Direct process

The direct process, also called the direct synthesis, Rochow process, and Müller-Rochow process is the most common method for preparing organosilicon compounds on an industrial scale. It was first reported independently by Eugene G. Rochow and Richard Müller in the 1940s.

The process involves copper-catalyzed reactions of alkyl halides with elemental silicon, which take place in a fluidized bed reactor. Although theoretically possible with any alkyl halide, the best results in terms of selectivity and yield occur with chloromethane (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 of dimethyldichlorosilane (Me₂SiCl₂) is produced annually using this process.

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

Reaction and mechanism

The relevant reactions are (Me = CH₃):

x MeCl + Si → Me₃SiCl, Me₂SiCl₂, MeSiCl₃, Me₄Si₂Cl₂, …

Dimethyldichlorosilane (Me₂SiCl₂) is of particular value (precursor to silicones), but trimethylsilyl chloride (Me₃SiCl) and methyltrichlorosilane (MeSiCl₃) are also valuable.

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

Product distribution and isolation

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. 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.

Dichlorodimethylsilane is the major product of the reaction, as is expected, being obtained in about 70–90% yield. The next most abundant product is methyltrichlorosilane (MeSiCl₃), at 5–15% of the total. Other products include Me₃SiCl (2–4%), MeHSiCl₂ (1–4%), and Me₂HSiCl (0.1–0.5%).

The Me₂SiCl₂ is purified by fractional 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.