kimura, S.G. J. Membr. Sci. 1980, 6, 339. Way, J.D.; Noble, R.D.; Flynn, T.M.; Sloan, E.D. J. Membr. Sci. 1982, 12, 239, and references therein. Warren, D.C. Anal. Chem. 1984, 56, 1523A. Folkner, C.A.; Noble, R.D. J. Membr. Sci. 1983, 12, 289. Kemena, L.L.; Noble, R.D.; Kemp, N.J. J. Membr. Sci. 1983, 15, 259. Scholander, P.F. Science 1960, 131, 585. Kohl, A.L.; Riesenfeld, F.C. Gas Purification; McGraw-Hill: New York, 1970. Smith, D.R.; Quinn, J.A. AIChE. J. 1980, 26, 112.




11. Yukimasa, H.; Sawai, H.; Takizawa, T. Makromol. Chem. 1979, 180, 1681. 12. Bdzil, B.; Carlier, C.C.; Frisch, H.L.; Ward, W.J. III; Breiter, M.W. J. Phys. Chem. 1973, 77, 846. 13. Ward, W.J. III AIChE. J. 1970, 16, 405. 14. Ward, W.J. III Nature 1970, 227, 162. 15. Hartley, F.R. Chem. Rev. 1973, 73, 163. 16. Stynes, D.V.; James, B.R. JACS 1974, 96, 2733. 17. Baldwin, D.A.; Pfeiffer, R.M.; Reichgott, D.W.; Rose, N.J. J. Am. Chem. Soc. 1973, 95, 5152. 18. Koval, C.A.; Noble, R.D.; Way, J.D.; Louie, B.; Reyes, Z.E.; Bateman, B.R.; Horn, G.M.; Reed, D.L. Inorg. Chem. 1985, 24, 1147. 19. Bateman, B.R.; Way, J.D.; Larson, K.M. Sep. Sci. Tech. 1984, 19, 21. 20. Jackels, S.C.; Harris, L.J. Inorg. Syn. 1983, 22, 107. 21. Drago, R.S.; Balkus, K.J. Inorg. Chem. 1986, 25, 718. 22. Nishide, H.; Ohyangi, M.; Okada, O.; Tsuchida, E. Macromol. 1986, 19, 496. 23. Nafion is the registered trademark for an IEM produced by DuPont deMours, Inc. IEM materials are also available from other manufacturers. 24. E l l i o t t , C.M.; Redepenning, J.G. J. Electrochem. Soc. 1984, 181 137. 25. Way, J.D.; Noble, R.D.; Reid, D.L.; Ginley, G.M.; AIChE. J., in press, 1986. 26. Bard, A.J.; Faulkner, L.R. "Electrochemical Methods"; John Wiley and Sons: New York, 1981, p. 144, 667, 668. 27. Steck, Α.; Yeager, H.L. Anal. Chem., 1979, 51, 862. 28. Helfferich, F. "Ion Exchange"; McGraw-Hill: New York; 1961, p. 339-420. 29. Lieber, C.M.; Lewis, N.S. J. Am. Chem. Soc. 1985, 107, 7170. RECEIVED January 9, 1987


Liquid Membranes - American Chemical Society

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