Iron, isomorphous substitution

Iron can be introduced into the frameworks of zeotype materials during their hydrothermal syntheses. Iron isomorphously substitutes for some silicon atoms in the framework if this route is chosen. The Si, Al, and Fe contents depend on the composition of the synthesis gel. The nature and distribution of the iron species in a particular sample strongly depend on the activation treatments that are appHed after the synthesis. 

Zinc, Cu and Ni have similar ionic radii and electron configurations (Table 5.6). Due to the similarity of the ionic radii of these three metals with Fe and Mg, Zn, Cu and Ni are capable of isomorphous substitution of Fe2+ and Mg2+ in the layer silicates. Due to differences in the electronegativity, however, isomorphous substitution of Cu2+ in silicates may be limited by the greater Pauling electronegativity of Cu2+ (2.0), whereas Zn2+ (1.6) and Ni2+ (1.8) are relatively more readily substituted for Fe2+ (1.8) or Mg2+ (1.3) (McBride, 1981). The three metals also readily coprecipitate with and form solid solutions in iron oxides (Lindsay, 1979 Table 5.7). 

Another characteristic of 2 1 clays is isomorphous substitution, where iso means same and morphous means shape. During the formation of clay, cations other than aluminum and silicon become incorporated into the structure. In order for this to work and still ensure a stable clay, the cation must be about the same size as either aluminum or silicon, hence the term isomorphous. There are a limited number of cations that satisfy this requirement. For silicon, aluminum as Al3+ and iron as Fe3+ will tit without causing too much distortion of the clay structure. For aluminum, iron as Fe3+, magnesium as Mg2+, zinc as Zn2+, and iron as Fe2+ will fit without causing too much structural distortion (see Figure 3.4). 

Isomorphous substitution of iron oxides is important for several reasons. In the electronics industry, trace amounts (dopants) of elements such as Nb and Ge are incorporated in hematite to improve its semiconductor properties. Dopants are also added to assist the reduction of iron ores. In nature, iron oxides can act as sinks for potentially toxic M", M and M heavy metals. Investigation of the phenomenon of isomorphous substitution has also helped to establish a better understanding of the geochemical and environmental pathways followed by Al and various trace elements. Empirical relationships (e. g. Fe and V) are often found between the Fe oxide content of a weathered soil profile and the levels of various trace elements. Such relationships may indicate similarities in the geochemical behaviour of the elements and, particularly for Al/Fe, reflect the environment in which the oxides have formed (see chap. 16). 

Difluorobenzenes are isomerized under gas-phase conditions in the presence of metallosilicates, containing the structure of pentasil zeolites with isomorphic substitution of some silicon atoms by aluminum, gallium, or iron.4 A German patent describes the isomerization of l-bromo-2,4-difluorobenzene to l-bromo-3,5-difluorobenzene in pentasil-type zeolites in an autoclave at 320 C and 25 x 105 Pa for 1 h, giving 29% conversion and 73% selectivity.5... 

Other metal sulfides, such as galena (PbS) and sphalerite (ZnS), may affect the mobility of arsenic in anoxic environments. However, immobilization depends on surface complexation rather than precipitation. In contrast to iron (oxy)(hydr)oxides (discussed later), As(III) adsorption on galena and sphalerite increases with pH (Bostick, Fendorf and Manning, 2003). Surface complexation does not occur by isomorphic substitution of lead or zinc, or by a ligand exchange mechanism. Instead, multinuclear, inner-surface arsenic-thiosulfide complexes probably form on galena or sphalerite surfaces (Bostick, Fendorf and Manning, 2003). 

It is assumed that some exchange is due to isomorphous substitution but this has not been proven. Schofield and Samson (1953) calculated that only one Al3+ need replace one Si4+ in 400 unit cells to afford an exchange capacity of 2 mequiv./lOOg. There is enough excess Al3+ in most kaolinites to account for 10 times this exchange capacity. Thus it appears likely that most of the excess Al3+ does not substitute in the tetrahedral sheet. The iron-rich kaolinite described by Kunze and Bradley (1964)has an exchange capacity of 60 mequiv./lOO g however, it is likely that much of this is due to the presence of iron oxides. 

Barium reacts with metal oxides and hydroxides in soil and is subsequently adsorbed onto soil particulates (Hem 1959 Rai et al. 1984). Adsorption onto metal oxides in soils and sediments probably acts as a control over the concentration of barium in natural waters (Bodek et al. 1988). Under typical environmental conditions, barium displaces other adsorbed alkaline earth metals from MnO2, SiO2, and TiO2 (Rai et al. 1984). However, barium is displaced from Al203 by other alkaline earth metals (Rai et al. 1984). The ionic radius of the barium ion in its typical valence state (Ba+) makes isomorphous substitution possible only with strontium and generally not with the other members of the alkaline earth elements (Kirkpatrick 1978). Among the other elements that occur with barium in nature, substitution is common only with potassium but not with the smaller ions of sodium, iron, manganese, aluminum, and silicon (Kirkpatrick 1978). 

Fe with the template ion. DTA studies indicate that Fe-faujasites have lower thermal stability than their Al—analogs.The (OH) vibration frequency shifts from 3540 and 3630 to 3570 and 3643 cm respectively on isomorphous substitution of Al by Fe. Relative changes in the intensity of the ESR peak at g = 4.3 at low temperatures also support the conclusion that iron can be inserted in the fauja-site lattice positions. 

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... 

In the practical syntheses of ferrisilicates, isomorphous substitution of iron usually is not complete, thus the starting material may already contain some amount of extraframework ions. At the following steps (e.g. at the conversion of the synthesized Na form to H-form, or at the activation or during catalytic processess) the portion of the... 

The investigation of ferrisilicates under reducing conditions clearly reveal the difference of Fe(lll)-Td and Fe(lll)-Oh coordinations. Isomorphously substituted ions in Td sites withstand reduction in a broad temperature range and preserve their oxidation number. The decrease in the portion is more rapid for the Fe(lll)-Oh state (characteristic probably for the extra-framework emplacement) in dependence on temperature, and the component disappears already at 770 K even in 4 kPa hydrogen, as ESR data attest. This is in accordance with the extension of Loewenstein s rule to iron (there are no adjacent [Fe04/2] primary building units in the lattice). The extra-framework Fe(lll)-Oh components are more accessible to hydrogen, thus they can be reduced to ferrous state more easily. 

The replacement of aluminium by bivalent cations such as Zn + and Mn2+ in aluminophosphate molecular sieves results in a negative charged framework and gives rise to the formation of acidic sites, which are known to act catalytically. Most transition metals do not fulfil the requirements for isomorphous substitution and are found to be extra-framework. There are only three well-established cases of isomorphous substitution iron, cobalt and zinc. ... 

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