US2608593A - Fluorination process - Google Patents

Description

Patented Aug. 26, 1952 FLUORINATION PROCESS Robert 'D. Fowler and Harry G. Anderson, Baltimore, Md., assignors to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Application August 13, 1946, SerialNo. 690,339

1 Claim. (01. 260--653) The present invention relates to a process for producing fluorocarbons and, more particularly, to aprocess for preparing fiuorocarbons from hydrocarbons having a wide range of molecular weights.

Highlyfluorinated hydrocarbons, especially completely fluorinated hydrocarbons and hydrocarbons having not more than one to three unsubstituted hydrogens, are particularly valuable for use as lubricating oils in processes where it is desirable to provide inert lubricants. It has recently been found that hydrocarbons may be highly fluorinated by passing vapors of said hydrocarbons into contact with fluorides of certain metals, such as cobalt, manganese and silver, in their higher valence state at temperatures between 150 C. and 400 C. Although such processes in general produce high yields of the low molecular weight hydrocarbons (say, up to heptane), low yields have been obtained by the application of these processes to hydrocarbons of high molecular weight, say, of the order of about 17 to about 23 carbon atoms per molecule. Thus, rather limited yields of fluorinated hydrocarbons of the desired viscosity have been prepared with metal fluorides formerly employed. The viscosities of the oils produced heretofore in major amounts are lower than are required for certain uses.

It is an object of the present invention to provide a simple and inexpensive process for producing fluorocarbons of desired molecular weights in relatively high yield.

It is another object of this invention to provide a process for preparing fluorocarbons of relatively high viscosity from hydrocarbons.

It is also an object of the invention to provide a fluorinating process for a wide range of hydrocarbons which produces a fluorocarbon product with relatively low production of scission and fluorinolysis products.

According to the present invention, hydrocarbons are contacted with cerium tetrafiuoride at relatively high temperatures to produce fluorocarbons. The hydrocarbons in vapor phase may be passed over and/or through cerium tetrafluoride, or the hydrocarbons may be contacted in the liquid phase with the cerium tetrafluoride with or without a suitable diluent or solvent for said hydrocarbons.

While the present invention is more particularly concerned with the fluorination of hydrocarbons of moderately high molecular Weight, such as those having about 17 to about 23 carbon atoms per molecule, preferably those boiling between about 250 C. and about 400 C., it is also applicable to the fluorination of hydro;- carbons of lower or ofhigher carbon content,- including those having about 6 to about 30 carbons with a corresponding extensionof the boiling range. The term hydrocarbon may be understood to include hydrocarbons of the paraffinic, olefinic, aromatic and naphthenic series."

The cerium tetrafluoride is preferably in come minuted form and may be prepared by tfluoriria-' tion of cerium trifluoride with elemental. fluorine. In practice, it is. desirable that temperatures be as low as possible during the early stages of reaction because of the relative instability of the reactant molecules at this time, and that the temperature increase as the molecule becomes more stable in order to assure complete fluorination. In the intermittent vapor phase operation described herein, this may be accomplished by having the front end of the reaction cylinder at the lower temperature and the exit end at the higher, and in a true batch process would be accomplished by raising the temperature during the course of reaction.

The extent of this temperature range depends principally upon thefeed stock. It is necessary in vapor phase reaction that the lowest temperature be high enough to vaporize the feed stock and also be high enough to begin the substitution reaction. The upper end of the temperature range is such as to give the desired; degree of fluorination, and is limited only by the: increase in molecular scission or by practical considerations or equipment life and power consumption.

Thus, when fluorinating heptane with CeFi, the temperature ranges from between about and about 300 C. along the reactor, but in the case of a lubricating ;oil feed, it was necessary that even the front end be above300 C. to insure continued vaporization and. the final temperature (at the exit end of the reactor) was frequently as high as 400-450 C.

Better yields of fluorocarbons are produced by the process of this invention than have been found possible when using fluorides of cobalt or manganese, the yield running between 39% and 40% of the theoretical yield when operating in the vapor phase. Moreover. lubricants of relatively high viscosity are produced.

