Thedirect process, also called thedirect synthesis,Rochow process, andMüller-Rochow process is the most common technology for preparingorganosilicon compounds on an industrial scale. It was first reported independently byEugene G. Rochow andRichard Müller in the 1940s.[1][2]
The process involves copper-catalyzed reactions of alkyl halides with elemental silicon, which take place in afluidized bed reactor. Although theoretically possible with any alkyl halide, the best results in terms of selectivity and yield occur withchloromethane (CH3Cl). Typical conditions are 300 °C and 2–5 bar. These conditions allow for 90–98% conversion for silicon and 30–90% for chloromethane. Approximately 1.4 Mton ofdimethyldichlorosilane (Me2SiCl2) is produced annually using this process.[3]
Few companies actually carry out the Rochow process, because of the complex technology and high capital requirements. Since thesilicon iscrushed prior to reaction in afluidized bed, the companies practicing this technology are referred to assilicon crushers.[4]
The relevant reactions are (Me = CH3):
Dimethyldichlorosilane (Me2SiCl2) is of particular value (precursor tosilicones), buttrimethylsilyl chloride (Me3SiCl) andmethyltrichlorosilane (MeSiCl3) are also valuable.[5]: 371
The mechanism of the direct process is still not well understood, despite much research.Copper plays an important role, and almost certainly forms anintermetallic with the approximate composition Cu3Si.[6] This intermediate facilitates the formation of the Si-Cl and Si-Me bonds. It is proposed that close proximity of the Si-Cl to a copper-chloromethane "adduct" allows for formation of the Me-SiCl units. Transfer of a second chloromethane allows for the release of the Me2SiCl2. Thus, copper is oxidized from the zero oxidation state and then reduced to regenerate the catalyst.[1]
The chain reaction can be terminated in many ways. These termination processes give rise to the other products that are seen in the reaction. For example, combining two Si-Cl groups gives the SiCl2 group, which undergoes Cu-catalyzed reaction with MeCl to give MeSiCl3.[1]
In addition to copper, the catalyst optimally contains promoter metals that facilitate the reaction.Tin is necessary, and synergistic withzinc; but extremely minute quantities of many other promoter metals appear to influence the reaction. Among the many promoter metals, iron, aluminum, titanium, manganese, nickel, lead, phosphorus,[7] antimony, magnesium, calcium, bismuth, arsenic, and cadmium have been mentioned.[1][3]
The major product for the direct process should be dichlorodimethylsilane, Me2SiCl2. However, many other products are formed. Unlike most reactions, this distribution is actually desirable because the product isolation is very efficient.[1] Each methylchlorosilane has specific and often substantial applications. Me2SiCl2 is the most useful. It is the precursor for the majority of silicon products produced on an industrial scale. The other products are used in the preparation of siloxane polymers as well as specialized applications.[1]
Dichlorodimethylsilane is the major product of the reaction, as is expected, being obtained in about 70–90% yield. The next most abundant product ismethyltrichlorosilane (MeSiCl3), at 5–15% of the total. Other products include Me3SiCl (2–4%), MeHSiCl2 (1–4%), and Me2HSiCl (0.1–0.5%).[1]
The Me2SiCl2 is purified byfractional distillation. Although the boiling points of the various chloromethylsilanes are similar (Me2SiCl2: 70 °C, MeSiCl3: 66 °C, Me3SiCl: 57 °C, MeHSiCl2: 41 °C, Me2HSiCl: 35 °C), the distillation utilizes columns with high separating capacities, connected in series. The purity of the products crucially affects the production of siloxane polymers, otherwise chain branching arises.[1]
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