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. since the acid chloride compound may be more readily The improvement is of too high a magnitude to be ex or cheaply prepared from the original source materials. plained by any variations in other conditions. More In either case, the hydrocarbon acid fluoride compound over, the improvement was found to be present even when is used as the actual starting compound (dissolved in the large variations were made in operating conditions; such liquid HF) that is electrochemically fluorinated. Simi as variations in concentration of organic starting ma larly, use can be made of the hydrocarbon acid bromide terial ranging from 4% to 30%, variations in concen or iodide compound, which likewise react in liquid HF to tration of sodium fluoride (or other conductivity additive yield the acid fluoride compound, releasing HBr or HI or carrier electrolyte), variations in temperature ranging (which are low-boiling gases insoluble in the HF). from 10 to 40 C., etc. (It is not meant to imply that Thus the starting compound added to the liquid HF these are operative limit ranges; they merely indicate a is in any case a hydrocarbon acid halide, R(COX)m, range of experimentation which has established that the where X stands for F, Cl, Br or I. When the chloride, improvement is not limited to some particular combination bromide or iodide is added it is converted to the fluoride, of operating conditions.) which is the actual starting compound for the electro 5 Experiments have been run using alkyl monocarboxylic chemical process. acid fluoride starting compounds (CnH2n+1COF) having Pure anhydrous liquid HF is non-conductive. Hydro from two to ten carbon atoms in the molecule to carbon acid fluorides (acyl fluorides) such as acetyl fluo produce the corresponding fluorocarbon acid fluorides ride (CH3COF) and butyryl fluoride (C3H7COF) are (Cn F2n+1COF) having from two to ten carbon atoms in soluble but do not ionize therein, and therefore a pure i,i. the molecule; which were hydrolyzed to produce the cor anhydrous solution is non-conductive. This is in contrast responding acids (CnF2n+1COOH), ranging from trifluo to the corresponding acids and their anhydrides which even roacetic acid (CF3COOH, having a B. P. of 72° C.) to in pure form can be added to pure anhydrous liquid HF to perfluorocapric acid (CgF19COOH, having a B. P. of 218 provide conductive solutions. The necessary conductivity C.). Both the normal and the iso-perfluorobutyric acids of the solution to permit of efficient current flow in the were made in this way, using butyryl fluoride and iso electrochemical cell, can be provided by including a small butyryl fluoride, respectively, as starting compounds. The amount (e. g., 0.1 to 5%) of sodium fluoride as a con unsaturated starting compound crotonyl fluoride, ductivity additive (carrier electrolyte). The use of con C3H5COF, was also used in making perfluorobutyryl acid ductivity additives in conjunction with non-conductive or fluoride, C3FCOF, which was hydrolyzed to yield per ganic starting compounds was described in the aforesaid () fluorobutyric acid, CaFCOOH. patent of J. H. Simons, No. 2,519,983 (see, especially, In an experiment using phthalyl fluoride, C6H4 (COF)2, columns 9-11), and need not be elaborated upon. A as the starting compound, a mixture of two different acid small amount of acid or a trace of water can also be fluoride product compounds was obtained, namely, perfluo employed for this purpose, for example. In fact, the ma rocyclohexane-dicarboxylic acid fluoride, C6F10 (COF)2, terials employed in practicing the present process will and perfluorocyclohexanecarboxylic acid fluoride, often be found to contain impurities (such as traces of C6F11COF; and these were hydrolyzed to produce the acid or water or both) which will serve as carrier electro corresponding acids, perfluorocyclohexane-dicarboxylic lytes and make unnecessary the deliberate addition of a acid, C6F10 (COOH)2, and perfluorocyclohexane-car conductivity additive. boxylic acid, C6F11COOH. As shown by "Example 4,” adequate conductivity has Examples of aliphatic fluorocarbon polycarboxylic acids been obtained without using a conductivity additive in the that have been made by employing the present invention case of starting compounds containing a substantial num are perfluorosuccinic acid, (CF2)2(COOH)2, produced ber of carbon atoms in the molecule. The scientific ex from (CF2)2(COF)2 using succinyl fluoride, planation is in doubt, since it may be that the higher com (CH2)2(COF)2, as the starting compound; perfluoroadip pounds ionize sufficiently to provide adequate conductivity ic acid (CF2)4(COOH)2, produced from (CF2)4(COF)2 2,717,871 5 using adipyl fluoride, (CH2)4(COF)2, as the starting 