Aluminum trichloride catalyzed hydrogenation of high-boiling residue from direct process

Abstract: A process is described for the production of monosilanes from the high-boiling residue resulting from the reaction of organohalides with silicon metalloid in a process typically referred to as the "direct process." This process comprises forming a mixture comprising an organotrihalosilane and the high-boiling residue in the presence of both hydrogen gas and a catalytic amount of aluminum trichloride. The process results in the consumption of the organotrihalosilane rather than an increase which typically occurs in processes for hydrogenation of the high-boiling residue. At least a portion of the catalytic amount of aluminum trichloride 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 production of monosilanes from the high-boiling residue resulting from the reaction of organohalides with silicon metalloid in the "direct process." Our process comprises forming a mixture comprising an organotrihalosilane and the high-boiling residue in the presence of both hydrogen gas and a catalytic amount of aluminum trichloride. This process results in the consumption of the organotrihalosilane rather than an increase which typically occurs in processes for hydrogenation of the high-boiling residue. At least a portion of the catalytic amount of aluminum trichloride may be formed in situ during conduct of the direct process and 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 chloromonosilanes include dimethyldichlorosilane, methyltrichlorosilane and trimethylchlorosilane, there is obtained a residue which boils above the organohalosilanes, that is above 70°C. This residue is hereinafter referred to as "high-boiling residue." The "direct process" is well 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. This 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 can constitute as much as five weight percent of the resultant product. Therefore, it is desirable to convert the high-boiling residue into commercially desirable products to both reduce waste disposal and to improve raw material utilization. U.S. Patent 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. U.S. Patent 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. This patent states that the disilane may be part of a mixture from the direct process. It further reports that when the disilane was a methylchlorosilane, the resulting product contained four to 28 weight percent of methyltrichlorosilane. Generally, organotrihalosilanes such as methyltrichlorosilane have limited commercial utility and for this reason the claimed process has limited usefulness. U.S. Patent 4,079,071 discloses a process for preparing high yields of hydrosilanes by reacting methylchloropolysilanes with hydrogen gas under pressure at a temperature of from 25°C. 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 include copper metal, copper salts and complexes of copper salts with organic ligands. In some cases, it reports that up to 29 weight percent of methyltrichlorosilane was formed. U.S. Patent 4,393,229 claims 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 if desired. U.S. Patent 5,175,329 shows a process for the production of organosilanes from high-boiling residues of 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. 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 while resulting in a net consumption of organotrihalosilane. We have unexpectedly found that this objective is met by contacting the high-boiling residue with an organotrihalosilane, hydrogen gas and a catalytic amount of aluminum trichloride at a temperature within a range of 150°C. to 500°C. Furthermore, at least a portion of the catalytic amount of aluminum trichloride can be formed in situ during conduct of the direct process and isolation of the resulting monosilanes. The present invention provides a process for converting a high-boiling residue resulting from the reaction of an organohalide with silicon metalloid to monosilanes. This process comprises (A) forming a mixture comprising a high-boiling residue resulting from the reaction of an organohalide with silicon metalloid and an organotrihalosilane described by formula RSiX 3 (1) where R is selected from a group consisting of alkyls comprising one to six carbon atoms, aryls, alkoxys comprising one to six carbon atoms, trimethylsilyl and