Clay active sites exchangeable cations

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. 

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 ( )). 

Both Lewis and Brdnsted acid sites exist on pillared clays. The acidity depends on the exchanged cations, the preparation method and the starting clay [8], It is known that surface acidity is important for SCR reaction of NO by NH3 [4, 5, 9]. Different pillared clays were synthesized and tested for their activities in the SCR NO [10, 11]. Research on titanium pillared clays was initiated by Sterte [12], who first reported the synthesis of titanium pillared montmorillonite using TiCU solution in hydrochloric acid. Bernier et al. 

As sulfur-containing organic molecules are known catalyst poisons because of strong adsorption, acylation of thioethers is difficult to obtain. The reaction between thioanisole and AAN can, however, be performed in the presence of ion-exchange resins that are more robust to deactiva-tion. The process is carried out in a Parr autoclave in 1,2-dichloro-ethane at 70°C. Under these conditions, other acid catalysts such as sulfated zirconia and KIO clay do not show any noticeable activity. Only the cation exchange resin catalysts, which contain Bronsted sites, are effective. Among these, Amberlyst-15 shows maximum conversion because it... 

The oxidation of various sulfides 424 with iodosylbenzene in the presence of catalytic amounts of quaternary ammonium bromides affords the respective sulfoxides 425 in high yields (Scheme 3.171) [521]. The best catalytic effect in this reaction is observed when oxidation is carried out in a nonpolar solvent (toluene, hexane, dichloromethane) in the presence of trace amounts of water and 10 mol% of cetyltrimethylammonium bromide (CTAB). Iodosylbenzene can also be activated in the solid state by pulverization with natural clays or silica gels [522, 523], The oxidation of various alkyl aryl sulfides with (PhIO) supported on natural (montmorillonite, KSF and bentonite clays) as well as cation-exchanged KlO-montmorillonite clays affords sulfoxides in excellent yields. A mechanism involving depolymerization of (PhIO) by the acidic SiOH sites on the clay is proposed for this reaction [522], Organic sulfides are also selectively oxidized to sulfoxides by the solid reagent system PhI(OAc)2-alumina [524], or by Phl(OAc)2 in water in the presence ofKBr [525],... 

Because of their large surface-to-volume ratio and high metabolic activity, microorganisms are important vectors in the introduction of heavy metal and radionuclide pollutants into food webs. As discussed in Chapter 5, heavy metals in soils and sediments tend to be immobilized by precipitation at neutral to alkaline pH and/or adsorption to cation exchange sites of clay minerals. Microbial production of acid and chelating agents can reverse this adsorption and mobilize toxic metals. Microbial metabolism products that can chelate metals include... 

Na", K+) in the interlayer region are replaced (exchanged) by more acidic metal cations (Al+, Fe+, Fe+, Mg ) leached from the octahedral layer, and 2) surface area and porosity are increased by the dissolution process that opens up previously inaccessible sites within the clay structure. These consequences, discussed in more detail in Section 4.4.3, constitute the key aspects of the acid-activation process. 

The enhancement in catalytic activity of cations such as Zn(II) which has been achieved both through ion exchange as well as deposition of Zn(II) salts onto clay surfaces led to studies of the acidity and catalytic activity of such ions when incorporated directly into the lattice sites of synthetic clay minerals. Luca et al. showed that Lewis acid sites are generated on Zn2+-substituted fluoro-hectorite.27 The Zn2+-substituted fluorohectorite was synthesised by a sol-gel route. The sol was allowed to crystallise in a Parr autoclave at 250 °C for 24 hours. The Lewis acid sites were identified as Zn2+ at the edges of the fluorohectorite crystallites and were active towards the Friedel-Crafts alkylation of benzene with benzyl chloride. 

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