Smectite clays, layer lattice structures

Here we report the synthesis and catalytic application of a new porous clay heterostructure material derived from synthetic saponite as the layered host. Saponite is a tetrahedrally charged smectite clay wherein the aluminum substitutes for silicon in the tetrahedral sheet of the 2 1 layer lattice structure. In alumina - pillared form saponite is an effective solid acid catalyst [8-10], but its catalytic utility is limited in part by a pore structure in the micropore domain. The PCH form of saponite should be much more accessible for large molecule catalysis. Accordingly, Friedel-Crafts alkylation of bulky 2, 4-di-tert-butylphenol (DBP) (molecular size (A) 9.5x6.1x4.4) with cinnamyl alcohol to produce 6,8-di-tert-butyl-2, 3-dihydro[4H] benzopyran (molecular size (A) 13.5x7.9x 4.9) was used as a probe reaction for SAP-PCH. This large substrate reaction also was selected in part because only mesoporous molecular sieves are known to provide the accessible acid sites for catalysis [11]. Conventional zeolites and pillared clays are poor catalysts for this reaction because the reagents cannot readily access the small micropores. 

Among all layered silicate clays, the smectite family of 2 1 layer lattice structures are preeminent in their ability to adsorb organic molecules and to catalyze their chemical transformations. All metal oxides in the soil environment may exhibit some degree of surface reactivity. However, the adsorptivity and reactivity of typical smectites are facilitated by their relatively high internal surface areas 700 m2/g) and external surface areas (10-50 m2/g). 

We have been investigating the use of imogolite as a pillaring agent for smectite clays with layer lattice structures ". The regular intercalation of the tubes within the layered host results in the formation of a tubular silicate-layered silicate (TSLS) complex. These new nanocomposite materials may be viewed as pillared clays in which the pillars themselves are microporous. Significantly, the TSLS structure is thermally stable up to 450 C when montmorillonite is selected as the layered host . 

Smectite clays are a family of complex layered oxides with 2 1 layer lattice structures analogous to those of muscovite, phlogopite and other mica minerals [6]. Figure 2 illustrates the 2 1 structure wherein a central M04(0H)2 octahedral sheet is symmetrically cross-linked above and below to two tetrahedral MO4 sheets. Aluminum, iron, magnesiiun and sometimes lithium... 

Figure 1. A schematic representation of the oxygen positions defining the mica-like layer lattice structure of a smectite clay. In hectorite, the tetrahedral positions are occupied by silicon, magnesium and lithium occupy octahedral positions. The gallery cations in the pristine mineral are alkali metal or alkaline earth cations.

The immobilization of metal complex catalysts on polymers and inorganic oxides has received considerable attention as a means of combining the best advantages of homogeneous and hetereo-geneous catalysis (1-6). The swelling layer lattice silicates known as smectite clay minerals have added an important new dimension to metal complex Immobilization. These compounds have mica-type structures in which two-dimensional silicate sheets are separated by monolayers of alkali metal or alkaline earth cations (7). The structure of a typical smectite, hectorite, is illustrated in Figure 1. 

The smectite group [see Table 9.1 and Fig. 9.3(a)] includes any clay whose interlayer repeat distance (thickness of individual T 0 T layers plus interlayer spacing) expands to 17 A on treatment with ethylene glycol (cf. Drever 1988). This is indicative of a structure in which the number of interlayer cations is smaller than 0.65 (Greenland and Hayes 1978), and is usually about 0.2 to 0.5 per formula unit Ojo(OH)2 (Drever 1988). The interlayer cations are adsorbed, and are necessary to balance the unsatisfied net charge (usually negative) of the clay crystal lattice caused by structural substitutions or vacancies in the octahedral and/or tetrahedral layers. For example Mg or another divalent cation may substitute for Al in the octahedral layer, or Al or Fe may replace Si" " in the tetrahedral layer. 

The smectite clays, for example, montmorillonite, hectorite, beidellite, and the mica minerals, for example, talc and pyrophillite possess a structure which is exemplified in Figure 67. The host lattice is formally composed of three sublayers two tetrahedral Si/O and one octahedral M/O, OH (M = Al, Mg) central layer. These layers bear an excess negative charge and compensation of these charges is achieved by the presence of interlayer cations. 

In many layer structures, such as clay minerals, the extent of lattice adjustment on entry of guest molecules is intermediate between the behaviors of zeolites and of clathrates. The layers remain intact, but the distance between them changes substantially (8). For water-free smectite crystals the d(001) distance is 9.4 A. The van der Waals diameter of a water molecule is 2.8 A so that, in batavite, for example, the water layer in the Na form is about 14.8 — 9.4 = 5.4 A thick, corresponding with two monolayers. 

Smectites are water swellable clays that have a platy structure. Smectite is the mineralogical term for a group of clays, which includes montmorillonite, hectorite, and saponite. Most smectites are more commonly known under the geological term bentonite. By convention, bentonite is understood to be an ore or product with a substantial smectite content. The range of possible chemical variations in the basic smectite trilayer lattice starts with montmorillonite, the high-aluminum end member. Montmorillonite is composed of a central alumina octahedral layer sandwiched between tetrahedral silica layers. This is identical to the dioctahedral pyrophyllite structure except for small... 

At the other end of the smectite series are the high-magnesium members, hectorite and saponite. These clays possess a talc structure, with a trioctahedral magnesia l er sandwiched between the silica layers. Swellability results from minor substitution of aluminum for silicon in saponite or Hthimn for magnesium in hectorite. As with montmorillonite, the type of exchangeable cation determines the degree of swelling. Hectorite also has partial substitution of lattice hydroxyls by fluorine. 

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