trifluoropropyl and X is halogen; and (B) contacting the mixture with hydrogen gas at a gauge pressure of 0.345 to 69.0 MPa (50 to 10,000 psi) in the presence of a catalytic amount of aluminum trichloride at a temperature within a range of 150°C. to 500°C. The present process can further comprise (C) recovering organosilanes of formula R y H z SiX 4-y-z (2) where R and X are as described above, y=0 to 3, z=0 to 3 and y+z=0 to 3. The present process may be run in any standard pressurizable reactor suitable for contact with halosilanes. The process is run as either a batch process or a continuous process. The process may be run, for example, in a stirred-bed reactor, continuous stirred-tank reactor, a bubble-column reactor, a trickle-bed reactor or a plug-flow reactor. The present process converts a high-boiling residue resulting from the reaction of an organohalide with silicon metalloid to useful monosilanes. In a typical process for reacting an organohalide with silicon metalloid, the process is conducted at a temperature of 300°C. to 350°C. in the presence of suitable catalysts and gaseous product, feed materials and fine particulates are continuously removed from the process. The removed materials are subsequently distilled to recover organohalosilanes, 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 organohalosilanes 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 a methyl group or chlorine atom and the disilane contains two to four methyl groups per molecule; 15 to 25 wt% silmethylenes described by formula Q 3 SiCH 2 SiQ 3 , where Q is as above and the silmethylene contains two to four methyl groups per molecule; silalkylenes described by formula Q 3 Si(SiQ 2 ) a (CH 2 ) b (SiQ 2 ) c SiQ 3 , where Q is also as above, the silalkylene contains two to four methyl groups per molecule, a=0 to 4, b=1 to 3, c=0 to 4 and a+c≥1; 5 to 15 wt% of other high-boiling silicon-containing compounds; catalyst carry 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 and compounds thereof. While it is known that high-boiling residues can be treated with a hydrogenation catalyst and hydrogen gas to produce monosilanes, an undesirable consequence of this process is the production of organotrihalosilanes which have limited commercial utility and are therefore an unsuitable product. U.S. Patent 5,175,329 teaches a hydrogenation process for converting disilanes into monosilanes that results in a net consumption of organotrihalosilane. However, both a hydrogenation catalyst and a redistribution catalyst are required to achieve this result. Unexpectedly, we have found a process for converting a high-boiling residue to monosilanes that results in the consumption of organotrihalosilane and which requires only aluminum trichloride as a catalyst. However, we do not intend to preclude the possible presence of other materials in the high-boiling residue which also may be active as a catalyst, co-catalyst, promotor or the like in our claimed process. In our process, a mixture of the high-boiling residues described above is formed with an organotrihalosilane as described by formula (1). The mixture is formed external to the reactor and added to the reactor or it may be formed by adding the individual components to the reactor. The organotrihalosilane contains one substituent R, where R is selected from a group consisting of alkyls comprising one to six carbon atoms, aryls, alkoxys comprising one to six carbon atoms, trimethylsilyl and trifluoropropyl. 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 organotrihalosilane contains three halogen substituents, X, where X can be any halogen. Preferred is when X is chlorine. The organotrihalosilane is, for example, methyltrichlorosilane, ethyltrichlorosilane, benzyltrichlorosilane, methyltribromosilane, methyl-triiodosilane and methyltrifluorosilane. Preferred is when the organotrihalosilane is methyltrichlorosilane. The weight percent of organotrihalosilane in the mixture with the high-boiling residue is not critical to the present process. Generally, a mixture where the organotrihalosilane is 0.1 to 95 weight percent of the total mixture is considered useful. Preferred is where the organotrihalosilane is 30 to 50 weight percent of the mixture. The mixture is then contacted with hydrogen gas at a gauge pressure of 0.345 to 69.0 MPa (50 to 10,000 psi). Preferred is a hydrogen gas gauge pressure of 