Solid superacids Nafion

Solid superacidic Nafion-H was also found to be effective in the hydration of acyclic alkenes.16 Isopropyl alcohol was produced with 97% selectivity in hydrating propylene17 at 150°C, whereas isobutylene yielded tert-butyl alcohol18 with 84% selectivity at 96°C. 

The solid superacidic Nafion-Hhas also been found to catalyze effectively nitration reactions with various reagents.503 The nitrating agents employed were n-butyl nitrate, acetone cyanohydrin nitrate, and fuming nitric acid. In nitric acid nitrations, sulfuric acid can be substituted by Nafion-H and the water formed is azeotropically removed during the reaction (azeotropic nitration).503... 

Develop a solid superacid-Nafion composite membrane electrode assembly (MEA) with the following components ... 

Many superacid-catalyzed reactions were found to be carried out advantageously not only using liquid superacids but also over solid superacids, including Nafion-H or certain zeolites. We extensively studied the catalytic activity of Nafion-H and related solid acid catalysts (including supported perfluorooctanesulfonic acid and its higher ho-... 

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. 

Substantial progress has been made to carry out alkylation in the gas phase over solid superacid catalysts. Nafion-H, a perfluorinated resin-sulfonic acid, for example, catalyzes the methylation of benzene and methylbenzenes with methyl alcohol under relatively mild conditions. The reaction shows low substrate selectivity.203... 

Electrophilic chlorination and bromination of methane over supported noble metals9-11 (Pt on A1203, Pd on BaSC ) and solid superacid catalysts9-13 (e.g., TaOF3 on alumina, Nafion-H, zeolites, SbF5-graphite, sulfated zirconia) have been studied (see Section 3.4.2). Monosubstitution with selectivities better than... 

Olah, G.A., Yamato, T., Iyer, P.S. and Prakash, G.K.S., Catalysis by solid superacids. 20. Nafion-H catalyzed reductive cleavage of acetals and ketals to ethers with triethylsilane, /. Org. Chem., 1986, 51, 2826. 

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. 

Subsequently, selective ionic chlorination of methane to methyl chloride was achieved in the gas phase over solid superacid catalysts.526 For example, chlorination of methane in excess chlorine over Nafion-H and SbF5-graphite gave methyl chloride with 88% and 98% selectivity, respectively (185°C and 180°C, 18% and 7% conversion).527 Similarly, electrophilic bromination of alkanes has also been carried out528 (Scheme 5.53). 

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. 

The topic of catalysis with Nafion has recently been reviewed in detail (56). Apart from using Nafion-H primarily as a solid superacid catalyst, a number of reports have described the use of functionalized Nafion derivatives by metal cation exchange to achieve various types of organic reaction. These include a bifunctional catalyst (acid and cation site), a heterogeneous perfluorosulfonate salt (only cation sites), and a trifunctional... 

Solid catalysts can be used at elevated temperatures, though their acidities are much weaker than those of liquid ones. From this point of view, solid superacids based on Lewis acids and liquid superacids discussed in Sections II—1V are not sufficiently stable Nafion-H is also unsatisfactory, its maximum operating temperature being below 200°C. A new type of the sulfate-supported metal oxides is more stable because of preparatory heat treatment at high temperatures, but elimination of the sulfate is sometimes observed during reaction, thus it is hoped to synthesize superacids with the system of metal oxides. Another type of superacid, tungsten or molybdenum oxide supported on zirconia, has been prepared by a new preparation method, and its stability is satisfactory so far. It is hoped that the preparation method will be extensively applied to other metal oxides for new solid superacids. 

Recently, various kinds of solid superacids have been developed. The first group is metal oxides and mixed oxides containing a small amount of sulfate ion, and those modified with platinum. The second group is metal oxides, mixed oxides, graphite, metal salts, etc. treated or combined with antimony fluoride or aluminum chloride. The third group is perfluorinated polymer sulfuric acid (Nafion-H). The fourth and fifth groups are H-ZSM-5 and a type of heteropolyacids, respectively. The last group is simply mixed oxides. 

The solid superacids of the second group which have a possibility of leaching or evaporating of halogen compounds seem to be environmentally undesirable as catalysts. These superacids and Nafion-H have been extensively reviewed [1-5]. H-ZSM-5 and heteropolyacids are discussed in Sections 2.3.3 and 2.1.7 of this handbook, respectively. Therefore, the superacids of the first group for which many papers have been contributed recently and the industrial application of which is promising, are mainly described here and the other superacids are dealt with only briefly. 

Olah, G. A., Arvanaghi, M., Krishnamurthy, V. V. Heterogeneous catalysis by solid superacids. 17. Polymeric perfluorinated resin sulfonic acid (Nafion-H) catalyzed Fries rearrangement of aryl esters. J. Org. Chem. 1983,48, 3359-3360. 

