Aluminosilicates framework composition

The preparation of silicon-rich zeolites, such as zeolite Y, can be achieved by varying the composition of the starting materials but can also be done by subsequent removal of aluminium from a synthesized aluminosilicate framework using a chemical treatment. Several different methods are available, including extraction of the aluminium by mineral acid, and extraction using complexing agents. 

C. Determination of the Composition of the Aluminosilicate Framework Using 29Si MAS NMR... 

The total exchange capacity (TEC) of an aluminosilicate zeolite is a function of the framework composition that is, the Si/Al ratio. It is easy to obtain a numerical relation between the TEC, C, in milliequivalent per gram, and the number of A1 atoms per framework unit cell, NAl [5]... 

Zeotypes comprise a variety of different framework compositions. The most prominent group are the zeolites, crystalline aluminosilicates containing channel-like voids with apertures from 4 to ca. 15 A. Zeolites have important uses as catalysts and catalyst supports, as ion-exchangers and in separation processes. Aluminosilicates with cage-like voids are called clathralites in analogy to clathrate hydrates [19]. 

In the above-studied systems, the aluminosilicate framework merely acts as a porous, electrochemically inert support embedding the redox-active guests. In contrast to this, the presence of electroactive atoms in the composition of the aluminosilicates renders the solid electroactive. This applies to the case of titanium... 

The framework composition also affects the stability of a material. For example, a high silica zeolite usually has a higher thermal stability than does the corresponding aluminosilicate, an aluminosilicate lends to be more stable than an aluminophosphale, and a gallophosphale is generally more sensitive to moisture than is an aluminophosphate. 

Zeolites [64] are crystalline aluminosilicates with a three dimensional micro-porous framework formed by corner sharing SiO and AIO4. (i.e. TO4) tetra-hedra. A framework with Si02 composition is stoichiometrically neutral. The substitution of Si by Al in such a silicate framework, results in an excess negative charge, which is compensated by cations or protons. Zeolites have unique adsorption and catalytic properties. Their diversity in framework composition and structure type leads to almost unlimited design opportunities. 

For aluminosilicate zeolites that have more than one T-site in the unit cell (by far the majority), signals from all the Q4 silicons with the same number of second coordination shell aluminium atoms may overlap, and then the formula given previously for the determination of framework composition still holds. Further details of the distribution of aluminium (or gallium) within a zeolite framework can also be obtained from the Si spectrum in this case. For aluminium-rich zeolites the aluminium may order on the short range in... 

Synthesis of zeolite analogs based on other framework compositions have come quite far afield from the original goal. Though the synthesis of all of these phases has utilized methods that have been initially developed to produce aluminosilicate zeolites and silicate molecular sieves, the chemistry of the individual... 

It follows that through its chemical composition and geometry the zeolite host exerts a considerable influence over the structure and stability of ionic clusters in zeolites and does not act merely as a convenient inert host. In particular, the formation of ionic clusters is much more likely to occur in aluminosilicates with a high aluminium content and a sizable anionic framework charge than in neutral aluminophosphate molecular sieves. In fact, a significant proportion of zeolite ionic clusters are found in just three aluminosilicate frameworks, each of which contains the sodalite cage structural unit (Fig. 1). 

Zeolites are aluminosilicates characterized by a network of silicon and aluminum tetrahedra with the general formula Mx(A102)x(Si02)Y. The M are cations that are necessary to balance the formal negative charge on the aluminum atoms. The tetrahedra are linked to form repeating cavities or channels of well-defined size and shape. Materials with porous structures similar to zeolites but with other atoms in the framework (P, V, Ti, etc.), as a class are referred to as zeotypes. The structure committee of the International Zeolite Association (IZA http //www.iza-online.org/) has assigned, as of July 1st 2007, 176 framework codes (three capital letters) to these materials. These mnemonic codes do not depend on the composition (i.e. the distribution of different atom types) but only describe the three-dimensional labyrinth of framework atoms. 

The past nearly six decades have seen a chronological progression in molecular sieve materials from the aluminosilicate zeolites to microporous silica polymorphs, microporous aluminophosphate-based polymorphs, metallosilicate and metaHo-phosphate compositions, octahedral-tetrahedral frameworks, mesoporous molecular sieves and most recently hybrid metal organic frameworks (MOFs). A brief discussion of the historical progression is reviewed here. For a more detailed description prior to 2001 the reader is referred to [1]. The robustness of the field is evident from the fact that publications and patents are steadily increasing each year. 

The early evolution of aluminosilicate zeolites, in the 1950s to 1970s, is summarized in Table 1.2, based on increasing framework Si/Al composition. The four somewhat arbitrary categories are (i) low , (ii) intermediate , (hi) high silica zeolites and (iv) silica molecular sieves. 

Table 1.4 lists some of the major new structures reported in the 1990s. Interestingly, as organic SDAs tended to dominate discovery of new frameworks, there were no new aluminum-rich synthetic zeoUtes reported in either the 1980s or the 1990s. The new aluminosilicate structures were all high silica or pure sihca in composition. It awaited the 2000s for new aluminosilicate zeolite materials with low to medium Si/Al to be reported (see below). 

---