2.1 to 10.3 MPa (300 to 1500 psi). More preferred is a pressure of 4.1 to 7.6 MPa (600 to 1100 psi). The mixture, in the presence of hydrogen gas, is contacted with a catalytic amount of aluminum trichloride. By "catalytic amount" we mean an amount of aluminum trichloride sufficient to facilitate the conversion of silicon containing compounds in the high-boiling residue to monosilanes. A preferred catalytic amount of aluminum trichloride is that sufficient to facilitate the conversion of polysilanes, such as methylchlorodisilanes, silmethylenes and silalkylenes in the high-boiling residue to monosilanes. Generally, 0.01 to 10 weight percent of aluminum trichloride, based on the combined weight of the aluminum trichloride and the high-boiling residue, is considered useful in the present process. Preferred is 0.1 to 2.0 weight percent of aluminum trichloride on the same basis. 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 isolation of the monosilane fraction to form the high-boiling fraction. 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 organohalide feed. The catalytic amount of aluminum trichloride is 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°C. to 500°C. Preferred is a temperature of 275°C. to 425°C. Most preferred is 300°C. to 350°C. Organosilanes 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 can contain zero to three substituents R, where R is as above. The monosilane can contain zero to three hydrogen atoms substituted on each silicon atom. They can also contain one to four halogens substituted on each silicon atom. A preferred monosilane is selected from dimethyldichlorosilane or methyldichlorosilane. Example 1 The ability of aluminum trichloride to catalyze the hydrogenation of a high-boiling residue (HBR) was evaluated in a stirred-batch reactor. The reactor was a 450 ml, pneumatically stirred, Parr Bomb reactor. An amount of 0.15 mole (Mol) 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 and adsorbed twice with equal volumes of activated carbon. Elemental metals analysis of the adsorbed high-boiling residue showed the presence of 0.0082 weight percent aluminum. A typical composition along with a range for major silicon containing compounds found in the filtered high-boiling residue is provided in Table 1. Composition of High-Boiling Residue Component Weight % Range (Wt%) MeCl 2 SiOSiCl 2 Me 2 0-5 Me 2 ClSiSiClMe 2 3 0-9 Me 2 ClSiSiMeCl 2 22 10-40 MeCl 2 SiSiMeCl 2 32 20-50 Me 2 ClSiCH 2 SiClMe 2 2 0-9 Me 2 ClSiCH 2 SiCl 2 Me 5 0-15 MeCl 2 SiCH 2 SiCl 2 Me 10 5-20 Me 3 SiCl 0 0-4 MeHSiCl 2 0 0-2 EtMeSiCl 2 2 0-5 iPrMeSiCl 2 1 0-5 Non-elutables 21 10-40 In Table 1, Me is methyl, Et is ethyl, iPr is isopropyl and non-elutables are those materials which do not elute from a GC column. Two runs were made. The first run was a base run without the addition of aluminum trichloride. In the second run, 0.55 wt% of aluminum chloride, based on high-boiling residue weight, was added to the reactor. The amount of hydrogen gas and methyltrichlorosilane (Mono) added to the reactor for each run is provided in Table 2. For each run, the reactor was heated to 325°C. and stirred for 2.8 hours. 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 also reported in Table 2. In Table 2, the "Total Silane Yield" is calculated as the weight silane in the product, minus the weight of methyltrichlorosilane added to the process and the difference is divided by the weight of HBR added to the process, with the dividend multiplied by 100. "HBR Conversion" is reported as weight percent of HBR converted to product. Further reported in Table 2 is the weight percent of silmethylene conversion, disilane conversion and high-boiling species conversion to other products. The weight percent conversion of methyltrichlorosilane to other products is reported in Table 2 in the row labelled "Mono. Conv." A positive number for methyltrichlorosilane conversion indicates a consumption of methyltrichlorosilane in the process. The product distribution for each run is listed in Table 2 in the columns labelled "Net Monosilane Product Distribution." The net monosilane product distribution is calculated as the weight percent each species of monosilane represents of the total monosilanes present in the product after subtracting out the amount of methyltrichlorosilane present in the product. Aluminum Trichloride Catalyzed Hydrogenation of HBR Run 1 Run 2 Temp. (°C) 325 325 Pressure (MPa) 9.2 (1333 psi) 8.6 (1252 psi) Time (h) 2.8 2.8 Catalyst (wt.