Process for converting high-boiling residue from direct process to monosilanes

Abstract: Described herein is a process for the production of monosilanes from the high-boiling residue resulting from the reaction of organochlorides with silicon metalloid in a process typically referred to as the "direct process." The process comprises contacting a mixture comprising the high-boiling residue, an organotrichlorosilane and hydrogen chloride with a catalytic amount of catalyst composition effective in promoting the formation of monosilanes from said residue. A preferred catalyst composition comprises aluminum trichloride, at least a portion of which is formed in situ during conduct of the direct process and isolation of the high-boiling residue. Typically, the monosilane product of the present process comprises almost exclusively methyltrichlorosilane and tetrachlorosilane.

Description: The present invention is a process for the production of monosilanes from the high-boiling residue resulting from the reaction of organochlorides with silicon metalloid in a process typically referred to as the "direct process." This process comprises contacting a mixture comprising the high-boiling residue, an organotrichlorosilane and hydrogen chloride with a catalytic amount of a catalyst composition effective in promoting the formation of monosilanes from the high-boiling residue. A preferred catalyst composition for use in the present process comprises aluminum trichloride, at least a portion of which may be formed in situ during the direct process and isolation of the high-boiling residue. In the preparation of organochlorosilanes by the direct process, a complex mixture is formed which is typically distilled to separate monosilanes from other components present in the mixture. For example, in the "direct process," in addition to the monosilanes, which in the case of the chloromethylsilanes include dimethyldichlorosilane, methyltrichlorosilane and trimethylchlorosilane, there is obtained a residue which boils above the organochlorosilanes, that is above 70°C. This residue is hereinafter referred to as "high-boiling residue." The "direct process" is described in the patent literature, for example, U.S. Patents 2,380,995 and 2,488,487. The residue remaining after distillation overhead of the monosilanes is a complex mixture comprising higher boiling silicon containing compounds which have, for example, SiSi, SiOSi and SiCSi linkages in the molecules. The residue may also contain silicon particulates and metals or compounds thereof. Typical high-boiling residues obtained from distillation of product from the direct process are described, for example, in U.S. Patents 2,598,435 and 2,681,355. In current commercial operations for performing the direct process, the high-boiling residue constitutes as much as five weight percent of the resultant product. Therefore, it is desirable to convert the high-boiling residue into commercially acceptable products to both reduce waste disposal and to improve raw material utilization. U.S. Patent 4,393,229 describes a process for converting alkyl-rich disilanes in a residue obtained from the manufacture of alkylhalosilanes to halogen-rich polysilanes. The process comprises treating an alkyl-rich disilane-containing residue with an alkyltrihalosilane or silicon tetrahalide in the presence of a catalyst and a catalytic amount of a hydrosilane reaction promoter at an elevated temperature. Aluminum trichloride is a useful catalyst in this process when used with a hydrosilane promoter. The resulting halogen-rich polysilanes can, in a separate step, be cleaved to form monosilanes. U.S. Patent 5,175,329 discloses a process for the production of organosilanes from the high-boiling residue resulting from the direct process that results in a net consumption of organotrichlorosilane. In that process, the high-boiling residue is contacted with an organotrichlorosilane and hydrogen gas in the presence of both a hydrogenation catalyst and a redistribution catalyst. U.S. Patent 5,430,168 teaches a process of forming a mixture comprising an organotrihalosilane and a high-boiling residue in the presence of hydrogen gas and a catalytic amount of aluminum trichloride. This process results in a net consumption of the organotrihalosilane and the formation of monosilanes. U.S. Patent 2,681,355 shows that a high-boiling residue resulting from the direct process can be contacted with hydrogen chloride at elevated temperatures to form methylchlorosilanes. U.S. Patent 2,709,176 claims heating an organohalogenpolysilane containing a silicon-silicon linkage at an elevated temperature in the presence of a tertiary organic amine and a hydrogen halide to effect cleavage of the silicon-silicon bond and to thereby form organohalogen-monosilanes. U.S. Patent 5,292,912 reveals a process comprising contacting a high-boiling residue from the direct process with hydrogen chloride at a temperature within a range of 250 to 1000°C. in the presence of a catalyst selected from the group consisting of activated carbon, platinum supported on alumina, zeolite, aluminum trichloride and aluminum trichloride supported on a support selected from the group consisting of carbon, alumina and silica. An object of the present invention is to provide a process where the high-boiling residue from the direct process for producing organohalosilanes is converted into commercially useful monosilanes. An advantage of our process is that