Preparation of FLUOROCARBONS by catalytic fluorination o f hydrocarbons George H. C a d y ' , Aristid V. GrOsse*, E. J. Barber', L. L. Burger', and Z. D. Sheldon3 WAR RESEARCH LABOR4TORIES, COLUMBIA UNIVERSITY, NEW YORK, N. Y.
THE technique for catalytic fluorination has been iniproved to such an extent that a desired saturated fluorocarbon may be produced in moderate to high yield b v the reaction of the corresponding hydrocarbon with elementary fluorine. The hydrocarbon vapor and the fluorine are each diluted by nitrogen and are gradually mixed iu the presence of fine copper turnings or ribbon coated with a thin layer of silver fluorides. This procedure has been used to produce straight or branchedchain fluorocarbons ranging from perfluorobutane to perfluorohexadecane and naphthenic fluorocarbons from C6Flz to Cl8F30. Volatile hydrocarbon lubricating oils have also been fluorinated. It appears probable that the fluorocarbons are produced by the action of the fluorinating agent, AgFz, upon the hydrocarbon vapor and that the supply of the silver difluoride in maintained by the reaction: 2AgFz f Fz ---.f 2AgF2.
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N J A N J A R Y 1942, a growing war project at Columbia University added a research group whose assignment was to learn
to prepare liquid fluorocarbons, including oils. The group started its work by trying out methods which had been used by others to produce such materials in very low yields. The first tried was the direct fluorination of hot, finely divided carbon under the reaction conditions described by Simons and Block ( 4 ) . The experiences of these authors were duplicated, and it was confirmed that the principal product of the reaction was carbon tetrafluoride. Higher fluorocarbons such as C,Fl" and C$Flawere produced, but the yields were very small. Another method tried a short time later was the vapor-phase reaction of fluorine with a hydrocarbon in the presence of Cu-gauze. At first the procedure of Fukuhara and Bigelow ( 2 ) for the fluorination of benzene was followed closely. As in their work, the fluorocarbon product was composed largely of a compound represented by the formula CsPlz. The yield of this compound was small but considerably greater than in the cme of the combustion of carbon in fluorine. The process was then studied intensively, and particular efforts were devoted to the development of the fluorinating catalyst, with the result that improvement sin the technique greatly increased the yields of desired products. During the development, the method was used to fluorinate many pure hydrocarbons and mixtures of hydrocarbons such as light lubricating oils. A wide variety of pure fluorocarbon compounds and mixtures was obtained. These ranged from gases through volatile fluid liquids to lubricating oils and rosinlike tars Crystalline waxes resembling paraffin were also produced in some cases. This development occurred simultaneously with that of a group working under R. D. Fowler a t The Johns Hopkins University on the fluorination of tiydrocarbons by metal fluorides. It was ten1
Present address, University of Washington, Seattle, Wash
3
Present address, University of Rochester, Rochester, N Y.
* Present address, Roudry Process Corporation, Marcus Hook, Pa.
tatively decided that the method using cdbalt trifluoride was superior for producing such fluorocarbons of moderate molecular weight as CsF,,, C F,,, and CeF,,, but that the catalytic technique way to be preferred for producing lubricating oils. Accordingly, the first industrial production of fluorocarbon oils, as conducted by the Du Pont Company, made use of the catalytic method. EQUIPMEYT AND OPERATIhG COhDITIONS
The system used consisted of the folloning parts: a group of cells for producing fluorine; tanks, valves, flowmeters, etc., for adding controlled streams of nitrogen to the gases before they were caused to react; an evaporator for the hydrocarbon capable of being held a t a nearly constant temperature and constructed to permit a steady stream of nitrogen t o sweep along a uniform flow of hydrocarbon vapor, a reaction vessel filled with catalyst and electrically heated t o the desired temperature, and a system of traps for collecting the reaction products. Figure 1 shows the arrangement of these parts. Small chip-shaped copper turnings covered with a coating of metallic silver were used as the catalyst. The reaction vessel was filled with this material. Before the system was put into operation, the silver coating was converted to AgFz by passing an excess of fluorine through the chamber. Traps for collecting the product were held a t different temperatures by cooling with either water, solid carbon dioxide in trichloroethylene, or liquid oxygen. Materials of low volatility collected in the first trap while hydrogen fluoride and volatile fluorocarbons condensed in the second. The fluorocarbons of lowest boiling point, which condensed in the third trap, were collected only to learn something about the extent of breaking of the chains and rings of carbon atoms during the fluorination reaction. Fluorccarbon products of each fluorination reaction, which collected in the first two traps, were combined and later