Clay minerals Bronsted acidity

The surfaces of clay minerals can catalyze the polymerization of organic compounds through a free radical-cationic initiation process. This type of reaction is believed to be initiated by the abstraction of an electron by Lewis acid sites on mineral surfaces however, Bronsted acidity has also been shown to be important in certain cases (see Chapter 22). 

Hydrolysis reactions occur by nucleophilic attack at a carbon single bond, involving either the water molecule directly or the hydronium or hydroxyl ion. The most favorable conditions for hydrolysis, e.g. acidic or alkaline solutions, depend on the nature of the bond which is to be cleaved. Mineral surfaces that have Bronsted acidity have been shown to catalyze hydrolysis reactions. Examples of hydrolysis reactions which may be catalyzed by the surfaces of minerals in soils include peptide bond formation by amino acids which are adsorbed on clay mineral surfaces and the degradation of pesticides (see Chapter 22). 

Clay minerals behave like Bronsted acids, donating protons, or as Lewis acids (Sect. 6.3), accepting electron pairs. Catalytic reactions on clay surfaces involve surface Bronsted and Lewis acidity and the hydrolysis of organic molecules, which is affected by the type of clay and the clay-saturating cation involved in the reaction. Dissociation of water molecules coordinated to surface, clay-bound cations contributes to the formation active protons, which is expressed as a Bronsted acidity. This process is affected by the clay hydration status, the polarizing power of the surface bond, and structural cations on mineral colloids (Mortland 1970, 1986). On the other hand, ions such as A1 and Fe, which are exposed at the edge of mineral clay coUoids, induce the formation of Lewis acidity (McBride 1994). 

It is important to distinguish these novel reactions, which take place between the individual sheet silicates and involve the intercalation of one or more of the reactants, from those catalysed by clay mineral surfaces. While details of the mechanism of reactions involving intercalation in sheet silicates are not fully understood, it is generally known that acid sites present in sheet silicates (Bronsted, as well as Lewis) are involved in the reactions. Protonated intermediates such as carbonium or oxonium ions play a crucial role in such reactions. 

The activity of clay minerals, proven in the reactivity of terrestrial (15-16), and postulated in Martian (j ) soils, is disproportionate to their quantity, relative to other minerals. This is the result of several factors small particle size, high specific surface area, Bronsted and Lewis acidity, redox and other potentially catalytically active sites common to clay minerals, and a limited capacity for size exclusion (which is influenced by the number and valence of exchangeable cations ( )). 

The catalytic effects of clays on hydrolysis processes is generally associated with the acidic pH values measured at clay mineral surfaces. Numerous studies have demonstrated that the surface pH of clay minerals can be as much as 2 to 3 units lower than the bulk solution (Mortland, 1970 Bailey et al., 1968 Frenkel, 1974 Karickhoff and Bailey, 1976). The Bronsted acidity of clays arises primarily from the dissociation of water coordinated to exchangeable cations (2.115). 

Bronsted acidity of mineral surfaces arises from proton transfer when exchangeable cations induce dissociation of interlayer water. In general, Bronsted acidity increases with increasing cation chargeiradius ratio. That clay minerals also can behave as strong Bronsted acids is primarily attributable to the presence of acidic exchangeable cations (1,6), such as Ar. Examples of the Bronsted acidity of mineral surfaces include protonation of ammonia and the formation of pyridinium ions upon sorption at acidic Bronsted acid sites (12). 

It has been postulated that the presence of clay in soil promotes degradation of polymers. The hydrolysis would be catalysed by surface Bronsted and Lewis acidities associated with clay minerals [25]. This intriguing hypothesis, which has been developed to explain the behaviour of a specific class of polymers (silicone polymers), could also be extended to other classes of carbon-based polymers. 

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