Zeolites classification

A collection of detailed reviews covering all aspects of zeolite classification, synthesis, modification, characterization and applications. 

Figure 2.2 Classification of different types of porous materials, (a) A purely microporous zeolite is considered as a non-hierarchical system according to the single level of porosity, (b) Fragmentation of the zeolite into nanocrystals engenders a network of mesopores constituting the intercrystalline space, leading to an interconnected hierarchical system. Intraconnected...

With over 600 currently known zeolites and new ones discovered every year, it is useful to have a general classification of structures endorsed by the lUPAC. In this system each structure has three letters, for example EAU for faujasites, MFI for ZSM-5 and MOR for mordenite. Within a given structure there can still be many different zeolites, as the composition may vary. 

Thus, methylacetylene and H2S lead to a classification of the zeolite basicity NaLSX < KLSX < CsLSX that again, well accounts for the change of the negative charge of the basic framework oxygen atoms with change of the nature of the cations. 

The range of the variables can be appreciated from Table 2.7, which presents examples of fibrous varieties from several of the zeolite mineral groups (using the classification system presented by Flanigen, 1977). 

Chalcedony AM 67 1248 (1982) also camelian, sard, amethyst, flint, chert Zeolites DHZ 1V 351 Breck (1978) Reviews 4 [Classification after Breck (1974), as found in Reviews 4, pp. 19-52]... 

This classification gives a reasonable account of the data except for BaX zeolite found in group B (monovalent adsorption sites). Barium zeolite does not lead to the heterolytic dissociation of water and is not likely to produce monovalent adsorption sites, in contrast to the other alkaline-earth zeolites. 

Classification of Zeolites According to Secondary Building Units... 

Until now we have mainly treated adsorption onto non-porous surfaces. In reality, most industrial and many natural materials are porous Textiles, paper, bricks, sand, porous rocks, food products, zeolites etc. We start our discussion with a classification of pores according to their size, which is recommended by IUPAC ... 

Shape selective reactions are typically carried out over zeolites, molecular sieves and other porous materials. There are three major classifications of shape selectivity including (1) reactant shape selectivity where reactants of sizes less than the pore size of the support are allowed to enter the pores to react over active sites, (2) product shape selectivity where products of sizes smaller than the pore dimensions can leave the catalyst and (3) transition state shape selectivity where sizes of pores can influence the types of transition states that may form. Other materials like porphyrins, vesicles, micelles, cryptands and cage complexes have been shown to control product selectivities by shape selective processes. 

Post-synthesis methods (pore-size engineering) allow an existing shape-selectivity effect to be intensified, and also a new one to be established. However, normally not only the pore size will be influenced by most of these methods, but also the catalytic activity. Vansant [104] gives a classification of post-synthesis modification methods which covers the entire range of zeolite applications (gas separation, gas purification, encapsulation of gases and catalysis). 

Intracrystalline sorption is normally of Type 1 in Brunauer s classification ( V7) and isotherm contours therefore resemble those according to Langmuir s isotherm equation. This can describe actual isotherms well enough (18) to be of value in predicting, through Equations 5 or 6, some features of zeolite chemistry. 

In our opinion, the main factor which governs the acidity of bridged OH groups in zeolites is the chemical one. If the local nature of Bronsted sites is taken into account, the following rational classification can be proposed for the bridged OH groups of zeolites with regard to their acidity and Si/Al ration in the framework (34). 

Molecular sieve catalysts that have been used for the conversion of methanol to hydrocarbons fall into two general classifications. Most of the initial research was done using ZSM-5 (MFI), a medium-pore size zeolite with a three dimensional pore system consisting of straight (5.6 x 5.3 A) and sinusoidal channels (5.5 x 5.1 A). While most of this work was directed at the conversion of methanol to liquid hydrocarbons for addition to gasoline, it was found that the product slate could be shifted toward light olefins by the use of low pressure and short contact times. 

Adsorption/separation processes are based on adsorption isotherms (thermodynamics) and intracrystalline diffusivity (kinetics). Figure 16.1 illustrates various shapes of adsorption isotherms depending on the VOC nature, trichloroethylene (TCE) and tetrachloroethylene (PCE), and of the zeolite, MFI with Si/Al > 500 and FAU (Si/Al > 100) (14). The isotherms of VOCs adsorbed on FAU present a more or less S-shape which corresponds to type V of the IUPAC classification. In contrast, the isotherms of VOCs on MFI are more of type I, with the additional particularity of a step at 4 molecules per u.c. for PCE adsorption. The... 

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