6 compound; and perfluorosebacic acid, (CF2)8(COOH)2, pounds of anhydrous liquid HF, and 0.9 pound of NaF. produced from (CF2)3(COF)2 using sebacyl fluoride, The average concentration of the n-butyryl fluoride was (CH2)8(COF)2, as the starting compound. Perfluoro 5% and a total of 444 pounds was added during the run sebacic acid was described and claimed in a copending of 711 hours. The current averaged 1670 amperes, the application of R. A. Guenthner, since issued as Patent voltage averaged 5.9 volts, and the average current den No. 2,606,206 (August 5, 1952). sity was about 15 amperes per sq. ft. Our process has particularly notable commercial value A total of 416 pounds of crude acid was produced in making acids containing from four to ten carbon atoms which analyzed 93% CF3(CF2)2COOH, and 7% in the molecule, which constitute an important class as to utility and as to which the previously obtained low yields and CF3COOH. Thus 387 pounds of heptafluorobutyric stood as a strong obstacle to extensive commercial use. acid were produced. The average production rate of the Example I latter was 0.33 pound per 1000 ampere hours. The yield Use was made of an iron-cathode nickel-anode pilot was 35.8% based on the n-butyryl fluoride charged to the plant cell which had an anode surface area of about 110 5 cell. square feet. (A photograph of this pilot plant cell ap A similar run was made using n-butyric acid pears on page 418 of the book "Fluorine Chemistry,” CH3(CF2)2COOH edited by J. H. Simons, published in 1950 by Academic as the starting compound. A total of 907 pounds was Press Inc., New York City.) The cell was initially charged added for replenishment during the run. A total of with 13 pounds of acetyl fluoride (CH3COF) and about 522.8 pounds of crude acid was produced which analyzed 330 pounds of anhydrous liquid HF, to which 7.5 pounds 65% CF3(CF2)9COOH, and 35% CF3CF2COOH and of sodium fluoride were added as a carrier electrolyte. CF3COOH. The average production rate of the hepta The CH3COF and HF were replenished from time to fluorobutyric acid was 0.144 pound per 1000 ampere time during the run to substantially maintain the initial hours. The yield was 15.3% based on the n-butyric acid concentration. The cell was operated at a pressure of charged to the cell. about 3 p. s. i. gauge (i. e., slightly above atmospheric pressure) and the cell temperature was about 20° C. The This comparison shows how strikingly the present proc average concentration of CH3COF was about 4.5%. The ess increases the acid fluoride production rate per 1000 average current value was 1955 amperes and the average ampere hours (electrical efficiency), the relative yield of voltage value was 5.45 volts. The average current density the desired acid versus lower by-product acids, and the was 18 amperes/sq. ft. The duration of the run was 1145 yield of acid relative to starting compound employed. hours. Example 4 The gas mixture from the cell was led through a series The same pilot plant cell was used in this run. The of low temperature condensers to condense out the bulk initial charge was 350 pounds of anhydrous liquid HF of the HF which was drained back to the cell. The exit and 35 pounds of caprylyl chloride, CH3(CH2)6COCl, gas mixture, after warming to room temperature, was both of which were replenished during the run of 537 passed through a packed tower countercurrently to a de hours. A total of 442 pounds of additional caprylyl scending flow of water. The bottom effluent was an chloride was added. The additions of caprylyl chloride aqueous solution containing the CF3COOH and HF formed were made slowly and the cell was vented to release the by hydrolysis of the CF3COF. This was processed to re HCl which instantly forms and is insoluble in the HF, cover the trifluoroacetic acid (CF3COOH) in pure form. so as to avoid an explosive reaction. The cell was oper During the run 1750.5 pounds of acetyl fluoride were ated at a pressure of 6 p.s. i. gauge and at a temperature consumed (this being the total amount added to the cell for of about 24° C. An average current of 1650 amperes at replenishment). A total of 2276.9 pounds of trifluoro 45 5.8 volts was passed through the cell. No conductivity acetic acid was produced, determined from analysis of the additive was found necessary due, possibly, to the pres effluent solution from the tower. The conversion of the ence of impurities which performed the function of a CF3COF to CF3COOH was essentially quantitative. The carrier electrolyte, or, possibly, to ionization of this start average production rate of CF3COOH was 1.015 pounds ing compound, or both. per 1000 ampere hours. The yield of acid (and hence 50 The liquid product, consisting of a mixture of fluoro also the yield of CF3COF) was 