Three-layer sheet aluminosilicates

Three-layer sheet aluminosilicates, when exchanged into the acidic form, are far less active as hydroisomerization catalysts than zeolites having a comparable surface proton density. However, introducing Ni or Co into the octahedral positions of the Al layer in synthetic beidellite results in hydroisomerization catalysts of an activity similar to that of a zeolite. 

Tetrahedra linked via three vertices correspond to a composition MX1 1X3 2 or MX2 5 = M2X5. Small units consisting of four tetrahedra are known in P4O10, but most important are the layer structures in the numerous sheet silicates and aluminosilicates with anions of the compositions and [AlSiOj-]. Because the terminal vertices of the single... 

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.

Pyrophyllite is the simplest layer aluminosilicate in which two tetrahedral Si04 layers are condensed on to the octahedral A106 layer to produce a three-sheet layer, the composition of the unit cell being [Al2(OH)2(Si205)2]2. Another ideal structure in which A1 is replaced by Mg is that of talc. In both cases the three-sheet layer is electrically neutral and the layers are stacked in the ABAB... sequence. Because of the cohesive strength of this ideal structure, neither pyrophyllite nor talc occurs in the form of the very fine particles which generally characterize clay minerals. 

When considering the natural mineral form, clays mainly consist of aluminosilicates alternated silicate and gibbsite sheets constitute the basic components of clays. In the silicate layer, three oxygen atoms of the tetrahedral Si04 units are shared by different units to form the layered structure reported in Fig. 7.1a. Free oxygen atoms, shown just above each Si atom in the figure, do not belong to one sheet, but may serve to link two adjacent sheets with each other. 

When dehydrated, the crystals are submicroporous. being penetrated by water and small ions. They have a layerlike structure which, in some types, expands and contracts during ion exchange or hydration-dehydration like the layerlike clays. The layer structures thus differ from the zeolitetype aluminosilicates, which have a rigid, three-dimensional lattice. The unusual, extremely thin alica sheets from a synthetic sodium polysilicate are shown in Figure 5.24 (495). 

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