Perfluorinated Ionomer Membranes - American Chemical Society

19 General Applications of Perfluorinated Ionomer Membranes

BRIAN KIPLING

Downloaded by UNIV OF LEEDS on July 6, 2015 | http://pubs.acs.org Publication Date: February 4, 1982 | doi: 10.1021/bk-1982-0180.ch019

University of Calgary, Department of Chemistry, Calgary, Alberta T2N 1N4 Canada

Described here are applications other than industrial chlora l k a l i , water electrolysis and fuel c e l l s . The applications are divided into six general classes. Separators The use of Nafion as a separator in cells for chloralkali electrolysis i s described elsewhere. The exceptional chemical i n ertness and thermal s t a b i l i t y coupled with favourable e l e c t r i c a l conductivity being of particular advantage i n this application. These properties have been exploited in a number of other electrochemical applications. The RAI Research Corporation also offers a range of battery separators under the name of Permion (1) . These membranes are made by radiation grafting of a suitably active group onto an inert base film. The active groups include weak acids such as acrylic and substituted acrylic acid and stronger acidic groups such as sulfonated styrene. The base film can be Teflon R, polyethylene or polypropylene. They are thus not s t r i c t l y perfluorinated membranes as i s Nafion, but in chemical inertness and in many physical properties such as e l e c t r i c a l conductivity and ion flux are useful as separators i n batteries. Grot (2, 3, 4) has summarized the properties of some perfluorinated membranes used as separators, mainly for chloralkali electrolysis, and also described regeneration of chromic acid solutions. Solutions of chromium(VI) in sulfuric acid are used in a number of industrial processes. For example i n etching plastics prior to metallizing. The used solution contains chromium(III) which can be e l e c t r o l y t i c a l l y reoxidized to chromium(VI) using lead anodes. Nafion i n tubular form (80 mm diameter) i s used as the separator i n cylindrical c e l l s . Sulfuric acid i s used as the catholyte, the permselectivity of the Nafion ensuring that little sulfate ion migrates to the chromium(VI) solution. In the regeneration of chrome plating solutions even small amounts of sulfate migrating across the separator would upset the c r i t i c a l ratio

0097-6156/82/0180-0475$05.00/0 © 1982 American Chemical Society In Perfluorinated Ionomer Membranes; Eisenberg, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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H^SO^rCrOs ( u s u a l l y around 1:100) and an o r g a n i c a c i d i s used as c a t h o l y t e , the s m a l l amounts o f organic anion which do migrate across the membrane a r e o x i d i z e d by the chromic a c i d . An a d d i t i o n a l problem i n the r e g e n e r a t i o n of chrome p l a t i n g s o l u t i o n s i s r e moval o f low c o n c e n t r a t i o n s o f c a t i o n s such as F e , C u , N i . The N a f i o n separator a l l o w s d i a l y s i s o f the n o n - o x i d i z a b l e c a t i o n s to the cathode, and even though e f f i c i e n c y o f removal i s low due to the r e l a t i v e l y h i g h c o n c e n t r a t i o n o f hydrogen i o n s , (the hydrogen i o n c a r r i e s the bulk o f t h e c u r r e n t ) , the process i s s t i l l economically a t t r a c t i v e because the amounts o f t h e unwanted cations are usually small. In chromic a c i d s o l u t i o n s c o n t a i n i n g l a r g e amounts o f unwanted c a t i o n s , e.g. s o l u t i o n s a f t e r use f o r e t c h i n g p r i n t e d c i r c u i t s c o n t a i n l a r g e amounts of copper(II) i o n s and r e g e n e r a t i o n o f t h e spent s o l u t i o n s w i t h no pretreatment r e s u l t s i n very low e f f i c i e n c i e s i n terms o f copper(II) i o n removal. About 8-13% of the copper has been removed when a l l the chromium(III) has been r e o x i d i z e d t o chromium(VI) ( 4 ) . Regeneration c e l l s f o r chromic a c i d s o l u t i o n s based on the c y l i n d r i c a l geometry u s i n g t u b u l a r N a f i o n are commercially a v a i l a b l e from AMJ Chemicals. (54). A simi l a r process, though the nature of the membrane was not s p e c i f i e d was d e s c r i b e d by Belobaba (6) f o r removing c a t i o n s such as chromi u m ( I I I ) and n i c k e l ( I I ) from spent e l e c t r o p l a t i n g s o l u t i o n s . N a f i o n i n c y l i n d r i c a l form i s a l s o s u p p l i e d by C.G. Processi n g Inc. ( 5 ) , f o r use i n e l e c t r o w i n n i n g o f g o l d . The membrane serves s e v e r a l purposes i n e l e c t r o l y s i s c e l l s where gold i n conc e n t r a t e d c h l o r i d e (or cyanide) s o l u t i o n i s deposited on s t e e l wool cathodes. The membrane prevents m i g r a t i o n o f t h e a n i o n i c gold complex t o the anode, so t h a t c u r r e n t i s c a r r i e d almost e n t i r e l y by sodium ions from t h e sodium hydroxide a n o l y t e . C h l o r i d e i o n s are a l s o prevented from reaching the anode, thus a v o i d i n g c o r r o s i o n problem w i t h t h e s t a i n l e s s s t e e l e l e c t r o d e s , and f i n a l l y oxygen gas, evolved a t the anode i s kept out of t h e c a t h o l y t e where i t would i n t e r f e r e w i t h the d e p o s i t i o n o f t h e gold (53). The r e l a t i v e l y high temperatures and h i g h current d e n s i t i e s l i m i t t h e range of a v a i l a b l e membranes t o those of t h e N a f i o n type which have the a p p r o p r i a t e combination of p r o p e r t i e s . On a somewhat d i f f e r e n t s c a l e i s the c o n s t r u c t i o n of separa t o r tubes d e s c r i b e d by Harrar and Sherry ( 7 ) . These tubes a r e made from Nafion ( t u b u l a r form) w i t h e i t h e r K e l - F o r g l a s s p l u s T e f z e l and a r e intended f o r use i n e l e c t r o l y s i s c e l l s f o r c o n t r o l l e d p o t e n t i a l e l e c t r o l y s i s o r f o r coulometry. N a f i o n t u b i n g 4.3 mm diameter ( i n t e r n a l ) i s attached t o a l e n g t h o f g l a s s u s i n g T e f z e l . The N a f i o n end of t h e tube i s sealed w i t h a plug of boros i l i c a t e g l a s s r o d producing a composite of a g l a s s tube w i t h a s e c t i o n o f Nafion tube ( F i g u r e 1) which serves t o i s o l a t e the counter e l e c t r o d e from the working e l e c t r o d e compartment. E l e c t rochemical c h a r a c t e r i s t i c s of t h e tubes were measured and t h e use of these tubes i n coulometry and voltammetry i n d i c a t e d (see a l s o r e f s 35, 37). 3 +


