Crystalline silica/silicate materials

Among various crystalline silica/silicate materials, zeolites provide some unique features as the shell of hollow particles high thermal stability, microporosity. 

Crystalline Silica. Quartz sand is of course the principal raw material for the production of glass (qv). Cristobalite and 3-quartz are used in glass ceramics (qv), ie, ceramics produced by the controlled crystallization of glass. Silica is a main constituent of ceramics (qv). For example, refractory silica brick containing small amounts of A O is used as roof brick for open-hearth furnaces at temperatures >1600° C (see Refractories). Silica sand or flour (ground quartz) is the raw material for soluble silicates, such as sodium silicate, which is consistently ranked as one of the top 50 U.S. industrial chemicals (98) (see Silicon compounds, synthetic inorganic silicates). 

The first discovered member of the group of crystalline microporous materials made of oxides of titanium and silicon is titanium silicalite-1 (TS-1). TS-1 has attracted much interest for its unique catalytic properties it is also of interest by virtue of the proposal that Tiiv assumes tetrahedral coordination in substituting for SiIV in framework positions of crystalline silica, as stated above. To clarify this point, many detailed studies of the TS-1 structure have been carried out. An outcome of the work was the discovery of new crystalline microporous titanium silicates. 

Amorphous Ti/SiCL oxides and crystalline Ti zeolites are two classes of well-studied solid Ti catalysts (11-14). In both classes, a Lewis-acidic Ti atom is anchored to the surrounding siliceous matrix by Si-O-Ti bonds. The oxidant of choice for Ti zeolites such as titanium silicalite 1 (TS-1) and 11-/1 is H2O2, whereas the amorphous, silica-based materials function optimally with organic peroxides such as /-butyl hydroperoxide (/-BuOOH) or ethyl benzene hydroperoxide. However, there are strictly no homogeneous analogues of these materials, and they therefore do not fit within the context of anchoring of homogeneous catalysts. 

Understanding of the chemistry of autoclave processes is due primarily to the work of Kalousek and co-workers (K32,K59-K62). Above about I. SO C. for the time scales of a few hours that are used in practice, two features of cement hydration chemistry are added to those relevant at lower temperatures. Firstly, the hydration products tend to crystallize in the absence of reactive silica, C-S-H tends to be replaced by a structurally unrelated, crystalline phase, a-CjS hydrate. Secondly, the range of siliceous materials having effective pozzolanic properties is widened, and includes quartz and various other crystalline minerals, if sufficiently finely ground. 

Phase studies in the siliceous portion of the lithia system and the lithia-soda system resulted in the synthesis of mordenites with the same coexisting phases—analcimes, phillipsites, quartz, opaline silica, and crystalline alkali silicates—as had been found in the soda system. Whereas the starting materials used as reactants are not critical parameters in the synthesis of these zeolites in the soda system, the choice of reactants is a predominant factor in lithia-containing systems to produce these phases. The mechanism is not understood yet but the sensitivity of these... 

Exposure to silicaceous dusts is a major occupational health concern in numerons industries. Much of the silicon in the earth is combined with other elements to form silicates. Various anions and cations are substitnted into the crystalline silica matrix. Minerals such as kaolin, an aluminum silicate, and talc, a magnesium silicate, are silicate minerals that are used in their pnre form. Many minerals of commercial importance are composed of mixtures of silica and silicates, such as feldspar and muscovite, in an infinite range of combinations. The pulmonary disease associated with silica exposure is greatly influenced by the silicate content of the material. It has been estimated that approximately 8.2 million workers in the United States are potentially exposed to crystalline silica dust each year. 

A variety of mineral-type materials are inorganic polymers [Ray, 1978]. Silica [(SiCLL] is found in nature in various crystalline forms, including sand, quartz, and agate. The various crystalline forms of silica consist of three-dimensional, highly crosslinked polymer chains composed of Si04 tetrahedra where each oxygen atom is bonded to two silicon atoms and each silicon atoms is bonded to four oxygen atoms. Silicates, found in most clay, rocks, and... 

Remarkably, in 2002, Inagaki and co-workers reported that, starting from 1,2-bis (triethoxysilyl)benzene as a siliceous precursor, mesoporous benzene-silica with crystal-like pore walls (Ph-PMO) can be prepared (Fig. 2) [35]. Owing to their crystallinity, these new hybrid organic-inorganic materials were much more stable in water than the amorphous mesoporous silica-supported sulfonic sites described above [36-39]. 

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