Solid superacids acidity estimation

Solid acid catalysts such as mixed oxides (chalcides) have been used extensively for many years in the petroleum industry and organic synthesis. Their main advantage compared with liquid acid catalysts is the ease of separation from the reaction mixture, which allows continuous operation, as well as regeneration and reutilization of the catalyst. Furthermore, the heterogeneous solid catalysts can lead to high selectivity or specific activity. Due to the heterogeneity of solid superacids, accurate acidity measurements are difficult to carry out and to interpret. Up until now, the most useful way to estimate the acidity of a solid catalyst is to test its catalytic activity in well-known acid-catalyzed reactions. 

Figure 1.3. Acidity ranges for the most common superacids. The solid and open bars are measured using indicators the broken bar is estimated by kinetic measurements numbers in parentheses indicate mol% Lewis acid.

What makes Nafion so useful for catalytic purposes is the superacidity of the acidic form of the polymers. Because the sulfonic acid group is attached to a highly electron-withdrawing perfluoroalkyl backbone, a relatively high polarization of the -OH bond results. Because methods for measuring solution acidity cannot be directly applied to heterogeneous solid materials, determination of the acidity of Nafion-H is difficult. Nevertheless, studies indicate that the acidic character of Nafion-H resin is comparable with that of 100% sulfuric acid, because the estimated Hammett Hq acidity function value of Nafion-H, approximately —12, is comparable with that of 100% sulfuric acid [1]. 

Besides the above reactions, any kind of acid-catalyzed reactions such as cracking of cumene, alkylation of benzene with propene, hydration of olefins, isomerization of cyclopropane, esterification of acetic acid with ethanol, etc. can be used for the estimation of the acidic property of solid acids. Skeletal isomerization of li-butane to -butane is used to check whether a solid acid has superacidity, since the isomerization is known not to be catalyzed even by 100% sulfuric acid. However, it should be noticed that the differentiation between acid strength and acid amount is not easy from the measurement of catalytic activity for an acid-catalyzed reaction. Characterization of acid catalysts by use of model reactions has been reviewed recently by Guisnet. ... 

Very weakly basic anions like the sulfonates (803 " and S03CF3 ) and the fluoroantimonates (SbF ", Sb2Fn , or Sb3Fig ) allow the stabilisation and study of highly electrophilic cations, both in the respective acid or superacid solutions or in solid, thermally stable compounds. The examples discussed here include main-group cations like a) (CH3)2Sn2+, which is studied by 1 l Sn Mdssbauer spectroscopy and allows the estimation of anion nucleophilicity or b) the molecular cations 02", Br2 , and l2" studied by magnetochemical techniques. 

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