Classifications

• • C—CHEMISTRY; METALLURGY
  • C07—ORGANIC CHEMISTRY
  • C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
  • C07C17/00—Preparation of halogenated hydrocarbons
  • C07C17/26—Preparation of halogenated hydrocarbons by reactions involving an increase in the number of carbon atoms in the skeleton
  • C07C17/263—Preparation of halogenated hydrocarbons by reactions involving an increase in the number of carbon atoms in the skeleton by condensation reactions
  • C07C17/269—Preparation of halogenated hydrocarbons by reactions involving an increase in the number of carbon atoms in the skeleton by condensation reactions of only halogenated hydrocarbons

Definitions

• This invention relates to the manufacture of perfiuoroolefins, and more particularly to a process suitable for the co-synthesis of tetrafluoroethylene and hexafiuoropropylene.
• pyrolysis is employed hereinafter to signify the conversion of one chemical species to another by the action of heat, regardless of whether the molecular complexities of the final products are greater or less than that of the starting material.
• fiuorocarbons is employed herein to denote hexafluoropropylene, tetrafluoroethylene, and perfluorocyclobutane.
• Tetrafiuoroethylene and hexafiuoropropylene may be polymerized or copolymerized with each other and with other vinyl monomers to form an exceedingly valuable series of resinous products.
• Such products as polytetrafiuoroethylene and copolymers of tetrafluoroethylene with hexafiuoropropylene are particularly valuable, being inert to almost all known chemicals with the exception of molten alkali metals, having extremely low coefiicients of friction and having an extremely wide range of use temperature.
• Perfiuorocyclobutane is valuable as a refrigerant and propellant and may also be converted to tetrafluoroethylene and hexafiuoropropylene.
• the yields of the various species referred to herein are calculated as the weight percent obtained from chlorodifluoromethane calculated on the basis of carbon and fluorine atoms only.
• chlorodifiuoromethane may be pyrolysed to give tetrafluoroethylene, for example, as described by Downing in US. Patent 2,551,573 issued May 8, 1951.
• hexa fiuoropropylene can be prepared directly from chlorodifiuoromethane by pyrolysis in high yield.
• perfluorocyclobutane is formed by the pyrolysis of chlorodifiuoromethane.
• An object of the present invention is to produce tetrafluoroethylene and hexafluoropropylene suitable for the production of fluorocarbon resins. Since tetrafluoroethylene may be readily obtained by modification of the .process of the present invention in high yield, it is more especially an object to obtain hexafluoropropylene in economic yield directly from chlorodifluoromethane.
• the above objects are achieved by the pyrolysis of a feed comprising chlorodifiuoromethane at a pyrolytic conversion between 86% and 94% based on the chlorodifiuoromethane charged.
• the temperature should be maintained in the range between about 700 C. and 900 C., and the pressure is preferably maintained between 0.5 and 1.2 atmospheres absolute.
• the pyrolysate is then cooled and tetrafiuoroethylene and hexafluoropropylene are separated from the product.
• Another modification of this invention comprises separating octofluorocyclobutane and chlorodifluoromethane from the reaction product and recycling these compounds in the feed.
• the octofluorocyclobutane product may be pyrolysed separately and the pyrolysis products of the octofluorocyclobutane may be added to the pyrolysis product of of the chlorodifiuoromethane.
• Yet another modification of this invention is to employ a feed consisting of crude chlorodifiuoromethane containing hydrofluoric acid.
• the conversion process may be controlled by controlling the temperature or by controlling the time of the reaction, or in a flow-type system by the rate of flow of the reactants, or all of these variables may be employed to control the conversion. From a practical standpoint it is generally preferable to employ a tubular furnace, control the temperature, and preferably also the temperature profile as explained hereinafter, and then control the degree of conversion by controlling the rate of flow of the reactant stream to the pyrolysis furnace. This controlling operation may be performed by hand, on the basis of periodic analyses of the product.
• the lower limit of conversion 86% appears to be substantially independent of other reaction conditions.
