Aluminosilicates ring structures

Calculated structure of hydrogen adsorbed on aluminosilicate rings (a) 5-ring and (b) 6-ring. Adapted from Figure 1 in Torres etal. (2007a). 

It is helpful in the discussion to describe silicate structures using the Q nomenclature, where Q represents [SiOJ tetrahedra and the superscript n the number of Q units in the second coordination sphere. Thus, isolated [SiO ] " are represented as Q and those fully connected to other Q units as Q. In general, minerals based on Q , Q and units are decomposed by acids. Such minerals are those containing isolated silicate ions, the orthosilicates, SiO (Q ) the pyrosilicates, Si O " (Q ) ring and chain silicates, (SiOg) (Q ). Certain sheet and three-dimensional silicates can also yield gels with acids if they contain sites vulnerable to acid attack. This occurs with aluminosilicates provided the Al/Si ratio is at least 2 3 when attack occurs at A1 sites, with scission of the network (Murata, 1943). 

MCM-22 zeolite (IZA code MWW) presents a novel and original structure which is composed of two independent pore systems a 2-dimensional, sinusoidal, 10-ring intralayer channels system and an interlayer system with 12-ring supercages, accessible through 10-ring apertures [1], This unique pore structure of the aluminosilicate MCM-22 zeolites makes them interesting catalysts for a wide variety of acid-catalyzed reactions, e.g. alkylation of aromatics [2],... 

Flanigen monitored the changes in the IR spectra that occur during the synthesis of NaX zeolite from a sodium aluminosilicate gel. The appearance of absorption bands due to the formation of structural units in the zeoUte as the crystallization of NaX proceeded were observed [93]. In particular, the growth of a band around 575 cm indicated the formation of double six-rings which is one of the structural sub-units of X zeolite. 

Fig. 2.12 Structural components and variations in the micas. (A) Plan view of the continuous aluminosilicate sheet (T), [Si,Al205] , a portion of the mica structure. (B) Stereographic representation of an idealized mica. The structure is composed of continuous layers containing two tetrahedral aluminosilicate sheets (T) that enclose octahedrally coordinated cations, or Mg (O). This layer or sandwich," the T-O-T or 2 1 aggregate, is held together by or Na ions. (C) The two possible positions (I and II) of octahedral cations in the micas. Sets of three locations for each are superimposed on the tetrahedral hexagonal aluminosilicate sheet. (D) The three possible directions of intralayer shift when octahedral set I (upper) or II (lower) are occupied. The dashed lines and circles represent ions below the plane of the paper. (E) Distorted hexagonal rings of apical oxygens in the tetrahedral sheet of dioctahedral micas compared with the undistorted positions of the apical oxygens in the tetrahedral sheet of trioctahedral micas.

Since the main peaks are 3.25A, 4.2-4.6A and 7.0A, the most reasonable unit structure of the sodium aluminosilicate hydrogel is the 4-member chain illustrated in Fig, 11. Many 4-member chains easily form sodalite cages as shown in Fig. 12. In the case of a simpler silica structure, the double 4-member ring (cube) was recognized by Sakka et al C5). 

Silicon, the second most abundant element in the earth s crust, is obtained by reducing silica sand (Si02) with coke. It is purified for use in the semiconductor industry by zone refining. In the silicates, SiC>4 tetrahedra share common O atoms to give silicon oxoanions with ring, chain, layer, and extended three-dimensional structures. In aluminosilicates, such as KAlSi30g, Al3+ replaces some of the Si4+. 

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