Reduction reactions silica

The equilibrium levels of the reaction products are very small, but both can dissolve in liquid sodium, and sodium oxide can form compounds with silica. As a consequence, the reaction moves to the right, leading to further reduction of silica. Nevertheless, vitreous silica crucibles have been used sucessfully for containing molten antimony (850°C), copper (1 210°C), gallium (1 100°C), germanium (1 100°C), lead (500°C) and tin (900 C). 

The thermograms of Cu reduction in silica-, alumina-, titania- and zirconia-supported catalysts show only one pe the maximum of which is reported in Table 3. The amount of hydrogen consumed by the r uction corresponds, within experimental error, to the theoretical amount required for the reaction ... 

A number of other sulphoxide reduction reactions bear mentioning. The first, due to Marchelli and coworkers , is a very simple procedure whereby the sulphoxide is refluxed with t-butyl bromide and chloroform. A useful range of sulphoxides was studied and distillation of the reaction mixture (or percolation through a column of silica gel) gave pure sulphides in yields of > 90%. The procedure is appealing because of its experimental simplicity, and its use of a relatively inexpensive reagent. It may not be very successful with sterically hindered sulphoxides and the authors do not comment on this possibility. The mechanism of this reduction reaction is akin to that of BBrj (cf. Section II.A.3), except that the bromine trap is provided by a second mole of t-butyl bromide, as shown in equation (13) ... 

Silicon is generally considered to be a congener of carbon and this is also reflected in the evolution of silicon as a reducing agent for metal oxides. Silicon forms a fairly stable solid oxide silica or silicon dioxide (Si02) and also a stable gaseous oxide silicon monoxide (SiO), both of which can be useful in oxide reduction reactions. 

More recently, during research aimed at supporting the highly linear selective hydroformylation catalyst [Rh(H)(Xantphos)(CO)2] onto a silica support, the presence of a cationic rhodium precursor in equilibrium with the desired rhodium hydride hydroformylation catalyst was observed. The presence of this complex gave the resulting catalyst considerable hydrogenation activity such that high yields of linear nonanol could be obtained from oct-1-ene by domino hy-droformylation-reduction reaction [75]. 

Silica is reduced to silicon at 1300—1400°C by hydrogen, carbon, and a variety of metallic elements. Gaseous silicon monoxide is also formed. At pressures of >40 MPa (400 atm), in the presence of aluminum and aluminum halides, silica can be converted to silane in high yields by reaction with hydrogen (15). Silicon itself is not hydrogenated under these conditions. The formation of silicon by reduction of silica with carbon is important in the technical preparation of the element and its alloys and in the preparation of silicon carbide in the electric furnace. Reduction with lithium and sodium occurs at 200—250°C, with the formation of metal oxide and silicate. At 800—900°C, silica is reduced by calcium, magnesium, and aluminum. Other metals reported to reduce silica to the element include manganese, iron, niobium, uranium, lanthanum, cerium, and neodymium (16). 

Production. Silicon is typically produced in a three-electrode, a-c submerged electric arc furnace by the carbothermic reduction of silicon dioxide (quartz) with carbonaceous reducing agents. The reductants consist of a mixture of coal (qv), charcoal, petroleum coke, and wood chips. Petroleum coke, if used, accounts for less than 10% of the total carbon requirements. Low ash bituminous coal, having a fixed carbon content of 55—70% and ash content of <4%, provides a majority of the required carbon. Typical carbon contribution is 65%. Charcoal, as a reductant, is highly reactive and varies in fixed carbon from 70—92%. Wood chips are added to the reductant mix to increase the raw material mix porosity, which improves the SiO (g) to solid carbon reaction. Silica is added to the furnace in the form of quartz, quartzite, or gravel. The key quartz requirements are friability and thermal stability. Depending on the desired silicon quality, the total oxide impurities in quartz may vary from 0.5—1%. 

Crude elemental silicon can be obtained by reduction of silica sand with coke in the electric furnace (reaction 17.33) and may be adequate for making ferrosilicon alloys (Section 16.7.5) or silicones (Section 3.5). The high purity silicon used for electronic chips can be made from silica via silicon tetrachloride, which, like TiCU, is a volatile liquid (bp 57 °C) susceptible to hydrolysis but readily purifiable by fractional distillation. Indeed, the procedure for silicon resembles the Kroll process for titanium, except that an argon atmosphere is not necessary ... 

