Process for converting polymeric silicon containing compounds to monosilanes

Abstract: A process for converting a high-boiling fraction, resulting from the reaction of methyl chloride with silicon metalloid in a process typically referred to as the "direct process", to monosilanes. Said process comprises forming a mixture comprising a low-boiling fraction comprising monosilanes and a high-boiling fraction comprising polymeric silicon containing compounds and contacting the mixture with hydrogen gas in the presence of a catalyst effective in promoting conversion of the polymeric silicon containing compounds to monosilanes. The process results in conversion of the high-boiling fraction to monosilanes. At least a portion of the catalyst present may be formed in situ during conduct of the direct process and isolation of the fractions.

Description: The present invention is a process for converting a high-boiling fraction, resulting from the reaction of methyl chloride with silicon metalloid in a process referred to as the "direct process", to monosilanes. Said process comprises forming a mixture comprising a low-boiling fraction comprising monosilanes and a high-boiling fraction comprising polymeric silicon containing compounds and contacting the mixture with hydrogen gas in the presence of a catalyst effective in promoting conversion of the polymeric silicon containing compounds to monosilanes. This process results in conversion of the high-boiling fraction to monosilanes. At least a portion of the catalyst required for conduct of the process may be formed in situ during conduct of the direct process and isolation of the fractions. In the preparation of methylchlorosilanes by the direct process, a complex mixture is formed which is typically distilled to separate methylchlorosilanes from other components present in the mixture. After the methylchlorosilanes are distilled from the mixture, monosilane and disilane by-product fractions remain. The monosilane fraction which boils below 45°C. is hereinafter referred to as "low-boiling fraction" and the disilane fraction which boils above 80°C. is hereinafter referred to as "high-boiling fraction." In current commercial operations for performing the direct process, the high-boiling fraction alone can constitute as much as five weight percent of the resultant product. Therefore, it is desirable to convert the high-boiling fraction along with the low-boiling fraction into commercially desirable products to reduce by-product disposal and to improve raw material utilization. The "direct process" is well described in the patent literature in U.S. Patents 2,380,995 and 2,488,487. The high-boiling fraction remaining after the overhead distillation of the monosilanes is a complex mixture comprising higher boiling silicon containing compounds which have SiSi, SiOSi and SiCSi linkages in the molecules. The high-boiling fraction may also contain particulate silicon and metals or compounds thereof. Typical high-boiling residues obtained from the direct process distillation product are described in U.S. Patents 2,598,435 and 2,681,355. U.S. Patent 2,606,811 teaches a hydrogenation process where a compound containing a halogen and the Si-Si bond are heated to a temperature of at least 300°C. in the presence of hydrogen. The resultant products are monosilanes. U.S. Patent 3,639,105 describes a process where hydrosilanes are produced by contacting a disilane with hydrogen gas under pressure and heating the mixture in the presence of a transition metal catalyst such as palladium on charcoal. Said disilane may be part of a mixture from the direct process. When the disilane was a methylchlorodisilane, the resulting product contained four to 28 weight percent methyltrichlorosilane. Generally, organotrihalosilanes such as methyltrichlorosilane have limited commercial usefulness and thus this process has limited usefulness. U.S. Patent 4,079,071 claims a process for preparing hydrosilanes in high yields by reacting methylchloropolysilanes with hydrogen gas under pressure at a temperature of 25°C. to 350°C. in the presence of a copper catalyst. The methylchloropolysilanes can be those typically created as direct process by-products. Useful copper catalysts include copper metal, copper salts and complexes of copper salts with organic ligands. In some