Faujasite, framework structure

The silicon-substituted aluminium phosphate SAPO-37 has a faujasite framework structure and shows a sharp tetrahedral and octahedral Al resonance, with an underlying broad tetrahedral resonance (Fyfe et al. 1995). By using Al—> P and Al Si TEDOR experiments, Fyfe et al. (1995) have shown that the main Si environment in SAPO-37 is Si[4Al], and have been able to identify the environments of the... 

Figure 6.9 The Na+ sites in the faujasite framework structure. Reproduced with permission from [17], Copyright (1980) Chemical Journal of Chinese Universities...

The framework structures and pore cross-sections of two types of zeolites are shown. (Top) A Faujasite-type zeolite has a three-dimensional channel system with pores of at least 7.4 A in diameter. A pore is formed by 12 oxygen atoms in a ring. (Bottom) ZSM-5 zeolite has interconnected channels running in one direction, with pores 5.6 A in diameter. ZSM-5 pores are formed by 10 oxygen atoms in a ring. Reprinted with permission from Chemical Engineering Progress, 84(2), February 1988, 32. 

Figure 4.1 The framework structure of a Faujasite-type zeolite and simplified structure representations thereof (top ball and stick model, middle simplified stick model, and bottom comparison stick model and polyhedra model).

The isomorphous replacement of aluminum by gallium in the framework structure of zeolites (beta, MFI, offretite, faujasite) offers new opportunities for modified acidity and subsequently modified catalytic activity such as enhanced selectivity toward aromatic hydrocarbons [249,250]. The Ga + ions in zeolites can occupy tetrahedral framework sites (T) and nonframework cationic positions. 

Other framework structures based on zeolites have also been synthesized which contain atoms other than aluminium and silicon, such as boron, gallium, germanium, and phosphorus, which are tetrahedrally coordinated by oxygen. Such compounds are known as zeotypes. Pure aluminium phosphate, commonly called ALPO, and its derivatives, can take the same structural forms as some of the zeolites such as sodalite (SOD), faujasite (FAU), and chabazite (CHA) (e.g., ALPO-20 is isostructural... 

Figure 4- Probable Si, Al ordering schemes in double 6-ring unit of faujasite-type structures. Al positions are marked by circles. B requires 96 AVs and C and D require 64 AVs per unit cell. The space group symmetry of the framework is given for each arrangement of the Al atoms. Numbering refers to the nonequivalent T atoms in the common subgroup F222...

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. 

Nature of acidic sites. The location of the acidic hydroxyl groups in the faujasite structure has been the subject of numerous investigations and much discussion. The results of adsorption experiments with several molecules led Eberly (170) to conclude that the 3550-cm-1 hydroxyl absorption band represented hydroxyl groups located in the hexagonal prisms of the faujasite framework [(Si sites (171)], where they were relatively inac-... 

The structure of the zeolite faujasite is shown in following figure. In this convention representation the framework structure is shown without depicting, atoms directly. Each line represents an SiOSi or Si O Al connection. Four lines meet at tetrahedral vertices representing the positions of the four-coordinate Si or A1 atoms. Spacefilling models of this zeolite show that the pores can accommodate molecules up to about 750 pm in diameter. 

The separation of fructose from glucose illustrates the interaction between the framework structure and the cation (Fig. 5) (50). Ca2+ is known to form complexes with sugar molecules such as fructose. Thus, Ca —Y shows a high selectivity for fructose over glucose. However, Ca—X does not exhibit high selectivity. On the other hand, K—X shows selectivity for glucose over fructose. This polar nature of faujasites and their unique shape-selective properties, more than the molecular-sieving properties, make them most useful as practical adsorbents. 

Figure 10-2 Framework structures and pore cross sections of two types of zeolites. (Top) Faujasite-type zeolite has a three-dimensional channel system with pores at least 7.4 8, in diameter. A pore is formed by 12 oxygen atoms in a ring.

FIGURE 3. Building blocks and simple topological representation of the framework structure of faujasite zeolites. 

Other framework structures containing atoms such as aluminium and phosphorus tetrahedrally coordinated by oxygen have been synthesised and are given the generic name zeotypes. Pure aluminium phosphate (commonly known as ALPO) and its derivatives have been found to take the same structural forms as some of the zeolites, such as sodalite and faujasite, as well as some novel structures. The metal-aluminium phosphates can be formed with metals such as Li, Be, Mg, Mn, Fe and Zn replacing some of the aluminium and these are called MeALPOs. If the compound contains silicon or silicon and a metal, partially replacing aluminium or phosphorus leads to SAPOs and MeSAPOs. 

Figure 6.17 An example of a zeolite framework structure (faujasite). The aluminosilicate framework surrounds void spaces which contain the charge balancing cations. Access to these voids is restricted by the size of the apertures between pores. (Adapted from [204].)... 

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