71% based upon the carbons and of fully fluorinated acid fluorides, was acetyl fluoride charged to the cell. drained from the cell and from traps located beneath the A closely similar run was made using acetic anhydride, low-temperature condensers, and totalled 745.1 pounds. (CH3CO)2O, as the starting compound charged to the cell, This material was treated with water to hydrolyze the and provides a comparison. In this case the production 55 acid fluorides, and the perfluorocaprylic acid rate of CF3COOH was 0.484 pound per 1000 ampere hours. The yield of acid (and hence also the yield of CF3COF) was 38% based upon the acetic anhydride was produced in a yield of 134 pounds. charged to the cell. In a similar run using caprylic acid, CH3(CH2)6COOH, Example 2 60 as the starting compound, the yield of perfluorocaprylic In order further to compare the actual yields of tri acid was only about one-third as great. fluoroacetyl fluoride, CF3COF, based on the acetyl fluoride, Similar runs were made to compare caproyl chloride, CH3COF, consumed in the cell, laboratory experiments CH3(CH2)4COCl, and caproic acid, CH3(CH2)4COOH, were made using a 50-ampere cell, and the gas was ana as starting compounds. A threefold increase in yield of lyzed to determine the production of CF3COF. Yields of 65 perfluorocaproic acid, CF3(CF2)4COOH, was obtained 75 to 85% were consistently obtained even with substan when using the caproyl chloride. tial variations in operating conditions. However, when The same trend, but even more striking, was observed using acetic anhydride as the starting compound, the high in the production of perfluorodibasic acids. Thus a four est yield obtained was 45%, and the highest yield obtained fold increase in yield of perfluoroadipic acid when using acetic acid was 20%. 70 Example 3 was obtained when using adipyl chloride, (CH2)4(COCl)2, In this experiment the procedure was similar to that of cell.rather than adipic acid, (CH2)4(COOH)2, to charge the Example 1. The pilot plant cell was charged with 13 We claim: pounds of n-butyryl fluoride, CH3(CH2)2COF, 330 75 1. An electrochemical process of making fluorocarbon 2,717,871 7 c. acid fluoride derivatives by electrolyzing, in a cell con 3. An electrochemical process of making fluorocarbon taining an electrode pack having nickel anodes, a current acid fluoride derivatives by electrolyzing, in a cell con conducting solution comprising anhydrous liquid hydro taining an electrode pack having nickel anodes, a current gen fluoride mixed with an appropriate organic starting conducting solution comprising anhydrous liquid hydro compound, the cell being operated at an average tempera 5 gen fluoride mixed with an appropriate organic starting ture which is not greatly below 0° C. and at an average compound, the cell being operated at an average tem voltage which does not exceed approximately 6 volts, such perature which is not greatly below 0° C. and at an aver that a fluorocarbon acid fluoride product is obtained in a age voltage which does not exceed approximately 6 volts, useful yield, and converting the fluorocarbon acid fluoride such that a fluorocarbon acid fluoride product is obtained product to a derivative thereof which is recovered, char i0 in a useful yield, and converting the fluorocarbon acid acterized by electrolyzing a hydrocarbon acid fluoride fluoride product to a derivative thereof which is recovered, starting compound. characterized by electrolyzing a hydrocarbon acid fluoride 2. An electrochemical process of making fluorocarbon starting compound having from four to ten carbon atoms acid fluoride derivatives by electrolyzing, in a cell contain in the molecule. ing an electrode pack having nickel anodes, a current conducting solution comprising anhydrous liquid hydro Rafarances Cited in the file of this patent gen fluoride mixed with an appropriate organic starting j JTE) SATES PATENTS compound, the cell being operated at an average tem 2,519,983 Simons ------Aug. 22, 1950 perature which is not greatly below 0° C. and at an aver 2,567,011 Diesslin et al. ------Sept. 4, 1951 age voltage which does not exceed approximately 6 volts, 2) 2,606,206 Guenthner ------Aug. 5, 1952 such that a fluorocarbon acid fluoride product is obtained in a useful yield, and converting the fluorocarbon acid OTHER REFERENCES fluoride product to a derivative thereof which is recovered, Simons et al.: Journal Electrochemical Society, vol. 95 characterized by adding to the liquid hydrogen fluoride a (February 1949), pp. 53-54. hydrocarbon acid halide to provide a dissolved hydrocar- 23 Kauck et al.: Industrial and Engineering Chemistry, bon acid fluoride and electrolyzing it as substantially the vol. 43 (October 1951), pp. 2332-2334. only organic starting material employed.