Silicon-aluminum aluminosilicate framework

It is generally accepted that aluminum is tetrahedrally coordinated in aluminosilicate glasses and melts, provided that cations such as alkali metals or alkaline earth metals are present in sufficient amounts to charge compensate the replacement of Si by AF+ [i.e., Si + —> AP" -t- (1/ )M" +, where monovalent or divalent M ions occur in nonframework sites associated with the tetrahedral aluminosilicate framework]. Experimental data for aluminosilicate glasses derived from both low-angle x-ray-scattering experiments and EXAFS studies show that as aluminum is substituted for silicon, the average T (tetrahedrally coordinated ion) -O bond... 

Zeolites and molecular sieves are crystalline porous metal oxides possessing enormous internal surface area (—1000 mVg), where every metal cation is either an internal or external surface site [3]. Zeolites are composed of an aluminosilicate framework of the type M ,/ [(A102)jc(Si02) ] zH20 (where n is the charge on the metal cation, which is typically Na, K, Ca ). Molecular sieves differ from the aluminosilicate zeolites because they possess framework atoms other than aluminum or silicon (P, Ga, Ti, V, etc.). Zeolites and molecular sieves possess a wide variety of properties pore size, pore shape, dimen-... 

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. 

Zeolite molecular sieves are composed of silicon and aluminum and can be natural or manmade minerals. Molecular sieves are crystalline, hydrated aluminosilicates of (most commonly) sodium, calcium, potassium, and magnesium. The alumininosilicate portion of the structure is a three-dimensional open framework consisting of a network of A104 and Si04 tetrahedra linked to each other by sharing all of the oxygens (Sherman, 1978). Zeolites may be represented by the empirical formula... 

Two adjacent aluminum tetrahedra, sharing a corner, contribute two bonds of strength f to the shared oxygen atom. The total of (which might be increased by a small amount by bonds from alkali or alkaline-earth ions) represents a deviation from the electrostatic valence rule such that in general in aluminosilicates of the tetrahedral framework type the Al/Si ratio does not exceed 1, and when it equals 1 there is good ordering, with alternation of the aluminum and silicon tetrahedra. 

It has been reported by F.A. Mumpton (1978) that more than 1000 occurrences of zeolite minerals in over 40 countries have been discovered since 1950. In addition to the hydrated aluminosilicate species, new minerals related to zeolites have been discovered, including the porous clathrasUs such as Melonophlogite (a silica only framework). Species in which the aluminum or silicon has been replaced by other elements (such as phosphorus, iron, and berylhum) have also been discovered as exemplified by viscite, a sihcoaluminophosphate related to analcime. At the present there are 3 8 different natural framework topologies, as shown in Table 25. 

An important class of minerals called aluminosilicates results from the replacement of some of the Si atoms in silicates with aluminum (Al) atoms. Aluminum is the third most abundant element in the earth s crust (8% by mass), where it occurs largely in the form of aluminosilicates. Aluminum in minerals can be a simple cation (Al ), or it can replace silicon in tetrahedral coordination. When it replaces silicon, it contributes only three electrons to the bonding framework in place of the four electrons of Si atoms. The additional required electron is supplied by the ionization of a metal atom such as sodium (Na) or potassium (K) the resulting alkali-metal ions occupy nearby sites in the aluminosilicate structure. 

Nearly all syntheses of zeolites and microporous aluminophosphates have limitations to gel composition and other parameters. For example, some zeolites with special compositions such as high-silica Y zeolite and low-silica ZSM-5 cannot be directly synthesized. A secondary framework modification is necessary for their preparation. For instance, dealuminization, isomorphous substitution of extraneous silicon for aluminum, and removal of the sodium process in Y zeolite are necessary to prepare ultra-stable zeolite Y (USY) isomorphous replacement of framework atoms of boron with aluminum in a presynthesized silicon-boron structure is often used to prepare some specific aluminosilicate zeolites that cannot be directly synthesized, such as Al-SSZ-24 (AFI) and Al-CIT-1. Secondary synthesis (post-treatment) will be discussed in detail in Chapter 6. 

In framework aluminosilicates, there are five possibilities, described by the formula Si(nAl), where n = 0, 1, 2, 3 or 4. These five basic units of Si(nAl) express the fact that each silicon atom is linked, via oxygens, to n aluminum... 

In some ways zeolites are similar in their properties in that they also provide size and shape selectivity because of the presence of pores within the three-dimensional structure. However in the zeolites these pores are generally much larger so that larger molecules can be incorporated into the structure. Zeolites are crystalline aluminosilicates which occur both in nature and as synthetic structures [204]. The zeolite framework is made up of aluminum and silicon atoms tetrahedrally coordinated by oxygen, giving a framework stoichiometry of MO2. For each aluminum atom within the framework there is a formal charge of —1 which is compensated by counterbalancing cations within pores in the structure. Typically these may be cations such as Na" ", Ca ", NH4, or HsO". ... 

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