Organic compounds -- part of the class 532-570 series – Organic compounds – Silicon containing
Reexamination Certificate
2001-07-25
2002-02-05
Shaver, Paul F. (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Silicon containing
C556S467000, C556S469000
Reexamination Certificate
active
06344578
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a process for preparing alkylchlorosilanes from the liquid constituents of the residues from the direct synthesis of alkylchlorosilanes.
2. Background Art
In the direct synthesis of alkylchlorosilanes of the general formula R
a
H
b
SiCl
4−a−b
where a is 1-4 and b is 0, 1 or 2 from silicon metal and alkyl chlorides R—Cl, where R is methyl is particularly preferred, oligosilanes, carbosilanes, siloxanes and high-boiling decomposition products are formed as by-products. Furthermore, very fine solids from the direct synthesis which are not retained by cyclones and filters are also present in the distillation residue. The solids comprise silicon, metal chlorides, e.g. AlCl
3
, metal silicides and carbon.
The predominant part of these residues constitute oligosilanes, in particular the disilanes R
c
Cl
6−c
Si
2
, where c is 0-6. For this reason, processes which convert the disilanes into monosilanes have been developed. Conversion may be achieved, for example, by amine-catalyzed cleavage with hydrogen chloride. However, only disilanes having fewer than 4 methyl groups can be cleaved by this process. Furthermore, the disilanes must be separated beforehand from the solid residues, since these residues, for example aluminum chloride, act as catalyst poisons.
In order to also recycle uncleavable disilanes, processes in which uncleavable disilanes are converted into cleavable disilanes and then cleaved, e.g. as described in U.S. Pat. No. 4,393,229, or in which these disilanes are cleaved directly by means of HCl over specific catalysts, usually comprising noble metals, e.g. as described in U.S. Pat. No. 5,502,230, have been developed. A disadvantage of metal-catalyzed reactions is always the tendency for the catalysts to be poisoned by impurities from the residue.
U.S. Pat. No. 2,681,355 describes a process in which relatively high-boiling residues from the direct synthesis of methylchlorosilanes are converted into monomeric silanes purely thermally, in the absence of catalysts, by employing hydrogen chloride at temperatures of 400-900° C. in an autoclave or in a tubular reactor. The patent emphasizes the low tendency to form carbon deposits as a great advantage when using a tubular reactor, as a result of which the process is said to be operable for a prolonged period. However, this does not apply when treating residues which, because of their solids content, quickly lead to blockage of reaction tubes.
U.S. Pat. No. 5,877,337 describes a process which allows even solids-containing residues from the direct synthesis of organochlorosilanes to be worked up at low pressures and yet allows the organosilicon components to be converted into useful silanes. This objective is achieved by thermal cleavage of the direct synthesis residues with hydrogen chloride at temperatures of 300-800° C. in a tubular reactor having rotating internals. The rotary motion of the internals strips off caked deposits on the reactor walls caused by carbonization or by deposition of solid components, thus preventing blockage of the reactor. However, this process is technically difficult to implement and is therefore correspondingly costly.
U.S. Pat. No. 5,592,912 describes a process for the cleavage of direct synthesis high-boiling residue with HCl in the presence of catalysts such as activated carbon, or supported aluminum chloride, platinum, or palladium compounds. Supports described are aluminum oxide, silica, zeolites and active carbon. The process can be carried out as a fixed-bed process or in a fluidized bed.
The preparation of trichlorosilane and tetrachlorosilane mixtures from silicon and hydrogen chloride in a fluidized bed has been described, for example, in U.S. Pat. No. 4,130,632.
The processes mentioned above suffer from the disadvantage of a high outlay in terms of apparatus required to solve the carbonization problem or the use of specific catalysts, sometimes based on noble metals, which make the process more expensive.
SUMMARY OF THE INVENTION
It is an object of the invention to provide an improved process which allows work up of solids-containing residues from the direct synthesis of alkylchlorosilanes, and which allows the organosilicon components to be converted into usable silanes.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
The invention provides a process for preparing alkylchlorosilanes from the liquid constituents of the residues from the direct synthesis of alkylchlorosilanes which have a boiling point above 70° C. at 1013 hPa and comprise disilanes, in which the residues are heated with hydrogen chloride and silicon at temperatures of at least 300° C., wherein at least 10% by weight of trichlorosilane and/or tetrachlorosilane, based on the weight of the alkylchlorosilanes formed, are formed concurrently. The process is, in particular, simple to carry out, and allows very long running times.
Hydrogen chloride and Si metal react in an exothermic reaction to produce a mixture of trichlorosilane and tetrachlorosilane. The equilibrium composition of the mixture is dependent on the reaction temperature. The reaction enthalpy is utilized to heat the liquid constituents of the residues from the direct synthesis of alkylchlorosilanes fed into the reactor to reaction temperatures at which the cleavage by means of hydrogen chloride to form monosilanes then takes place. In the process, it is preferred that at least 20% by weight, in particular at least 30% by weight, of trichlorosilane and/or tetrachlorosilane, based on the weight of the alkylchlorosilanes formed, are formed at the same time. The product obtained is a mixture of trichlorosilane and tetrachlorosilane together with varying proportions of organochlorosilanes, depending on the composition of the liquid constituents of the residues. The silicon is preferably present as a fixed bed or, in particular, in a fluidized bed in the process.
The amount of HCl can be varied within a wide range and, in the case of fluidized-bed reactors, the lower limit is determined essentially by the minimum amount required for maintaining the fluidized bed, while the upper limit is determined essentially by the need to avoid excessive discharge of dust from the fluidized bed. Both are highly dependent on the particle size and the geometry of the reactor. In fixed-bed reactors, the amount of HCl is limited by plant engineering considerations and economic constraints, e.g. by the loss of HCl. The amount of hydrogen chloride used is at least the molar equivalent amount of the disilanes present in the residue and the trichlorosilanes and/or tetrachlorosilanes to be formed. The residue and the hydrogen chloride may both be preheated, or may be metered at ambient temperature into the reactor, with the streams preferably being metered in continuously. When heating is utilized, the residue can also be metered in as a gas/liquid mixture. Preference is given to using a fluidized-bed reactor. Preference is further given to using customary plants for the preparation of trichlorosilane and tetrachlorosilane.
The alkylchlorosilanes prepared by cleavage of disilanes from the liquid constituents of the residues from the direct synthesis of alkylchlorosilanes preferably have the above general formula where R is a methyl, ethyl, butyl or propyl radical, in particular a methyl radical. Most preferably, methyltrichlorosilane, dimethylchlorosilane, trimethylchlorosilane, methyldichlorosilane, or dimethylchlorosilane are prepared.
The target product of the direct synthesis of methylchlorosilanes is generally dimethyldichlorosilane which has a boiling point of 70° C. and usually makes up more than 80% of the crude mixture and is purified by distillation. The residues from the direct synthesis having a boiling point above 70° C., preferably at least 80° C., in particular at least 100° C., comprise, inter alia, monosilanes, e.g. ethyldichlorosilane, ethylmethyldichlorosilane, ethyltrichlorosilane or i-propyltrichlorosilane. The proportions can fluctuate greatly and are usually less t
Köhler Bernd
Mautner Konrad
Tamme Gudrun
Brooks & Kushman P.C.
Shaver Paul F.
Wacker-Chemie GmbH
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