Perfluorinated sulfonic acid

To solve some of the environmental problems of mixed-acid nitration, we were able to replaee sulfuric acid with solid superacid catalysts. This allowed us to develop a novel, clean, azeotropic nitration of aromatics with nitric acid over solid perfluorinated sulfonic acid catalysts (Nafion-H). The water formed is continuously azeotroped off by an excess of aromatics, thus preventing dilution of acid. Because the disposal of spent acids of nitration represents a serious environmental problem, the use of solid aeid eatalysts is a significant improvement. 

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. 

Xie, T., Hayden, C., Olson, K. and Healy, J. 2005. Chemical degradation mechanism of perfluorinated sulfonic acid ionomer. In Advances in materials for proton exchange membrane fuel cell systems, Pacific Grove, CA, Feb. 20-23, abstract 24. 

Using proton exchange membranes as electrolytes that are quasi-solid may cause a problem with respect to the perfect wetting of the catalyst particles. In spite of this (initial) difficulty of developing solid polymer membrane fuel cells, water-swollen perfluorinated sulfonic acid polymers such as the commercial Nation have been used for fuel cells very early since they offer the following advantages ... 

Chromium(III) and cerium(IV) impregnated Nafion K (a perfluorinated sulfonic acid resin) were used as catalysts f°r the themoselective oxidation of a variety of alcohols using TBHP or NaBr03 as tlie oxy9en donor A e.g. 

The following subchapters cover various solid superacids, including perfluorinated sulfonic acid resins (Nafion resins). Furthermore, in the past, various attempts have been made to obtain solid superacids by either (a) enhancing the intrinsic acidity of a solid acid by treatment with a suitable co-acid or (b) physically or chemically binding a liquid superacid to an otherwise inert surface. We will briefly review some of these attempts because most of these catalysts rapidly lose activity and need to be regenerated. 

The new Nafion-nanocomposite catalysts are produced by DuPont and marketed as Nation SAC materials with Nation loading between 10% and 20%. Additional information for perfluorinated sulfonic acid resin nanocomposites including characterization by a variety of physical and chemical methods can be found in a recent... 

The acidity of perfluorinated sulfonic acids can be increased further by complexa-tion with Lewis acid fluorides, such as SbF5, TaF5, and NbF5.183 They have been found to be effective catalysts for n-hexane, n-heptane isomerization, alkylation of benzene, and transalkylation of alkylbenzenes (see Chapter 5). 

Figure 5.15. Superacidic perfluorinated sulfonic acid functions anchored to silica surfaces.

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>. 

This review summarizes the recent works on syntheses of solid superacids and their catalytic action, including Lewis acids and liquid superacids in the solid state, as discussed in Sections Il-IV. Sections VI and VII describe new types of solid superacids we have studied in this decade sulfate-supported metal oxides and tungsten or molybdenum oxide supported on zirconia. Perfluorinated sulfonic acid, based on the acid form of DuPont s Nafion brand ion membrane resin, is also gaining interest as a solid superacid catalyst Nafion-H-catalyzed reactions are reviewed in Section V. 

A convenient solid of perfluorinated-sulfonic acid can be made readily from DuPont s commercially available Nafion brand ion membrane resins. Powder granules of the 1200-EW polymer, Nafion 501, have been used most frequently in catalytic applications the price in the K+ form of the perfluorosulfonic salt, 501X, was 650/kg in 1981. Because only the potassium salt derivative is commercially available, the salt is converted to the free sulfonic acid by treatment with mineral acid. A standard procedure for the conversion is described below. This procedure also serves to regenerate the resin in various catalytic cycles. 

Nafion, a perfluorinated sulfonic acid (PFSA) polymer electrolyte developed and produced by the E. I. Dupont Company, has been extensively studied as a fuel cell membrane. Despite its age, it remains the industry standard membrane because of its relatively high proton conductivity, toughness and quick start capabilities. Attempts to build upon the strengths of Nafion have resulted in a class of PFSA polymer electrolytes, including the short-side-chain (SSC) PFSA polymer electrolyte, originally synthesized by Dow and now produced by Solvay Solexis. Stracturally, PFSA polymer... 

Mechanistic Aspects of Proton Conductivity (Nahon and Perfluorinated Sulfonic Acids).774... 

Short Side Chain Perfluorinated Sulfonic Acid Membranes.782... 

MECHANISTIC ASPECTS OF PROTON CONDUCTIVITY (NAFION AND PERFLUORINATED SULFONIC ACIDS)... 

FCs must cost around 45/kW to be competitive with the internal combustion engine. Perfluorinated sulfonic acid membranes currently cost 100-200/KW resulting in PEM FC prices estimated to be of the order of 1000/KW. 

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