Reducing cyclic organosiloxanes in a recirculating process stream

Abstract: A process for continuously reducing the amount of cyclic organosiloxane in a recirculating process stream is disclosed. The process comprises washing a process stream in a wash step to reduce chloride content of the process stream, distilling the process stream into a low-boiling fraction comprising low molecular weight cyclic organosiloxanes and an inert solvent and a high-boiling fraction comprising linear organosiloxanes and high molecular weight cyclic organosiloxanes and reequilibrating the overhead low-boiling fraction in the presence of a reequilibration catalyst to form a reequilibration mixture comprising high molecular weight cyclic organosiloxanes and the inert solvent and recycling the reequilibration mixture to the wash step.

Description: Our present invention is a process for continuously reducing the amount of cyclic organosiloxane in a recirculating process stream. Dimethyldichlorosilane produced by the direct process contains small amounts of heptane as a by-product. When the dimethyldichlorosilane is hydrolyzed to form polysiloxanes, the by-product heptane remains as an unwanted odor causing contaminant. The heptane is subsequently removed by passing the hydrolyzate through a distillation column where the hydrolyzate is recovered as a high-boiling bottom stream and a low-boiling overhead stream comprising heptane and lower boiling siloxanes are recycled back into the process. The low boiling siloxanes, which consist mostly of hexamethylcyclotrisiloxane, are recycled back into the process and as a consequence builds up over time to as much as 25% to 33% of the recycle stream. As the hexamethylcyclotrisiloxane content of the recycle stream increases, it significantly impacts the capacity of the column used to remove heptane from the hydrolyzate. As a result of hexamethylcyclotrisiloxane build-up, additional capital is required to make the column large enough to handle the recycled hexamethylcyclotrisiloxane. When the recycle streams' hexamethylcyclotrisiloxane concentration becomes too great, the recycle stream must be removed from the process for disposal as a hazardous waste. Therefore, a process is needed to remove hexamethylcyclotrisiloxane from the recycle stream, while allowing the heptane to be recycled. U.S. Patent 5,395,956 describes a process for preparing cyclic organohydrogensiloxanes by contacting an organohydrogendichlorosilane with a stoichiometric equivalent of water to form a hydrolyzate. The hydrolyzate is diluted in an inert solvent and contacted with an acidic rearrangement catalyst to effect formation of cyclic organohydrogensiloxanes. The cyclic organohydrogensiloxanes are separated from the inert solvent and linear organohydrogensiloxanes and the linear organohydrogensiloxanes are then recycled to the process for further contact with the acidic rearrangement catalyst. U.S. Patent 3,714,213 describes a process for preparing cyclic methylhydrogensiloxanes by contacting linear methylhydrogen siloxanes with an acid catalyst absorbed on a carrier. The process requires the presence of high molecular weight chain termination groups. The yield of tetramethylcyclotetrasiloxane is reported to be 73 percent. U.S. Patent 4,895,967 describes a method for making cyclic organohydrogensiloxanes by contacting a linear organohydrogensiloxane with a heated bed of a cracking catalyst at reduced pressure. The resulting volatile cyclic organohydrogensiloxane is recovered. A typical yield for the method is reported to be 85 percent. EP-A-738 732 discloses a process for preparing cyclic polysiloxanes by continuously passing a feed comprising linear polysiloxanes through a reactor containing a solid acidic catalyst to produce a mixture comprising volatile cyclic polysiloxanes, nonvolatile reaction products, and unreacted linear polysiloxanes. The volatile products can be recovered and the nonvolatile reaction products and unreacted linear polysiloxanes are recycled to the polysiloxane feed. US-A-4,197,251 discloses a process for the production of octamethylcyclotetrasiloxane by reacting dimethylsiloxane in the presence of an alkaline catalyst, an inert solvent and recycled by-produced cyclic dimethylsiloxanes. We have discovered that hexamethylcyclotrisiloxane can be removed from a process recycle stream to eliminate hexamethylcyclotrisiloxane build-up and the solvent recycled back to the process. Our invention provides a process for continuously removing hexamethylcyclotrisiloxane from a recirculating process stream and passing it through a reequilibration reactor containing a catalyst to reequilibrate the hexamethylcyclotrisiloxane to higher molecular weight cyclic organosiloxanes. The present invention is a process for continuously reducing the amount of cyclic organosiloxane in a recirculating process stream comprising a continuous process for reducing the amount of cyclic organosiloxane in a recirculating process stream comprising, (A) washing a process stream resulting from i) hydrolysis of a silane or mixtures of silanes corresponding to the formula R 2 SiCl 2 , ii) adding an inert solvent to the hydrolysis mixture, iii) phase separation of the hydrolysis mixture into the process stream comprising organosiloxanes and the inert solvent and an aqueous hydrogen chloride layer