Catalysis with a Perfluorinated Ion-Exchange ... - ACS Publications

3 Catalysis with a Perfluorinated Ion-Exchange Polymer F. J. Waller

Downloaded by DICLE UNIV on November 12, 2014 | http://pubs.acs.org Publication Date: May 5, 1986 | doi: 10.1021/bk-1986-0308.ch003

Central Research & Development Department, Experimental Station, E. I. du Pont de Nemours & Company, Wilmington, DE 19898

NAFION i s a perfluorinated ion-exchange polymer (PFIEP). Because of i t s chemically inert backbone, the heterogeneous resin has been used as a strong acid catalyst for a wide variety of reactions i n synthetic organic chemistry. The polymeric catalyst offers an advantage over homogeneous analogs because of i t s ease of separation from reaction mixtures. Also in many instances, there are improved yields or an increase i n selectivity over other catalysts, such as poly(styrenesulfonic acid) resins. The r e s i n , NAFION ( l a ) , has been used as a heterogeneous s t r o n g a c i d by many r e s e a r c h e r s b u t most e x t e n s i v e l y by G. A. O l a h . I t i s t h e o b j e c t i v e o f t h i s r e v i e w t o summarize t h e c u r r e n t l i t e r a t u r e w i t h r e s p e c t t o t h e s y n t h e t i c a p p l i c a t i o n s o f PFIEP. I n a d d i t i o n , o t h e r uses f o r NAFION a r e mentioned b r i e f l y . The r e f e r e n c e s c i t e d a r e r e p r e s e n t a t i v e o f t h e p u b l i c a t i o n s from b o t h s c i e n t i f i c j o u r n a l s and p a t e n t s . T h i s r e v i e w w i l l d e a l p r i m a r i l y w i t h c a t a l y s i s d e r i v e d from NAFION i n t h e a c i d form. NAFION has t h e g e n e r a l s t r u c t u r e shown i n F i g u r e 1.

F i g u r e 1. G e n e r a l

s t r u c t u r e o f NAFION.

A s e r i e s o f c o m p o s i t i o n s may be produced i n w h i c h n can be as l o w as 5 and as h i g h as 1 3 . 5 . The lower v a l u e o f n c o r r e s p o n d s t o an 0097 6156/ 86/0308 O042$07.50/ 0 © 1986 American Chemical Society

In Polymeric Reagents and Catalysts; Ford, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.


3.

WALLER

Catalysis with a Perfluorinated

Ion-Exchange

Polymer

equivalent weight of 950 and the higher value to 1800. The value of x i s about 1000. NAFION i s prepared from a copolymer of tetrafluoroethene and perfluoro[2-(fluorosulfonylethoxy)-propyl v i n y l ether ( l b ) . The perfluorinated v i n y l ether i s produced by reacting tetrafluoroethene with S0~ to form a c y c l i c sultone which subsequently rearranges to ffuorocarbonylmethanesulfonyl f l u o r i d e . The linear analog reacts with two moles of hexafluoropropylene oxide to y i e l d a compound with a terminal 1-fluorocarbonyltrifluoroethoxy group. This group loses carbonyl f l u o r i d e on heating with Na^CO^ to give the perfluorinated v i n y l ether ( l c ) . The copolymer resin i n the sulfonyl f l u o r i d e form i s base hydrolyzed to the a l k a l i form and then a c i d i f i e d to the sulfonic acid form. Polymeric perfluorinated s u l f o n i c acids offer a variety of advantages over their homogeneous analogs, for example trifluoromethanesulfonic acid, foremost of those being catalyst separation. In many instances, there are also improved y i e l d s or an increase i n s e l e c t i v i t y . The greatest challenge i s finding applications for PFIEP where the c a t a l y s i s i s unique. This i s an important consideration i n view of catalyst cost. If r e a c t i v i t y or s e l e c t i v i t y are only marginally better than less expensive c a t a l y s t s , the incentive to use NAFION i s l o s t . NAFION i n the powder form can presently be purchased from two sources: A l d r i c h Chemical Company and C. S. Processing Company (2). Though l i t e r a t u r e references report polymeric perfluorinated s u l f o n i c acids with equivalent weights of 900, 1000, or 1200, the above two vendors s e l l NAFION only i n the acid form with an equivalent weight of 1100. In spite of the low number of mequiv. per gram and incomplete a c c e s s i b i l i t y of acid s i t e s for reaction, the u t i l i t y of NAFION as a strong acid catalyst i s remarkable. It has been estimated that approximately 50% of the acid s i t e s are not accessible for reactions involving molecules larger than NH^ i n non-swelling solvents (3). Therefore, coating of PFIEP on a s o l i d support to increase surface area or other methods to increase porosity of the resin should enhance activity. Perfluorinated ion exchange polymers i n the potassium or acid form should be exchanged with n i t r i c acid, 3N, four or f i v e times at approximately 75°C and then dried at M 1 0 ° C for 3-4 hours before use. This w i l l insure that the PFIEP i s completely converted to the acid form, free of any organics or metal ion contamination and nearly anhydrous. Exposure to moisture w i l l result i n a less e f f e c t i v e catalyst i f maximum Bronsted a c i d i t y i s required for a p a r t i c u l a r a p p l i c a t i o n . The perfluorinated backbone of NAFION provides a r e s i n with chemical and thermal s t a b i l i t y s i m i l a r to that of TEFLON fluorocarbon. Unlike TEFLON, the polymer i s permeable to many cations and polar compounds but impermeable to anions and non-polar species. Cation size and ionic properties determine mobility through the polymer. The polymeric fluorinated backbone makes PFIEP highly resistant to attack from strong oxidizing and reducing acids, strong bases and oxidizing/reducing agents. The fluorinated resins have a greater thermal s t a b i l i t y than t h e i r non-fluorinated analogs and can be used at temperatures approaching 175°C. Table I summarizes a few selected


44

POLYMERIC REAGENTS A N D CATALYSTS

Table I. Properties of NAFION and AMBERLYST 15 Property meq/g


Maximum operating temperature Backbone Structure Surface area (m g )

NAFION

AMBERLYST 15

0 | (EW 1100) 180-190°C

4.7 (dry)

