Conversion of high-boiling residue from direct process to monosilanes

Abstract: 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 present process comprises forming a mixture comprising an organosilane and the high-boiling residue and contacting the mixture in the presence of hydrogen gas with a catalytic amount of a 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 may be formed in situ during conduct of the direct process and isolation of the high-boiling residue.

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 forming a mixture comprising an organosilane and the high-boiling residue and contacting the mixture in the presence of hydrogen gas 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 our process comprises aluminum trichloride, at least a portion of which is formed in situ during the direct process and the isolation of the high-boiling residue. In the preparation of organohalosilanes 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 methylchlorosilanes include dimethyldichlorosilane, methyltrichlorosilane and trimethylchlorosilane, there is obtained a residue which boils above the methylchlorosilanes, 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, US-A-2,380,995 and US-A-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 US-A-2,598,435 and US-A-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. US-A-2,606,811 teaches a hydrogenation process where a compound containing a halogen and the Si-Si bond is heated to a temperature of at least 300°C. in the presence of hydrogen. The resultant products are monosilanes. US-A-3,639,105 describes a process where hydrosilanes are produced by contacting a disilane with hydrogen gas under pressure and then heating the mixture in the presence of a transition metal catalyst such as palladium on charcoal. Therein, the disilane may be part of a mixture from the direct process. Further, when the disilane was a methylchlorodisilane, the resulting product contained four to 28 weight percent of methyltrichlorosilane. Generally, organotrihalosilanes such as methyltrichlorosilane have limited commercial usefulness and for this reason limit the usefulness of that process patent. US-A-4,079,071 claims a process for preparing high yields of hydrosilanes by reacting methylchloropolysilanes with hydrogen gas under pressure at a temperature of from 25 to 350°C. in the presence of a copper catalyst. These methylchloropolysilanes are those typically created as by-products of the direct process. Useful copper catalysts therein include copper metal, copper salts and complexes of copper salts with organic ligands. In some cases up to 29 weight percent of methyltrichlorosilane was formed by that patent. US-A-4,393,229 provides 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 that process when used with a hydrosilane promoter. The resulting halogen-rich polysilanes can, in a separate step, be cleaved to form monosilanes by said teachings. US-A-5,175,329 reveals a process for the production of organosilanes from the high-boiling residue of the direct process that actually results in a net consumption of organotrichlorosilane. In said 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. US-A-5,430,168 is another process for production of monosilanes from the high-boiling residue resulting from the direct process. That process comprises forming a mixture comprising an organotrihalosilane and the high-boiling residue in the presence of hydrogen gas and a catalytic amount of aluminum trichloride. An object of the present invention is to provide a simple process where the high-boiling residue from a direct process for producing organohalosilanes is converted into commercially useful monosilanes. We have found that this objective is met by contacting the high-boiling residue with an organosilane, hydrogen gas and a catalytic amount of a catalyst composition effective in promoting the formation of monosilanes from the high-boiling residue. The present process is performed with the high-boiling residue, organosilane and hydrogen gas being fed to a reactor as co-feeds forming a mixture which then contacts the catalyst composition to effect formation of monosilanes. The process results in unique product distributions that depend upon the organosilane feed. Therefore monosilane product resulting from the use of a specific organosilane is used as a co-feed in a subsequent conduct of the process, thereby providing a means of controlling the monosilanes produced to match commercial demand. The present invention is a process for converting a