If attention at first is confined to the production of methyl silicone from the previously accepted raw materials, the chemical processes must include reduction of silica to silicon, preparation of the methyl chloride from methane or methanol, reaction of the methyl chloride with silicon, and hydrolysis of the methylchlorosilanes. If the same conventions are used as in the discussion of the,Grignard method, and the methanol process for methyl chloride is elected, the steps are ... 

The carbothermic reduction of silica is believed to follow [25] a reaction mechanism given by... 

Metal oxide catalysts can be classified as oxides of transition elements or as oxides of other typical metals. Typical transition elements include Cr, Fe, Co, Mo, and V, whose oxides catalyze oxidation and reduction reactions by changing the oxidation state of the metal ion. For selective oxidation of hydrocarbons, mixed oxides containing Mo and V are widely used. Oxides of other metals (acidic oxides such as silica and silica-alumina, basic oxides such as CaO and MgO, and amphoteric oxides such as alumina) catalyze acid or base reactions such as alkylation, isomerization, and hydration-dehydration. 

Silicon differs from carbon in that all of the element existing in nature is in an oxidized form, as SiCb or silicate minerals. The first step in the synthesis of sihcones then becomes the reduction of silica to an active reduced form of sihcon. The usual starting point is the electrothermal reduction of Si02 with carbon (equation 1). The elemental sihcon can be converted to reactive SiCLi by chlorination (equation 2) or to hydrochlorosilanes by reaction with HCl (equation 3). The importance of the latter materials in silicone technology will be described later. 

The formation of silicon nitride whiskers was observed in several different reactions, including vapor deposition, CVD, and growth from a melt. However, only the following techniques are considered to have commercial significance nitriding of metallic silicon or silicon-silica mixture, carbothermal reduction of silica with simultaneous nitridation, and thermal decomposition of silicon halides. 

Some ceramic materials are not found widely or at all in nature, and thus are synthesized for use. To prepare more complex ceramic compositions such as perovskites of general structural formula ABO3, and ferrites, of formula MFc204, the individual oxides or salts of the cations A, B, and M are often combined as powders and then reacted at high temperature by a solid-state diffusion mechanism. Silicon nitride (Si3N4) can be manufactured from either the nitridation of silicon metal or from the reaction of silicon tetrachloride with ammonia. Silicon carbide (SiC) is obtained from the reduction of silica with a carbon containing source. 

These reactions prevent the pH from rising much above 7 even as reduction reactions continue in the soil. The process of Fe reduction in soils dissolves silica and phosphate adsorbed on ferric oxides and precipitated in Fe -rich minerals. The fainter... 

The field of applications is therefore very similar to the one with alumina, reactants being impregnated on silica gels prior to reactions. As typical examples, they are applied in reduction reactions [43] especially in silica-gel supported zinc borohydride [44], oxidations including mainly KMn04/Si02 [45] and several cases of anionic activations in dry media [46]. 

PHRQPITZ retains the original logic of PHREEQE concerning oxidation-reduction reactions, but because an internally-consistent data base of Pitzer interaction parameters for multiple oxidation states is not currently available, geochemical redox reactions may not be attempted in PHRQPITZ. All redox equilibria used with PHREEQE have been removed from the PHRQPITZ data base. Because of the lack of Pitzer interaction parameters for aqueous aluminum and silica species, calculations with aluminosilicates are not possible in PHRQPITZ. 

Studies (90, 91) with Cr02/Si02 catalyst have shown that formation of a surface chromate takes place by reaction of Cr02 and surface silanol groups on silica (Reaction 17). Reaction of this chemisorbed chromate with ethylene results in an oxidation-reduction reaction (90-95) with formation of a low-valent chromium center (Reaction 18). Proposals for Cr(II) as the active site are based on studies of the catalyst after reduction by ethylene, carbon monoxide, or hydrogen. One study (93. 94) showed that the polymerization rate increased with the fraction of Cr(II) in the catalyst. Another study (92) showed by polarography that the chromium is reduced to a divalent state by ethylene. 

Compared to the Grignard route, the direct process reaction requires only the reduction of silica to silicon and the production of methyl chloride. A form of the overall reaction (related to equation 2) is shown in equation 3. On the basis of this equation there... 

A recent innovative method of handling alkali metals is to absorb them into silica gel, thus providing a convenient source of the metals as powerful reducing agents. Foreseeable applications of these materials are in the use of continuous-flow columns for reduction reactions in, for example, the pharmaceutical industry. The silica gel-alkali metal powders react quantitatively with water, liberating H2 since the powders are easily handled and stored, they have the potential to act as a supply-on-demand source of H2. ... 

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