In carrying out the reaction in the vapor phase, it is preferred to have the cerium tetrafluoride in a rotary reaction cylinder equipped with agitator blades and to pass the hydrocarbon to be .fluorinated in vapor phase in intimate contact with the cerium tetrafluoride thereinalthough most closed chemical reactors may be employed, if desired. The cerium tetrafluoride may only partially occupy the volume of the cylinder or may fill the same substantially entirely. If desired, an inert gas, such as nitrogen or a vaporized fluorinated hydrocarbon (usually of low molecular weight), may be employed as a diluent as and may be passed into the reaction cylinder in admixture with the vaporized hydrocarbon. A slow rotation of the cylinder is preferably provided for obviating channelling of the gaseous hydrocarbon through the cerium tetrafluoride. Also the same result is produced if the reaction cylinder remains stationary and the agitation blades are rotated.

The fluorination of the cerium trifluoride to form cerium tetrafiuoride may be carried out in the same reactor by passing fluorine over cerium trifluoride. Economical operation is achieved by a recuperative process comprising fluorinating the cerous fluoride to ceric fluoride as described, thereafter stopping the flow of fluorine, passing the hydrocarbon vapor over the resulting cerium tetrafluoride to fluorinate the hydrocarbon with concomitant reduction of the cerium tetrafluoride to trifluoride, then sweeping out with nitrogen or other inert gas, and again fluorinating the cerium trifluoride to cerium tetrafluoride for the next run. Thus, in this operation, the cerium compound need never be removed from the reactor, and hence suifers virtually no quantitative loss in the process, but is re-utilized as often as is required. Care must be taken, however, thoroughly to purge the reaction vessel of organic compounds with the inert gas between operations. If such care is not taken, the difficulty of operation is increased and the yield is decreased. Preferably, it is advisable to prevent direct contact of fluorine and hydrocarbon.

Upon the completion of the fluorination reaction, the vaporized fluorocarbon and the hydrogen fluoride formed in the reaction are removed from the reaction chamber together and are condensed in a cooling chamber. Two liquid phases are formed, and these may be separated by settling, decantation and/or centrifugation. Thereafter, each layer may be separately purified by fractional distillation or fractional condensation, or, if desired, the original vapors as removed from the reaction chamber may be fractionally condensed. Any other suitable method of isolating the reaction products from the reacted mixture may also be employed.

The process may be carried out in batch or in continuous operation, as well as in the intermittent operation described. Thus, vaporized hydrocarbonsmay be passed into contact with cerium tetrafluoride in a reaction vessel, which is thereafter closed and heated to the reaction temperature, the products of the reaction being withdrawn after a suitable time period. Again, if desired, the hydrocarbons may be continuously passed through a reaction cylinder of suitable le th in countercurrent or concurrent flow with respect to a moving bed or stream of cerium tetrafluoride. at atmospheric, superatmospheric or reduced pressures.

In carrying out the fluorination of hydrocar- The reaction may be carried out 4 normal atmospheric pressure, although the use of an autoclave and high pressures permits the fluorination by this procedure of lower-boiling reactants. Some variation in temperature is permissible bearing in mind the heretofore discussed practical considerations. I

As noted supra, fluorocarbon solvents may be employed as reaction media, and those found particularly suitable for carrying out the present process are the highly fluorinated carbon compounds boiling above about C. and which are fluid at temperatures in the vicinity of their boiling points. Liquid fluorocarbon fractions boiling between 180 C. and 300 C. may be employed with satisfactory results.

The fluorocarbon solvent may contain a small proportion of elements other than carbon and fluorine. Thus, compounds containing one or two atoms of residual hydrogen in the molecule may be used, especially as part of this hydrogen is usually replaced by fluorine during the process. The character of this side reaction is generally not sufliciently pronounced to affect the main reaction or to interfereseriously with its control. Likewise, small proportions of other elements such as chlorine, bromine, oxygen or nitrogen may be present in the molecule without materially affecting the predominantly fluorocarbon character of the solvent.

The occurrence of some fiuorinolysis during the process of the invention is not unusual, and byproducts having a boiling range below that of the desired fluorination products are thereby produced. These by-products, after separation from the principal fluorination products, may be satisfactorily used as reaction media in the inventive process and may be recycled. A proportion of the desired fluorination products themselves may also be employed as fluorocarbon solvents, their use having the advantage that the introduction of foreign materials is thereby avoided. The process may be carried out in cyclic manner, with a part of the product of each fluorination being returned to the next successive fluorination for use as a fluorocarbon solvent therein.

