Crystalline aluminosilicates, example

The FPI principle can also be used to develop thin-film-coating-based chemical sensors. For example, a thin layer of zeolite film has been coated to a cleaved endface of a single-mode fiber to form a low-finesse FPI sensor for chemical detection. Zeolite presents a group of crystalline aluminosilicate materials with uniform subnanometer or nanometer scale pores. Traditionally, porous zeolite materials have been used as adsorbents, catalysts, and molecular sieves for molecular or ionic separation, electrode modification, and selectivity enhancement for chemical sensors. Recently, it has been revealed that zeolites possess a unique combination of chemical and optical properties. When properly integrated with a photonic device, these unique properties may be fully utilized to develop miniaturized optical chemical sensors with high sensitivity and potentially high selectivity for various in situ monitoring applications. 

C at pressures of about 250—400 kPa (36—58 psi). The two types of catalysts, the amorphous silica—alumina (52) and the crystalline aluminosilicates called molecular sieves or zeolites (53), exhibit strong carboniumion activity. Although there are natural zeolites, over 100 synthetic zeolites have been synthesized and characterized (54). Many of these synthetic zeolites have replaced alumina with other metal oxides to vary catalyst acidity to effect different type catalytic reactions, for example, isomerization. Zeolite catalysts strongly promote carboniumion cracking along with isomerization, disproportionation, cyclization, and proton transfer reactions. Because butylene yields depend on the catalyst and process conditions, Table 7 shows only approximations. 

More extensive discussions for several reaction types is provided by Germain [14], Most catalytic cracking today utilizes zeolite catalysts. These are crystalline aluminosilicates that contain cages, often of molecular dimensions, that can physically block, branched chain molecules, for example (often called molecular sieves). Some of the above ideas undoubtedly apply, but the prediction of the selectivity is now much more complicated. (They are also much more active as catalysts.) Some aspects of their properties are reviewed by Venuto [15], where more than 50 different reactions catalyzed by zeolites are listed. 

Because the accelerating effect of the solid catalyst comes from the surface atoms, it is desirable to have the largest surface possible, meaning a high degree of dispersion of the substance in question. In most cases, very small particles of catalytically active material such as platinum or rhodium are applied for stability to highly porous carrier materials with specific surfaces of several hundred square meters per gram. Aluminum oxide, silicon dioxide, activated carbon, as well as zeolites (crystalline aluminosilicates with numerous submicroscopic pores and canals) are all suitable for this purpose. Examples of such supported catalysts are... 

Zeolite membranes indicate inorganic membranes with a selective/cata-lytic layer composed of a zeolite which is crystalline aluminosilicate with the feature of a high ordered porous structure with size comparable to molecular dimension. An example of the use of zeolites as a catalyst in a multi-phase membrane reactor can be found in Shukla and Kumar (2004) who have immobilized a lipase on a zeolite-clay composite membrane by using glu-taraldehyde as a bifunctional ligand in order to carry out the hydrolysis of olive oil. An application of a zeolite-based membrane in a three-phase membrane reactor has been reported by Wu et al. (1998), where TS-1 zeoUte crystallites were embedded in a polydimethylsiloxane (PDMS) membrane in order to catalyse the oxyfunctionalization of n-hexane (from a gas phase) with hydrogen peroxide (from a liquid phase). 

The most widely used catalysts for acid-catalyzed aldol condensations are the molecular sieve zeolites, for example, crystalline aluminosilicates of group I and II elements, in which the latter have been replaced by protons. The surface protons confer Br0nsted acidity. Among the acidic zeolites we can mention HZSM-5 (pentasil zeolite), HY (faujasite), or HM (mordenite). Recently, polystyrene-supported sulfonic acids such as those of the macroreticular strongly acidic cation-exchange resins (59) and acid-base functionalized mesoporous materials such as amine and sulfonic acid-containing SBA-15 (60) have been shown to promote the acid-catalyzed aldol condensation of aldehydes with ketones at low temperatures. 

Clays are finely divided crystalline aluminosilicates. Swelling clays (e.g. montmorillonite) are widely used as thickening agents in the formulation of, for example,... 

The most important class of inorganic adsorbents is the zeolites, a subclass of molecular sieves. These are crystalline aluminosilicates with specific pore sizes located within small crystals. Two common classes have simple cubic crystals (type A) or body-centered cubic crystals (type X). Sometimes, the type is assigned a number equal to a nominal pore size in the crystals. For example, zeolite 5A with a nominal 5 A pore size is used to separate normal from branched paraffins. 

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