Silica lattice, isomorphous substitution

The acidic centers, both the Br0nsted and Lewis types, are generated by the isomorphous substitution of trivalent aluminum for a tetravalent silicon in the silica lattice.61-63 Additionally, carbocations may be formed by protonation of alkenes,43,52,56,61 which explains their higher reactivity in catalytic cracking. 

Isomorphous substitution with a suitable transition metal, notably Ti, produces an oxidation catalyst. The larger ionic radius, coupled with the preference of Ti for octahedral coordination, produces strain in the silica lattice, favouring the splitting of SiO-Ti bonds by a protic molecule, e.g., H2O... 

TS-1 Titanium silicalite-1 material obtained by the isomorphous substitution of Ti for Si in the lattice of the pure silica Silicalite-1 (S-1). 

A silica-alumina is formed upon isomorphous substitution of a trivalent aluminum atom for a tetrava-lent surface silicon atom in the silica lattice. The commensurate incomplete coordination of the aluminum atom generates charged structural sites that, in the absence of surface hydroxyl groups, behave as Lewis acid and base sites(7). The presence of a proton as a counterion or a hydroxyl group as occurs upon hydrolysis can produce strong Bronsted acid sites(2 15, 16). Removal of the hydroxyl groups may convert Bronsted sites to Lewis sites reversibly(9, 15). Similar complexities as alluded to previously with the alumina surface arise in a detailed discussion of the nature of the active sites on silica-alumina so that several kinds of sites are usually invoked in interpretations of experimental measurements of surface acidity and catalytic activity(14, 15). 

Bronsted or the Lewis type, owe their existence in silica-alumina catalysts to an isomorphous substitution of trivalent aluminum for tetravalent silicon in the silica lattice (Hansford, 42 Thomas, 18 Tamele, 35). Such an isomorphous substitution would lead to a structure somewhat as follows ... 

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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