Aluminosilicates

Aluminosilicates represent solid acids (42). In general, the nature of their acidity is quite clear and is associated with the chemical peculiarity of the substitution of a tetrahedral Si4 + ion by an Al3 + ion in the silicate structure. At the same time, the detailed structure of BASs and LASs in a particular system still remains the subject of experimental studies. Recently quantum-chemical methods have also come to find ever-increasing use in this field. To 

The original literature devoted to the structures and properties of zeolites was reviewed in Rabo (69). Therefore, in what follows we shall discuss only some aspects which are relevant to a quantum-chemical treatment. 

The crystalline structure of the most intreresting forms of zeolites is now well investigated. It involves the primary structural units in the form of tetrahedra T04 occupied either by Si or A1 atoms. The secondary units are constituted by the tetrahedra joined by their corners. Such units include 4, 5, 6, and 8-membered rings, hexagonal prisms, etc. 

Bronsted acidity, characteristic of H forms of zeolites, is associated with bridged hydroxyl groups. Another type of center is represented by the cations in the cationic forms of zeolites. They are mainly located within 4- and 6-membered rings. Just these two types of sites have comprised the principal subject of quantum-chemical studies. They will be discussed below. 

A simple cluster model of a bridged hydroxyl group in a zeolite is cluster 3. Such a cluster with A = H was used by Chuvylkin et al. (70) as early as 1975 to discuss the properties of possible intermediate structures in the catalytic isomerization of butenes on aluminosilicate surfaces in terms of CNDO/2 approximation. Mikheikin et al. (34) have used a similar cluster with terminal pseudo-atoms A to study the Bronsted acidity of zeolites and its dependence on the Si/AI ratio. 

Aluminosilicates are the active components of amorphous silica—alumina catalysts and of crystalline, well-defined compounds, called zeolites. Amorphous silica—alumina catalysts and similar mixed oxide preparations have been developed for cracking (see Sect. 2.5) and quite early [36,37] their high acid strength, comparable with that of sulphuric acid, was connected with their catalytic activity. Methods for the determination of the distribution of the acid sites according to their strength have been found, e.g. by titration with f-butylamine in a non-aqueous medium using adsorbed Hammett indicators for the H0 scale [38], 

The original view, that in the reaction of silica with aluminium hydroxides a strong aluminosilicic acid, which possesses a dissociable proton (e.g. ref. 2), is formed has not been proved. H-aluminosilicates are unstable and spontaneously convert to aluminium aluminosilicates [19]. 

Aluminosilicates are formed by the combination of AI2O3 and Si02- We have seen in mica fhaf A1 ions substitute for ions. For each of this substitution, 

Therefore, the most likely structure is the sheet structure. 

Silica occurs widely in Nature as quartz, often in large transparent crystals of characteristic shape but also in the translucent agglomerations of microscopic crystals known as chalcedony, which includes cherts and flint. Other natural crystalline varieties of Si02 include tridymite and cristobalite (opal is a semiprecious stone that consists of microcrystalline, hydrous cristobalite). All forms of silica involve three-dimensional networks of corner-linked Si04 tetrahedra. 

If we were to replace some of the Si + ions with Al +, it would still be possible to have a three-dimensional Si—0—A1 network, but cations would be needed to counterbalance the now anionic structural framework. Thus, for each substituent Al , we must add, say, an Na , a K , or half a Ca + ion. The feldspars, which along with quartz and micas (see below) are typical constituents of granites, can be viewed in this way  

K[AlSi30g] orthoclase Na[AlSi308] albite Ca[Al2Si208] anorthite. 

Plagioclase is a solid solution of albite in anorthite (or vice versa). 

The micas have layer structures in which silicate sheets are combined with aluminate units the aluminum ions can be octahedrally as well as tetrahedrally coordinated. For example, the mica muscovite contains both octahedral and tetrahedral Al  

Kulikov, Izvest. Akad. Nauk S.S.S.R., Neorg. Materialy, 1973, 9, 604. 

The crystal structure of a natural sample of a 2M2 lepidolite (a mica-type mineral) from the island of Elba (Italy) has been determined by singlecrystal X-ray diffraction techniques and compared with that of a second 2M2 lepidolite from Rozna (Czechoslovakia) and other related structures. The two sets of unit-cell parameters are collated in Table 38. The most interesting features which emerge from the comparison are the constant presence of remarkable octahedral ordering and the almost identical composition of the tetrahedral sheets (Elba Si3 3oAlo.64 Rozna Sis soAlo.ei) 

investigation of both single-crystal and polycrystalline samples of hydrosodalite Na4[Al3Si30x2](OH),3H20 has been undertaken on the basis of a detailed analysis of the spectra, a model has been proposed for 

747 Yu. Galitskii, V. N. Shcherbakov, and S. P. Gabuda, Soviet Phys. Cryst., 1973,17, 

Above 800 C an important chemical change takes place with the formation of one of the three aluminosilicate polymorphs (Al SiOj), i.e., andalusite, kyanite, or sillimanite, and free silica according to the overall chemical reaction  

If firing is carried out above 1595 C, the highly refractory mineral muttite then forms (see mullite) with an additional liberation of free sihca that melts according to the following chemical reaction  

The incorporation of foreign ions into the crystal lattice of the mullite for refractory applications has been examined in depth. These investigations established the content of ions of a specific size that can be incorporated into the mullite lattice without destroying it. 

For example, up to 9% ions with a radius of 0.5-0.7 A can be incorporated into mullite, 2Si02 3Al203. 

Mullite also exhibits a different morphology compared with other crystals. The double einer-chain silicate demonstrates preferred crystallization in a needlelike habit. If crystallization is uncontrolled, undesirable large mullite needles may occur in the glass-ceramic as a secondary reaction in the ceramic-forming process. This type of crystallization can be initiated by surface nucleation. The properties of such a material, its strength in particular, are negatively influenced. 

