Process for purifying halosilanes

Abstract: A process for purifying halosilanes consisting of contacting a mixture comprising a halosilane and a hydrocarbon with silica gel, thereby reducing the hydrocarbon content of the mixture. The present process is particularly useful for removing hydrocarbon contaminates from chlorosilanes, where the hydrocarbon contaminates have a boiling point similar to that of the chlorosilanes.

Description: The present invention is a process for the purification of halosilanes. The process consists of contacting a mixture comprising a halosilane and a hydrocarbon with silica gel, thereby reducing the hydrocarbon content of the mixture. The present process is particularly useful for removing hydrocarbon contaminates from chlorosilanes, where the hydrocarbon contaminates have a boiling point similar to that of the chlorosilanes. Hydrocarbon contaminates in halosilanes can create quality problems such as undesirable odor and color not only in the halosilanes, but in products made from the halosilanes. In addition, in cyclic processes using halosilanes as feed, where unreacted materials are being recovered and fed back to the process, hydrocarbons can build up in the process leading to a decrease in process capacity and operational control. Often it is hard to remove these hydrocarbon contaminates from the halosilanes by standard processes such as distillation because of similar boiling points. The present process is particularly useful for removing hydrocarbon contaminates from halosilanes prepared by the reaction of an organohalide with silicon metalloid in the presence of a suitable catalyst, as was originally described by US-A-2,380,995. The use of adsorbents to recover hydrocarbons from air and hydrocarbon mixtures generated during the production and transfer of petroleum products is described, for example, in US-A-4,056,369; US-A-4,066,423; US-A-4,462,811 and US-A- 4,338,101. A typical adsorbent used to recover the hydrocarbons is activated carbon. US-A-4,421,532 also describes a process for recovering volatile organic substances from industrial waste gases. The process involves the passing of the waste gas through a bed of an adsorbent such as activated carbon, silica gel, alumina or molecular sieve to remove volatile organic substances and then regenerating the adsorbent bed by stripping with steam or a hot gas. Similarly, US-A-5,445,742 claims a process for purification of halosilanes. The process consists of contacting a mixture comprising a halosilane and a hydrocarbon with an adsorbent selective for the hydrocarbon. This latter patent teaches that the adsorbent is characterized by being (1) hydrophobic and (2) organophilic and by having (3) a neutral surface, and (4) no polarizable pendant groups. Examples of useful adsorbents taught by the above patent are activated carbon, carbon molecular sieves and high silica zeolites. US-A-4,156,689 discloses a process for the purification of hydrogen-containing silanes and siloxanes containing no phenyl substituent groups comprising contacting the silanes and siloxanes with an adsorbent bed formed from material selected from charcoal and molecular sieves and removing the purified silanes and siloxanes from the adsorbent bed. The present inventors have found that hydrocarbon contaminates present in halosilane liquids and gases can be reduced by contacting the halosilane liquid or gas with silica gel. The described art does not recognize that silica gel can selectively adsorb hydrocarbon contaminates from halosilane liquids and gases. The present invention is a process for reducing hydrocarbon content of halosilanes. The process comprises contacting a mixture comprising a halosilane and a hydrocarbon with silica gel, thereby reducing the hydrocarbon content of the mixture. Preferred halosilanes from which hydrocarbons can be adsorbed are described by formula R a H b SiX 4-a-b , where a=0 to 3, b=0 to 3, a+b=0 to 3, X is a halogen and R is a monovalent hydrocarbon radical comprising one to 12 carbon atoms. The preferred halogen, X, is chlorine. R is, for example, methyl, ethyl, propyl, tert-butyl, vinyl, allyl and phenyl. The halosilane can be, for example, trimethylchlorosilane, dimethyldichlorosilane, methyltrichlorosilane, dimethylchlorosilane, phenylmethyldichlorosilane, phenyltrichlorosilane, trichlorosilane, tetrachlorosilane, methylvinyldichlorosilane and dimethyldivinylchlorosilane. The present process is particularly useful for removing hydrocarbon contaminates from methylchlorosilane mixtures comprising dimethyldichlorosilane and methyltrichorosilane. Hydrocarbons which are removed by the present process are those which are typically found in trace amounts in halosilanes after distillation processes. The specific types of hydrocarbons present in the halosilanes will depend upon the particular halosilane and its separation history. For purposes of this invention the term "hydrocarbons" includes saturated hydrocarbons, unsaturated hydrocarbons and halogenated hydrocarbons. The present process is especially useful for removing those hydrocarbons having boiling points similar to the halosilane of interest, where the hydrocarbons cannot readily be separated by distillation. The present process is especially useful for removing hydrocarbons comprising one to 12 carbon atoms from chlorosilanes. The mixture comprising a halosilane and a hydrocarbon is contacted with silica gel, thereby reducing the hydrocarbon content of the mixture. Silica gels useful in the present process can be of the regular-density, intermediate-density or low-density types. Such silica gels are described, for example, in Kirk-Othmer, Encyclopedia of Chemical Technology, Vol. 20, Third Edition, p. 773-774, John Wiley & Sons, NY, 1982. It is preferred that the silica gel have a surface area of at least 200 m 2 /g. More preferred is when the silica gel is of the regular-density type and has a surface area greater than 500 m 2 /g. A preferred silica gel is one which contains 0.1 to 10 weight percent alumina (Al 2 O 3 ). Even more preferred is when the silica gel contains one to five weight percent alumina. Since chlorosilanes readily hydrolyze on contact with water, it may be necessary to at least partially dry the silica gel prior to use. Drying of the silica gel can be effected by standard methods known in the art such as by heating under reduced pressure. An example of a useful method for drying the silica gel is described in the examples herein. The mixture comprising a halosilane and a hydrocarbon can be contacted with the silica gel by standard methods for contacting a gas or liquid halosilane with a solid. The process can be run as a batch, semi-continuous or continuous process. Preferred is when the mixture comprising a halosilane and a hydrocarbon is a liquid when contacted with the silica gel. In one embodiment of the present process, once the silica gel becomes saturated with hydrocarbon and breakthrough occurs, the silica gel may be desorbed and reused in the process. The process may be run as a continuous process using multiple beds of silica gel, where adsorption and desorption of the beds is staged to provide a continuous process. Adsorption and desorption of the silica gel can be accomplished by standard methods such as a pressure swing adsorption and desorption process, a temperature swing adsorption and desorption process or a combination of pressure and temperature swing adsorption and desorption processes. The method of desorption of the silica gel is not critical to the present process and may be any of those methods known in the art for desorbing adsorbents. The length of time the mixture containing halosilane and hydrocarbon is in contact with the silica gel will depend upon the particular silica gel used, the hydrocarbon to be adsorbed and the concentration of hydrocarbon. Examples of useful contact times are provided in the examples herein. Generally, any contact time sufficient for any or all of the hydrocarbon to be adsorbed from the mixture is considered useful. A number of silica gels were evaluated for their ability to adsorb hydrocarbon contaminates from a methylchlorosilane mixture. The silica gels tested were products of Engelhard, Charlotte, NC. The manufacturer's designation for each of the silica gels along with typical physical properties is provided in Table 1. Silica Gels Tested Name Surface Area (m 2 /g) Pore Vol. (cm 3 /g) Ave. Pore Size (nm) Alumina (%Wt) Sorbead™ WS 650 0.40 2.5 3 Sorbead™ H 750 0.47 2.5 3 Sorbead™ R 750 0.35 2.0 3 Sorbead™ W 275 0.30 - 10 Sorbead™ AF 700 0.40 2.5 0.3 Sorbead™ AC 750 0.60 - - Fifteen to 20 grams of a silica gel was placed in a 50 ml flask. The flask was heated at about 350°C. under vacuum at 4x10 3 Pa (30 mm Hg) for six to eight hours to dry. The flask was cooled and purged with dry nitrogen. Thirty grams of a chlorosilane mixture comprising methyltrichlorosilane, 12,000 ppm dimethyldichlorosilane and hydrocarbon contaminates as described in Table 2 were injected into the cooled flask through a rubber septum. The concentrations of hydrocarbon contaminates given in Table 2 are average values for the methylchlorosilane stream from which the methylchlorosilane mixture was sampled. Hydrocarbon Contaminates Hydrocarbon ppm Trans-2-hexene 7 2-Methyl-2-pentene 6 Cis-3-methyl-2-pentene 20 Trans-3-methyl-2-pentene 50 2,3-Dimethyl-2-butene 85 2-Chloro-2-methylpentane 5 2-Chloro-2,3-dimethylbutane 30 3-Chloro-3-methylpentane 30 The flask was shaken at room temperature for 16 hours. A liquid sample was taken from each flask and analyzed by gas chromatography using a flame ionization detector (GC-FID). None of the hydrocarbons described in Table 2 were detected in any of the liquid samples after adsorption with the silica gels described in Table 1.