high-boiling residue resulting from the reaction of an organochloride with silicon metalloid to monosilanes. The process comprises: (A) forming a mixture comprising a high-boiling residue resulting from the reaction of an organochloride with silicon metalloid and an organosilane selected from dimethyldichlorosilane, methyldichlorosilane, trimethylchlorosilane and tetramethylsilane; and then (B) contacting the mixture with hydrogen gas in the presence of a catalytic amount of a catalyst composition effective in promoting the formation of monosilanes from the high-boiling residue at a temperature within a range of 150 to 500°C. and a total reactor gauge pressure within a range of 0.69 - 34.5 MPa (100 to 5,000 psi). The present process further comprises: (C) recovering monosilanes described by formula R y H z SiCl 4-y-z (2) where R is as described above, y=0 to 4, z=0 to 3 and y+z=0 to 4. The present process is run in any standard pressurizable reactor suitable for contact with chlorosilanes. The process is run as a batch process or as a continuous process. The process is 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 is useful for converting a high-boiling residue resulting from the reaction of an organochloride with silicon metalloid to useful monosilanes. In a typical process, the process is conducted at a temperature of 300 to 350°C. in the presence of suitable catalysts and gaseous product and the 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 the present process is one with a boiling point above 70°C. resulting from the distillation of methylchlorosilanes 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 weight percent 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, 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 weight percent other high-boiling silicon-containing compounds; catalyst carry over from the direct process such as copper and compounds of copper; particulates containing silicon; and low levels of metals such as aluminum, calcium and iron and compounds thereof. As discussed previously, it is known that the high-boiling residue can be treated with a hydrogenation catalyst and hydrogen gas to produce monosilanes. However, a consequence of this hydrogenation process is the production of organotrihalosilanes which have limited commercial utility and therefore are an undesirable product. Our claimed process results in unique product distributions which depend upon the organosilane feed. Therefore monosilane product resulting from the use of a specific organosilane is used as a co-feed in a subsequent conduct of the process, thereby providing a means of controlling the types of monosilanes produced to match commercial needs. For example, when the organosilane feed is dimethyldichlorosilane, a major product of the process is methyldichlorosilane and a smaller amount of dimethylchlorosilane. The methyldichlorosilane is then used as a feed in a subsequent stage of the process resulting in dimethyldichlorosilane being a major product. In such a manner the amount of methyldichlorosilane, dimethylchlorosilane and dimethyldichlorosilane produced by our process is controlled to match commercial demand. In the claimed process, a mixture of the high-boiling residue described above is formed with the organosilane. This mixture is formed external to the reactor and added to the reactor or it may be formed by adding the individual components to said reactor. The organosilane is selected from a group consisting of dimethyldichlorosilane, methyldichlorosilane, trimethylchlorosilane and tetramethylsilane. The weight percent of organosilane in the mixture with the high-boiling residue is not critical to the present process. Generally, a mixture where the organosilane is 0.1 to 95 weight percent of the mixture is considered useful. Preferred is where the organosilane is 30 to 80 weight percent of the mixture. The mixture is contacted with hydrogen gas at a gauge reactor pressure within a range of 0.69 - 34.5 MPa (100 to 5,000 psi). Preferred is a reactor gauge pressure within a range of 2.07 - 10.3 MPa (300 to 1500 psi). More preferred is a reactor gauge pressure of 4.14 - 7.6 MPa (600 to 1100 psi). The above cited reactor gauge pressures refer to the total pressure within the reactor. The amount of hydrogen gas added to the process is not critical and is any amount sufficient to effect an acceptable level of hydrogenation. Generally 0.05 to 10 weight percent of hydrogen gas is considered useful in the process, based upon the total weight of the mixture, hydrogen gas and