Method for preparing organohalosilanes

Abstract: The present invention provides a method for reducing the concentration of methyltrichlorosilane obtained during the reaction of silicon metal with methyl chloride without substantially decreasing the combined yield of methyldichlorosilane, dimethylchlorosilane and dimethyldichlorosilane. The methyl chloride is blended with from 0.05 up to about 44 mole percent of hydrogen, based on combination of methyl chloride and hydrogen. At hydrogen concentrations above about 12 mole percent there is a substantial increase in the methyldichlorosilane content of the reaction product while the concentration of methyltrichlorosilane in the reaction product is maintained below about 4 percent by weight.

Description: This invention relates to the preparation of organohalosilanes. More particularly, this invention relates to reducing the yield of undesired monoorganotrichlorosilane produced when preparing organochlorosilanes using a method referred to in the art as the "direct process". This reduction is accomplished without either adversely affecting the yield of other desirable organochlorosilanes, particularly the diorganodichlorosilane or generating substantial quantities of undesirable inorganic chlorosilanes and organochlorosilanes. A second embodiment of this invention provides a method for achieving all of the foregoing objectives while increasing the yield of the corresponding monoorganodichlorosilane. The preparation of organohalosilanes by the reaction of an alkyl or aryl halide with silicon metal in the presence of various catalysts is known as the "direct process". The halide portion is typically chlorine, but can also be bromine or iodine. Application of the direct process to the preparation of organohalosilanes was first disclosed by Rochow and his co-workers, beginning in the mid-1940's. The art describes numerous improvements to this direct process. For various reasons, including cost and availability of starting materials, alkylchlorosilanes, particularly methyl- and ethylchlorosilanes, have become the organohalosilanes most frequently prepared by the direct process. The present invention has therefore been described by reference to this class of alkylchlorosilanes using the corresponding alkyl chlorides. It should be understood that while preferred embodiments of the present invention are directed primarily to the preparation of certain methylchlorosilanes by reacting methyl chloride and silicon, the invention is not to be so limited. When methyl chloride, represented by the formula MeCl, and silicon metal are reacted using the catalysts and reaction conditions described in the prior art, the resultant products include but are not limited to MeHSiCl 2 , Me 2 SiCl 2 , Me 3 SiCl, MeSiCl 3 , Me 2 HSiCl, HSiCl 3 and SiCl 4 , where Me represents the methyl radical. By an appropriate selection of catalyst and reaction conditions it is possible to obtain dimethyldichlorosilane, Me 2 SiCl 2 , as the major component, often 90 weight % or more, in the final product mixture. Methyldichlorosilane, MeHSiCl 2 typically constitutes about 1 weight percent of the product under these conditions, which are designed to optimize the yield of Me 2 SiCl 2 . The reaction product also typically contains a significant concentration of methyltrichlorosilane, MeSiCl 3 , which in many instances is not a desired product. The prior art does not provide a method for decreasing the relative amount of methyltrichlorosilane in a direct process product mixture without a substantial decrease in the yield of dimethyldichlorosilane. It is known to blend the methyl chloride with five weight percent or more, based on methyl chloride, of hydrogen as a means for increasing the relative yield of methyldichlorosilane in the final product, however, the yield of dimethyldichlorosilane is more than correspondingly reduced and substantial quantities of undesirable organic chlorosilanes are produced. US-A 2 380 998 is concerned with the direct process for the production of organosilicon halides. According to this reference, an alkyl halide is passed over heated silicon or silicon copper alloy, the alkyl halide gas being mixed with hydrogen. From GB-A 1 089 726 it was known to prepare methylchlorosilanes and methylhydrochlorosilanes using the direct process wherein the catalyst used is a combination of copper and a salt of a metal of the eighth group of the periodic system of elements, optionally together with zinc chloride. Using this method, the amount of the undesired trichlorosilane is up to 25%. In Chemical Abstracts, Vol. 78, 43573s (1973), the effect of the addition of hydrogen in an amount of 0 to 45 mole % on the reaction of MeCl with Si-Cu is evaluated. The yield of the desired silanes MeSiHCl 2 and Me 2 SiCl 2 is about 60% at all concentrations of hydrogen added. In the Journal of Organometallic Chemistry, 99 (1975), 371 to 377, the direct synthesis of methyldichlorosilane and dimethylchlorosilane, i.e. hydrogen-containing methylchlorosilanes, is described. It was found that cadmium chloride and zinc chloride do not promote the formation of hydrogen-containing products. An objective of the present invention is to provide a method