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In Perfluorinated Ionomer Membranes; Eisenberg, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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GLASS

TEFZEL

• NAFION

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Figure 1. Nafion/'glass/Tejzel

BOROSILICATE separator.


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Sakagami, Kato, H i r a i and Murayana (8) have used Nafion 110 as a separator and c o l l e c t o r i n the e l e c t r o d i a l y s i s of p r o t e i n . A s o l u t i o n , or homogenized d i s p e r s i o n of the p r o t e i n , i s contained i n a compartment separated from the e l e c t r o d e s by sheets of the Nafion membrane. (A c y l i n d r i c a l geometry w i t h t u b u l a r Nafion would seem a more convenient arrangement h e r e ) . On passage of current the p r o t e i n migrates, ( d i r e c t i o n depends upon the p r o t e i n and the pH of the d i s p e r s i o n ) , and c o l l e c t s as a homogenous f i l m on the membrane surface - from which i t could be e a s i l y removed.


Energy Sources Although the membranes used i n some of the a p p l i c a t i o n s l i s t ed here are not of the p e r f l u o r i n a t e d v a r i e t y , the chemical i n e r t ness, h i g h e l e c t r i c a l conductance and r e l a t i v e l y high s o l u t e f l u x confer t e c h n i c a l advantages over most n o n - f l u o r i n a t e d membranes. The use of membranes i n energy sources can be d i v i d e d i n t o two c a t e g o r i e s . As e l e c t r o d e separators i n c o n v e n t i o n a l b a t t e r i e s , the Nafion membranes would seem e s p e c i a l l y u s e f u l f o r nonaqueous e l e c t r o l y t e systems because of t h e i r i n e r t n e s s to most organic s o l vents. The Permion membranes based on t e f l o n w i t h s u l f o n a t e exchange s i t e s seem to have p o t e n t i a l a p p l i c a t i o n here too. Dampier (9) evaluated a number of membranes w i t h respect to e l e c t r i c a l r e s i s t a n c e , t r a n s f e r e n c e numbers and i n t e r d i f f u s i o n r a t e s f o r l i t h i u m ( I ) , c o p p e r ( I I ) , bromide, p e r c h l o r a t e i n propylene carbonate, but d i d not at that time c o n s i d e r Nafion o r Permion membranes. Lopez, K i p l i n g and Yeager (10) reported r e s u l t s of t r a n s p o r t s t u d i e s f o r sodium, cesium and i o d i d e ions i n propylene carbonate which showed t h a t nonaqueous a p r o t i c s o l v e n t s may remove the advantage of h i g h i o n f l u x , though the f l u x i n p r o t i c s o l v e n t s such as methanol i s s t i l l q u i t e h i g h . L a t e r work (11) suggested that the presence of even s m a l l amounts of water may have a pronounced e f f e c t on r a t e s of i o n t r a n s p o r t . K r a t o c h v i l and B e t t y (12) used an anion exchange membrane i n a c e t o n i t r i l e f o r a b a t t e r y based on c o p p e r ( I I ) - ( I ) and copper ( 0 ) - ( I ) couples. The membrane was e f f e c t i v e i n p r e v e n t i n g m i g r a t i o n of copper c a t i o n s . The c e l l was intended as a power source at low temperatures. In a d d i t i o n to uses i n c o n v e n t i o n a l type b a t t e r i e s , the use of i o n exchange membranes i n d i a l y t i c b a t t e r i e s has been proposed by s e v e r a l groups. Based on e a r l y experiments of Manecke (13) the d i a l y t i c b a t t e r y i s a device f o r e x t r a c t i n g the f r e e energy of d i l u t i o n of s a l i n e water. The p r i n c i p l e s were discussed by Clamp i t t and K i v i a t (14) who proposed a scheme f o r d i r e c t generation of e l e c t r i c i t y u s i n g an e l e c t r o c h e m i c a l c o n c e n t r a t i o n c e l l . Fresh water and s a l i n e s o l u t i o n s are separated by an i o n exchange membrane. A r e v e r s i b l e c h l o r i d e e l e c t r o d e i s placed i n each compartment and the E.M.F. of such a c e l l i s a f u n c t i o n of the d i f f e r e n c e i n c o n c e n t r a t i o n between the two s o l u t i o n s . Ion t r a n s p o r t across the membrane gives r i s e to a current f l o w . This proposal was examined i n more depth by Weinstein and L e i t z (15), who amongst


