Framework aluminosilicate

Zeolites have much more open aluminosilicate frameworks than feldspars and this enables them to take up loosely bound water or other small molecules in their structure. Indeed, the name zeolite was coined by the mineralogist... 

Most of the papers available on desilication are devoted to the alkaline treatment of ZSM-5 zeolites. Since a large number of zeolites consist of aluminosilicate frameworks and framework Al plays a crucial role in the mesoporosity development during desilication, this methodology should be suitable for extrapolation to other... 

Zeolites are structurally related to colorless sodalite, Na4Cl[Al3Si3012], and to deeply colored ultramarines. These have aluminosilicate frameworks that enclose cations but no water molecules (Fig. 16.25). Their special feature is the additional presence of anions in the hollows, e.g. Cl-, S()4, S2, or S. The two last-mentioned species are colored radical ions (green and blue, respectively) that are responsible for the brilliant colors. The best-known representative is the blue mineral lapis lazuli, Na4S (.[Al3Si3012], which is also produced industrially and serves as color pigment. 

The guest molecules experience different potential depending on the nature and the spatial distribution of the ions and the structural modifications in the aluminosilicate framework associated with the Si-Al substitution. Accordingly, the diffusive process can be different [1], The efficiency of migration of guest molecules depends on several factors the Si/Al ratio, the nature of the extra framework cations, the presence of sorbed water molecules, the temperature, and the sorbate concentration [1]. 

Figure 10a, Scheme of the aluminosilicate framework of a typical faujasitic zeolite Si/Al ratio of LI8 (arbitrarily chosen to illustrate the ordering among tetrahedral sites) before (left half) and after (right half) exposure to SiCls, which dealuminates the zeolite (see Figure 10b),... 

Klinowski, J., Carr, S.W., Tarling, S.E. and Barnes, P. (1987). Magic-angle-spinning NMR shows the aluminosilicate framework of ultramarine to be disordered. Nature... 

Since this initial work there has been a plethora of literature on mesoporous molecular sieves. In addition to the silica and aluminosilicate frameworks similar mesoporous structures of metal oxides now include the oxides of Fe, Ti, V, Sb, Zr, Mn, W and others. Templates have been expanded to include nonionic, neutral surfactants and block copolymers. Pore sizes have broadened to the macroscopic size, in excess of 40 nm in diameter. A recent detailed review of the mesoporous molecular sieves is given in ref [73]. Vartuli and Degnan have reported a Mobil M41S mesoporous-based catalyst in commercial use, but to date the application has not been publicly identified.[74]. 

In zeolite synthesis, large cations such as tetramethylammonium (NMe4" ) and tetrapropylammonium (N(C3H7)4" ) can be used as a template around which the aluminosilicate framework crystallizes with large cavities to accommodate the ion. On subsequent heating the cation is pyrolysed, but the structure retains the cavities. Such structures formed around a single molecule template, with pore sizes between 200 and 2000 pm, are known as microporous. 

The formation of novel silicon-rich synthetic zeolites has been facilitated by the use of templates, such as large quaternary ammonium cations instead of Na+. For instance, the tetramethylammonium cation, [(CH3)4N], is used in the synthesis of ZK-4. The aluminosilicate framework condenses around this large cation, which can subsequently be removed by chemical or thermal decomposition. ZSM-5 is produced in a similar way using the tetra-.n-propyl ammonium ion. Only a limited number of large cations can fit into the zeolite framework, and this severely reduces the number of [AIO4] tetrahedra that can be present, producing a silicon-rich structure. 

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. 

Zeolites are intrinsically microporous aluminosilicates of the general formula [(A102) t(Si02) ] mH20 and may be considered as open structures of silica in which aluminium has been substituted in a fraction x/(x + y) of the tetrahedral sites. The net negative charge of the aluminosilicate framework is neutralized by exchangeable... 

The effective area of the anionic aluminosilicate framework in the pores of a zeolite is at least 100 times the external surface area, and it can be as high as 1000 m2 g-1. Consequently zeolites are unusually effective as catalysts for reactions that are favored by aluminosilicate surfaces. Substitution of Sj4+ Al3+ in a silica framework makes it acidic and, potentially, coordinatively unsaturated. Suppose, for example, that we heat the NH4+ form of a zeolite. Ammonia is driven off, and one H+ remains to counterbalance each Al3+ that has substituted for a silicon. The protons are attached to oxygens of the aluminosilicate framework ... 

The zeolites are aluminosilicate framework minerals of general formula M", [AI4Sil0lr+>JJ -zH20.y They are characterized by open structures that permit exchange of catioas and water molecules (Fig. 16.2). In the synthetic zeolites the aperture and channel sizes may sometimes be controlled by a sort of template synthesis—the zeolite is synthesized around a particular organoammonium canon. This yields channels of the desired size. The zeolite framework thus behaves in some ways like a clathrate cage about a guest molecule (Chapter 8). The synthesis of zeolites also involves several other factors such as the Al/Si ratio, the pH. the temperature and pressure, and the presence or absence of seed crystals - ... 

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. 

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