Fluorocarbon Non-porous ML8

Polystyrene Macroreticular 50

M50°C

EW = equivalent weight

c h a r a c t e r i s t i c s of NAFION and AMBERLYST 15 C4), a poly(styrenesulfonic acid) r e s i n available from Rohm and Haas. Between 180-190°C, NAFION powder i n the acid form has a tendency to flow and thus fuse together. At elevated temperatures, 210-220°C, the polymer loses s u l f o n i c acid groups ( 5 ) . PFIEP has hydrophilic [SO-H] and hydrophobic [CF CF ] regions i n close proximity. These polymers have the a b i l i t y to sorb r e l a t i v e l y large quantities of water and other p r o t i c solvents despite the predominance of the hydrophobic regions. This solvation causes swelling of the s o l i d . This morphology of the perfluorinated membrane suggests a framework where the hy4rated -SO^ groups and counterion clusters which are roughly 40A i n diameter are interconnected by short channels approximately 10A i n diameter. This porous i o n i c system i s immersed i n a f l u o r o carbon backbone network ( 6 ) . The analogy between this biphasic structure and t h e ^ t r u c t u r e of reversed micelles has been substantiated by Na magnetic resonance studies. This configuration minimizes both the hydrophobic i n t e r a c t i o n of water with the backbone and the e l e c t r o s t a t i c repulsion of proximate sulfonate groups (7). The a c i d i t y , H , of a dried, methylene chloride swollen, perfluorinated sulfon?c acid side chain i s about -6 (8). It has been suggested that NAFION has an a c i d i t y between -11 and -13 (9). The difference between the measured and a reasonable value of -11 i s that the methylene chloride-swollen r e s i n may not have been completely anhydrous. For comparison, t r i f l i c acid and AMBERLYST 15 have H values of -14.6 and -2.16, respectively (9-10). ° However, i t should be recognized that there are several additional classes of catalysts associated with metal cation ion-exchanged polymers. These include 1) a p a r t i a l l y cationexchanged polymer, 2) a completely cation-exchanged polymer, and 3) a cation-exchanged polymer where the metal cation i s coordinated to another ligand. The general catalyst classes are shown i n Figure 2. A discussion of this topic w i l l appear elsewhere (11). 2

2

0


3.

WALLER


Resin Resin Resin Resin Figure 2.

Ion-Exchange

Polymer

[SO~H] [SO^H, (SO ) M ] [(S(L) M V * [(SOp^M (ligand Z

Classes of metal cation ion-exchanged

resins

Synthetic organic applications.


Acylation. Acylation of benzene and substituted benzenes (equation 1 ) , are conveniently c a r r i e d out by r e f l u x i n g a xc.H.coei + CH C^H 64 365 0

c

^

> xe.H.coe.H.CH. + HCI 6 4 6 4 3

(i)

mixture of the benzoyl chloride, arene, and NAFION. The benzophenones are Isolated by f i l t e r i n g the hot reaction mixture to remove the r e s i n and followed by removing the excess arene by distillation. Table II summarizes several examples of the acylation of toluene with substituted benzoyl chloride (12)•

Table I I . X

Yield

H 4-CH2-F 4-F 3-C1 3

81 83 87 87 82

(%)

Acylation of toluene Isomer D i s t r i b u t i o n (o:m:p) 22.4:3.1:74.5 28.7:3.1:68.2 16.7:2.9:80.4 21.5:3.4:75.1 22.6:1.1:76.3

Acetic anhydride with a c e t i c acid has also been used to acetylate reactive alkylbenzenes l i k e mesitylene, equation 2. In t h i s case, the y i e l d of the product was 72%.

COCH

3

Benzoylation of jp-xylene (equation 3) at 135°C f o r



46

P O L Y M E R I C REAGENTS A N D CATALYSTS

6 hours provided 2,5-diraethylbenzophenone i n 85% y i e l d (13). The NAFION c a t a l y s t , removed by f i l t r a t i o n , was recycled without noticeable loss i n a c t i v i t y . However below 100°C, the catalyst showed less a c t i v i t y . An increase i n the equivalent weight of NAFION from 1100 to 1790 resulted i n a marked decrease i n the rate of reaction. In 2 days, only a 16% y i e l d of 2,5-dimethylbenzophenone was obtained. The reaction of thiophene with a c y c l i c acid anhydrides i n the presence of NAFION affords 2-acylthiophenes i n moderate to good yields (equation 4) (14). In p a r t i c u l a r , when the r a t i o

of acid anhydride to catalyst i s 125 and the reaction i s run i n refluxing CH^Cl^t ketones were obtained as shown i n Table I I I . Under the same set of reaction conditions, AMBERLYST 15 gave

Table I I I . Acylation of thiophene R Me Et n-Pr n-Pr

Yield

(%)

76 81 75

* AMBERLYST 15

lower y i e l d s . It was noted that deactivation of NAFION occurred with repeated use. The loss of the c a t a l y t i c a c t i v i t y seems to be ascribable to the adsorption of polymeric materials on the catalyst. Rearrangements. Fries rearrangement. The Fries rearrangement of phenol esters to hydroxyphenyl ketones, shown i n equation 5, proceeds i n dry refluxing nitrobenzene with yields between 63-75% depending upon the nature of R and R*.


3.

WALLER


Ion-Exchange

Polymer


(5)

In a t y p i c a l experiment, phenyl benzoate was converted to hydroxyphenyl ketone, o/p::l/2 i n 73% y i e l d (15). Pinacol rearrangement. The second rearrangement involves the preparation of ketones from 1,2-diols (equation 6 ) . Excellent

OH R

R

OH

| l _ e _ c _ R R„

2

| _

3

>

R

3

(6) R

R_ 3

O

_ R 2

0

3

y i e l d s , 82-92%, have been reported using NAFION as the acid catalyst (16). Examples of this transformation include tetramethylethylene g l y c o l to pinacolone, tetraphenylethylene g l y c o l to triphenylacetophenone and dicyclohexyl-1,1'-diol to spiro[5,6]dodecane-2-one. Rupe reaction. Rupe reaction converts alkynyl t e r t i a r y alcohols to a,B-unsaturated carbonyl compounds (17). Table IV summarizes the u t i l i t y of NAFION. Yields are based upon i s o l a t e d product and vary from 60 to 88%. This method suppresses the normal by-product formation from the polymerization of a,B-unsaturated compounds, giving considerable synthetic advantage. Unsaturated alcohols to aldehydes. The l a s t example of a NAFION-catalyzed rearrangement i s shown i n equation 7. Gaseous a l l y l alcohols, when passed over the resin at 170-190°C

^° RCH*CHCH OH 2

-

> RCH^CH^C^

rearrange to aldehydes (18). isomerizations at 195°C.