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other f a c t o r s looked a t power output as a f u n c t i o n of s a l t concent r a t i o n on the freshwater s i d e . A compromise must be s t r u c k , given a constant s a l i n e c o n c e n t r a t i o n f o r sea water, between maximum c o n c e n t r a t i o n d i f f e r e n c e ( g i v i n g maximum p o t e n t i a l d i f f e r e n c e ) and i n c r e a s i n g i n t e r n a l r e s i s t a n c e as the s a l t c o n c e n t r a t i o n of the freshwater i s reduced t o a s m a l l value. An optimum of 0.026 M sodium c h l o r i d e f o r the freshwater s i d e i s somewhat higher than average r i v e r water (0.01 M) . Other f a c t o r s such as membrane r e s i s t a n c e , water t r a n s p o r t e t c . were examined i n l e s s e r d e t a i l but Weinstein and L e i t z attempted an economic prognosis based on current costs and concluded that major advances i n membrane t e c h n o l ogy were needed i f the d i a l y t i c b a t t e r y were to become a commerc i a l p o s s i b i l i t y . The a v a i l a b i l i t y o f adequate q u a n t i t i e s of r i v e r water may a l s o be a l i m i t i n g f a c t o r . Forgacs and 0 B r i a n (16) have proposed a s i m i l a r model f o r a d i a l y t i c b a t t e r y and made more o p t i m i s t i c p r e d i c t i o n s , though without s p e c i f y i n g membrane types or c o s t s . A device based upon the d i f f e r e n c e i n s a l t conc e n t r a t i o n between f r e s h water and sea water but u t i l i z i n g osmotic pressure r a t h e r than a p o t e n t i a l generation was proposed by Norman (17) . H i s osmotic pump u s i n g a reverse osmosis membrane i s theore t i c a l l y e q u i v a l e n t to a w a t e r f a l l of height 225 m, and a device for e x t r a c t i n g u s e f u l work from t h i s osmotic pressure was d e s c r i bed.


1

Electrodes Ion s e l e c t i v e e l e c t r o d e s u s i n g i o n exchange membranes have been i n v e s t i g a t e d over a long p e r i o d of time (18). Two major problems of such e l e c t r o d e s are l a c k of s e l e c t i v i t y and short l i f e times. Polymer membrane e l e c t r o d e s have u s u a l l y i n c o r p o r a t e d a p l a s t i c i z e r t o increase i o n i c m o b i l i t y i n the membrane and an e l e c t r o a c t i v e s p e c i e s . The r a t e of l e a c h i n g of these components c o n t r o l s the l i f e t i m e o f the e l e c t r o d e (19, 20). The unique morphology of Nafion polymers (11, 21, 22) coupled w i t h , or perhaps r e s u l t i n g i n , high s e l e c t i v i t i e s (23) prompted M a r t i n and F r e i s e r to f o l l o w t h e i r e a r l i e r work on a dinonylnaphthalenensulfonate membrane (24) w i t h an i n v e s t i g a t i o n of Nafion 120 as a membrane i n i o n s e l e c t i v e e l e c t r o d e s (25). I o n i c m o b i l i t i e s i n Nafion are s u f f i c i e n t l y high to o b v i a t e the need f o r p l a s t i c i z e r s and the i o n exchange s i t e s are c o v a l e n t l y bound to the polymer backbone so that i n p r i n c i p l e at l e a s t l i f e t i m e s should be i n f i n i t e . M a r t i n and F r e i s e r prepared e l e c t r o d e s by s e a l i n g a d i s c of Nafion 120 to the end of a g l a s s tube. The tube was then f i l l e d w i t h a p p r o p r i a t e i n t e r n a l reference s o l u t i o n . They r e p o r t Nerns t i a n response f o r e l e c t r o d e s f o r cesium i o n and f o r t e t r a b u t y l ammonium i o n and present s e l e c t i v i t y data f o r these e l e c t r o d e s i n presence of v a r i o u s other i o n s . S i m i l a r i t i e s i n s e l e c t i v i t y sequences t o the DNNS e l e c t r o d e i n d i c a t e that s o l v e n t e x t r a c t i o n cons i d e r a t i o n s may be s i g n i f i c a n t i n determining s e l e c t i v i t y , perhaps from p a r t i t i o n between h y d r o p h i l i c and h y d r o p h o l i c regions w i t h i n


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the Nafion membrane. The use of a strong c a t i o n exchange membrane i n an i o n s e l e c t i v e e l e c t r o d e was reported by Inokuma, Ochiar, Endo and H i i r o (26) f o r c o n t r o l of p i c k l i n g bath s o l u t i o n s c o n t a i n ing n i t r i c and h y d r o f l u o r i c a c i d s . A p e r f l u o r i n a t e d membrane i s p a r t i c u l a r l y s u i t a b l e here because of i t s chemical i n e r t n e s s . A n a l y t i c a l Preconcentration

Techniques (Donnan D i a l y s i s )

The p e r m s e l e c t i v i t y of i o n exchange membranes can be e x p l o i t ed as a preconcentration technique. Thus suppose two s o l u t i o n s (1 and 2) are separated by a c a t i o n exchange membrane which i s t o t a l l y impermeable to anions. I n i t i a l l y s o l u t i o n 1 contains A ions and s o l u t i o n 2 contains B i o n s . I t can be shown (27, 28) that at e q u i l i b r i u m the f o l l o w i n g r e l a t i o n s h i p holds Downloaded by UNIV OF LEEDS on July 6, 2015 | http://pubs.acs.org Publication Date: February 4, 1982 | doi: 10.1021/bk-1982-0180.ch019