in a wash step to reduce chloride content of the process stream (B) distilling the process stream into a low-boiling fraction comprising low molecular weight cyclic organosiloxanes described by formula (R 2 SiO) n and an inert solvent and a high-boiling fraction comprising linear organosiloxanes and high molecular weight cyclic organosiloxanes described by formula (R 2 SiO) n , (C) reequilibrating the low-boiling fraction in the presence of a reequilibration catalyst to form a reequilibration mixture comprising high molecular weight cyclic organosiloxanes and the inert solvent, and (D) recycling the reequilibration mixture to the wash step, where each R is selected from saturated monovalent hydrocarbons comprising one to 12 carbon atoms or an aryl radical, n is less than 5 and m is 4 to 14. The process stream of the present process is formed by reacting silanes with water to form a hydrolysis mixture. The silanes useful in the process may be a single species of silane as described by formula R 2 SiCl 2 or a mixture of such silanes. R is selected from saturated monovalent hydrocarbons comprising one to 12 carbon atoms and aryl radicals. R can be, for example, methyl, ethyl, propyl, isopropyl, butyl, tertiary butyl, sec-butyl, hexyl, cyclohexyl, dodecyl, phenyl, tolyl and naphtyl. Preferably, R is selected from methyl or phenyl. Most preferred is when R is methyl. The preferred silane is dimethyldichlorosilane. In the hydrolysis process the silane is contacted with 1000% to 2000% excess of the stoichiometric equivalent of water in a hydrolysis reactor forming a hydrolysis mixture comprising cyclic and linear organosiloxanes and hydrogen chloride. The cyclic organosiloxanes formed by the hydrolysis process are described by formulas (R 2 SiO) n and (R 2 SiO) m and the linear organosiloxanes are described by formula HO(R 2 SiO) q H, where R is as previously defined, n is less than 5, m = 4 to 14 and q = 2 to 50. Examples of cyclic siloxanes are hexamethylcyclotrisiloxane (D 3 ), octamethylcyclotetrasiloxane (D 4 ) and dodecamethylcyclohexasiloxane. Examples of linear siloxanes are 1,3-dihydroxytetramethyldisiloxane, 1,5-dihydroxyhexamethyltrisiloxane and 1,7-dihydroxyoctamethyltetrasiloxane. The hydrolysis mixture is phase separated into a phase comprising the cyclic and linear organosiloxanes and a phase comprising aqueous hydrogen chloride. An inert solvent is added to the hydrolysis mixture to facilitate phase separation. The term "inert" means a solvent which acts as a diluent and does not otherwise have significant reaction in the process. Examples of suitable inert solvents are hydrocarbons, such as, hexane, heptane, benzene, toluene, xylene, THF and diethyl ether. The preferred solvent is heptane. The amount of solvent useful in the separation process is 2 to 15 weight percent based on the weight of the hydrolysis mixture. Preferred is 5 to 10 weight percent on the same basis. The inert solvent increases the density difference between the organosiloxanes and the hydrogen chloride such that within the phase separator, the hydrolysis mixture separates by gravity into an upper process stream comprising organosiloxanes and inert solvent and a lower aqueous hydrogen chloride layer. The lower aqueous hydrogen chloride layer is withdrawn and recycled back to the hydrolysis process. In the present process, the process stream as described above is water washed by passing the process stream through a wash step. By adding water to the process stream, the hydrogen chloride becomes soluble in the water forming an upper organosiloxane and solvent phase and a lower hydrogen chloride acid phase. The lower hydrogen chloride acid phase is removed form the process stream and recycled back to the hydrolysis process. The chloride content of the process stream is reduced to 100 ppm. The washing step may be repeated to further reduce the process stream chloride content to 0.5 ppm. After washing, the process stream is distilled, resulting in a low-boiling fraction comprising low molecular weight cyclic organosiloxanes described by formula (R 2 SiO) n and the inert solvent and a high-boiling fraction comprising linear organosiloxanes and high molecular weight cyclic organosiloxanes described by formula (R 2 SiO) where each R is independently selected from saturated monovalent hydrocarbons comprising one to 12 carbon atoms or an aryl radical, n is less than 5 and m is 4 to 14. The low molecular weight cyclic organosiloxanes consist mostly of hexamethylcyclotrisiloxane and smaller amounts of octamethylcyclotetrasiloxane. The high-boiling fraction is withdrawn from the distillation column and collected in a suitable container. The low-boiling fraction comprising the low molecular weight cyclic organosiloxanes and inert solvent distilled from the process stream are passed through a reequilibration reactor containing a reequilibration catalyst to reequilibrate the low molecular weight cyclic organosiloxanes forming a reequilibration mixture comprising high molecular weight cyclic organosiloxanes described by formula (R 2 SiO) m , where R and m are as previously defined. The amount of inert solvent remaining in the low-boiling fraction of the process stream is 20 to 95 weight percent based on the weight of the low-boiling fraction of the process stream. For use in the present process, it is