cases, up to 29 weight percent methyltrichlorosilane was formed. U.S. Patent 4,393,229 teaches 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 said process when used with a hydrosilane reaction promoter. The resulting halogen-rich polysilanes can, in a separate step, be cleaved to form monosilanes. U.S. Patent 5,175,329 describes 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 this 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 reveals a process for the production of monosilanes from the high-boiling residue resulting from the "direct process." Said 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. This process results in consumption of the organotrihalosilane and conversion of the high-boiling residue to useful monosilanes. The present invention is a process for converting a high-boiling fraction, resulting from the reaction of methyl chloride with silicon metalloid in a process typically referred to as the direct process, to monosilanes. Said process comprises forming a mixture comprising a low-boiling fraction comprising monosilanes and a high-boiling fraction comprising polymeric silicon containing compounds and contacting the mixture with hydrogen gas in the presence of a catalyst effective in promoting conversion of the polymeric silicon containing compounds to monosilanes. This process results in conversion of the high-boiling fraction to monosilanes. At least a portion of the catalyst present in the process may be formed in situ during conduct of the direct process and isolation of the fractions. The present invention is a process for converting a low-boiling fraction and a high-boiling fraction resulting from the reaction of methyl chloride with silicon metalloid to form monosilanes. Said process comprises forming a mixture comprising a low-boiling fraction comprising monosilanes having a boiling point below 45°C., resulting from the reaction of methyl chloride with silicon metalloid; and a high-boiling fraction comprising polymeric silicon containing compounds having a boiling point above 80°C., resulting from the reaction of methyl chloride with silicon metalloid. The mixture is contacted with hydrogen gas at a pressure of 345 kPa to 68,900 kPa in the presence of a catalyst effective in promoting conversion of the polymeric silicon containing compounds to monosilanes at a temperature of 150°C. to 500°C. The present process may be run in any standard pressurizable reactor suitable for contact with chlorosilanes. The process may be run as a batch process or as a continuous process. The process may be run in 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 fraction, resulting from the reaction of methyl chloride with silicon metalloid, to useful monosilanes. The direct process for reacting methyl chloride with silicon metalloid produces a mixture of methylchlorosilanes, methylchlorodisilanes and silmethylenes. This mixture is then distilled to produce relatively pure dimethyldichlorosilane, methyltrichlorosilane, trimethylchlorosilane and methyldichlorosilane. The remaining by-product stream can be further distilled to produce a low-boiling fraction and a high-boiling fraction. The low-boiling fraction comprises monosilanes having a boiling point below 45°C. A typical weight percent composition of a low-boiling fraction resulting from the reaction of methyl chloride with silicon metalloid is 4 wt% methylchloride, 1 wt% methylchlorosilane, 18 wt% tetramethylsilane, 13 wt% trichlorosilane, 41 wt% dimethylchlorosilane, 0.4 wt% methyldichlorosilane and 23 wt% isopentane. The high-boiling fraction comprises polymeric silicon containing compounds having a boiling point above 80°C. The present process is useful for converting this high-boiling fraction to useful monosilanes. The polymeric silicon containing compounds include disilanes, namely, Me 2 ClSiSiClMe 2 , Me 2 ClSiSiMeCl 2 , MeCl 2 SiSiMeCl 2 and silmethylenes, namely, Me 2 ClSiCH 2 SiClMe 2 , Me 2 ClSiCH 2 SiCl 2 Me and MeCl 2 SiCH 2 SiCl 2 Me. A typical composition for a high-boiling fraction comprises 50-60 wt% of disilanes of formula Si 2 Q 6 , where each Q is independently selected from a group consisting of methyl and 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+c≥1; 5 to 15 weight percent of other high-boiling silicon-containing compounds; catalysts carried over from