+

a

A,l

a

A,2

=

^BJ, a

B,2 +

+

where a denotes a c t i v i t i e s , A and Β denotes species A and B i n s o l u t i o n s 1 and 2 on d i f f e r e n t sides of the membrane. I f the i n i t i a l c o n c e n t r a t i o n of A i s l a r g e and i n i t i a l c o n c e n t r a t i o n of B very s m a l l , then at e q u i l i b r i u m a r e l a t i v e l y small f r a c t i o n of A w i l l have moved from s o l u t i o n 1 to s o l u t i o n 2 but a r e l a t i v e l y l a r g e f r a c t i o n of B w i l l have moved from s o l u t i o n 2 to s o l u t i o n 1. By s u i t a b l e choice of volumes f o r the two s o l u t i o n s a high degree of c o n c e n t r a t i o n of B can be achieved. Similar considerations apply to anions using an anion exchange membrane. This technique i s o f t e n r e f e r r e d to as Donnan d i a l y s i s . In p r a c t i c e t o t a l im p e r m e a b i l i t y of coion (anion i n the case of a c a t i o n exchange membrane) i s not achieved and so although the c a t i o n e q u i l i b r i u m s i t u a t i o n described i s observed i t i s e v e n t u a l l y superseded by complete i o n i c e q u i l i b r i u m as anions slowly permeate the membrane u n t i l both c a t i o n and anion concentrations are equal on each s i d e of the membrane. P e r m s e l e c t i v i t y i s a r e s u l t of r e l a t i v e d i f f u s i o n r a t e s of c a t i o n s and anions through the membrane and holds only under Donnan e x c l u s i o n c o n d i t i o n s , i . e . i s most e f f e c t i v e i n low i o n i c strength s o l u t i o n s . B l a e d e l and Haupert (28) demonstrated the f e a s i b i l i t y of using t h i s phenomenon as a preconcentration technique using isotope t r a c e r s t u d i e s on the ions Na , C s , Z n , and l a t e r B l a e d e l and Christensen (29) extended the work to i n c l u d e the anions I " and ΗΡθζ. . They found anion transport to be much slower than the pre v i o u s l y reported c a t i o n t r a n s p o r t . Coion transport i n the anion exchange membranes was much higher and apparently dependent on the a n i o n i c charge of the bulk e l e c t r o l y t e . Further s t u d i e s with more r e c e n t l y a v a i l a b l e membranes (1) seem to be needed. B l a e d e l and K i s s e l (30) used e l e c t r o d e s responsive to the d e s i r e d i o n wrapped with an i o n exchange membrane. For example a glass e l e c t r o d e wrapped with a c a t i o n membrane, Permion P1010, +

+

+

+

+

+

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responds to hydrogen ions when immersed i n a s o l u t i o n . The small volume of s o l u t i o n contained between the membrane and the glass bulb provides enrichment f a c t o r s of about one hundred and thus extends the s e n s i t i v i t y range f o r the e l e c t r o d e . The use of i o n exchange membranes as a p r e c o n c e n t r a t i o n dev i c e assumes no s e l e c t i v e b i n d i n g of the ions of i n t e r e s t . B l a e d e l and Niemann (31) examined three commercially a v a i l a b l e membranes by a v a r i e t y of techniques and found evidence of impurity groups i n the membranes "capable of b i n d i n g cations ( s p e c i f i c a l l y copper ( I I ) ) very s t r o n g l y by mechanisms other than the i o n exchange mechanism". Such e f f e c t s would v i t i a t e the use of these membranes i n p r e c o n c e n t r a t i o n techniques. No d e f i n i t i v e i d e n t i f i c a t i o n of the groups was presented but carboxylate and o l e f i n i c groups were detected s p e c t r o s c o p i c a l l y . Of the three membranes examined Nafion XR170 appeared to be the most homogeneous with respect to exchange s i t e and b i n d i n g of copper(II) i o n s . Preconcentration of copper at concentrations of around 10 M p r i o r to determination by atomic absorption was reported by Cox and Dinunzio (32). Using Nafion 125, Permion P1010 or Permion 4010 small volume r e c e i v e r c e l l s were separated from l a r g e r volume samples. A f t e r a f i x e d time the s o l u t i o n i n the r e c e i v e r was analyzed f o r copper, and by s u i t a b l e adjustment of c o n d i t i o n s the copper concentration of the r e c e i v e r s o l u t i o n was shown to be a l i n e a r f u n c t i o n of copper content i n the o r i g i n a l sample. The authors found curious e f f e c t s on adding c e r t a i n cations to the sample s o l u t i o n . M g or A l caused marked a c c e l e r a t i o n i n the r a t e of C u t r a n s p o r t , an e f f e c t which i t i s tempting to l i n k as does Twardowski (37) with the B l a e d e l and Niemann impurity s i t e s (31), but which Cox and DiNunzio c l a i m i s not f e a s i b l e because potassium t r a n s p o r t i s s i m i l a r l y a c c e l e r a t e d by a d d i t i o n of sodium or magnesium ions to the sample s o l u t i o n . Optimum c o n d i t i o n s with respect to composition of r e c e i v e r s o l u t i o n , sample s o l u t i o n , temperature and s t i r r i n g rate were e s t a b l i s h e d i n order to provide a p r e c i s e method f o r i o n p r e c o n c e n t r a t i o n . 5

