Solid superacids acid strength

Several metal oxides could be used as acid catalysts, although zeolites and zeo-types are mainly preferred as an alternative to liquid acids (Figure 13.1). This is a consequence of the possibility of tuning the acidity of microporous materials as well as the shape selectivity observed with zeolites that have favored their use in new catalytic processes. However, a solid with similar or higher acid strength than 100% sulfuric acid (the so-called superacid materials) could be preferred in some processes. From these solid catalysts, nation, heteropolyoxometalates, or sulfated metal oxides have been extensively studied in the last ten years (Figure 13.2). Their so-called superacid character has favored their use in a large number of acid reactions alkane isomerization, alkylation of isobutene, or aromatic hydrocarbons with olefins, acylation, nitrations, and so forth. 

Figure 13.2 Acid strength of acid and superacid solids determined by Hammet method. For a comparable purpose, the acid strengths of some liquid acids are also included. (After Ref. 12.)...

In the case of C4-hydrocarbons, the use of acid or superacid solids will depend on both the acid strength required in each reaction and the reaction conditions required to optimize the thermodynamic equilibrium (Figure 13.3). For example, catalysts with very high acid strength could be substituted for a solid with a lower acidity by increasing reaction temperature. This has been proposed in both the isomerization of lineal alkanes and in the alkylation of isobutene with olefins, although the thermodynamic equilibrium should also be considered. 

In conclusion, more efficient and clean solid (acid and superacid) catalysts will be used in the coming years to reduce not only the emission of environmentally harmful products but also the use of noxious catalysts. The optimal catalytic systems will be determined from the nature of acid strength of its active sites, the nature of the reaction, and the reaction conditions. 

In addition, a new physicochemical method to characterize the acid strength of acid and superacid solids also will be required in order to evaluate and compare acid strengths with those obtained with liquid acids. 

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. 

As excellent candidates for design at the atomic or molecular level, heteropoly catalysts have proven to be of value in fundamental studies as well as practical applications. But it is also true that much remains to be done. Efforts to establish methodologies for design of practical catalysts are still under way. The acid strength and acid site density can be controlled quite well both in solution and in the solid state, but the redox properties in the solid state are much less well understood because of the lack of sufficient thermal stability of mixed-metal (mixed-addenda) heteropolyanions. The acid strengths of some solid heteropolyacids have been suggested to reach the range of superacids, but they... 

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

The acidity strength of solid acids is still not well known and is difficult to measure. Claims of superacidity in solids are numerous and will be discussed later in Chapter 2. Among the reviews related to acidity characterization of solids, those of Corma, Farneth and Gorte, and Fripiat and Dumesic are quite significantly representative.30... 

Subsequently, the same authors138 described the preparation of a solid superacid catalyst with acid strength of H0 = —16 with a sulfuric acid-treated zirconium oxide. They exposed Zr(OH)4 to 1A sulfuric acid and calcined it in air at approximately 600°C. The obtained catalyst was able to isomerize (and crack) butane at room temperature. The acidity was examined by the color change method using Hammett indicators added to a powdered sample placed in sulfuryl chloride. The... 

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>. 

Table 3. Acid strength of supported solid superacid (SO3/Z1O2/ Si02).

TPD using base adsorbents such as ammonia and pyridine has been one of the common techniques to evaluate the amount and strength of acid sites on solid acids. However, the desorption temperature of ammonia and pyridine adsorbed on solid superacids is so high (500 °C and above) owing to the strong interaction that the adsorbed compounds are decomposed or oxidized before desorption. The surface structure can even be destroyed by reactions with probe molecules [120-122]. 

A new method has involved the use of Ar as a probe atom. We tried Ar as a probe TPD and found that Ar-TPD could be applied to the evaluation of the acid strength of solid superacids. 

Chemically inert, solid acid catalysts that have strong and even superacid characters are needed. The role and the specific mechanism of protic and Lewis acid site interactions must be elucidated by both theoretical modeling and experimentation. Based on an analogy with the chemistry of molecular acids, the interacting H+ Lewis-acid system offers the best chance to achieve high acid strengths. 

To begin we would like to define what we mean when we refer to a very strong base. It is appropriate to compare the notion of a very strong base with that of a very strong acid, a superacid. Superacids in solution or in the solid state are so strongly acidic that they are capable of protonating a hydrocarbon molecule their acid strength Hq < — 17. 

Many types of solid acid and base catalysts are known.11 Superacids are those that are at least as strong as 100% sulfuric acid.12 The acid strengths are measured using basic indicators and are assigned a Hammett acidity function, H0- Table 6.1 lists some superacids, with the strongest at the top. 

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