Preparation of alkoxysilanes

Abstract: A process is described for preparing alkoxysilanes by the contact of silicon with a solution of hydrogen fluoride in a liquid primary or secondary alcohol. The hydrogen fluoride activates the silicon causing the formation of alkoxysilanes to occur more rapidly and can provide selectivity for the production of dialkoxysilanes and trialkoxysilanes. The process may be run in the presence of copper or a copper compound as a catalyst. The process is advantageously run using atomized silicon.

Description: The present invention is a process for preparing alkoxysilanes by the contact of silicon with a solution of hydrogen fluoride in a liquid primary or secondary alcohol. The hydrogen fluoride activates the silicon causing the formation of alkoxysilanes to occur more rapidly and can provide selectivity for the production of dialkoxysilanes and trialkoxysilanes. The process may be run in the presence of copper or a copper compound as a catalyst. The process is advantageously run using atomized silicon. Silicon is known to react with methanol in the presence of a strong base to form tetramethoxysilane in near quantative yield. For example, Japanese Patent No. J6-3041-482-A, February 22, 1988, describes a process where silicon is reacted with methanol in the presence of sodium methoxide to form tetramethoxysilane. The process was run at 140°C., for one hour, with the reported yield of tetramethoxysilane being about 95%, based on silicon added to the process. This process does not produce significant quantities of dialkoxysilanes or trialkoxysilanes. EPA 0 280 517 describes a process for producing trialkoxysilanes comprising an activation step where metallic silicon and copper catalyst are activated, a reaction step where an alcohol is contacted with metallic silicon and the copper catalyst so as to allow it to react with the metallic silicon, and a purification step. A halogenide is introduced into, and allowed to contact with, the reaction system and/or mixture in one or more of the process steps. The objective of the present invention is to provide a process that can provide significant yields of dialkoxysilanes and trialkoxysilanes at faster rates and shorter reaction times than those previously reported in the literature. This objective is achieved by using a solution of hydrogen fluoride in alcohol to activate the silicon and make the silicon more reactive with the alcohol. The reaction of the activated silicon with the alcohol is further facilitated by the addition of copper or copper compounds to the process as a catalyst. In addition, particulate silicon prepared by atomization has been found to be considerably more reactive in the process than ground particulate silicon. The alkoxysilanes obtained by the present process contain reactive alkoxy groups and in some cases hydrogen. These reactive constituents make the alkoxysilanes useful as intermediates for preparing various organic silicon compounds or hydride functional polysiloxanes. The present invention is a process for preparing alkoxysilanes by the contact of silicon with a solution of hydrogen fluoride in a primary or secondary alcohol. The hydrogen fluoride activates the silicon causing the formation of alkoxysilanes to occur more rapidly and can provide selectivity for the production of dialkoxysilanes and trialkoxysilanes. The process may be run in the presence of copper or a copper compound as a catalyst. The process is advantageously run using atomized silicon. The present invention is a process for preparing alkoxysilanes of formula (RO) n SiH 4-n , where R is a primary or secondary alkyl radical and n=2, 3 or 4. The process comprises: contacting silicon with a solution of hydrogen fluoride in an alcohol of formula ROH, where R is as previously described. The process is conducted at a temperature within a range of about 50°C. to 300°C. In an additional embodiment of the present invention, the process may be conducted in the presence of a catalyst selected from the group consisting of copper and copper compounds. Alkoxysilanes which can be prepared by the present process include, for