Hydroxylated framework structure

FIGURE 7.26 Idealized structure of the layers in a smectite clay (a) showing only the oxygen/hydroxyl framework (b) also depicting aluminium and silicon positions. 

The Hb NMR spectrum of this sample contains a single narrow resonance centered at -3.2 ppm, which is characteristic of boron in a tetrahedral coordination environment in the framework structure. The Si nmr spectra of a synthetically prepared siliceous mordenite with the same Si/Al ratio is shown in Figure 8. No CP resonances are present, Which indicates that hydroxyl nest concentration in this material is very low compared to the acid treated sample. These data confirm that hydroxyl nests, generated by the removal of A1 from the zeolite structure, are reactive sites for isomorphous substitution. Aluminum deficient, preformed zeolites which do not contain hydroxyl nests, i.e. synthetically prepared samples, do not undergo isomorphous substitution when treated in a similar fashion. 

The final product is a crystalline silica rich in hydroxyl defects but having essentially the framework structure of the parent zeolite 4, 13, 30). The defects render it polar, but the electrostatic components of the heats of immersion are reduced. The extent to which smooth correlations are obtained for qn and Qh is shown in Figure 6 for clinoptilolite and mor-d ite, respectively. Equally clear correlations between A1 content and AH have been observed for CO2 m these 2 dealuminized zeolites, whereas for the nonpolar Kr molecule, AH remained virtually independent of the... 

Titanium silicalite-1 (TS-1), first synthesized in 1983, is well known for its outstanding ability to catalyze various oxidation and hydroxylation reactions. This catalytic activity is ascribed to the presence of Ti atoms in the zeolite. Knowledge of the effect of the Ti atoms on the framework structure and of the location of the Ti atoms in the zeolite would be useful in understanding the catalytic properties of TS-1. Although TS-1 has been characterized extensively, the location of the Ti atoms in the zeolite is still under discussion. The maximum amount of framework Ti has been reported to be 2.5 Ti atoms per... 

Dealumination and silicon-enrichment reaction of (NH SiF6 (AHFS) with zeolites Besides the hydrothermal method for preparation of ultra-stable Y zeolite (USY), Breck and Skeels[23] in 1983 invented a new secondary synthesis method for silicon-enriched zeolites. This method uses an ammonium hexafluorosilicate solution to remove the aluminum atoms from the framework structure of Y zeolite to the solution, and to insert silicon atoms back into the Al-removal vacancies in the framework so as to form a more or less perfect Y zeolite with a high Si/Al ratio. In comparison with the USY prepared by the hydrothermal method, the framework silicon-enriched Y zeolite obtained through the current technique possesses fewer framework hydroxyl vacancies, and the resulting zeolite has an ideal crystal lattice, and hence higher structural stability. Meanwhile, there... 

Infrared spectroscopy has been a valuable technique for exploring zeolite structures. It is useful for studying the nature of hydroxyl groups in zeolites, the interaction of cations with adsorbed molecules, and the fundamental framework structures of zeolites. 

Framework Hydroxyls. The structural nature of framework hydroxyls is particularly important because they probably act as Bronsted acid catalysts. The literature on catalysis is much too extensive to be reviewed here, and comments will be restricted to x-ray and infrared evidence on the location and nature of framework hydroxyls. Reviews of infrared data are given in this volume by Rabo and Poutsma and by Ward (see also 66, 67),... 

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. 

It was also shown that thermal treatment of an ammonium zeolite under steam causes not only framework dealumination, but also a structural rearrangement in the zeolite framework. The defect sites left by dealumination are filled to a large extent by silica, which leads to a very stable, highly silicious framework (11,12) (Figure IB) Defect sites not filled by silica are occupied by "hydroxyl nests" (13). 

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