Faujasites zeolite structures

The structure-type 37 is the Faujasite zeolite structure analog (30). It has a three-dimensional (3-D) system of pores running perpendicular to each other. The apertures are defined by 12-ring windows with a 0.74-nm diameter. 

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 )...

Zeolite, or more properly, faujasite, is the key ingredient of the FCC catalyst. It provides product selectivity and much of the catalytic activity. The catalyst s performance largely depends on the nature and quality of the zeolite. Understanding the zeolite structure, types, cracking mechanism, and properties is essential in choosing the right catalyst to produce the desired yields. 

Zeolites are used in various applications such as household detergents, desiccants and as catalysts. In the mid-1960s, Rabo and coworkers at Union Carbide and Plank and coworkers at Mobil demonstrated that faujasitic zeolites were very interesting solid acid catalysts. Since then, a wealth of zeolite-catalyzed reactions of hydrocarbons has been discovered. Eor fundamental catalysis they offer the advantage that the crystal structure is known, and that the catalytically active sites are thus well defined. The fact that zeolites posses well-defined pore systems in which the catalytically active sites are embedded in a defined way gives them some similarity to enzymes. 

The linking pattern of two zeolites is shown in Fig. 16.24. They have the /I-cage as one of their building blocks, that is, a truncated octahedron, a polyhedron with 24 vertices and 14 faces. In the synthetic zeolite A (Linde A) the /3-cages form a cubic primitive lattice, and are joined by cubes. j3-Cages distributed in the same manner as the atoms in diamond and linked by hexagonal prisms make up the structure of faujasite (zeolite X). 

New Zeolitic Structures. Multiply twinned faujasitic zeolites (typically zeolite-Y) have recently been shown (30, 31) to be capable, by recurrent twinning on 111 planes, to generate a new, hexagonal zeolite in which tunnels replace the interconnected cages of the parent cubic structure. 

Early attempts to utilize the high acid activity of faujasite zeolite catalysts for direct xylene isomerization suffered from low selectivity. Considerable improvement was obtained first by using a large pore zeolite (7) catalyst and subsequently in several process modifications that use ZSM-5 as catalyst (2). In the following we will show how these selectivity differences can be related to structural differences of the various zeolites. 

The connectivity (topology) of the zeolite framework is characteristic for a given zeolite type, whereas the composition of the framework and the type of extra-framework species can vary. Each zeolite structure type is denoted by a three-letter code [4], As an example, Faujasite-type zeolites have the structure type FAU. The pores and cages of the different zeolites are thus formed by modifications of the TO4 connectivity of the zeolite framework. 

Zeolite structures are designated by a three capital-letter code, for example, FAU stands for the faujasite structure, to which the well-known X and Y zeolites belong. A very useful short notation is used for the description of the pore system(s) each pore network is characterized by the channel directions, the number of atoms (in bold type) in the apertures, the crystallographic free diameter of the apermre (in A), asterisks (1, 2, or 3) indicating whether the systems is one-, two-, or three-dimensional. To completely specify the pore system, the eventual presence of cages (or channel intersections) should be indicated, along with their... 

Caglione, A.)., Cannan, T.R., Greenlay, N., and Hinchey, R.). (1994) Process for preparing low silica forms of zeolites having the faujasite type structure. US Patent 5,366,720. 

Zeolites can be ion-exchanged with cations or impregnated with various metals to modify their performance for use in applications such as separations, adsorption and catalysis. For example, faujasite zeolites exchanged with Na, Li, K, Ca, Rb, Cs, Mg, Sr, Ba, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ru, Pd, Ag, Cd, In, Pt, H, Pb, La, Ce, Nd, Gd, Dy and Yb have been made and studied due to their use in separation and catalysis [135]. The ability to determine the distributions of these cations in the zeolitic structure is one of the key parameters needed in understanding adsorption mechanisms and molecular selectivities. Little has compiled an excellent reference... 

Faujasites of the X and Y type are the most frequently studied zeolite structure type for this reachon. Because the key step in the reachon is a hydride hansfer, zeolites with low Si02/Al203 ratios are favored. The other preferred characteristic is a large pore opening and hence a low diffusion barrier to product diffusion. These details and others were reviewed recently by Feller et al. [50] (Table 12.8). 

