Organic compounds -- part of the class 532-570 series – Organic compounds – Silicon containing
Reexamination Certificate
1999-11-30
2001-01-16
Shaver, Paul F. (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Silicon containing
Reexamination Certificate
active
06175029
ABSTRACT:
The subject of the present invention is an improved process for obtaining organosilanes and it relates in particular to an improved process for obtaining organosilanes in which a so-called redistribution reaction is involved. More particularly, the present invention relates to an improved process for obtaining organosilanes, involving a redistribution reaction between a chlorinated organohydrosilane and an organo-substituted and optionally chlorinated silane to give a product comprising a redistributed chlorinated organohydrosilane which is extracted from the reaction medium by distillation.
Without limitation, the present invention is aimed quite especially at a redistribution reaction between an alkylhydrodichlorosilane and a trialkylchlorosilane to give a product comprising a redistributed dialkylhydrochlorosilane. This redistributed dialkylhydrochlorosilane is a synthesis reagent which is highly prized in a great many and varied applications, examples being the preparation of organosilicon monomers or more condensed base compounds.
The dialkylhydrochlorosilane is one of the by-products of the synthesis of alkylchlorosilanes in accordance with a conventional and well-known procedure which consists in reacting alkyl chloride with silicon in the presence of a copper catalyst to form alkylchlorosilanes. In this synthesis, the dialkyldichlorosilane is the main product. In addition to the dialkylhydrochlorosilane by-product specified above, compounds of the trialkylchlorosilane, alkyltrichlorosilane and alkylhydrodichlorosilane type are also obtained.
In view of the industrial interest of these products in the chemistry of silicones and especially of dialkylhydrochlorosilanes such as dimethylhydrochlorosilane, numerous proposals have seen the light of day for methods of obtaining these by-products. One of the few proposals which has proved itself in this respect is that which consists in carrying out a redistribution reaction between, for example, an alkylhydrodichlorosilane and a trialkylchlorosilane or between an alkylhydrodichlorosilane and a tetraalkylsilane. This redistribution leads to the specified dialkylhydrochlorosilanes which are extracted from the reaction medium by distillation.
In this context, numerous redistribution reactions of organosilanes, which cut and redistribute the silicon-alkyl, silicon-chlorine and/or silicon-hydrogen bonds, in the presence of various catalysts such as Lewis acids, are known. French Patent FR-A-2 119 477 is a good illustration of this technique for preparing dialkylhydrochlorosilanes by redistribution/distillation. In accordance with the teaching of this patent, methylhydrodichlorosilane and trimethylchlorosilane are reacted in a molar ratio of methylhydrodichlorosilane to trimethylchlorosilane of the order of 0.5 and in the presence of a catalyst consisting of AlCl
3
. The reaction mixture is placed in a reactor under an autogenous pressure of the order of 3 to 5×10
5
Pa and is held for several hours at a temperature of the order of 85 to 170° C. The Applicant has repeated this prior art method and has observed that it involves a reaction in homogeneous catalysis in which the reacting substances and the catalyst form a single liquid phase. After the redistribution reaction has been implemented, the redistributed dimethylhydrochlorosilane is separated from the reaction mixture by distillation and, at the end of the distillation step, a distillation residue comprising the catalyst is left. The said residue is generally in the form of a suspension, since a greater or lesser proportion of the catalyst (which will depend on the amount employed at the start) is in the solid state in the residue. At the end of the process, owing to the dual state of the catalyst (solid state and dissolved state), the separation of AlCl
3
is made difficult, which considerably complicates the implementation of the process, and it is therefore quicker to destroy the catalyst by conducting an acidic or basic hydrolysis of the distillation residue. It must, however, be borne in mind that results of this kind are not satisfactory from the standpoint of industrial profitability: the aluminium chloride cannot be recycled, owing to its hydrolysis, and, furthermore, it presents the problem of aqueous effluents which can be awkward.
In the light of this knowledge, one of the essential objectives of the present invention consists in the development of a novel process for obtaining organosilanes which involves a heterogeneously catalysed redistribution reaction between a chlorinated organohydrosilane and an organo-substituted and optionally chlorinated silane. This novel process must employ a catalyst which remains in the solid state in the presence of the reacting substances and must lead, at the end of the reaction, to a reaction medium comprising a solid phase, the catalyst, which can easily be recycled in its entirety into a new operation, and a liquid phase comprising a redistributed chlorinated organohydrosilane which is recovered in conventional manner by distillation (for example).
Another essential objective of the invention is to provide a process of the type specified above which is particularly simple to implement and economic.
These various objectives are achieved by the implementation of the process according to the invention, which involves the use of an effective amount of an alumina-based catalyst.
The document EP-A-0 743 315 proposed a process for redistributing a mixture of methylsilanes by heterogeneous catalysis over alumina for the purpose of increasing the dimethylhydrochlorosilane and/or trimethylchlorosilane concentration of the said mixture. The starting materials referred to in this prior art relate only to mixtures of methylchlorosilanes (approximately 5 in number) and tetramethylsilane having low boiling points (methylsilanes boiling between 35 and 70° C. under atmospheric pressure are involved) as are separated by distillation of the crude mixture obtained from the direct Rochow synthesis, which consists in reacting methyl chloride over a catalyst mass composed of silicon and a catalyst. This prior art says absolutely nothing about the possibility of employing heterogeneous catalysis over alumina to aid a specific redistribution reaction, as implemented in the process according to the invention, in which two pure organosilanes, separated beforehand from their preparation medium and consisting of a chlorinated organohydrosilane and an organo-substituted and optionally chlorinated silane, are brought into contact.
The merit of the Applicant was not limited to this demonstration of the possibility of employing heterogeneous catalysis over alumina in order to promote a specific redistribution reaction between two organosilanes taken in the pure state. In fact, entirely surprisingly, the Applicant also found that the alumina must meet well-defined characteristics in order to perform correctly its role of heterogeneous catalyst in a specific redistribution reaction.
The present invention therefore provides a process for obtaining organosilanes, comprising a redistribution reaction between a chlorinated organohydrosilane of formula (1) (R)
a
(H)
b
SiCl
4-a-b
and an organo-substituted and optionally chlorinated silane of formula (2) (R′)
c
SiCl
4-c
, in which formulae a 1 or 2, b=1 or 2, a+b≦3, c=1, 2, 3 or 4, the symbols R and R′ are identical or different and each represent a linear or branched C
1
-C
6
alkyl radical or a C
6
-C
12
aryl radical, the said redistribution reaction proceeding in the presence of an effective amount of a catalyst based on a metal derivative, and the said process being characterized in that the catalyst remains in the solid state in the presence of the reacting silanes (1) and (2) and consists of an alumina which has an alkali metal M or alkaline earth metal M′ content, expressed in ppm of oxide M
2
O or M′O relative to the weight of catalyst (alumina comprising in particular M or M′), of less than or equal to 500 ppm.
In accordance with one preferred embodi
Burns Doane Swecker & Mathis L.L.P.
Rhodia Chimie
Shaver Paul F.
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