Methylchlorosilanes synthesis

CuCl, especially in a single crystal form, is extensively used as an optical material for its special optical properties. Orel et al. [2] first proposed a new method to obtain CuCl particles by the reduction of Cu with ascorbic acid. Several dispersants were used in the reduction and monodispersed CuCl particles can be obtained by selecting the proper dispersant and reduction conditions. In this work, the above method was used to modify the traditional process of CuCl preparation, namely, by reducing the Cu " with sodium sulfite to obtain the highly active CuCl catalyst to be used in the direct process of methylchlorosilane synthesis. 

Highly active CuCl catalysts for the direct process of methylchlorosilane synthesis were prepared by reducing Cu with a sodium sulfite solution in the presence of dispersing agents. Several well-known dispersants, e.g. SDBS, were used in this study. When SDBS was used, a catalyst in the form of small flakes was obtained that gave the best performance in reactivity, product selectivity and silicon conversion. This provides a convenient way to prepare the CuCl catalyst for use in industrial production. 

The average composition of the mixture obtained in methylchlorosilane synthesis is (in %) ... 

The Miiller-Rochow-Synthesis [16,17] (direct synthesis of methylchlorosilanes) provides as byproduct a high boiling fraction consisting essentially of 1,1,2-trimethyltrichlorodisilane and 1,2-dimethyltetrachlorodisilane [18]. Starting with these disilanes Wacker-Chemie has developed different ways to produce silicon carbide [19, 21] and silicon carbonitride [22] fibers. 

Methylbenzene halogen complex of, 3 122 iodine monochloridecomplese, 3 109 Methylchlorosilanes hydrolysis, 42 149-150, 157 pyrolysis products of, 7 356-363 Methylcobalamin, 19 151, 152 Methyl-coenzyme M reductase, 32 323-325 EPR spectra, 32 323, 325 F43 and, 32 323-324 function, 32 324-325 Methyl-CoM reductase, 32 329 Methyl cyanide, osmium carbonyl complexes, reaction, 30 198-201 Methylcyclophosphazene salts, 21 70 synthesis, 21 109... 

Methylchlorosilane serves as the initial stock for synthesis of Enterosgel. Enterosgel, i.e. polymethylsiloxane, is a hydrogel of methylsilicic acid obtained in the process of its poly condensation (Fig. 21.1), when molecnles of methylsilicic acid lose water and form silanol (=Si-0-Si=) bonds. 

The advantages offered by this process are (1) a one-step synthesis to poly(methylchlorosilanes), (2) low cost dimers and (3) fiber processing that leads to phase pure SiC fibers. The primary drawbacks to the Dow-Corning process are (1) a low polymer yield (15-20%), (2) a multistep process and (3) the high cost of LiAUTj reduction of the chlorinated polymer. 

The disilane fraction refers to a fraction boiling over the range of about 150°-l 60° C which is obtained by fractionation of the higher boiling fraction of methylchlorosilanes produced by the so-called direct synthesis (141, 214). It is composed mainly of 1,1,2-trichlorotrimethyldisilane and 1,1,2,2-tetrachlorodimethyldisilane, somewhat contaminated by siloxanes (6, 22, 27, 114,125,126, 134). 

Cermak, J., Franc, J. Identification of some compounds formed in the direct synthesis of methylchlorosilanes. Collection Czech. Chem. Commun. 30, 3278 (1965). - Anal. Abstr. 14, 764 (1967). 

Methylchlorosilanes are more efficiently produced from methylchloride obtained in this way, because this product contains considerably lesser amounts of impurities, which inhibit direct synthesis. 

The latter activation technique uses more silicon in the process of direct synthesis (70-75% against 30-40% for nonactivated mass) and considerably increases the yield of dialkyldichlorosilanes. For example, if for the normal mass during the synthesis of methylchlorosilanes the yield of di-methyldichlorosilane varies from 30 to 45% in time, for the mass activated by zinc chloride the yield grows up to 60-75%. However, this activation technique also has disadvantages ... 

Raw stock for the direct synthesis of methylchlorosilanes, methylchlo-ride, has such impurities as moisture, methyl alcohol, oxygen, sulfur dioxide, methylenechloride, dimethyl ether, carbon oxide and dioxide, etc. Most of them negatively affect the synthesis of methylchlorosilanes harmful impurities are chemisorbed on the active centres of contact mass and foul the copper catalyst, which naturally inhibits the reaction of methyl-chloride with contact mass. A similar situation is observed in the direct synthesis of ethylchlorosilanes. 

Selectivity of contact mass. Since the most important products in the synthesis of organochlorosilanes are diorganodichlorosilanes, it is natural that the increase of their yield receives much attention. The selective formation of diorganodichlorosilanes when organic chlorine derivatives interact with contact mass is connected with the purity of the reactants, silicon above all. High yield of dimethyldichlorosilane in the direct synthesis of methylchlorosilanes largely depends on the presence of noticeable quantities of aluminum in contact mass. The yield of dimethyldichlorosilane in the presence of aluminum as a rule decreases due to the formation of trimethylchlorosilane in the reaction with pure (semiconductor) silicon in the presence of copper trimethylchlorosilane is virtually not formed. 

Thus, the methylation of dimethyldichlorosilane becomes very significant in the processes of direct synthesis, whereas the total amount of trimethylchlorosilane, which is formed according to the reactions of dis-proportioning and methylation, may reach 60-65%. Thus, in the direct synthesis of methylchlorosilanes the introduction of significant amounts of AI or its compounds into contact mass reduces the yield of dimethyldichlorosilane and respectively increases the yield of trimethylchlorosilane. 

