Chlorosilanes methylchlorosilane

The most important industrially utilized silicon compounds include chlorosilanes, methylchlorosilanes, silicones, silicon dioxide and silicic acids in different forms, silicates in the form of glass, water glass, enamel frits, silicate fillers, zeolites, silicon carbide and silicon nitride. 

The gas chromatographic separation of chlorosilanes, methylchlorosilanes and their associated siloxanes have been reported404 on 11 different stationary phases. Temperature programming is necessary for the elution of the siloxanes but many of the stationary phases reported in the literature cannot be used under these conditions because of their high volatility at elevated temperatures. Among the liquids or gums which were used as stationary phases, the most effective were dimethyl, diethyl and dibutyl phthalates. 

Direct Process. Passing methyl chloride through a fluidized bed of copper and siUcon yields a mixture of chlorosilanes. The rate of methylchlorosilane (MCS) production and chemical selectivity, as determined by the ratio of dimethydichlorosilane to the other compounds formed, are significantly affected by trace elements in the catalyst bed very pure copper and siUcon gives poor yield and selectivity (22). 

A number of metal and metalloid halides have been separated with rather conventional arrangements. For example, Keller and Freiser separated SnCl4, TiCl4, NbClj, and TaCIs at 200°C using a copper column packed with squalane on Chromsorb P (a modified diatomaceous earth).51 A variety of chlorosilanes and methylchlorosilanes have been separated using silicone oil plus diethyl ph-thalate as the stationary-phase and thermal conductivity detectors.52... 

Not only octadecyltrichlorosilane is unreactive towards dry silica at room temperature. This is also the case for the chlorosilanes and the methylchlorosilanes. It was stated earlier that the vapour phase reaction occurs at elevated temperatures (> 473 K). This high-temperature constraint limits potential gas phase silanizing agents to those which have a high thermal stability and sufficient vapour pressure. 

Apart from antimony, there are other good promoters of the direct synthesis of methylchlorosilanes, which increase the yield of dimethyldichlorosilane, such as arsenic and zinc chloride. If it is necessary to increase the yield of alkylhydridechlorosilanes, one should use univalent copper chloride, cobalt, and titanium. The addition of tin or lead into contact mass increases the yield of dimethyldichlorosilane up to 70% the yield of ethyldi-chlorosilane is increased to 50-80% when contact mass receives 0.5-2% of calcium silicide (Ca2Si). In the synthesis of phenylchlorosilanes effective promoters are zinc, cadmium, mercury or their compounds. In particular, the introduction of zinc oxide (up to 4%) into contact mass may increase the diphenyldichlorosilane content up to 50%, and the introduction of a mixture of zinc oxide and cadmium chloride, even up to 80%. 

Industrial chlorination of methyltrichlorosilane and trimethylchlorosi-lane is conducted by a similar technique. It should be borne in mind, however, that the chlorination speed of methyl groups greatly depends on the number of methyl radicals in the original methylchlorosilanes. Trimethyl-chlorosilane is the easiest to chlorinate, methyltrichlorosilane is the hardest. For example, the chlorination speed at the transition from dimethyldi-chlorosilane to trimethylchlorosilane increases 9-fold. 

Among the silicon halides, the chlorosilanes have attracted by far the greatest attention. Silicon tetrachloride, trichlorosilane and the methylchlorosilanes are produced on an industrial scale and are used, for instance, in the production of pure silicon or silicones. 

The halogen methyl-chlorosilanes were prepared by photochlorination or photobromination of methylchlorosilanes. The compounds... 

As in other preparative methods for organosilicon compounds, the direct synthesis produces a mixture of methylchlorosilanes rather than the single compound shown in equation 3. Besides dimethyl-dichlorosilane, the mixture usually contains silicon tetrachloride, tri-chlorosilane, methyltrichlorosilane, methyldichlorosilane, trimethyl-chlorosilane, and even silicon tetramethyl. Under proper conditions, dimethyldichlorosilane is the principal product. Of the other compounds, methyltrichlorosilane usually is next in abundance this substance finds use in the cross-linked methyl silicone resins, or it can be methylated further by the Grignard method to increase the yield of dimethyldichlorosilane. There is no way of recycling it in the direct process, and so supplemental operations are required for the conversion. The interconversion of this and the other minor products of the direct synthesis, involving the exchange of methyl and chlorine groups as desired, has been a special study in itself.10... 

