Solid superacids

The acid-base character of solids was studied very early by Tanabe s group31,32 and was first described in a landmark volume.33 

Solid superacids can be further divided into various groups depending on the nature of the acid sites. The acidity may be a property of the solid as part of its chemical structure (possessing Lewis or Brpnsted sites the acidity of the latter can be further enhanced by complexing with Lewis acids). Solid superacids can also be obtained by deposition on or intercalation of strong acids into an otherwise inert or low-acidity support. 

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

Enhanced acidity solids including Brpnsted and Lewis acid-modified metal oxides and mixed oxides, as well as metal salts complexed with Lewis acids. 

As with previous classifications, these are also arbitrary and are suggested for practical discussion of an otherwise rather complex field. The superacid character of 

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Acids are not limited to liquid (or gaseous) systems. Solid acids also play a significant role. Acidic oxides such as silica, silica-alumina, etc. are used extensively as solid acid catalysts. New solid acid systems that are stronger than those used conventionally are frequently called solid superacids. 

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. 

In these (and other) solid superacid catalyst systems, bi- or multi-dentate interactions are thns possible, forming highly reactive intermediates. This amounts to the solid-state equivalent of protosolvation resulting in superelectrophilic activation. 

Solid Superacids. Most large-scale petrochemical and chemical industrial processes ate preferably done, whenever possible, over soHd catalysts. SoHd acid systems have been developed with considerably higher acidity than those of acidic oxides. Graphite-intercalated AlCl is an effective sohd Friedel-Crafts catalyst but loses catalytic activity because of partial hydrolysis and leaching of the Lewis acid halide from the graphite. Aluminum chloride can also be complexed to sulfonate polystyrene resins but again the stabiUty of the catalyst is limited. 

Solid superacid catalysts, proposed as replacements for catalysts such as hydrogen fluoride and aluminum chloride for processes such as alkylation and acylation (Misono and Okuhara, 1993). 

Misono, M., and T. Okuhara (1993). Solid Superacid Catalysts. Chemtech, November, 23-29. 

Proc. 13th Intern. Conf. Catal. ( New Trends in Solid Superacids and Superbases ,... 

T. Ishida, T. Yamaguchi, and K. Tanabe, Acid property of sulfur-promoted zirconium-oxide on silica as solid superacid, Chem. Lett. 1869—1872 (1988). 

These reactions can also be performed over a strong acid catalyst at reaction temperatures that are lower than over zeolites. Because of this, isomerization of M-butane over Zr02-supported sulfate catalysts was initially proposed by Hino and Arata. They proposed these catalysts as being effective in butane isomerization at room temperature, a reaction that does not take place, even in 100% sulphuric acid. For this reason, these catalysts were considered as solid superacids, since they are active and selective in the isomerization of n-butane to isobutane at... 

For a monograph, sec Olah Prakash Sommer Superacids Wiley New York, 1985. For a review, sec Gillespie Peel Adv. Phys. Org. Chem. 1971, 9, 1-24. For a review of solid superacids, see Arata Adv. fatal. 1990,37. 165-211. For a review of methods of measuring superacidity, see Jost Sommer Rev. Chem. Interned. 1988, 9, 171-199. 

Acylium ions can be formed in superacid solutions from carboxylic acids and acyl halides (8). They are among the best characterized carbenium ions, and single-crystal X-ray structures of a number of them have been determined as BFf, SbFg, or TaClfi salts (135-139). Solid-state NMR characterization of these species on AlBr3 and other solid superacids was described earlier in this review. 

Improved yields of adamantane up to 65% were achieved by using superacidic systems [HF-SbF5, CF3S020H-B(0S02CF3)3, various solid superacids]40"42 and chlorinated Pt on alumina.43 More recently near-quantitative isomerization with conjugate superacids promoted by 1-haloadamantanes as the source of 1-ada-mantyl cation and sonication was reported.44... 

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... 

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. 

Halogenation. Fluorination, chlorination, and bromination of alkanes catalyzed by superacids have been reported.1,2 Reactions may be carried out in the liquid phase, or in the gas phase over solid superacids or supported noble metal catalysts. High selectivity and relatively mild reaction conditions are the main features of these transformations. 

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... 

Solid superacids may be made by treating ordinary solid add catalysts with strong Br0nsted or Lewis acids. For example, if freshly precipitated titanium hydroxide or zirconium hydroxide is treated with sulfuric acid and calcined in air at 500 °C. a very active solid acid catalyst results. The solids consist mainly of the metal dioxides with sulfate ions coordinated to the metal ions on the surface. Likewise, a superacid solid catalyst can be made by treating these metal oxides with antimony penlafluonde. Both catalysts contain both Br nsted and Lewis acid sites, and they arc sufficiently active to catalyze the isomerization of n-butane at room temperature.26... 

Hull and Conant in 1927 showed that weak organic bases (ketones and aldehydes) will form salts with perchloric acid in nonaqueous solvents. This results from the ability of perchlonc aad in nonaqueous systems to protonate these weak bases. These early investigators called such a system a superacid. Some authorities believe that any protic acid that is stronger than sulfunc aad (100%) should be typed as a superaad. Based upon this criterion, fluorosulfuric arid and trifluoro-methanesulfonic acid, among others, are so classified. Acidic oxides (silica and silica-aluminai have been used as solid acid catalysts for many years. Within the last few years, solid acid systems of considerably greater strength have been developed and can he classified as solid superacids. 

T. Yamaguchi, T. Jin, T. Ishida, K. Tanabe, Structural identification of acid sites of sulfur promoted solid superacid and construction of its structure on Silica Support, Mat.Chem.Phy., 17 (1987) 3-19. 

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. 

Fig. 18. Acid strengths of liquid and solid superacids. (From Ref. 128.)...

On the basis of its catalytic activity in isobutane conversion, this catalyst was described as zirconia-supported oleum,117 but this hypothesis implies that the reaction mechanism is known, which is not the case. On the same basis, Fraenkel118 suggested also that SZ was a very strong solid superacid. 

It appears that despite the lack of reliability of acidity determination of solid acids by spectroscopic means and in the absence of knowledge of the nature of the initial step in alkane activation by solid acids the qualification of superacid solids has been and continues to be used, despite the absence of a clear definition of solid superacidity. 

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