US3306940A - Process for the manufacture of perfluoroolefins - Google Patents

Description

United States Patent Ofiice 3,306,940 Patented Feb. 28, 1967 3,306,940 PROCESS FOR THE MANUFACTURE OF PERFLUOROOLEFTNS Ronald Harry Halliwell, Parkersburg, W. Va., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Filed Sept. 28, 1960, Ser. No. 58,895

7 Claims. (Cl. 260653.3)

This invention relates to the manufacture of perfiuoroolefins, and more particularly to a process suitable for the co-synthesis of tetrafluoroethylene and hexafiuoropropylene.

The term 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.

The expression useful 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.

It has been known heretofore that tetrafluoroethylene may be pyrolysed to give hexafiuoropropylene. Under certain critical conditions high yields may be obtained. See, for example, US. Patent 2,758,138 issued to D. A. Nelson on August 7, 1954.

It has also been known that chlorodifiuoromethane may be pyrolysed to give tetrafluoroethylene, for example, as described by Downing in US. Patent 2,551,573 issued May 8, 1951.

It has not been known heretofore, however, that hexa fiuoropropylene can be prepared directly from chlorodifiuoromethane by pyrolysis in high yield.

It has been discovered that when chlorodifiuoromethane is pyrolysed at low conversion so that a high yield of tetrafluoroethylene is formed, a small amount of hexafiuoropropylene is also formed. The quantity thus formed is too small to be of economic significance. As the percentage conversion is increased by increasing the temperature, or the contact time, or both, the amount of hexafiuoropropylene formed also increases. On the other hand, the unwanted side products, hereinafter called unrecoverables, also increase, and at a rate twice as great as the rate of increase of the hexafiuoropropylene. However, in the range between 86% conversion and 94% conversion a highly surprising effect has been found. In that range the concentration of hexafluoropropylene suddenly increases until the proportion of hexafiuoropropylene is comparable with and may exceed the concentration of tetrafluoroethylene. Moreover, the increase in yield with conversion is accompanied by an increase of 0.8 part of unrecoverables per part of hexafluoropropylene as compared with over 2.0 parts of unrecoverables per part of hexafiuoropropylene at lower conversion. Above about 90% conversion, the yield of unwanted products itself shows a sharp increase, and beyond about 94% conversion, the yield becomes less favorable economically. Again it has been found that, at conversions above 94%, carbon deposits form and rapidly choke the tubular furnaces which are the preferred form of reactor for the process of this invention.

In addition to tetrafluoroethylene and hexafiuoropropylene, perfluorocyclobutane is formed by the pyrolysis of chlorodifiuoromethane.

At conversions below about about two parts of perfluorocyclobutane are formed for each part of hexafiuoropropylene. As the conversion is increased to a level within the range from about 86% to 94%, the percentage of perfiuorocyclobutane in the useful product decreases rapidly, substantially vanishing at about 94% conversion.

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.

Other objects will be apparent hereinafter.

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.

It has also been discovered that greater overall yields can be obtained in the process of this invention by employing a tubular reactor and maintaining an ascending temperature profile along the reaction path. Another modification of this invention comprises separating octofluorocyclobutane and chlorodifluoromethane from the reaction product and recycling these compounds in the feed. In yet another modification, 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.

In the process of this invention it is highly critical to maintain the level of conversion of chlorodifluoromethane between 86% and 94%. Generally speaking, 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. It is preferable, however, to monitor the product stream, measuring the concentration (and hence conversion) of the chlorodifiuoromethane by such methods as mass spectrometry, infrared spectrometry, gas chromatography and the like, and hence electrically control a valve regulating the flow of reactant to the furnace in order to maintain the level measured in the product stream at the predetermined value. It will be understood that many other methods which will be obvious to one skilled in the art may be employed to control the level of conversion of chlorodifiuormethane.

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 increased efiiciency of the reaction, however, which may amount to as much as 15% greater yield to useful fluorocarbons at 0.1 atmosphere in comparison to results at 1 atmosphere, is counterbalanced to an increasing extent, as partial pressure of the chlorodifiuoromethane is lowered by the decrease in convenience associated with operation at the lowered partial pressures.

For the above reason, 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.

For the above reasons 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.

Generally speaking, 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. In any case 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. Preferably 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.

A study of temperature profiles along the furnace led to the surprising discovery that a furnace temperature between 700 C. and 900 C. which ascended along the length of the furnace in the direction of gas flow gave improved overall yields. Particularly good yields of useful fiuorocarbons are obtained when the furnace temperature is maintained at a temperature between 700 C. and 800 C. over a major and initial portion of its length and at a 4 temperature between 800 C. and 900 C. over a minor and final portion of its length.

