2,732,410 United States Patent Office Patented Jan. 24, 1956 2 The process can be carried out in various ways. Thus, 2,732,410 the vaporized halogen fluoride can be passed, if desired PROCESS FOR PREPARNG TETRAFLUORO with an inert carrier gas such as nitrogen, argon or helium, ETHYLENE BY REACTING CARBON AND through a column of carbon heated at a temperature of A BINARY HALOGEN FLUORDE at least 1500 C. in a suitable reactor, e. g., a graphite Mark W. Farlow, Holly Oak, and Earl L. Muetterties, tube placed inside a resistance furnace or an induction Hockessin, Dei, assignors to E. I. du Post de Neois's furnace. The gaseous reaction products are then imme and Company, Wilmington, Del, a corporation of Dela diately treated, as described below, to remove any un Ware reacted halogen fluoride and any free chlorine or bromine, 0 in order to minimize or eliminate the possibility of their No Drawing. Application January 12, 1955, reacting with the tetrafluoroethylene present in the re Serial No. 481,482 action product. A preferred mode of operation consists in 4 Claims. (C. 260-653) reacting the halogen fluoride with the carbon electrodes of a carbon arc, where the temperature is estimated to be in This invention relates to a new process of preparing 15 the range of 2500 to 3500-4000 C., and again immediate compounds containing only carbon and fluorine, or car ly removing from the effluent gas any halogen fluoride and bon, fluorine and another halogen, such compounds be free reactive halogen which may be present. Preferably, ing called hereinafter fluorocarbons and halofluorocar the vaporized halogen fluoride is passed through the car bons. More particularly, the invention relates to a new bon arc, for example in an apparatus of the type de process of preparing tetrafluoroethylene. 20 Scribed below. However, it is also possible to operate The fluorocarbons are known to possess considerable with the carbon arc submerged in the liquid halogen usefulness in many fields of applied chemistry. For ex fluoride in a suitably designed apparatus permitting rapid ample, they have demonstrated utility as dielectrics, re escape of the volatile halocarbons containing the tetra frigerant liquids and ingredients. of insecticidal composi fluoroethylene so that the effluent gas can be treated at tions (e. g., as propellants). Tetrafluoroethylene has 25 once to free it from entrained halogen fluoride and already achieved outstanding commercial success in the halogen. form of its polymer. The halofluorocarbons are also The removal of the unreacted halogen fluoride and free extremely valuable compounds. For example, the chloro chiorine or bromine from the reaction product can be fluorocarbons find extensive use as refrigerant materials accomplished in various ways. The most effective method and the bromofluorocarbons as fire-extinguishing liquids. 30 consists in treating the effluent gas, immediately after it This invention has as an object a new process for the leaves the reaction zone, with a chemical agent which de preparation of tetrafluoroethylene. A further object is a stroys the halogen fluoride by hydrolysis or metathesis. process for the preparation of perhalocarbons containing Such agents include water, solid alkali metal hydroxides fuorine. Other objects will appear hereinafter. Such as potassium or sodium hydroxides, aqueous solu These objects are accomplished by the present invention 35 tions of the alkali metal hydroxides, carbonates, or sul wherein tetrafluoroethylene and other compounds contain fites, and the like. Suitable neutralizing agents for the ing only carbon and halogen are synthesized by bringing unreacted halogen fluoride and corresponding halogen are a binary halogen fluoride, i. e., a binary compound of the metal iodides and particularly the alkali metal iodides, fluorine with another halogen, in contact with carbon particularly potassium iodide and sodium iodide, which at a temperature above 1500 C., and removing any un 40 can be used in the Solid state or in aqueous solutions. reacted halogen fluoride and any free halogen of atomic These iodides and particularly the iodides of alkali form number less than 53 from the reaction product as soon ing metals, i. e., the alkali metals and alkaline earth as possible after contact with the carbon. imetals are particularly useful when chlorine fluorides or in this reaction, the effluent gas after passage through bromine fluorides are used. These chemical absorbents the reaction Zone normally contains, besides the reaction are used in amounts at least stoichiometrically equivalent products, unchanged halogen fluoride and the free halo to the amount of halogen fluoride used for reacting with gen, other than fluorine, corresponding thereto (fluorine the carbon. Another method of removing the unchanged is not present since it reacts instantly with the carbon). halogen fluoride and halogen from the reaction product In order to obtain the desired tetrafluoroethylene by this consists in