the direct process such as copper and compounds of copper; particulate containing silicon; and low levels of metals such as aluminum, calcium, iron and compounds thereof. A typical weight percent composition of a high-boiling fraction resulting from the reaction of methyl chloride with silicon metalloid is 4 wt% Me 2 ClSiSiClMe 2 , 31 wt% Me 2 ClSiSiMeCl 2 , 46 wt% MeCl 2 SiSiMeCl 2 , 1 wt% Me 2 ClSiCH 2 SiClMe 2 , 3 wt% Me 2 ClSiCH 2 SiCl 2 Me, 7 wt% MeCl 2 SiCH 2 SiCl 2 Me and 9 wt% other. In the mixture of the present process, the ratio of low-boiling fraction to the high-boiling fraction is not critical. Normally, a mixture where the low-boiling fraction is 0.1 to 95 weight percent of the mixture is considered useful. Preferred is where the low-boiling fraction is 30 to 50 weight percent of the mixture. The mixture is contacted with hydrogen gas at a pressure of 345 kPa to 68,900 kPa. Preferred is a hydrogen gas pressure of 2,000 to 10,000 kPa. Most preferred is a hydrogen gas pressure of 4,000 to 7,500 kPa. The catalysts useful in the present process are those catalysts effective in promoting conversion of the polymeric silicon containing compounds to monosilanes. Generally, Lewis Acids or equivalents may be used as a catalyst. Catalysts effective in promoting conversion of the polymeric silicon containing compounds to monosilanes include aluminum trichloride, iron trichloride, antimony pentachloride, zirconium tetrachloride, potassium aluminum tetrachloride, quaternary phosphonium halides, quaternary ammonium halides, ammonium halides, cuprous chloride, boric acid and boron halides. The catalyst can be heterogeneous catalysts including alumina, aluminosilicates, zeolites silica and activated carbon. A preferred catalyst is aluminum trichloride. The amount of catalyst useful in the present process is any amount effective in promoting conversion of the polymeric silicon containing compounds to monosilanes. Preferably, the catalyst concentration is 0.05 to 15 weight percent of the high-boiling fraction comprising the polymeric silicon containing compounds. Most preferable, the catalyst concentration is 0.5 to 5 weight percent of the high-boiling fraction comprising the polymeric silicon containing compounds. The catalyst is added to the mixture of low-boiling fraction and high-boiling fraction as a separate component or, preferably, is present from conducting the direct process. When the catalyst is present in the mixture from the direct process, additional catalyst may be added to said mixture if required to promote conversion of the polymeric silicon containing compounds to monosilanes. Preferably, the additional catalyst added to the mixture is the same as the catalyst present from the direct process. However, a catalyst which is different than the catalyst present from the direct process may be added. When the catalyst is aluminum trichloride, the catalyst concentration is 0.05 to 15 weight percent of the high-boiling fraction comprising the polymeric silicon containing compounds. Preferably, the aluminum trichloride concentration is 0.5 to 5 weight percent of the high-boiling fraction comprising the polymeric silicon containing compounds. The aluminum trichloride is added to the process as the compound or formed in situ by the addition of materials that form aluminum trichloride. The source of the aluminum and chlorine necessary to form the aluminum trichloride is the raw materials used in the direct process, particularly the silicon metalloid and methyl chloride feed. The aluminum trichloride catalyst can be a combination of added aluminum trichloride and in situ formed aluminum trichloride remaining in the mixture as isolated from the direct process. The present process is conducted at a temperature of 150°C to 500°C. Preferred is a temperature of 275°C. to 425°C. Most preferred is a temperature of 300°C. to 350°C. Monosilanes described by formula Me a H b SiCl 4-a-b are recovered from the present process; where Me is methyl, a=0 to 4, b=0 to 3 and a+b=0 to 4. Preferred monosilanes are dimethyldichlorosilane and methyldichlorosilane. The monosilanes are separated by standard methods for separating liquid mixtures, for example, distillation. The following examples are provided to illustrate the present invention. Example 1 The ability to convert a high-boiling