example, dimethoxysilane, trimethoxysilane, tetramethoxysilane, diethoxysilane, triethoxysilane, tetraethoxysilane, diisopropoxysilane, triisopropoxysilane, tetraisopropoxysilane, dipropoxysilane, tripropoxysilane, tetrapropoxysilane, dibutoxysilane, tributoxysilane, diisobutoxysilane, triisobutoxysilane, dioctoxysilane, trioctoxysilane, didodecoxysilane and tridodecoxysilane. The preferred alkoxysilane products are dialkoxysilanes and trialkoxysilanes. Most preferred, is when the alkoxysilane is dimethoxysilane and trimethoxysilane. The alkoxysilanes are formed by contacting silicon with a solution of hydrogen fluoride in an alcohol. It is preferred that the silicon be of metallurgical grade or greater purity. By metallurgical grade is meant, a silicon mass which contains greater than about 98 percent elemental silicon. The silicon may be in the form of a powder, particles, flakes, chips or chunks. The physical size and shape of the silicon is not important to the present invention and can be varied within wide limits without significant effect on the process. Preferred is a particulate silicon with a particle size of less than about 60 mesh. Larger particle sizes may be used, but the reduction in available surface area may reduce the rate of product formation. The lower range for particle size is limited only by the ability to produce and handle the particulate silicon. In general, particle sizes greater than about 10µ in diameter are considered useful for the process. The silicon may be alloyed with minor amounts of metals, for example, copper, aluminium, calcium, phosphorous and zinc. However, no particular advantage is perceived in the instant process for silicon alloys. Atomized silicon prepared by the method described by Liles, U.S. Patent No. 4,701,289, issued October 20, 1987, has been found to be particularly active in the described process. A preferred atomized silicon is prepared by the method of Liles, where the volatile coolant is water. The inventors believe that the rapid cooling of the atomized silicon creates increased crystal defects which serve as reactive sites for the reaction of silicon with alcohol. The silicon is contacted with a solution of hydrogen fluoride in liquid alcohol. The solution of hydrogen fluoride a liquid alcohol can be formed by dissolving hydrogen fluoride gas in the liquid alcohol or by adding an acid hydrofluoride salt to the alcohol. By acid hydrofluoride salt, is meant any salt which upon dissolution in the alcohol forms hydrogen fluoride in solution. The acid hydrofluoride salt can be, for example, NH 4 F·HF, NaF·HF and KF·HF. Preferred is a solution formed by dissolving hydrogen fluoride gas in a liquid alcohol. The inventors have found that when the solution of hydrogen fluoride in alcohol is formed by this method, the initial induction period observed for the process is shortened, in comparison to when an acid hydrofluoride salt is used to form the solution and dialkoxysilane is a product. The inventors postulate that the hydrogen fluoride activates the silicon by removing the oxide coating that forms on silicon, thus making the silicon more readily available for reaction with the alcohol. Therefore, the concentration of hydrogen fluoride is not considered critical and can be varied within a wide range with minimal effect on the rate of the instant process. A useful concentration of hydrogen fluoride in the alcohol is in a range of about 0.01 N to 6.0 N. A preferred concentration of hydrogen fluoride in the alcohol is within a range of about 0.10 N to 1.0 N. The alcohol can be any alcohol of formula ROH, where R is a primary or secondary alkyl radical. Preferred is when R is a primary or secondary alkyl radical of less than 12 carbon atoms. More preferred is when R is a primary or secondary alkyl radical of less than seven carbon atoms. The alcohol can be, for example, methanol, ethanol, propanol, isopropanol, butanol, isobutanol, octanol and dodecanol. The preferred alcohol is selected from the group consisting of methanol and ethanol, with methanol being most preferred. Contact of the silicon with the solution of hydrogen fluoride in a liquid alcohol can be effected by standard means for contacting solids and liquids under pressure. The process can be conducted, for example, in a fixed-bed, a stirred-bed, a fluidized-bed or a vibrating-bed