Direct process

The Direct Process, also called the Direct Synthesis, Rochow Process, and Müller-Rochow Process is the most common technology for preparing organosilicon compounds on an industrial scale. It was first reported independently by Eugene G. Rochow and Richard Müller in the 1940s. The reaction involves a copper-catalyzed reaction of alkyl halides with silicon metal, which takes place in a fluidized bed reactor. Although theoretically possible with any type of alkyl halide, the best results in terms of selectivity and yield occur with methyl chloride. 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 of dimethyldichlorosilane (Me₂SiCl₂) is produced annually using this process.

Few companies actually carry out the Rochow process, because of the complicated process technology and high capital requirements. Since the silicon is prior to reaction in a fluidized bed, the companies practicing this technology are referred to as “silicon crushers”.

The relevant reactions are (Me = CH₃):

The mechanism of the direct process is still not well understood, despite much research. Copper plays an important role. The copper and silicon form an intermetallics with the approximate composition Cu₃Si. 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.

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.

In addition to copper, the catalyst optimally contains promoter metals that facilitate the reaction. Among the many promoter metals, zinc, tin, antimony, magnesium, calcium, bismuth, arsenic and cadmium have been mentioned.

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