Silicone industry, direct process

Silicones, an important item of commerce, are widely available commercially (9,494). The principal manufacturers of silicone operate direct-process reactors to produce dimethyl dichi orosilane and, ultimately, polydimethyl siloxane. Typical plants produce more than 450 t per year. The siUcone industry is a global enterprise in the 1990s, with principal producers in the United States (Dow Coming, GE, and OSi), Europe (Wacker Chemie, Hbls, Rhc ne-Poulenc, and Bayer), and Southeast Asia (Shin-Etsu, Toshiba SiUcones, and Dow Coming, Japan). Table 15 Hsts the approximate sales of the principal producers for 1991. 

The direct, copper catalyzed reaction of methyl chlorid with silicon, the Direct Process , is the most important reaction for the production of methylsilicon intermediates. A similar reaction with hydrogen chloride yields trichlorosilane, which in turn is the basis for trifiinctional intermediates and also for polycrystalline silicon, the raw material for the semiconductor industry. This Direct Process has been practiced for fifty years and much progress has been made by many towards understanding and optimizing the process, but much still remains to be learned about this complex series of heterogeneous reactions. 

Organosilanes, especially dimethyldichlorosilane (M2), are important chemicals used in the silicone industries. The direct reaction of silicon with an organic halide to produce the corresponding organosilanes as a gas-solid-solid catalytic reaction was first disclosed by Rochow [1]. In the reaction, a copper-containing precursor first reacts with silicon particles to form the catalytically active component, which is a copper-silicon alloy, the exact state of which is still under discussion. As the reaction proceeds. Si in the alloy is consumed, which is followed by the release of copper. This copper diffuses into the Si lattice to form new reaction centers until deactivation occurs. The main reaction of the direct process is ... 

Highly active CuCl catalysts for the direct process of methylchlorosilane synthesis were prepared by reducing Cu with a sodium sulfite solution in the presence of dispersing agents. Several well-known dispersants, e.g. SDBS, were used in this study. When SDBS was used, a catalyst in the form of small flakes was obtained that gave the best performance in reactivity, product selectivity and silicon conversion. This provides a convenient way to prepare the CuCl catalyst for use in industrial production. 

Other markets for char include iron, steel, and sili-con/ferro-silicon industries. Char can be used as a reducing agent in direct reduction of iron. Ferro-silicon and metallurgical-grade silicon metal are produced carbothermally in electric furnaces. Silica is mixed with coke, either iron ore or scrap steel (in the case of ferro-silicon), and sawdust or charcoal in order to form a charge. The charge is then processed by the furnace to create the desired product. Char can be substituted for the coke as a source of reducing carbon for this process. Some plants in Norway are known to have used coal-char in the production of silicon-based metal products as late as mid-1990.5 The use of char in this industry is not practiced due to lack of char supply. 

More than 60 years after its simultaneous discovery by Rochow and Muller, the direct reaction of copper-activated silicon with alkyl chlorides is arguably still the most important industrial process for the preparation of basic organosilanes. An inspiring historic account highlighting the significance of this seminal work has been given by Seyferth.12 A comprehensive review on the subject has been written by Jung and Yoo.13 The most recent work associated with the direct process is concerned with the role of metallic promoters, such as Zn and Cd, as well as mechanistic aspects.14... 

The example of the first category is the formation of alkyl- and arylchlorosilanes in the so-called direct process (DP). The process was discovered over 60 years ago by Rochow in the United States, and, independently, by Muller in Germany, and it is still the most important reaction in organosilicon chemistry. In fact, it is at the very basis of the silicone industry, being the primary source of organochlorosilane precursors (mostly methylchlorosilanes, comprising over 90% of the total) in the production of silicone oligomers and polymers. 

After more than fifty years of industrial use, the direct reaction of methyl chloride with silicon, which underlies the entire silicone industry, is not understood. Promising recent experiments on this process are likely to be continued, and should at least settle the question of the nature of the intermediates. Are silylenes important in the process, either in the gas phase or at the silicon surface ... 

A common feature of all these compounds is their tetrahedral structure at the silicon atom which is bound to four oxygen neighbors. A tremendous breakthrough in the history of silicon-based polymers has been achieved by the invention of the Direct Process by Muller and Rochow resulting in the industrial production of methyl chlorosilanes with hydrolytically stable Si-C bonds besides very reactive Si-Cl bonds which serve as building units for a wide variety of polydimethyl siloxanes including silicon fluids, resins, and elastomers. 

Transition metals have already established a prominent role in synthetic silicon chemistry [1 - 5]. This is well illustrated by the Direct Process, which is a copper-mediated combination of elemental silicon and methyl chloride to produce methylchlorosilanes, and primarily dimethyldichlorosilane. This process is practiced on a large, worldwide scale, and is the basis for the silicones industry [6]. Other transition metal-catalyzed reactions that have proven to be synthetically usefiil include hydrosilation [7], silane alcdiolysis [8], and additions of Si-Si bonds to alkenes [9]. However, transition metal catalysis still holds considerable promise for enabling the production of new silicon-based compounds and materials. For example, transition metal-based catalysts may promote the direct conversion of elemental silicon to organosilanes via reactions with organic compounds such as ethers. In addition, they may play a strong role in the future... 