reactor. The process can be conducted as a batch process or a continuous process. An effective temperature for effecting the reaction of silicon with methanol containing hydrogen fluoride in solution is within a range of about 50°C. to 300°C. A preferred temperature range is about 100°C. to 200°C. The most preferred temperature is in a range of about 130°C. to 170°C. In the described process, copper and copper compounds can act as a catalyst to facilitate the reaction of the silicon with the hydrogen fluoride containing alcohol solution. The inventors believe that the copper complexes with reactive sites on the silicon to facilitate the reaction of the silicon with the methanol. Therefore, as long as sufficient copper is in solution to saturate the reactive site of the silicon the concentration of copper can be varied within wide limits. The copper can be added to the alcohol as elemental copper or as a copper compound. A useful concentration of copper, either added to the process as elemental copper or as a copper compound, is within a range of about 0.01 to 6.0 weight percent of the silicon. A preferred concentration of copper is within a range of about 1.0 to 3.0 weight percent of the silicon. The copper compound can be any inorganic or organic compound of copper capable of being reduced to copper in the described process. The inorganic compound of copper can be selected from a group consisting of, for example, copper(I) and copper(II) halides, oxides, sulfates, sulfides and cyanides. The inorganic compound of copper can be, for example, Cu(I) chloride, Cu(I) bromide, Cu(I) iodide, Cu(I) fluoride, Cu(II) chloride, Cu(II) bromide, Cu(II) iodide, Cu(II) fluoride, Cu(I) oxide, Cu(II) oxide, Cu(I) sulfate, Cu(II) sulfate, Cu(I) sulfide, Cu(II) sulfide, Cu(I) cyanide, Cu(II) cyanide. The preferred inorganic compounds of copper as catalyst for the present process are copper halides. The preferred copper halide is Cu(I) chloride. The organic compounds of copper can be for example Cu(II) methoxide, Cu(II) ethoxide, Cu(II) allyloxide, Cu(II) acetate, Cu(II) stearate, Cu(II) tetramethylheptanedionate, Cu(II) acetylacetonate, Cu(II) napthanate and Cu(II) phenylate. The preferred copper catalyst for the present process is powdered copper metal with an average particle diameter of less than about 100µ. Example 1 The effects of an acid hydrofluoride salt and a copper catalyst on the reaction of methanol with silicon to form methoxysilanes was evaluated. For each sample tested, about 0.1 g of ground metallurgical grade silicon was transferred to a graduate cylinder. The silicon had a particle size of less than 45 µm (320 mesh). Five milligrams of the copper compound and/or NH 4 HF 2 was added to the graduate cylinder. About 0.43 ml of methanol was added to the graduate cylinder and the contents shaken to form a slurry. The slurry was quickly transferred to a thick walled Pyrex Brand tube and then chilled in dry ice to about -78°C. The tube containing the mixture was purged with argon and sealed. The tubes were then heated in a tube furnace at 150°C. for the times presented in Table 1. At the end of proscribed heating period each tube was removed from the furnace, cooled to room temperature, chilled in a dry ice bath to about -78°C., then opened and the contents transferred to a vial for analysis. For analysis, each sample was centrifuged and the liquid supernate analyzed by gas liquid chromatography using a mass spectrometer as a detector (GLC-MS). The results of this analysis are presented in Table 1. The time, in hours (h), that the sample was heated at 150°C. is presented in the first column of Table 1. The column labelled "Cat./Act." lists the copper compounds tested as catalyst and/or the acid hydrofluoride salt serving as an activator for the process. The percent conversion of methanol is presented in the column labelled "% Conv. MeOH." The area percent of alkoxsilanes formed by the process, as determined by GLC-MS, is presented under the columns labelled "Area % Product." Effects of Copper Catalyst And an Acid Hydrofluoride Salt on The Reaction of Methanol with Silicon Area % Product Time(h) Cat./Act. % Conv. MeOH (MeO) 2 SiH 2 (MeO) 3 SiH (MeO) 4 Si 3 None 0 - - - 3 CuCl 2 0 - - - 3 CuF 2 0 - - - 3 CuO 0 - - - 3 CuBr 0 - - - 3 NH 4 Cl 0 - - - 3 CuSO 4 0 - - - 2 NH 4 HF 2 13 - 62 27 10 NH 4 HF 2 50 - 68 24 2 CuCl 2 /NH 4 HF 2 56 - 61 24 2 CuCl/NH 4 HF 2 29 - 74 21 3 CuCl/NH 4 HF 2 54 - 88 11 4 CuCl/NH 4 HF 2 81 - 82 13 5 CuCl/NH 4 HF 2 100 - 89 9 The data presented in Table 1 demonstrate that, under the process conditions, methanol does not react with silicon in the presence or absence of copper to form methoxysilanes. Under the process conditions, when methanol is contacted with silicon in the presence of NH 4 HF 2 methoxysilanes are formed and the rate of formation of methoxysilanes is increased when a copper compound is added as a catalyst. Example 2 The effect of using atomized silicon as a starting material for the process was evaluated. The atomized silicon was prepared in a manner similar to that described in Liles, U.S. Patent No. 4,701,289, issued October 20, 1987. The process of contacting the atomized silicon with methanol was similar to that of Example 1. The results. are presented in Table 2. The headings of Table 2 are the same as for Table 1. Reaction of Atomized Silicon With Methanol in The Presence of Copper And an Acid Hydrofluoride Salt Area % Product Time(h) Cat./Act. % Conv. MeOH (MeO) 2 SiH 2 (MeO) 3 SiH (MeO) 4 Si 3 None 0 - - - 1 CuCl/NH 4 HF 2 0 - - - 2.0 CuCl/NH 4 HF 2 49 - 73 20 2.5 CuCl/NH 4 HF 2 63 - 62 11 2.5 CuCl/NH 4 HF 2 80 - 84 28 These results, when compared to the data presented in Table 1, demonstrate that the atomized silicon is more reactive than ground silicon in the presence of CuCl and NH 4 HF 2 . Example 3 Hydrogen fluoride gas was bubbled through liquid methanol to form a solution of hydrogen fluoride. The methanol solution of hydrogen fluoride was contacted with ground metallurgical grade silicon and atomized silicon in the presence of CuCl to produce methoxysilanes. The ground metallurgical grade silicon was the same as that described in Example 1 and the atomized silicon was the same as described in Example 2. In addition, a sample of silicon alloy with two weight percent copper and atomized as described in Example 2, was tested. The reaction procedure was similar to that described in Example 1. The normality of the hydrogen fluoride in the methanol was determined by titration. The results are presented in Table 3. The type of silicon is designated as: (B) ground metallurgical grade silicon, (C) atomized silicon and (E) atomized silicon-copper alloy. The hydrogen fluoride normalities tested are presented in Table 3 under the heading "HF Norm.". All other heading are as previously described for Table 1. Effects of Hydrogen Fluoride Solution in Methanol on Methoxysilane Formation in The Presence of Copper Compound Area % Product Si Type Time (h) Cat./Act. HF Norm. % Conv. MeOH (MeO) 2 SiH 2 (MeO) 3 SiH (MeO) 4 Si B 5 - 0.4 31 16 63 12 B 11 - 0.4 83 6 79 13 B 6 - 0.5 35 14 70 16 B 13 - 0.5 78 4 82 13 B 2.5 CuCl 0.15 30 6 64 18 B 5.5 CuCl 0.15 100 7 79 10 B 2 CuCl 0.5 21 17 62 12 B 3.5 CuCl 0.5 57 11 67 14 B 5 CuCl 0.5 93 8 75 10 B 5.5 CuCl 0.5 100 7 73 14 C 1 CuCl 0.5 53 10 70 12 C 2 CuCl 0.5 85 10 75 10 E 1.5 CuCl 0.15 46 15 68 12 E 2.5 CuCl 0.15 93 5 78 13 E 1 CuCl 0.5 35 2 72 17 E 2 CuCl 0.5 60 11 62 14 The data presented in Table 3 demonstrate the effectiveness of a solution of hydrogen fluoride in methanol in facilitating the reaction of methanol with silicon, both in the presence and absence of copper catalyst. Only minor differences are noted in the reaction rates and product distribution when the process is run with between 0.15 and 0.5 normality hydrogen fluoride. Of particular significance is the production of dimethoxysilane when a solution of hydrogen fluoride is used to activate the silicon. Example 4 The effect of the size of the silicon particles on the reaction with methanol was evaluated. The silicon tested was an atomized silicon-copper alloy containing one weight percent copper, prepared as described in Example 2. The particulate silicon was screened into the size ranges described in Table 4. The particulate silicon was contacted with methanol containing 0.3N hydrogen fluoride and 5 weight percent CuCl, based on the weight of silicon, by the procedure described in Example 1. The reaction temperature was 150°C. The