Zeolite 12-membered ring

Large-pore zeolites (12-membered rings up to 0.75 nm)—e.g., zeolites Y and L, EMT, mordenite. Beta, and AIPO4-5. 

The problem of accessibility in microporous solids is extreme in zero-dimensional zeolite structures such as clathrasils, that is, zeolite-related materials consisting of window-connected cages. The pore openings in these caged structures are restricted to six-membered rings of [Si04] units at most, which corresponds to pore diameters of approximately 0.2 nm [58]. These pores are too small for the removal of templates and, afterward, are impenetrable to typical sorptive molecules for characterization such as N2 and Ar or reactants such as hydrocarbons. Therefore, the intrinsic... 

The exact nature of the zeolite is determined by the reaction conditions, the silica to alumina ratio and the base used. For example zeolite /3, a class of zeolites with relatively large pores, in the range of 0.7 nm, of which mordenite is an example, are usually made using tetraethylammonium hydroxide as the base. This acts as a template for the formation of 12-membered ring apertures (Figure 4.3). 

Since their development in 1974 ZSM-5 zeolites have had considerable commercial success. ZSM-5 has a 10-membered ring-pore aperture of 0.55 nm (hence the 5 in ZSM-5), which is an ideal dimension for carrying out selective transformations on small aromatic substrates. Being the feedstock for PET, / -xylene is the most useful of the xylene isomers. The Bronsted acid form of ZSM-5, H-ZSM-5, is used to produce p-xylene selectively through toluene alkylation with methanol, xylene isomerization and toluene disproportionation (Figure 4.4). This is an example of a product selective reaction in which the reactant (toluene) is small enough to enter the pore but some of the initial products formed (o and w-xylene) are too large to diffuse rapidly out of the pore. /7-Xylene can, however. 

Figure 5.29. Example of the structure of a zeolite every corner is a Si or Al ion, with O ions between (halfway between the connecting lines). The structure is a combination of 4-, 6-, and 8-membered rings.

The Si04 tetrahedra can be arranged into several silicate units, e.g. squares, six-or eight-membered rings, called secondary building blocks. Zeolite structures are then built up by joining a selection of building blocks into periodic structures. 

This paper describes the successful incorporation of molybdenum and molybdenum-nickel clusters into zeolites with 12-membered ring by aqueous ion exchange and application of the resulting materials to HDS reaction of benzothiophene. Stoichiometry of the ion exchange was examined by elemental analysis. UV-visible spectroscopy and EXAFS measurements were carried out to investigate the structure of molybdenum species loaded on zeolites. 

According to the chemical analysis and coordination distances, the Rietveld refinement of the crystal structure at room temperature revealed 1.2 Co2+ atoms per unit cell at the Col and Co2 sites, whereas the 1.4 Ag+ cations are spread over the Co3 site, from now on referred to as Ag5 for clarity, and two new sites, Ag2 and Ag3, located near Co2 in the 10-membered ring (Fig. 3). In addition, for this catalyst the presence of Ag° clusters outside the zeolite structure was recognized by the detection of a strong reflection at about 40° 28. In agreement with the lower Ag content, in Ag2.7Co2.8AF the Ag3 site... 

Figure 11. The truncated octahedron building block (also termed sodalite cage,f or p-cage ) (a) tetrahedral atoms (usually Si or Al) are located at the corners of the polygons with oxygen atoms halfway between them. Illustration of the linkage, through double four-membered rings, of two truncated octahedra (b) and the structure of zeolite-A (c).

Figure 12. The structure of zeolite-A formed by linking truncated octahedra through double four-membered rings (a), the sodalite structure formed by direct face-sharing of four-membered rings in the neighboring truncated octahedra (b), and the faujasite structure formed by linking the truncated octahedra through double six-membered rings (c).

Zeolite Socony Mobil constructed from five-membered-ring building units... 

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