As in the vapor phase operation, the liquid phase procedure may also be carried out in batch, intermittent, or continuous operation. It is preferred to conduct the process in each of such operations to provide gradually rising temperatures during the fluorination. Good agitation promotes uniform reaction and reduces the desirable proportion of solvent, where such solvent is employed. In batch operation, the gradual addition of the hydrocarbon to a mixture of the solvent and the cerium tetrafluoride permits a lower ratio of solvent to final product or to put it diiferently, a higher ratio of solvent to unreacted hydrocarbon. This ratio may, in general, vary between about four parts by weights of fluorocarbon solvent to one part of hydrocarbon and about thirty parts to one, respectively.

The following examples are illustrative of the present invention but it will be understood that the inventionis not limited thereto:

Example 1 A lubricating oil with an average molecular weight of approximately 300 (Standard Oil #557; D101 45) was vaporized at a rate of about 4 ml. liquid per minute. This vaporization was accomplished at about 375 C. and a slow stream of nitrogen (1 L/min.) was used to sweep the hydrocarbon vapor through the vaporizer. The two gases were passed into and through a horizontal steel cylinder 8 feet long and 8 inches Internal Diameter containing'about 165 lbs. of CeF-i constantly agitated by paddles fixed to an axially rotating shaft. The contents of the reaction cylinder were maintained at a temperature of about 300 C. at the front, rising to about 425 C. at the exit.

The eiiluent gases were passed through a watercooled condenser Where the bulk of the resulting fiuorinated product and hydrogen fluoride condensed. In order to remove final traces of these products from the permanent gas, the efliuent from the first condenser was passed through another condenser at a lower temperature (-25 C.)

The liquid in both condensers separated into two layers, the lower of which comprised the larger portion of the fiuorinated oil and some hydrogen fluoride. This product was drawn off by gravity, neutralized with dilute NaOH solution and distilled, cuts being taken at specific temperature intervals, in order to obtain a fraction meeting the desired specifications.

In this run, 683 gms. of crude neutralized product were obtained from 500 gms. of oil fed. Of this material, 72 gms. boiled below 160 (3., 131 gms. boiled between 160-225 C., and 460 gms. boiled above 225 C. with a 20 gm. loss.

Example 2 Equipment and technique identical with that described in Example 1 except that the feed stock was a lower boiling oil (Standard Oil #1338; XCT White Oil) was employed. The crude product from the first run was 814 gms. and a from the second 965 gms. These were combined for distillation, and it was found that 139 gms. boiled below 160 C. and 1601 gms. boiled above 160 C. with a loss of 39 gms.

Example 3 Fifteen hundred m1. of fluorocarbon material boiling at 105-200 C. was placed in a copper kettle equipped with a diphenyl oxide heading jacket and a stirrer. To this was added 15 gms. of Cenco Hyvac vacuum pump oil and 1200 gms. of CeF4.

This mixture was stirred at 90 C. for 12 hours and then filtered. The CeFr was leached several times with Freon 113 and the washings added to the original filtrate. Upon distillation 27.5 gms. of material were found to boil above the original upper limit of the fluorocarbon solvent 6 (200 0.). Unlike the starting hydrocarbon this light yellow material was heavier than water.

Although the present invention has been described with reference to specific embodiments, it will be appreciated that variations and modifications thereof may be made and that equivalents may be substituted therein.

We claim:

A method for the perfluorination of a hydrocarbon, to replace, with fluorine, all hydrogen atoms attached to carbon therein, while retain ing the carbon structure of the starting compound, which includes the steps of: (1) vaporizing a hydrocarbon, (2) maintaining solid cerium tetrafluoride in a reaction zone at a temperature between about degrees and about 450 degrees centigrade and above the boiling point of the starting hydrocarbon, (3) causing the vapor of the hydrocarbon to react with the cerium tetrafiuoride in the reaction zone, with replacement, with fluorine, of all hydrogen atoms attached to carbon in the starting hydrocarbon, and (4) separating from the reaction product a perfluoro organic compound having the carbon structure of the starting hydrocarbon.

ROBERT D. FOWLER. HARRY C. ANDERSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES Moissan, Comptes Rendus, vol. 130, pp. 622- 2'7 (1900).

Wartenberg, Zeit. Anorg. Allgem. Chem, vol. 244, pp. 337-347 (1940).

Ruff et a1., Zeit. Anorg. Allgem. Chem, vol. 183, pp. 193-213 (1929).

Ruff et al., Zeit. Anorg. Allgem. Chem,

vol. 219, pp. 143-48 (1934).