Zeolite catalysts play a vital role in modern industrial catalysis. The varied acidity and microporosity properties of this class of inorganic oxides allow them to be applied to a wide variety of commercially important industrial processes. The acid sites of zeolites and other acidic molecular sieves are easier to manipulate than those of other solid acid catalysts by controlling material properties, such as the framework Si/Al ratio or level of cation exchange. The uniform pore size of the crystalline framework provides a consistent environment that improves the selectivity of the acid-catalyzed transformations that form C-C bonds. The zeoHte structure can also inhibit the formation of heavy coke molecules (such as medium-pore MFl in the Cyclar process or MTG process) or the desorption of undesired large by-products (such as small-pore SAPO-34 in MTO). While faujasite, morden-ite, beta and MFl remain the most widely used zeolite structures for industrial applications, the past decade has seen new structures, such as SAPO-34 and MWW, provide improved performance in specific applications. It is clear that the continued search for more active, selective and stable catalysts for industrially important chemical reactions will include the synthesis and application of new zeolite materials. 

Faujasite-type zeolite structures have maximum symmetry Fd3m, and all the 192 T atoms per unit cell of the A structure are symmetrically equivalent. The observed Si/Al ratios of synthetic faujasite-type species vary within a range from slightly over 1 up to 2.5 (and occasionally above). Unmodified species thus normally contain between 48 and almost 96 A1 atoms per unit cell. The almost continuous range in A1 content does not by itself rule out any kind of Si, A1 order. Discontinuities in the plot of the cell dimensions against the number of A1 atoms per unit cell have been reported by several investigators (11, 12). The observed discontinuity at around 64 Al, in particular, has been related to Si, A1 ordering (12). Full details and references on faujasite-type zeolite structures can be found in the comprehensive and critical review by Smith (13). 

Smith, J. V., Faujasite-Type Structures Aluminosilicate Framework Positions of Cations and Molecules, Paper 15, presented at the Second International Conference on Molecular Sieve Zeolites, Worcester, Mass., 1970 Advan. Chem. Ser. (1971) 101, 171. 

Structural Determinations. Si-0 and Al-0 vibrations at 1200-350 cm-1 give information on zeolite structure (21-25). Qualitatively, the resolution of the bands around 1150 and 1050 cm-1, the intensity and sharpness of the bands around 580 and 390 cm-1, and the presence of a shoulder at ca. 500 cm-1 are characteristic of the faujasite structure. 

Zeolites. Unlike silica and clay, zeolites possess interior structures that are uniform and well defined in shape and size. In spite of this, inhomogeneity in the microenvironment around a guest included in a faujasite zeolite may arise for two reasons variation in the occupancy number within a cage and the presence of sites of varying microenvironment. Even at low loading levels, the... 

The open three-dimensional nature of zeolite structures permits diffusion of reactant molecules into the interior voids in the crystal and accounts for the high effective surface area of these materials. Faujasitic zeolites have channels of about 8-A diameter connecting cavities of 13-A diameter (supercages) in a three-dimensional network. The zeolite mor-denite has parallel channels with a diameter of about 7-A. The intracrystalline surface of the zeolite is, therefore, accessible to molecules with kinetic diameters equal to or smaller than the channel diameters. 

Figure 9.6 Topologies of zeolite structure types, (a) Sodalite (b) Linde type A (c) faujasite (zeolite X and Y) (d) AlP04-5 and (e) ZSM-5. The vertices represent the positions of AI04 or Si()4 letrahedra while straight lines represent Si-O-Si or Si-O-Al linkages. (Reproduced with permission from [5]).

The formation and location of the active complex is obviously strongly dependent on the zeolite topology and composition indeed, among the many zeolite structures examined, only the faujasite topology with the charge density of zeolite Y was found stabilize a maximum number of active sites. 

Fig. 4. Line representations of zeolite structure (a) sodalite cage, or truncated octahedron (b) type A zeolite unit cell (c) unit cell of types X and Y, or faujasite (d) cation sites in type A (there are 8 1,3 II, and 12 III sites per unit cell) (e) cation sites in types X and Y (161,32 Y, 32 II, 32 IT, 48 III, and 32 IIT, sites per unit cell).

In order to obtain more specific qualitative information for above acid attack dealu-mination in USY zeolite, the pople s CNDO/2 calculations were made on a model of the six ring cluster with a TtO,(OH)i structure unit (T represents A1 atom or Si atom) in Fig, 1 (ref. 4, ref. 7) which simu-lates the Sr and Si sites of faujasite zeolite. 

We describe a systematic investigation of various synthesis variables that usually affect the crystallization of faujasite-type structures from Si, Al, Na, tetraethylammonium (TEA) hydrogels.A careful control of parameters such as the composition of the precursor hydrogel, temperature and crystallization time is needed to selectively prepare and stabilize pure zeolite ZSM-20 in high yield. 

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