United States Patent (19) 11) Patent Number: 4,552,973 Feldner et al. (45) Date of Patent: Nov. 12, 1985 54 PROCESS FOR THE PREPARATION OF 2,647,912 8/1953 Barry et al. ......................... 556/469 DMETHYLDCHLOROSLANE 2,717,257 9/1955 Bluestein ............................. 556/469 (75. Inventors: Kurt Feldner; Wolfgang Grape, both 3,384,652 5/1968 Hamilton ............................ 556/469 of Cologne, Fed. Rep. of Germany 4,477,631 5/1984 Faure et al. ......................... 556/469 Primary Examiner-Paul F. Shaver 73 Assignee: Bayer Aktiengesellschaft, Attorney, Agent, or Firm-Sprung, Horn, Kramer & Leverkusen, Fed. Rep. of Germany Woods (21) Appl. No.: 713,501 57 ABSTRACT 22 Filed: Mar. 19, 1985 A process for the preparation of dimethyldichlorosilane (30) Foreign Application Priority Data from the low-boiling and high-boiling by-products of the direct synthesis of methylchlorosilane, comprising Mar. 23, 1984 (DE) Fed. Rep. of Germany ....... 3410644 reacting methyltrichlorosilane simultaneously with the 51) Int. Cl." ................................................ C07F 7/12 low-boiling components having a high content of 52 U.S.C. .................................................... 556/469 methyl groups and with the high-boiling non-cleavable 58 Field of Search ......................................... 556/469 components, in the presence of a catalyst, at a tempera 56) References Cited ture between about 250 C. and 400 C. and under a U.S. PATENT DOCUMENTS pressure of up to 100 bar. 2,647,136 7/1953 Sauer ................................... 556/469 4 Claims, No Drawings 4,552,973 1. 2 minum chloride which, however, is converted to the PROCESS FOR THE PREPARATION OF known catalyst AlCl3 by hydrogen chloride gas fed DIMETHYTLDICHLOROSLANE simultaneously into the reaction mixture. German Offenlegungsschrift (German Published The present invention relates to a process for the Specification) No. 3,314,734 describes the reaction of preparation of dimethyldichlorosilane from unavoida disilanes which have a high methyl content and do not ble products of the direct synthesis of methylchlorosi undergo cleavage reactions, with methyltrichlorosilane, lanes, in which low-boiling components having a high with the aim of producing dimethyldichlorosilane and methyl content as well as non-cleavable high-boiling disilanes which have a high chlorine content and un residues together with methyltrichlorosilane are con 10 dergo cleavage reactions. Since a declared aim of this verted to, in particular, dimethyldichlorosilane, which process is to convert disilanes which cannot undergo is a valuable compound. cleavage reactions to cleavable disilanes, the reaction is In the preparation of dimethyldichlorosilane by di restricted to temperatures up to about 175 C. and pres rect synthesis (Miller-Rochow method), a substantial sures up to 3 atmospheres, in order to avoid side reac amount of unavoidable products are formed in addition 15 tions. Possible side reactions are in principle familiar to to the desired product. The direct method is described one skilled in the art. Free-radical reactions of polysi in the patent literature, for example in U.S. Patent Spec lanes are described in, for example, German Ausleges ifications Nos. 2,380,995 and 2,488,487. chrift (German Published Specification) No. 2,618,246, The most important by-product formed, in terms of and the possible participation of Lewis acids, such as, amount, is methyltrichlorosilane, which is formed in 20 for example, aluminum chloride, in such free-radical amounts which cannot be utilized economically for the reactions is described in, for example, German Offen preparation of silicones. The bulk of the methyltri legungsschrift (German Published Specification) No. chlorosilane is currently used for the preparation of 3,136,786. Under the reaction conditions which are pyrogenic silica; however, this process constitutes an described in DOS (German Published Specification) unsatisfactory provisional solution, since valuable 25 No. 3,314,734 as being advantageous, the stated side methyl groups introduced into the system undergo reactions do not take place. However, the consequence combustion to form CO2 and water. of choosing the reaction conditions 175 C/3 atmo In addition to methyltrichlorosilane, other by-pro spheres is that substantial amounts of aluminum chlo ducts are formed in the direct synthesis of dimethyldi ride are required as catalyst. In some of the examples chlorosilane: 30 published, the amount of catalyst is about 20%, relative (a) Unavoidable low-boiling products having a boil to the silane mixture employed, without taking into ing point below 40 C., such as, for example, tetrame account the hydrogeniosilane added as a promoter. thylsilane (TMS), dimethylmonochlorosilane and 2 The catalyst can be worked up only with consider methylbut-2-ene. able effort, and this procedure is therefore uneconomi (b) Compounds which have a higher boiling point 35 cal. Furthermore, a continuous procedure with homo than the monosilanes, that is to say a boiling point from geneous catalysis is scarcely possible for such a large above 70 C. to about 180° C.; these compounds are amount of catalyst, owing to the low solubility of the referred to below either as "residue" or as "high-boiling aluminum chloride in the reaction mixture. Hence, the fraction'. This residue is a complex mixture of com process described in DOS (German Published Specifi pounds which contain SiSi, SiOSi and SiCH2Sibonds in cation) No; 3,314,734 is hardly suitable as a method of the molecules. Typical residues are described in U.S. preparing dimethyldichlorosilane in a continuously Patent Specifications Nos. 2,599,435 and 2,681,355. working reactor. A large number of methods have been suggested for It was therefore the object of the present invention to working up these unavoidable products (see, for exam provide a method for reducing the amounts of waste ple, German Offenlegungsschrift (German Published 45 products in the direct synthesis of methylchlorosilane, Specification) No. 2,950,402; R. Calas et al., Journal of these waste products hitherto essentially being de Organometallic Chemistry, 225, 117 (1982)). stroyed. In certain circumstances, the compounds trimethyl It was furthermore the object of the present invention monochlorosilane and methyldichlorsilane, which are to find a process for the preparation of dimethyldichlo normally included among the useful products in the 50 rosilane from unavoidable products of the direct syn direct method, are also formed in amounts which can thesis of methylchlorosilane. not be utilized economically. It was furthermore the object of the present invention According to U.S. Patent Specification No. reasonably to utilize the methyl groups which are pres 2,786,861, alkylchlorosilanes were converted in the ent in bonded form in the low-boiling residues, such as presence of a catalyst, such as aluminum chloride, and it 55 tetramethylsilane (TMS) and dimethylchlorosilane, and was found that this conversion could be carried out in in the high-boiling non-cleavable residue, and have an advantageous manner at relatively low temperatures hitherto been destroyed. if a compound which promotes this conversion, that is A further object of the present invention is to convert to say a hydrogenosilane, was used. A serious disadvan the compound methyldichlorosilane, if required, to tage of this method is that extremely large amounts of 60 valuable compounds. aluminum chloride are required as the catalyst, at least Finally, it is intended that the process for the prepara 10%, relative to the silane mixture to be converted. tion of dimethyldichlorosilane from unavoidable prod The transfer of methyl groups from tetramethylsilane ucts of the direct synthesis of methylchlorosilane be to hexachlorodisilane or 1,1,2,2-tetrachloro-1,2-dime designed in such a way that it can be carried out by a thyldisilane or mixtures of disilanes having a high chlo 65 continuous procedure. rine content is described in, for example, German Offen The present invention therefore relates to a process legungsschrift (German Published Specification) No. for the preparation of dimethyldichlorosilane from the 3,208,829. The catalyst used in this case is an organoalu unavoidable low-boiling and high-boiling products of 4,552,973 3 4. the direct synthesis of methylchlorosilane, character elevated temperature, in order to produce dimethyldi ized in that methyltrichlorosilane, together with the chlorosilane. After the monomeric methylsilanes and low-boiling fractions having a high content of methyl methylchlorosilanes which have boiling points up to groups and the high-boiling non-cleavable fractions, is about 70° C. have been removed from the reaction converted at temperatures between 250 C. and 400 C. and under pressures up to 100 bar, in the presence of a product, a residue, a high-boiling fraction, remains. It catalyst. has been found that valuable fractions have remained in By means of the process according to the invention, this residue, and various methods have been used in "waste products', as obtained in the Rochow synthesis, order to utilise the silane present in this residue. are converted in high yield to the valuable compound 10 U.S. Patent Specifications Nos. 2,709,176 and dimethyldichlorosilane by reaction with one another in 2,842,580 describe processes for the cleavage of the the presence of a catalyst. polysilanes present in the residue. While the polysilanes Any fraction obtained from the Rochow-Muller