• the upper limit of about 94% represents a limit determined by economic yield and operability of the process and is somewhat dependent on the exact reaction variables selected. Generally, it is preferable to operate well below this limit of conversion, although operation is feasible up to the limit. The most pronounced eflfects are produced by pressure. Increasing the pressure reduces the yield of useful fiuorocarbons substantially, and consequently lowers the limit of conversion at which the process is usefully operable. On the other hand, at very low pressure, improved overall yields of useful fluorocarbon may be obtained, although the amount of hexafluoropropylene does not increase.
• the process may be operated with advantage at partial pressures 0.1 to 2 atmospheres, but preferably in most instances at 0.5 to 1.2 atmospheres.
• the process should be operated at a partial pressure of chlorodifluoromethane between 0.1 atmospheres and 2 atmospheres and preferably between 0.5 and 1.2 atmospheres.
• the partial pressure may be varied by operating the process under increased or reduced pressure by methods well known to those skilled in the art, or by the dilution of the chlorodifluoromethane with a chemically unreactive gas such as nitrogen, or certain unreactive gases to be described hereinafter.
• the temperature at which pyrolysis takes place is not highly critical but a temperature in the range between about 700 C. and 900 C. should be employed. In this temperature range the contact time required is between about 0.1 second and 10 seconds. It is therefore highly convenient to employ a flow system for the pyrolysis whereby the reactant vapors are passed through a long furnace, and the pyrolytic reaction may be controlled by controlling the rate of flow or the heat input as explained hereinabove.
• the process of the present invention is a pyrolytic reaction which depends on the transfer of heat to the reactant gas, and is accomplished by the agency of heat. Any material which will withstand the necessary temperatures, pressures and the chemical action of the reactants, reaction products, or intermediates at the aforesaid temperatures and pressures may be employed for the construction of the furnace.
• the noble metals are particularly pre* ferred as materials of construction, or as a lining of the surface exposed to the reaction, but it will be realized that other materials may be employed, e.g. silver, carbon or Iconel.
• the furnace should be constructed to have a high surface to volume ratio and a long length in order to promote effective heat transfer to the gas at short residence time.
• the furnace should have a surface/ volume ratio of at least 5 inches-
• the pyrolysis furnace may be heated by any convenient means. Electrical resistance heaters have been employed very successfully but other heating means, such as natural or coal gas, oil and the like, may be used to heat the furnaces.
• the chlorodifiuoromethane employed in the process of this invention as the starting material is an article of commerce. It may be manufactured at low cost by the catalytic fluorination of chloroform with hydrofluoric acid.
• the reaction product is distilled, and a crude chlorodifluoromethane is obtained which contains some 2% by weight of hydrofluoric acid which distills with the chlorodifluoromethane as an azeotrope.
• the hydrofluoric acid may be removed from the crude material by washing with water, and with alkali metal hydroxide solution, or by other suitable procedures. It has been found, however, that crude chlorodifluoromethane containing about 2% of hydrofluoric acid may be employed in the practice of this invention without appreciable loss in the yield of useful fluorocarbons.
• the perfiuorocyclobutane which is formed by the process of the reaction can be pyrolysed to give tetrafluoroethylene and hexafluoropropylene in good yield.
• tetrafluoroethylene and hexafluoropropylene in good yield.
• the greatest yields of hexafiuoropropylene are obtained when the perfiuorocyclobutane is pyrolysed at high conversion, although there appears to be no critical range of conversion, as has been found in the case of chlorodifluoromethane.
• the perfiuorocyclobutane may be separated, pyrolysed at a temperature between about 700 C. and 900 C. and the reaction product added to the efiiuent from the pyrolysis furnace employed for the pyrolysis of chlorodifluoromethane.
• the perfluorocyclobutane may be pyrolysed concurrently with the chlorodifluoromethane by adding the perfiuorocyclobutane separated from the product stream to the reagent chlorodifluoromethane entering the pyrolysis furnace.
• perfiuorocyclobutane is pyrolysed at high conversion, and that very little yield loss takes place on account of side reactions. The small yield loss encountered in this modification is offset by the simplicity of the resultant equipment.
