Experimental Techniques Applied to Solid Acids

Since solid acid catalysts are used extensively in chemical industry, particularly in the petroleum field, a reliable method for measuring the acidity of solids would be extremely useful. The main difficulty to start with is that the activity coefficients for solid species are unknown and thus no thermodynamic acidity function can be properly defined. On the other hand, because the solid by definition is heterogeneous, acidic and basic sites can coexist with variable strength. The surface area available for colorimetric determinations may have widely different acidic properties from the bulk material this is especially true for well-structured solids like zeolites. It is also not possible to establish a true acid-base equilibrium. 

Moreover, the accessibility of sites causes discrepancies in the different methods and to measure the acidity of solids. Because several reviews on this subject have been published in recent years,27-29 we will just illustrate this problem with sulfated zirconia (SZ), probably the most studied single catalyst in the last 20 years. 

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. 

Results of temperature-programmed desorption (TPD) of ammonia125 or argon126 were attributed to superacid sites. TPD of very weak bases such as substituted benzenes has been used successfully to compare the superacid character of a series 

More recently, the reactivity of SZ has been assigned to its oxidizing ability,129-131 which should not be surprising because it has often been considered as SO3 adsorbed on zirconium oxide. However, that sulfated zirconia is not only an oxidant but also a strong protic acid has been demonstrated by Sommer, Walspurger, and co-workers132 on the basis H/D exchange experiments with neopentane. 

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