Method for preparing organohalosilanes

Abstract: The present invention provides a method for increasing the yield of methyldichlorosilane obtained from the reaction of silicon metal with methyl chloride without substantially decreasing the combined yield of methyldichlorosilane and dimethyldichlorosilane. This is achieved by blending the methyl chloride with an amount of hydrogen chloride sufficient to accomplish this without forming significant amounts of undesirable SiCl₄ and/or HSiCl₃. In accordance with a preferred embodiment of the present method the concentration of hydrogen chloride is from 0.1 to about 3 weight percent, based on methyl chloride. Most preferably the hydrogen chloride is present only from the first 50% to about 90% of the methyl chloride addition.

Description: This invention relates to the preparation of organohalosilanes. More particularly, this invention relates to a method for increasing the yield of monoorganodichlorosilanes using a method referred to in the art as the "direct process" without either adversely affecting the yield of other desirable organochlorosilanes, particularly the combined yield of monoorganodichlorosilane and the corresponding diorganodihalosilanes, or generating substantial quantities of undesirable inorganic halosilanes and organotrihalosilanes. Chemical Abstracts, Vol. 73, No. 131 073f teaches the mechanism of methyldichlorosilane formation during direct synthesis. However, there is little information other than the fact that HCl can impact the reaction. From Chemical Abstracts, Vol. 82, No. 171 178m it is known to use hydrochloride in the direct process. However, it is only said that the yields of EtSiCl₂H and Et₂SiCl₂ will improve by reacting EtCl with Si in the presence of a promoter, which is for example AlF₃, in the presence of HCl. Nothing is said if the improvement results from AlF₃ or HCl or the combination of the two. The present invention is based on the discovery that when amounts of hydrogen chloride smaller than reported in the prior art are blended with at least a portion of the methyl chloride that is reacted with silicon metal to form methylhalosilanes the yield of MeHSiCl₂ is increased without substantially decreasing the combined yield of Me₂SiCl₂ and MeHSiCl₂ or forming significant amounts of the undesirable methyltrichlorosilane or the inorganic chlorosilanes SiCl₄ and/or HSiCl₃. In accordance with a preferred embodiment, the hydrogen chloride is present only during the first half of the methyl chloride addition. This invention provides an improved process for the preparation of dimethyldichlorosilane and methyldichlorosilane by the addition of methyl chloride to a reactor containing a reaction mass comprising silicon metal and a suitable catalyst at a temperature of from 250 to about 350°C, characterized by homogeneously blending at least the initial portion of the methyl chloride with 0,01 to 5 weight % of hydrogen chloride based on the methyl chloride, in order to increase the yield of methyldichlorosilane without substantially reducing the combined yield of dimethyldichlorosilane and methyldichlorosilane, producing more than 5 weight percent of methyltrichlorosilane, based on the total reaction product, or producing more than trace amounts of inorganic chlorosilanes selected from at least one of trichlorosilane and tetrachlorosilane; wherein the reaction mass comprises (a) metallurgical grade silicon, (b) a catalytically effective amount of copper or a copper compound in combination with at least one member selected from the group consisting of zinc and zinc compounds, tin and tin compounds and a reaction promoter selected from the group consisting of elemental phosphorus, phosphorus-containing alloys and metal phosphides. The concentration of hydrogen chloride in the methyl chloride reactant that will increase the yield of methyldichlorosilane without producing significant amounts of inorganic chlorosilanes will depend, at least to some extent, on the conditions under which the methyl chloride and silicon metal are reacted. These conditions include but are not limited to catalyst(s), the presence of reaction promoters, temperature, purity of the reactants and type of reactor. The optimum concentration of hydrogen chloride can be determined with a minimum of experimentation by those of ordinary skill in the art of organohalosilane preparation having knowledge of the present invention. Specifically, using a preferred set of reaction conditions described in the examples that form part of this specification, the present inventors discovered that while