Silicon-aluminium exchange

The regioselectivity of the double nucleophilic addition of ketene silyl acetals to a,/3-unsaturated imines has been found to be highly dependent on the subtle difference in the reactivities of the ketene silyl acetals the factors are mainly derived from the ability of the ketene silyl acetals to undergo the silicon-aluminium exchange reaction, where the aluminium enolate preferentially undergoes 1,4-addition.209... 

Figure C2.12.1. Origin of ion exchange capacity in zeolites. Since every oxygen atom contributes one negative charge to the tetrahedron incoriDorated in the framework, the silicon tetrahedron carries no net charge while the aluminium tetrahedron carries a net charge of-1 which is compensated by cations M.

The tlrree impurities, iron, silicon and aluminium are present in the metal produced by the Kroll reduction of zirconium tetrachloride by magnesium to the extent of about 1100 ppm. After dre iodide refining process tire levels of these impurities are 350, 130 aird 700ppm respectively. The relative stabilities of the iodides of these metals compared to that of zirconium can be calculated from the exchange reactions... 

Black, J. R., Etch Pit Formation in Silicon at Al-Si Contacts Due to the Transport of Silicon in Aluminium by Momentum Exchange with Conducting Electrons , J. Electrochem. Soc., 115, 242c (1968)... 

Silicon tetrachloride, SiCl4, when passed over fragments of felspar (a silicate of aluminium and calcium) heated to whiteness in a porcelain tube, exchanges chlorine for oxygen, and yields a liquid boiling at about 137°, of the formula... 

The basic unit of a zeolitic structure is the TO4 tetrahedron, where T is normally a silicon or aluminium atom/ion (or phosphorus in an aluminophosphate). In this section we deal with the aluminosilicate zeolites, which have the general formula M4/ [(A102)J/Si02) ].mH20. The zeolite framework is composed of [(AlO (SiOj) ] and M is a non-framework, exchangeable cation. 

Although many soil scientists had considered the possible mechanisms which soils employ for the retention [fixation] of phosphorus, it remained for Haseman et al. (1950) to demonstrate that phosphorus could — and in experimental situations did — replace the silicon of micas and clay minerals in order to form crystalline hydrous aluminium phosphates of sodium, ammonium and potassium. Prior to experimentation by this group, associated with the laboratories of the Tennessee Valley Authority (TVA), most authors attributed the retention of phosphorus by soils to combination with calcium to produce fairly insoluble minerals to adsorptive, exchangeable combination with silicate minerals and to formation of phosphates of iron... 

The effect of exchanging Cd " into zeolite LTA has been studied by Cd NMR, enabling the identification of 2 different cadmium coordination states in the hydrated form of the zeolite. The effect of cadmium exchange on the silicon and aluminium environments of this zeolite was also investigated using Si and Al NMR spectroscopy (Eldewick et al. 1999). 

Alkaline treatment of Fe-MFI zeolites prepared via ion exchange or isomorphous substitution leads to combined micro- and mesoporous Fe-MFI structures. The preparation method highly determines the impact of the alkaline treatment. Iron in framework positions (iron sihcahte) directs the silicon extraction towards controlled mesopore formation similarly to framework aluminium in ZSM-5. Iron in non-framework positions inhibits leaching of silicon to the filtrate. Higher concentrations of non-framework iron, as obtained in ion-exchanged Fe-ZSM-5 catalysts, lead to a lower mesoporosity development. TTie iron nanoparticles appear to remain unchanged upon Eilkaline treatment and speculatively isolated and oligonuclear iron species are held responsible for the restricted silicon dissolution and mesoporosity development. 

The alternative tooth-coloured material, the glass-ionomer cement, has also been widely studied, especially in terms of its bioactivity. This arises from its ability to exchange ions with its surroundings when placed in the mouth. Typical conventional glass-ionomers have been shown to release sodium, silicon and phosphorus under neutral conditions, and also calcium and aluminium under acidic conditions [48]. The non-metals are assumed to be released as sihcate, Si03 ", and phosphate, PO/, respectively. In addition, they release fluoride [49], a process that is capable of continuing for several years [50]. 

The incorporation of trivalent cations of charge less than 4 + in place of silicon atoms imparts a net negative charge to the framework and gives the potential for properties of ion exchange and solid acidity, as does the inclusion of aluminium, but, in general, the soUd acidity of such materials is weaker and the substituting cations are more likely than aluminium to leave the framework. 

Fig. 2 Model structure of layered silicates (montmorillonite) where usually silicon sits in the tetrahedral locations of the oxygen network. The octahedral positions may variously be iron, aluminium, magnesium or lithium, and the exchangeable cation in the gallery is given by M " [4]...

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