Definition and Classification of Solid Acids and Bases

In general terms, a solid acid may be understood to be a solid on which the color of a basic indicator changes or a solid on which a base is chemically adsorbed. More strictly, following both the Bronsted and Lewis definitions, a solid acid shows a tendency to donate a proton or to accept an electron pair, whereas a solid base tends to accept a proton or to donate an electron pair. These definitions are adequate for an understanding of the acid-base phenomena shown by various solids, and are convenient for a clear description of solid acid and base catalysis.

In accordance with the above definitions, a summarized list of solid acids and bases is given in Tables 1.1 and 1.2, The first group of solid acids in Table 1.1 includes naturally occurring clay minerals. The main constituents are silica and alumina. Various types of synthetic zeolites such as zeolites X,Y,A, ZMS-5, ZSM-11, etc. have been reported to show characteristic catalytic activities and selectivities. The well-known solid acid, synthetic silica-alumina, is listed in the seventh group, which also includes the many oxide mixtures which have recently been found to display acidic properties and catalytic activity. In the fifth and sixth groups are included many inorganic chemicals such as metal oxides, sulfides, sulfates, nitrates, phosphates and halides. Many have been found to show characteristic selectivities as catalysts.

Of the solid bases listed in Table 1.2, special mention should be made of the alkaline earth metal oxides in the fourth group and mixed metal oxides in the sixth group, whose basic properties and catalytic action have been recently found to be striking and interesting. A solid superacid is defined as a solid whose acid strength is higher than the acid strength of 100% sulfuric acid. Since the acid strength of 100% sulfuric acid expressed by the Hammett acidity function, Ho, is - 11.9, a solid of Ho < - 11.9 is called a solid superacid. The kinds of solid superacids are shown in Table 1.3. The groups 1 through 6 include acids supported on various solids.

On the other hand, a solid superbase is defined as a solid whose base strength expressed by the basicity function, H-, is higher than + 26. The basis of the definition has been described in the literature.') The kinds of solid superbases are shown in Table 1.4 together with their preparation method and pretreatment temperature.

REFERENCES

1. K. Tanabe, in: Catabsis by Acids andBases, (eds. B. Imelik, C. Naccache, C. Coudurier, Y. Ben Taarit, J. C. Vedrine) Elsevier, Amsterdam, 1985, p.1.

Terms: Condiciones

May: Puede

Understood: Entendido

Following: Siguiente

Both: Ambos

Whereas: Mientras

Phenomena: Fenómenos

Shows: Mostrar

Naturally: Naturalmente

Clay: Arcilla

Synthetic zeolites:  Zeolitas sintéticas

Such:  Tal

Well-known:  Bien conocido

Mixed: Mixto

Striking:  Llamativo

Strength: Fuerza

higher:  Superior

kinds:   Tipos