Conversions, of silica

But the Direct Process has now been practiced with little fundamental change for over fifty years. Conversion of silica (sand) to diorganopolysiloxane such as dimethylpolysiloxane is accomplished through a series of reactions very simplisticly represented as follows ... 

Conversion of silica gel to quartz in up to 40wt-% NaCl, NaF, and KCl. Little salt effect observed. 

N.Takeuchi, S.Yamane, S.lshida and H.Nanri. Conversion of silica gel and silica glass mixed with various metal oxides into quartz, J. Non-Cryst. Solids, 203, 369—ilA (1996). 

In the early stages of conversion of silica to particles, where the concentrations of oligomers are decreasing, it is probable that once a certain concentration of nuclei has been reached, they remain constant in number as they grow in size. When oligomers have been consumed, the particles then grow in size at the expense of the smaller particles, and thus decrease in number. 

The actual conversions of silica are influenced by the impurities in clay, meaning that the actual conversion temperatures are different than those published in textbooks for pure silica. For example, fireclay brick can contain cristobalite if they are fired above about 2250°F (1230°C). The transformation... 

The catalytic degradation of HDPE and PP was carried out in a powder-particle fluidised bed with an inside diameter of 25 mm. Two catalysts were used as the medium fluidisation particles. These were F9 (with a composition of silica alumina sodium oxide of 32 48 20 wt.percent) and silica/alumina (SA, with a composition of silica alumina of 71 29 wt.percent). Liquid fuel was produced in high yield. SA produced a higher yield of liquid fuel nd a more valuable gas product between temperatures of400-550C. Over 86 wt.percent conversion... 

The overall pathway for the conversion of the unsaturated azido ether 281 to 2,5-dihydrooxazoles 282 involves first formation of the dipolar cycloaddition product 287, which thermolyzes to oxazoline 282 or is converted by silica gel to oxazolinoaziridine 288. While thermolysis or acid-catalyzed decomposition of triazolines to a mixture of imine and aziridine is well-documented [71,73], this chemoselective decomposition, depending on whether thermolysis or exposure to silica gel is used, is unprecedented. It is postulated that acidic surface sites on silica catalyze the triazoline decomposition via an intermediate resembling 289, which prefers to close to an aziridine 288. On the other hand, thermolysis of 287 may proceed via 290 (or the corresponding diradical) in which hydrogen migration is favored over ring closure. 

As described in the previous section, the silica-alumina catalyst covered with the silicalite membrane showed exceUent p-xylene selectivity in disproportionation of toluene [37] at the expense of activity, because the thickness of the sihcahte-1 membrane was large (40 pm), limiting the diffusion of the products. In addition, the catalytic activity of silica-alumina was not so high. To solve these problems, Miyamoto et al. [41 -43] have developed a novel composite zeohte catalyst consisting of a zeolite crystal with an inactive thin layer. In Miyamoto s study [41], a sihcahte-1 layer was grown on proton-exchanged ZSM-5 crystals (silicalite/H-ZSM-5) [42]. The silicalite/H-ZSM-5 catalysts showed excellent para-selectivity of >99.9%, compared to the 63.1% for the uncoated sample, and independent of the toluene conversion. 

Sticking with nylon production, high-silica pentasil zeolites are used by Sumitomo to overcome environmental issues associated with the conversion of cyclohexanone oxime to caprolactam (Chapter 1, Scheme 1.4). 

In our previous paper [6], the authors have demonstrated that production of fine silica powder is possible by phase hydrolpis of tetramethoxysilane fTEMS). In this communication, we report the effect of the shape of reactor and operational condition, especially mixing condition on conversion of the reaction and properties especially diameter of produced silica fines. 

When a reverse procedure was applied, i.e. enzymatic acetylation of racemic 3, formed in situ from the appropriate aldehydes and thiols, the reaction proceeded under the conditions of dynamic kinetic resolution and gave enantiomerically enriched acetates 2 with 65-90% yields and with ees up to 95% (Equation 2). It must be mentioned that the addition of silica proved crucial, as in its absence no racemization of the initially formed substrates 3 occurred and the reaction stopped at the 50% conversion. 

The commercial process for the production of vinyl acetate monomer (VAM) has evolved over the years. In the 1930s, Wacker developed a process based upon the gas-phase conversion of acetylene and acetic acid over a zinc acetate carbon-supported catalyst. This chemistry and process eventually gave way in the late 1960s to a more economically favorable gas-phase conversion of ethylene and acetic acid over a palladium-based silica-supported catalyst. Today, most of the world s vinyl acetate is derived from the ethylene-based process. The end uses of vinyl acetate are diverse and range from die protective laminate film used in automotive safety glass to polymer-based paints and adhesives. 

Activity of the Keggin HPW/Si02 catalyst in terms of the conversions of ethylene and acetic acid and production of ethyl acetate vs. reaction time is displayed in Figure 3. Besides ethyl acetate, ethanol and diethyl ether are also produced. It can be seen that the catalyst is quite stable over the 17 hour period on stream. Activities of the other silica-supported Keggin... 

Extending these ideas to enzymatic catalysis, Jiang et al. reported the use of protamine-silica hybrid microcapsules in combination with a host gel-like bead structure to encapsulate several enzymes individually in the enzymatic conversion of C02 to methanol [20]. They used a layer-by-layer (LbL) method where alternately charged layers were deposited on an enzyme-containing CaC03 core. The layers, however, were not polyelectrolytes, but protamine and silica (Scheme 5.6). 

The temperature resistance of the polysiloxane on the samples was tested by stepwise heating up to 500°C. Whereas the pure hydrolysis product undergoes a complete thermal degradation via oxidative conversion of the CH3-Si groups into HO-Si groups, the polysiloxane persists on the silica sample. This stabilization effect most likely results from the covalent attachment of the methyl-polysiloxane. 

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