To avoid uncontrolled crystallization in the development of a material exhibiting specific optical properties MacDowell and Beall (1969), and Beall (1992, 1993) produced a mullite glass-ceramic in the Si02—AI2O3—B2O3— system. The composition of this optically optimized material is shown in Table 2-5. 

Composition (in wt%) of the Muiiite Giass-Ceramic Used in Soiar Ceiis (Beall and Pinckney, 1999) Additives 0.1 wt% CrgOg, 0.4 wt% AS2O5  

The other interesting ehemical character is that the silicon atom in [SiO ] may be replaced by cations of similar sizes without significant modification of the structure. The most important one is aluminum Al(III). However, when this happens, the unit [AlOJ will now carry 5- electric charge instead of 4-. That means that cations (M(I) or M(II)) must be added to compensate the electric charge difference when Si(IV) is substituted by Al(III), or that an O unit is replaced by an OH unit. Al(III)-substitution occurs often in amphibole-type minerals. The amphiboles contain [Si Oj J repeating units as mentioned earlier (Fig. 14.3), and A1 can replace Si up to the extent of [AlSi30 J. The extent of Al-substitution depends on the condition of formation of such minerals. Minerals of hornblende has the composition of Ca3Na, (Mg,Fe,Al)3[(Al, 

By far the most important rock minerals are feldspars, which constitute about 60% of the igneous rock. They are essentially made of three fundamental types of minerals albite NaAlSi30g, orthoclase KAlSi30g, and anorthite CaAl SiO. Their structures are continuous three-dimensional network of [SiO ] and [AlO ] tetrahedrons, interspersed by positively charged sodium (Na(I)), potassium (K(I)), or calcium (Ca(II)). Albite and anorthite mix in arbitrary proportions such mixed minerals are known as plagioclase feldspars. Albite and orthoclase also make mixtures they are alkali feldspars. 

Another type of widely occurring aluminosilicates is classified as zeolite. Zeolites have much more open structures than feldspars, and, as a result, they can take up loosely bound water or other small molecules into their cavity. Now zeolites are artificially created, which have specific chemical characteristic and holes that accommodate only a specific substance. 

Several different zeolite structures (0) sodalite. If) Linde A, and (c) faujasite. (From D. A. whan, Chemfetiy in Britain, voL 17, p. 532. Copyright d 1981. Reproduced by permission of The Royal Society of Chemistiy.) 

A commercial water softener, (o) The calcium and magnesium ions of hard water are trapped in an ion-exchange resin or a zeolite and are replaced by sodium ions. 

Micas are also aluminosilicates. For example, in muscovite, KAl2[AlSi30io] (0H)2, the extra cations are located between the aluminosilicate anionic sheets. This material can be split up into sheets so thin that 1,000 are needed to make a pile only 1 inch high. Vermiculite is a hydrated mica that splits off into soft flakes, scales, and layers when dehydrated. It makes an excellent packing material or soil conditioner. A variety of materials such as clays, feldspars, and talc are also aluminosilicates, as is the absorbent used as cat litter. 

---

A tremendous variety of structures is known, and some of the three-dimensional network ones are porous enough to show the same type of swelling phenomena as the layer structures—and also ion exchange behavior. The zeolites fall in this last category and have been studied extensively, both as ion exchangers and as gas adsorbents (e.g.. Refs. 185 and 186). As an example, Goulding and Talibudeen have reported on isotherms and calorimetric heats of Ca -K exchange for several aluminosilicates [187]. 

The traditional definition of a zeolite refers to microporous, crystalline, hydrated aluminosilicates with a tliree-dimensional framework consisting of comer-linked SiO or AlO tetrahedra, although today the definition is used in a much broader sense, comprising microporous crystalline solids containing a variety of elements as tetrahedral building units. The aluminosilicate-based zeolites are represented by the empirical fonmila... 

Additional to tire aluminosilicate-based zeolites, a number of otlier crystalline microporous tliree-dimensional oxides have been syntliesized [25]. Most prominent among tliese are tire aluminophosphates (ALPO series) [26,... 

Bibby D M, Milestone N B and Aldridge L P 1979 Silicalite-2 a silica analogue of the aluminosilicate zeolite ZSM-11 Nature 280 664-5... 

Guth J-L and Kessler H 1999 Synthesis of aluminosilicate zeolites and related silica-based materials Catalysis and Zeolites, Fundamentals and Applications ed J Weitkamp and L Puppe (Berlin Springer) pp 1-52... 

Aluminium is not found free but its compounds are so widespread that it is the most abundant metal in the earth s crust. Aluminosilicates such as clay, kaolin (or china clay), mica and feldspar are well known and widely distributed. The oxide. AI2O3. occurs (anhydrous) as corundum and emery, and (hydrated) as bauxite. Cryolite. Na,AlF. (sodium hexafluoroaluminate). is found extensively in Greenland. 

Zeolite A [1318-02-1] Zeolite (aluminosilicate) Zeolite catalysts Zeolite L Zeolites... 

Aluminosilicate glasses are used commercially because they can be chemically strengthened and withstand high temperatures. Thus apphcations include airplane windows, frangible containers, lamp envelopes, and flat panel display devices. 

Aluminosilicates. These silicates consist of frameworks of silica and alumina tetrahedra linked at all corners to form three-dimensional networks familiar examples are the common rock-forming minerals quartz and feldspar. Framework silicates generally form blocky crystals, more isotropic... 

Table 3. Some Synthetic Zeolites Prepared from Sodium Aluminosilicate Gels...

---