separated into cuts of different volatility by fractional distillation. Th.e first lubricating oils were separated by a simple distillation at 10 mm. of mercury pressure. Several of the more volatile compounds were purified by careful fractional distillation. Desirable conditions for conducting the fluorination to obtain the best yield of a fluorocarbon having the same number of carbon atoms per molecule as the original hydrocarbon were found to be the following: The catalyst should be finely divided, of uniform porosity, and large channels for the flow of gas. A catalyst coated with silver fluorides should be used in preference to copper only (cobalt fluorides also behave as a catalyst). The streams of fluorine and hydrocarbon vapor should be introduced a t opposite sides of the reaction vessel (approximately 3 inches apart) and allowed t o mingle with each other as they flow side by side through the bed of catalyst. A slight excess of fluorine should be used so that the proportion of partially fluorinated compounds in the product is small. An inert diluent such as nitrogen should be added t o the reacting gases in a proportion of about two volumes of diluent per volume of fluorine. Although the reaction vessel should preferably be held a t about 200" C., it is possible, but not advisable from the standpoint of R ithout
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agent and be responsible for the beiieficial action of copper gauze, originally introduced by Fredenhagen and Cadenbach ( 1 ) . Others have suggested that the copper may simply serve the useful function of removing heat and thereby allowing the reaction to occur without combustion and extensive breaking of the chains and rings of carbon atoms. Fukuhara and Bigelow (2) discussed the fluorination of benzene in the presence of copper gauze and proposed a free radical mechanism in which the copper plays no part. The f l m of metal fluoride on the metallic catalyst appears to serve as a fluorinating agent, and a fluorocarbon such as CsF,, then results from a series of reactions which may be summarized by two equations of the following types:
+ Fz +2AgFz +C6F12 + 6 H F + 18AgF
2AgF 18AgFz
PRODUCT
Figure 1. Apparatus for Catalytic Fluorination of Hydrocarbons
the life of the catalyst, to increase the temperature to as much as 326" C. in order that hydrocarbons of low volatility may be fluorinated. The catalyst should not be used a t temperatures below about 140" C. because a t low temperatures the surface becomes coxted Kith a film of polymerized fluorocarbon and loses its activity. Reacting gases should not be passed a t rates which raise the catalyst above its desired operating temperature. The catalyst base should have high heat conductivity and heat capacity, so as to prevent the formation of overheated zones. TYPICAL EXAMPLES OF CATALYTIC FLUORINATION
Among the different hydrocarbons fluorinated JTere those listed in Table 1. Each of them \\-as caused to react with a small excess of fluorine a t the temperature indicated, and the yield of product was calculated on the basis of the amount n hich might theoretically have been obtained from the quantity of hydrocarbon used. In the first four cases each of the principal pure fluorocarbon compounds was identified by analysis and molecular weight. The product from anthracene was not purified and definitely identified, but it is almost certain that the principal compound mas CI4Fz4. The light hydrocarbon oils which w r e fluorinated yielded products of wide boiling range, including moderate proportions of fluorocarbons corresponding in number of carbon atoms per molecule t o the original materials. Experience with the catalytic fluorination of many hydr ocarhons has led to the following conclusions: ( a ) FThen members of a .series of hydrocarbons, such as the normal paraffins, are caused t o react, the yields of fluorocarbon become less as the number of carbon atoms in the molecule of hydrocarbon becomes greater; ( b ) the more stable the type of hydrocarbon, the higher the yields of desired product (aromatic hydrocarbons can be fluorinated with less degradation than paraffin hydrocarbons) ; (c) a partially fluorinated hydrocarbon can be carried to complete fluorination more efficiently than the pure hydrocarbon. NATURE OF CATALYTIC REACTION
Miller, Calfee, and Bigelon (3) have suggested that a film of cupric fluoride coating the metal may behave as a fluorinating
+
These equations are not intended t o express the mechanism of the reaction in any way except to indicate that fluorine forms an active fluorinating agent, AgF,, q-hich in turn reacts with the hydrocarbon or partially fluorinated hydrocarbon to form the fluorocarbon eventually. Obviously, side reactions should occur as the result of direct action of fluorine upon the pure or partially fluorinated hydrocarbon, for these materials must contact each other. Evidence for side reactions is found in the fact that more degradation and polymerization occurs during catalytic fluorination than during fluorination of a hydrocarbon by AgFl or CoF3. It is somewhat surprising that the products of catalytic fluorinstion predominate as they do over those formed by the direct action of fluorine.
