Cavity size, zeolites

Because the size of the interlayers can be easily varied by incorporation of complex moieties of different sizes, these clays (montmorillonite, hectorite) may nevertheless compete as catalyst supports with zeolites which have a rigid, predetermined cavity size. 

Supramolecular concepts involved in the size- and shape-selective aspects of the channels and cavities of zeolites are used to control the selectivity of reactions of species produced by photoexcitation of molecules encapsulated within zeolites. The photochemistry of ketones in zeolites has been extensively studied. Photoexcitation of ketones adsorbed on zeolites at room temperature produces radical species by the Norrish type 1 reaction. A geminate (born together) radical pair is initially produced by photolysis of the ketone, and the control of the reaction products of such radicals is determined by the initial supramolecular structure... 

Values for channel and cavity sizes for various zeolites and zeotypes are listed in Table 7.1. 

The results reported here and in earlier publications in this series suggest that cavity size and limitations to molecular motion play a dominant role in the photochemistry and photophysics of alkyl aryl ketones included in zeolites. In the case of Silicalite the size and polarity of various substituted 8-phenylpropiophenones seem to determine the efficiency of inclusion and ultimately of luminescence. The same factors, relating to size and mobility can be expected to play an important role in the use of zeolites as catalysts for other reactions, whether these are photochemical or thermal processes. In this sense studies with 8-phenylpropiophenones may lead to considerable information on adsorption sites and on the freedom (or lack of it) of molecular motion as well as on the accessibility of these sites to other reactants. Recent work from Turro s laboratory has shown that pyrene aldehyde can be used to probe the nature of inclusion sites in various zeolites (27) dibenzyl-ketones were also used as probes on porous silica (28). 

NaY zeolite and V205 react at temperatures as low as 700 K, although both compounds are stable at this temperature. For low R values, the V2C>5 structure disappears, whereas that of the zeolite does not seem to be damaged nor is the cavity size reduced. However some cavity entrances are blocked by a compound containing vanadium atoms and, in particular, a few tetravalent vanadium atoms. Some tetravalent vanadium atoms have also been observed by Occelli and Stencel (25, 26) in much more complex reaction mixtures. 

The term molecular sieve describes a material having pores that closely match the dimensions of a specific molecule. The best-known molecular sieves are composites of microcrystalline zeolites embedded in an inert clay binder. Zeolites are composed of regular clusters of tetrahedral aluminosilicates, with varying percentages of bound cations and water molecules, whose crystal structures incorporate small molecule-sized cavities. Because zeolite pore size is different for each of the numerous different crystal structures in this family, the size-selective nature can be tailored for specific applicatimis. Studies of the transport of liquid and gaseous organic species in molecular sieves indicate that the diffusion rate and equilibrium concentration of sorbed analyte are sensitive functions of their molecular dimensions, as well as zeolite pore size and shsqre [110]. 

In the case of zeolites, studies have also been performed to distinguish sites located at the external surface from those present in the cavities, and also to distinguish the position in different cavities for zeolites with a complex pore structure. This can be performed using molecules having different molecular sizes but similar chemical behavior. Some of the useful molecules are shown in Scheme 3.6. 

The open framework structure of the zeolites allows small molecules to be absorbed into their structures. The size and shape of the molecules absorbed depends on the structure of the zeolite, and hence the geometry of its pores. For example, zeolite A readily absorbs water but ethanol is excluded. Figure 7.17 demonstrates the effect of cavity size on the molecules which can be absorbed. 

Figure 7.17 Effect of cavity size on absorbed species for zeolite A.

Zeolite as a catalyst is very useful for organic reactions, providing high selectivity. As they are available in different cavity sizes, they are very effective in... 

Sometimes the choice of zeolite can determine which product is obtained. This is true in the alkylation of 4-methylimidazole with methanol (6.34) in the vapor phase.197 The product from a Meerwein-Ponnodorff-Ver-ley reduction (6.35) varied with the zeolite catalyst.198 High selectivity for acrolein, propylene, and allyl ether from allyl alcohol (6.36) can be obtained by using different zeolites.199 A combination of acid or base strength and cavity size may be responsible for these effects. 

Product selectivity arises when, corresponding to the cavity size of a zeolite, only products of a certain size and shape that can exit from the pore system are formed. Well-known examples of product selectivity are the methylation of toluene (Fig. 7-5 b) and the disproportionation of toluene on ZSM-5. 

The cavity size of the faujasite zeolite agrees well with the diameter of the phthalocyanine (Fig. 8-3) [54]. Phthalocyanines included in MCM-41 are found in the columnar-orientated detergent (used as templates for synthesis of the... 

One of the most elegant synthetic approaches to using the internal cavities of zeolites as nanometre size reactors is that of the ship-in-a-bottle synthesis of metal complexes within zeolite cages, which are then too large to escape through the cage windows. The term was initially coined by Herron to describe metal complexes, such as those with salen-(bis(salicylidene)ethylendiamine-) " or phthalocyanine (Scheme 6.9) that were formed in the supercages of faujasitic zeolites. Zeolites X and Y are most commonly used, but the fully... 

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