the yield of methyldichlorosilane appears proportional to the concentration of hydrogen chloride introduced together with the methyl chloride up to a hydrogen chloride concentration of about 3 weight percent, based on methyl chloride, the combined concentration of undesirable HSiCl₃ and tetrachlorosilane does not become significant, i.e. exceed about 2 weight percent of the total reaction product, within this concentration range for hydrogen chloride. In addition, the concentration of MeSiCl₃, another undesirable product increases significantly at about 1% HCl. Because of difficulties with product purification and other problems associated with the presence of inorganic chlorosilanes, it is preferred to maintain the total amount of these inorganic silanes below about 1 weight percent. This can usually be achieved by using between about 0.01 and about 5 weight % HCl, based on the methyl chloride. This range will vary depending upon reaction conditions and the capacity of the reactor. For particular laboratory scale fluidized bed reactors described in the accompanying examples the upper limit for hydrogen chloride concentration is typically about 1 weight percent. The present inventors discovered that under the reaction conditions described in the following sections of this specification when the amount of hydrogen chloride introduced into the reactor is from 0.01 to about 1 weight percent, based on methyl chloride 1) the concentration of inorganic halosilanes can be maintained at or below trace amounts 2) the concentration of methyldichlorosilane in the product is substantially increased 3) the combined concentration of dimethyldichlorosilane and methyldichlorosilane in the reaction product is at least 90 weight percent and 4) the concentration of undesirable methyltrichlorosilane is typically below 5%. This concentration range for the hydrogen chloride therefore represents a preferred embodiment of the present method. When the present method is conducted in a batch or semi-batch mode the present inventors found it desirable to have more of the hydrogen chloride present during the initial methyl chloride addition and reduce or eliminate hydrogen chloride during the terminal portion of the methyl chloride addition. In accordance with a particularly preferred embodiment hydrogen chloride is present in only the initial 50 to about 90 weight % of the methyl chloride added to the reactor. The optimum concentration range for the hydrogen chloride is at least partially dependent on the type of equipment and reaction conditions used in practicing the present method. Suitable equipment for conducting the direct process include fixed bed, stirred bed and fluid bed reactors. Any of these reactors can be operated in a continuous or batch mode. It is within the scope of the present invention to utilize the reactor described in United States Patent No. 3,133,109, which issued to Dotson on May 12, 1964 or the one described by Maas et al. in United States Patent No. 4,218,387. The particle size of the fluidized material should be within the range typically used for the direct process. Dotson in United States Patent No. 3,133,109 discloses particle size range of from 20 to 200 microns. Depending upon the capacity of the reactor, a range of from 1 to 200 microns is preferred for the present method. The temperature range used for the direct process is typically from 250 to about 350°C. Temperatures within the range of from 260 to about 330°C. are preferred to optimize yields of the desired methylchlorosilanes. The art pertaining to the direct process for preparing alkylhalosilanes discloses a variety of catalysts and promoters suitable for use in the direct process. United States Patent No. 4,500,724, which issued to Ward et al. discloses catalysts for the production of organohalosilanes comprising copper and copper oxides, tin or tin-containing compounds and zinc or zinc-containing compounds. Halm et al. in U.S. Patent No. 4,602,101, issued July 22, 1986, discloses a method for controlling a process for the manufacture of alkylhalosilanes, said process comprising contacting an all halide with metallurgical grade silicon, at a temperature of 250-350°C., in the presence of tin or tin compounds and copper or copper compounds, wherein there is present as a promoter phosphorous or phosphorous-containing compounds. Other suitable catalysts and promoters include but are not limited to calcium, barium, titanium, zirconium, cadmium, lead, bismuth, arsenic, nickel, antimony, silver, brass, cobalt, iron, carbon and aluminum. Any of these promoters can be used