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Oct. 5, 1965 A Oct. 5, 1965 A. C. KNIGHT 3,210,430 PREPARATION OF TETRAFLUOROETHYLENE Filed May 24, l963 INVENTOR ALAN C. KNIGHT BY ATTZRNEY 3,210,430 United States Patent Office Patented Oct. 5, 1965 2 The process of the present invention is based on the PREPARATION OF 320,430fifAFLUOROETHYLENE discovery that brominated by-products formed in the Alan Campiel Knight, Wilmington, Dei, assignor to E. E. various reactions involved in the process can either be du Pont de Nemoirs and Company, Winington, e. recycled or can be decomposed to result in the recovery a corporation of Delaware of bromine to such a degree that substantially no bro Filed May 24, 1963, Ser. No. 285,88 imine i.e., less than 3% is lost in the process. Hence, 3 Cains. (C. 260-653.3) the process requires only the use of methane, oxygen and hydrogen fluoride and produces substantially no This application is a continuation in part of S.N. 137, products other than tetrafluoroethylene. The process of 685 filed September 12, 1961, now abandoned. O the present invention differs significantly from a process The present invention relates to the preparation of based on an analogous system using, instead of bromine, tetrafluoroethylene, and, more particularly, to a process chlorine. In such a process carbon tetrachloride is for the preparation of tetrafluoroethylene which employs formed as a by-product which makes the complete re substantially only methane, oxygen and hydrogen fluoride covery of the chlorine for purposes of recycle difficult if and produces essentially no unfluorinated side products. 5 not impossible because of the difficulties encountered in Tetrafluoroethylene is an unsaturated fluorocarbon the oxidation of carbon tetrachloride to chlorine which compound which is produced commercially on a large only gives rise to low yields of chlorine. The major re scale and is principally used as a monomer in the forma actions involved in the process of the present invention tion of fluorocarbon polymers, such as polytetrafluoro are illustrated by the following equations. ethylene, a plastic well known for its chemical stability 20 and high temperature properties. Many methods have been described in the literature for the preparation of tetrafluoroethylene, all of which in some way involve a pyrolysis as a result of which the tetrafluoroethylene is formed. The nature of the common reactions by which 25 the tetrafluoroethylene is formed, however, is such that a substantial number of unfluorinated by-products are formed. These by-products generally add to the cost of The bromination of methane to result in bromoform the preparation of tetrafluoroethylene, since these by may also be combined with the oxidation of hydrogen products cannot be sold economically, or, if Sold, do not 30 bromide to bromine and water in a single reaction ves result in a financial return which approaches the cost of sel if this is desirable. The reaction for combined process making them in the tetrafluoroethylene process, since gen steps (1) and (4) would then be as follows: erally these by-products are made more economically by different methods. It is, therefore, an object of the present invention to 35 The process of the present invention is further illus provide an improved process for the manufacture of tetra trated by the attached drawing which shows in schematic fluoroethylene. It is another object of the present inven form a flow sheet of the process discovered. In this flow tion to provide a process for the preparation of tetra sheet the bromination of methane and the oxidation of fluoroethylene which does not result in any significant un hydrogen bromide are treated as separate process steps. fluorinated by-product formation. Still a further object 40 The various modifications necessary to combine these two of the present invention is to provide a process for the process steps will be apparent to one skilled in the art. preparation of tetrafluoroethylene using low cost start Referring to the drawing, methane, bromine and re ing materials, namely methane, oxygen and hydrogen cycled methane, bromomethane and dibromomethane are fluoride. Other objects will become apparent hereinafter. charged to the reactor 1. The reactor is preferably a The objects of the present invention are accomplished 45 nickel tube furnace containing an inert heat transfer solid by a process which comprises reacting a mixture of Such as silicon carbide, which is maintained at a tem methane and recycled bromomethane and dibromo perature of 300 to 700° C., and, preferably at a tem methane with bromine at a temperature of 300 to 700 perature of 450 to 500 C. The pressure in the reactor C. in a pyrolysis furnace, separating the resulting hydro is not critical and is selected to be of most convenience gen bromide, carbon tetrabromide, bromoform, dibromo 50 in the overall process. In general, the