Lattice site, silica

II. The dissolution rate is controlled by interdiffusion of hydrogen or hydronium ions and species contained in lattice sites within the interior of the silicate phase. This process results in a leached layer consisting mainly of silica and alumina. Such a layer may retain the original silicate structure (W, 11) or may represent a collapsed or hydrated layer (12, 13). 

Acid-treated clays were the first catalysts used in catalytic cracking processes, but have been replaced by synthetic amorphous silica-alumina, which is more active and stable. Incorporating zeolites (crystalline alumina-silica) with the silica/alumina catalyst improves selectivity towards aromatics. These catalysts have both Fewis and Bronsted acid sites that promote carbonium ion formation. An important structural feature of zeolites is the presence of holes in the crystal lattice, which are formed by the silica-alumina tetrahedra. Each tetrahedron is made of four oxygen anions with either an aluminum or a silicon cation in the center. Each oxygen anion with a -2 oxidation state is shared between either two silicon, two aluminum, or an aluminum and a silicon cation. 

Simple Models. The surface chemical properties of clay minerals may often be interpreted in terms of the surface chemistry of the structural components, that is, sheets of tetrahedral silica, octahedral aluminum oxide (gibbsite) or magnesium hydroxide (brucite). In the discrete site model, the cation exchange framework, held together by lattice or interlayer attraction forces, exposes fixed charges as anionic sites. 

Aluminium oxide exists in many crystalline modifications, usually designated by Greek letters, some with hexagonal and some with cubic lattices (cf. refs. 11 and 24). The best known and mostly used forms are a- and 7-alumina but practical catalysts are seldom pure crystallographic specimens. This makes the surface chemistry of aluminas rather complicated. Moreover, the catalytic activity of alumina depends very much on impurities. Small amounts of sodium (0.08—0.65%) poison the active centres for isomerisation but do not affect dehydration of alcohols [10]. On the other hand, traces of sulphates and silica may increase the number of strong acidic sites and change the activity pattern. 

Substitution of aluminum for silicon in a silica lattice increases the concentration of both Lewis and Bronsted acidic sites over that of pure silica [35]. These sites, together with those created by other minor glass constituents, create the surface acidity of glass fibers that we observed in this work and has been noted by previous investigators [9-17], We consider this acidity to be the primary effect that we observed in this study. 

A unique way of identifying acid sites in amorphous silica-alumina was tried by Bourne et al. (128). These authors decided to synthesize, then characterize, two extreme types of acid site structures that they felt existed in commercial silica-aluminas. The two catalyst types consisted of low concentrations (<1.4% wt) of aluminum atoms incorporated (a) on the surface of silica gel (termed aluminum-on-silica) and (b) within the silica lattice (termed aluminum-in-silica). From infrared measurements of pyridine chemisorbed on the two materials, they conclude that dehydrated aluminum-on-silica contains only Lewis acid sites and that dehy-... 

Infrared spectral studies of rare earth (RE) ion-exchanged faujasites have been reported by Rabo et al. (214), Christner et al. (217), Ward (211, 212), and Bolton (218). Distinct hydroxyl absorption bands are observed at 3740, 3640, and 3522 cm-1 after calcination at temperatures in the range of 340° to 450°C. As previously discussed, the hydroxyl groups at 3740 cm-1 are attributed to silanol groups either located at lattice termination sites or arising from amorphous silica associated with the structure. The hydroxyl groups that form the 3522 cm-1 band are nonacidic to pyridine or piperidine and are thought to be associated with the rare earth cations. 

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