Claims: A process for reducing hydrocarbon content of halosilanes, the process comprising: contacting a mixture comprising a halosilane and a hydrocarbon with silica gel, thereby reducing the hydrocarbon content of the mixture. A process according to claim 1 where the mixture is a liquid when contacted with the silica gel. A process according to claim 2 where the halosilane is a chlorosilane described by formula R a H b SiCl 4-a-b , a=0 to 3, b=0 to 3, a+b=0 to 3 and each R is a monovalent hydrocarbon radical comprising one to 12 carbon atoms. A process according to claim 3 where each R is independently selected from a group consisting of methyl, vinyl and phenyl. A process according to claim 1 where the process is run as a continuous process using multiple beds of silica gel and adsorption and desorption of the beds is staged to provide a continuous process. A process according to claim 1 where the silica gel comprises 0.1 to 10 weight percent alumina. A process according to claim 1 where the silica gel is of a regular-density type and has a surface area greater than 500 m 2 /g and contains one to five weight percent alumina and the hydrocarbon comprises one to 12 carbon atoms. A process according to claim 1 further comprising the steps: (B) desorbing the silica gel to remove the hydrocarbon and (C) recycling the silica gel to the process. A process according to claim 8 where desorbing the silica gel is accomplished by reducing the pressure below that pressure at which the mixture was contacted with the silica gel. A process according to claim 8 where desorbing the silica gel is accomplished by raising the temperature of the process higher than that at which the mixture was contacted with the silica gel.

Inventors: MCINTYRE MICHAEL ANDREW, WILDING OLIVER K, MCINTYRE, MICHAEL ANDREW, WILDING, OLIVER K.
Applicants: DOW CORNING, DOW CORNING CORPORATION
Application year: 1996
Main classification: C07F7
IPC codes: B01J20, C07B63, C07F7