• the separation of the products of pyrolysis may be attained by distillation in an efficient fractionation column, and by extractive distillation in the presence of hydrocarbons, aromatic hydrocarbons or chlorinated hydrocarbons.
• the first of these furnaces was a small laboratory furnace which consisted of a pure silver tube 30 inches in length and 0.5 inch in diameter, and having a wall thickness of inch.
• Two electrical resistance furnaces of 750 watt rated capacity were employed to heat this tube (which was maintained in a horizontal position), the heating zone of each was 12 inches.
• the furnace heaters were controlled with an autotransformer.
• Furnace temperatures were measured with a Chromel-Alumel thermocouple located in the center of the furnace.
• the reactant gas was taken from a cylinder, metered and mixed as needed with other gases.
• the gases that emerged from the furnace were passed through a coil of inch Inconel tubing immersed in a water bath.
• thermocouples Welded to the walls of the pyrolysis tube were measured with the aid of thermocouples Welded to the walls of the pyrolysis tube.
• Examples 1 to 10 which are collected together for comparison in Table I, demonstrate the importance of the level of conversion in the process of this invention. Inspection of this table reveals the surprising increase in the concentration of hexafluoropropylene in the pyrolysis product which takes place at conversion levels above 86%. Again, the sharp decrease in the yield of useful fiuorocarbons with increasing conversion and the formation of increasing amounts of unrecoverables is clearly shown in the same table.
• a process for the co-synthesis of hexafiuoropropylene and tetrafluoroethylene which comprises passing Wt. percent (CF) of CHCIF in feed-Wt.percent (CF) in product 100 Wt. percent (CF) of CHCIF in feed Percent conversion (C 1 Wt. percent C 1 in feed Wt. percent C F in product X Wt. percent C F in feed chlorodifiuoromethane through a tubular furnace, said furnace being maintained at a temperature between 700 C. and 900 C., said chlorodifiuoromethane being maintained at a partial pressure between 0.5 and 1.2 atmos- Feed Wt. Percent Percent Product, Wt.
• Th1s was pyrolysed at a conof the said chlorodifiuoromethane iigg g i ggg f i gi g 33 32 52 i gz if 4.
• the process of claim 2 wherein the temperature 2nd run under identical c c mdifions was made with pure major and initial perm-m Pf the said furnace is mam- CHCIF Th Sulta t oducts (Tabl Were tamed at a temperature w1th1n the range between 700 6 re n Pr 6 C. and 800 C.
• a process for the co-synthesis of hexafiuoropropyl- 23% 35% ene and tetrafiuoroethylene which comprises passing a 3:0 6:1 mixture containing chlorodifiuoromethane and perfiuoro- 2&6 2&2 cyclobutane through a tubular furnace, said tubular furnace being maintained at a temperature within the range From this table it can be seen that the overall yield of b t 700 C, d 900 C, aid chlorodifluoromethane useful fluorocarbon is substantially the same from chlobeing at a partial pressure within the range between 0.5 rodifluoromethane containing 2% of HF as from Pur atmospheres and 1.2 atmospheres, controlling the rate of chlorodimethane.
• the Process of this invention is extremely Valuable for furnace to maintain a level of conversion between 86% the production of tetrafiuoroethylene and hexafiuoroproand 94% based on the chlorodifluoromethane charged, pylene which may be copolymerized with each other Or cooling the reaction product emerging from the said with other vinyl monomers, using free radical catalysts to pyrolysis furnace, separating perfiuorocyclobutane, hexaeffect the polymerization.
• the present process offers fiuoropropylene and tetrafiuoroethylene from the said great advantages in simplicity of operation and of ecoreaction product, and recycling the perfiuorocyclobutane nomy in the necessary plant required to effect the synto the mixture passed into the said furnace.
• thesis of hexafluoropropylene 7.
• 260-6533 tetrafluoroethylene, pyrolysing the said perfiuorocyclo- 3,009,966 11/1961 Hauptschein et a1.

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• Chemical & Material Sciences (AREA)
• Organic Chemistry (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Organic Low-Molecular-Weight Compounds And Preparation Thereof (AREA)

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Citations (9)

• 0
  • NL NL269647D patent/NL269647A/xx unknown
  • NL NL122622D patent/NL122622C/xx active
• 1960
  • 1960-09-28 US US58895A patent/US3306940A/en not_active Expired - Lifetime
• 1961
  • 1961-09-26 GB GB34422/61A patent/GB917093A/en not_active Expired
  • 1961-09-27 BE BE608590A patent/BE608590A/fr unknown
  • 1961-09-28 DE DEP27951A patent/DE1170935B/de active Pending

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