(7)

Table V presents examples of three

American Chemical Society Library 1155 16th St.,

N.W.

In Polymeric Reagents and Catalysts; Ford, W.; Washington. O.C. 2CD36 ACS Symposium Series; American Chemical Society: Washington, DC, 1986.



Table IV, a-Ethynyl alcohol

Rupe rearrangement

a,$-unsaturated carbonyl compound

Y i e l d (%)

0 OH

c=c: OH

O

84

CH CH — C - C=CH J Z j OH 0

0

CH^CH = C — CCH, j

83

j


3.

WALLER


Table V. Alcohol

Polymer

Isomerizatton of a l l y ! alcohols Yield(%)

Contact Time(sec)

allyl 2-methylallyl crotyl


Ion-Exchange

60 88 55

8 3 8

Ether synthesis. C y c l i c ethers. As shown i n equation 8, c y c l i c ethers are conveniently prepared by the dehydration of 1,4- and 1,5-diols at 135°C. Excellent yields (_>86%) are obtained and no solvent i s required (19).

OH RCH(CH

OH

CCH > 2 n

) CHR

+

2 n

H

(8)

2°

1,4-Butanediol, 1,5-pentanediol, and ethylene g l y c o l are dehydrated to give tetrahydrofuran, tetrahydropyran and 1,4dioxane, respectively. The f i l t e r e d NAFION i s treated with acetone, deionized water, and dried at 150°C to give a catalyst with o r i g i n a l a c t i v i t y . Polymeric ethers. Poly(tetramethylene ether)glycol, PTMEG, can be prepared by the polymerization of THF with NAFION. Conversion of 56% at 25°C i s possible after 65 hours (20). The product, PTMEG, has a molecular weight (number average) of about 1000. Analogous chemistry can be applied to prepare ester endcapped copolyether g l y c o l by copolymerizing THF and propylene oxide with acetic anhydride i n the presence of NAFION. The acetate end-capped copolyether g l y c o l contains 8 mol % of propylene oxide units (21). A non-end-capped copolymer can also be prepared under similar conditions but without acetic anhydride (22). Acetals and Ketals. There have been several reports of protecting aldehydes and ketones v i a acetals and ketals prepared from trimethyl orthoformate (equation 9) 1,2-ethanedithiol and 1,3-butanediol, or 1,3-propanediol.

+

rcwaVa - c c T >

W

0 0

^

+

H C 0

2

C H

3

(9)



Excellent yields are obtained f o r dimethyl acetals and ketals. Table VI, or ethylene dithioketals (Table VII) ( 2 3 ) . The

Table V I .

Dimethyl acetal and ketal formation

—

—


CH CH^ H

Yield (%)

-(CH ) n-C H Ph Ph Z

98 83 93 87

3

D

Table VII.

Ethylene d i t h i o k e t a l formation

R

R

{

Yield (%)

2

-CCH ) -(CHp^PhCH PhCH_ Ph CH Ph Ph 2

91 100 100 9b 100

6

9

1

3

ketones and aldehydes are readily deprotected by hydrolysis which also occurs very readily with NAFION as a c a t a l y s t . The ethylenedithioketals are obtained by refluxing a solution of the carbonyl compound with 1,2-ethanedithiol i n benzene with azeotropic removal of water. 2-Vinyl-1,3-dioxane has been reported to be prepared from acrolein, 1,3-propanediol and NAFION at a rate of 16 M per hour at 82% a c r o l e i n conversion (24). Aldehyde diacetates. 1,1-Diacetates of aldehydes can be prepared by using soluble p r o t l c acids. With a c a t a l y t i c amount of NAFION, the same diacetates can be obtained by vigorously s t i r r i n g equivalent amounts of the aldehyde and freshly d i s t i l l e d acetic anhydride (25). The general reaction i n equation 10 i s exemplified by the summary i n Table VIII. The yields are good with aromatic aldehydes and alkanals. RCH0 + (CH CO) 0 3

2

> RCH(0 CCH ) 2

3

2

(10)

A c y c l i c ethers. Aryl ethers can be prepared i n modest y i e l d by reacting the phenol, methanol, and NAFION at 120-150°C f o r 5 hours (26). Table IX summarizes several examples. Substituting


WALLER

Catalysis

with a Perfluorinated

Table VIII. R

Polymer

Aldehyde diacetate formation Reaction Time(h)

Yield(%)

Ph

1

99

73

CH

4 3 4

°

H

ci c 3


Ion-Exchange

Table IX. Phenol phenol pyrocatechol B-naphthol

Temp(°C)

120 150 125

90

67

Aryl ether formation Product

Yield(%)

anisole guaiacol B-naphthol monoether

14.9 18.5 13.7

AMBERLYST 15 under similar conditions produced only a 1% y i e l d of guaiacol from pyrocatechol. Hydroquinone i s converted to mono-t-butylhydroquinone and di-t^-butylhydroquinone as shown i n equation 11. During a two-hour reaction at 75°C, the

(11) OH

OtBu

hydroquinone and isobutylene conversion was 27 and 66%, respectively (27). The reaction pathway changes i f phenol and methanol are passed over the polymeric catalyst i n the gas phase at 205°C. Not only anisole and methyl anisoles but also cresols and xylenols are formed. In a t y p i c a l experiment, a product composition, i n mol%, i s 37.3% unreacted phenol, 37.2% anisole, 9.7% methyl anisoles, 1% dimethyl anisoles, 10.4% cresols and 4.4% xylenols. The C-methylated products were shown to occur v i a fast i n i t i a l O-methylation of the phenol followed by an i n t e r molecular rearrangement of the a r y l methyl ethers to methyl phenols (28). NAFION also catalyzes the formation of diisopropyl ether when isopropyl alcohol i s passed over the catalyst at 110°C. The


POLYMERIC REAGENTS AND CATALYSTS

52


s e l e c t i v i t y to the ether i s 92% at a 26% conversion of isopropyl alcohol (29). The perfluorinated ion exchange resin also acts as a catalyst i n the reaction between isobutylene and methanol at 80°C to y i e l d methyl t-butyl ether. The y i e l d based on methanol i s 81.8% (30). The recovered catalyst had no decrease i n the number of acid s i t e s . However, the same reaction with AMBERLYST 15 gave a s i m i l a r y i e l d , 83.1%, but the mequiv. per gram dry catalyst decreased from 5.10 to 4.83, suggesting possible a l k y l a t i o n of the benzene rings i n the polystyrene r e s i n . AMBERLYST 15 i s used commercially as a catalyst to manufacture methyl _t-butyl ether. Methoxymethyl ethers are prepared by exchanging alcohols with excess dimethoxymethane (equation 12) i n the presence of NAFION. ROH + (CH 0) CH 3

2

2

^

> ROCH OCH + CH^H 2

(12)

3

The reaction has a convenient rate at a reflux temperature of 41°C (31). Table X summarizes several examples of this reaction. This method requires only f i l t r a t i o n to remove the r e s i n with no aqueous workup.

