Process for preparing acyloxysilanes

Abstract: A process for the preparation of acyloxysilanes is described. It comprises contacting a chlorosilane with a carboxylic acid in a film to form an equilibrium mixture of acyloxysilane and hydrogen chloride. The film is heated at a temperature sufficient to cause vaporization of the hydrogen chloride from the equilibrium mixture thereby increasing the yield of acyloxysilane. In a preferred process, the process is run in a falling-film reactor or a wiped-film reactor.

Description: The present invention is a process for the preparation of acyloxysilanes. The process comprises contacting a chlorosilane with a carboxylic acid in a film to form an equilibrium mixture of acyloxysilane and hydrogen chloride. The film is heated at a temperature sufficient to cause vaporization of the hydrogen chloride from the equilibrium mixture thereby increasing the yield of acyloxysilane. In a preferred process, our process is run in a falling-film reactor or a wiped-film reactor. Acyloxysilanes are well known crosslinking agents for one-part, room-temperature, vulcanizable, silicone rubber compositions. Common acyloxysilane crosslinking agents are methyltriacetoxysilane or ethyltriacetoxysilane. Acyloxysilane crosslinking agents are made by the reaction of an appropriate chlorosilane with a carboxylic anhydride or with a carboxylic acid. The process is known to be an equilibrium reaction as described by the equation SiCl + AcOH <-> SiOAc + HCl , where Ac denotes an acyl radical. A reactive column distillation process for preparing acyloxysilanes by reacting a chlorosilane with a carboxylic acid or carboxylic acid anhydride in the presence of an iron complexing agent is disclosed in U.S. Patent 3,974,198. In this process, an aliphatic carboxylic acid, such as glacial acetic acid, is added at the top of a distilling column to a refluxing mixture of chlorosilane in an organic solvent, such as benzene, containing an iron complexing agent. After addition completion of the aliphatic carboxylic acid, the solvent is removed by conventional distillation. Thereafter, the acyloxysilane compound is isolated from the mixture. U.S. Patent 4,176,130 describes a reactive distillation column process where an aliphatic carboxylic acid in the vapor phase is passed upwards from the bottom of a column filled with raschig rings countercurrent to a flow of chlorosilanes. U.S. Patent 4,332,956 claims a reactive distillation column process where acyloxysilanes are prepared by reacting a chlorosilane with an aliphatic carboxylic acid in a column at an elevated temperature where the aliphatic carboxylic acid in the vapor phase reacts with the chlorosilane in the column. The carboxylic acid is introduced therein to the column by a plurality of feed streams, where at least one feed stream of aliphatic carboxylic acid is located above the chlorosilane feed stream and one carboxylic acid feed stream is positioned below the chlorosilane feed stream. In these reactive distillation processes, the formation of dimer at the bottom of the column is a significant side reaction that results from the thermal decomposition of monomeric acyloxysilanes, from a reaction of the chlorosilane with acyloxysilane or from both. When this happens the yield of acyloxysilane product is reduced. Also, gels can form in the reactive distillation column causing plugging. These detrimental processes are catalyzed by the presence of hydrogen chloride in the reaction mixture as well as by excess acetic acid. The conduct of our reaction as a film process provides for good heat transfer to the film and good mass transfer, thereby effecting rapid removal of hydrogen chloride from the reaction mixture. This results not only in a shifting of the equilibrium of the reaction to favor formation of acyloxysilanes, but also reduces undesired dimer formation as catalyzed by hydrogen chloride. In addition, the conduct of our reaction as a film process allows the use of lower reactor surface temperatures and shorter residence times than those required in reactive distillation columns. A reduction in reactor surface temperatures and residence times reduce undesired side reactions, such as dimer formation and gelation. The present invention is a process for preparation of acyloxysilanes. Our process comprises: (A) contacting a chlorosilane described by formula R 1 a SiCl 4-a with a carboxylic