it is run with no need to isolate the more chlorinated intermediates, such as chlorodisilanes and to further treat them to produce monosilanes. Another advantage of our process is that it converts both disilanes and other silicon containing species comprising the high-boiling residue, such as silmethylenes or silalkylenes, to monosilanes. Typically, the monosilane product of the present process comprises almost exclusively methyltrichlorosilane and tetrachlorosilane. The present invention is a process for the production of monosilanes from the high-boiling residue resulting from the reaction of organochlorides with silicon metalloid as the "direct process." The latter process comprises contacting a mixture of the high-boiling residue, an organotrichlorosilane and hydrogen chloride with a catalytic amount of catalyst composition effective in promoting the formation of monosilanes from the high-boiling residue. A preferred catalyst composition comprises aluminum trichloride, at least a portion of which is formed in situ during the direct process and the subsequent isolation of the high-boiling residue. Surprisingly, the monosilane product of the present process comprises almost exclusively methyltrichlorosilane and tetrachlorosilane. The present invention provides a process for converting a high-boiling residue resulting from the reaction of an organochloride with silicon metalloid to monosilanes. Our process comprises contacting a mixture comprising a high-boiling residue resulting from the reaction of an organochloride with silicon metalloid, an organotrichlorosilane described by formula RSiCl 3 (1) where R is selected from the group consisting of alkyls comprising one to six carbon atoms; aryls; alkoxys comprising one to six carbon atoms; trimethylsilyl and trifluoropropyl; and hydrogen chloride with a catalytic amount of a catalyst composition effective in promoting the formation of monosilanes from said high-boiling residue at a temperature within a range of 150 to 500°C. and at a gauge pressure within a range of 0.69 - 34.5 MPa (100 to 5,000 psi). The present process can further comprise recovering monosilanes described by formula R y H z SiCl 4-y-z (2) where R is described above, y=0 to 4, z=0 to 3 and y+z=0 to 4. The claimed process is run as a one-step process with no additional processing steps being required to form monosilanes from said high-boiling residue. This process is run in any standard pressurizable reactor suitable for contact with chlorosilanes. The process may be run as either a batch process or a continuous process. The process may be run, for example, in a stirred-bed reactor, a continuous stirred-tank reactor, a bubble-column reactor, a trickle-bed reactor or a plug-flow reactor. The present process unexpectedly converts a high-boiling residue resulting from the reaction of an organochloride with silicon metalloid to useful monosilanes. In a typical process for reacting an organochloride with silicon metalloid, the process is conducted at a temperature of 300 to 350°C. in the presence of suitable catalysts and gaseous product. Therein, feed along with fine particulates are continuously removed from the process. The removed materials are subsequently distilled to recover organochlorosilanes, leaving a "high-boiling residue." A preferred high-boiling residue for use in our process is one with a boiling point above 70°C. resulting from the distillation of organochlorosilanes from the reaction product of methyl chloride with silicon metalloid. A typical composition for such a high-boiling residue comprises: 50-60 wt% of disilanes of formula Si 2 Q 6 , where each Q is independently selected from methyl or chlorine and the disilane contains two to four methyl substituents per molecule; 15 to 25 wt% of silmethylenes described by formula Q 3 SiCH 2 SiQ 3 , where Q is as previously described and the silmethylene contains two to four methyl substituents per molecule; silalkylenes described by formula Q 3 Si(SiQ 2 ) a (CH 2 ) b (SiQ 2 ) c SiQ 3 , where Q is as previously described and the silalkylene contains two to four methyl substituents per molecule, a=0 to 4, b=1 to 3, c=0 to 4 and a+b+c≥1; 5 to 15 wt% of other high-boiling silicon-containing compounds; catalysts carried over from the direct process such as copper or compounds of copper; particulates containing silicon; and low levels of metals such as aluminum, calcium and iron or compounds thereof. In the present process, a mixture of the high-boiling residue described above is formed with an organotrichlorosilane described by formula (1). The mixture is formed external to the reactor and is then added to the reactor or it may be formed by adding the individual components to the reactor. The organotrichlorosilane contains one substituent R, where R is selected from the group consisting of alkyls comprising one to six carbon atoms; aryls; alkoxys comprising one to six carbon atoms; trimethylsilyl and trifluoropropyl. Substituent R is, for example, methyl, ethyl, propyl, isopropyl, tert-butyl, methoxy, ethoxy, phenyl, tolyl, naphthyl, trimethylsilyl and trifluoropropyl. Preferred is when R is methyl. The organotrichlorosilane is, for example, methyltrichlorosilane, ethyltrichlorosilane and benzyltrichloro-silane. Preferred is when the organotrichlorosilane is methyltrichlorosilane. The weight percent of organotrichlorosilane in the mixture with the high-boiling residue is not critical to the present process. Generally, a