Table X. R

Epoxide opening.

Methoxymethyl ethers from dimethoxymethane Reaction Time(h)

Yield(%)

10 10 10 16

90 90 96 65

The hydration, equation 13, or methanolysis of

(13)

epoxides can be carried out with NAFION i n good y i e l d s (66-81%) with minimal polymerization (32). Several examples are shown i n Table XI. NAFION allows the reaction to be conducted under mild conditions without heating of the reaction mixtures. The c a t a l y t i c hydration of ethylene oxide with NAFION or supported NAFION at 92°C showed a 56% conversion of ethylene oxide with a 94% s e l e c t i v i t y to ethylene g l y c o l (33). Only a 10.1% conversion of ethylene oxide i s realized when a poly-


WALLER


Ion-Exchange

Polymer


3.


53

54



(styrenesulfotiic acid) r e s i n i s used as a c a t a l y s t . Apparently, the high a c i d i t y of the polymeric perfluorinated s u l f o n i c acid i s preserved even when the r e s i n i s hydrated. Under the above conditions, about 90 wt % of water e x i s t s which at f i r s t thought would make the a c i d i t y of the two catalysts the same due to the l e v e l i n g action of water. This phenomenon i s not completely understood, but may be due to protection of some of the acid s i t e s by the hydrophobic portions of the polymer backbone. E s t e r i f i c a t i o n . E s t e r i f i c a t i o n s with NAFION have been carried out i n both the l i q u i d and gas phase. The reaction of a c r y l i c acid and ethanol at 68°C with the catalyst i n tubular form (approximately 25 and 35 mils inside and outside diameter, respectively) has been studied to obtain the forward rate constant i n this second order reversible reaction (34). A rate constant of 4,2 x 10 M min was obtained from the a n a l y s i s . U t i l i z a t i o n of s u l f u r i c acid i n the same e s t e r i f i c a t i o n reaction gave a k * 6.15 x 10" M~ min" at 82°C i n d i c a t i n g that the polymeric catalyst was as good as H S0^. When a mixture of a saturated carboxylic acid and an alcohol were passed over PFIEP at 95-125°C with a contact time 1

2

R j C O ^ + R 0H ~

* R C0 R

2

1

2

2

+ H0

(14)

2

of V> sec, high y i e l d s of the esters were obtained according to reaction 14. Table XII i l l u s t r a t e s the scope of the reaction.

Table XII. R

CH

E s t e r i f i c a t i o n catalyzed by NAFION R

l

CH CH

3

Yield(%)

2

3

2

CH.CH

n-C^ C

H

^ 5 11 CH CH^

i-e H; CH CH 3

2

n-C H C

H

1" 4 9

96 93 82 98

100 20

Primary and secondary alcohols gave excellent y i e l d s of ester (35). However, t e r t i a r y alcohols gave poor results because the predominant pathway was dehydration followed by polymer formation. Ethyl acetate can be formed over a NAFION catalyst from a c e t i c acid and ethylene i n the vapor phase at 135°C. Conversions were 48% based on acetic acid and after 100 hours, the a c t i v i t y of the catalyst was undiminished (36). The reverse of the above reaction has been demonstrated by the pyrolysis of ethyl acetate to ethylene at 185°C with NAFION tubing (37). The conversion of ethyl acetate i s 19.4%, and ethene i s the only gaseous product. Without the polymeric c a t a l y s t , the pyrolysis


3.

WALLER


Ion-Exchange

Polymer

took place at 700°C over glass helices with 100% conversion of ethyl acetate but the e x i t i n g gaseous stream consisted of ethylene (96.5%), C0 (1.52%), CH (1.09%), ethane (0.54%), and CO (0.33%).


?

Hydration. NAFION tubing of approximately 1/16 inch outside diameter has been used for the hydration of isobutylene to t-butyl alcohol at 96°C. The PFIEP tube was contained within a second p l a s t i c tube of 1/8 inch outside diameter. Liquid water was passed through the inner bore of the membrane tube while isobutene gas was passed through the void space between the two tubes. At least 84 wt % of the formed organics was t^butyl alcohol. The remaining 16 wt % consisted of isobutene dimer and trimer (38). If water i s not passed through the inner bore, 83 wt % of the organics was the isobutene dimer. The remaining material was the trimer. Hydration of a c y c l i c o l e f i n s can be carried out readily with NAFION i n a fixed bed tube reactor at 150°C. Because of thermodynamic consideration, propene conversion i s 16% with a isopropyl alcohol s e l e c t i v i t y of 97% (39). Condensation of acetone. The condensation of phenol and acetone to bisphenol-A (equation 15) i s catalyzed by polymeric perfluorinated sulfonic acid which has been p a r t i a l l y neutralized OH

(15)

with 2-mercaptoethYlamine (40). When 30% of the acid s i t e s are converted to R^SO^ NH^ CH CH SH and the r e s i n i s used as a c a t a l y s t , bisphenol-A was obtained i n 100% s e l e c t i v i t y at an acetone conversion of 88%. The bisphenol-A had an isomer d i s t r i b u t i o n of 97.5 and 2.5% for £,£*-bisphenol-A and £,£*-bisphenol-A, respectively. Acetone also undergoes an aldol condensation followed by dehydration to give mesityl oxide (equation 16). 2

2(CH ) CO 3

2

> (CH ) C-CHCOCH + Hy) 3

2

3

(16)

The reaction i s limited by thermodynamic equilibrium, so t y p i c a l yields for mesityl oxide, given i n Table XIII, approach 20% when the reaction i s catalyzed by NAFION (41). Crossed aldol condensation of acetone and benzaldehyde gave 4-phenyl-3-penten-2-one i n 27% y i e l d after 36 hours at 60°C.



