The present invention relates to a process for making redistribution products in residues containing polysilanes, and more particularly, to a process for converting alkyl-rich polysilanes in residues obtained from the manufacture of alkylhalosilanes, into more valuable products. Typically, when the halogen is chlorine, the useful or more valuable products are such monosilanes as dimethyldichlorosilane, trimethylchlorosilane, methyltrichlorosilane and the like.
In the preparation of alkylhalosilanes, various polysilane products are formed during the reaction and remain in the residue after the separation of the monosilanes. For example, in the commercial method known as the "direct process", in addition to the monosilanes, which in the case of the chloromonosilanes include dimethyldichlorosilane, methyltrichlorosilane, trimethylchlorosilane and the like, there is always obtained a variety of compounds which boil above the monosilanes, that is, above about 70.degree. C., which is hereafter referred to as "residue", high boiling fraction or crude fraction. The "direct process" is well described in the patent literature, for example, in U.S. Pat. Nos. 2,380,995 and 2,488,487. The residue is a complex mixture of compounds that include SiSi and SiOSi linkages in the molecules. Typical residues are described in U.S. Pat. Nos. 2,598,435 and 2,681,355.
In current commercial operations, the methylchlorosilane residues, that is, the residue boiling above the monosilanes, and which is formed, for example, during the reaction of methyl chloride and silicon, is cleaved to yield monosilanes plus a fraction that is non-cleavable. The cleavage of the organohalogenopolysilanes is described in U.S. Pat. No. 2,709,176 and U.S. Pat. No. 2,842,580, incorporated herein by reference. U.S. Pat. No. 2,709,176 describes the cleavage of the organohalogenopolysilanes by treating them with a hydrogen halide in the presence of an organic amine compound selected from the group consisting of heterocyclic tertiary organic amines, tertiary organic amines having the formula R.sub.3 N, where R is a member selected from the group consisting of aryl and alkyl groups, and salts of the tertiary amines of (1) and (2). U.S. Pat. No. 2,709,176 also describes the composition of a typical residue mixture containing the high boiling fraction, that is, the material boiling above the boiling point of dimethyldichlorosilane.
U.S. Pat. No. 2,842,580 also describes the cleavage of organohalogenopolysilanes by treating the organohalogenopolysilanes with a specified class of quaternary halides, such as quaternary ammonium halides and quaternary phosphonium halides. The fraction remaining after cleavage is generally disposed of by incineration. For both economic considerations and disposal considerations, it is desirable to eliminate or substantially reduce the amount of residue disposed of by incineration by converting the non-cleavable fraction of the residue to substituents which are cleavable.
In U.S. Pat. No. 2,598,435, methylhalogenopolysilanes containing silicon-silicon linkages and a silicon-bonded methyl group, were heated at a temperature of at least 250.degree. C., and generally at a temperature from 250.degree. C. to 800.degree. C. to form methylhalogenomonosilanes. In U.S. Pat. No. 2,598,435, it was found that the organohalogenopolysilanes could be degraded to lower molecular weight materials and in no way could have been predicted since it was found that attachment of organic groups to a silicon of a disilane, for instance, hexamethyldisilane, rendered such a compound thermally stable at temperatures as high as 500.degree. C. under pressure, and it was found that this stability decreased markedly if there were both a silicon-bonded hydrocarbon radical and a silicon-bonded halogen atom in the disilane. Thus, the cracking process described in U.S. Pat. No. 2,598,435 applied to halogen-rich polysilanes, for example, chlorine-rich disilanes, but it did not apply to the alkyl-rich polysilanes, for example, methyl-rich disilanes.
The redistribution of organosilanes is disclosed in U.S. Pat. No. 2,647,136 where both alkyl groups and halogen atoms are shifted from one silicon atom to another silicon atom as a result of effecting reaction at a temperature of about 250.degree. C. to 400.degree. C. between a first alkylhalogenosilane and a second alkylhalogenosilane. In U.S. Pat. No. 2,647,136, it was found that there is not only the migration of an alkyl group, but also the migration of a halogen atom, and the reaction proceeds without a catalyst at elevated temperatures. Aluminum chloride was the only catalyst found which would accelerate the reaction. In U.S. Pat. No. 3,793,357 redistribution of alkylhalosilanes wherein alkyl groups and halogen atoms are redistributed from one silicon atom to another silicon atom, is effected by using aluminum chloride catalyst and a catalytic amount of a catalytic promoter having at least one aromatic radical in the silane molecule. Compounds included within the scope of U.S. Pat. No. 3,793,357 include alkylhydrogensilanes, such as methyldichlorosilane, trichlorosilane, dimethylchlorosilane, and the like, in which redistribution can occur. In U.S. Pat. No. 3,135,778, it is disclosed that redistribution had been effected by using aluminum chloride as a catalyst in the presence of a silane containing silicon-bonded hydrogen atoms (hydrosilane) for the redistribution of organosilanes (monosilanes) to effect a redistribution reaction among the end products in order to increase the yields of the desired organosilanes and organohalogenosilanes. In U.S. Pat. No. 2,786,861, alkylchlorosilanes were redistributed in the presence of a catalyst, such as the redistribution catalyst, aluminum chloride, and it was found that the reaction could be carried out at lower temperatures in an advantageous manner over the prior art by the use of a catalytic promoter which catalytic promoter comprised a hydrogen silane. However, it is noted in U.S. Pat. No. 2,786,861 that the redistributions were used for making methyl-rich monosilanes rather than chlorine-rich disilanes. Thus, there is no prior disclosure of improving the amount of useful or more valuable product obtainable from the residues and crude fractions obtained from the manufacture of alkylhalosilanes or of treating alkyl-rich disilane- or polysilane-containing residues to produce redistribution products containing halogen-rich disilanes or polysilanes in said residues. Furthermore, there is no suggestion of simultaneously converting alkyl trihalosilanes to dialkyldihalosilanes while converting the alkyl-rich disilane or polysilane fractions of residues to the halogen-rich disilanes or polysilanes.