This example shows the importance of so-called activators, or promoters, for increasing the activity and selectivity of contact mass. These additives can sharply activate the reaction and shift it in a certain direction. Various substances have different effects on the activity of contact mass. For example, antimony has a positive effect on the direct synthesis of organochlorosilanes and increases the total yield of methylchlorosilanes, whereas lead and bismuth reduce the formation of these substances. How-ever, the positive effect of a promoter manifests itself only in a certain concentration, exceeding which transforms a positively acting additive into poison or an inhibitor of the reaction. For example, in a 0.002—0.005% concentration antimony is a promoter of the direct synthesis of methylchlorosilanes on the other hand, in a concentration higher than 0.005% it becomes poison. 

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%. 

Alloy A is used for the synthesis of methylchlorosilanes and other al-kylchlorosilanes alloy B is used for the synthesis of methylchlorosilanes with an increased yield of dimethyldichlorosilane alloy C is used for the synthesis of methyl- and ethylchlorosilanes alloy D is used for the synthesis of phenyltrichlorosilane. 

It follows from all the above-mentioned facts that the direct synthesis of methyl-, ethyl and phenylchlorosilanes is a complex heterophase process which depends on many factors and forms a compex reactive mixture. For example, in the direct synthesis of methylchlorosilanes there are about 130 compounds found and characterised. This does not mean, however, that in this or other definite synthesis all the 130 products are formed. The composition of the mixtures formed and the transformation degree of alkyl-chlorides and chlorobenzene in the synthesis of methyl-, ethyl and phenylchlorosilanes depend on the synthesis conditions, the type of the reactor used and many other factors. In spire of the complexity of the process and the variety of its products, the reaction of direct synthesis can nevertheless be directed (towards a preferential formation of a main product), changing the conditions for the preparation of contact mass, introducing various promoters into contact mass and changing the reaction conditions. 

Because direct synthesis in this case is very complex, the mechanism of this process has not been fully established. However, the following way of methylchlorosilane formation with the catalytic effect of copper on the reaction of methylchloride with silicon is the most probable. 

Fig. 3. Production diagram of methylchlorosilanes by direct synthesis technique 1 - container 2 - evaporator 3 - separator 4 - reactor 5 - filters 6-8 - condensers 9 - collector 10 - centrifugal pump 11 - Field tube.

The direct synthesis of methylchlorosilanes forms a condensate of the following composition 50-80% of methylchlorosilanes and 20-50% of methylchloride (unreacted). 

The basic diagram of ethylchlorosilane production by direct synthesis is similar to the production diagram of methylchlorosilanes given in Fig.3. After loading the reactor with contact mass, electrical heating is switched on and at 200 °C nitrogen is fed through the heater. Thus, the mass is... 

The investigations carried out on the organometallic synthesis of carbosilanes and presented in this paper indicate the progress achieved in this field in the years between 1967 and 1971 in cooperation with Dr. P. Schober, Dr. M. Hahnke and Dr. G. Maas. The yields of these syntheses are not all satisfactory yet, in most cases with the cyclization reaction. There is no doubt that the procedures described can be extended and simplified. However, it is also apparent that only some of the compounds (and their derivatives) which arise from the pyrolysis of the methylchlorosilanes and of tetramethylsilane can be obtained by organometallic synthesis38. The development of this field of chemistry requires extensive advances in methods for synthesis. 

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... 

Promoters are normally incorporated in quantities less than 1%. Zinc is known as one of the most effective promoters for the direct synthesis methylchlorosilanes.33 Cadmium is known as one of the most effective promoters for the direct synthesis of tris(sila)alkanes,20 bis(chlorosilyl)methanes,22 and tris(chlorosilyl)methane.23 In the direct reaction of (dichloromethyl)silane 6b, the total amounts (percentages) of (trisilyl)methane.23 In the direct reaction of (dichloromethyl)silane 6b, the total amounts (percentages) of (trisilyl)methane products 7b-10b in the... 

The proposed silylene mechanism gives an explanation for the high selectivity of (CH3)2SiCl2 formation in the "Direct Synthesis" of methylchlorosilanes (Miiller-Rochow process). Via an oxidative addition of CH3CI to methylsilylenes on the surface of a Cu/Si catalyst, (CH3)3SiCl2 is produced in a kinetically controlled process (Scheme 2). 

Summary The heterogeneous catalytic redistribution reaction of methylchlorodisilanes provides spinnable poly(methylchlorosilanes/-carbosilanes). Especially copolymers like poly(methylchlorosilanes-co-styrenes) are suitable polymers for melt spinning. The high reactivity caused by Si-Cl bonds enables oxygen free curing methods of the melt spun polymer filaments with ammonia. The synthesis is achieved without the employment of highly reactive metals and any solvents. The thus produced SiC fibers exhibit oxygen contents lower than 1 wt. %. 

The disadvantages of the synthesis routes 1-4 are the application of highly reactive and expensive metals (Li, Na, K, Mg) and the enormous quantity of solvents. Particularly, as result of the dehalocoupling reactions, the polymers are unreactive at room temperature. To overcome these problems we synthesized spinnable reactive poly(silanes/-carbosilanes) via heterogeneous catalytic disproportionation of methylchlorodisilanes which have been wasted as a byproduct of the "Direct synthesis" of methylchlorosilanes so far. 

Other more complex polymers have been employed as silicon carbide precursors. For instance, the mixture of methylchlorodisilanes obtained as a by-product in the direct synthesis of Me2SiCl2 can be redistributed with catalysts to give a polycyclic, partially cross-Uuked polymer with the approximate composition shown in equation (44). Pyrolysis of this precursor produces silicon carbide in good yield. Partially cross-linked polymers made by condensing vinylmethyldichlorosilane and other methylchlorosilanes with sodium also are efficient precursors for silicon carbide. 

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