Zeolites specific

The determination of the specific surface area of a zeolite is not trivial. Providers of zeolites typically give surface areas for their products, which were calculated from gas adsorption measurements applying the Brunauer-Emmet-Teller (BET) method. The BET method is based on a model assuming the successive formation of several layers of gas molecules on a given surface (multilayer adsorption). The specific surface area is then calculated from the amount of adsorbed molecules in the first layer. The space occupied by one adsorbed molecule is multiplied by the number of molecules, thus resulting in an area, which is assumed to be the best estimate for the surface area of the solid. The BET method provides a tool to calculate the number of molecules in the first layer. Unfortunately, it is based on a model assuming multilayer formation. Yet, the formation of multilayers is impossible in the narrow pores of zeolites. Specific surface areas of zeolites calculated by the BET method (often termed BET surface area) are therefore erroneous and should not be mistaken as the real surface areas of a material. Such numbers are more related to the pore volume of a zeolite rather than to their surface areas. 

Aluminosilicates, see Organic syntheses, using aluminosilicates Zeolites specific compounds... 

Very often the liquids to be processed may be contaminated with substances detrimental to some types of zeolites consequently a complete knowledge of the process stream composition and physical properties must be available before preliminary sieve selection can be made. In the absence of prior knowledge of separation factors, competitive co-adsorption, environmental stability, regeneration techniques, or irreversible zeolite contamination, zeoli te contamination, zeolite specification must be proceded by time-con-... 

The p-jump technique has been applied to metal-soil constituent reactions and to ion exchange kinetics on zeolites. Specifics of these studies are... 

Even in the absence of Lewis acid functions, zeolites can accelerate gas phase Diels-Alder reactions. This rate enhancement, for instance in the butadiene cyclodimerization, is attributed to a concentration effect inside the zeolite pores. The effect is however not zeolite-specific any adsorbent with affinity for dienes, such as a carbon molecular sieve, displays similar effects (5). 

Among the various conversions discussed in this section, deprotonation is clearly a general reaction of alkane radical cations. The interesting elimination or fragmentation reactions, on the other hand, seem to be zeolite-specific reactions without precedent in halogen containing matrices. 

In this context, we mention two zeolite-induced conversions of cyclopropane derivatives. Incorporation of tra -l,2-diphenylcyclopropane trans-Vi) and its 3,3-D2-isotopomer into the channels of a redox-active pentasil zeolite (Na-ZSM-5) generated exo,exo-l,3-diphenylallyl radical (24 ) and its 2-Di-isotopomer. This conversion is a zeolite-specific reaction it requires a series of reactions, including oxidation, ring opening, and deprotonation [70]. 

Catalytic dewaxing, developed in the 1970s by Mobil s immensely successful research program into zeolites, specifically ZSM-5 for this application, cracks paraffins and paraffinic groups in wax into light hydrocarbons and is applicable to the entire lube slate. 

Zeolite H-T catalyzed the ketonization of short-chain carboxylic acids. The formation of anhydrides is a side reaction, occurring on the outer surface of the zeolite ctystals. The propionic and butyric acid molecules seem to have the optimum size for a bimolecular ketonization reaction inside an erionite cavity. The ketonization of carboxylic acids is an example of zeolite specificity in catalysis, illustrating the necessity of strict adaptation of the transition state of the reaction to the intracrystalline porosity of the zeolite. 

Although spectroscopic measurements such as UV/visible spectroscopy and solid state MAS NMR can provide valuable information about clusters in zeolites, specific information relating to cluster nuclearity and charge is in general not available without recourse to theoretical calculations. To date few, if any, ionic clusters have been positively identified in this way. 

Activation of zeolites is a dehydration process aceomplished by the application of heat in a high vacuum. Some zeolite crystals show behavior opposite to that of activated carbon in that they selectively adsorb water in the presence of nonpolar solvents. Zeolites can be made to have specifie pore sizes that will increase their seleetive nature due to the size and orientation of the molecules to be adsorbed. Moleeules above a specific size could not enter the pores and therefore would not be adsorbed. 

Base exchange The property of the trading of cations shown by certain in- soluble namrally occurring materials (zeolites) and developed to a high degree of specificity and efficiency in synthetic resin adsorbents. 

Faujasite is a naturally occurring mineral, having a specific crystalline, alumina-silicate structure, used in the manufacturing of the FCC catalyst. Zeolite faujasite is a synthetic form of the mineral. 

Today n-paraffms are exclusively produced from the corresponding distillation cuts of paraffin-rich oils with the use of molecular sieves. Molecular sieves are synthetically manufactured aluminum silicates of the zeolite type, which after dehydration have hollow spaces of specific diameters with openings of specific diameters. The molecules are then able to penetrate the openings in the correct size and form and are held in the hollow spaces by electrostatic or van der Waals forces. The diameter of the zeolite type used for the production of paraffins is 5 A and is refined so that the n-paraffins (C5-C24) can penetrate the hollow spaces while the iso- and cyclic paraffins are unable to pass through [15]. 

Microporous catalysts are heterogeneous catalysts used in catalytic converters and for many other specialized applications, because of their very large surface areas and reaction specificity. Zeolites, for example, are microporous aluminosilicates (see Section 14.19) with three-dimensional structures riddled with hexagonal channels connected by tunnels (Fig. 13.38). The enclosed nature of the active sites in zeolites gives them a special advantage over other heterogeneous catalysts, because an intermediate can be held in place inside the channels until the products form. Moreover, the channels allow products to grow only to a particular size. 

Encapsulated rhodium complexes were prepared from Rh-exchanged NaY zeolite by complexation with (S)-prolinamide or M-tert-butyl-(S)-prolinamide [73,74]. Although these catalysts showed higher specific activity than their homogeneous counterparts in non-enantioselective hydrogenations, the hydrogenation of prochiral substrates, such as methyl (Z)-acetamidocinnamate [73] or ( )-2-methyl-2-pentenoic acid [74], led to low... 

Since then, organic amines, quaternary-ammonium bases, metal complexes, and other compounds have been extensively used in zeolite synthesis, acting as space fillers with low specificity, structure-directing agent, or true templates ... 

In this chapter, we Hmit ourselves to the topic of zeolite membranes in catalysis. Many types of membranes exist and each membrane has its specific field where it can be appHed best. Comparing polymeric and inorganic membranes reveals that for harsher conditions and high-temperature applications, inorganic membranes outperform polymeric membranes. In the field of heterogeneous catalYsis, elevated temperatures are quite common and therefore this is a field in which inorganic membranes could find excellent applications. 

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