2 +

3 +

2 +

The voltammetric determination of n i t r a t e f o l l o w i n g a preconcentration step using Permion 1025 was developed by Cox, Lundquist and Washinger (33). N i t r a t e i s exchanged from the samp l e s o l u t i o n through the membrane i n t o a s m a l l volume c e l l c o n t a i n i n g 0.1 M potassium c h l o r i d e and 0.01 M lanthanum c h l o r i d e . The method e l i m i n a t e s c a t i o n i c i n t e r f e r e n c e s with the voltammetry but does not s u c c e s s f u l l y deal with a n i o n i c i n t e r f e r e n c e s such as a s u l f a t e or n i t r i t e . Donnan d i a l y s i s of conjugate bases of weak acids presents an a d d i t i o n a l f a c t o r f o r c o n t r o l , namely pH. Cox and Cheng (34) i n v e s t i g a t e d p r e c o n c e n t r a t i o n of a number of anions i n t h i s category. Sample pH was adjusted to ensure that a major f r a c t i o n of the weak a c i d system was i n a n i o n i c form. Maximum enrichment f a c t o r , defined as r a t i o of r e c e i v e r c o n c e n t r a t i o n to sample conc e n t r a t i o n a f t e r some given, f i x e d time ( u s u a l l y 30 minutes), was obtained when pH of the r e c e i v e r s o l u t i o n was much l e s s than the



482


M E M B R A N E S

pK of the corresponding a c i d . For the anions c h l o r i d e , phosphate, arsenate, c h l o r a c e t a t e and pyruvate the enrichment f a c t o r of about seven was obtained, suggesting that t r a n s f e r r a t e across the memb r a n e - r e c e i v e r s o l u t i o n boundary may be the r a t e determining s t e p . The much lower enrichment f a c t o r o f about 3 f o r s u l f a t e i s a t t r i buted to s t r o n g i n t e r a c t i o n w i t h the methyl p y r i d i n i u m exchange s i t e s o f the Permion 1025 membrane. The presence o f s u l f a t e i o n i n the sample s o l u t i o n a l s o has the e f f e c t of decreasing e n r i c h ment f a c t o r s f o r other anions. Donnan d i a l y s i s not only serves as a p r e c o n c e n t r a t i o n process but i n a d d i t i o n provides m a t r i x normali z a t i o n f o r a n a l y t i c a l methods where the m a t r i x i s important. Cox and Twardowski (35) have d e s c r i b e d a voltammetric method o f a n a l y s i s f o r the metal ions C d ( I I ) , P b ( I I ) , Zn(II) and Cu(II) i n which a p r e l i m i n a r y d i a l y s i s u s i n g Permion 1010 removes i n t e r f e r ences from v a r i o u s s u r f a c t a n t s such as T r i l o n X-100 and l i g a n d s such as humic a c i d . The d i a l y s i s technique i s s i m i l a r to that described by Cox and DiNunzio (32) the r e c e i v e r s o l u t i o n being analyzed by anodic s t r i p p i n g voltammetry a f t e r a f i x e d contact time, v i a the membrane»with the sample s o l u t i o n . I n presence o f complexing agents (humic a c i d ) pH c o n t r o l o f the sample s o l u t i o n to c o n t r o l p r o p o r t i o n of uncomplexed metal i o n i s a l s o important, p o s s i b l y a l l o w i n g f o r s p e c i a t i o n i n a d d i t i o n to m a t r i x n o r m a l i z a t i o n and c o n c e n t r a t i o n enrichment. The a v a i l a b i l i t y o f N a f i o n i n t u b u l a r form, Nafion 811, was u t i l i z e d by Cox and Twardowski (36) i n a dynamic Donnan d i a l y s i s system where s o l u t i o n i s pumped through Nafion t u b i n g , a c o i l o f the t u b i n g being immersed i n the sample s o l u t i o n . The use of t u b i n g a l l o w s an i n c r e a s e of s u r f a c e area of membrane t o r e c e i v e r volume r a t i o and minimizes concentrat i o n p o l a r i z a t i o n on the r e c e i v e r membrane s u r f a c e thus g i v i n g much enhanced enrichment r a t i o s i n s h o r t e r contact times. I n a t y p i c a l experiment r e c e i v e r s o l u t i o n , 0.2 M MgSOij. p l u s 5 10~ M A l ( 8 0 ^ ) 3 was pumped through the t u b i n g , diameter 0.63 mm, l e n g t h 10 m at a r a t e o f about 6.0 mL min" g i v i n g enrichment f a c t o r s o f up to 27 i n d i a l y s i s times o f about 15 minutes. Considerably more e f f i c i e n t than f a c t o r s of 5-10 i n 1 hour o f s t a t i c d i a l y s i s . Cox and DiNunzio (3_2_) e s t a b l i s h e d an optimum composition f o r the r e c e i v e r s o l u t i o n , 0.2 M M g ( I I ) , 5 * IO" M A l ( I I I ) . Other r e c e i v e r s o l u t i o n s show a l e s s favourable c a t i o n t r a n s p o r t across membranes such as permion 1010, a g r a f t e d t e f l o n base onto styrene w i t h s u l f o n i c a c i d exchange s i t e s . For example 0.1 M N a shows enrichment f a c t o r s only 50% that of the above s o l u t i o n . This lower c a t i o n t r a n s p o r t r a t e i s a t t r i b u t e d t o i n t e r a c t i o n between the mobile c a t i o n s and the f i x e d exchange s i t e s . The f u n c t i o n of the m u l t i v a l e n t ions i s t o provide a s h i e l d between exchange s i t e s and mobile ions and thus lower residence times i n the membrane. Cox and Twardowski (38) i n an elegant experiment provide evidence s u p p o r t i n g t h i s view by a p p l y i n g on AC f i e l d across the membrane during d i a l y s i s w i t h a sodium n i t r a t e r e c e i v e r . A 5V cm"" s i n e wave at frequencies ranging from 10"" KHz t o 1 0 KHz was used during t r a n s p o r t of the ions C u ( I I ) , P b ( I I ) , C d ( I I ) , Z n ( I I ) . I t x

4

2

1

4

+

1

1

3


19.