for decreasing to below about 4 weight percent the concentration of methyltrichlorosilane in the product obtained from the reaction of silicon with methyl chloride and to achieve this without substantially reducing the yield of dimethyldichlorosilane. The objective of a second embodiment of this invention is to maintain this reduced level of methyltrichlorosilane while increasing the yield of methyldichlorosilane and dimethylchlorosilane. The present invention is based on the discovery that when amounts of hydrogen smaller than reported in the relevant literature are blended with methyl chloride and the resultant mixture is reacted with silicon metal in the presence of a suitable catalyst to form methylchlorosilanes, the yield of undesirable methyltrichlorosilane is reduced without adversely affecting the yield of desirable products, particularly dimethyldichlorosilane. The present invention provides a method for producing a mixture of methylchlorosilanes by the addition of methyl chloride to a reactor containing a reaction mass comprising silicon metal and copper catalyst at a temperature of from 250 to 350°C, characterized by homogenously blending said methyl chloride with an amount of hydrogen equivalent to from 0,5 to 12 mole percent, based on the combination of methyl chloride and hydrogen, to reduce the concentration of methyltrichlorosilane without substantially decreasing the concentration of dimethyldichlorosilane in the mixture, wherein the catalyst is combined with a reaction promoter and is selected from the group consisting of copper or a copper compound, zinc or a zinc compound and tin or a tin compound; copper or a copper compound and tin or a tin compound; copper or a copper compound, tin or a tin compound and arsenic or an arsenic compound; copper in the form of a mixture, alloy or compound, at least one member selected from the group consisting of tin, tin compounds, zinc and zinc compounds and at least one member selected from the group consisting of phosphorus, phosphorus compounds, metal-phosphorus alloys and metal phosphides; copper or a copper compound, tin or a tin compound, arsenic or an arsenic compound and phosphorus or a phosphorus compound; iron alloyed with silicon or an alloy of iron, aluminum and silicon, either copper or at least one copper compound and at least one member selected from the group consisting of tin, tin compounds, elemental phosphorus, metal phosphides, metal-phosphorus alloys, zinc and zinc compounds, where the concentrations of iron and aluminum do not exceed one weight percent, based on the initial weight of silicon; and a mixture of copper and zinc that is present as brass, cuprous chloride, tin and tin compounds and metal-phosphorus alloy. Copper in combination with tin optionally together with zinc is the preferred catalyst for this reaction. In accordance with the first embodiment of this invention, the concentration range of hydrogen in the methyl chloride reactant that will reduce the yield of methyltrichlorosilane without reducing the concentration of dimethyldichlorosilane, (CH 3 ) 2 SiCl 2, in the reaction product 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 the presence of catalyst(s) and reaction promoters, temperature, purity of the reactants and the configuration and size of the reactor. The optimum hydrogen concentration range for a given set of reaction conditions 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 and catalyst compositions described in the subsequent section of this specification, a surprisingly large reduction in the yield of methyltrichlorosilane is achieved using as little as 0.5 mole percent of hydrogen, based on the combination of hydrogen and methyl chloride (equivalent to 0.02 weight percent of hydrogen, based on methyl chloride). The yield of methyltrichlorosilane decreases with increasing hydrogen concentration up to a level about 12 mole percent (0.5 weight percent of hydrogen, based on methyl chloride). This is accompanied by small increases in the concentrations of methyldichlorosilane and dimethylchlorosilane. When the hydrogen concentration in a direct process reaction is increased from about 12 mole percent up to about 44 mole percent (3 weight percent hydrogen, based on methyl chloride), the prior art discussed in a preceding section of this specification shows a substantial increase in the yield of methyldichlorosilane that is proportional to the hydrogen concentration. The shortcoming of these prior art processes is that the increase in methyldichlorosilane is accompanied by a relatively high concentration of methyltrichlorosilane, which is typically an undesirable by-product. The present inventors have discovered that, in the presence of certain catalyst compositions, methyltrichlorosilane constitutes less than 4 weight percent of the reaction product while the combined yield of methyldichlorosilane, dimethylchlorosilane and dimethyldichlorosilane exceeds 90 weight percent of the reaction mixture. Suitable catalysts include the mixtures of copper with tin and, optionally, zinc that are described in detail in the following section of this specification as preferred catalysts for use in the present method. This method for