Chlorosilanes hydrogenation

Methods of dkect reduction of chlorosilanes using hydrogen at high temperatures have historically been inefficient processes (68—70). Significant process innovations, involving the hydrogenation of siUcon tetrachloride over Si—Cu at less than 2.45 MPa (500 psi), proceed in good conversion (71,72) and allow commercial processes. 

The ratio of cycHc to linear oligomers, as well as the chain length of the linear sdoxanes, is controlled by the conditions of hydrolysis, such as the ratio of chlorosilane to water, temperature, contact time, and solvents (60,61). Commercially, hydrolysis of dim ethyl dichi oro sil a n e is performed by either batch or a continuous process (62). In the typical industrial operation, the dimethyl dichi orosilane is mixed with 22% a2eotropic aqueous hydrochloric acid in a continuous reactor. The mixture of hydrolysate and 32% concentrated acid is separated in a decanter. After separation, the anhydrous hydrogen chloride is converted to methyl chloride, which is then reused in the direct process. The hydrolysate is washed for removal of residual acid, neutralized, dried, and filtered (63). The typical yield of cycHc oligomers is between 35 and 50%. The mixture of cycHc oligomers consists mainly of tetramer and pentamer. Only a small amount of cycHc trimer is formed. 

Mixing trichlorosilane, acetonitrile and diphenylsulphoxide, carried out at 10°C, detonated. This accident was put down to the exothermic addition reaction of the silicon-hydrogen bond on the carbon-nitrogen triple bond of nitrile. Other interpretations are possible for instance, the effect of traces of hydrogen chloride formed by the hydrolysis of chlorosilane on acetonitrile. 

Tetrachlorosilane was added to aqueous ethanol (the presence of water was accidental). There was no proper stirring during this operation, which led to the formation of two liquid layers of compounds that did not react. The very fast and exothermic reaction of the alcoholysis-hydrolysis of chlorosilane started violently and the large compoundion of hydrogen chloride caused the reactor to detonate. 

The self-organization of polysilanols in the presence of other hydrogen bond acceptors has been studied by several groups.512-516 Several other publications have dealt with the stepwise synthesis of siloxane and siloxanol chains.450,517-522 Recent work on fully condensed siloxane rings and silsesquioxane cages involves the non-aqueous hydrolysis of chlorosilanes - as well as mechanistic and structural studies. 

Bis(organosilyl) peroxides are prepared by nucleophilic substitution reactions of hydrogen peroxide with chlorosilanes in the presence of base . Thus, bis(triorganosilyl) peroxide has been prepared from the reaction of 98% hydrogen peroxide and chlorosilane with ammonia as an HCl acceptor (equation 3). Bis(triphenylsilyl) peroxide 2 can also be prepared by the reaction of hydrogen peroxide and triphenyl silylamine (equation 4). 

This reaction is not a simple one. There are a number of intermediate chlorosilanes generated by competing reactions (10). The process is sensitive both to the thermodynamics and kinetics of the chemical reactions, and to the fluid mechanics (qv) of the gas flow in the reactor. The overall procedure involves purging the reactor with hydrogen gas, raising the temperature of the reactor, cleaning the wafers with a brief HC1 etch, and replacing the HQ with the silicon source gas. A complete process cycle can take up to an hour. 

Certain reactions of silyl—alkali metal compounds, such as coupling of (CH3) (C6H5)3 BSiLi with appropriate chlorosilanes (51) and hydrolysis of [(CH3)3Si]3SiLi (63), lead to the formation of organopolysilanes with the silicon-hydrogen bond. There have been few reports of such synthesis. 

The silica is evacuated and exposed to the vapour of a nitrogen-containing base which forms a strong hydrogen bond with the surface silanols. Excess base is removed by evacuation and the silica is then placed in contact with the chlorosilane. It is the evacuation of the excess amine which suppresses polymerization. 

The reaction should be conducted in the presence of catalytic amounts of AICI3 at 220°C, passing through methyldiphenylchlorosilane anhydrous hydrogen chloride at the speed of 0.2 m3/min. The unreacted methyltri-chlorosilane and chlorobenzene, as well as xylene, are regenerated and returned to production, and the intermediate higher distillates extracted at rectification are used to obtain oligomethylphenylsiloxanes. 

The presence of the chlorine atom in the methyl group of dimethyldi-chlorosilane increases the speed of hydrogen replacement in it that is why the chlorination of dimethyldichlorosilane with the preference of obtaining methyl(chloromethyl)dichlorosilane should be conducted superficially (to the conversion degree of 8-14%). The yield of methyl(chloromethyl)dichlorosilane will be 70-80% for the reacted dimethyldichlorosilane, and the unreacted dimethyldichlorosilane is returned into the cycle. 

Similarly to triacetoxymethylsilane, acetylation of chlorosilanes with metal acetates can also yield tetraacetoxysilane and other al-kyl(aryl)acetoxysilanes. The physicochemical properties of important acy-loxyorganosilanes are given in Table 13. The practical value of acetoxysi-lanes is that their hydrolysis, unlike the hydrolysis of organochlorosilanes, forms weak acetic acid, rather than hydrogen chloride. That is why acetox-ysilanes can be used to waterproof various materials (textiles, paper, etc.). Alkyl(aryl)acetoxysilanes are also used to obtain some silicone varnishes and as hardeners for low-molecular silicone elastomers. 

Ethylchlorosilanes (ethyltrichlorosi-lane, diethyldichloro silane, triethyl-chlorosilane, etc. or a mixture of them) are colourless or yellowish liquids. They are easily hydrolysed, releasing hydrogen chloride. MAC=1 mgW. 

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