preferred that the inert solvent comprise 30 to 90 weight percent of the low-boiling fraction of the process stream. Most preferred is when the inert solvent comprises 60 to 85 weight percent of the low-boiling fraction of the process stream. The reequilibration catalysts useful in the present process are catalysts, such as, trifluoromethane sulfonic acid, HCl activated clays, sulfuric acid activated clays and sulfonic acid ion exchange resins, such as sulfonated divinylbenzenestyrene copolymer resin. Contact of the catalyst with the low-boiling fraction can be effected by standard means for contacting liquids with solids, for example, in a fixed-bed, a stirred-bed or a fluid-bed reactor. The process can be run as a continuous, semi-continuous or batch process. Preferred is when the present process is run as a continuous process using a fixed-bed of the reequilibration catalyst. The low-boiling fraction comprising the low molecular weight cyclic organosiloxanes are contacted with a reequilibration catalyst which facilitates reequilibration of the lower boiling cyclic siloxanes, such as hexamethylcyclotrisiloxane and octamethylcyclotetrasiloxane into a mixture of high molecular weight cyclic organosiloxanes comprising a reequilibration mixture. The high molecular weight cyclic organosiloxanes can then be removed from the process during distillation. The reequilibration mixture comprising the high molecular weight cyclic organosiloxanes and inert solvent are recycled back to the wash step. The following examples are provided to illustrate the present invention. The examples are not intended to limit the present claims. Example 1 Evaluation of the reequilibration of hexamethylcyclotrisiloxane and octamethylcyclotetrasiloxane using trifluoromethane sulfonic acid as a catalyst. A 10 ml sample comprising 26 wt% hexamethylcyclotrisiloxane, 13 wt% octamethylcyclotetrasiloxane and 61 wt% heptane was placed in a glass vial. Then, 15 drops of trifluoromethane sulfonic acid was added to the vial. The vial was closed, shaken and allowed to sit for 2 hours. An aliquot of the original sample and the reequilibrated sample were analyzed by gas chromatography (GC) using a thermal conductivity detector (TCD) and the results are reported in Table 1. Component Before Reequilibration wt% After Reequilibration wt% D3 26 0 D4 13 18 D5 11 D6 4 D7 1 D8 0.4 D9 0.2 D10 0.2 Example 2 Evaluation of the reequilibration of hexamethylcyclotrisiloxane and octamethylcyclotetrasiloxane using an Amberlyst® A-15 resin bead catalyst bed. A catalyst bed was prepared by pouring Amberlyst® A-15 resin beads, Rohm and Haas, Philadelphia, Pa., into a glass tube having a catalyst bed 2 cm in diameter and 13 cm high. A sample comprising 22.3 wt% of hexamethylcyclotrisiloxane, 11.7 wt% of octamethylcyclotetrasiloxane and 61 wt% heptane was passed through the catalyst bed into a receiving vessel. After passing through the catalyst bed, the sample was analyzed by GC-TCD and passed through the catalyst bed again. This procedure was repeated six times with GC-TCD being conducted after each pass. The residence time of the sample in the catalyst bed was 5 minutes per pass. The results of the analysis are reported in Table 2 in wt%. Component Initial Sample Pass 1 Pass 2 Pass 3 Pass 4 Pass 5 Pass 6 Pass 7 D3 22.3 3.9 0.2 0.1 0.2 0.2 0 0 D4 11.7 12.2 11.3 11 10.8 10.6 10.4 10.6 D5 4.2 5.9 6.9 7.2 7.1 7.1 7.2 D6 2.5 2.7 2.7 2.5 2.5 2.4 2.5 D7 0.4 0.6 0.7 0.7 0.7 0.7 0.7 D8 0.2 0.2 0.3 0.3 0.3 0.3 0.3 D9 0.1 0.2 0.2 0.2 0.2 0.2 D10 0.1 0.1 0.1 D11 0.1 D12 0.1 D13 0.1

Claims: A continuous process for reducing the amount of cyclic organosiloxane in a recirculating process stream comprising, (A) washing a process stream resulting from i) hydrolysis of a silane or mixtures of silanes corresponding to the formula R 2 SiCl 2 , ii) adding an inert solvent to the hydrolysis mixture, iii) phase separation of the hydrolysis mixture into the process stream comprising organosiloxanes and the inert solvent and an aqueous hydrogen chloride layer in a wash step to reduce chloride content of the process stream, (B) distilling the process stream into a low-boiling fraction comprising low molecular weight cyclic organosiloxanes described by formula (R 2 SiO) n and an inert solvent and a high-boiling fraction comprising linear organosiloxanes and high molecular weight cyclic organosiloxanes described by formula (R 2 SiO) m , (C) reequilibrating the low-boiling fraction in the presence of a reequilibration catalyst to form a reequilibration mixture comprising high molecular weight cyclic organosiloxanes and the inert solvent, and (D) recycling the reequilibration mixture to the wash step, where each R is selected from saturated monovalent hydrocarbons comprising one to 12 carbon atoms or an aryl radical, n is less than 5 and m is 4 to 14. A process according to claim 1 where the reequilibration catalyst is a sulfonated divinylbenzenestyrene copolymer resin or trifluoromethane sulfonic acid. A process according to claim 1 where the amount of inert solvent in the low-boiling fraction is 20 to 95 weight percent based on the weight of the low-boiling fraction.

Inventors: BRAMER DAVID HAROLD, WOOD LARRY HERBERT, BRAMER, DAVID HAROLD, WOOD, LARRY HERBERT
Applicants: DOW CORNING
Application year: 1999
Main classification: C08G77
IPC codes: C07F7, C08G77