Aluminosilicates bridged hydroxyls

A simple cluster model of a bridged hydroxyl group in a zeolite is cluster 3. Such a cluster with A = H was used by Chuvylkin et al. (70) as early as 1975 to discuss the properties of possible intermediate structures in the catalytic isomerization of butenes on aluminosilicate surfaces in terms of CNDO/2 approximation. Mikheikin et al. (34) have used a similar cluster with terminal pseudo-atoms A to study the Bronsted acidity of zeolites and its dependence on the Si/AI ratio. 

The catalytic activity of aluminosilicates in typical reactions of acid catalysis is determined by two types of BASs bridged hydroxyl groups and... 

The chemisorption of olefins on an aluminosilicate catalyst is also believed to proceed by a mechanism similar to that shown in Figure 5-33. As shown in Equation 5-69, the olefin couples with an acid/base pair, that is, a bridging hydroxyl group and a lattice oxygen center on the surface, probably as the result of a direct geometrical correspondence. 

Phosphorus has been most successfully substituted by silicon, although there are many reports that other elements, including titanium and vanadium, can be introduced. The direct replacement of isolated phosphorus atoms by silicon atoms results in a negatively charged framework and, upon calcination, an add site. Since this can take the form of a bridging hydroxyl between aluminium and silicon atoms, it is similar in character to those found in aluminosilicate zeolites. 

Proposed decomposition of tetraalkylammonium ions in organically modified clays by Hofmaim P-elimlnation. Although the aluminosilicate oxygens are less basic than the amine, loss of the organic components by volatilization could lead to the formation of strongly acidic bridging hydroxyls. 

The motivation to replace aluminium in aluminosilicate zeolite structures by other elements arose from the need to adjust their properties to intended applications. Since the nature and strength of the bridging hydroxyl groups (Si - OH - T, T = Al, Fe, Ga, B,. ..) depend on T atom, and thus the proton - T distance and resulting acid strength of the modified material. At comparable bond angles, the proton - T distance decreases in order Fe, Ga, Al. This means that electrostatic repulsion between the proton and T increases in the same way, and that the acidity is expected to increase in the same order [48]. These changing in the acidity are easily accessible by adsorption calorimetry technique. 

The structural framework of silica [5] is based on interconnected Si(-0->4 tetrahedra, with some OH terminations at the surface. Aluminas are built primarily of A1(-0-)4 tetrahedra and AK-O-) octahedra, with some surface-OH terminations [6]. The fundamental framework structure of aluminosilicates include primarily Si(-0-)4 and AK-O-) units plus charge compensating bridging or framework hydroxyl groups, namely structure I, which shows formal charges on oxygen and aluminum [7]. On the basis of these kinds of structures and various so-called defect structures that may include octahedral and three-coordinate aluminum sites, the investigation by NMR of a wide variety of nuclei can be anticipated. Numerous NMR studies based on Si, Al, H, H, and O have been reported on silica, alumina, and silica-alumina systems. 

Pure silica end-members may be considered as special cases of aluminosilicate zeolites. They may be prepared directly from hydrothermal synthesis and in some cases from aluminosilicates by post-synthetic treatment. For example, the pure silica analogue of ZSM-5 (Silicalite-1) is readily prepared by direct synthesis, whereas purely siliceous zeolite Y can only be obtained by postsynthetic treatment (Chapter 6). The microstructures present in these solids depend on the way in which they are prepared. For direct preparation routes the presence or absence of fluoride as a mineraliser in the preparation (see Chapter 5) determines whether the framework is prepared defect-free or with high concentrations of terminal silanol (SiOH) hydroxyls, where silicon is attached to three bridging oxygen atoms and a hydroxyl group. Post-crystallisation preparation of pure silica zeolites can be achieved by treatment of appropriate starting materials with silicon tetrachloride or by removal of aluminium from the aluminosilicate framework by heating the ammonium form in steam (Chapter 6). 

If the hydrated metal monomer is a much stronger acid than the water-bridged dimer, the proton is released after dissociation of the bridge and can recycle. For these cases, flic proton adsorption is catalytic and these pathways may be particularly important for oxide minerals with polymers of oxyanions, such as aluminosilicates and phosphates. There is, for example, no such thing as a t -OH2 site on monomeric Si(IV) as Si(OH2)4 dissociates immediately to form silicic acid Si(OH)4 (aq), which has co-ordinated hydroxyls. 

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