Isomorphic substitution distribution

If isomorphic substitution of Si(IV) by AI(III) occurs in the tetrahedral sheet, the resulting negative charge can distribute itself over the three oxygen atoms of the tetrahedron (in which the Si has been substituted) the charge is localized and relatively strong inner-sphere surface complexes (Fig. 3.10a) can be formed. 

Incorporation in cement minerals will lead to a similar relationship, which may be described by a distribution ratio with the exception that uptake may be much greater than that of surface sorption ( ). Such mechanisms may apply to C-S-H, AFt, and AFm phases. The mechanism of incorporation may be by isomorphic substitution of a particular species within a crystal lattice. A good example here is the exchange of SO in ettringite for another anion. Another possibility is the adsorption to sites within a crystal structure, as may occur at silicate sites within C-S-H. 

These three-layer silicates are characterized by permanent surface charges (due to isomorphic substitutions). Therefore the binding of cations is assumed to be caused by stoichiometric ion exchange of interlayer ions. These concepts hold well for alkaline and earth-alkaline cations their adsorption and their ionic strength dependence can be characterized by distribution coefficients derived from ion exchange theory. It has been known for some time, however, that... 

From the electrochemical point of view, an important class of materials is that constituted by aluminosilicates incorporating cobalt, iron, etc., centers. In the case of Fe-based zeolites with Mobil Five structure (FeZSM-5) materials, different forms of iron can coexist. These include isolated ions either in framework positions (isomorphously substituting silicon centers), isolated ions in cationic positions in zeolite channels, binuclear and oligonuclear iron complexes in extra-framework positions, iron oxide nanoparticles (size <2 nm), and large iron oxide particles (FcjOj) in a wide distribution (up to 25 nm in size) located in the surface of the zeolite crystal (Perez-Ramirez et al., 2002). The electrochemistry of such materials will be reviewed in Chapter 8. 

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). 

Charged areas on the mineral surface that arise from unsatisfied bonds and isomorphic substitution help to retain polar molecules. In addition, the asymmetrical distribution of orbital electrons in O and OH groups produces local negative and positive (polar) areas. Both the charged and the polar surfaces actively adsorb polar molecules by hydrogen bonding and by van der Waals forces. 

The preparation method of iron-zeolites has been recomized as critical in order to obtain reproducible catalysts with a desired performance." A distribution of iron species is normally obtained upon activation of catalysts by available methods. Suppressing clustering of iron species into iron oxide is convenient, since these species are proven inactive at low temperatures in the various reactions catalyzed by Fe-zeolites. " Steam activation of isomorphously substituted FeMFI zeolites enables a certain control of the degree of iron clustering, and thus on the relative amount of certain species in the final catalyst, as compared toother methods. A rather unique achievement has been attained here with Fe-silicalite (873 K), in view of the remarkable uniform nature of extraframework species in isolated positions. A minor association of iron species is present... 

However, the incorporation of metal cations whose valence is different from that of A1 or P leads to the formation of electronically unsaturated sites, as schematically shown in Figure 3. This addition of aliovalent metal cations into the lattice of AlPO-n generates solid acidity and ion-exchange sites. There are numerous reports on the incorporation of many different metal cations into the lattice of AlPO-n. Table 2 summarizes the reported isomorphous substituted AlPO-n. The family of AlPO-n substituted with metal cations is generally called metal aluminophosphates (MeAPO-n). The typical metal cations substituted into AlPO-n are Li, B, Be, Mg, Ti, Mn, Fe, Co, Zn, Ga, Ge, Si, and As. The Si-substituted AlPO-n is called a silicoaluminophosphate and denoted as SAPO-n, where n also means the framework structure, and it is distinct from the MeAPO-n materials.SAPO-n exhibits both structural diversity and compositional variation. In particular, the crystal structure of SAPO-n is of greatest interest, because the distribution of the Si atom in the framework is quite complicated. Some crystal structures, such as SAPO-40, are only found in SAPO-n and not in AlPO-n or zeolite. The mole... 

The effect of the type of isomorphously substituted atoms (Al, Fe, B) on the number of isolated heteroatoms, their pairs and on the distribution of these pairs in the framework of ZSM-5 structure was investigated using UV-Vis spectroscopy of Co ions. It was found that the distribution of heteroatoms in the framework of ZSM-5 is not random and depends on the type and concentration of heteroatoms. The number of pairs of boron is significantly lower compared to the number of Al and Fe pairs for similar concentrations of heteroatoms. All Al, Fe and B pairs are present in three different rings forming, thus, three different sites for divalent cations. 

Isomorphous substitution of boron into the MFI structure to produce [B]ZSM-5 was carried out and investigated via FTIR micro-spectroscopy ( infrared microscopy ) of single crystals by Jansen et al. [308]. Typical bands were observed at 1380 and 905 cm. The integrated band intensity of the 905 cm" band was used as a quantitative measure of the boron content, and the boron distribution in the single crystals was shown to be homogeneous. 

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