acid described by formula R 2 COOH in a film thereby forming acyloxysilane and hydrogen chloride, (B) vaporizing the hydrogen chloride from the film to facilitate formation of the acyloxysilane, and (C) recovering the acyloxysilane; where each R 1 is independently selected from hydrogen atom or monovalent hydrocarbon radicals comprising one to eight carbon atoms, R 2 is selected from hydrogen atom or alkyl radicals comprising one to eight carbon atoms and a = 0, 1, 2 or 3. Chlorosilanes useful in our process are described by formula (1), where each R 1 is independently selected from hydrogen or monovalent hydrocarbon radicals comprising one to eight carbon atoms. In addition to hydrogen, R 1 may be the same or different, substituted and unsubstituted, monovalent hydrocarbon radicals comprising one to eight carbon atoms. R 1 is, for example, alkyl radicals such as methyl, ethyl, propyl, isopropyl, butyl, sec-butyl, octyl and 2-ethylhexyl; alkenyl radicals such as vinyl or allyl; hexadienyl radicals; cycloalkyl radicals such as cyclopentyl, cyclohexyl and cycloheptyl; aromatic hydrocarbon radicals such as phenyl or naphthyl; aralkyl radicals such as benzyl or phenylethyl; alkaryl radicals such as tolyl or dimethylphenyl; and substituted hydrocarbon radicals such as chloromethyl, 3-chloropropyl and 3,3,3-trifluoropropyl. In formula (1) it is preferred that a=1. Preferred chlorosilanes for use in this process are selected from methyltrichlorosilane or ethyltrichlorosilane. The chlorosilane of formula (1) is contacted with a carboxylic acid described by formula (2). In formula (2) each R 2 is selected from hydrogen atom or alkyl radicals comprising one to eight carbon atoms. R 2 is, for example, methyl, ethyl, propyl, tert-butyl, sec-butyl and octyl. Preferred is when R 2 is methyl and the carboxylic acid is acetic acid. The molar ratio of the carboxylic acid to silicon-bonded chlorine atoms of the chlorosilane in the thin film is not critical to the practice of our invention. However, it is preferred that the molar ratio of carboxylic acid to silicon-bonded chlorine be within a range of 40 to 200 percent of stoichiometric equivalence. More preferred is when the molar ratio of carboxylic acid to silicon-bonded chlorine is near stoichiometric equivalence. The chlorosilane and the carboxylic acid are contacted in a film. The term "film" includes the coating or spreading of a bulk liquid onto a surface to increase the surface area of the bulk liquid and to thereby increase mass transfer of components from the liquid to a vapor phase. The carboxylic acid, chlorosilane or both may be preheated to a temperature below their boiling point prior to contact. The method of forming the film is not critical and may be any of those known in the art. The benefit of our present process is realized by the efficient transfer of heat and mass transfer within a film causing a rapid vaporization and removal of hydrogen chloride from the film. The vaporization and removal of hydrogen chloride from the film results in a shift of the chemical equilibrium of the reaction to favor production of acyloxysilanes. This film is formed, for example, in a falling film evaporator reactor or in a wiped film evaporator reactor. Because of operation simplicity, a falling film evaporator reactor is generally preferred for our process. Examples of such reactors are described, for example, in Kirk-Othmer, Encyclopedia of Chemical Technology , Fourth Edition, Vol. 9, p. 965-968, (1994); and in Mehra, "Selecting Evaporators," Chemical Engineering, February 3, 1986, p. 56-72. The film forming reactor may also be connected to a reboiler to facilitate conversion of lower-boiling materials that exit the reactor to higher-boiling materials within the reactor. The reactor may additionally be used as a multiple pass reactor, where materials exiting the reactor are recycled to the reactor to effect further reaction of the materials. Film thickness and flow rates will depend upon such factors as minimum wetting rate for the surface on which the thin film is formed and the flooding point. Standard methods for determining these parameters are described, for example, in Perry et al., Perry's Chemical Engineers' Handbook, 6th ed., McGraw-Hill, NY, p. 5-59; and in York et al., Chemical Engineering Progress, October 1992, p. 93-98. Hydrogen chloride formed as a result of contact of the chlorosilane with