56

Table XIII.


Time(h)

Acetone to mesityl oxide at 60°C Yield of mesityl oxide

I

To75

6 12 24

18.1 19.0 19.7

(%)

Intramolecular condensation. The formation of anthraquinone from o-benzoylbenzoic acid i s an example of i n t e r n a l c y c l i z a t i o n (equation 17). At 150° for 3 hours, the conversion of

o-benzoylbenzoic was 60% with 78% anthraquinone s e l e c t i v i t y (42) . NAFION can also intramolecular c y c l i z e 2,5-hexanedione to 2,5-dimethylfuran and 3-methyl-2-cyclopentenone at 150°C i n 29 and 2% y i e l d , respectively (41). Ollgomerlzation. NAFION has been used i n the c a t i o n i c o l i g o merization of styrene. Oligomers range from dimer to hexamers (43) . With a soluble perfluorinated acid, CF^SO^H, a linear dimer was the primary product i n non-polar solvents. The s o l i d , strong acid catalyst retained c a t a l y t i c a c t i v i t y on repeated reaction, had higher a c t i v i t y than that of a conventional poly(styrenesulfonic acid) r e s i n and was v i r t u a l l y free of solvent e f f e c t s on the reaction rate and product composition. The perfluorinated polymeric catalyst was not very e f f e c t i v e when compared with CF~S0~H; a ten-fold excess of NAFION r e l a t i v e to CF-SO-H was required to give oligomerizations with a similar rate In CC1, at 50°C. NAFION has been used as an ollgomerlzation catalyst for decene-1 (44). A t y p i c a l procedure consisted of heating the catalyst (35 g) and decene-1 (140 g) at 120°C for 0.5 hours. After f i l t e r i n g o f f the c a t a l y s t , and p a r t i a l decene-1 removal, the reaction mixture (114 g) consisted of C-10 (6.8%), C-20 (70.6%), C-30 (19.1%), and C-40 (3.4%). The amount of oligomers was 75.9%. Other o l e f i n s that can be oligomerized s i m i l a r l y are 7-tetradecene, octene-2 and decene-5.


3.

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Ion-Exchange

Polymer

N i t r a t i o n . NAFION resins catalyze the n i t r a t i o n of alkylbenzene with several different reagents: HNO^, NÔ,, n-BuNO^ and acetone cyanohydrin n i t r a t e (ACN). Table XIV compares the

Table XIV.

Nitration of toluene


% isomer r a t i o Reagent

Solvent

o

m

P

HNO M n-BuNO ACN

toluene cci toluene toluene

56 49 50 47

4 6 3 3

40 45 47 50

4

various reagents for the n i t r a t i o n of toluene (45-46). The cleanest reaction occurs with n-butyl n i t r a t e or ACN because a l l by-products are v o l a t i l e materials. The n i t r o compounds are isolated simply by f i l t r a t i o n of the catalyst without need f o r aqeuous basic washing or workup. In general, the yields were above 80% except for NÔ^. In a special case, 9-nitroanthracene can transfer a n i t r o group because the strong p e r i - i n t e r a c t i o n of the n i t r o group with the two neighboring hydrogens t i l t the group out of the plane of the aromatic r i n g . Transfer n i t r a t i o n yields are generally low ( ArR + C0 + HC1 2

(19)


58


oxalates. For a comparison, see Table XV. When using a l k y l halides, polymer formation i s minimized by keeping the reaction temperature between 155-200°C.

Table XV. A l k y l a t i n g Agent

A l k y l a t i o n of aromatics

Aromatic

Temp(°C) o

CICO^CH ClCOÊt (co Et) EtCt 9

7


2

toluene toluene toluene toluene

70-72 90 110 195

Isomer % m

48 46 48 4. 1

26 26 24 62.1

P 26 28 28 33.8

O l e f i n (5) and alcohols are other potential a l k y l a t i n g agents. Studies have been done on the gas phase methylation and ethylation of benzene over the polymeric perfluorinated sulfonic a c i d . At 185°C, methylation of benzene with methyl alcohol gives toluene i n only 4.1% (50). On the other hand, ethylation of benzene at 175°C gave ethylbenzene i n 88% s e l e c t i v i t y with 100% conversion of ethylene at a weight hourly space v e l o c i t y , WHSV, of 8.0 (51). Naphthalene has been alkylated with propene over the polymeric catalyst i n the gas phase. At 220°C, the y i e l d of isopropylnaphthalene i s 37% with the 0-isomer being 90% (52)• The a l k y l a t i o n of aromatics i s often complicated by competing side reactions such as isomerization and disproportionation of methyl benzenes (53-55). Investigations, including a k i n e t i c study on the isomerization of m-xylene, have been reported i n the l i t e r a t u r e (56-57). In a l l cases, NAFION has been used as a catalyst. Diels-Alder reaction. NAFION c a t a l y s i s allows the Diels-Alder reaction to be run at a lower reaction temperature. Reactions of anthracene with maleic anhydride, p-benzoquinone, dimethyl maleate, and dimethyl fumarate were carried out at 60-80°C i n either refluxing CHC1- or benzene. Table XVI gives % y i e l d of the various adducts (58).

Table XVI. Dienophile maleic anhydride p-benzoquinone dimethyl maleate dimethyl fumarate

Diels-Alder adducts Rxn

time(h) 5 2 15 16

Yield(%) 91 92 95 94


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WALLER


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Polymer

In general, excellent results were obtained. Even i f the diene i s e a s i l y polymerized, NAFION p r e f e r e n t i a l l y catalyzes the Diels-Alder reaction. As an example, isoprene and p-benzoquinone react at room temperature to give 80% of the adduct a f t e r 25 hours. Deacylation. Acylation of substituted benzenes with aroyl chloride has been described before. I t i s possible f o r deacetylation and decarboxylation of aromatic substrates to take place when unfavorable ortho s t e r i c interaction with methyl substituents seem to provide the driving force f o r the reaction. Table XVII summarizes a few examples of these reactions (59).


