Crystalline silica matrix

One milestone in the development of the process was the discovery of a new epoxidation catalyst made of titanium grafted onto silica, in 1998 the catalyst performs outstandingly in the reaction, with a minimal loss of cumene. Ti-silica catalysts have remarkably high epoxidation activity when a high dispersion of highly active tetrahedral titanium in a hydrophobic crystalline silica matrix is achieved [7]. The activity of these Ti-silica catalysts varies greatly according to the peroxides, olefins or solvent systems used. 

A. M. Schneider and P. Behrens, Chem. Mater., 10,679 (1998). Molecular Mechanics Study on Organometallic Complexes in Crystalline Silica Matrixes Using the ESFF (Extensible Systematic Force Field). 

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. 

Quartz on filter media in a clay matrix is also available from NIST. The SRM 2679a is certified for quartz at three levels 30.8, 80.2 and 202.7 [xg/filter respectively. Respirable silica in powder form is also issued by NIST SRMs 1878a and 1879a are crystalline silica materials with particles in the respirable range and they are intended for use in X-ray diffraction and infrared spectroscopy. 

Tranter, T. J., Aloy, A. S., Sapozhnikova, N. V., Knecht, D. A. Todd, T. A. 2002. Porous crystalline silica (Gubka) as an inorganic support matrix for novel sorbents. Materials Research Society Symposium Proceedings, 713, 907-913. 

Silica and aluminosilicate fibers that have been exposed to temperatures above 1100°C undergo partial conversion to mullite and cristobalite (1). Cristobalite is a form of crystalline silica that can cause silicosis, a form of pneumoconiosis. IARC has determined that cristobalite should be classified as 2A, a probable carcinogen. The amount of cristobalite formed, the size of the crystals, and the nature of the vitreous matrix in which they are embedded are time- and temperature-dependent. Under normal use conditions, refractory ceramic fibers are exposed to a temperature gradient, thus only the hottest surfaces of the material may contain appreciable cristobalite. Manufacturers Material Safety Data Sheets (MSDS) should be consulted prior to handling RCF materials. 

Fillers present a potential inhalation and dermal contact hazard. They can cause mechanical damage to the skin, which may aggravate the irritant effect of other chemicals and additives. When fillers are handled in a liquid epoxy matrix or in a cured epoxy, their inhalation hazard is low. However, inhalation exposure to fillers can occur when they are handled in the dry state or when one is machining or grinding cured epoxy products. Inhalation exposure to fillers such as crystalline silica or fiberglass may result in delayed lung injury. Asbestos fillers have long been abandoned from use for these reasons. 

Recently, Boilot et al. utilized sol-gel method as the subsequent annealing treatment for the preparation of YV04 Eu " NPs (Mialon et al., 2008). At room temperature, water-phase precipitation always produced inorganic NPs with low crystallinity, so the author re-dispersed the crude YV04 Eu + NPs into a polymeric silica sol for sol-gel thermal annealing. The silica matrix could prevent the aggregation and growth of NPs even... 

Si02 and AIPO4 are isoelectronic and isostructural. They have the same crystalline forms, and transitions between these forms follow the same order and even occur at similar temperatures. Thus, the phase diagrams for both are quite similar. On the periodic table, silicon is bordered by aluminum on the left, and phosphorus on the right. Si4+ in the silica matrix can be replaced by alternating Al3+ and P5+ in the analogous... 

The seareh for zeolitie materials with pore sizes larger than lOA recently resulted in a new class of materials termed mesoporous molecular sieves (known by the designation M41S), which are prepared using surfactant micelles as templating agents [17]. The pore diameter of these materials is tunable in a wide range of about 30 to lOOA but the matrix consists of amorphous rather than crystalline silica walls. Incorporation of hetero-atoms (Al,... 

In the present work, we consider the two approaches for synthesis of nanoparticles designed for metal particles and being in the progress for ultraflne semiconductors. They allow to fabricate nanocomposites of the type nanoparticles-in-dielectrics with amorphous and crystalline matrices. The first one is based on the sol-gel technique producing dielectric silica films with nanoparticles incorporated within silica matrix [1]. Nanoparticles provide an optical response of the material due to the plasmon resonance [2] with variable spectral position and band shape. In the second approach nanoparticles are produced within the crystalline zeolite matrices which stabilize both the few-atomic clusters (e.g., Agg) and metal particles in the size range of 1-20 nm [3], Chemical routes of their synthesis admit easy control of size and optical properties. The metal nanoparticles in zeolites can be transformed into semiconductors without destroy of the zeolite matrix and with incorporation of zeolite microcrystals into transparent silica films. This construction... 

Following preparation (via condensation of titanium and silicon-alkoxide precursors around a tetrapropylammonium hydroxide template), the material is hydrothermally treated to produce a highly crystalline solid with a distinct X-ray diffraction profile analogous to that of ZSM-5. It contains isolated titanium sites in a silica matrix. These function significantly differently from bulk titanium dioxide, which is inactive in selective oxidation reactions using hydrogen peroxide, as the latter promotes the thermal decomposition of hydrogen peroxide. ... 

Here, it has to be mentioned that more or less porous and ordered, the amorphous state of silica mesoporous matrix is the only one that is suitable to be used in biological purposes, because crystalline silica was found to be toxic. Santarelly and coworkers (2004) found that the incubation of human endothelial cells with crystalline - silica leads to alteration typical of apoptosis. Anyway further studies are... 

The health and environmental issues for nanoclays specifically are rninimal and manageable. Sister organoday products have been used for many years in a host of industrial and consumer products. The perception that nanoclays are somehow different because of the prefix nano may be the problem. Nanoclays only become nano when they are placed in a host-polymer matrix, whereupon they cannot be separated or distinguished form the bulk polymer and other constituents. Crystalline silica is a naturally occurring component that may be present in commercial alkyl quaternary ammonium bentonite (CAS No. 68953-58-2) at concentrations <0.5% [8]. Crystalline silica dust (see also Chapter 19) when inhaled is a health hazard in humans and is regulated to very low permissible exposure limits. 

With the development of nanomaterials concerns exist about their toxicity and health impacts. Because of the size of nanoparticulates, concerns arise about then-ability to penetrate tissue and cause harmful effects ranging from mild irritation to more serious tumor developments. In the case of nanoclays, one has to consider that nanoclays only become nanosized after incorporation and exfoUation in a polymer matrix. In a practical sense, the presence of crystalline silica, a material that may be found in clays, is probably more of a real hazard. Crystalline silica is regulated to extremely low levels in commercial day materials [112] (see also Chapter 19). 

Matrix. That part of a ceramic raw material or product in which the larger crystals or aggregates are embedded. A fired silica refractory, for example, consists of crystalline silica (quartz, cristobalite and tridymite) set in a glassy matrix a fireclay refractory may consist of quartz and mullitic grog (q.v.) set in a largely amorphous matrix. (Also sometimes known as the ground-mass.)... 

---