Structure of a zeolite

Basically, zeolites consist of Si04 and AIO4 tetrahedra (Fig. 5.28), which can be arranged by sharing 0-corner atoms in many different ways to build a crystalline lattice (Fig. 5.29). 

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. 

The same periodic structures can also be formed from alternating AIO4 and PO4 tetrahedra the resulting aluminophosphates are not called zeolites but AlPOs. Zeolites are made by hydrothermal synthesis under pressure in autoclaves, in the presence of template molecules such as tetramethylammonium, which act as structure directing agents. 

Zeolites exhibit various pore systems. Zeolitel L (LTL) has parallel one-dimensional channels, Mordenite (MOR) has two different one-dimensional parallel chan- 

Common name 3-letter code Channels Window or channel diameter (nm) Pore volume (mLg- ) Si/AI 

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Figure 10.4. Outline structures of (a) zeolite A, (b) its homologue faujasite, (c) the channel network of the tubular zeolite ZSM-5.

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 three-dimensional framework structure of a zeolite is formed by linking BBUs in an infinite repeating lattice. The framework structure for zeolite type A (framework code LTA) is shown in Figure 2.4. Figure 2.4a shows the T-atom connectivity. Figure 2.4b is the same view with all rings of size 6 and smaller filled in... 

If the framework structure of a zeolite remains constant, the cation exchange capacity is inversely related to thd Si/Al ratio. Furthermore, fine tuning of the adsorptive and catalytic properties can be achieved by adjustment of the size and valency of the exchangeable cations. Dealumination of certain silica-rich zeolites can be achieved by acid treatment and the resulting hydrophobic zeolites then become suitable for the removal of organic molecules from aqueous solutions or from moist gases. 

Figure 4.30 Structure of a zeolite material, (a) Internal view-atoms, (b) internal view of windows, (c) central-cavities and windows.

Figure 14. (a) Structure of a zeolite-entrapped perfluorinated ruthenium phthalocyanine (RuFi6Pc) complex used in epoxidation reactions, (b) Comparison of turnover numbers for cyclohexane oxidation using ruthenium phthalocyanine (RuPc), RuFi Pc and zeolite X-entrapped RuFi Pc. 

OH groups (or coordinated water molecules) connected to the outer or iimer surface of the porous structure of a zeolite or in lattice defects can be detected by the - 29sj cross-polarization technique. Strong intensity enhancements are observed in the CP spectra for the resonances of (TO)3SiOH or (TO)2Si(OH)2 but not of Si(OT)4 environments (2). 

FIGURE 6.9 Microporous membrane structures (a) resulting from packing and sintering of ceramic nanoparticles and (b) ultramicroporous channels in the crystalline structure of a zeolite. 

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. 

However, while from these general considerations it is clear that both acidity and pore structure of a zeolite affect the rate of formation of carbonaceous compounds (other factors being held constant), it is generally impossible to quantify the effect of each of these parameters because of the difficulty in obtaining zeolite samples with identical acidities and with different pore structures or vice-versa. Having said that, some examples are given below which illustrate within these limitations the respective roles of acidity and of pore structure. 

FIGURE 4.1 Three-dimensional representation of the framework structure of (a) zeolite A and (b) X and Y. 

The determination of the structure of a zeolite with a new framework type remains a challenge to the powder method. Nonetheless, significant advances have been made in this area, and an increasing proportion of the new framework types are solved this way. While the techniques are rather sophisticated and beyond the scope of this chapter, it is perhaps important to know that methods of structure determination from powder diffraction data do exist and that all is not lost if single crystals of a new material cannot be synthesized. It may still be possible to solve the structure from the powder data [39,40,41]. 

Computational techniques in general arc used in various areas of zeolite research. In fact, just such a basic question as what is the structure of a zeolite relies on the zeolite modeling. Currently, the methods of computational chemistry are used in investigation of almost any property of zeolites, including, e. g., zeolite structures, zeolite characterizations (modeling, e. g., the UV-vis, IR, NMR, or ESR spectra), and catalytic activities. 

If one considers the rather complicated crystal structure of a zeolite, then the assumption that it contains sites or cells of only one type to accommodate sorbate molecules does not seem very realistic. 

It is possible that in silica-alumina many of the active sites are located in dead-end pores. Thus, a little coke at one end may kill many active sites, while coke in the three-dimensional pore structure of a zeolite would have little effect on access to the interior. 

Scheme 3.1 The FOCUS program can use data from both real space and redproeal (diffraction) space, and from eleetron microseopy as well as powder X-ray diffraction (PXRD). It has successfully solved the structure of a zeolite with 24 independent silicon positions, TNU-9.

FIGURE 1.6. Schematic representation showing framework structures of a) zeolite A, (b) zeolites X and Y, (c) erionite and (d) chabaziie. (To translate these schematic diagrams into the actual structure, a Si or Al is placed at each vertex and an 0 at or near the center of each line.)... 

The structure of a zeolitic framework and the chemical nature of the host lattice lead to a wide variety of industrial uses, especially catalytic and screening processes within the petroleum industry. The size of the pores is one of the most crucial factors in determining zeolite use. The pore size is determined by the number of T-atoms defining the entrance (ring-size). For example. Figure 4.1(b) shows an 8-membered ring entrance at the front of the cavity. If the pore is too large or too small, then the activity of the material may be adversely... 

Figure 1.3 The cubo-octahedral unit of faujasite zeolites, structure of A zeolite, and X and Y zeolites (from left to right). (Barrer 196 )...

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. 

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