the carboxylic acid is conveniently vaporized from the film. Vaporization of the hydrogen chloride is effected by heating the thin film, by reducing pressure over the thin film or by a combination of both. It is preferred that vaporization of the hydrogen chloride from the film be effected by heating the film. The film can be heated by standard methods, for example, passing a heated media such as a gas, water or silicone oil through a jacket contacting the wall supporting the film. Generally, it is preferred that the temperature of the film be as great as possible without causing significant vaporization of the chlorosilane fed to the reactor. For example, when the chlorosilane is methyltrichlorosilane, a preferred temperature for a heating media provided to a jacketed reactor is 85°C. to 100°C. and when the chlorosilane is ethyltrichlorosilane, a preferred temperature for the heating media is 95°C. to 115°C. The hydrogen chloride vaporized from the present process is removed from the reactor by standard methods, for example, venting. It can then be collected and used as a feed to other processes. Acyloxysilanes are readily recovered from our process. These acyloxysilanes are described by reference to formula 1, where one or more of the chlorine substituents of the silicon atom of the chlorosilane is replaced by an acyloxy radical. Preferred acyloxysilanes of our process are methyltriacetoxysilane and ethyltriacetoxysilane. Recovery of acyloxysilanes from our process may consist of simply retaining an acyloxysilane liquid mixture resulting from the contact of a chlorosilane with an acyloxysilane in a thin film. It can also consist of using the acyloxysilane liquid mixture from our process as a feed, for example, to a reactive distillation column to effect further reaction of the mixture. Recovery of the acyloxysilane can even consist of standard separation processes, such as distillation, to further isolate the acyloxysilanes. The following examples are provided to illustrate the present invention. These examples do not limit the scope of the present claims. Example 1 The reaction of acetic acid with methyltrichlorosilane in a falling film evaporator reactor was evaluated. The reactor comprised a jacketed, glass, 2.5 cm internal diameter tube 58 cm long. Syltherm™ heat exchange fluid, Dow Corning Corporation, Midland, MI, was circulated through the reactor jacket at a temperature of 95°C. A mixture comprising a 4.2:1 molar ratio (40 percent stoichiometric excess) of acetic acid and trichlorosilane was fed to the top of the reactor, where the mixture was distributed in a film by a 3.3 cm length wiper blade. The mixture was fed to the reactor at 25 mL/min. The bottom fraction from the reactor was collected and passed through the reactor a total of seven passes. The product from the reactor was analyzed by gas chromatography using a thermal conductivity detector (GC-TCD). After seven passes, 75 percent of the methyltrichlorosilane had been converted to acetoxysilanes and 49 percent of the added chlorine was removed as hydrogen chloride. Example 2 The effect of acetic acid concentration on the reaction with methyltrichorosilane in a falling-film evaporator reactor was evaluated. The reactor and procedure was similar to that described in Example 1. The molar ratios of acetic acid to methyltrichlorosilane tested were: 2:1, 3:1 and 4.2:1. The bottom fraction from the reactor was passed through the reactor 5 times and then analyzed as described in Example 1. The fraction of methyltrichlorosilane converted to acetoxysilanes is reported in Table 1. Effects of Acetic Acid Concentration Molar Ratio Methyltrichlorosilane Conv. 2:1 0.51 3:1 0.64 5:1 0.73 Example 3 The reaction of acetic acid with methyltrichlorosilane in a wiped film evaporator reactor connected to a reboiler was evaluated in a series of runs. The reactor comprised a 15 cm length jacketed, glass, tube with a 7.6 cm inside diameter. Syltherm™ heat exchange fluid was circulated through the reactor jacket at various temperatures between 65°C. and 96°C. A reboiler heated to 125°C. was connected to the bottom of the reactor. A mixture comprising a 3:1 molar ratio of acetic acid to methyltrichlorosilane was fed to the top of the reactor. A wiped film of the mixture was formed in the reactor by means of three rotating Teflon™ blades which extend the length of the reactor. The residence time of