percent of original methanol remaining in the reaction tube at various times was assayed by GLC-MS. The results are presented in Table 4 in the column labelled "%Conv. MeOH." Effect of Silicon Particle Size on Reaction With Methanol in The Presence of Hydrogen Fluoride and Copper Catalyst Micrometres Size Si (Mesh) Time (h) % Conv. MeOH 250-149 (60-100) 0.5 27 250-149 (60-100) 1.5 65 250-149 (60-100) 2.0 79.5 250-149 (60-100) 3.0 100 149-74 (100-200) 0.5 29.2 149-74 (100-200) 1.0 42 149-74 (100-200) 2.0 76.7 149-74 (100-200) 3.0 100 ≤ 44 (≥325) 1.0 48.3 ≤ 44 (≥325) 1.5 67.5 ≤ 44 (≥325) 2.0 82.4 The data of Table 4 demonstrate that particle size, within the range tested, had little effect on the described process. Example 5 The reaction of metallurgical grade silicon with isopropyl alcohol in the presence of hydrogen fluoride, with and without copper catalyst was evaluated. The process was conducted similar to that described in Example 1. Ground metallurgical grade silicon was contacted with isopropyl alcohol at a molar ratio of silicon to isopropyl alcohol of two and at a temperature of 150°C. The isopropyl alcohol contained 0.4N hydrogen fluoride in solution, prepared by bubbling hydrogen fluoride gas through the alcohol. CuCl was added as catalyst at a concentration of two weight percent, based upon the weight of silicon. The percent of the original isopropyl alcohol remaining in the sample tube at various times was determined by GLC-MS. The results are expressed in Table 5 under the heading "% Conv. i-PrOH." Reaction of Silicon With Isopropyl Alcohol in The Presence of Hydrogen Fluoride and Copper Catalyst Cat./Act. Time (Day) % Conv. i-PrOH -/- 4 0 -/HF 2 25 -/HF 2.5 30 -/HF 3 35 -/HF 4 48 CuCl/HF 0.8 21 CuCl/HF 1.5 34 CuCl/HF 2 46 Example 6 The reaction of metallurgical grade silicon with normal propyl alcohol in the presence of hydrogen fluoride, with and without copper catalyst was evaluated. The process was conducted similar to that described in Example 1. Ground metallurgical grade silicon was contacted with n-propyl alcohol at a molar ratio of silicon to n-propyl alcohol of two and at a temperature of 150°C. The n-propyl alcohol contained 0.4N hydrogen fluoride in solution, prepared by bubbling hydrogen fluoride through the alcohol. CuCl was added as catalyst at a concentration of two weight percent, based upon the weight of silicon. The percent of the original n-propyl alcohol remaining in the sample tube at various times was determined by GLC-MS. The results are expressed in Table 5 under the heading "% Conv. n-PrOH." Reaction of Silicon With n-Propyl Alcohol in The Presence of Hydrogen Fluoride and Copper Catalyst Cat./Act. Time (h) % Conv. n-PrOH /HF 8 34 /HF 16 68 /HF 24 99 CuCl/HF 5 41 CuCl/HF 8 80 CuCl/HF 10 97

Claims: A process for preparing alkoxysilanes of the formula (RO) n SiH 4-n , where R is a primary or secondary alkyl radical and n=2, 3 or 4; the process comprising contacting silicon with a solution of hydrogen fluoride in a liquid alcohol of formula ROH, where R is as previously described, at a temperature within a range of 50°C. to 300°C. A process according to claim 1 where the solution of hydrogen fluoride in a liquid alcohol is prepared by dissolving hydrogen fluoride gas in the alcohol at a concentration within a range of 0.01 N to 6.0 N. A process according to claim 1 where the solution of hydrogen fluoride in a liquid alcohol is formed by dissolving an acid hydrofluoride salt in the alcohol. A process according to claim 1, wherein the contacting is effected in the presence of a catalyst selected from the group consisting of copper and copper compounds. A process according to claim 4 where the solution of hydrogen fluoride in a liquid alcohol is prepared by dissolving hydrogen fluoride gas in the liquid alcohol at a concentration within a range of 0.01 N to 6.0 N. A process according to claim 4 where the solution of hydrogen fluoride in a liquid alcohol is formed by dissolving an acid hydrofluoride salt in the liquid alcohol.

Inventors: NGUYEN BINH THANH, SPEIER JOHN LEOPOLD, NGUYEN, BINH THANH, SPEIER, JOHN LEOPOLD
Applicants: DOW CORNING, DOW CORNING CORPORATION
Application year: 1992
Main classification: C07F7
IPC codes: C07B61, B01J27, C07F7, B01J23