Other synthetic uses of NAFION. (p-Tolyl)phosphorous d l c h l o r i d e . Phosphorus t r i c h l o r i d e reacts with toluene (60) i n the presence of NAFION to y i e l d (p-tolyl)phosphorous dlchloride and (o-tolyl)phosphorous dlchloride (equation 20). The conversion of toluene was 26%.

(20)

p / o = 17

Photolsomerization on NAFION. The course of cis/trans photoisomerizations can be modified by surfaces. NAFION has been examined as a s o l i d acid to catalyze the photolsomerization of ethyl cinnamate (equation 21). C0 Et 2

Ph

w Ph

CO Et 2 (21)

300 nm

Irradiation of trans-ethyl cinnamate afforded a photostationarystate composition of 73% c i s - and 27% trans-ethyl cinnamate. Without the s o l i d catalyst or with FSO^H, the c i s composition was 42% or 39%, respectively. Apparently, there i s selective complexation or adsorption of the s t a r t i n g material by the s o l i d acid (61).


60


Table XVII. Ketone or Acid

Deacylation and decarboxylation of aromatics Tempt~C)

Time(h)

Product(% Yield)


GOMe


WALLER


Ion-Exchange

Polymer


The perfluorinated polymer also functions as a photocatalyst i n the photolsomerization of 3-methylene-l,2,4,5,6,6hexamethylcyclohexa-1,4-diene (equation 22). No photolsomerization took place when the i r r a d i a t i o n was

performed i n the absence of NAFION (8). The heptamethylbenzenium cation i s formed f i r s t with the strong acid catalyst i n a dark reaction, and i r r a d i a t i o n then converts the benzenium ion to mostly vinylcyclopentadiene. Carbonylation chemistry. The carbonylation of formaldehyde i n aqueous jp-dioxane at 150°C and 3000 psig yields hydroxyacetic acid (equation 23), i n about 70% y i e l d (62). CH 0 + CO + H 0 2

(23)

-> HOCH C0 H

2

2

2

P r i n c i p a l by-products are methyl formate and polyhydroxyl aldehydes from condensation of formaldehyde. The NAFION used as a catalyst i n this reaction swells controllably i n the organic polar solvent. The actual products of the reaction are dehydrated forms of g l y c o l i c acid and the water-to-acid r a t i o influences the product y i e l d . The hydroxyacetic acid formed i s a precursor to ethylene g l y c o l . Immobilized reagents. NAFION can be converted into a hypohalite functionalized polymer as shown i n equation 24 by treatment of PFIEP[SO^H] + GIF

(24)

> PFIEP[S0 C1] + HF < -30°C 3

the acid form of PFIEP with GIF below -30°C. The corresponding PFIEP[S0«Br] can be formed i n an analogous reaction. PFIEP[SO^Br] reacts with CH^Br to form PFIEP[SO^Hj (63). The acid form of NAFION can also be reacted with chlorotrimethylsilane to form a polymer-supported s i l y l a t i n g agent, equation 25. Incorporation of up to 0.8 mmol of the PFIEP[S0 H] + ( C H ) S i C l 3

3

3

-> PFIEP[S0 Si(CH ) ] 3

3

3

(25)

s i l y l group per gram of r e s i n has been achieved (64). Unlike monomer analogs, this reagent does not fume i n a i r . Reaction with compounds possessing an active hydrogen such as ethanol, acetic acid, ethanethiol, or diethylamine, transforms them into the corresponding t r i m e t h y l s i l y l d e r i v a t i v e s .



62

The FITS reagent, (perfluoroalkyl)aryliodonium t r i f l u o r o sulfonates, can also be immobilized on NAFION (65). In p a r t i c u l a r , bis(trifluoroacetoxy)iodoperfluoroalkanes react with PFIEP[SO^H] as i n equation 26. The immobilized reagent,

PFIEP[S0 H] — J

R 1(0 CCF ) ±-=~> PFIEP[S0 IR ] PhH | Ph

(26)

J

R. « n-C F„ , f m 2m + 1 f

PFIEP[S0 IC F _],upon heating i n thiophene at 80°C i n o

Q


J J O X/

Ph the presence of pyridine gave a-C F --thiophene i n 95% y i e l d . Q

Supported NAFION. In order to increase the a c t i v i t y of the acid s i t e s by achieving better dispersion, NAFION has been supported on s i l i c a g e l , silica/alumina, alumina, porous glass and Chromosorb T (f luoropolymer support) . -These supports can have either low or high surface area and various pore diameters (50-600A). Catalysts prepared i n this fashion have been used i n the a l k y l a t i o n of benzene, isomerization of normal alkanes and disproportionation of toluene. Table XVIII summarizes the results on the a l k y l a t i o n of benzene with ethene f o r NAFION and several supported catalysts (66-68).

Table XVIII. Support Temp (°C) Ethene Conv (%) PhEt s e l (%) WHSV

—

175 100 80 1.0

A l k y l a t i o n of benzene with ethene Silica 185 99 91 1.8

Porous glass 188 95 87 3

Alumina 200 99 85.5 3

The weight hourly space v e l o c i t y , WHSV, measures the a c t i v i t y of the catalyst and suggests that coating NAFION on a support increases the number of accessible acid s i t e s . The primary method f o r dispersing PFIEP i s by impregnating supports with a l c o h o l i c solutions of soluble PFIEP i n the acid form. A l t e r n a t i v e l y , the polymeric sulfonyl fluoride precursor i s a thermoplastic and can be extruded into thin films or blended with a powder support and extruded into various shapes (69). In addition, the sulfonyl f l u o r i d e precursor can be coextruded with a polyethylene r e s i n to form d i f f e r e n t l y shaped parts (70). After extrusion, the sulfonyl f l u o r i d e form i s converted to the active sulfonic acid form.


