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
- 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
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