Zeolites and Related Structures

The general formula for the composition of a zeolite is M J(A102) f(Si02), ] mU20 

FIGURE 7.1 The zeolite building units. Two Si04/A104 tetrahedra linked by corner sharing. 

FIGURE 7.2 The structure of quartz (a) as a ball and stick representation and (b) as linked [S104] tetrahedra. 

FIGURE 7.3 (a) 6-ring containing two A1 and four Si atoms, (b) computer model of the 6-ring, and (c) shorthand version of the same 6-ring. 

FIGURE 7.5 The relationship between an octahedron, a truncated octahedron, a cuhoctahedron, and a truncated cuhoctahedron. 

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The naming of zeolites and related structures has been somewhat unsystematic. Some structures were named after the parent minerals (e.g., sodalite, faujasite), while others were named by researchers, or after the projects which synthesized them (e.g., ZSM [Zeolite Socony Mobil]). Unfortunately, this led to the same zeolites synthesized by different routes and bearing different names—in some cases, up to 20 different trade names ... 

Figure 14. Water void volume in cc/cc of crystal vs. tetrahedra density in tetrahedra 1000/A3 for synthetic zeolites and related structures of this study adapted from unpublished work of D.W. Breck...

Sorption into, release from and diffusion inside microporous and mesoporous materials are of paramount interest in view of separation processes and catalysis by zeolites and related structures. Thus, volume 7 of the handbook-like series Molecular Sieves - Science and Technology is exclusively devoted to the phenomena of adsorption into, desorption out of and diffusion in the pores of zeolite crystalhtes. 

Xu, R., Pang, W., Huo, O., and Chen, J. (2007) Chemistry of Zeolites and Related Porous Materials, Synthesis and Structure, John Wiley Sons (Asia) Pte. Ltd. 

The technique of solid-state NMR used to characterize supported vanadium oxide catalysts has been recently identified as a powerful tool (22, 23). NMR is well suited for the structural analysis of disordered systems, such as the two-dimensional surface vanadium-oxygen complexes to be present on the surfaces, since only the local environment of the nucleus under study is probed by this method. The nucleus is very amenable to solid-state NMR investigations, because of its natural abundance (99.76%) and favourable relaxation characteristics. A good amount of work has already been reported on this technique (19, 20, 22, 23). Similarly, the development of MAS technique has made H NMR an another powerful tool for characterizing Br 6nsted acidity of zeolites and related catalysts. In addition to the structural information provided by this method direct proportionality of the signal intensity to the number of contributing nuclei makes it a very useful technique for quantitative studies. 

Let us briefly examine modelling of solids, taking silicates as a case study. In order to model silicate structures, it is necessary to develop accurate potentials for the various silicate forms. Such potentials should be able to predict the known structures of zeolites and related materials accurately. Attempts have been made by a number of workers in this direction. The Si04 tetrahedron is the basic building block in silicates. While in... 

The high thermal stability of zeolites and related micro-porous solids is one of their most attractive features. Whilst it Is clear that materials with organic components cannot withstand ultra-high temperatures, quite respectable compositional stability can be achieved. Thus the [Er(TMA)] polymer mentioned above shows no weight loss in its TGA curve before 550°C. However for porous solids another key issue is that of structural stability. Many open framework coordination polymers lose their crystalline structure upon mild heating, or even evacuation, through loss of guest molecules. 

Mineral Surfaces. Organic matter is chemically adsorbed (deriva-tized) at the surfaces of clay minerals, zeolites, and related minerals (105) and is at times protected, concentrated, and degraded by contact with the solid surfaces. For example, porphyrins are protected (106), as are optically active amino acids by montmorillonite (107). This may result in part from the position of the organic matter in lattice spaces, as shown by Stevenson and Cheng (108) for proteinaceous substances keyed into hexagonal holes on interlamellar surfaces of expanding lattice clays, or from the fact that there are ordered structures at solid-water interfaces (109). 

We have already explained the structure of zeolites and related materials (see Section 2.5.1). Diffusion in zeolites is a very important industrial problem and, consequently, it has been comprehensively... 

Zeolites and related aluminosilicates constitute a vital family of microporous materials with immense applications in catalysis, sorption and separation processes [1-3]. The discovery of aluminophosphates is an important landmark in the science of these materials [4], All these materials are, in general, synthesized under hydrothermal conditions by making use of template molecules [2]. The template molecules are usually organic amines and they are involved in the formation of these framework structures in more ways than one. While it is difficult to pinpoint the exact manner in which the amines participate or direct the formation of these inorganic structures, it is generally believed that their size and shape are crucial in determining the pore structure. In recent years, a variety of open-framework structures formed by divalent metal phosphates... 

Some important metal oxide materials that have used molecular and supramole-cular templates to direct structure formation are the zeolites and related semi-crystalline aluminosilicates. In this section we shall discuss the use of ammonium cations that direct formation of microporous zeolites and finish with some of the possibilities that exist with the use of surfactant systems and molecular aggregates to create mesoporous structure. Excellent books and reviews are suggested for additional reading into the detailed description of the art [58-60]. The intention of this section is to briefly introduce this area and describe the types of materials being produced using various imprinting techniques in metal oxide materials. 

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