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. Generally speaking, it has been found that 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. If it is desired to increase the yield of hexafluoropropylene and tetrafiuoroethylene at the expense of perfluorocyclobutane, 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.

It has also been discovered that 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. Under the conditions required for the pyrolysis of chlorodifiuoromethane it has been established that 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 invention is further illustrated by the following examples, which are given by way of illustration only, and are not intended to define the scope of the invention.

EXAMPLES In the following examples three furnaces were employed. 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. Samples of the cooled gases were then taken in gas sampling bottle mounted in a by-pass line of the line leading to an aspirator. The samples were analyzed, using gas chromotography. The gas chromotography column was standardized by using pure components. A constant volume syringe was employed to inject a series of standard volumes of air and pure compound. The factors relative to air were computed for the various compounds to be analyzed and hence the concentration of the components in the mixture were determined, assuming a linear relationship between the concentration of each component and the malt height on the chromatogram. The validity of this analysis was verified by analytical distillation techniques. 5

less steel tubular furnace to which were strapped three resistance heaters having a rated capacity of 7.5 kilowatts each. The assembly was contained inside a 5 inch mild 6 EFFECT OF FURNACE TEMPERATURE ON THE PYROLYSIS OF CHClF The eifect of furnace temperature on the yield of useful fluorocarbon and in the distribution of the useful products is shown in Table III; (Examples 15 and 16). These examples represent two runs made with furnace No. 2, under substantially identical conditions with the exception of the temperature profile. In both cases a contact time in the pyrolysis tube of 2.1 seconds was employed.

TABLE III Temperature, 0.:

(i) 2 ft. from inlet end. 720 810 steel p1pe which was covered with 1% inches of asbestos (u) 5ftfrom inlet end 735 815 laf q (111) 8 ft. from inlet end 750 775 ms 1 j (iv) lift. from inlet end 764 764 The third furnace was similar to the second furnace (v) 14 It. from inlet end (2 it. from F except that the heaters were replaced with six resistance at heaters, each having a rated capacity of 4.75 kilowatts, 82F; 38.0 12.1 and was then capable of more uniform heating. if The temperature in the second and third furnaces were Percent 011cm Couverted 90.0 87.7 Yield to useful fluorocarbons 73. 2 52.0

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.

It will be noted that not only is the overall yield to useful fluorocarbons decreased substantially when a descending furnace temperature profile is employed rather than an ascending temperature profile, but that the distribution of products is also unfavorable, a much larger proportion of C 1 being formed at the expense of the tetrafiuoroethylene and hexafiuoropropylene. C 1 may, of course, be converted to the unsaturated compounds TABLE I Example Furnace Contact Time, Temp. Percent C onv. C21 (3 1% CE; Yield to Useful Secs. Fluorocarbons 1 1. 8 687 38. 4 93. 1 1.1 3. 2 97. 4 2 -2 797 69.1 78.9 3. 7 9. 7 92.3 2 -2 841 81. 9 63. 8 7. 5 13. 8 85. 1 2 -2 907 89. O 31. 2 31. 9 6. 5 69. h 3 -2 806 86. 2 49. 7 14. 6 l4. 4 7S. 7 3 866 93. 6 46. 3 17.5 14. 3 78. 1 1 1. 8 869 91. 9 31. 2 36. 8 4. 8 72. 8 1 1. 8 881 94. 9 12. 8 49. 4 3. 1 65. 3 1 0. 19 923 92. O 48. 5 29. 2 2. 7 80. 4 1 0.19 931 93.0 36 4 36. 5 3.2 76. 1

The effect of pressure on the products of pyrolysis. chlorodifiuoromethane are shown in Examples 11 to 14, collected together in Table II.

C 1 and C 1 by subsequent pyrolysis, but this involves further loss of valuable material in the second pyrolytic step, and furthermore decreases the overall throughout TABLE II Partial Pres- Product Composition Yield to Example Contact Time, Hot Spot Total Pressure sure in Atmos- Useful Percent Conv.

Secs. Temp. in Atmospheres phcres of Fluorocarbons CHCIFB CzF-l C Fs CAFE From Table II it can be seen that the partial pressure of the CHCIF is the major factor, rather than the total pressure, and hence that the eifect of reduced pressure 70 of the pyrolytic synthesis equipment, when the unsaturated fluorocarbon C F and C F are alone required for the production of the polymer resins.

RECYCLE OF C F IN THE PYROLYSIS OF CHCIF Examples 17 to 22 were collected in Table IV to show the effect of adding perfiuorocyclobutaue to the feed of chlorodifluoromethane to the pyrolysis furnace.

The following definitions were employed to compute the degree of conversion of chlorodifiuoromethane and perfluorocyclobutane.

