Rochow process

In 1940 Rochow discovered the direct process, also cabed the methylchlorosilane (MCS) process, in which methyl chloride is passed over a bed of sibcon and copper to produce a variety of methylchlorosilanes, including dim ethyl dichi oro sil a n e [75-78-5] (CH2)2SiCl2. Working independently, Mbber made a similar discovery in Germany. Consequently, the process is frequently cabed the Rochow process and sometimes the Rochow-Mbber reaction. 

A new approach to understanding the Rochow process synthesis of methylcWorosilanes from CH3 + Cl monolayers on CusSi in vacuum... 

The Rochow Process refers to the synthesis of liquid methylcWorosilanes from metallurgical silicon powder and gaseous methyl cWoride ... 

The evolution of methylchlorosilanes between 450 and 600 K is consistent with the 550 - 600 K typical for the catalytic Rochow Process [3]. It is also reasonably consistent with the evolution of methylchlorosilanes at 500 - 750 K reported by Frank and Falconer for a temperature programmed reaction study of the monolayer remaining on a CuaSi surface after catalytic formation of methylchlorosilanes from CHaCl at higher pressures [5]. Both of these observations suggest that the monolayer formed by methyl and chlorine adsorption on pure CuaSi is similar to that present on active catalysts. For reference, methylchlorosilanes bond quite weakly to tiie surface and desorb at 180 - 220 K. It can thus be concluded that the rate-determining step in the evolution of methylchlorosilanes at 450 - 600 K is a surface reaction rather an product desorption. 

Figure 2. Comparison of the relative yields of methylchlorosilanes from methyl + chlorine monolayers on Cu3Si and doped CuaSi samples with typical yields for the catalytic Rochow process.

Professor Mailer and I have enjoyed cordial relations for over thirty years, but this is the first time he and I have been honored together for our separate and almost simultaneous discovery of what has become known as the Mttller-Rochow Process. It is a great pleasure, and a source of much personal satisfaction to us, that we have both been awarded the "Wacker Silicon-Preis" during the stimulating and enjoyable "Munich Silicon Days 92". 

This contribution to the broad field of organosilicon chemistry including molecules and materials, marks a very unusual anniversary, the existence of which effectively contradicts a historical statement by the famous organosilicon pioneer F. S. Kipping, who believed 80 years ago that organosilicon chemistry would never gain industrial and commercial importance. Fifty years after the invention of the Miiller-Rochow process, the silicon industry achieved an annual worldwide turnover of US 4.700.000.000. This proves impressively that the basic process - independently developed by R. Muller and E. G. Rochow in 1941/2 - can be considered to be the most important innovation for organosilicon research work in industry and university. 

The synthesis of organosilicones and organosilicone surfactants has been well described elsewhere [36-39] and hence only a brief review is given here. Industrially the manufacture of silicones is performed stepwise via the alkylchlorosilanes, produced through the reaction of elemental silicon with methyl chloride (the Muller—Rochow Process) [40,41]. Inclusion of HC1 and/or H2(g) into the reaction mixture, as in Eq. (1.2), yields CH3HSiCl2, the precursor to the organofunctional silanes, and therefore the silicone surfactants ... 

Silicones are made by hydrolysis of organochlorosilanes RnSiCl4-n, which are produced from elemental silicon (obtainable from the carbon reduction of silica, i.e., sand, Section 17.7) by the Rochow process ... 

Industrially, however, organosilicon and organotin chlorides axe usually obtained by the direct Rochow process (Section 3.5.2) ... 

This relatively complicated reaction has been replaced by the so-called Direct Process or Rochow Process, 12,16,22 which starts from elemental silicon. It is illustrated by the reaction... 

These compounds are of special importance because of their hydrolytic reactions. They may be obtained by normal Grignard procedures from SiCU, or, in the case of the methyl derivatives, by the Rochow process, in which methyl chloride is passed over a heated, copper-activated silicon ... 

The proposed silylene mechanism gives an explanation for the high selectivity of (CH3)2SiCl2 formation in the "Direct Synthesis" of methylchlorosilanes (Miiller-Rochow process). Via an oxidative addition of CH3CI to methylsilylenes on the surface of a Cu/Si catalyst, (CH3)3SiCl2 is produced in a kinetically controlled process (Scheme 2). 

Scheme 2. Oxidative addition of CH3CI by "MOIIer-Rochow process"...

The Rochow Process. Rochow found that alkyl and aryl halides react directly with silicon when their vapors contacted silicon at elevated temperatures to produce complex mixtures of organosilicon halides. The reaction is promoted by a wide variety of metals from both the main group and the transition series, but the most efficient catalyst is copper. The most studied reaction of this type is the reaction between methyl chloride and silicon to give dimethyldichlorosilane and methyltrichlorosilane. Dimethyldichloro-silane is major feedstock silane for methylsilicon polymers. 

Catalytic (Lewis base-induced) disproportionation of tetrachlorodimethyldisilane or dichlorotetramethylsilane is another efficient process to generate oligomeric silanes without formation of solid by-products. - ° The inexpensive starting compounds are obtained as a by-product during the synthesis of chlorosilanes in the Muller-Rochow process. Depending on the molecular structure, that is, the number of chlorine atoms per silicon atom, mixtures of linear or branched polymers form. 

Latest developments concern the quenching of liquid silicon by feeding a jet of molten silicon into water (water granulation) or casting into cooled ingot molds. The material thus produced exhibits an improved reactivity in the synthesis of methylchlorosilanes (Rochow process). 

Metallurgical grade silicon plays an important role as an alloy constituent in aluminum alloys. Addition of 2-25% improve the casting properties of aluminum in the manufacture of castings for example for engine blocks or cylinder heads. The utilization of metallurgical grade silicon in the manufacture of methylchlorosi lanes and the silicones produced therewith by direct synthesis (Rochow process) is covered in Chapter 4. 

Complete hydrolysis (excess water) is either carried out continuously in the liquid phase with ca. 25% hydrochloric acid or in the gas phase at temperatures of ca. lOO C. In liquid phase hydrolysis cyclic or linear dimethylsiloxane oligomers are produced in the ratio 1 I to 1 2 depending upon process design. Ca. 30% hydrochloric is produced as a byproduct in liquid phase hydrolysis. This can be recycled in the Rochow process by using it to produce chloromethane by reacting it with methanol, thereby recycling the chlorine. 

The starting materials for the manufacture of diorganopolysiloxanes are the diorganodi-chlorosilanes. Dimethyldichlorosilane, which is the most important one, is made industrially by the Rochow process from methyl chloride and silicon metal in the presence of a copper catalyst at 250-300 °C. 

Silicon tetraalkyl and tetraaryl derivatives (R4Si), as well as alkyl or aryl silicon halides (R SiCl4 , n = 1-3) can be prepared by reaction types 18.38-18.42. Note that variation in stoichiometry provides flexibility in synthesis, although the product specificity may be influenced by steric requirements of the organic substituents. Reaction 18.38 is used industrially (the Rochow process). 

---