catalyst composition present in the process. Preferred is when the hydrogen gas is added to our process at 1 to 5 weight percent, on the same basis. The mixture comprising the high-boiling residue and organosilane, in the presence of hydrogen gas, is then contacted with a catalytic amount of a catalyst composition effective in promoting formation of monosilane from said high-boiling residue. We believe that our process requires a catalytic composition which simultaneously promotes redistribution of alkyl and chlorine groups between silicon atoms, hydrogenation and scission of silicon-silicon bonds and optionally silicon-carbon bonds. Therefore, the catalyst composition comprises one or more chemical entities providing the described activities to the reaction of these components. Typically, any Lewis Acid or its equivalent are used to provide redistribution activity to a catalyst composition. Examples of useful chemical entities to effect redistribution are those described in U.S. Patents 4,393,229 and 5,175,329. Specific examples of such chemical entities to effect redistribution in our 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 useful chemical entities in the catalyst composition to effect hydrogenation include those hydrogenation catalysts described by U.S. Patent 5,175,329. The chemical entity for providing hydrogenation activity to the catalyst composition herein is, for example, aluminum trichloride, antimony pentachloride, nickel, supported nickel, organometallic nickel compounds, complexed nickel salts, inorganic nickel compounds, palladium, supported palladium, organometallic palladium compounds, complexed palladium salts, inorganic palladium compounds, platinum, supported platinum, organometallic platinum compounds, complexed platinum salts and inorganic platinum compounds. Usually, those chemical entities which promote redistribution of alkyl groups and chlorine atoms between silicon atoms and those chemical entities which promote hydrogenation in our claimed process will also promote scission of silicon-silicon bonds, and optionally silicon-carbon bonds. Therefore, it is not necessary to add additional chemical entities to our 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 our catalyst composition. In the present process, it is preferred that the catalyst composition comprise a single chemical entity which promotes redistribution, hydrogenation and bond scission under our process conditions. Such chemical entities include aluminum trichloride and antimony pentachloride. The present process requires the presence of a "catalytic amount" of the catalyst composition as described above. "Catalytic amount" herein means an amount of catalyst composition sufficient to facilitate the conversion of silicon containing compounds in said high-boiling residue to monosilanes. A preferred catalytic amount of catalyst composition is that sufficient to facilitate the conversion of polysilanes, for example, methylchlorosilanes, 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, 0.01 to 20 weight percent of catalyst composition, based on the combined weight of the catalyst composition and the high-boiling residue, is useful in the present process. When aluminum trichloride or antimony pentachloride comprises the catalyst composition, a preferred weight of catalyst composition in our process is within a range of 0.5 to 5 weight percent, on the same basis. A preferred catalyst composition for use in our process is aluminum trichloride. It may be added to the process as the 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 the aluminum trichloride can be from 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 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 within a range of 275 to 425°C. Most preferred is a temperature of 300 to 350°C. Monosilanes as described by formula (2) are recovered from the present process. The monosilanes are separated by standard methods for separating liquid mixtures, for example, distillation. These monosilanes contain zero to four substituents R, where R is as previously described. The monosilane can also contain zero to three hydrogens substituted on each silicon atom. The monosilanes can further contain zero to four chlorine atoms substituted on each silicon atom. A preferred monosilane is selected from the group consisting of dimethyldichlorosilane, methyldichlorosilane and dimethylchlorosilane. The following examples are provided to illustrate the present invention. In the examples, Me represents the methyl radical. Examples The ability of