Percent conversion (CHClF arating tetrafiuoroethylene and hexafiuoropropylene from the reaction product.

2. 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. Percent Example P erceut C HClFz C4Fs N0. C4Fs converted converted CzFi C3F6 Unrecoverables 91 30. 9 37. 2 31. 9 90.9 99. 3 29. 9 40. 4 29. 7 89. 3 79. 2 35. 9 39. 3 24. s 30 90. 4 100 22 47. 4 30. s 45 39. 7 78.6 39 39.1 21. 9 100 8O 51 41 8 EXAMPLE 23.-PYROLYSIS OF CRUDE CHC1F2 pheres; and controlling the temperature of the said furnace and the rate of fiow of the said chlorodifiuoro- .crude CHClFfi i hyfidlstfnailon fi g methane to maintain a level of conversion within the pg g g 8 53 ytlc i p a 6 $3. 2; range between 86% and 94%, cooling the pyrolysis prodg r0 f am 6 2; emp Oye g g g net and separating tetrafluoroethylene and hexafluoro- Tif c fr rfl e i di i f coi' lsist d t if g n a eotrt ic mixture propylene from the Said reaction product i t 987 b f CHCIF p d b 40 3. The process of claim 2 wherein the temperature colitalmng a on Y 0 2 an 0 y of the said furnace ascends along the direction of flow weight of hydrofluoric acid. 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. and the temperature of a minor and final Plg ggg g g lggfi SQESEQ EX S SS portion is m aintained at a temperature within the range between 800 C. and 900 C. (1) (2) 5. The process of claim 2 wherein the chlorodifluoro- Yield (Wt.percent) methane contains about 2% by weight of hydrofluoric Crude CHClFz Pure CHClFz id 6. 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. fiow of the said mixture and the temperature of the said 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. A process for the co-synthesis of hexafiuoropropyl- Iclaim: ene and tetrafiuoroethylene which comprises passing 1. A process for the co-synthesis of hexafiuoropropylchlorodifiuoromethane through a first tubular furnace, said ene and tetrafluoroethylene which comprises pyrolysing furnace being maintained at a temperature within the chlorodifiuoromethane at a temperature in the range berange between 700 C. and 900 C., said chlorodifluorotween 700 C. and 900 C. at a partial pressure between methane being at a partial pressure between 0.5 to 1.2 0.1 and 2.0 atmospheres and at a conversion level beatmospheres controlling the rate of flow of said chlorotwecn 86% and 94% based on the chlorodifluoromethane difiuoromethane to maintain the level of conversion withh ged; 9 9 reaction product and thereafter sepin the range between 86% and 94% cooling the products 9 10 of reaction from the said first furnace and separating 2,979,539 4/1961 Errede et a1. 260653.3 therefrom perfluorocyclobutane, hexafluoropropylene and 2,994,723 8/ 1961 Scherrer et a1. 260-6533 tetrafluoroethylene, pyrolysing the said perfiuorocyclo- 3,009,966 11/1961 Hauptschein et a1. 260653.3 butane in a second furnace at a temperature in the range 3,016,405 1/1962 Lovejoy 260-6533 between 700 C. and 900 C. and adding the reaction 5 3,022,357 2/1962 Kasper 260-6533 product from the second furnace to the reaction product OTHER REFERENCES of the first furnace.

Wiist: 1,108,205, printed June 8, 1961, 260-648F References Cited by the Examiner (German apphcatlon K1 120)' UNITED STATES PATENTS LEON ZITVER, Primary Examiner. 2,404,374 7/ 1946 Harmon 260-648 ALPHONSOD SULLIVAN E 2,551,573 5/1951 Downing et a1. 260 653.3 f 2,617,836 11/1952 Pcaflson et aL 2 0 I. W. WILLIAMS, B. HELFIN, Assistant Exammers.

2,758,138 8/1954 Nelson 260-6533 15

Claims (1)

1. A PROCESS FOR THE CO-SYNTHESIS OF HEXAFLUOROPORPYLENE AND TETRAFLUOROETHYLENE WHICH COMPRISES PYROLYSING CHLORODIFLUOROMETHANE AT A TEMPERATURE IN THE RANGE BETWEEN 700* C. AND 900* C. AT A PARTIAL PRESSURE BETWEEN 0.1 AND 2.0 ATMOSPHERES AND AT A CONVERSION LEVEL BETWEEN 86% AND 94% BASED ON THE CHLORODIFLUOROMETHANE CHARHED, COOLING THE REACTIO PRODUCT AND THEREAFTER SEPARATING TETRAFLUOROETHYLENE AND HEXAFLUOROPROPYLENE FROM THE REACTION PRODUCT.