Acid Nafion

Perfluonnated Resinsulfonic Acid (Nafion H) Catalysis in Synthesis Olah, G A Iyer P S, Prakash, G K S Synthesis 513-531 181... 

Perfluorinated Resinsulfonic Acid (Nafion-H) Catalysis in Synthesis ... 

Polymeric resin sulfonic acids including sulfonic acid resins complexed with Lewis acids and perfluorinated polymer resin acids (Nafion-H and Nafion-silica nanocomposites). 

Dehydration of alcohol to alkenes usually does not require superacidic conditions. Surprisingly, however, perfluroalkylcyclopentanols could not be dehydrated with para-toluenesulfonic acid, but alkenes could be isolated with the use of the stronger acid Nafion-H albeit in low yields737 [Eq. (5.282)]. 

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). 

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. 

Yeo, R.S., Dual cohesive energy densities of perfluorosulphonic acid (Nafion) membrane, Polymer, 21, 432, 1980. 

Hasegawa and Higashimura showed that perfluorinated resinsulphonic acid , Nafion-H, is an effective catalyst for the oligomerisation of styrene. 

Nafion is another choice of polymer support for Sc-based Lewis acids. Nafion-Sc catalyst is readily prepared by treatment of Nafion with ScCb 6H2O in acetonitrile under reflux [116]. Nafion-Sc catalyst has been found to be effective in several synthetic reactions including allylation of carbonyl compounds with tetraallyltin, Diels-Alder reaction, Friedel-Crafts acylation, and imino Diels-Alder reactions. The use of Nafion-Sc in flow systems has also been tested. 

Not only do these acids present an economic advantage over conventional acid technology, but there are also frequent selectivity eidiancements and changes in reactivity associated with the use of these heterogeneous catalysts. A number of different types of solid acids have been involved in synthetically useful reactions. They range from the hydrogen forms of various ion exchange resins and the perflourinated resin sulfonic acid, Nafion-H, to the amorphous acidic oxides, silica and aluminum silicate,the crystalline zeolites > and the natural clays. -9... 

The sulfonic acid resins such as Dowex-50 and Amberlyst-15 have been used to promote the alkylation of the more active aromatic rings but attempts to increase their acidity generally resulted in the degradation of the solid. 2 The more strongly acidic perfluorinated resin sulfonic acid, Nafion-H,2>3 has, however, been used to promote the alkylation of benzene and other aromatic compounds. Nafion-H catalyzed the vapor phase reaction between toluene and methanol. When nm at 185°C a 12% yield of the isomeric xylenes was obtained with the ortho isomer the major product. 0 Methylation of phenol at 205°C over this catalyst gave, at 63% conversion. 37% anisole and 10% of a mixture of the ortho and para cresols in a 2 1 ratio. Reaction of anisole with methanol under these conditions resulted in a 14% selectivity to the methyl anisoles at 40% conversion, with the ortho and para isomers formed in nearly equal amounts. ... 

Huorinated resinesulphonic acid (nafion-H). U provides an exceptionally clean way of preparing nitroaromatic compounds. Nitro compounds can be isolated by a simple filtration of the catalyst, without the need of any aqueous basic washing or workup. AU by-products are removed during the reaction, as they are volatile. By way of azeotropic removal of water, the nitration can be brought to a very high yield. 

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., Rung, A. P. Synthetic methods and reactions 98. Improved solid super acid (Nafion-H) catalyzed Rupe rearrangement of a-ethynyl alcohols to a,P-unsaturated carbonyl compounds. Synfhes/s 1981,473-474. 

To minimize the indane, 99, formation, dimerization was conducted in two-phase systems containing toluenesulfonic acid,354 sulfuric acid,355 356 electrophilic transition-metal complexes,357 the polymeric solid-state acid Nafion,358 359 metal oxide solid-state catalysts such as tungstophosphoric acid,360 various zeolites,361 362 mixed oxides,363 and montmorillonite clay in the presence of organic solvents.364 365 The major limitation of the cationic approach, however, is the unavoidable formation of internal isomer 100. Since isomer 100 is inert in radical polymerization, the lower the content of isomer 100, the higher activity of the 98 mixture. Even in the very best cases, its presence is never less than 5—15%. 