in their elemental form or as compounds that contain the element and are capable of liberating the element under the reaction conditions of the direct process. Preferred catalyst/promoter compositions include but are not limited to: 1. (a) Copper or a copper compound and (b) zinc or a zinc compound; 2. (a) Copper or a copper compound, (b) zinc or a zinc compound and (c) tin or a tin compound; 3. (a) Copper or a copper compound, (b) tin or a tin compound, and (c) optionally arsenic or an arsenic compound; 4. (a) Copper in the form of a mixture, alloy or compound, (b) at least one member selected from the group consisting of tin, tin compounds, zinc and zinc compounds, and (c) at least one of phosphorus, phosphorus compounds, phosphorus-containing alloys and metal phosphides; 5. (a) Copper or a copper compound, (b) tin or a tin compound, (c) arsenic or an arsenic compound, and (d) phosphorus or a phosphorus compound; 6. (a) Iron alloyed with silicon or a mixture of iron and aluminum alloyed with silicon, (b) copper or at least one copper compound, optionally at least one of the following (c) tin and tin compounds, (d) phosphorus in the form of elemental phosphorus, at least one metal phosphide or a phosphorus-containing alloy, and (e) zinc or a zinc compound; and 7. (a) A mixture of copper and zinc that is present as brass, (b) cuprous chloride (c) tin and tin compounds and (d) a metal phosphorus alloy. The metal portion of the metal phosphorus alloy or phosphide is preferably aluminum, calcium, copper or zinc. The present catalysts and promoters are typically used in amounts as low as several parts per million. Up to 10 weight percent, based on initial silicon, of some of the aforementioned catalysts have been used. For best results, the purity of the silicon should be at least 95% but less than 100%. A metallurgical grade of silicon is preferred. For optimum results the silicon is in a particulate form. The following examples describe preferred embodiments of the present invention with respect to types and concentrations of reactants, catalysts, promoters, process conditions and equipment and should not be interpreted as limiting the present invention as defined in the accompanying claims. Unless otherwise indicated all parts and percentages in the example are by weight, all quantities expressed in parts per million are based on the weight of all materials initially charged to the reactor together with the silicon. The reactions between silicon and methyl chloride was conducted in a fluidized bed reactor of the type described in U.S. Patent No. 3,133,109 to Dotson. The temperature of the sand bath was 315°C. and each heating period, equivalent to the reaction time, was 44 hours in duration. Metallurgical grade silicon (Globe Metallurgical, Inc. Beverly, Ohio) was employed which contained aluminum (0.22%), calcium (0.046%) and iron (0.34%). The hydrogen chloride used was of 99.999% minimum purity, obtained from Matheson Gas Products, Dayton, Ohio. The methyl chloride and hydrogen chloride were individually metered using calibrated flowmeters. When hydrogen chloride was used, the two gas streams were combined and passed through a static mixer to ensure proper blending prior to being introduced into the reactor. The material used as the reaction mass was prepared by blending the following ingredients to homogeneity in a suitable container: 100 parts of silicon, 6.48 parts of cuprous chloride, 600 parts per million (ppm) brass (a 1/1 weight ratio alloy of copper and zinc), 30 ppm tin and 2000 ppm copper phosphorus alloy containing 13.5 weight percent phosphorus. The resultant mixture of ingredients was mixed shaking vigorously for 2 to 3 minutes. This mixture was then charged to the reactor, following which the reactor was closed and placed in the 315°C. sand bath. At this time a stream of nitrogen was passed through the reactor. The sand bath was continuously fluidized to maintain a constant temperature within the reactor. When the temperature of the reactor reached about 315°C. the nitrogen was replaced with a stream of gaseous methyl chloride as the fluidizing medium. The flow of methyl chloride was continued for 44 hours. When hydrogen chloride was added, it was blended with the methyl chloride throughout the entire methyl chloride addition or only for the first 28 hours, at which time the hydrogen chloride addition was discontinued. The products emerging from the reactor were condensed and collected in previously weighed cold traps immersed in a bath of dry ice and isopropanol. The liquid collected in the tubes was then