Nafion sulfonic acid groups

Solid Polymer E,kctroljte. The electrolyte in soHd polymer electrolyte (SPE) units is Nafion, a soHd polymer developed by Du Pont, which has sulfonic acid groups attached to the polymer backbone. Electrodes are deposited on each side of the polymer sheet. H" ions produced at the anode move across the polymer to the cathode, and produce hydrogen. The OH ions at the anode produce oxygen. These units have relatively low internal resistances and can operate at higher temperatures than conventional alkaline electrolysis units. SPE units are now offered commercially. 

Nafion-H, a perfluorinated sulfonic acid resin, is another strongly acidic solid that has been explored as alkylation catalyst. Rprvik et al. (204) examined unsupported Nafion-H with a nominal surface area of 0.2 m2/g (surface area of a swellable polymer is difficult to define) in isobutane/2-butene alkylation at 353 K and compared it with a CeY zeolite. The zeolite gave a better alkylate and higher conversion than Nafion-H, which produced significant amounts of octenes and heavy-end products. The low surface area of the resin and questions about the accessibility of the sulfonic acid groups probably make the comparison inadequate. 

In Nafion, the hydrophobic perfluorinated segments of the polymer are incompatible with the hydrophilic sulfonic acid groups and thus phase separation occurs. When exposed to water, the hydrophilic domains swell to provide channels for proton transport, whereas the hydrophobic domains provide mechanical integrity and, at least in the case of lower lEC samples. 

By treating Nafion (NR-50), a perfluorinated acidic ion exchanger based on sulfonic acid groups, with scandium(III) chloride hexahydrate Kobayashi et al. generated a solid scandium-derived catalyst (29) (Nafion-Sc) that proved to be effective in al-lylation reactions of carbonyl compounds with tetraallyltin (Scheme 4.15). Since the catalyst is stable in both organic solvents and water, even unprotected carbohydrates could be transformed directly in aqueous solvents. The resulting homo-allylic alcohols were separated by simple filtration [97]. 

All work has been accomplished using perfluorosulfonate ion exchange membranes manufactured by E.l. duPont and sold under the trade name Nafion. Nafion is a copolymer of polytetrafluoroethy-lene (PTFE) and polysulfonylfluoride vinyl ether containing pendent sulfonic acid groups. The sulfonic acid groups are chemically bound to the perfluorocarbon backbone. 

Nafion membrane materials were originally developed for electrochemical applications.155 The acid strength (—Ho) is estimated to be between 11 and 13156 that is, the acidity of Nafion is comparable to that of concentrated sulfuric acid. In these systems, the sulfonic acid group is attached to a CF2 or CF group in a perfluorinated backbone. 

Two significant drawbacks of Nafion-H in catalytic applications are its very low surface area (0.02 m2 g ) and the hindered accessibility of the active sites (sulfonic acid groups) located inside the pockets of the polymeric backbone. Consequently, the specific activity of Nafion, namely, the number of reacting molecules per unit weight,... 

The most important use of /3-sultones is for the preparation of fluorinated polymers such as Nafion 64. These solid acid catalysts containing perfluorinated sulfonic acid groups have been known for many years and the presence of the electron-withdrawing F atoms increases the acid strength of the terminal sulfonic acid groups, which become comparable to that of pure sulfuric acid. Prior to the last decade, Nafion had been in use as a superacid, a fuel cell electrolyte and as a membrane-ion separator <1996CHEC-II(1B)1083>. 

Figure 12. Three examples of complex, H or M (e g., Li+) conducting electrolytes. Top. Nafion consists of an organic backbone to which sulfonic acid groups are covalently bonded.

Carrying out experiments with equal ratios of starting material 47 to sulfonic acid group of the specific catalysts reveals the same pattern as obtained in the experiments described above. Figure 5.2 shows the results obtained over some Nafion/silica composite materials with different amounts of Nafion compared to the Amberlyst ion-exchange resin. 

It seems, due to the very high activity of the Nafion/silica based catalysts, that these reactions conditions are not suitable for observing any differences in activity between the various Nafion/silica materials. All composites with a ratio of starting material to sulfonic acid group of about 1,000 reveal complete conversion. 

The most successful solid protonic membrane for fuel cell use is NAFION membrane (saturated with aqueous acid solution) used in kW-size fuel cells in the US Gemini space program up to 150 °C. NAFION is a perfluorinated polymer with sulfonic acid groups. 

Figure 4 The Nafion backbone and side chains. The sulfonic acid groups are hydrophilic and cluster together. This gives Nafion a relatively porous structure. Nafion swells in polar solvents, such as methanol and water...

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