Organized media zeolites

Dynamic Kinetic Resolution. Another typical acid-catalysed reaction is the racemisation of chiral alcohols, due to inversion at the chiral carbon. This can actually be made use of in the formation of enantiopure compounds, by dynamic kinetic resolution using an enzyme, such as a lipase, that catalyses enantioseleetive esterification in an organic medium. By coupling zeolite Beta-catalysed intereonversion of benzylic alcohol enantiomers with enzyme-catalysed esterifieation of only one of the enantiomeric alcohols, almost complete eon version to enantiopure ester ean be achieved. ... 

Table 1.4 lists some of the major new structures reported in the 1990s. Interestingly, as organic SDAs tended to dominate discovery of new frameworks, there were no new aluminum-rich synthetic zeoUtes reported in either the 1980s or the 1990s. The new aluminosilicate structures were all high silica or pure sihca in composition. It awaited the 2000s for new aluminosilicate zeolite materials with low to medium Si/Al to be reported (see below). 

From the above examples it is clear that reaction cavity provided by an organized or confining medium has unique features that mimics some of the features of proteins. While crystals and zeolites provide reaction cavities that are inflexible, there is a whole spectrum of organized and confined media (e.g., micelles, host-guest complexes, monolayers and bilayers, liquid crystals etc.,) that allow different degrees of freedom to the reactant molecules. These systems demonstrate clever usage of favorable entropy that is so important in natural systems. One should keep in mind... 

An ideal approach to achieving chiral induction in a constrained medium such as zeolite would be to make use of a chiral medium. No zeolite that can accommodate organic molecules, currently exists in a stable chiral form. Though zeolite beta and titanosilicate ETS-10 have unstable chiral polymorphs, no pure enantiomorphous forms have been isolated. Although many other zeolites can, theoretically, exist in chiral forms (e.g., ZSM-5 and ZSM-11) none has been isolated in such a state. In the absence of readily available chiral zeolites, one is left with the choice of creating an asymmetric environment within zeolites by the adsorption of chiral organic molecules. 

We are interested in the role of organic ions and organic molecules in zeolite synthesis. The use of organic molecules in zeolite synthesis mixtures in addition to inorganic bases is known to result in the medium-pore-size low-aluminum-content zeolites, such as ZSM-5, that revolutionized several processes of importance to the oil-refining and petrochemical industry. 

Expressions 2 and 3 show that, in order to overcome this energy difference, the micropore cavities should be largely filled with adsorbed molecules. As mentioned earlier, in cases where low-alumina-content materials have been directly synthesized, high values for 6 are invariably found. This confirms Barrer s postulation. The dominant interaction that governs narrow- and medium-pore zeolite synthesis is the strong interaction of the occluded organic molecule with the micropore wall. 

Of interest with respect to this hypothesis is the significant difference in heat of paraffin adsorption between the medium-pore zeolite silicalite and large-pore, de—aluminated faujasite. The heat of paraffin adsorption is much smaller in the case of the de-aluminated faujasite, which has so far had to be prepared by an indirect route, than for silicalite, which can be synthesized direct in the presence of an organic molecule. The difference, which increases with chain length, is of the order of 5 kJ/mol per CH2 unit, and may be ascribed to the optimum fit of hydrocarbon and channel in the case of the medium-pore zeolite (H, 12). ... 

Molecular design and rational synthesis of inorganic microporous crystalline materials are frontier subjects in the fields of zeolites science and molecular engineering. Zeolite synthesis is an active field of research because zeolites with uniform micropores are important in many industrial processes in catalysis, adsorption, and separation, and are finding new applications in electronics, magnetism, chemical sensors, and medicine, etc.12 91 Synthesis of such materials typically involves crystallization from a gel medium under hydrothermal/solvothermal conditions in the presence of organic amines as... 

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