Silicate aluminium substitution

Colloidal aluminium magnesium silicate (Veegum ) and bentonite are both aluminum silicates, but in Veegum a much higher percentage aluminium is substituted by magnesium. Because of the constant quality aluniinium magnesium silicate is preferred to bentonite. 

We alluded earlier to the variety of structural modifications which may he observed in sheet silicates. Clearly it is a matter of considerable in jortance to he able to determine if, for example, the aluminium content within a clay arises p a ely from octahedral substitution (as in montmorillonite) or whether there is some tetrahedral component (as in heidellite). a1 MASNMR readily provides the necessary answers. Figvire 1 illustrates the a1 spectrum for a synthetic heidellite material with Na as charge balancing cation. Aluminium in two distinct chemical environments is observed, with chemical shifts corresponding to octahedrally and tetrahedrally co-ordinated aluminium. 

We have also investigated the properties of several of our nanostructured catalysts as solid acids in reactions such as the dehydration of alcohols and transesterification reactions [99]. One of the best examples of atomically dispersed solid acid catalysts is aluminosilicates [100]. When aluminium is substituted into silicate frameworks and remains isolated from other A1 centers it can behave as a strong acid site [101]. 

Recently, the preparation of metallosilicates with MFI structure, which are composed of silicone oxide and metal oxide substituted isomorphously to aluminium oxide, has been studied actively [1,2]. It is expected that acid sites of different strength from those of aluminosilicate are generated when some tri-valent elements other than aluminium are introduced into the framework of silicalite. The Bronsted acid sites of metallosilicates must be Si(0H)Me, so the facility of heterogeneous rupture of the OH bond should be due to the properties of the metal element. Therefore, the acidity of metallosilicate could be controlled by choosing the metal element. Moreover, the transition-metal elements introduced into the zeolite framework play specific catalytic roles. For example, Ti-silicate with MFI structure has the high activity and selectivity for the hydroxylation of phenol to produce catechol and hydroquinon [3],... 

The silicon ion in silicates can be replaced by an aluminium ion, when, of course, the charge of one of the positive ions must be increased. This means that, in a silicate, a part of the silicon can be replaced by aluminium, provided that at the same time an equal quantity of the sodium is replaced by calcium, so that the sum of the charges remains the same. Such substitutions of ions A+ by ions of about the same size, greatly increase the range of compositions of the silicates. 

In the silicates, the tetrahedral SiCU units are either isolated or share corners with other tetrahedra, giving rise to an enormous variety of structures. In many silicates, silicon may be replaced to a certain extent by other elements, such as aluminium, so the structures of silicates are further extended to cover cases where such partial substitution occurs. 

We have earlier addressed the problem of the post-synthesis insertion of aluminium in zeolites ZSM-5 (12) and Y (Hamdan, H. Sulikowski, B. Klinowski, J. T.Phvs.Chem.. (in press)). The substitution of gallium in silicalite-n has also been achieved (13). It was therefore of considerable interest to establish whether boron can also be incorporated into silicate frameworks after the completion of synthesis. We report isomorphous substitution of boron into zeolite ZSM-5 by mild hydrothermal treatment with borate species. 

The disparity in size of the aluminate and the silicate tetrahedra must be the reason why, at least for some frameworks, the range of Si/Al ratios, and therefore the extent of the post-synthesis isomorphous substitution of Al for Si is limited (27). For boron, with the ionic radius of 0.23 A as compared with 0.51 A for aluminium, the disparity in size is even greater (2). Quantum chemical calculations predict that the tetrahedral coordination of aluminium is favoured in comparison with BO4 groupings (32.33). An attempt to insert boron into the framework of ferrierite (34). a structurally related zeolite, was unsuccessful. 

The first study using a wet cell, made at high w/c ratios, showed tubular growths radiating from the cement grains, which were considered to have formed by a silicate garden mechanism (D14). Later work showed that they were rich in calcium, aluminium and sulphur, and that they did not form if CjS was substituted for cement (BlOl). They have not been observed in the more recent studies made at normal w/c ratios, and do not appear to be a significant feature of normal cement hydration. 

A second feature of silicate crystal chemistry is peculiar to silicates alone, and arises accidentally from the particular value of the radius of the aluminium ion. The A1 0 radius ratio of 0 36 is so close to the critical value of 0 3 for transition from 6- to 4-co-ordination that this ion can occur in both conditions, sometimes in the same structure. When 4-co-ordinated the aluminium ion replaces silicon, and such replacement is purely random and may be of indefinite extent. For every aluminium ion so introduced a corresponding substitution of Ca2+ for Na+, Al3+ for Mg2+ or Fe3+ for Fe2+ must simultaneously occur else-... 

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