increasing the yield of methyldichlorosilane and dimethylchlorosilane constitutes the second embodiment of the present invention. In addition to decreased yields of methyldichlorosilane and dimethyldichlorosilane, a second reason for not using relatively large concentrations of hydrogen in the present method is that only a small fraction of the hydrogen reacts. The remainder must be either vented or recycled. The presence of unreacted hydrogen in combination with a number of other materials, including unreacted methyl chloride, may make recycling of the effluent gas stream economically unattractive and venting of large volumes of the gas stream hazardous. A third reason is a reduction in the quantity of silicon converted during the reaction. The combination of a high yield of dimethyldichlorosilane and a reduction in the concentration of methyltrichlorosilane achieved using the present method is believed due to 1) the mixture of hydrogen and methyl chloride that is reacted with silicon and 2) the presence of a catalyst composition that will suppress the formation of methyltrichlorosilane during this reaction. Catalysts suitable for use in the present method include but are not limited to copper or a copper compound in combination with tin optionally together with zinc. The tin and zinc can be in the form of the metals or compounds of these metals. 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 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. In accordance with the teaching of this patent the concentration of methyltrichlorosilane in a silicon/methyl chloride reaction product is reduced using specified ratios of copper, zinc and tin as the catalyst for the reaction. Halm et al. in U.S. Patent No. 4,602,101, issued July 22, 1986, discloses catalysts for controlling product selectivity and increasing silicon conversion during the reaction of an alkyl halide with metallurgical grade silicon, at a temperature of 250-350°C. The catalysts for this reaction are combinations of copper or a copper compound with tin or a tin compound. The reaction mass also contains phosphorous or phosphorous-containing compounds as reaction promoters. The concentration of phosphorus or phosphorus compound is from 25 to 2500 parts by weight per million parts (ppm) of initial reaction mass, the concentration of copper is from 0.2 to 10 weight percent, based on the initial weight in the reaction mass and the concentration of tin is from 5 to 200 ppm. The concentration limits on phosphorus, copper and tin disclosed in the aforementioned patent to Halm et al. also apply to the reaction mixtures of the present invention. The reaction mass can also contain up to about one weight percent each of aluminum and/or iron based on weight of initial silicon. The catalyst/promoter compositions useful for carrying out the invention are: 1. (a) Copper or a copper compound, (b) zinc or a zinc compound and (c) tin or a tin compound; 2. (a) Copper or a copper compound, (b) tin or a tin compound and (c) optionally arsenic or an arsenic compound; 3. (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 member selected from the group consisting of phosphorus, phosphorus compounds, phosphorus-containing alloys and metal phosphides; 4. (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; 5. (a) Iron alloyed with silicon or an alloy of silicon, iron and aluminum, where the concentrations of iron and aluminum do not exceed about one weight percent, based on weight of initial silicon, (b) copper or at least one copper compound and optionally at least one member selected from the group consisting of tin, tin compounds, zinc, zinc compounds, elemental phosphorus, metal phosphides and phosphorus-containing alloys; and 6. (a) A mixture of copper and zinc that is present as brass, (b) cuprous chloride, (c) tin and/or at least one tin compound 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. Unless otherwise specified in the preceding specification, up to 10 weight percent, based on initial reaction mass, of many of the aforementioned catalysts can be 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 optimum hydrogen concentration range 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. The aforementioned Dotson patent discloses a 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 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 following general procedure was used in the examples. The reaction 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 used to heat the reactor 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 was of 99.999% minimum purity, obtained from Matheson Gas Products, Dayton, Ohio. The methyl chloride and hydrogen were individually metered using calibrated flowmeters. When hydrogen was used the 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 of a copper phosphorus alloy containing 13.5 weight percent phosphorus. The resultant mixture of ingredients was homogenized by shaking it 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 as a fluidizing medium. 