fraction with in situ formed aluminum trichloride as catalyst was evaluated in a stirred-tank batch reactor. The reactor was a 600 ml, pneumatically stirred, Parr Bomb reactor. A mixture comprising 37 grams of low-boiling distillation fraction (LBF) and 56.5 grams of a high-boiling distillation fraction (HBF), from the direct process for the preparation of methylchlorosilanes by the reaction of methyl chloride with silicon metalloid, was added to the reactor. The aluminum trichloride present in the mixture was in situ formed during preparation and isolation of monosilanes. Hydrogen gas at 2758 kPa was added to the reactor while stirring and the reactor was heated to 340°C. for 1.5 hours. A sample from the reactor was analyzed by gas chromatography (GC) using a thermal conductivity detector (TCD). The LBF components, HBF components, initial component weight percent and analysis results indicated by the final component weight percent are reported in Table 1. Mixture Composition Components Initial Component Weight % Final Component Weight % LBF MeCl 1.48 0.05 MeH 2 SiCl 0.35 3.88 Me 4 Si 7.05 1.21 HSiCl 3 5.16 1.03 Me 2 HSiCl 16.33 4.97 MeHSiCl 2 0.15 17.32 Me 3 SiCl 0 4.42 MeSiCl 3 0 6.73 Me 2 SiCl 2 5 24.63 HBF Me 2 ClSiSiClMe 2 3.39 0.17 Me 2 ClSiSiMeCl 2 16.84 0 MeCl 2 SiSiMeCl 2 21.73 0 Me 2 ClSiCH 2 SiClMe 2 0 0.17 Me 2 ClSiCH 2 SiCl 2 Me 5 1.25 MeCl 2 SiCH 2 SiCl 2 Me 0 1.73 Example 2 The ability to convert a high-boiling fraction with in situ formed aluminum trichloride as catalyst was evaluated in a stirred-tank batch reactor. The reactor was a 600 ml, pneumatically stirred, Parr Bomb reactor. A mixture comprising 40.6 grams of low-boiling distillation fraction (LBF) and 47.4 grams of a high-boiling distillation fraction (HBF), from the direct process for the preparation of methylchlorosilanes by the reaction of methyl chloride with silicon metalloid, was added to the reactor. The aluminum trichloride present in the mixture was in situ formed during preparation and isolation of monosilanes. Hydrogen gas at 2758 kPa was added to the reactor while stirring and the reactor was heated to 340°C. for 1.5 hours. A sample from the reactor was analyzed by gas chromatography (GC) using a thermal conductivity detector (TCD). The LBF components, HBF components, initial component weight percent and analysis results indicated by the final component weight percent are reported in Table 2. Mixture Composition Components Initial Component Weight % Final Component Weight % LBF MeCl 2.12 0.05 MeH 2 SiCl 0.54 4.91 Me 4 Si 9.94 0.09 HSiCl 3 7.04 0.97 Me 2 HSiCl 22.53 7.02 MeHSiCl 2 0.21 20.55 Me 3 SiCl 0 3.63 MeSiCl 3 0 7.70 Me 2 SiCl 2 0 31.50 HBF Me 2 ClSiSiClMe 2 1.61 0.10 Me 2 ClSiSiMeCl 2 13.77 0.02 MeCl 2 SiSiMeCl 2 20.47 0 Me 2 ClSiCH 2 SiClMe 2 0.31 0.14 Me 2 ClSiCH 2 SiCl 2 Me 1.55 1.22 MeCl 2 SiCH 2 SiCl 2 Me 3.29 1.70

Claims: A process for converting a high-boiling fraction resulting from the reaction of methyl chloride with silicon metalloid to monosilanes, the process comprising: (A) forming a mixture comprising a low-boiling fraction comprising monosilanes having a boiling point below 45°C. resulting from the reaction of methyl chloride with silicon metalloid and a high-boiling fraction comprising polymeric silicon containing compounds having a boiling point above 80°C. resulting from the reaction of methyl chloride with silicon metalloid; and (B) contacting said mixture with hydrogen gas at a pressure of 345 kPa to 68,900 kPa in the presence of a catalyst effective in promoting conversion of the polymeric silicon containing compounds to monosilanes at a temperature of 150 to 500°C. A process according to claim 1 further comprising (C) recovering monosilanes selected from dimethyldichlorosilane or methyldichlorosilane. A process according to claim 1 where the catalyst is aluminum trichloride, iron trichloride, antimony pentachloride, zirconium tetrachloride, potassium aluminum tetrachloride, quaternary phosphonium halides, quaternary ammonium halides, ammonium halides, cuprous chloride, boric acid and boron halides.

Inventors: CRUM BRUCE ROBERT, FREEBURNE STEVEN KERRY, WOOD LARRY HERBERT, CRUM, BRUCE ROBERT, FREEBURNE, STEVEN KERRY, WOOD, LARRY HERBERT
Applicants: DOW CORNING, DOW CORNING CORPORATION
Application year: 1999
Main classification: C07F7
IPC codes: C07F7