transferred to bottles cooled with dry ice/isopropanol and then injected into the sample chamber of a gas chromatograph using a previously cooled syringe. The chromatograph was used to determine the types and concentration of reaction products. Example 1 - (Control Example) This example typifies the product distribution obtained in the absence of hydrogen chloride. The product distributions from two runs performed without the addition of hydrogen chloride were determined and the results were averaged. The averaged values, reported here and summarized in Table 1, were: 92.1% Me₂SiCl₂ 1.4% MeHSiCl₂ 4.1% MeSiCl₃ The remaining material was a mixture of other methylchlorosilanes. There were no detectable amounts of HSiCl₃ or SiCl₄ present. Example 2 This example demonstrates the advantages of blending from 0.2 to 2 weight percent hydrogen chloride with the methyl chloride reactant. The reaction described in example 1 was repeated with the exception that the amounts of hydrogen chloride listed in Table 1 were blended with the methyl chloride throughout the entire methyl chloride addition. The reaction products contained the indicated amounts of HSiCl₃ and/or SiCl₄. The quantity of HCl employed and the percentage of the principal products are shown in the following table, based on an average of two runs. The remaining material in the reaction product was a mixture of other methylchlorosilanes. The data reported in Table 1 demonstrate that at a hydrogen chloride concentration of 0.85 weight percent the amount of MeHSiCl₂ more than quadrupled from the value reported in Example 1 with only a small reduction in the combined yield of Me₂SiCl₂ and MeHSiCl₂ and only trace amounts of HSiCl₃ and SiCl₄. When the hydrogen chloride concentration was increased to 1.0%, the yield of both MeHSiCl₂ and the undesirable HSiCl₃ and SiCl₄ increased significantly. Example 3 This example demonstrates the advantage with respect to yield of MeHSiCl₂ obtained adding hydrogen chloride during only the first portion of the methyl chloride addition. The procedure described in example 2 was repeated, with the exception that 1 weight percent of hydrogen chloride was blended with the methyl chloride only during the first 28 hours of the methyl chloride addition, at which time the addition of hydrogen chloride was discontinued. The product was collected throughout the reaction and exhibited the following composition, expressed as percent by weight of the total product mixture: Me₂SiCl₂ MeHSiCl₂ MeSiCl₃ HSiCl₃ SiCl₄ 87.7 4.7 5.16 0.17 0.00 As in the previous examples, the remaining yield was a mixture of methylchlorosilanes. Although the total amount of hydrogen chloride added was 0.64 weight percent, based on total methyl chloride, the 4.71 weight percent yield of MeHSiCl₂ is higher than the value of 3.54 percent obtained when 0.72 weight percent of hydrogen chloride was added during the entire methyl chloride addition with no reduction in the combined yield of methyldichlorosilane and dimethyldichlorosilane and only trace amounts of the undesirable inorganic chlorosilanes.

Claims: A process for the preparation of dimethyldichlorosilane and methyldichlorosilane by the addition of methyl chloride to a reactor containing a reaction mass comprising silicon metal and a suitable catalyst at a temperature of from 250 to about 350°C, characterized by homogeneously blending at least the initial portion of the methyl chloride with 0,01 to 5 weight % of hydrogen chloride based on the methyl chloride, in order to increase the yield of methyldichlorosilane without substantially reducing the combined yield of dimethyldichlorosilane and methyldichlorosilane, producing more than 5 weight percent of methyltrichlorosilane, based on the total reaction product, or producing more than trace amounts of inorganic chlorosilanes selected from at least one of trichlorosilane and tetrachlorosilane; wherein the reaction mass comprises (a) metallurgical grade silicon, (b) a catalytically effective amount of copper or a copper compound in combination with at least one member selected from the group consisting of zinc and zinc compounds, tin and tin compounds and a reaction promoter selected from the group consisting of elemental phosphorus, phosphorus-containing alloys and metal phosphides. A method according to claim 1 where hydrogen chloride is added during the entire reaction.

Inventors: HALM ROLAND LEE, ZAPP REGIE HAROLD, HALM, ROLAND LEE, ZAPP, REGIE HAROLD
Applicants: DOW CORNING, DOW CORNING CORPORATION
Application year: 1990
Main classification: C07F7
IPC codes: C07B61, B01J23, C07F7, B01J27