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 was added, it was blended with the methyl chloride throughout the entire methyl chloride addition. The products emerging from the reactor were condensed and collected in previously weighed cold traps. The liquid collected in the traps was then transferred to cooled bottles 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 This example (Control example) typifies the product distribution obtained in the absence of hydrogen. The product distributions from two runs performed without the addition of hydrogen were determined and the results were averaged. The averaged values, reported here and summarized in Table 1, were: 91.9% Dimethyldichlorosilane (Me 2 SiCl 2 ) 1.3% Methyldichlorosilane (MeHSiCl 2) 0.2% Dimethylchlorosilane (Me 2 HSiCl) 4.1% Methyltrichlorosilane (MeSiCl 3) The remaining material was a mixture of other methylchlorosilanes. There were no detectable amounts of HSiCl 3 or SiCl 4 present. Example 2 This example demonstrates the advantages of blending from 0.2 to 5 weight percent of hydrogen with the methyl chloride reactant. Using the General Procedure described in the foregoing specification, the amounts of hydrogen listed in Table 1 were blended with the methyl chloride throughout the entire methyl chloride addition. The concentration of hydrogen in the methyl chloride stream and concentrations of four of the principal products are shown in the following table. The product concentrations are based on an average of two runs. The table includes data from two sets of two runs using a hydrogen concentration of 1 percent. The remaining material in the reaction product was a mixture of other methylchlorosilanes. No detectable amounts of inorganic silicon halides were produced. The data reported in Table 1 demonstrate that 1) the concentration of methyltrichlorosilane is substantially reduced from 4.1 weight percent to 2.9 weight percent at hydrogen concentrations of 0.0 and 0.2 weight percent, respectively and 2) there is a substantial increase in the yield of MeHSiCl 2 at a hydrogen concentration of 0.45 weight percent. At a hydrogen content of 3 weight percent the concentration of methyldichlorosilane increased to 14.1 percent, with only a 3 percent decrease in the combined yield of this silane and dimethyldichlorosilane relative to the combined yield in the absence of hydrogen and an increase in the combined yield of Me 2 SiCl 2 , MeHSiCl 2 and Me 2 HSiCl. The concentration of methyltrichlorosilane decreased from 4.1 percent in the absence of hydrogen to 2.1 percent using from 1 to 3 percent hydrogen in the methyl chloride feed. Products (Weight % of Total Product) Desired Undesired H 2 added (Mole %) (Wt %) Me 2 SiCl 2 MeHSiCl 2 Me 2 HSiCl Combined Yield MeSiCl 3 0 0.00 92.1 1.3 0.3 93.7 4.1 6 0.20 92.2 1.6 0.7 94.5 2.9 12 0.45 90.7 3.2 1.0 94.9 2.9 21 1.00 81.6 10.2 4.3 96.1 2.1 21 1.00 84.9 8.4 2.8 96.1 2.1 44 3.00 77.2 14.1 4.4 95.7 2.1 57 5.00 76.2 13.6 6.3 96.1 1.8

Claims: A method for producing a mixture of methylchlorosilanes by the addition of methyl chloride to a reactor containing a reaction mass comprising silicon metal and copper catalyst at a temperature of from 250 to 350°C, characterized by homogenously blending said methyl chloride with an amount of hydrogen equivalent to from 0,5 to 12 mole percent, based on the combination of methyl chloride and hydrogen, to reduce the concentration of methyltrichlorosilane without substantially decreasing the concentration of dimethyldichlorosilane in the mixture, wherein the catalyst is combined with a reaction promoter and is selected from the group consisting of copper or a copper compound, zinc or a zinc compound and tin or a tin compound; copper or a copper compound and tin or a tin compound; copper or a copper compound, tin or a tin compound and arsenic or an arsenic compound; copper in the form of a mixture, alloy or compound, at least one member selected from the group consisting of tin, tin compounds, zinc and zinc compounds and at least one member selected from the group consisting of phosphorus, phosphorus compounds, metal-phosphorus alloys and metal phosphides; copper or a copper compound, tin or a tin compound, arsenic or an arsenic compound and phosphorus or a phosphorus compound; iron alloyed with silicon or an alloy of iron, aluminum and silicon, either copper or at least one copper compound and at least one member selected from the group consisting of tin, tin compounds, elemental phosphorus, metal phosphides, metal-phosphorus alloys, zinc and zinc compounds, where the concentrations of iron and aluminum do not exceed one weight percent, based on the initial weight of silicon; and a mixture of copper and zinc that is present as brass, cuprous chloride, tin and tin compounds and metal-phosphorus alloy. A method according to claim 1 wherein the reaction between the methyl chloride and silicon is conducted at a temperature of from 260 to about 330°C. in the presence of a catalyst comprising copper or a copper compound and tin or a tin compound optionally in combination with zinc or a zinc compound. A method according to claim 2 where the yield of methyltrichlorosilane does not exceed four percent of the combined weight of methylchlorosilanes produced in accordance with said method and said reaction mass includes a reaction promoter.

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