United States Patent Office Patented Sept
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2,717,871 United States Patent Office Patented Sept. 13, 1955 2 pure form by a suitable procedure. Numerous other 2,717,871 derivatives can be made from these initial derivatives. ELECTROCHEMICAL PRODUCTION OF FLUCR0. Unsaturated acids as well as saturated acids can be used CARBON ACID FLUORE)E DERVATIVES as starting compounds and saturation is produced by Harold M. Scholberg, St. Paul, Min, and fugh G. 5 fluorine addition during the electrochemical fluorination. Bryce, Hudson, Wis., assignors to Minnesota Mining The electrochemical process is not limited to the pro & Manufacturing Company, St. Paul, Minn., a corpo" duction of monocarboxylate compounds. The hydrocar ration of Delaware bon polycarboxylic acids (and their anhydrides) can be fluorinated to produce the corresponding fluorocarbon acid No Drawing. Application February 1, 1952, fluorides, the hydrogen atoms and the hydroxyl groups Serial No. 269,584 of the starting acid being replaced by fluorine atoms. The fluorocarbon acid fluorides can be generically represented 3 Claims. (Cl. 204-59) by the formula: This invention relates to our discovery of a new and Rf (COF)m useful process of making saturated fluorocarbon acid fluo where n is an integer having a value of 1 for monocar rides, which are converted to derivatives thereof and re boxylic acid fluorides, a value of 2 for dicarboxylic acid covered as such. It is an improvement upon the electro fluorides, etc. chemical procedures described in the U.S. patents of J. H. The process as heretofore described and used, outlined Simons, No. 2,519,983 (August 22, 1950), and A. R. above, has the economic disadvantage of producing rela Diesslin, E. A. Kauck and J. H. Simons, No. 2,567,011 20 tively low yields of the fluorocarbon acid fluoride com (September 4, 1951), and further described in a paper pound corresponding to the hydrocarbon acid (or its an by E. A. Kauck and A. R. Diesslin, published by the hydride) used as the starting compound. Even in the American Chemical Society in Industrial and Engineer most favorable case, the production of trifluoroacetyl fluo ing Chemistry, vol.43, pp. 2332-2334 (October 1951). ride (CF3COF) from acetic acid (CH3COOH) or its an These references describe an electrochemical fluorina hydride, the consumed acid starting compound is less than tion process of making saturated fluorocarbon acid fluo 50% converted to CF3COF, due to molecular fragmenta rides, which (in the case of monocarboxylic acid fluorides) tion and partial fluorination resulting in substantial yields can be represented by the formulas RfCOF or of CF4, CF3H, OF2, COF2 and CO2. In the case of higher acids, still other by-product compounds are produced and 22 the yield of the desired acid fluoride (corresponding to the RfC starting compound) decreases rapidly with increase in Yp number of carbon atoms. The situation is even more where Rf stands for a saturated fluorocarbon group (cyclic unfavorable in the case of polycarboxylate compounds. or non-cyclic) consisting solely of carbon and fluorine. The yields when using hydrocarbon acids as starting com The non-cyclic (aliphatic) compounds have the formula: pounds are materially lower than when using the anhy CF-1COF, and the cyclic compounds have the for drides of the acids as starting compounds. These observa mula: CaF2n-1COF. These compounds may also be tions are based on a great many laboratory and pilot plant termed saturated perfluorocarboxylic acid fluorides, and runs by Minnesota Mining & Manufacturing Company saturated perfluoroacyl fluorides. 4) (St. Paul, Minnesota) wherein numerous operating vari ables and expedients have been studied in the attempt to electrolyzingThe procedure a current-conducting described in these solution references comprising involves an improve yields. hydrous liquid hydrogen fluoride to which has been added The importance of this from the commercial produc a hydrocarbon carboxylic acid (or its anhydride), by pass tion standpoint is apparent in view of the high prices which ing direct-current through the solution at a cell voltage it has been necessary to charge for fluorocarbon com which is insufficient to generate molecular (free elemen 45 pounds and which have seriously limited their acceptance tal) fluorine under the existing conditions, but which is except for special applications. It has been necessary to sufficient to cause the formation of the desired fully fluo charge from $10.00 per pound to $50.00 per pound and rinated acid fluoride at a useful rate. Use is made of a upwards for fluorocarbon acids. single-compartment cell without diaphragms. The elec It is evident, therefore, that any innovation which can trode pack consists of alternating and closely-spaced iron materially increase the yields of the electrochemical proc and nickel plates, serving as cathode and anode electrodes, ess is of great value in promoting the usage of fluorocar respectively. The cell can be conveniently operated at bon compounds, which are unique and have many fields substantially atmospheric pressure and at temperatures in of utility that could be served if not too expensive. the neighborhood of 0 to 20 C. The applied cell voltage We have discovered a modification of the above-de is approximately 5 to 6 volts. 