cooling it quickly to a temperature low enough process, it has been found essential to remove these con 50 to condense the halogen fluoride and halogen but not low taminants from the reaction product as soon as possible enough to condense the tetrafluoroethylene. This cooling after it leaves the reaction zone. If this is not done, tetra procedure is satisfactory when using bromine fluoride or fiuoroethylene is not obtained because the halogen fluo iodine fluoride, but with the highly reactive and more rides and the more reactive halogens (chlorine and bro 55 volatile chlorine fluoride it should be replaced or com mine) add rapidly to the double bond of tetrafluoroethyl plemented by chemical removal. ene. The presence of free iodine in the reaction product Regardless of whether physical or chemical means are can be tolerated since it is less reactive with tetrafluoro adopted to remove the halogen fluoride and free halogen ethylene than the halogens of lower atomic number. from the reaction product, this removal should be done The halogen fluorides suitable for use in the process of 60 as quickly as possible after effluent gas leaves the reac this invention include chlorine I fluoride, CIF; chlorine tion Zone. It is not possible to state, with extreme Ii fluoride, ClF3; bromine I, III and V fluorides, BrF, accuracy, the critical time limit but, for best results as BrF3, and BrF5, respectively; iodine V fluoride, IFs, and regards the yield of tetrafluoroethylene, it is desirable iodine VII fluoride, IFi. These materials, which are that the effluent gas be treated for halogen fluoride re gases or low boiling liquids, can be prepared by methods moval within five seconds after leaving the reaction zone, described in the literature. One of them, iodine V fluo 65 and preferably within one second. ride, F5, can be made without having resort to elemental Moreover, conversions to tetrafluoroethylene are im fluorine by reacting iodine with silver I fluoride. For this proved when the effluent gas is cooled as quickly as the reason, and also because it leads to good conversions to physical features of the apparatus permit, since rapid halocarbons containing very substantial amounts of tetra quenching minimizes thermal conversion of tetrafluoro fluoroethylene, iodine V fluoride is the preferred starting 70 ethylene to other products. More specifically, it is material for use in this invention. desirable to cool the reaction product to a temperature 2,732,410 3. 4 below about 400 C, within a very short time after it dichlorodifluoromethane, bromotrifluoromethane, dibro leaves the reaction zone, e. g., within ten seconds and modifluoromethane, and iodotrifluoromethane. Higher preferably within less than five seconds. halocarbons, such as hexafluoropropane and octafluro It is desirable to carry out the reaction with an excess propane, are formed in Smaller amounts. These various of carbon relative to the halogen fluoride, in order to products can be separated by fractionation in a low convert as much as possible of the latter to fluorocarbons temperature, high pressure stili. and halofluorocarbois. There is suitably used at least 3, The following examples in which parts are by weight and preferably at least 10, gram atoms of carbon per are illustrative of the invention. mole of halogen fluoride. A much greater excess of car bon can be used if desired. O Example I Any form of carbon, whether amorphous or crystal 8tomine fluoride, BrF3, was reacted with the carbon line, is suitable for the purpose of this invention. Thus, electrodes of a carbon arc as follows: The anode was there can be used coal, graphite, charcoal, the various a hollow graphite cylinder, Ag inch outside diameter and forms of carbon black Such as lamp black, acetylene BA6 inch inside diameter, mounted on a copper tube. The black, bone black, etc. In general, higher conversions 5 cathode was a solid 546 inch graphite cylinder mounted are obtained with active carbon, of which many well on a copper tube having perforations near the end holding known varieties are available commercially. Active cat the cathodie. The cathode was positioned with its end bon is very finely divided, porous carbon having a total aearly fitish with the open end of the hollow anode. surface area of at least 20 square meters per grain The electrodes were mounted in a water-cooled, gas (Hassler, "Active Carbon, Chemical Publishing Co. 1951, 20 tight glass jacket which was evacuated to a pressure of p. 127). When using the carbon arc, the activity or a few tenths of a millimeter of mercury. In the opera state of subdivision of the carbon is apparently of no tion of this type of arc, the incoming gas flows out of consequence, but the carbon must, of course, possess the perforations in the copper tube around the carbon sufficient conductivity. The carbon need not be rigor cathode and enters the hollow anode, passing through ously pure and it may, for example, contain the normal 25 the burning arc at this point.