Abstract:
High boiling residue from the direct synthesis of alkylchlorosilanes are converted in large part to monosilanes by heating the residue by passage of alternating current in a pressurized reactor.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is the U.S. national phase of PCT Appln. No. PCT/EP2009/051771 filed Feb. 16, 2009 which claims priority to German application DE 10 2008 000 410.3 filed Feb. 26, 2008. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a process for producing alkylchlorosilanes from the residues from the direct synthesis of alkylchlorosilanes via heating by alternating current. 
     2. Description of the Related Art 
     When metallic silicon and alkyl chlorides RCl, where R is an alkyl moiety, are used in the direct synthesis of alkylchlorosilanes of the general formula R a H b SiCl 4-a-b  in which a is 1, 2, 3, or 4 and b is 0, 1, or 2, the by products comprise oligosilanes, carbosilanes, siloxanes, and high-boiling-point cracking products. The residues also comprise solids derived from the direct synthesis process which are very fine and are not retained by cyclones and filters. These solids are composed of silicon, metals, and compounds of these, metal silicides, and carbon black. 
     Oligosilanes make up the main part of the residues, and in particular the disilanes of the general formula R c Cl 6-c Si 2 , in which c is from 0 to 6. However, silane compounds having more than 2 Si—Si bonds can also be present, examples being trisilanes or silamethylenes. In all these formulae, silicon is tetravalent. 
     U.S. Pat. No. 2,598,435 describes a process in which methylchloromonosilanes are obtained from residues from the direct synthesis of methylchlorosilanes which comprise methylhalopolysilanes, at temperatures of from 250 to 800° C. 
     A wide variety of specifications, e.g. EP 1505070 and publications cited therein, describe processes in which residues from the direct synthesis of alkylchlorosilanes can be cleaved to give alkylchloromonosilanes, using additions of HCl and/or H 2  and sometimes using complex catalyst systems. U.S. Pat. No. 2,681,355 describes a continuous process in which residues from the direct synthesis of methylchlorosilanes are reacted in an externally heated tube with HCl at temperatures of from 400 to 900° C. to form monomeric silanes, without catalysis and by a purely thermal route. Large amounts of by products are formed here, examples being carbosilanes and polymers, and also solids, because of carbonization of the residues on hot reactor walls. 
     DE 19711693 describes a continuous process in which alkylchlorosilanes are produced from the residues from the direct synthesis of alkylchlorosilanes, which comprise liquid constituents with a boiling point &gt;70° C. and solids, using HCl at from 300 to 800° C. in a tubular reactor with rotatable internals. The rotatable internals are needed for scraping to remove caked material produced by carbonization on the reactor walls. 
     SUMMARY OF THE INVENTION 
     It was an object to provide a process which can produce alkylchlorosilanes from the residues from the direct synthesis of alkylchlorosilanes and which provides a high yield of alkylchlorosilanes purely via thermal treatment and without further additives, e.g. hydrogen chloride, hydrogen, or catalysts, and which at the same time inhibits carbonization of the residues and formation of deposits on hot reactor walls. These and other objects are achieved by heating the residue in a pressurized reactor by passage of an alternating electric current. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention thus provides a process for producing alkylchlorosilanes from the residues from the direct synthesis of alkylchlorosilanes, which comprise liquid constituents with a boiling point of at least 70° C. at 1013 hPa and further constituents, including metals and compounds thereof, in which the reaction mixture is heated in a pressurized reactor to a reaction temperature of from 200 to 500° C. via passage of alternating current. 
     The process uses residues from the direct synthesis of alkylchlorosilanes to provide useful monosilanes. It is preferable here that relatively high-molecular-weight constituents of the residues are cleaved to give monosilanes. The process preferably requires no use of catalysts and no use of further starting materials. Deposits of solids on the reactor walls are very substantially avoided. 
     Methylchlorosilanes of the general formula Me a H b SiCl 4-a-b  are preferably produced, in which Me is a methyl moiety, a is 1, 2, or 3, and b is 0 or 1. 
     The residues from the direct synthesis consist in essence of volatile constituents with a boiling point of at least 70° C., in particular at least 100° C., at 1013 hPa, and metals or compounds of these, in dissolved or finely suspended form. In particular, the residues comprise metals from the group of Al, Cu, Zn, Sn, Fe, Ti, and/or compounds of these. The residues can also comprise further solids, e.g. silicon and carbon black. The residue mostly comprises disilanes. 
     The residues are introduced batchwise or continuously into a pressurized reactor and heated to the temperature required for reaction via passage of alternating current. 
     The reactor is preferably operated at at least 250° C. and with preference at at most 350° C. 
     The pressure is preferably at least 20 bar, more preferably at least 30 bar, and preferably at most 100 bar, in particular at most 80 bar. 
     The reaction product is preferably condensed, and if appropriate freed from solids, and can be returned to the alkylchlorosilane mixture produced in the direct synthesis process, or else can be separately isolated to give pure substances. 
     EXAMPLE 
     Unless otherwise stated, all of the amounts and percentages stated are based on weight. Me means a methyl moiety. 
     A high-boiling-point residue from the direct synthesis of methylchlorosilanes was used and according to GC was composed of:
     54% of disilanes (mixture of 1,1,1,2,2,2-hexamethyl-disilane, 1-chloropentamethyldisilane, 1,1-dichloro-tetramethyldisilane, 1,2-dichlorotetramethyldisilane, 1,1,2-trichlorotrimethyldisilane, and 1,1,2,2-tetra-chlorodimethyldisilane),   9% of trisilanes of the general formula   Cl a Me 3-a Si(Cl b Me 2-b )SiSiCl c Me 3-c , where each of a and c is from 0 to 3, and b is from 0 to 2,   7% of silamethylenes (carbosilanes) of the general formula Cl a Me 3-a SiCH 2 SiMe 3-b Cl b , where each of a and b is from 0 to 3,   2% of metals and metal compounds (in particular Al, Zn, Sn, Fe, Ti),   2% of alkylchlorosilanes, and also   26% of unidentified compounds, each at low concentration.   

     The high-boiling-point residue was fed in liquid form into an autoclave of volume 0.92 1. The reaction mixture was heated to 270° C. via passage of alternating current and kept at this temperature for 180 min. 
     The autoclave was cooled to ambient temperature. The product mixture was removed and analyzed by GC:
     51.7% of methylchlorosilanes, of which   40.4% of dichlorodimethylsilane,   7.2% of trichloromethylsilane,   2.5% of chlorotrimethylsilane,   1.4% of dichloromethylsilane,   0.5% of uncleaved disilanes,   6.8% of silamethylenes,   41% of unidentified compounds.   

     There was only very little solid deposited on the wall of the reactor and on the base of the reactor. The solid was in the form of fine particulates and could easily be removed mechanically.