55 scribed process by which, as the result of using different The fluorocarbon acid fluoride product of the cell op starting compounds, the yields of trifluoroacetyl fluoride eration is relatively insoluble in the electrolyte solution can be substantially doubled, and the yields of higher com and either settles to the bottom of the cell from which it pounds can be improved in an even greater ratio, as can be drained with other fluorocarbon products of the compared with the yields obtained when using the an process, or is volatilized and evolves from the cell in ad 60 hydrides of hydrocarbon acids as starting compounds. mixture with the hydrogen and other gaseous products, de The improvement is even more marked when comparison pending upon its volatility. The fluorocarbon acid fluoride is made with the use of hydrocarbon acids as starting com compounds are very reactive and the normal procedure is pounds. Furthermore, the acid fluoride product yield to promptly convert them to derivatives without isolating per unit of electrical energy (electrical efficiency) is more them first in pure form, and recovering the derivatives. A than doubled. A further advantage of the present pro cell product mixture containing the fluorocarbon acid fluo cedure is that there is no formation of OF2 (oxygen fluo ride can be hydrolyzed with water to produce the corre ride), apart from what may be formed from impurities, sponding fluorocarbon acid (RCOOH), or can be reacted and the formation of COF2 (caronby fluoride) is mark with ammonia to produce the amide (RfCONH2), or can edly decreased. 70 In this new procedure, we employ as the starting com be reacted with an alcohol to produce an ester (RfCOOR), pound the acid fluoride of the hydrocarbon carboxylic for example. The derivative can then be recovered in acid, that is, the hydrocarbon acyl fluoride, rather than 2,717,871 4. 3 without the need of any other material. Regardless of the acid itself (or its anhydride). Despite the higher scientific speculation as to the whys and wherefores, the cost of the acid fluoride as compared with the acid (or its practical usage of the process can in any case be Success anhydride) there is a very substantial net economic gain fully accomplished by providing a current-conducting because of the extent of gain in the yield and in the solution, a conductivity additive being used when found electrical efficiency. to be needed or desirable. According to one procedure the hydrocarbon acid fluo It was a surprising discovery that very high yields and ride starting compound is directly added to the liquid efficiencies can be obtained when using acetyl fluoride HF of the cell. It is converted by the electrochemical as the starting compound in conjunction with a con process to the corresponding fluorocarbon acid fluoride ductivity additive, since previously it had been the ex compound as indicated by: perience that the non-ionizable organic starting com pounds (that require the use of an additive to provide a where R is a hydrocarbon group (saturated or unsatu carrier electrolyte) cannot be electrochemically fluori rated), Rf is the corresponding saturated fluorocarbon nated in as high yields and efficiencies as can those which group (resulting from complete fluorination), and m is an ionize in the HiF and per se provide adequate conductivity. integer. J. H. Simons had published a negative report on evidence Another procedure is to add the corresponding hydro of formation of trifluoroacetyl fluoride (CF3COF) by the carbon acid chloride compound to the liquid HF (either electrochemical process, using acetyl chloride (CH3COCl) before or after the latter is introduced into the cell), as the starting compound in conjunction with sodium 2 : fiuoride to provide conductivity (J. H. Simons et al., whereupon reaction occurs (even when no current is flow Journal of the Electrochemical Society, vol. 95, No. 2, ing) by which the chlorine atoms are replaced by fluo February 1949, pp. 47-67, see especially pp. 53-54). The rine atoms, with evolution of HCl, resulting in a solution particular circumstances of his laboratory experiment were of hydrocarbon acid fluoride in the liquid HF. The HCl quite different from the operating conditions and pro is insoluble in the liquid HF and is released as fast as cedures of the electrochemical process as employed in our formed. As before, electrochemical fluorination then re- 2 work and in plant operations. sults in the production of the fluorocarbon acid fluoride The reality of the substantial increase in yield obtained product. These two steps can be indicated by: by our process, and of the reduction in manufacturing cost, has been demonstrated by many laboratory experi ments and pilot plant runs wherein all other variables This two step procedure has an advantage in many cases were kept as constant as possible in making comparisons.