Clay properties Lewis base

The presence of Lewis and Bronsted acid sites gives rise to a variety of chemical reactions. Amines, or other Lewis bases, readily adsorb and the use of fatty amines to render the clay organophilic has been applied for many years to modify properties in a number of applications [19,20]. Because of these reactive sites, kaolin will enter into organic reactions and, of particular interest for plastics, will catalyse the polymerisation of certain monomers. Sometimes depolymerisation can occur and it can promote the dehydrochlorination reaction of PVC [21]. 

Mesoporous solids including silicas and acid-treated clays can be functionalised at their surfaces so as to provide high local concentrations of active sites. These sites can be introduced by post-modification or via sol-gel preparations. In this way a range of novel materials with useful catalytic and other properties can be prepared. One of the most valuable applications for these materials is as replacements for environmentally hazardous reagents including corrosive mineral and Lewis acids, caustic bases and toxic metallic compounds. 

Mg-Al mixed oxides obtained by thermal decomposition of anionic clays of hydrotalcite structure, present acidic or basic surface properties depending on their chemical composition [1]. These materials contain the metal components in close interaction thereby promoting bifunctional reactions that are catalyzed by Bronsted base-Lewis acid pairs. Among others, hydrotalcite-derived mixed oxides promote aldol condensations [2], alkylations [3] and alcohol eliminations reactions [1]. In particular, we have reported that Mg-Al mixed oxides efficiently catalyze the gas-phase self-condensation of acetone to a,P-unsaturated ketones such as mesityl oxides and isophorone [4]. Unfortunately, in coupling reactions like aldol condensations, basic catalysts are often deactivated either by the presence of byproducts such as water in the gas phase or by coke build up through secondary side reactions. Deactivation has traditionally limited the potential of solid basic catalysts to replace environmentally problematic and corrosive liquid bases. However, few works in the literature deal with the deactivation of solid bases under reaction conditions. Studies relating the concerted and sequential pathways required in the deactivation mechanism with the acid-base properties of the catalyst surface are specially lacking. 

For cracking reactions, combinations of zeolites, alumina, clay, and silica are used as the catalyst. These acidic materials, which contain both Br0nstead and Lewis acidic sites, initiate a complex set of carbonium- and carbenium ion-based reactions. Note that carbonium ions are protonated alkyl groups (e.g., C Hg ), while carbenium ions refer to alkyl cations (e.g., To enhance the acidic properties, rare... 

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