KIPLING

A pplicaîions of lonomer

Membranes

483

was argued t h a t i n c r e a s i n g frequency would i n c r e a s e the r a t e of d i s s o c i a t i o n of c a t i o n - s u l f o n a t e bonds and u l t i m a t e l y residence times of the mobile c a t i o n s would reach a d i f f u s i o n c o n t r o l l e d l i m i t . The t r a n s p o r t of i o n s through the membrane was measured i n terms of enrichment f a c t o r s (see e a r l i e r ) and reached a l i m i t i n g value at about 1 0 KHz of around 4.0. The value observed using the m u l t i v a l e n t r e c e i v e r s o l u t i o n . For the l a t t e r on a p p l i e d f i e l d has r e l a t i v e l y l i t t l e e f f e c t on the enrichment f a c t o r . The use of t u b u l a r Donnan d i a l y s i s systems has s t i m u l a t e d attempts to provide p r e d i c t i v e models, p r i n c i p a l l y f o r i n d u s t r i a l l y o r i e n t e d a p p l i c a t i o n s . Ng and Snyder (39) have r e c e n t l y publ i s h e d one such attempt a p p l i e d to d i a l y s i s of n i c k e l ( I I ) i n t o a s u l f u r i c a c i d r e c e i v e r s o l u t i o n . C o r r e l a t i o n s between mass t r a n s port c o e f f i c i e n t and Reynolds number are r e p o r t e d , and the f a c t o r s c o n t r o l l i n g t r a n s p o r t over a range of n i c k e l c o n c e n t r a t i o n s are discussed. P r e c o n c e n t r a t i o n i n a s l i g h t l y d i f f e r e n t way i s d e s c r i b e d by E i s n e r and Mark (40) who e q u i l i b r a t e d s m a l l areas of c a t i o n exchange membranes w i t h sample s o l u t i o n s and then used the membrane as a source of ions f o r d e p o s i t i o n i n an anodic s t r i p p i n g v o l t a mmetry system. The c o n c e n t r a t i o n of the i o n i n the membrane i s l i n e a r l y r e l a t e d to i t s c o n c e n t r a t i o n i n the bulk sample s o l u t i o n . The p r e - e q u i l i b r a t e d membrane was a l s o analyzed by neutron a c t i v a t i o n thus extending the range of ions f o r which the technique i s u s e f u l . Data are quoted f o r A g , C u , Z n , C o and I n , a l l of which show favourable d i s t r i b u t i o n f o r the membrane phase. E q u i l i b r a t i o n times are i n v e r s e l y p r o p o r t i o n a l to c o n c e n t r a t i o n ranging from s e v e r a l minutes at 10 M to one day or more at 10~ M. The method a f f o r d s a convenient s e p a r a t i o n from n o n i o n i c and a n i o n i c species which i n t e r f e r e w i t h the measurement technique. A s i m i l a r p r e c o n c e n t r a t i o n process was developed by Lochmuller, G a l b r a i t h and Walter (41) f o r the a n a l y s i s of water f o r t r a c e metals. The membrane a f t e r e q u i l i b r a t i o n w i t h the water sample i s i n t h i s case analyzed by proton induced X-ray emission. Claimed advantages of the l a t t e r technique are a wider range of a p p l i c a b i l i t y than neutron a c t i v a t i o n , e a s i e r a p p l i c a b i l i t y to r a p i d r o u t i n e a n a l y s i s than anodic s t r i p p i n g and g r e a t e r s e n s i t i v i t y than c o n v e n t i o n a l X-ray f l u o r e s c e n c e spectroscopy.


2

+

2 +

2 +

2 +

_l+

3 +

6

Catalysis Superacids such as Magic a c i d (42), a system c o n t a i n i n g antimony p e n t a f l u o r i d e and f l u o r o s u l f o n i c a c i d and designated a superacid because i t i s a more ready e l e c t r o n p a i r acceptor than anhydrous aluminium c h l o r i d e (43), have u s e f u l c a t a l y t i c propert i e s i n s y n t h e t i c organic chemistry where the r e a c t i o n i n v o l v e s a c a r b o c a t i o n i n t e r m e d i a t e . The extremely low n u c l e o p h i l i c i t y of the counter i o n of such a c i d s make i t p o s s i b l e to prepare c a t i o n s such as the t e r t i a r y b u t y l ( 0 Η ) ΰ w i t h an a p p r e c i a b l y long l i f e time i n superacid s o l u t i o n s whereas such c a t i o n s are too r e a c t i v e +

