Base catalysis using clays

For a variety of reasons, silica and alumina structures are important frameworks for the subject of heterogeneous catalysis 11-3]. Aluminas, and perhaps to a lesser extent silicas, are employed directly as cracking catalysts or as substrates for assorted catalytic systems (see Chapter 4). The silylation of silica surfaces also provides a strategy for immobilizing a catalytic center that has been found useful in the context of homogeneous catalysis [4], Furthermore, many heterogeneous catalytic systems based on zeolites, clays, or silica-aluminas have aluminosilicate frameworks for which silica and alumina structures serve as structural prototypes. 

Another silicon-based porous solid which has been used in heterogeneous catalysis of organic reactions is silica gel. However, unlike zeolites and clays, silica gel is... 

The use of solid bases as catalysts in organic synthesis is less well-developed than solid-acid catalysis but is becoming increasingly popular [18]. For example, hy-drotalcite anionic clays [19] and mesoporous silicas modified by surface attachment of organic bases [20] are effective and recyclable catalysts for aldol, Knoe-venagel, and related condensations that are widely used in fine chemical synthesis. 

Catalysis by sohd acids and bases is another area of current interest for the industrial manufacture of fine chemicals. In many cases strong mineral or Lewis acids (i. e. H2SO4 or AICI3) which are often used in stochiometric quantities, can be replaced by recyclable solid acids such as zeolites and clays. 

Besides clay-based nanocomposites, there has been huge discussion on the metallic and semiconductor-based hybrid materials. The ability of polymer materials to assemble into nanostructures describes the use of polymers providing exquisite order to nanoparticles. Finally, a discussion on potential applications of polymer—nanoparticle composites with a special focus on the use of dendrite polymers and nanoparticles for catalysis should follow (Polymer-Nanoparticle Composites Part 1 (Nanotechnology), 2010) (Figure 1.15). 

On September 25-30, 1988 in Los Angeles, California the first ACS Symposium on zeolite synthesis emphasized the importance that gel chemistiy, zeolite nucleation, crystal growth, crystallization kinetics, and structure-directing phenomena have in understanding zeolite (and molecular sieve) synthesis. The objectives of a similar ACS Symposium held in New York on August 25-30, 1990 where expanded to include papers on pillared clay synthesis and on the synthesis of other microporous materials that could be used in catalyst preparation. About 90% of all the chemical processes in the U.S. are based on catalysis and today catalysts have become indispensable to petroleum refining, an industry that in 1990 had sales of 140 billion (U.S. Dept, of Commerce U.S. Industrial Outlook, 1991). 

Stable nano/mesoporous materials with mixed oxides such as zeolites, clays, and other minerals are widely used in various fields catalysis, adsorption, ion-exchange, separation, etc. because of their catalytic activity in acid/base and redox (e.g., materials with titania phase) reactions, ability to sorb selective molecules of diverse types, participate in ion-exchange reactions, providing sieve effects, etc. (Tanabe 1970, Grandjean and Laszld 1989, Rocha and Anderson 2000, Cundy and Cox 2005, Tao et al. 2006). The most important processes that utilize the selective properties of these materials are alkylation and isomerization of aromatic hydrocarbons as well as conversion of methanol to hydrocarbons, and some other reactions. Silicalite is an extreme type of the materials with the ZSM-5 zeolite structure but whose aluminum content is negligible. Therefore, unlike conventional zeolites, silicalite does not possess ion-exchange properties, and its surface has a weak affinity to water. 

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