Solvented silicone systems may be hazardous with regard to health or fire. Further, primers are flammable and require care in handling. Methylchlorosilanes are flammable and corrosive because of the liberation of hydrochloric acid on hydrolysis other chlorosilanes are less flammable but are hazardous chemicals and can irritate the eyes, respiratory organs and skin. Protective gloves should be used. The material should not be allowed to make contact with the eyes or skin. Inhalation of vapours should be avoided especially in systems which evolve irritant by-products such as acetic acid or amines. 

By far the most important chlorosilanes, on the basis of quantity (> 90%), utilized in industrial hydroly.sis and methanolysis, are the methylchlorosilanes, followed by the chlorosilanes, which contain exclusively, or in combination with methyl groups, the ligands H-, C6H5-, CH2=CH- and CF3-CH2-CH2- linked to a silicon atom. 

Methylchlorosilanes are used in the manufacture of a variety of resins, elastomers, and silicone oils. They are produced as a mixture of chlorosilanes, mainly dimethyldichlorosilane, by the reaction between silicon and methyl chloride by a direct route discovered independently by Rochow (1945) and Muller (1950). In this route, metallic copper, with or without promoters, is used to accelerate the reactions. The form of copper is important and depends on its preparation and association with the silicon phase. The whole system of solids comprising silicon metal, copper... 

TABLE 8. Recommended standard enthalpies of formation of methylchlorosilanes and methyl(hydrido)chlorosilanes... 

The direct process is also quite versatile and can be fine-tuned to prepare other types of chlorosilanes also. Ethylchlorosilanes could also be similarly prepared analogous to the synthesis of methylchlorosilanes. Preparation of phenylchlorosilanes required a slight modification of the catalyst. Addition of a mixture of HCl and MeCl to silicon affords mixtures of methylchlorosilanes along with MeSiHCU- Simultaneously, along with Rochow, but independently, Richard Mueller in Germany had also come out with a direct process, initially for preparing HSiCls, and later for methylchlorosilanes [7]. 

Compared to the Grignard process, the direct process is not very versatile. In practice, it is satisfactory for the manufacture of only methyl- and phenylchloro-silanes. (For the latter, silver is the preferred catalyst and a reaction temperature of about 400°C is used.) When the process is used for other chlorosilanes, yields are unacceptably low. However, methylchlorosilanes are the chlorosilanes most widely used in the manufacture of commercial silicones and the direct process has become the dominant process. 

In a typical process, the chlorosilane blend is dissolved in a solvent such as toluene or xylene and then stirred with water. When the blend contains mainly methylchlorosilanes, reaction is very rapid and exothermic and cooling may be... 

Burson and Kennerfound that the dimethyl-, diethyl- and dibutyl-phthalates were the most effective of the phthalate ester stationary phases in making complete separations of chlorosilanes and methylchlorosilanes. The maximum temperature for these three phases is less than lOO C and, consequently, considerable column bleed is evident when the program nears this temperature. Dinonyl-phthalate has a higher temperature limit, but is not so effective in resolving the methyltrichlorosilane and dimethyldichlorosilanes. Dipropyltetrachlorphthalate is even less effective. 

Burson and Kenner determined the purity of trichlorosilane and silicon tetrachloride with the SF-96 column. DC-LSX-3-0295 tri-fluoropropyl silicone gum was found to be the best for analysing samples of the methylchlorosilanes. Figure 57 shows a chromatogram of a sample of methyltrichlorosilane containing as impurities 0.02% silicon tetrachloride, 0.03% methyldichlorosilane, 0.04% trimethyl-chlorosilane, 0.12% dimethyldichlorosilane, and 0.07% 1,1,3,3,-tetra-chloro-1,3-dimethyldisiloxane. The concentrations of these impurities were determined by comparison of peak areas with standards prepared by adding known amounts of these impurities to methyltrichlorosilane of 99.99% purity. 

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