Cavities, typical, zeolites

Figure C2.12.4. Typical polyhedra found in zeolites (a) sodalite cage found in sodalite, zeolite A or faujasite (b) cancrinite or a-cage found in cancrinite, erionite, offretite or gmelinite (c) the 5-ring polyhedron found in ZSM-5 and ZSM-11 (d) the large cavity of the faujasite stmcture and (e) the a-cage fonning the large cavity in zeolite A.

Smith and coworkers recently proposed a specific and novel mineral-based solution to the problem of dilution and diffusion of prebiotic reactants. They have suggested [132-134] the uptake of organics within the micron-sized three-dimensional cross-linked network of pores found to exist within the top 50 xm, or so, of alumina-depleted, silica-rich weathered feldspar surfaces. These surfaces incorporate cavities typically about 0.5 pm in diameter along with cross inter-connections of about 0.2 pm. The nominal area of the weathered feldspar surface is apparently multiplied by a factor of about 130 arising from this network. The similarity of these pores to the catalytic sites in zeolite-type materials is pointedly mentioned. 

Zeolites are microporous crystalline materials with pores that have about the same size as small molecules like water or n-hexane (pore size is usually 3-12 A). The structure of a zeolite is based on a covalently bonded TO4 tetrahedra in which the tetrahedral atom T is usually Silicium or Aluminum. The very famous Lowenstine rule only allows the existence of zeolites with a Silicium/Aluminum ratio of at least 1. As all corners of a tetrahedrcd have connections to other tetrahedra, a three dimensional pore network of channels and/or cavities is formed. Currently, these are about 100 different zeolite structures [1], several of these Ccin be found in nature. To clarify the topology of a typical zeolite, the pore structure of the zeolite Silicalite [2] is shown in figure 1.1. This zeolite has a three dimensional network of straight and zigzag channels that cross at the intersections. 

For 200years following their discovery by Cronstedt, zeolite minerals (or natural zeohtes) were known to occur typically as minor conshtuents in vugs or cavities in basaltic and volcanic rock. Such occurrences precluded their being obtained in mineable quanhhes for commercial use. From the late 1950s to 1962 major geologic discoveries revealed the widespread occurrence of a number of natural... 

Zeolites are crystalline aluminosilicates whose primary structure is formed by Si04 and A104 tetrahedra sharing the edges . Their tertiary structure forms uniform channels and cavities of molecular dimensions that are repeated along the zeolite lattice. Due to the lower valence of the aluminium relative to silicon, the excess negative charge (one per A1 atom) is balanced by alkali metal cations, mainly Na". An important class of the zeolite family are the faujasites, known as zeolites X and Y, which have the typical composition for the unit cell as follows ... 

In the last few years, computer graphics with colour display are being more commonly used not only to visualize complex structures better, but also to examine unusual structural features, defects and transformations as well as reactions. In Fig. 1.45, we show the presence of a Nal" cluster within the sodalite cage of zeolite Y as depicted by computer graphics the cluster fits well within the cavity bounded by the van der Waals surface (net) of the framework atoms. The immense power of computer graphics has been exploited widely in recent years. Structural transitions in solids and sorbate dynamics in zeolites are typical areas where computer simulation and graphics have been used (Ramdas et al., 1984 Rao et al., 1992). 

Recently, Ramamurthy and colleagues demonstrated that certain zeolites, including Na-ZSM-5, spontaneously oxidize a variety of olefmic or aromatic substrates (Fig. 1) [34, 35], Zeolites have been utilized frequently as supporting matrix materials [36-38], These materials contain host cavities of well defined geometries and allow molecules of appropriate shapes to be incorporated. Typically, the host contained in the zeolite is oxidized by exposure to ionizing radiation (vide infra), and the resulting radical cation is protected against ion... 

In electrides, electrons behave as anions. In a typical organic electride Cs+ (15-crown-5)2 e, the electrons occupy cavities connected by narrow channels. The first electride stable at room temperature was made by ionizing Cs into Cs+ and e within the channels of an aU-sUica zeohte. Petkov et confirmed that Cs+ and a low-density electron gas are present in the one-dimensional nanopores of the sihceous zeolite ITQ-4 (Si32064). The PDF technique provided direct structural evidence that Cs intercalated in the form of Cs+ ions arranged in short zigzag chains (see Figure 10). This structural model... 

As emphasized in the text, the importance of a zeolite derives from its function. It is not only important to realize a structure which retains its cavities upon removal of the guest but should also demonstrate reversible absorption and desorption of guests, selectivity in the intake of guest molecules and stability under conditions of a typical reaction. 

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