TABLEI. TYPICAL EX.+lfPLES
O F CATALYTIC
FLUORISATION
Total Yield Yield of Av of Fluorocarbon Desired Temp. of Boiling above Product, 3laterial Rezction, 20" C., 7' of Desired Approx. % of Fluorinated C. Theoretical Product Theoretical CeHa 265 75 58 n-Heptane 135 75 62 CeHsCFs 200 90 85 200 CeHd(CFs)r 93 87 Anthracene 300 43 73 Oil A'L 290 68 15 Oil J b 300 76 19 Quaker State 300 60 12 a 4 light acid-treated naphthenic lubricating oil which distilled between 185' and 215' C. a t 10 mm. pressure. b An aromatic gas oil (Mirando) which distilled between 377' and 414' C. a t one atmosphere. This sample and oil A were supplied by the Standard Oil Company of 4 e w Jersey. A light Pennsylvania paraffin-base lubricating oil. d Cut boiling from 150' to 200' C. a t 10 mm. Hg
Catalytic fluorination resembles fluorination by ;igFo or CoF3, in that the reaction occurs best a t an elevated temperature and produces a saturated fluorocarbon containing the same number of carbon atoms (presumably in the same sort of arrangement) as the parent hydrocarbon. Direct gas-phase fluorination of a hydrocarbon vapor is a very different sort of reaction. It occurs rapidly even at temperatures beloiv room temperature, and leads principally t o the formation of products containing either fen er or many more carbon atoms per molecule than the parent hydrocarbon. .it high temperatures the hydrocttrbon burns in the fluorine and the main products are hydrogen fluoride and carbon tetrafluoride. At low temperatures the principal products are hydrogen fluoride and a rejinous, highly fluorinated polymer. 4CKNOW LEDGMENT
The authoi. are grateful for the help and encouragement furnished by the many persons who cooperated in the study of fluorocarbons. In particular, thanks are extended to the following men in the group at Columbia University: Irving B. Oneson, Homer Priest, Albert Myerson, Helmut M. Haendler, Vaughn Englehardt, Robert Cranston, li. D. Kirschenbaum, Donald H. Stewart, Lyle J. Hals, Robert Henig, Lloyd Roth, and the administra-
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tive officials H. C. Urey, J. R. Dunning, W. F. Libby, and R . H. Crist. Other persons t o whom the authors are grateful are Raphael Rosen of the Standard Oil Company of S e w Jersey, R. D. Fowler of The Johns Hopkins University, W. S. Calcott and A. F. Benning of the D u Pont Company, L. A. Bigelow and P. M. Gross of Duke University, J. H. Simons of Pennsylvania State College, E: T. McBee and H. B. Hass of Purdue University, Manson Benedict of The Kellex Corporation, A. L. Henne of The Ohio State University, W. T. Miller of Cornel1 University, F. H. Reed and G. C . Finger of the University of Illinois, and Henry Eyring of Princeton University. This work was performed in partial fiilfillment of an Office of Scientific Resixarch and Development Contract.
Vol. 39, No. 3
LITERATURE CITED
(1) Fredenhagen, K., and Cadenbach, G., Ber.. 67, 928-36 (1934). (2) Fukuhara, S . , and Bigelom, L. A , , J . A m . Chem. Soc., 63, 2792, (1941).
(3) Miller, IT. T., Calfee, J . D . , and Bigelow, L. .A., Ibid., 59, 198 (1,937). (4) Sinioiii, J. H.. aid Block, L.P., Ihid.,61,2962 (1939).
PRESFXTED hefurc t h e Sgiiipo.iuni on rluorinc Chemistry l i s i l a i r r 50, Dirision of Industrial and Engineering Chemistry, 110th Meeting of tltc ~ V E R I C S C H E U I C ASOCIETY, L Chicago, Ill.
SYNTHESIS OF FLUOROCARBONS R. D. Fowler, W.B. Burford 111, J. 31. Hamilton, Jr.I, R. G. Sweet2, C. E. Weber3, J. S. Kasper'. and I. Litant4 T H E JOHNS H O P K l h S UhIVERSITY, BALTIMORE, >ID.