pressure varies methane and bromomethane, recycling the dibromo from /2 to 6 atmospheres. A typical bromination reac methane and bromomethane and reacting the bromo tion is shown in the following examples. form with hydrogen fluoride at a temperature of 100 to 500 C., in the presence of a fluorination catalyst, pref Example I erably selected from the group consisting of chromium 55 A 1 x 30' nickel pipe packed with silicon carbide pel III oxide and titanium tetrafluoride, separating the re lets was heated by a split tube furnace to a temperature Sulting hydrogen bromide and bromodifluoromethane, :of 450 to 500 C. In the period of one hour, 1.089 pyrolyzing the bromodifluoromethane at a temperature g./moles of methane and 2.38 g/moles of bromine were of 400 to 1000 C., separating the resulting tetrafluoro passed through the reactor. The pressure was main ethylene and hydrogen bromide, recycling unconverted 60 tained essentially at atmospheric pressure. The product bromodifiuoromethane, oxidizing hydrogen bromide and collected during this period consisted of 0.0717 g/moles carbon tetrabromide formed in the foregoing steps with of methyl bromide, 0.1472 g./moles of methylene bro molecular oxygen to bromine at a temperature of 300 to imide, 0.504 g/moles of bromoform, 0.0452 g/moles 500° C. and separating and recycling the resulting bro 65 of carbon tetrabromide, 1.771 g/moles of hydrogen bro c. mide, and 0.643 g/moles of methane. 3,210,430 3 4. Example II about 400 to about 600° C. Various methods of pre paring this catalyst have been described in the literature. Using the apparatus and temperature and pressure cont Although the preferred catalyst is the described chro ditions described in Example I, dibromomethane and mium (III) oxide catalyst, other well known fluorination bromine were fed at a mole ratio of 1.07 to the reactor catalysts, such as titanium tetrafluoride, antimony fluo for a contact time of 5.9 seconds. The resulting prod rides and chromium fluoride, may be used. Using the uct distribution was 22.3 mol percent dibromomethane, chromium (III) oxide catalyst the molar ratio of the an 72.7 mol percent bromoform, and 4.7 mol percent car hydrous hydrogen fluoride to the bromoform charged to bon tetrabromide. No bromomethane was isolated. the reactor may be varied over the range of 1 to 10 moles 0 of hydrogen fluoride for each mole of the bromoform, Example III the preferred ratio being determined by the amount of Using the equipment described in Example I at the fluorination desired. Usually an amount of hydrogen pressures and temperatures disclosed, 0.643 mole per fluoride is used that is at least the stoichiometric amount hour of methane, 0.214 mole per hour of bromomethane, needed for the fluorination of the bromoform to difluoro 0.232 mole per hour of dibromomethane, and 2.84 moles bromomethane, and, preferably, about a 50 molar per per hour of bromine were fed into the reactor for a con cent excess. The process is simple and easily carried out tact time of one second. There was obtained 0.0204 by passing the gaseous mixture over or through the cat mole per hour of bromomethane, 0.0942 mole per hour alyst maintained at the desired temperature in a conven of dibromomethane, 0.6372 mole per hour of bromoform, tional reactor of the type commonly employed for a re 0.1206 mole per hour of carbon tetrabromide, 1.890 moles 20 action of this character. Usually the gaseous mixture per hour of hydrogen bromide, 1.15 moles per hour of will be passed through a bed of the catalyst in a reaction bromine and 0.232 mole per hour of methane. tube. The reaction tube may be constructed of any metal The foregoing examples demonstrate the reaction in or any other material that is inert to anyhdrous hydrogen the bromination reactor. It is, in general, preferred to fluoride, and is resistant to the temperature employed for maintain an excess ratio of methane to bromine in the 25 extended periods of time. Suitable materials for con system, preferably from 5:1 to 1:1. Using an excess of struction of the reaction tube include Hastelloy C. nickel methane results in two advantages, a low concentration alloy, Inconel nickel alloy, nickel, stainless steel, platinum of carbon tetrabromide in the product and also a com and fused refractory alumina, such as alundum. The plete conversion of bromine to brominated reaction fluorination reaction is further illustrated by the following products. 30 examples. The product from the bromination reactor compris Example IV ing essentially methane, brominated methanes and hy Into a nickel tube, 28' x 1' heated to a temperature drogen bromide are passed into a multistage hydrogen of 250° C. by a salt bath, was charged hydrogen fluoride bromide absorber 2 in which the product is contacted with and bromoform in a molar ratio of 2.45 for a contact an azeotrope solution of hydrogen bromide in water.
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