Summary The Direct Process discovered by Rochow and Muller around 1940 is the basic reaction used to produce methylchlorosilanes, which are the monomeric intermediates used for production of silicones. An understanding of the elementary reactions, the nature of active sites and the action of promotors does not nearly come close to the performance level of the industrial process and the economic importance. The silylene-mechanism is a useful model to understand the complex product mixture from the reaction of silicon with chloromethane. 

The Direct Process is the reaction of silicon with chloromethane to form methylchlorosilanes (Eq.l). This reaction is unique, in that it is the only solid-catalyzed gas-solid reaction applied in the chemical industry. The Direct Process was first discovered by Rochow [1] and independently Muller [2] around 1940. 

The first industrial production of methylchlorosilanes using the Direct Process started in 1947 in the USA (General Electric, Waterford), in Germany between 1951 and 1955 three companies entered into it. During the intervening five decades the Direct Process became a worldwide utilized process in the fast growing world of silicones and in 1993 the production of methylchlorosilanes passed 1 000 0001 per year in the Western World. In 1995 the production of methylchlorosilanes was about 1 250 000 t, for which 1 000 000 t chloromethane and 300 000 t silicon were consumed. The vale of the silicones market in 1995 was about 6 billion US. These figures emphazise the economic importance of the Direct Process in the silicones industry. 

Even after five decades of industrial utilization no alternative method is coining in sight to substitute the Direct Process. Neither the reaction in liquid phase nor the insertion reaction of silicon monoxide SiO can compete with the way we produce MCS today. 

The process of synthesizing high-molecular-weight copolymers by the polymerization of mixed cyclics is well established and widely used in the silicone industry. However, the microstructure which depends on several reaction parameters is not easily predictable. The way in which the sequences of the siloxane units are built up is directed by the relative reactivities of the monomers and the active chain-ends. In this process the different cyclics are mixed together and copolymerized. The reaction is initiated by basic or acidic catalysts and a stepwise addition polymerization kinetic scheme is followed. Cyclotrisiloxanes are most frequently used in these copolymerizations since the chain growth mechanism dominates the kinetics and redistribution reactions involving the polymer chain are of negligible importance. Several different copolymers may be obtained by this process. They will be monodisperse and free from cyclics and their microstructure can be varied from pure block to pure random copolymers. 

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 19.39-19.43. Note that variation in stoichiometry provides flexibility in synthesis, although the product speciflcity may be influenced by steric requirements of the organic substituents. Reaction 19.39 is used industrially (the Rochow or Direct process). 

Silicon alloyed with a copper catalyst and promoter substances reacts with methyl chloride (at temperatures around 300 °C) to give a mixture of methylchlorosilanes in the industrial direct synthesis. Dimethyldichlorosilane represents the most important target in this process. Since Rochow [1] and Mtiller [2] discovered this direct synthesis route for the silicon-promoter-catalyst system, many investigations were done to increase the activity as well as the selectivity and to clarify the mechanism. Zinc, tin, and phosphorus, beside other substances, were found to give effects [3-6]. The goal of this research work is to find out whether there are relationships between the electronic effect of phosphorus, tin, boron, or indium doping of silicon and its reactivity as well as selectivity in direct synthesis. Characterization of the electronic state of the variously doped silicon relies on photo-EMF measurements. 

Silicones are made from silicon and methyl chloride in a process known as the direct reaction or direct process. This reaction yields methyl chlorosilanes. They are distilled for purification and the dimethyldichlorosilane is hydrolyzed to give PDMS. This product can be formulated into thousands of different products, which are sold to every major industrial segment. 

Direct Process. Passing methyl chloride vapors through a bed of copper and silicon at temperatures near 300°C 5delds a mixture of methylchlorosilanes. The efficiency of this process is critically dependent on reactor design and chemistry. Optimization of the fluidized-bed reactor is a critical part of the industrial process (22). The rate of methylchlorosilane (MCS) production and selectivity for dichlorodimethylsilane are significantly affected by trace elements in the catalyst bed very pure copper and silicon give poor rate and selectivity (23-25). 

The silicones industry got its start in the late 1930 s (1,2) and became viable after Rochow s 1940 discovery of the direct process which reacts elemental silicon with MeCl to produce methylchlorosilanes (3,4). This chapter attempts to summarize some of the steps which take place in the process of converting sand into silicones. The vignette chosen for this summary is the production of a platinum-cure, so-called addition cured silicone. This brief review will make use of the M, D shorthand wherein an M group is MojSiO- and a D group is -MojSiO-. Substituents on silicon other than Me are represented with a superscript so that M stands for (H2C=CH)Me2SiO- and DH stands for -(Me)(H)SiO- (5). Figure 1 summarizes the entire process covered in this review. 

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