%) - 0.55 Mono (mole) 0.15 0.15 HBR (mole) 0.15 0.18 Hydrogen (mole) 0.95 0.92 Total Silane Yield 17.3 83.2 HBR Conv. 36.7 91.0 Disilane Conv. 37.0 98.9 Silmethylene Conv. 14.1 54.9 Non-elutables Conv. 32.3 70.9 Mono Conv. 0.0 18.6 Net Monosilane Product Distribution (Wt%) MeH 2 SiCl 7.0 8.9 Me 4 Si 0.0 0.0 HSiCl 3 1.4 4.0 Me 2 HSiCl 9.5 3.1 MeHSiCl 2 48.8 57.1 SiCl 4 0.5 0.1 Me 3 SiCl 2.2 0.9 Me 2 SiCl 2 30.2 25.9 Example 2 The effect of in situ formed aluminum trichloride on the hydrogenation of a high-boiling residue in a stirred-batch reactor was evaluated. The reactor was the same as described in Example 1. An amount of 0.13 mole of a high-boiling residue resulting from the reaction of methyl chloride with silicon metalloid and boiling above 70°C. was added to the reactor. The high-boiling residue was filtered to reduce particulates and had a composition similar to that described in Table 1. For comparison purposes, a sample of the high-boiling residue was adsorbed twice with equal volumes of activated characoal and the testing of this material is reported in Table 3 as Run 3. After adsorption, the elemental analysis of the high-boiling residue showed the presence of 0.0003 weight percent aluminum. Elemental analysis of the test sample (Run 4) showed the presence of 0.76 weight percent aluminum. The conditions under which the test and comparison runs were made are described in Table 3. The product from the two runs was analyzed by GC-TCD. These results are also reported in Table 3. The headings for Table 3 are as described for Table 2. In Situ Formed Aluminum Trichloride Catalyzed Hydrogenation of HBR Run 3 Run 4 Temp. (°C) 325 325 Pressure (MPa) 7.2 (1046 psi) 7.3 (1056 psi) Time (h) 1.0 1.0 Mono (mole) 0.13 0.13 HBR (mole) 0.13 0.13 Hydrogen (mole) 0.60 0.60 Total Silane Yield 0.0 100.0 HBR Conv. 39.0 89.6 Disilane Conv. 25.9 93.1 Silmethylene Conv. 9.6 30.1 Mono Conv. 5.2 28.8 Net Monosilane Product Distribution (Wt%) MeH 2 SiCl 0.0 5.6 Me 4 Si 0.0 0.0 HSiCl 3 0.0 3.2 Me 2 HSiCl 0.0 3.1 MeHSiCl 2 5.5 48.6 SiCl 4 3.4 0.3 Me 3 SiCl 8.8 0.7 Me 2 SiCl 2 82.3 38.3

Claims: A process for converting a high-boiling residue resulting from the reaction of an organohalide with silicon metalloid to monosilanes, the process comprising (A) forming a mixture comprising said high-boiling residue and an organotrihalosilane described by formula RSiX 3 , where R is selected from a group consisting of alkyls comprising one to six carbon atoms, aryls, alkoxys comprising one to six carbon atoms, trimethylsilyl and trifluoropropyl and X is halogen; and (B) contacting the mixture with hydrogen gas at a gauge pressure of 0.345 to 69.0 MPa (50 to 10,000 psi) in the presence of a catalyst essentially consisting of aluminum trichloride at a temperature within a range of 150°C. to 500°C. A process according to claim 1 further comprising (C) recovering monosilanes of formula R y H z SiX 4-y-z where R and X are as previously described, y=0 to 3, z=0 to 3 and y+z=0 to 3. A process according to claim 2 where the high-boiling residue has a boiling point above 70°C. and is a distillation fraction resulting from the distillation of the reaction product of methyl chloride with silicon metalloid. A process according to claim 3 where the monosilane is selected from dimethyldichlorosilane or methyldichlorosilane. A process according to claim 1 where the organotrihalosilane is 0.1 to 95 weight percent of the mixture comprising the high-boiling residue and the organotrihalosilane. A process according to claim 1 where 0.01 to 10 weight percent of aluminum trichloride, based on the combined weight of the aluminum trichloride and the high-boiling residue, is present in the process. A process according to claim 1 where a portion of the catalytic amount of aluminum trichloride is formed in situ during formation of the high-boiling residue.

Inventors: FERGUSON STEPHEN PAUL, JARVIS ROBERT FRANK JR, NAASZ BRYAN MICHAEL, OLTMANNS KIMBERLEY K, WARRICK GORDON L, WHITELEY DARREL LESLIE, FERGUSON, STEPHEN PAUL, JARVIS, ROBERT FRANK JR., NAASZ, BRYAN MICHAEL, OLTMANNS, KIMBERLEY K., WARRICK, GORDON L., WHITELEY, DARREL LESLIE
Applicants: DOW CORNING, DOW CORNING CORPORATION
Application year: 1995
Main classification: C07F7
IPC codes: B01J27, C07F7, C07B61