.4 PRACTICAL and general synthesis for completely fluorinated organic compounds was de\ eloped; it consists of reaction between cobalt trifluoride and vaporized hydrocarbons or fluorohydrocarbons at elelated temperatures. .-i number of pure fluorocarbons, ranging from perfluoron-butane to perfluorocetane, were produced and characterized. Certain reaction by-products were identified. Perfluoro-n-heptane was obtained in jields up to 789" from n-heptane, and perfluorodimethjlcjclohexane i n 8894 4 ields from bis(trifluorometh>1)beiizeiie. Crude yields for these two feed stocks were 91 aud 9 7 7 ~respectilely. ~
With high boiliup feed stocks, such as light lubricating oils, jieltls of fully fluorinated material in excess of 50% ere obtained. The Tariables of the reaction were studied, and iniproled equipment was designed on the basis of the results. These rariables included reaction temperature. dilution of the hjdrocarbon with inert gas, contact time, and degree of CoF3 exhaustion. 4 rough measurement was made of the heat of the reaction between fluorine and CoF2, and the result used to calculate the heat of the reaction between the resulting CoF3 aiid hjdrocarhons.
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bons. There n-ere indications that these materials would be chemically inert, and there was interest in their possible use as fluids for the recoil mechanisms of large guns. Interest in the possibility of using these compounds for various purposes in the separation of uranium isotopes was also manifested; if these compounds could be made available on a large scale, it appeared that the separation of uranium isotopes would be greatly facilitated. Until October 1941 no pract'ical general method for the synthesis of perfluorocarbons had been reported in the literature. Several reviews of the subject had been written, among the more recent being those of Bockemueller (4) and Henne ( 1 4 ) . The syntheses reported could be divided roughly into two classes: those involving indirect techniques, such as a reaction between an organic halide and an inorganic fluoride, and those involving direct reaction between carbon and fluorine or the direct replacementof hydrogen by fluorine. The indirect techniques had been successful in introducing a fen fluorine atoms into organic molecules. Ruff and Keim (19) and Simons and Block (80) in 1930 and 1939, respectively, reported that the fluorination of elementary carbon resulted in the production of small amounts of fluorocarbons containing more than one carbon atom, in addition to the expected carbon tetrafluoride. A number of a t t e m p h had been made t o substitute hydrogen in hydrocarbons with fluorine by reaction between the hydrocarbons and elementary fluorine. Among the more successful were a vapor-phase method described by Bigelow (3)and a liquid-phase method described by Fredenhagen and Cadenbach ( 1 1 ) . In both of these cases, hon-ever, the
resulting products consisted of a coniplicated mixture of polymers and degradation products with only very small yields of the desired fluorocarbon. It was suggested by Roger Adanis and Ralph Connor that the n-ork of Ruff and Keirn and of Simons and Block he repeated in the hope of increasing the yields of heavier fluorocarbons. This was done. utilizing Sorite sugar charcoal, various other forms of carbon, and silicon carbide. The temperatures Tvere varied from 400" to 550" C., and the concentration of fluorine varied over a wide range by nitrogen dilution. However, in all cases S0-9070 of the product was a gas at room temperature, and the greater portion of the liquid product boiled helow 100" C . Although small amounts of higher fluorocarbons-including a little oily and a little waxlike material-were produced in these esperimentq, the results gave little promise of a practical synthei;is. Ruff (18) discovered a number of metallic fluorides in which the metal ion possessed a high ralence. These compound.; could be made only by the use of elementary fluorine, and Ruff showed that they were strong oxidizing agents. MnF3, AgF2, and CoF3 were three of these fluorides. They had been prepared in thi. laboratory previous to October 1941 and had heen used as fluorinating agents for converting CF,into CFs. It Peemed possible then that these compounds might react n-ith hydrocarbons to replace hydrogen by fluorine as does elementary fluorine, and that the reaction n-ould he 1e.s drastic. The replacement of hydrogen n-ith fluorine in hydrocarbons n-ould therefore essentially be accomplished by the uqe of elementary fluorine, hut the process v-ould be in two steps: fir-t, the fluorination of the lon-er metallic fluoridr to the hitrher:
]Present address, E . I. d u P o n t de Sernours & Company, Inc., KImington, Del. 2 Present address, The Linde Air Products Company, Tonawanda, N . T. Present address, General Electric Company, Schenectady, N. Y. 'Present address, New York University, S e w York, 1..Y .
and, second, the replacement of hydrogen with fluorine in the hydrocarbon by the higher fluoride:
S THE fall of 1941 work was undertaken in this laboratory on a general and practical method for the synthesis of fluorocar-
