Molecular sieves, pore structure

Fig. 1. Schematic diagram of molecular sieve pore structure. See Table 1...

Table 1 small- Principal characteristics of pore molecular sieves pore structure for some... 

New Adsorbent Materials. Silicalite and other hydrophobic molecular sieves, the new family of A1P04 molecular sieves, and steadily increasing families of other new molecular sieves (including structures with much larger pores than those now commercially available), as well as new carbon molecular sieves and pillared interlayer clays (PILCS), will become more available tor commercial applications, including adsorption. Adsorbents with enhanced performance, both highly selective physical adsorbents and easily regenerated, weak chemisorbents will be developed, as will new rate-selective adsorbents. 

Molecular sieve type Structure type code [269] Pore size largest channel (A) Channel Dimensions Isotopic framework structures... 

Under the mineralogical name zeolite such sieves occur naturally. For technical purposes due to their higher uniformity only synthetic zeolites are used [10], In the empirical formula Me is an exchangeable cation of the valence n (zeolites are cation exchangers). Molecular sieves have a very regular and orderly crystal structure, which is characterized by a three-dimensional system of cavities with a diameter of 11 A. These cavities are interconnected by pores with a constant diameter. The value of this diameter depends on the type of the exchangeable cation Me. It is 5 A, if in the above formula Me stands for 75% Na+ and 25% Ca2+. 

Because such guest molecules usually interact with the frameworks through H-bonds, van der Waals s forces, or sometimes coordination bonds, it is crucial to remove the templates properly to form structurally stable, free-pore molecular sieves. 

Since the discovery by researchers at Mobil of a new family of crystalline mesoporous materials (1), a large effort has been expended on synthesis, characterization, and catalytic evalrration (2). MCM-41 is a one-dimerrsiorral, hexagonal structure. MCM-48 is a cubic structine with two, norrintersecting pore systems (3). MCM-50 is a layered stractme with silica sheets between the layers (4). Many scientists also looked into other mesoporous materials, of note the HMS (Hexagonal Molecular Sieve) family (5) and SBA-15 (acronym derived from Santa Barbara University) (6), bnt to date few materials have been both catalytically significant and inexpensive to synthesize. 

Molecular sieves (zeolites) are artificially prepared aluminosilicates of alXali metals. The most common types for gas chromatography are molecular sieve 5A, a calcium aluminosilicate with an effective pore diameter of 0.5 nm, and molecular sieve 13X, a sodium aluminosilicate with an effective pore diameter of 1 nm. The molecular sieves have a tunnel-liXe pore structure with the pore size being dependent on the geometrical structure of the zeolite and the size of the cation. The pores are essentially microporous as the cross-sectional diameter of the channels is of similar dimensions to those of small molecules. This also contrilsutes to the enormous surface area of these materials. Two features primarily govern retention on molecular sieves. The size of the analyte idiich determines whether it can enter the porous... 

In the above discussion, we have presumed that the tortuosity factor t is characteristic of the pore structure but not of the diffusing molecules. However, when the size of the diffusing molecule begins to approach the dimensions of the pore, one expects the solid to exert a retarding influence on the flux and this effect may also be incorporated in the tortuosity factor. This situation is likely to be significant in dealing with catalysis by zeolites (molecular sieves). 

Cancrinites are one of the rarest members of the feldspathoid group, classified as such due to its low silicon content. However, cancrinite is also classified as a zeolite, due to its open pore structure, which confers molecular sieve properties [1], Likewise, variable sodium carbonate and NaOH concentrations in the hydrothermal synthesis of cancrinite could direct the synthesis of the intermediate phase or the disordered cancrinite formation [2], The intermediate phase is described as a phase between cancrinite and sodalite [3], The disordered cancrinite is an intermediate phase which is much closer to the cancrinite structure than sodalite structure [2],... 

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