Silica compounds structural chemistry

Existence/nonexistence of a compound is certainly an important topic of chemistry, but depends on the state-of-the-art of chemical synthesis and survey of natural occurrence. In Chapters 7 and 8 we shall consider other properties of compounds, including less variable properties. We shall try to deduce experimental properties horn the chemical structure and vice versa. For this purpose it is crucial to generate in silica molecular structures with prescribed structural properties in silica, i.e. to generate virtual molecular libraries. The most important tools for this task will be described in Chapter 5. 

Tetravalent silicon is the only structural feature in all silicon sources in nature, e.g. the silicates and silica even elemental silicon exhibits tetravalency. Tetravalent silicon is considered to be an ana-logon to its group 14 homologue carbon and in fact there are a lot of similarities in the chemistry of both elements. Furthermore, silicon is tetravalent in all industrially used compounds, e.g. silanes, polymers, ceramics, and fumed silica. Also the reactions of subvalent and / or low coordinated silicon compounds normally lead back to tetravalent silicon species. It is therefore not surprising that more than 90% of the relevant literature deals with tetravalent silicon. The following examples illustrate why "ordinary" tetravalent silicon is still an attractive field for research activities Simple and small tetravalent silicon compounds - sometimes very difficult to synthesize - are used by theoreticians and preparative chemists as model compounds for a deeper insight into structural features and the study of the reactivity influenced by different substituents on the silicon center. As an example for industrial applications, the chemical vapor decomposition (CVD) of appropriate silicon precursors to produce thin ceramic coatings on various substrates may be mentioned. 

Structures based on networks of corner-shared tetrahedra of oxygen ions round a central cation are some of the most important in crystal chemistry. Silica-based ceramics belong to such a class of compounds. The cristobalite form of silica has two modifications, a (low) and (high), which are separated... 

Abstract. Structural and adsorption characteristics of various adsorbents such as fumed silicas, silica gels, activated carbons and carbon/silicas were analyzed. The adsorption of a variety of compounds reveals the effects of adsorbent grain size, specific surface area, pore volume, pore size distribution, surface chemistry, conditions of adsorbent synthesis and pre-treatment. Both dynamic (nonequilibrium) and static (equilibrium) adsorption conditions are addressed. 

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