Organic compounds -- part of the class 532-570 series – Organic compounds – Silicon containing
Reexamination Certificate
2000-03-13
2001-01-16
Shaver, Paul F. (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Silicon containing
Reexamination Certificate
active
06175030
ABSTRACT:
TECHNICAL FIELD
The invention relates to a process for the direct synthesis of methylchlorosilanes using a catalyst composition comprising pyrogenic silicic acid.
BACKGROUND ART
Processes for the preparation of methylchlorosilanes by reacting silicon with chloromethane in the direct Muller-Rochow synthesis in the presence of suitable catalysts and catalyst combinations have already been disclosed. For example, U.S. Reissue Pat. No. 33,452 describes a direct synthetic method using a catalyst combination of copper or copper compounds with zinc and tin co-catalysts. The ratio of copper, zinc and tin catalysts to one another has a considerable effect on the process, in particular on productivity and selectivity, while the form in which the catalysts are introduced into the catalyst composition, for example as metal, alloys or compounds, is of secondary importance.
U.S. Pat. No. 4,554,370 describes a process in which silicon is reacted with chloromethane in the presence of copper(I) chloride as catalyst, and in the presence of pyrogenic silicic acid as an anti-agglomerant, to give methylchlorosilanes. The presence of pyrogenic silicic acid is intended to prevent the formation of agglomerates in the “contact mass”, composed of silicon and catalysts.
DISCLOSURE OF INVENTION
The principle aim of the direct synthesis is to prepare dimethyldichlorosilane, required for the preparation of linear polysiloxanes, as inexpensively as possible, in the most environmentally friendly manner possible, in high yields, and with a high space/performance ratio. A process which requires lesser amounts of catalysts for comparable selectivity and reactivity is both less expensive and also more environmentally friendly, since the spent catalysts must either be reprocessed or converted into a form suitable for disposal to landfill. The object of the present invention is to provide a process for the direct synthesis of methylchlorosilanes by the M{umlaut over (u)}ller-Rochow method in which, while retaining reactivity and selectivity, the concentration of copper catalyst or promoter in the catalyst composition can be reduced. Thus the invention relates to a process for the direct synthesis of methylchlorosilanes wherein chloromethane is reacted with a catalyst composition comprising silicon, copper catalyst, zinc promoter and pyrogenic silicic acid. The addition of pyrogenic silicic acid to the catalyst composition allows the concentration of copper catalyst and/or zinc promoter to be reduced without having an adverse effect on reactivity and/or selectivity. Moreover, the pyrogenic silicic acid used in the process is enviroinentally acceptable without further treatment since it consists exclusively of SiO
2
.
BEST MODE FOR CARRYING OUT THE INVENTION
The organochlorosilane process of the subject invention can be carried out batchwise or continuously. In industrial production, only the continuous version is used. “Continuous” means that the amounts of silicon reacted and the amount of catalysts and promoters discharged with the reaction dust are continuously replenished, preferably as a pre-mixed catalyst composition. The continuous direct synthesis is preferably carried out in fluidized-bed reactors, in which chloromethane is simultaneously used as both fluidizing medium and reactant.
The silicon required is ground to a powder in advance and mixed with copper catalyst and promoters to give the catalyst composition. Silicon is preferably employed in a maximum particle size of 700 &mgr;m, particularly preferably in a maximum particle size of 500 &mgr;m and a minimum particle size of 20 &mgr;m. The silicon employed usually has a purity of >98%.
A production campaign for the continuous direct synthesis is started with the induction phase. To commence the induction phase, methyl chloride is passed into the heated catalyst composition. The induction phase is followed by the initiation phase, in which the crude silane formation commences. The reaction initially proceeds with low selectivity and reactivity. The stable production phase is subsequently reached. Silicon and, where necessary, catalysts and promoters/co-catalysts are continuously replenished. The production campaign ends when chloromethane is no longer passed into the catalyst composition.
In the case of continuous operation of a reactor, the production rates, based on the target product dimethyldichlorosilane, drop in a production campaign after a substantially stable production phase. For this reason, the production campaign must be terminated after a certain time. A production campaign usually lasts from only a few days to several weeks. After termination of a production campaign, the reactor is emptied, refilled with silicon, copper catalyst and promoters/co-catalysts, and restored to the reaction conditions.
In the direct synthesis, unreacted chloromethane, gaseous methylchlorosilanes and any entrained particles exit the reactor. The entrained particles consist of reacted silicon particles, fine silicon particles, catalysts, and promoters/co-catalysts. If desired, the entrained particles can be separated from the gas stream via one or more cyclones, it being possible for large entrained particles of the catalyst composition to be fed back into the reactor. The silane is subsequently separated from residual dust components and unreacted chloromethane and fed to distillation. Purified, unreacted chloromethane can be recirculated into the reactor.
The process is preferably carried out in a fluidized-bed reactor, preferably at a temperature in the range from 250 to 400° C., in particular at from 250 to 360° C. The process is usually carried out at the pressure of the ambient atmosphere, i.e. at from about 0.1 MPa to 0.5 MPa, since this requires the least complication although higher pressures can also be used.
The process can also be carried out using inert gases, such as, for example, nitrogen or argon. Preferably, no inert gas is used.
In a preferred embodiment, the amount and velocity of the gas stream is selected such that a fluidized bed of catalyst composition and gas is formed in the reactor. The mixture of silicon, catalysts, promoters and pyrogenic silicic acid is referred to as the “catalyst composition”. Unreacted chloromethane and inert gas, if used, and gaseous methyIchlorosilanes leave the reactor. If desired, the entrained particles can be separated from the gas stream via one or more cyclones, large entrained particles of the catalyst composition being fed back into the reactor. The catalyst composition is prepared by simple mixing of the individual components at room temperature. Treatment of the catalyst composition before introduction into the reactor is possible, but is not carried out in the preferred embodiment.
In a particularly preferred embodiment, the pyrogenic silicic acid is firstly mixed with the copper catalyst and/or the zinc promoter, since this increases the flowability of the catalyst system and, depending on the silicic acid employed, reduces the hygroscopic action of various catalyst components and thus considerably improves the handling properties.
In the process according to the invention, the form of the copper (a) is preferably selected from metallic copper, copper alloys, copper oxide and copper chloride. Copper oxide can be, for example, copper in the form of copper oxide mixtures and in the form of copper(II) oxide. Copper chloride can be in the form of CuCl or in the form of CuCl
2
, with corresponding mixtures also being possible. In a preferred embodiment, the copper is employed as copper oxide and/or as CuCl. It is preferred to use from 0.3 to 10% by weight, more preferably from 0.5 to 7% by weight, and most preferably from 0.5 to 4.5% by weight of copper catalyst, based on metallic copper and silicon.
In the process according to the invention, zinc (b) is preferably employed in the form of metallic zinc, or alternatively as an alloy with copper and, if desired, further promoters, or in the form of zinc oxide or zinc chloride. The amount of zinc employed is preferably from 0.3 to 60% by weight, more pre
Goetze Ulrich
Kalchauer Wilfried
Straussberger Herbert
Streckel Willibald
Brooks & Kushman P.C.
Shaver Paul F.
Wacker-Chemie GmbH
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