Alumo-silicates

The following review is concerned with the synthetic and structural chemistry of molecular alumo-siloxanes, which combine in a molecular entity the elements aluminum and silicon connected by oxygen. They may be regarded as molecular counterparts of alumo-silicates, which have attracted considerable attention owing to their solid-state cage structures (see for example zeolites).1 3 Numerous applications have been found for these solid-state materials for instance the holes and pores can be used in different separation techniques.4,5 Recently the channel and pore structures of zeolites and other porous materials have been used as templates for nano-structured materials and for catalytical purposes.6 9... 

For commercial processes, formed supports are more useful. Compared with other supports, fumed oxide supports showed new catalytic effects [41]. Some intensively investigated applications for these supports are abstracted in the following. SiC>2 pellets have been successfully introduced in a new generation of precious metal supports in vinylacetate monomer production [42]. This resulted in better selcctivities and an up to 50% higher space-time yield compared with supports based on natural alumo-silicates. In alkene hydration fumed silica pellets serve as a support for phosphoric acid. In this case, an increased catalyst lifetime and a higher space-time yield were observed [43]. Pyrogenic TiC>2 powder can be used as a starting material for the manufacture of monolithic catalysts [44] for the selective reduction of NOv with ammonia. 

The second most important class of molecular sieves besides the alumo-silicates are without any doubt the alumophosphates and their derivatives containing elements other than aluminum and/or additional elements in the framework. Chapter 6 authored by R. Szostak is a review covering the synthesis of these molecular sieve phosphates. 

Sodium alumo silicate glass RF cyl. magn. sputtering Na" [8]... 

In lithium-alumo-silicate glass ceramics a higher lithium concentration due to ion exchange is reached within the surface layer than within the glass. This leads to a higher content of crystal phase in this exchanged layer during the ceramization, because lithium is incorporated into the crystal layer. 

The lithium ion exchange has hardly gained acceptance in practical applications for prestressing lithium-alumo-silicate glass ceramics. On the one hand, the compressive stresses are lower in comparison to the ion exchange with potassium below T, and on the other hand, most Li salts attack the surface so that the articles cannot be used any more. 

The first crystal structure evidence of a molecular cage entity within an alumo-silicon oxide was reported in 1987 with tetrakis(tertamethylammonium)alumino-silicate.53 Although there is no organic ligand on the silicon atom, the compound contains a separated silsesquioxane-like Si4Al4012 unit (half of the silicon atoms of the silsesquioxane being substituted by aluminum atoms) with all silicon and... 

Fig. 3.12. Specific electrical resistivity of a Pyrex-type glass (1), Neutral-glass 4.9 (2), alkaline free Alumo-Boro-Silicate glass (3), E-fiber glass (4) and an alkaline poor Alumo-Boro-Silicate glass (5) melt as function of the temperature [486]...

---