3

3



484

PERFLUORINATED

IONOMER

M E M B R A N E S

to e x i s t i n l e s s a c i d i c media. The h i g h l y a c i d i c c h a r a c t e r of superacids a l s o promotes p r o t o n a t i o n o f very weak bases such as alkanes as a p r e l i m i n a r y stage o f i s o m e r i z a t i o n , a l k y l a t i o n and p o l y m e r i z a t i o n r e a c t i o n s (43). The wide a p p l i c a b i l i t y o f superacids encouraged attempts t o prepare them i n the s o l i d phase by absorbing antimony f l u o r i d e and f l u o r o s u l f o n i c a c i d on supports such as f l u o r i n a t e d g r a p h i t e or a f l u o r i n a t e d p o l y o l e f i n e r e s i n (45). N a f i o n i s an obvious candidate f o r such systems and has been used as a c a t a l y s t f o r a v a r i e t y o f r e a c t i o n s . Olah (43) reviewed some of the e a r l i e r applications to isomerization, polymerization, transbromination, n i t r a t i o n , a c e t a l i z a t i o n and h y d r a t i o n , and used N a f i o n ( i n t h e hydrogen i o n form) f o r gas phase a l k y l a t i o n o f benzene and a l k y l benzenes. N a f i o n showed h i g h e r c a t a l y t i c a c t i v i t y then other s o l i d phase s u p e r a c i d c a t a l y s t s thus e n a b l i n g lower temperatures and pressures t o be used and hence " c l e a n e r " products. C a t a l y s t l i f e t i m e s were a l s o l o n g e r at moderate temperatures (below 200°C) but a t 220°C N a f i o n decomposed w i t h l o s s o f s u l f o n a t e groups and c a t a l y t i c a c t i v i t y . The same c a t a l y s t has a l s o been reported t o be e f f e c t i v e i n s o l u t i o n phase e s t e r i f i c a t i o n r e a c t i o n s (46). Beltrame, C a r n i t i and N e s p o l i (47) used N a f i o n as a c a t a l y s t f o r the i s o m e r i z a t i o n o f m-xylene and Olah (48) has used N a f i o n as a c a t a l y s t f o r F r i e d e l - C r a f t r e a c t i o n s o f toluene and phenol w i t h alkylchloroformâtes and o x a l a t e s . The chemical i n e r t n e s s ( a t l e a s t below 220°C) of t h i s heterogeneous c a t a l y s t c o n f e r s obvious advantages over more c o n v e n t i o n a l homogeneous s u p e r a c i d c a t a l y s t s (43, 4 9 ) . Permeation D i s t i l l a t i o n N a f i o n e x h i b i t s a r e l a t i v e l y h i g h d i f f u s i o n r a t e f o r water vapour and t h i s p r o p e r t y has been used f o r removal of water from gas streams (50). The technique of Permeation d i s t i l l a t i o n i s a form o f counter current e x t r a c t i o n w i t h a N a f i o n membrane a c t i n g as the boundary between a moist sample gas stream f l o w i n g i n one d i r e c t i o n and a dryer purge gas stream f l o w i n g i n the counter d i r e c t i o n . The p r i n c i p l e i s shown i n Figure 2. Moist sample gas flows through t h e N a f i o n tube which i s enclosed i n a s h e l l . The s h e l l may be o f s t e e l , p o l y e t h y l e n e o r any s u i t a b l e m a t e r i a l . A dry purge gas i s pumped i n t o t h e s h e l l a t C. Water vapor from the moist gas permeates the w a l l s o f the N a f i o n tube and i s removed by the flow o f dry gas on the s h e l l s i d e o f the tube. The gas stream e x i t i n g from the tube a t Β i s o f much lower water content than the i n i t i a l gas a t A. The wet purge gas a t D may be d r i e d f o r reuse or d i s c a r d e d . S u i t a b l e c o n t r o l o f temperature, pressure and gas flow r a t e s plus s e l e c t i o n o f tube l e n g t h and diameter enables a h i g h degree of d r y i n g t o be a t t a i n e d by t h i s simple technique. A commercially a v a i l a b l e u n i t based on t h i s concept i s marketed by Perma Pure Products Inc. (51). I n t h i s u n i t a bundle of N a f i o n tubes i s contained w i t h i n each s h e l l i n order to p r o v i d e a l a r g e



KIPLING

A pplications of lonomer

Membranes

(S) (A)

NAFION TUBE -

Figure 2. Per ma pure gas dryer.

—-| i t r

3

"

©


486


M E M B R A N E S

c a p a c i t y and tubes of v a r i o u s lengths are a v a i l a b l e (tube l e n g t h , f o r a given flow r a t e determines the extent of d r y i n g ) . The u n i t i s designed to operate at ambient temperatures and i s adaptable to v a r i o u s pressure ranges f o r both sample and purge gases. An a p p l i c a t i o n of the u n i t has been d e s c r i b e d by Baker (52). I n f r a - r e d spectroscopy of a i r samples at pressures up to ten atmo spheres r e q u i r e s removal, s e l e c t i v e l y , of water vapour. Using dry n i t r o g e n gas as a purge Baker found t h a t absorbance due to water i n a i r samples could be reduced about ten f o l d , thus a l l o w i n g use of regions of the s p e c t r a f o r d e t e c t i o n of t r a c e c o n s t i t u e n t s w i t h out r e f e r e n c e beam compensation. Water bands i n the r e g i o n 1900 to 1300 cm" which were extremely strong i n untreated samples were reduced to such low values that t r a c e s of s u l f u r d i o x i d e c o u l d be detected by i t s 1370 cm" band. The permeation technique i s not s p e c i f i c f o r water, the lower a l c o h o l s (up to hexanol), e s t e r s , ethers, amines and some ketones a l s o d i f f u s e through the Nafion tube. Many small i n o r g a n i c molecules however such as CO, C O 2 , S O 2 , CS2 do not d i f f u s e as w e l l as most hydrocarbons and l a r g e r organic molecules. The dryer does not remove p a r t i c u l a t e s or con densable o i l vapors or m i s t s . 1


1

Acknowle d gement s The author acknowledges w i t h g r a t i t u d e the c o n s i d e r a b l e a s s i s t a n c e provided by Dr. W.G. Grot of E.I. Du Pont De Nemours and Co. Inc. and by Dr. H.L. Yeager of the U n i v e r s i t y of Calgary i n the p r e p a r a t i o n of t h i s review. Literature Cited 1. 2. 3. 4. 5. 6.

7. 8. 9. 10. 11. 12. 13. 14. 15.

