Clays in Organic Synthesis – Preparation and Catalytic Applications

Clays are extraordinary layered materials whose structure is composed of stacked inorganic metal oxide layers built by connected sheets. These layers hold usually charge excess or deficiency which is balanced by the presence of either cations (cationic clays) or anions (anionic clays) at the interlayer spacing. By taking advantage of this relevant property, many works have been dedicated to explore this by introducing cations or anions inside the interlayer spacing of clays. This includes exchange from simple cations to complex structures such as large polyoxoions or even metal complexes. The resulting tuned materials find wide use in several catalytic applications with the advantage that in some cases the heterogeneous process overcomes the homogeneous one. Important chemical reactions such as the Suzuki and Heck coupling reactions or other such as Friedel-Crafts, Michael addition and Diels- Alder reactions can be successfully catalyzed by such materials. Oxidation reactions including epoxidation of olefins and oxidation of alcohols to carbonyl compounds are also important chemical processes where clays can be effective catalysts. From the economical point of view such catalysts feature high synthetic flexibility, tunable acid-base properties and ease of set-up and work-up, among others. These properties lead to the development of more efficient catalytic systems with improved yields and selectivity, which are valuable achievements in the present days by contributing towards the establishment of environmentally friendly technologies.