Clay minerals acidity, reaction promotion

It is believed that clay minerals promote organic reactions via an acid catalysis [2a]. They are often activated by doping with transition metals to enrich the number of Lewis-acid sites by cationic exchange [4]. Alternative radical pathways have also been proposed [5] in agreement with the observation that clay-catalyzed Diels-Alder reactions are accelerated in the presence of radical sources [6], Montmorillonite K-10 doped with Fe(III) efficiently catalyzes the Diels-Alder reaction of cyclopentadiene (1) with methyl vinyl ketone at room temperature [7] (Table 4.1). In water the diastereoselectivity is higher than in organic media in the absence of clay the cycloaddition proceeds at a much slower rate. 

One method for studying reactions that occur at the mineral-water interface is to concentrate the interface by drying a clay mineral slurry. Certain reactions are promoted by this process, for example, reactions that respond to Brflnsted acidity (1, 2). Repeated... 

Reactions Promoted by the Lewis and BrRested Acidity of Clay Minerals... 

Table III summarizes the parameters that affect Brrfnsted acid-catalyzed surface reactions. The range of reaction conditions investigated varies widely, from extreme dehydration at high temperatures in studies on the use of clay minerals as industrial catalysts, to fully saturated at ambient temperatures. Table IV lists reactions that have been shown or suggested to be promoted by Br nsted acidity of clay mineral surfaces along with representative examples. Studies have been concerned with the hydrolysis of organophosphate pesticides (70-72), triazines (73), or chemicals which specifically probe neutral, acid-, and base-catalyzed hydrolysis (74). Other reactions have been studied in the context of diagenesis or catagenesis of biological markers (22-24) or of chemical synthesis using clays as the catalysts (34, 36). Mechanistic interpretations of such reactions can be found in the comprehensive review by Solomon and Hawthorne (37).

The dissociation of water coordinated to exchangeable cations of clays results in Brtfnsted acidity. At low moisture content, the Brrfnsted sites may produce extreme acidities at the clay surface-As a result, acid-catalyzed reactions, such as hydrolysis, addition, elimination, and hydrogen exchange, are promoted. Base-catalyzed reactions are inhibited and neutral reactions are not influenced. Metal oxides and primary minerals can promote the oxidative polymerization of some substituted phenols to humic acid-like products, probably through OH radicals formed from the reaction between dissolved oxygen and Fe + sites in silicates. In general, clay minerals promote many of the reactions that also occur in homogenous acid or oxidant solutions. However, rates and selectivity may be different and difficult to predict under environmental conditions. This problem merits further study. 

Rate data for hydrolysis reactions in homogeneous aqueous solutions have been reviewed (79), but application of these data to environmental conditions involving mineral surfaces remains difficult due to the unknown effects sorption may have. Several studies have demonstrated that acid-catalyzed reactions are promoted if the substrate is sorbed at clay surfaces (70-74 and other works reviewed by Theng, 8), but inhibition may also occur if substrate hydrolysis is base-promoted (74). 

Clays are the most universal of all the minerals occurring at the surface of the earth and consequently allow both cheap and environmentally friendly organic chemistry [56, 58]. Natural clays were among the earliest solid acid catdysts used to promote cracking and isomerization reactions in the oil industry [59]. 

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