Sheet aluminosilicates

The intercalates of sheet aluminosilicates (clays) and of graphite are demonstrated to be efficient catalysts for a variety of reactions, and results obtained using several analytical techniques, including magic angle spinning KMR, are presented. For the clay family,... 

This i>aper describes two broad types of intercalation conpomds which are based on graphite on the one hand and sheet aluminosilicate (clay) hosts on the other. Taken together these provide a rich veiriety of examples of heterogeneously catalysed reactions. Appropriately cation exchanged clays, for example, act as efficient catalysts for a number of commercially important proton catalysed reactions (10-13) (see Table I). Graphite intercalates, whilst also capable of... 

Figure 1. Schematic illiistration of the structure of sheet aluminosilicates (After Brindley, G.W. MacEwan, D.M.C. In "Ceramics - a syn josium" Green, A.T. Stewart, G.H. Eds. Br. Ceramic Society 1953, 15).

Table II. Idealised coiqpositions of representative 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. 

More complex (and more common) structures result when some of the sili-con(IV) in silicates is replaced by aluminum(III) to form the aluminosilicates. The missing positive charge is made up by extra cations. These cations account for the difference in properties between the silicate talc and the aluminosilicate mica. One form of mica is KMg (Si1AlO10)(OH)2. In this mineral, the sheets of tetrahedra are held together by extra K+ ions. Although it cleaves neatly into transparent layers when the sheets are torn apart, mica is not slippery like talc (Fig. 14.40). Sheets of mica are used for windows in furnaces. 

FIGURE 14.40 The aluminosilicate mica cleaves into thin transparent sheets with high melting points. These properties allow it to be used for windows in furnaces. 

Many varieties of clay are aluminosilicates with a layered structure which consists of silica (SiOa" ) tetrahedral sheets bonded to alumina (AlOg ) octahedral ones. These sheets can be arranged in a variety of ways in smectite clays, a 2 1 ratio of the tetrahedral to the octahedral is observed. MMT and hectorite are the most common of smectite clays. 

Silicates also exist in which each silicon atom bonds to one outer oxygen and to three inner oxygen atoms. The result is a linked network in which every silicon atom forms three Si—O—Si links, giving a planar, sheet-like structure. The empirical formula of this silicate is S12 O5. In many minerals, aluminum atoms replace some of the silicon atoms to give aluminosilicates. The micas—one has the chemical formula... 

KMg3(AlSi3 Oio) (0H)2 —contain sheets in which every fourth silicon atom is replaced by an aluminum atom. Because of the planar arrangement of its aluminosilicate network, mica is easily broken into flakes. Figure 9-19 includes a photograph of mica. 

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). 

The hydroamination of alkenes has been performed in the presence of heterogeneous acidic catalysts such as zeolites, amorphous aluminosilicates, phosphates, mesoporous oxides, pillared interlayered clays (PILCs), amorphous oxides, acid-treated sheet silicates or NafioN-H resins. They can be used either under batch conditions or in continuous operation at high temperature (above 200°C) under high pressure (above 100 bar). 

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... 

Taking this one step further, perhaps even an inorganic gene may have been provided by clay mineral sources. Earliest clay samples are of a mineral called montmorillonite that consists of sheets of aluminosilicates in which Fe2+, Fe3+ and Mg2+ are substituted for some of the Al3+, and Al3+ is substituted for Si4+. The oxygen content of the layers does not change and the alternative valencies allow the production of positive and negatively charged layers. Dramatically, Paecht-Horowitz and co-workers showed that the amino acid adenylate could be polymerised with up to 50 units on the montmorillonite surface in aqueous solution. Similar condensation reactions for carbohydrates on hydrotalcite surfaces have... 

Figure 4.6 Layer structure of kaolinite. Edge view showing the aluminosilicate sheets, and the stacking arrangement of these sheets. (After Evans, 1966 Figure 11.11, by permission of Cambridge University Press.)...

Fig. 2.1 Configurations of the tetrahedral units and chain, double chain, and sheet structures in the silicate and aluminosilicate minerals. (A) Two-dimensional representation of a single silicate tetrahedron. (A ) Two-dimensional representation of an extended silicate chain. (B) Three-dimensional representations of single tetra-hedra in two orientations. The apexes of the tetrahedra point above or below the plane of the paper. (B ) Three-dimensional representations of extended silicate chains showing different orientations of the tetrahedra in two of the many possible configurations. Single chain pyroxenes (C), wollastonite (D), rhodonite (E). Double chains amphiboles (F). Sheets as found in the serpentines, micas, and clays (G).

ALUMINOSILICATES WITH SHEET STRUCTURES THAT FORM FIBERS... 

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.

Because the clays have a basic layered structure of aluminosilicate sheets the expectation was a platy habit. The creation of tubular forms was studied by Bates (1959). He postulated that the curved forms, observed by electron microscopic investigation of halloysite and chrysotile, originated through the relief of strain between sheets of unequal dimensions. 

Silica and aluminosilicate fibers that have been exposed to temperatures above 1100°C undergo partial conversion to mullite and cristobalite (1). Cristobalite is a form of crystalline silica that can cause silicosis, a form of pneumoconiosis. IARC has determined that cristobalite should be classified as 2A, a probable carcinogen. The amount of cristobalite formed, the size of the crystals, and the nature of the vitreous matrix in which they are embedded are time- and temperature-dependent. Under normal use conditions, refractory ceramic fibers are exposed to a temperature gradient, thus only the hottest surfaces of the material may contain appreciable cristobalite. Manufacturers Material Safety Data Sheets (MSDS) should be consulted prior to handling RCF materials. 

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