Methylchlorosilanes, Direct Process Reaction

In 1940 Rochow discovered the direct process, also cabed the methylchlorosilane (MCS) process, in which methyl chloride is passed over a bed of sibcon and copper to produce a variety of methylchlorosilanes, including dim ethyl dichi oro sil a n e [75-78-5] (CH2)2SiCl2. Working independently, Mbber made a similar discovery in Germany. Consequently, the process is frequently cabed the Rochow process and sometimes the Rochow-Mbber reaction. 

The example of the first category is the formation of alkyl- and arylchlorosilanes in the so-called direct process (DP). The process was discovered over 60 years ago by Rochow in the United States, and, independently, by Muller in Germany, and it is still the most important reaction in organosilicon chemistry. In fact, it is at the very basis of the silicone industry, being the primary source of organochlorosilane precursors (mostly methylchlorosilanes, comprising over 90% of the total) in the production of silicone oligomers and polymers. 

The polymers described in this chapter are derived from the residue of the manufacture of methylchlorosilanes by the direct process. Approximately 10% of the reaction product of methylchlorosilane manufacture is a residue (called the direct-process residue) (2). The fraction of direct-process residue boiling between 150 and 160 °C consists primarily of 1,2-dimethyltetra-chlorodisilane and 1,1,2-trimethyltrichlorodisilane (3). 

Transition metals have already established a prominent role in synthetic silicon chemistry [1 - 5]. This is well illustrated by the Direct Process, which is a copper-mediated combination of elemental silicon and methyl chloride to produce methylchlorosilanes, and primarily dimethyldichlorosilane. This process is practiced on a large, worldwide scale, and is the basis for the silicones industry [6]. Other transition metal-catalyzed reactions that have proven to be synthetically usefiil include hydrosilation [7], silane alcdiolysis [8], and additions of Si-Si bonds to alkenes [9]. However, transition metal catalysis still holds considerable promise for enabling the production of new silicon-based compounds and materials. For example, transition metal-based catalysts may promote the direct conversion of elemental silicon to organosilanes via reactions with organic compounds such as ethers. In addition, they may play a strong role in the future... 

Summary The Direct Process discovered by Rochow and Muller around 1940 is the basic reaction used to produce methylchlorosilanes, which are the monomeric intermediates used for production of silicones. An understanding of the elementary reactions, the nature of active sites and the action of promotors does not nearly come close to the performance level of the industrial process and the economic importance. The silylene-mechanism is a useful model to understand the complex product mixture from the reaction of silicon with chloromethane. 

The Direct Process is the reaction of silicon with chloromethane to form methylchlorosilanes (Eq.l). This reaction is unique, in that it is the only solid-catalyzed gas-solid reaction applied in the chemical industry. The Direct Process was first discovered by Rochow [1] and independently Muller [2] around 1940. 

Chloromethane can be produced by either chlorination of methane or reaction of methanol with hydrogen chloride. In an integrated production of silicones hydrogen chloride obtained by hydrolysis of methylchlorosilanes is recycled to the Direct Process via a chloromethane synthesis. The losses are compensated by make-up chloromethane or hydrogenchloride. Impurities, tike water, methanol, dimethyl ether or oxygen, must be kept at as low a level as possible. 

Methylchlorodisilanes are by-products of the direct-process. Commonly referred to as residue, these are formed at a level of about 4% of the total (CH3)2SiCl2 produced (In 1994 (CH3)2SiCl2 production was about 30,000 ton/year). Disilanes are key constituents of the residue, and novel reactions forming Si—Cl bonds have been described (67-69). Commercially, disilanes are redistributed to recover methylchlorosilane monomers (70). Some chemical reactions of direct-process disilanes are shown in Figure 2. Cleavage of Si—Si bonds with HCl has also been described and is practiced industrially (68). 

This direct process has been the focus of much research, particularly in Industrial laboratories during the last 40 years [8]. Published results show that it an extremely complex process with many competing reactions which is difficult to apply to systems other than methylchlorosilanes and trlchlorosllane. Key monomers for productfon of linear polydimethyl-siloxane are dimethyldichlorosilane and trimethylchlorosllane so in general the process has been optimized to produce these materials. Other monomers such as, phenylmethyldichlorosilane, methylvinyldichlorosilane, diphenyldichlorosilane and trlfluoropropylmethyldichlorosilane are required to modify the propertfes of polydimethylsiloxane as described later. These are produced by a variety of approaches depending upon the producer. 

The F3 cyclic species is obtained through hydrolysis of the corresponding silane, methyl(3,3,3-trifluoropropyl)dichlorosilane. This silane is the hydrosilylation adduct of 3,3,3-trifluoropropene and methyldichlorosilane, CH3HSiCl2, a minor product of the Rochow-Muller direct process for manufacturing methylchlorosilanes. The hydrosilylation reaction is catalyzed with transition metal complexes of platinum or rhodium such as Speier s catalyst and hexachloroplatinic acid in isopropanol. The reaction conditions are very similar to those employed with unfluorinated reagents, complicated only by the fact that 3,3,3-trifluoropropene is a gas. 

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. 

Compared to the Grignard and direct processes, the addition method has the advantage of yielding one principal product rather than a mixture. The process is also economically attractive since olefins are low-cost materials and suitable Si—H containing compounds (trichlorosilane and methyldichlorosilane) are by-products of the direct process. The addition process is versatile but cannot, of course, be used to prepare methylchlorosilanes. It is particularly suitable for the production of silanes with organofunctional groups useful reactions of this type include the following ... 

Phenylchlorosilanes can be prepared by a direct process from silicon and chlorobenzene with copper or silver catalyst, but yields are not so good as those of methylchlorosilanes. Alternative routes are Grignard or phenylsodium arylation of SiQ4, and the reaction between benzene and either H2SiCl2 or HSiCla in the presence of platinum or peroxide catalysts. The required intermediates are made from silicon-copper (or-iron) and HQ ... 

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. 

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. 

Although the direct reaction of elemental silicon with methyl chloride shown in Eq. (3) looks simple, it is a complicated reaction and gives many kinds of byproducts.7,8 The yield of methylchlorosilane obtained from the direct reaction varies, and depends upon the reaction conditions such as temperature, pressure, flow rate of reactants, and other processing conditions including particle size and impurities of elemental silicon, catalyst, promoter, reactor type, etc.7... 

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