Zeolites, synthetic crystal structure

Under the mineralogical name zeolite such sieves occur naturally. For technical purposes due to their higher uniformity only synthetic zeolites are used [10], In the empirical formula Me is an exchangeable cation of the valence n (zeolites are cation exchangers). Molecular sieves have a very regular and orderly crystal structure, which is characterized by a three-dimensional system of cavities with a diameter of 11 A. These cavities are interconnected by pores with a constant diameter. The value of this diameter depends on the type of the exchangeable cation Me. It is 5 A, if in the above formula Me stands for 75% Na+ and 25% Ca2+. 

B. (1994) Rietveld refinement of the crystal structure of the synthetic porous zincosihcate VPl-7. Zeolites, 14,... 

The discrepancy in numbers between natural and synthetic varieties is an expression of the usefulness of zeolitic materials in industry, a reflection of their unique physicochemical properties. The crystal chemistry of these aluminosilicates provides selective absorbtion and exchange of a remarkably wide range of molecules. Some zeolites have been called molecular sieves. This property is exploited in the purification and separation of various chemicals, such as in obtaining gasoline from crude petroleum, pollution control, or radioactive waste disposal (Mumpton, 1978). The synthesis of zeolites with a particular crystal structure, and thus specific absorbtion characteristics, has become very competitive (Fox, 1985). Small, often barely detectable, changes in composition and structure are now covered by patents. A brief review of the crystal chemistry of this mineral group illustrates their potential and introduces those that occur as fibers. 

Zeolite, zeolite group A collective term for a family of aluminosilicate minerals characterized by framework structures that allow easy access and exchange of cations and small molecules (see chapter 2), The name derives from the Greek terms zein, meaning to boil, and lithos, meaning stone. The term is also applied to synthetic materials of comparable composition, crystal structure, and physical properties (see chapter 2). 

N2 02, neopentane) in the zeolites A, X, L, mordenite, omega, and a synthetic offretite type have been determined from isotherms. These have been compared with the void volumes calculated from the known crystal structures. For most adsorbates the measured and calculated void volumes are in good agreement. However, helium and nitrogen exhibit anomalous behavior. A void volume-framework density relation for zeolites is given. 

In these experiments, synthetic zeolites of the faujasite-type without binding substance were used. Calcium and nickel-calcium samples in ionic form were obtained by ion exchange under conditions ensuring stability of the crystal structure (5). Platinum addition was carried out by ion exchange with Pt(NH3)6Cl4 (6). 

Fig. 1.1 The structure of the synthetic zeolite ZSM-5 (a) microscopic crystals (b) an electron micrograph of the area marked in (a) (c) the crystal structure of ZSM-5 related to the electron micrograph. ICourtesy of J. M. Thomas. Royal Institute of Chemistry.]...

Many zeolites were crystallized from strongly basic solution containing silicate and aluminate. In strongly basic solution, silicate exists as polymerized-state polysilicates with a variable distribution of molecular weight. Aluminate reacted with polysilicate to form various zeolite structures via condensation reaction under hydrothermal or solvothermal conditions. Therefore, in the study of the synthetic chemistry of zeolites, one of the key issues is to understand the existing state and reactivity of the polymerized silicate. 

Heterogeneous catalysis offers an opportunity to achieve selectivity based on molecular size and shape. Excellently suited for this purpose are zeolites with their crystal structure of regularly repeating unit cells with voids large enough to accommodate reactants. For catalysis, natural zeolites have been large replaced by synthetic products. Table 9.3 lists structural information on some of the most common natural and synthetic materials. 

The term zeolite was coined in 1756 by the Swedish mineralogist Cronstedt who observed that the mineral stilbite frothed and gave off steam when heated.1 The name zeolite comes from the Greek meaning boiling stone . Zeolites have a crystal structure which is constructed from T04 tetrahedra, where T is either Si or Al. In addition to the relatively small number of naturally occurring zeolites there is a wide range of synthetic materials.2... 

The object of this study was to apply mid-infrared spectroscopy to zeolite structural problems with the ultimate hope of using infrared, a relatively rapid and readily available analytical method, as a tool to characterize the framework structure and perhaps to detect the presence of the polyhedral building units present in zeolite frameworks. The mid-infrared region of the spectrum was used (1300 to 200 cm"1) since that region contains the fundamental vibrations of the framework (Si,Al) 04 tetrahedra and should reflect the framework structure. Infrared data in similar spectral regions have been published for many mineral zeolites (30) and a few synthetic zeolites (23, 49, 50). There is an extensive literature on infrared spectra of silica, silicates, and aluminosilicates (17). However, no systematic study of the infrared characteristics of zeolite frameworks as related to their crystal structure has appeared. 

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...

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