WALLER


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Polymer


Other uses of NAFION. In addition to the synthetic applications described above, NAFION has found other applications including coatings on electrodes (71-72), hydrogen production (73-74), f u e l c e l l s , and membranes for chemical devices (75-78). Some of these uses are outlined here. Because i t i s resistant to chemical attack, even by strong oxidants at elevated temperature, perfluorinated coatings on electrodes o f f e r a stable environment f o r attaching reactants. For example, a p y r o l y t i c graphite electrode was coated with NAFION (71). After e l e c t r o s t a t i c a l l y attaching metal complexes into the membranes, chemical reactions were carried out i n the membrane or at the membrane interface with the s o l u t i o n . Other studies have focused on the mechanism of charge transport through NAFION coated on glassy carbon electrodes and containing Cp FeTMA , Ru(bpy) , and Os(bpy) where Cp^FeTMA i s [(triethylammonio)methyl)ferrocene (72)• The polymeric perfluorinated s u l f o n i c acid has been used as a matrix for a system which combines semiconductor CdS c r y s t a l l i t e s and a Pt hydrogen-evolution catalyst i n a photocatalytic hydrogen generator (73-74). Upon photolysis of the p l a t i n i z e d CdS p a r t i c l e s i n the presence of a s a c r i f i c i a l electron donor, Na«S, the production of hydrogen gas by water reduction was observed. The number of moles of H produced with a t y p i c a l NAFION/CdS system exceeds the moles of CdS present by a factor greater than 100. NAFION systems have also been used to construct devices f o r 1) separating monofunctional carboxylic acids l i k e propionic, butyric or v a l e r i c acid from other acids (75), 2) n i t r a t i n g and sulfonating aromatics i n an annular tubular flow reactor where the inner tube i s NAFION and the outer tube i s TEFLON f l u o r o carbon resin (76), 3) sulfonating aromatics i n a two-compartment c e l l separated by a membrane sheet (77), and 4) a l k y l a t i n g aromatics with a tightly-packed bundle of p a r a l l e l tubular NAFION i n a s t a i n l e s s steel tube (78). 2

3

3

2

C h l o r - a l k a l l membranes. Several companies are or w i l l supply perfluorinated membranes to the c h l o r - a l k a l l industry. These companies are summarized i n Table XIX along with the type of membrane available (79-80). Du Pont and Asahi Glass have signed an agreement to cross-license patents ( l c ) . Presently, the industry workhorse i n the chlorine-caustic diaphragm c e l l i s the asbestos diaphragm. In the membrane c e l l , a cat ion-exchange membrane i s used instead of a diaphragm. Perfluorinated membranes w i l l o f f e r the c h l o r - a l k a l i industry energy savings and less p o l l u t i o n . Conclusion NAFION resins are e f f e c t i v e , heterogeneous, strong acid catalysts for many reactions useful i n organic synthesis. Literature



64

Table XIX.

C h l o r - a l k a l l membranes

Company Asahi Chemical Industry Asahi Glass Company Du Pont


Dow Tokuyama Soda Company

Membrane-Type surface carboxylation on sulfonate ionomer perfluorocarboxylate perfluorocarboxylate/ perfluorosulfonate perfluorosulfonate oxidize NAFION membrane surface

references highlighted i n t h i s review also suggest other unique applications for PFIEP. It i s the author's b e l i e f that research w i l l continue to be f r u i t f u l not only i n the areas mentioned i n this a r t i c l e but also those areas described elsewhere (11). NAFION has a l l the advantages of an insoluble catalyst but also has important inherent properties: a chemically inert polymeric framework and higher thermal s t a b i l i t y . It w i l l be interesting to see i f NAFION and other PFIEP s w i l l grow beyond their i n i t i a l applications i n the c h l o r - a l k a l l industry and find applications i n the marketplace as catalyst or unique support for c a t a l y s i s . ?

Literature Cited 1. a. Du Pont registered trademark for its NAFION perfluorinated membranes. Henceforth in this review, NAFION will refer to Du Pont's brand of perfluorinated ion exchange polymer resins. b. Connolly, D. J.; Gresham, W. F. (to E. I. du Pont de Nemours & Company) U.S. Patent 3,282,875 (November 1, 1966). c. Chem. Eng. News, March 15, 1982, 22-25. 2. NAFION in the form of powder, film, and alcohol solutions can be purchased from C. G. Processing, Inc., P. O. Box 133, Rockland, DE 19732. 3. Waller, F. J. Brit. Polymer J. 1984, 16, 239. 4. Rohm and Haas registered trademark. 5. Olah, G. A.; Kaspi, J.; Bukala, J. J. Org. Chem. 1977, 42, 4187. 6. Gierke, T. D.; Hsu, W. Y. in "Perfluorinated Ionomer Membranes," A. Eisenberg and H. L. Yeager, Ed., ACS Symp. Ser., No. 180, American Chemical Society: Washington, D.C., 1982, Chapter 13. 7. Komoroski, R. A.; Mauritz, K. A. J. Am. Chem. Soc. 1978, 100, 7487. 8. Childs, R. F.; Mika-Gibala, A. J. Org. Chem. 1982, 47, 4204.



