Isomorphous substitution montmorillonites

Montmorillonite is a laminar and expandable clay with wet binding properties and widely available throughout the world. The layers have permanent negative charges due to isomorphic substitutions. The scientific interest of montmorillonite lies in its physical and chemical properties as well as its low price. Consequently, the industrial application of montmorillonite is an attractive process [1]. On the other hand, among numerous reports published so far, crystallization of zeolite Beta draws much attention because of its unique characteristics, in particular, acidity and acid catalysis. It is reasonable to conceive that a catalyst system based on Beta/montmorillonite composite with suitable composition should provide a good catalytic capacity. 

In montmorillonite some of the Al3 + in the octahedral sublattice are replaced by Mg2+ ions, and in hectorite some of the Mg2 + in the octahedral sublattice are replaced by Li+ ions. With beidellite and saponite, however, the isomorphous substitution takes place in the tetrahedral sublattice with Al3 + replacing some of the Si4+ ions. The residual negative charges in the layers on montmorillonite, hectorite, beidellite, and saponite are counterbalanced in the natural state by coexisting interlamellar, hydrated cations, usually Na+,... 

As indicated in Table 1, the three 2 1 groups differ from one another in two principal ways. The layer charge decreases in the order illite > vermiculite > smectite, and the vermiculite group is further distinguished from the smectite group by the extent of isomorphic substitution in the tetrahedral sheets. Among the smectites, those in which substitution of Al for Si exceeds that of Fe2+ or Mg for Al are called beidellite, and those in which the reverse is true are called montmorillonite. The sample chemical formula in Table 1 for smectite thus represents montmorillonite. In any of these 2 1 clay... 

The crystal lattice of montmorillonite, similar to other 2 1 phyllosilicates, may have isomorphic substitutions both in the tetrahedral and octahedral positions. In the tetrahedral positions, the central tetravalent silicon can be substituted by trivalent aluminum ions in the octahedral positions, the trivalent aluminum ions can be substituted by bivalent (usually magnesium and iron(II)) cations of similar... 

Montmorillonite is a 2 1 clay with isomorphic substitutions mainly in the octahedral sheet and some substitutions in the tetrahedral sheets. When the clay is exchanged with monovalent ions, water and electrolyte ions can enter the interlayer spacing and delaminate the system. With Li+ or Na+ as the exchanging cations the delamination is almost complete, whereas with K+ or Cs+ the delamination is less effective.4849 At low pH, edge-to-face interactions can lead to the formation of aggregates. 

Montmorillonite clay s degree of isomorphic substitution and the ability to expand and contract in the presence of water inpart a high CEC. The expanding of the crystal lattice allows the ions between the crystal edges to become available for exchange. This give., montmorillonite clay a 17 to 20... 

The incorporation of Mg 2+ into OL-1 is consistent with the finding of Mg2+ in some natural birnessite materials. The Mg2+ isomorphous substitution in octahedral sites of OL-1 is also similar to the substitution of Mg2+ ions in octahedral layers of certain smectite clays where Mg2+ substitutes for some A 2+, such as in montmorillonite which has an ideal composition of... 

To state categorically that isomorphous substitution in the montmorillonites concerns only the octahedral layer would be misleading. Some tetrahedral replacement could and does occur, but for most minerals within this class the observed exchange capacity correlates closely with the degree of octahedral substitution. 

Aluminosilicates such as montmorillonite, kaolinite, illite, and vermiculite are solids that have structures readily accessible to counter ions. The excess negative charge resulting from isomorphic substitution of Al for Si is primarily distributed over the three adjacent surface O atoms of the layer, where it is electrically balanced by mobile, exchangeable cations- Thermodynamically, ion exchange can be interpreted in terms of the interlayer electrostatic interaction between surface charges and hydrated cations in accordance with the classical Eisenman theory (Eisenman, 1983). A comprehensive description has recently been given by Maes and Cremers (1986). 

Smectites, which are based on either the trioctahedral 2 1 (talc) or dioctahedral 2 1 (pyrophyllite) structure, differ from these neutral structures by the presence of isomorphous substitution in the octahedral or tetrahedral sheet. For example, the dioctahedral smectite, montmorillonite, has the general formula... 

Montmorillonite is a layered smectite clay. It has a central gibbsitic octahedral layer of alumina sandwiched between two tetrahedral layers of sUica. But usually isomorphous substitution occurs and natural clay has Al substituted by Mg and Fe leading to negative charge on the layers. The interlamellar space between the layers is occupied by hydrated cations usually Na", K Ca" etc. balancing the negative charge on the layCTS. 

Like the montmorillonites, the micas can be regarded as derived from talc or pyrophyllite by isomorphous substitution. Imagine that we start with pyrophyllite, i 2Si40io(OH)2, and replace one in four of the Si atoms by A1 this substitution would result in a charged lattice having the formula [Al2(Si8Al)Oio(OH)2] Note that the substituent A1 is written separately because it is in the silica, as distinct from the gibbsite layer. 

The theory of structural acids is largely due to Pauling (22). In any crystal lattice involving both n ative and positive ions, a net negative charge can be created by the isomorphous substitution of a positive ion of a valence lower than that of the substituted podtive ion. Thus, if an aluminum ion is substituted for a silicon ion in a silica lattice made up of silica tetrahedra, a tiivalent ion has been substituted for a quadrivalent ion and there results a positive valence deficiency of one for each aluminum ion so isomorphously introduced. In many naturally-occurring silica-alumina structures, this type of substitution has taken place. In all these systems the valence deficiency or net n ative charge in the crystal lattice is made up or satisfied by a positive ion at or near the point in the structure at which the substitution has taken place. Materials typical for these structural characteristics are natrolite and other natural zeolites, montmorillonites, and feldspars. 

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