Structure of montmorillonite

Of the naturally existing smectites, montmorillonite is a clay of major interest in the subsurface environment. Figure 1.5 shows the crystal structure of montmorillonite,... 

An isopoly cation, [A1i304(0H)24(0H2) 2]7+, has the Keggin structure and can be intercalated into the layer structure of montmorillonite clay,... 

Figure 4,6. Schematic structure of montmorillonite showing the planar sites, the interlayer sites, and edge sites (adapted from Jackson, 1964).

Fig. 10.16. Schematic representation of a) the layered structure of montmorillonite and b) pillared montmorillonite.

Figure 9.3 Diagram showing the structures of montmorillonite (a) and muscovite (b). After Clay mineralogy by R. E. Grim. Copyright 1968 by McGraw-Hill, Inc. Used with permission of The McGraw-Hill Book Companies.

Illite as a non-swelling clay mineral of layer structure plays a very important role in our studies. This special role is due to the fact that both sides of the surface of the silicate lamellae are made up of Si04-tetrahedron planar lattices, and this structure - even when hydrophobized - is identical with the surface structure of montmorillonite and vermiculite, both of which are of the swelling type. 

The catalytic application of montmorillonites may either be related to their acidity [5-7], or their swelling properties [8-9], Swelling, induced by careful selection of both the reaction medium and interlayer cations, enables the reactant molecules to enter the interlamellar region and undergo catalytic reaction on interstitial active sites. Moreover, the layered structure of montmorillonite may induce a substrate selectivity depending on the reactant size. Therefore, clay intercalated metal catalysts offer potential applications in the preparation of fine chemicals. 

The present study makes an attempt to increase mesoporosity within the pillared structure of montmorillonite and to study the catalytic property of the resulting material. Enhancement of mesoporous areas has been observed in a smectite seunple pillared by aluminium polyoxy cations by chemical modification in presence of varied amounts of a nonionic surfactant. The effeet of the enhaneement in mesoporous areas on the pillar density and the deaetivation of the pillared materials in a typical alkylation reaction has been studied. 

FIGURE 5.7. Schematic structure of montmorillonite. (From F. E. Bear, ed. 1964. Chemistry of the Soil. ACS Monograph Series No. 160.)... 

Figure 2. Crystal structure of montmorillonite clay. (Reproduced with permission from reference 14. Copyright 1977 John Wiley and Sons.)...

The interlayer regions of clay minerals also have cation-exchange capability and can be modified by organotransition metal compounds. Montmorillonite is typical of these materials and is readily intercalated with transition metal complex ions. The layered structure of montmorillonite is shown in Figure 3. 

Transport of materials in bentonite is controlled by the interlayer structure of montmorillonite lamellae and by interlayer cations. The governing equation of transport is given by... 

Fig. 10.13. Flake structure of montmorillonite (bentonite). (T, tetrahedron O, octahedron)...

FIGURE 33.1. Representation of the two to one structure of montmorillonite showing the centrai octahedraiiy coordinated Aiuminums sandwiched between two tetrahedraiiy coordinated siiicons. Atoms above and beiow the iayer are exchangeabie cations and their associated waters of hydration. 

Figure 8.1 Crystalline structure of montmorillonite (MMT) clay. (Reproduced with permission of Elsevier Ltd.)...

Fig. 1 Representation for the crystal structure of montmorillonite [14]. Copyright 2011. Reproduced by permission of Elsevier Science Ltd.

Figure 6.5 Structure of montmorillonite clay [53, http //mrsec.wisc.edu/Edetc/pmk/pages/ montmorillonite.html].

Ions in Clays, Fig. 1 Structure of montmorillonite clays. An octahedral (O) aluminuin oxide layer is sandwiched between two tetrahedral (T) silicon oxide layers. Substitutions of A1(III) by Mg(II) in the T layer result in a pmnanent negative charge compensated by counterions (here sodium Na ), located in the interlayer pores and can be hydrated by water molecules. Al and Mg atoms are in green. Si ia yellow, Na in blue, oxygen in red, and hydrogen in white... 

Figure 6.1 Structure of montmorillonite MMT [30] (left) and hydrotalcite HT [27] (right).

Figure 6.1 shows the lamellar structures of montmorillonite and hydrotalcite clays. Because the ions in the interlayer space can easily be replaced by foreign ions (cation/anion), these clays can be modified under appropriate conditions by intercalation with suitable ions to satisfy various practical applications. 

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