mixture where the organotrichlorosilane is 0.1 to 95 weight percent of the mixture is considered useful. Preferred is where the organotrichlorosilane is 30 to 50 weight percent of the mixture. The mixture further comprises hydrogen chloride, which is fed to the reactor as a gas. The concentration of hydrogen chloride fed to the reactor is not critical and can be any amount sufficient to effect a desirable level of hydrochlorination. Preferred is when the amount of hydrogen chloride fed to the reactor is within a range of 1 to 70 weight percent, based on the weight of the high-boiling residue fed to the reactor. Even more preferred is when the amount of hydrogen chloride fed to the reactor is within a range of 15 to 50 weight percent, on the same basis. The contacting within the present process is effected in a reactor under a gauge pressure of 0.69 - 34.5 MPa (100 to 5,000 psi) ensuring that at least a portion of the hydrogen chloride is in a liquid phase. More preferred is when the contacting is effected in a reactor at a gauge pressure of 2.07 - 10.3 MPa (300 to 1,500 psi). Most preferred is when said contact is effected at a gauge pressure of 4.14 - 7.6 MPa (600 to 1,100 psi). The mixture comprising the high-boiling residue, organotrichlorosilane and hydrogen chloride is contacted with a catalytic amount of a catalyst composition effective in promoting formation of monosilane from the high-boiling residue. This catalyst composition of the present process promotes the formation of monosilanes from the high-boiling residue. We believe that our process requires a catalytic composition which promotes redistribution of alkyl and chlorine groups between silicon atoms, hydrochlorination and scission of silicon-silicon bonds or, optionally, silicon-carbon bonds. Therefore, our catalyst composition comprises one or more chemical entities providing the described activities to our claimed process. Generally, any Lewis Acid or its equivalent may be used to provide redistribution activity to the catalyst composition. Examples of chemical entities useful to effect redistribution in the present process include those described by U.S. Patents 4,393,229 and 5,175,329. Examples of such chemical entities useful to effect redistribution in. the present process include aluminum trichloride, antimony pentachloride, zirconium tetrachloride, potassium aluminum tetrachloride, quaternary phosphonium halides, quaternary ammonium halides, ammonium halides, cuprous chloride, boric acid and boron halides. Examples of chemical entities useful in the present catalyst composition to effect hydrochlorination include aluminum trichloride, antimony pentachloride, cuprous chloride, copper metal, copper salts, complexes of copper salts with organic ligands and those hydrogenation catalysts described in U.S. Patent 5,175,329. Normally, those chemical entities which promote redistribution of alkyl and chlorine groups between silicon atoms and those chemical entities which promote hydrochlorination in the present process, will also promote scission of silicon-silicon bonds and optionally silicon-carbon bonds. Therefore, it is not usually necessary to add additional chemical entities to the catalyst composition to promote the scission of the silicon-silicon bonds and optionally silicon-carbon bonds. If such chemical entities are needed, a chemical entity such as aluminum trichloride or antimony pentachloride is added to the composition. In the present process it is preferred that the catalyst composition comprise a single chemical entity which promotes redistribution, hydrochlorination and bond scission under process conditions. Such single chemical entities include aluminum trichloride or antimony pentachloride. The present process requires the presence of a "catalytic amount" of a catalyst composition as described above. By "catalytic amount" it is meant an amount of the catalyst composition sufficient to facilitate the conversion of silicon containing compounds in the high-boiling residue to monosilanes. A preferred catalytic amount of catalyst composition is that sufficient to facilitate the conversion of polysilanes, for example methylchlorodisilanes, silmethylenes and silalkylenes in the high-boiling residue to monosilanes. The amount of catalyst composition required will depend upon the chemical entities comprising the catalyst composition and such amounts are easily determined by those skilled in the art. When aluminum trichloride or antimony pentachloride comprise the catalyst composition, from 0.01 to 10 weight percent of catalyst composition, based on the combined weight of the catalyst composition and the high-boiling residue, is considered useful in the present process. Preferred is when the aluminum trichloride or antimony pentachloride concentration is within a range of 0.5 to 5 weight percent, on the same basis. A preferred catalyst composition for use in the present process is aluminum trichloride. The aluminum trichloride may be added to the process as a compound or it may be formed in situ by the addition of materials that form aluminum trichloride. All or a portion of the catalytic amount of aluminum trichloride may be formed in situ during conduct of the direct process and the isolation of the monosilane fraction to form the high-boiling residue. The source of the aluminum and chlorine necessary to form aluminum trichloride is the raw materials used in the direct process, particularly the silicon metalloid and