aluminum trichloride to catalyze the conversion of a high-boiling residue was evaluated in a stirred-batch reactor using various methylchlorosilanes as co-feeds. The reactor was a 650 ml, pneumatically stirred, Parr Bomb reactor. Next, 0.15 mole of a high boiling residue from a direct process for the preparation of methylchlorosilanes by the reaction of methyl chloride with silicon metalloid was added to the reactor. The high-boiling residue was the fraction remaining in the bottom of a still after distilling off the monosilane fraction at 70°C. The high-boiling residue was filtered to reduce particulates. The major components of the high-boiling residue used in each run is presented in Table 1 on a weight percent basis. Major Components of High-Boiling Residue Component Run Designation C (Comparative) D (Comparative) E F G (Weight Percent) Cl 2 MeSiSiMe 3 0.4 0.7 1.2 0.5 0.5 ClMe 2 SiSiMe 2 Cl 6.2 2.2 1.1 2.2 2.4 Cl 2 MeSiSiMe 2 Cl 32.2 15.3 16.8 5.6 9.3 Cl 2 MeSiSiMeCl 2 53.5 15.3 16.9 15.4 15.2 ClMe 2 SiCH 2 SiMe 2 Cl 0.8 2.2 1.1 0.5 0.5 Cl 2 MeSiCH 2 SiMe 2 Cl 3.2 3.1 3.1 1.5 1.8 Cl 2 MeSiCH 2 SiMeCl 2 3.7 3.1 2.7 2.2 2.6 For each run, 3 to 4 weight percent of aluminum chloride, based on the high-boiling residue, was present in the high-boiling residue. Then, 0.9 to 1 mole of hydrogen gas, at a gauge pressure within a range of 6.9 - 9.7 MPa (1000 to 1400 psi) was added to the reactor for each run and the reactor was heated to 325°C. and stirred for 2.8 hours. The type and amount of organosilane fed to the reactor is as follows: Run C, none; Run D, 55 weight percent of MeSiCl 3 ; Run E, 54.2 weight percent of Me 2 SiCl 2 ; Run F, 70.3 weight percent of MeHSiCl 2 ; and Run G, 65 weight percent of Me 3 SiCl. At the end of each run a sample from the reactor was analyzed by gas chromatography (GC) using a thermal conductivity detector (TCD). The results of this analysis are reported in Table 2. The product distribution for each run is present in Table 2 as a net value calculated as the weight percent each species of monosilane represents of the total monosilanes present in the product. In Table 2, the weight percent of the high-boiling residue converted to monosilane product is reported in the row labelled "HBR Conversion". Product Conversion and Distribution Run Designation Monosilane c D E F G (Weight Percent) MeSiCl 3 22.3 0.0 25.2 39.2 3.8 Me 2 SiCl 2 27.2 28.3 1.0 45.1 64.3 Me 3 SiCl 1.2 0.7 9.6 1.6 0.0 MeHSiCl 2 34.7 48.6 43.9 0.0 9.9 Me 2 HSiCl 9.6 3.1 16.1 6.3 15.6 MeH 2 SiCl 4.3 5.6 4.1 7.6 2.4 HSiCl 3 0.8 0.0 0.7 0.3 0.0 HBR Conversion 66.2 81.2 94.4 89.5 89.5

Claims: A process for converting a high-boiling residue resulting from the reaction of an organochloride with silicon metalloid to monosilanes, the process comprising: (A) forming a mixture comprising said high-boiling residue and an organosilane selected from dimethyldichlorosilane, methyldichlorosilane, trimethylchlorosilane and tetramethylsilane; and then (B) contracting said mixture with hydrogen gas in the presence of a catalytic amount of a catalyst composition effective in promoting the formation of monosilanes from said residue at temperature within a range of 150 to 500°C. and a reactor 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 2, where the organosilane comprises 0.1 to 95 weight percent of said mixture. A process according to any of claims 1 to 3, comprising 0.05 to 10 weight percent of hydrogen gas, based on the total weight of the mixture, hydrogen gas and catalyst composition added to the process. A process according to any of claims 1 to 4, where the catalyst composition is selected from the group consisting of aluminum trichloride, antimony pentachloride and combinations thereof. A process according to claim 5, where the catalyst composition comprises 0.01 to 20 weight percent of the combined weight of the catalyst composition and said high-boiling residue. A process according to claim 5, where at least a portion of the catalytic amount of the aluminum trichloride is formed in situ. A process according to any of claims 1 to 7, 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 8 where the monosilane is selected from the group consisting of dimethyldichlorosilane, methyldichlorosilane and dimethylchlorosilane.

Inventors: BRINSON JONATHAN ASHLEY, FREEBURNE STEVEN KERRY, JARVIS ROBERT FRANK JR, BRINSON, JONATHAN ASHLEY, FREEBURNE, STEVEN KERRY, JARVIS, ROBERT FRANK, JR.
Applicants: DOW CORNING, DOW CORNING CORPORATION
Application year: 1997
Main classification: C07F7
IPC codes: C07F7, C07B61, B01J27