Critical molecular dimensions zeolites

Volumetric measurements of the products evolved when [Fe3(CO)i2] was sorbed in the zeolite, combined with infrared and ultraviolet spectra of the solid, also indicated the formation of [HFe3(CO)n] , as shown in Equation (4.2). The observation that the sorption of [Fe3(CO)i2] was mueh slower than that of [Fe2(CO)9] was inferred to be a consequence of their size differences. The critical molecular dimension of [Fe3(CO)i2] (ca. 10.5 x 7.5 A) is close to the diameter of the zeolite window (about 7.4 A). The anion was inferred to have been generated inside the zeolite supercages. 

Shape selective catalysis differentiates between reactants, products, or reaction intermediates according to their shape and size. If almost all catalytic sites are confined within the pore structure of a zeolite and if the pores are small, the fate of reactant molecules and the probability of forming product molecules are determined by molecular dimensions and configurations as well as by the types of catalytically active sites present. Only molecules whose dimensions are less than a critical size can enter the pores, have access to internal catalytic sites, and react there. Furthermore, only molecules that can leave the pores, appear in the final product. 

The results of catalyst testing runs are shown in Figures 1 and 2. With a critical molecular size of 0.5-0.55 nm for all three molecules involved in the reaction [5], the influence of pore dimensions is clearly seen as nickel deposited on a ZSM-5 support (average channel size of 0.55 nm [6]) does not deactivate rapidly, while nickel supported on USY zeolite (average channel size of 0.77 nm [6]) and nickel on mordenite (average chaimel size of 0.68 nm [6])... 

The key structural feature of the molecular sieves is the narrow, uniform, continuous channel system that becomes available after the zeolitic water has been driven off by heating and evacuation. Great thermal stability after dehydration has been observed in the rigid lattices of X- and Y-type faujasites, zeolite A, mordenite, and chabazite. The geometry of the internal channel and cavity system is characteristic of the individual zeolite. Entrance to the intracrystalline volume is through orifices (ranging from 3 to 9 A in the various zeolites) located periodically throughout the structure. It is thus apparent that access to the intrazeolitic environment is limited to molecules whose dimensions are less than a certain critical size. 

Carbon molecular sieves (CMS) are highly microporous materials having a preponderance of pores of < 1 nm. Among the various types of carbon, CMS materials represent one member of the family of activated carbons. CMS differ from activated carbons in the actual surface composition and the pore size distribution. Unlike CMS, activated carbons display far better detectable surface functionalities. CMS are finding a number of possible uses for the separation of air or other gases and in catalysis. CMS for use as air separation sorbents are usually made from activated carbons by a post-treatment that narrows the pore-size distribution to produce a material with a biomodal pore distribution having a predominance of pores < 0.6 nm [38]. Key to the performance of these materials is their size specific selectivity. CMS are similar to zeolites in that their porous structures have dimensions sized close to the critical dimensions of small to medium sized molecules, that is, the range between 0.3 and 1 nm. As a result, separations can be made on the basis of differences in molecular sizes and... 

Zeolite catalysts are composed of silicon and aluminum oxides in a crystal structure that is permeated by Intracrystalline pores and cavities of precise and uniform dimensions. Chemical reactions occur primarily within these pores. If the Intracrystalline structure is chosen to have certain precise dimensions, the ease of accommodation of reactant and product molecules will depend critically on the shape of the molecules. It is thus possible to generate molecular shape selective catalysts (12-16). 

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