organochloride feed. The catalytic amount of aluminum trichloride can be a combination of added aluminum trichloride and that in situ formed aluminum trichloride remaining in the high-boiling residue as isolated from the direct process. The present process is conducted at a temperature within a range of 150 to 500°C. Preferred is a temperature of 275 to 425°C. Most preferred is a temperature within a range of 300 to 375°C. Monosilanes as described by formula (2) are recovered from the present process. These monosilanes are separated by standard methods for separating liquid mixtures, for example, distillation. The monosilanes contain zero to four substituents R, where R is as previously described. The monosilane can also contain zero to three hydrogen atoms substituted on each silicon atom. The monosilanes can further contain zero to four chlorine atoms substituted on each silicon atom. Typically, the monosilane product of the present process comprises almost exclusively methyltrichlorosilane and tetrachlorosilane, with only minor amounts of other monosilanes being formed. Therefore, a preferred monosilane product of the present process is selected from methyltrichlorosilane or tetrachlorosilane. The following example is provided to illustrate the present invention. In the example, Me represents a methyl radical. Example The ability of aluminum trichloride to catalyze the conversion of a mixture comprising a high-boiling residue and methyltrichlorosilane in the presence of hydrogen chloride was evaluated in a stirred-batch reactor. The reactor was a 650 ml, pneumatically stirred, Parr Bomb reactor. A mixture comprising 0.15 mole of a filtered high-boiling residue from a direct process for the preparation of methylchlorosilanes by the reaction of methyl chloride with silicon metalloid and 62.5 weight percent of methyltrichlorosilane was added to this reactor. The major components of the high-boiling residue are presented in Table 1 on a weight percent basis. Major Components of High-Boiling Residue Component Weight Percent ClMe 2 SiSiMe 2 Cl 2.8 Cl 2 MeSiSiMe 2 Cl 16.8 Cl 2 MeSiSiMeCl 2 31.6 ClMe 2 SiCH 2 SiMe 2 Cl 4.5 Cl 2 MeSiCH 2 SiMe 2 Cl 18.5 Cl 2 MeSiCH 2 SiMeCl 2 25.8 Roughly, 3.5 weight percent of aluminum trichloride was present in the reactor, based on the weight of the high-boiling residue. Then, 0.3 to 0.4 mole of hydrogen chloride gas was fed to the reactor. The reactor was heated to 325°C. and stirred for 3 hours, with an internal-reactor gauge pressure of 4.8 - 6.9 MPa (700 to 1000 psi). At the end of the run, a sample from the reactor was analyzed by gas chromatography using a thermal conductivity detector. The results of this analysis are reported in Table 2. The conversion of disilane in the high-boiling residue to monosilanes was 100 percent. The conversion of silmethylenes in the high-boiling residue was 49 weight percent. The product distribution is presented in Table 2 is a net value calculated as the weight percent species of monosilane from the total monosilane present in the product. Monosilane Product Distribution Monosilane Weight Percent MeSiCl 3 56 SiCl 4 44

Claims: A process for converting high-boiling residue resulting from reaction of an organochloride with silicon metalloid to monosilanes, the process comprising contacting said residue, an organotrichlorosilane described by formula. RSiCl 3 where R is selected from the group consisting of alkyls comprising one to six carbon atoms, aryls, alkoxys comprising one to six carbon atoms, trimethylsilyl and trifluoropropyl; and hydrogen chloride with a catalytic amount of a catalyst composition effective in promoting the formation of monosilanes from said residue at a temperature within a range of 150 to 500°C. and a gauge pressure within a range of 0.69 - 34.5 MPa (100 to 5,000 psi). A process according to claim 1 where said residue has a boiling point above 70°C. and results from the distillation of methylchlorosilanes from the reaction product of methyl chloride with silicon metalloid. A process according to claim 1 or claim 2 where the organotrichlorosilane is methyltrichlorosilane. A process according to any of claims 1 to 3 where the organotrichlorosilane comprises 0.1 to 95 weight percent of said mixture. A process according to any of claims 1 to 4 where the amount of hydrogen chloride provided to the process is within a range of 1 to 70 weight percent, based on the weight of the high-boiling residue. A process according to any of claims 1 to 5 where the catalyst composition is selected from the group consisting of aluminum trichloride, antimony pentachloride and combinations thereof. A process according to claim 6 where the catalyst composition comprises 0.01 to 10 weight percent, based on the combined weight of the catalyst composition and said residue. A process according to claim 6 where at least a portion of the catalytic amount of aluminum trichloride is formed in situ. A process according to any of claims 1 to 8 further comprising recovering monosilanes described by formula R y H z SiCl 4-y-z where R is as previously described, y=0 to 4, z=0 to 3 and y+z=0 to 4. A process according to claim 9 where the monosilane is selected from methyltrichlorosilane or tetrachlorosilane.

Inventors: FREEBURNE STEVEN KERRY, JARVIS ROBERT FRANK JR, FREEBURNE, STEVEN KERRY, JARVIS, ROBERT FRANK, JR.
Applicants: DOW CORNING, DOW CORNING CORPORATION
Application year: 1997
Main classification: C07F7
IPC codes: C07F7, C01B33