Figure 18. Typical underwater stress relaxation results at various temperatures for Naflon acid, Nafion-Na, and Naflon-Ba. Reproduced with permission from Ref. 47. Copyright 1984 John Wiley Sons, Inc.

The synthesis of FITS reagents poses two principal difficulties (1) isolation of the FITS reagent from the reaction mixture often requires repeated yield-reducing recrystallization steps and (2) recovery of the expensive triflic acid obtained during the synthesis is difficult. A solution to these problems was found in the immobilization of the reagents [23] by means of a solid analog of triflic acid - Nafion-H resin [24] (Scheme 2.151). 

The perfluorinated sulfonic acid (Nafion) membranes have found a great variety of electrochemical applications. These include the SPE water electrolyzers, alkaline water elec-trolyzers, " hydrochloric acid electrolyzers, Na2S04... 

Figure 2. Neutron diffraction spectra of acid Nafion membrane for wet and dr samples. Instrument is D 1B. Incident wave-length is Ao = 2.56 A. The horizontal axis is expressed either in scattering angles (20) units, or in neutron momentum transfer (Q = tt/Xo stn0) units. Note the different vertical scales for the two samples. The separation between the amorphous and crystalline contributions to the large angle peak is shown for the dry sample. Temperature 25°C.

Figure 2. Room temperature sorption-desorption isotherms of acid Nafion. Key , room temperature dried absorption , 220° C dried primary absorption , 220° C dried desorption Q, 220° C dried secondary absorption.

Figure 7. Influence of the heating speed on the acid Nafion polymer containing 12%...

A CPC 600 calorimeter (15) has been used with heating or cooling speeds between 0.5 and 4°/mn. Acid Nafions 120 containing different water percentages have been studied. 

Another interesting result is given in fig. 13 in which is plotted the amount of desorbed water versus the annealing temperature for the 15 % H2O - acid Nafion system. From this figure it is shown that around 60 % of the total water content can be desorbed. We therefore have around 9 % desorbable water and around 7 % fixed water. These values are in close agreement with the two absorption regimes and also with the fact that no DSC peak has been observed for water content velow 8 % water. 

Figure 13, Examples of neutron quasielastic spectra of acid Nafion membrane for...

Figure 15. Half width at half maximum of the broadened component of the neutron quasielastic spectra obtained with an acid Nafion membrane containing 15% H20 by weight. The points are the widths of the best fit Lorentzian lines from spectra obtained with an incident wavelength of 10 A (Q), 11 A and 13 A (A). The full line is the theoretical width predicted by the model with diffusion in a sphere (with D = 1.8 X 10 5 cm2/s and a = 4.25 A). The two theoretical asymptotes for Q O and Q 00 are also shown (compare with Figure 2 of Reference 2 for more details). Half width at half maximum of the best fit Lorentzian lines to the spectra obtained with bulk water at 28° C (incident wavelength 10 A) is denoted by +. The straight line passing through the points (+) is the theoretical width predicted by the simple self diffusion model with Dt = 2.5 X 10-5 err /s. Note the different vertical scales for the Nafion sample and the bulk water sample.

The perfluorosulfonic acid (Nafion) membrane found its application in fuel cells long before its introduction to the chlor-alkali industry (26-28). The Nafion membrane is used as the solid polymer electrolyte (separator/electrolyte) in fuel cells. Figure 2 shows the schematic of such an SPE fuel cell. 

Figure 1. Water loss on heating an acid Nafion sample (at 3°C/min) under 10 t torr to 220°C C+, after 30 sec at 220°C)...

Figure 2. Sorption-desorption curves obtained after drying an acid Nafion sample at different temperatures the humidity fP/Po ...

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