material in the reboiler was approximately 15 minutes. At the end of fifteen minutes the contents of the reboiler were analyzed by GC-TCD. A linear regression was performed on the results and calculated values for methylacetoxydichlorosilane (SiOAc), methyldiacetoxychlorosilane (SiOAc 2 ) and methyltriacetoxysilane (SiOAc 3 ), as reported in Table 2, were determined for various temperatures. The value for each of the acetoxysilanes is reported as the mole percent of methyltrichlorosilane converted to each of the acetoxysilanes. The estimated total conversion of methyltrichlorosilane to acetoxysilanes is reported in the column labelled "Total" in Table 2. Effects of Temperature on Acetoxysilane Yield Mole Fraction of Acetoxysilane Temp. °C SiOAc SiOAc 2 SiOAc 3 Total 65 0.39 0.18 0.09 0.66 75 0.33 0.22 0.22 0.77 85 0.26 0.27 0.35 0.88 95 0.18 0.32 0.48 0.98 Example 4 The effects of heat on dimer formation during the reaction of acetic acid with methyltrichlorosilane was evaluated using a wiped film evaporator reactor with and without a reboiler. The reactor was the same as in Example 3. The reactor was heated by passing Syltherm™ heat exchange fluid through the reactor jacket at a temperature of 95°C. Feed to the reactor was a 3:1 molar mixture of acetic acid to methyltrichlorosilane. In a first run, a reboiler maintained at 125°C. was used to reflux the bottom fraction coming off the wiped film evaporator reactor for a period of 15 minutes. Conversion of methyltrichlorosilane to acetoxysilanes was 95 percent with 4 percent conversion to dimers. In a second run the bottoms coming off the reactor where collected in a flask and recycled through the reactor four times giving a total time in the reactor of 15 minutes. The conversion of methyltrichlorosilane to acetoxysilanes was 68 percent with 2 percent conversion to dimers. Example 5 The reaction of acetic acid with ethyltrichlorosilane in a wiped film evaporator reactor connected to a reboiler was evaluated. The evaluation was conducted in the reactor described in Example 3. The reactor was heated by passing Syltherm™ heat exchange fluid through the reactor jacket at a temperature of 94°C. The reboiler was maintained at a temperature of 125°C. A 3:1 molar ratio of acetic acid to ethyltrichlorosilane was fed to the reactor. Resident time of material in the reboiler was 15 minutes. At the end of 15 minutes, the content of the reboiler was analyzed by GC-TCD. Conversion of ethyl-trichlorosilane to acetoxysilanes was determined to be 87 percent.

Claims: A process for preparation of acyloxysilanes, the process comprising: (A) contacting a chlorosilane described by formula R 1 a SiCl 4-a with a carboxylic acid described by formula R 2 COOH in a film thereby forming an acyloxysilane and hydrogen chloride, (B) vaporizing the hydrogen chloride from the film to facilitate formation of the acyloxysilane, and (C) recovering the acyloxysilane; where each R 1 is independently selected from hydrogen atom or monovalent hydrocarbon radicals comprising one to eight carbon atoms, R 2 is selected from hydrogen atom or alkyl radicals comprising one to eight carbon atoms and a = 0, 1, 2 or 3. A process according to claim 1 where the chlorosilane is selected from methyltrichlorosilane or ethyltrichlorosilane. A process according to claim 1 where the molar ratio of carboxylic acid to silicon-bonded chlorine is within a range of 60 to 200 percent of stoichiometric equivalence. A process according to claim 1 where the film is formed in a falling-film evaporator reactor or a wiped-film evaporator reactor. A process according to claim 4 where the film exiting the apparatus is recycled to the reactor to effect further reaction. A process according to claim 1 where the acyloxysilane is selected from methyltriacetoxysilane or ethyltriacetoxysilane. A process according to claim 1 where the chlorosilane is selected from methyltrichlorosilane or ethyltrichlorosilane, the carboxylic acid is acetic acid, the molar ratio of carboxylic acid to silicon-bonded chlorine is near stoichiometric equivalence and the film is formed in a falling-film evaporator type apparatus.

Inventors: RASMUSSEN EDWARD THOMAS, SCHROEDER BARBARA ANN, WALKER MONICA ANN, RASMUSSEN, EDWARD THOMAS, SCHROEDER, BARBARA ANN, WALKER, MONICA ANN
Applicants: DOW CORNING, DOW CORNING CORPORATION
Application year: 1995
Main classification: C07F7
IPC codes: C07B61, C07F7