Olah, G. A., Reddy, V. P., Prakash, G. K. S. Catalysis by solid superacids. 26. Peterson (silyl-Wittig) methylenation of carbonyl compounds using Nafion-H catalyzed hydroxy-trimethylsilane elimination of P-hydroxysilanes. Synthesis 1991,29-30. 

Another type of solid superacid is based on perfluorinated resin sulfonic acids, such as the acid form of DuPont s Nafion resin, a copolymer of a perfluorinated epoxide and vinylsulfonic acid, or higher perflu-oroalkanesulfonic acids such as perfluorodecanesulfonic acid, CF3(CF2) 03H. Such solid catalysts were found to be very efficient in alkylation of aromatic hydrocarbons and other Friedel-Crafts reactions. A comprehensive review is available on the application of Nafion-H in organic catalysis. ... 

Nafion (Figure 4.12), a perfluorinated polymer containing pendant sulfonic acid groups, is generally considered to be a solid superacid whose pK ranges from -5 to -9. It was earlier shown to be an active catalyst for various organic reacfions such as alkylation, isomerization, disproportionation, transalkylation, acylation, nitration, hydration, rearrangement, and so on. ° ° ... 

Because of the advantages of using solid superacidic catalysts in electrophilic aromatic nitration and in acid-catalyzed reactions in general, Olah et al. have examined the mercury (Il)-promoted azeotropic nitration of aromatics using Nafion-H solid superacidic catalyst [142]. Azeotropic removal of water accelerates the rate of reaction by mitigating the dilution of nitric acid in a static reaction system. The yield of nitroaromatics varies from 48-77% (Table XXVIII). 

A solid superacid (stronger than H2SO4 100 %) such as Ti02-S04 as nanosized particles has been successfully used as a filler yielding membranes with conductivities up to 50 % higher than the recast Nafion membrane [88]. Neodymium oxide [90] and triflate [91], along erbium triflate [92] enhance the conductivity of Nafion between 30 % and 40 % when added in concentrations between 3 and 8 wt%. 

Wu Z, Sun G, Jin W, Hou H, Wang S, Xin Q (2008) Nafion and nano-size Ti02-S04 solid superacid composite membrane for direct methanol fuel cell. J membrane Sci 313 336-343... 

In homogeneous solutions, fluorosulphonic acid combined with antimony pentafluoride is used as a superacid having some useful catalytic properties in organic chemistry. In its protonated form, perfluorosulphonic Nafion powders are used as the catalyst for a variety of organic syntheses. The catalytic power is higher than that of other solid phase superacid catalysts. It enables lower temperatures and pressures to be used and its specificity is higher. Nafion is used as the solid superacid catalyst for the gas phase alkalination of aromatic hydrocarbons and liquid phase esterification or Friedel-Craft reactions ... 

Many acid-catalyzed reactions can be advantageously carried out using solid superacids instead of conventional acid systems. The reactions can be carried out in either the gaseous or the liquid phase. Using the example Nafion-H (a perfluoroalkane resin sulfonic acid, developed by DuPont) solid acid, several simple procedures were reported to carry out alkylation, transbromination, nitration, acetalization, hydration, and so on. 

In discussing superacids as catalysts for chemical reactions, we will review both liquid (Magic Acid, fluoroantimonic acid, etc.) and solid (Nafion-H, etc.) acid-catalyzed reactions, but not those of conventional Friedel-Crafts-type catalysts. The latter reactions have been extensively reviewed elsewhere (see G. A. Olah, Friedel-Crafts Chemistry, Wiley, New York, 1972 G. A. Olah, ed., Friedel-Crafts and Related Reactions, Vols. I-IV, Wiley-Interscience, New York, 1963-1965). 

The conventional resinsulfonic acids such as sulfonated polystyrenes (Dowex-50, Amberlite IR-112, and Permutit Q) are of moderate acidity with limited thermal stability. Therefore, they can be used only to catalyze alkylation of relatively reactive aromatic compounds (like phenol) with alkenes, alcohols, and alkyl halides. Nafion-H, however, has been found to be a suitable superacid catalyst in the 110-190°C temperature range to alkylate benzene with ethylene (vide infra) 16 Furthermore, various solid acid catalysts (ZSM-5, zeolite /3, MCM-22) are applied in industrial ethylbenzene technologies in the vapor phase.177... 

In addition to the discussed Br0nsted or Lewis superacidic activation in solution chemistry, there have been reports to suggest that superelec-trophilic species can be formed with solid acids, and even in biochemical systems. For example, Sommer and co-workers have found several examples in which HUSY zeolite has exhibited catalytic activity similar to liquid superacids (eqs 33-34).12 In the same study, the perfluorinated resinsulfonic acid Nafion-H (SAC-13) was found to give products consistent with the formation of the superelectrophile (36, eq 35). 

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>. 

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