The Müller-Rochow Synthesis of Chloromethylsilanes

Industrial synthesis of chloromethyl silanes is almost exclusively carried out by the Müller-Rochow process. In it, silicon is made to react with chloromethane. The reaction proceeds under the catalytic influence of copper at temperatures between 250 and 300 °C. The most important basic materials for the production of silicones are sand, petroleum and salt.

For the conversion to be economical, the silicon must meet certain requirements. It must be at least 97 % pure and the particle size should lie between 45 and 250 µm. Purer silicon would lead to more sluggish conversion rates while a purity level less than 95 % would yield a raw silane mixture of unfavorable composition.

Overall, the Müller-Rochow synthesis is very sensitive to foreign metals. It responds best to copper. Too high a copper fraction (between 20 % and 80 %) has a negative effect, though. While the reaction kick-starts very quickly, the silicon is consumed too quickly and the conversion rate is poor. Even traces of other metals, such as aluminum and lead, will lead to unwanted products. ZnCl2 and ZnSO4 are added in quantities of up to 0.2 % in an attempt to counteract this. Their goal is to increase the activity and the selectivity of the reaction. While traces of lead inhibit the reaction, antimony promotes it and will accelerate the conversion of the reagents if added in appropriate amounts.

Some gases also influence the synthesis by increasing the selectivity of the reaction. These include HCl and H2. The amount added determines whether the reaction mixture is rich in halogens or poor in halogens.
The catalyst consists of either silicon-copper alloys or a finely ground blend of the two metals. Precisely how the copper works is not known. However, it is generally assumed that the direct synthesis proceeds via the following intermediate state:

Since high space-time yields and high yields of the target molecule dichlorodimethylsilane are only possible at temperatures from 250 to 300 °C, and since the reaction is highly exothermic, a close watch has to be kept on the temperature. It is controlled by means of a cooling jacket over the reactor or by addition of inert gas.

It is just as important to intimately mix the catalyst in the stream of chloromethane. This prevents local overheating. Nowadays, fluid bed reactors are used almost exclusively as they meet all these processing requirements. The catalyst is placed in the reactor and heated to 280 °C. A powerful stream of chloromethane is then introduced at a tangent to cause turbulence. The turbulence caused by the gas counteracts any tendency for hot spots to occur. Solid particles can be separated off. At the end of the reactor, the raw silane mixture is condensed to separate it from unreacted chloromethane. The silane mixture is separated into its constituent silanes by means of column distillation while the excess chloromethane is fed back into the synthesis again.

The following diagram illustrates the Müller-Rochow process for synthesizing silanes:

The most favorable pressure range for silane synthesis is 1 to 5 bar.

Up to 40,000 metric t/y of raw silane can be produced in modern fluid-bed reactors. These silane mixtures have approximately the composition shown in the following diagram:

The raw silane mixture is purified by distillation in a series of downstream columns. The purity of dichlorodimethylsilane is critical. Even low fractions of trifunctional silanes would lead to crosslinking reactions later. Fractions of monofunctional silanes would have chain-terminating effects.

The following table provides an overview of the range of silanes that occurs and their boiling points:

The diagram at the left shows the typical structure of a distillation column with bubble-cap trays (the diagram shows a column for the fractionation of petroleum). Silane distillation requires columns with up to 200 trays and a reflux ratio of 1:500. To avert the risk of water breakthrough, they are air cooled. Problems occur during the separation of trichloromethylsilane from dichlorodimethylsilane since the boiling points differ only by 4 °C. To achieve the corresponding purity levels, the first raw-silane distillation has to be followed by a second fine distillation. Azeotropic mixtures of chlorotrimethylsilane and tetrachlorosilane are also difficult to separate.

High-boiling by-products can be converted with HCl under the catalytic influence of amines to the preferred product dichlorodimethylsilane. Chloromethylsilanes can also be converted to the desired dichlorodimethylsilane by treatment with AlCl3. Co-proportionation occurs between (CH3)3SiCl and CH3SiCl3.