RAI Research Co., Permion and Raipore Brochures. Grot, W.G. Chemie Ing. Tech. 1975, 47, 617. Grot, W.G. Chemie Ing. Tech. 1978, 50, 299. Grot, W.G. Case Western Symposium, October 1980. C.G. P r o c e s s i n g Inc., Box 133, Rockland, D.E. 19732. Belobaba, A.G.; Pevnitskaya, M.V.; Kozina, Α.Α.; Nefedova, G.Z.; F r e i d l i n , Y.G. I z v . S i b . Otd. Akad. Nauk. SSSR. Ser Khim Nauk 1980, ( 4 ) , 161. Harrar, J.E.; Sherry, R.J. A n a l . Chem. 1975, 47, 601. Sakagami, T.; Kato, T.; H i r a i , T.; Murayama, N. Eur. Pat. Appl. 11504, 28 May, 1980. Dampier, F.W. J . Appl. Electrochem. 1973, 3, 169. Lopez, M.; K i p l i n g , B.; Yeager, H.L. A n a l . Chem. 1977, 49, 629. Yeager, H.L.; K i p l i n g , B. J . Phys. Chem. 1979, 83, 1836. K r a t o c h v i l , B.; Betty, K.R. J . E l e c t . Soc. 1974, 121, 851. Manecke, G. Z. Phys. Chem. 1952, 201, 1. Clampitt, B.H.; K i v i a t , F.E. S c i . 1976, 191, 719. Weinstein, J.N.; L e i t z , F.B. S c i . 1976, 191, 557.


19.

16. 17. 18. 19.


20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54.

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of lonomer Membranes

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Forgacs, C.; O'Brien, R.N. Chem. in Can. 1979, 19. Norman, R.S. S c i . 1974, 186, 350. Parsons, J.S. Anal. Chem. 1958, 30 (7), 1262. C u t l e r , S.G.; Meares, P.; H a l l , D.G. J. E l e c t r o a n a l . Chem. 1977, 85, 145. Oesch, W.; Simon, W. Anal. Chem. 1980, 52, 692. Yeo, R.S.; Eisenberg, A. J. Appl. Polym. S c i . 1977, 21, 875. Gierke, T. 152nd Meeting of Electrochem. Soc., A t l a n t a , 1977. Steck, Α.; Yeager, H.L. Anal. Chem. 1980, 52, 1215. Martin, C.R.; F r e i s e r , H. Anal. Chem. 1980, 52, 562. Martin, C.R.; F r e i s e r , H. Anal. Chem. 1981, 53, 902. Inokuma, Y.; Ochiar, T.; Endo, J . ; H i i r o , K. Nippon Kagaku K a i s h i 1980 (10), 1469. H e l f f e r i c h , F. "Ion Exchange", McGraw Hill, New York, 1962. B l a e d e l , W.J.; Haupert, T.J. A n a l . Chem. 1966, 38, 1305. B l a e d e l , W.J.; Christensen, E.L. Anal. Chem. 1967, 39, 1262. B l a e d e l , W.J.; K i s s e l , T.R. Anal. Chem. 1972, 44, 2109. B l a e d e l , W.J.; Niemann, R.A. A n a l . Chem. 1975, 47, 1455. Cox, J.A.; DiNunzio, J.E. Anal. Chem. 1977, 49, 1272. Cox, J.A.; Lundquist, G.L.; Washinger, G. Anal. Chem. 1975, 47, 320. Cox, J.A.; Cheng, K.H. Anal. Chem. 1978, 50, 601. Cox, J.A.; Twardowski, Z. Anal. Chim. Acta. 1980, 119, 39. Cox, J.A.; Twardowski, Z. Anal. Chem. 1980, 52, 1503. Twardowski, Z. Ph.D. T h e s i s , S.I.U. 1980. Cox, J.A.; Twardowski, Z. Anal. L e t t e r s 1980, 13, 1283. Ng, P.K.; Snyder, D.D. Proc. of Symposium on Ion Exchange, Yeo, R.S.; Buck, R.P. Eds, Electrochem. Soc. 1981. E i s n e r , U.; Mark, H.B. Talanta, 1969, 16, 27. Lochmuller, C.H.; G a l b r a i t h , J.W.; Walter, R.L. Anal. Chem. 1974, 46, 440. Registered Trade Mark of C a t i o n i c s Inc., Columbia, S.C. Olah, G.A.; Prakash, G.K.S.; Sommer J. S c i . 1979, 206, 13. Olah, G.A.; Baker, E.B.; Evans, J.C.; T o g l y e s i , W.S.; McIntyre, J.S.; Bastieau, J . J . J.A.C.S. 1964, 86, 1360. Olah, G.A.; Kaspi, J . ; Bukala, J . J. Org. Chem. 1977, 42, 4187. Olah, G.A.; Keumi, T.; Meidar, D. Synth. 1978 (2), 929. Beltrame, P.; Beltrame, P.L.; C a r n i t i , P.; N e s p o l i , G. Ind. Eng. Chem. Prod. Res. Dev. 1980, 19, 205. Olah, G.A.; Meidar, D., Mulhotray, R.; Olah, J.A.; Narang, S.C. J . C a t a l . 1980, 61, 96. Olah, G.A. A l d r i c h i m i c a 1979, 12, (3), 43. Kertzman, J . ISA A i d 73425 (1973), 121. Perma Pure Products Inc., Box 70, Oceanport, New Jersey 07757. Baker, B.B. J. Am. Ind. Hyg. Ass. 1974, Nov. Personal communication from Anglo American Corporation of South A f r i c a L t d . A.M.J. Chemical Inc.

R E C E I V E D August7,1981.


Perfluorinated Ionomer Membranes - American Chemical Society

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