Direct process
Encyclopedia
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 crushed prior to reaction in a fluidized bed
, the companies practicing this technology are referred to as “silicon crushers”.
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.
Dichlorodimethylsilane is the major product of the reaction, as is expected, being obtained in about 70-90% yield. The next most abundant product is 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 chloro methyl silanes 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.
Organosilicon
Organosilicon compounds are organic compounds containing carbon silicon bonds. Organosilicon chemistry is the corresponding science exploring their properties and reactivity.Like carbon, the organically bound silicon is tetravalent and tetrahedral...
compounds on an industrial scale. It was first reported independently by Eugene G. Rochow
Eugene G. Rochow
Eugene George Rochow was an American inorganic chemist. Rochow worked on organosilicon chemistry; in the 1940s, he described the direct process, also known as the Rochow process or Müller-Rochow process....
and Richard Müller
Richard Müller (chemist)
Richard Gustav Müller was a German chemist. Independent of Eugene G. Rochow he discovered the Direct process of organosilicon compounds in 1941. The synthesis which is also known as Müller-Rochow process is the copper catalysed reaction of chloromethane with silicon. Müller was awarded the...
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
Chloromethane
Chloromethane, also called methyl chloride, R-40 or HCC 40, is a chemical compound of the group of organic compounds called haloalkanes. It was once widely used as a refrigerant. It is a colorless extremely flammable gas with a minorly sweet odor, which is, however, detected at possibly toxic levels...
. Approximately 1.4 Mton of dimethyldichlorosilane
Dimethyldichlorosilane
Dimethyldichlorosilane is a tetrahedral, organosilicon compound with the formula Si2Cl2. At room temperature it is a colorless liquid that readily reacts with water to form both linear and cyclic Si-O chains...
(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
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
is crushed prior to reaction in a fluidized bed
Fluidized bed
A fluidized bed is formed when a quantity of a solid particulate substance is placed under appropriate conditions to cause the solid/fluid mixture to behave as a fluid. This is usually achieved by the introduction of pressurized fluid through the particulate medium...
, 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, other products
The mechanism of the direct process is still not well understood, despite much research. Copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
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.
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 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
Fractional distillation
Fractional distillation is the separation of a mixture into its component parts, or fractions, such as in separating chemical compounds by their boiling point by heating them to a temperature at which several fractions of the compound will evaporate. It is a special type of distillation...
. Although the boiling points of the various chloro methyl silanes 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.