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9. Olah, G. A.; Prakash, G. K. S.; Sommer, J. Science 1979, 206, 13. 10. Rys, P.; Steinegger, W. J. J. Am. Chem. Soc. 1979, 101, 4801. 11. Waller, F. J. accepted for publication in Catalysis Reviews - Science and Engineering. 12. Olah, G. A.; Malhotra, R., Narang, S. C.; Olah, J. A. Synthesis 1978, 672. 13. Krespan, C. G. J. Org. Chem. 1979, 44, 4924. 14. Konishi, H.; Suetsugu, K.; Okano, T.; Kiji, J. Bull. Chem. Soc. Jpn. 1982, 55, 957. 15. Olah, G. A.; Arvanaghi, M.; Krishnamurthy, V. V. J. Org. Chem. 1983, 48, 3359. 16. Olah, G. A.; Meidar, D. Synthesis 1978, 358. 17. Olah, G. A.; Fung, A. P. Synthesis 1981, 473. 18. Olah, G. A.; Meidar, D.; Liang, G. J. Org. Chem. 1978, 43, 3890. 19. Olah, G. A.; Fung, A. P.; Malhotra, R. Synthesis 1981, 474. 20. Pruckmayr, G.; Weir, R. H. (to E. I. du Pont de Nemours & Company) U.S. Patent 4,120,903 (October 17, 1978). 21. Pruckmayr, G. (to E. I. du Pont de Nemours & Company) U.S. Patent 4,153,786 (May 8, 1979). 22. Pruckmayr, G. (to E. I. du Pont de Nemours & Company ) U.S. Patent 4,139,567 (February 13, 1979). 23. Olah, G. A.; Narang, S. C.; Meidar, D.; Salem, G. F. Synthesis 1981, 282. 24. Hughes, O. R. (to Celanese Corporation). U.S. patent 4,003,918 (January 18, 1977). 25. Olah, G. A.; Mehrotra, A. K. Synthesis 1982, 962. 26. Maggioni, P.; Minisci, F. GB Patent 2,085,004. 27. Malloy, T. P.; Engel, D. J. (to UOP Inc.) U.S. Patent 4,323,714 (April 6, 1982). 28. Kaspi, J.; Olah, G. A. J. Org. Chem. 1978, 43, 3142. 29. Olah, G. A. GB Patent 2,082,177. 30. Oyama, K.; Kihara, K., GB Patent 2,075,019. 31. Olah, G. A.; Husain, A.; Gupta, B. G.; Narang, S. C. Synthesis 1981, 471. 32. Olah, G. A.; Fung, A. P.; Meidar, D. Synthesis 1981, 280. 33. Kim, L. (to Shell Oil Company) U.S. Patent 4,165,440 (August 21, 1979). 34. Schreck, D. J. GB Patent 2,063,261. 35. Olah, G. A.; Keumi, T.; Meidar, D. Synthesis 1978, 929. 36. Gruffaz, M.; Micaelli, O. (to Rhone-Poulenc Industries) U.S. Patent 4,275,228 (June 23, 1981). 37. Schreck, D. J. (to Union Carbide Corporation) U.S. Patent 4,399,305 (August 16, 1983). 38. Cares, W. R. (to Petro-Tex Chemical Corporation) U.S. Patent 4,065,512 (December 27, 1977). 39. Olah, G. A. GB Patent 2,082,178. 40. McClure, J. D.; Neumann, F. E. (to Shell Oil Company) U.S. Patent 4,053,522 (October 11, 1977). 41. Pittman, C. U.; Liang, Y. J. Org. Chem. 1980, 45, 5048. 42. Nutt, M. O. (to Dow Chemical Company) U.S. Patent 4,304,724 (December 8, 1981). 43. Hasegawa, H.; Higashimura, T. Polymer J. 1979, 11, 737.


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66


44. Marquis, E. T.; Watts, L. W.; Brader, W. H.; Darden, J. W. GB Patent 2,089,832. 45. Olah, G. A.; Malhotra, R.; Narang, S. C. J. Org. Chem. 1978, 43, 4628. 46. Olah, G. A.; Narang, S. C. Synthesis 1978, 690. 47. Olah, G. A.; Narang, S. C.; Malhotra, R.; Olah, J. A. J. Am. Chem. Soc. 1979, 101, 1805. 48. Olah, G. A.; Meidar, D. Nouv. J. Chim. 1979, 3, 269. 49. Olah, G. A.; Meidar, D.; Malhotra, R.; Olah, J. A.; Narang, S. C. J. Catal. 1980, 61, 96. 50. Kaspi, J.; Montgomery, D. D.; Olah, G. A. J. Org. Chem. 1978, 43, 3147. 51. McClure, J. D. (to Shell Oil Company) U.S. Patent 4,041,090 (August 9, 1977). 52. Olah, G. A. GB Patent 2,090,856. 53. Olah, G. A.; Kaspi, J. Nouv. J. Chim. 1978, 2, 581. 54. McClure, J. D. (to Shell Oil Company) U.S. Patent 4,052,474 (October 4, 1977). 55. McClure, J. D. (to Shell Oil Company) U.S. Patent 4,022,847 (May 10, 1977). 56. Beltrame, P.; Beltrame, P. L.; Carniti, P.; Magnoni, M. Gazz. Chim. Ital. 1978, 108, 651. 57. Beltrame, P.; Beltrame, P. L.; Carniti, P.; Nespoli, G. Ind. Eng. Chem. Prod. Res. Dev. 1980, 19, 205. 58. Olah, G. A.; Meidar, D.; Fung, A. P. Synthesis 1979, 270. 59. Olah, G. A.; Laali, K.; Mehrotra, A. K. J. Org. Chem. 1983, 48, 3360. 60. Cozens, R. J.; Hogan, P. J.; Lalkham, M. J. (to Imperial Chemical Industries Limited), EP Patent 24128. 61. Childs, R. F.; Duffey, B.; Mika-Gibala, A. J. Org. Chem. 1984, 49, 4352. 62. Chem. Eng. News, April 11, 1983, 41-42. 63. Desmarteau, D. D. J. of Fluorine Chem. 1982, 21, 249. 64. Murata, S.; Noyori, R. Tetrahedron Lett. 1980, 21, 767. 65. Umemoto, T. Tetrahedron Lett. 1984, 25, 81. 66. McClure, J. D.; Brandenberger, S. G. (to Shell Oil Company) U.S. Patent 4,060,565 (November 29, 1977). 67. McClure, J. D.; Brandenberger, S. G. (to Shell Oil Company) U.S. Patent 4,065,515 (December 27, 1977). 68. McClure, J. D.; Brandenberger, S. G. (to Shell Oil Company) U.S. Patent 4,052,475 (October 4, 1977). 69. Peluso, S. L. Research Disclosure 1982, 221, 311. 70. Vaughan, R. J. (to Varen Technology) U.S. Patent 4,303,551 (December 1, 1981). 71. Rubinstein, I.; Bard, A. J. J. Am. Chem. Soc. 1980, 102, 6641. 72. White, H. S.; Leddy, J.; Bard, A. J. J. Am. Chem. Soc. 1982, 104, 4811. 73. Krishnan, M.; White, J. R.; Fox, M. A.; Bard, A. J. J. Am. Chem. Soc. 1983, 105, 7002. 74. Mau, A. W.; Huang, C. B.; Katuta, N.; Bard, A. J.; Campion, A.; Fox, M. A.; White, J. M.; Webber, S. E. J. Am. Chem. Soc. 1984, 106, 6537. 75. Chum, H. L.; Sopher, D. W. (to U.S. Department of Energy) U.S. Patent 4,476,025 (October 9, 1984).


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with a Perfluorinated

Ion-Exchange

Polymer

76. Vaughan, R. J. (to Varen Technology) U.S. Patent 3,976,704 (August 24, 1976). 77. Vaughan, R. J. (to Varen Technology) U.S. Patent 4,308,215 (December 29, 1981). 78. Vaughan, R. J. (to Varen Technology) U.S. Patent 4,316,997 (February 23, 1982). 79. Chemical Week August 25, 1982, 63-64. 80. Chemical Week November 17, 1982, 35-36. January 23, 1986


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