US2717871A - Electrochemical production of flucrocarbon acid fluoride derivatives - Google Patents

Description

2,71 7 ,8 7 1 Patented Sept. 13, 1955 ELECTROCHEMICAL PRODUCTION OF FLUGRO- CARBON ACID FLUORIDE DERIVATIVES Harold M. Scholberg, St. Paul, Minn., and Hugh G. Bryce, Hudson, Wis., assignors to Minnesota Mining & Manufacturing Company, St. Paul, Minn., a corpo ration of Delaware N Drawing. Application February 1, 1952, Serial No. 269,584

3 Claims. (Cl. 204-59) This invention relates to our discovery of a new and useful process of making saturated fluorocarbon acid fluorides, which are converted to derivatives thereof and recovered as such. It is an improvement upon the electrochemical procedures described in the U. S. patents of J. H. Simons, No. 2,519,983 (August 22, 1950), and A. R. Diesslin, E. A. Kauck and J. H. Simons, No. 2,567,011 (September 4, 1951), and further described in a paper by E. A. Kauck and A. R. Diesslin, published by the American Chemical Society in Industrial and Engineering Chemistry, vol. 43, pp. 2332-2334 (October 1951).

These references describe an electrochemical fluorination process of making saturated fluorocarbon acid fluorides, which (in the case of monoc'arboxylic acid fluorides) can be represented by the formulas RtCOF or /0 RiC where Rf stands for a saturated fluorocarbon group (cyclic or non-cyclic) consisting solely of carbon and fluorine. The non-cyclic (aliphatic) compounds have the formula: CnF2n+1COR and the cyclic compounds have the formula: CnFZn-ICOF. These compounds may also be termed saturated perfiuorocarboxylic acid fluorides, and saturated perfluoroacyl fluorides.

The procedure described in these references involves electrolyzing a current-conducting solution comprising anhydrous liquid hydrogen fluoride to which has been added a hydrocarbon carboxylic acid (or its anhydride), by passing direct-current through the solution at a cell voltage which is insuflicient to generate molecular (free elemental) fluorine under the existing conditions, but which is suflicient to cause the formation of the desired fully fluorinated acid fluoride at a useful rate. Use is made of a single-compartment cell without diaphragms. The electrode pack consists of alternating and closely-spaced iron and nickel plates, serving as cathode and anode electrodes, respectively. The cell can be conveniently operated at substantially atmospheric pressure and at temperatures in the neighborhood of 0 to C. The applied cell voltage is approximately 5 to 6 volts.

The fluorocarbon acid fluoride product of the cell operation is relatively insoluble in the electrolyte solution and either settles to the bottom of the cell from which it can be drained with other fluorocarbon products of the process, or is volatilized and evolves from the cell in admixture with the hydrogen and other gaseous products, depending upon its volatility. The fluorocarbon acid fluoride compounds are very reactive and the normal procedure is to promptly convert them to derivatives without isolating them first in pure form, and recovering the derivatives. A cell product mixture containing the fluorocarbon acid fluoride can be hydrolyzed with water to produce the corresponding fluorocarbon acid (RrCOOH), or can be reacted with ammonia to produce the amide (RrCONHz), or can be reacted with an alcohol to produce an ester (RfCOOR) for example. The derivative can then be recovered in ell) fluorides, the hydrogen atoms and the pure form by a suitable procedure. Numerous other derivatives can be made from these initial derivatives.

Unsaturated acids as wellas saturated acids can be used as starting compounds and saturation is produced by fluorine addition during the electrochemical fluorination.

The electrochemical process is not limited to the production of monocarboxylate compounds. The hydrocarbon polycarboxylic acids (and their anhydrides) can be fluorinated to produce the corresponding fluorocarbon acid hydroxyl groups of the starting acid being replaced by fluorine atoms. The fluorocarbon acid fluorides can be generically represented by the formula:

Where m is an integer having a value of 1 for monocarboxylic acid fluorides, a value of 2 for dicarboxylic acid fluorides, etc.

The process as heretofore described and used, outlined above, has the economic disadvantage of producing relatively low yields of the fluorocarbon acid fluoride compound corresponding to the hydrocarbon acid (or its anhydride) used as the starting compound. Even in the most favorable case, the production of trifluoroacetyl fluoride (CFsCOF) from acetic acid (CH3COOH) or its anhydride, the consumed acid starting compound is less than 50% converted to CFaCOF, due to molecular fragmentation and partial fluorination resulting in substantial yields of CR1, CFsH, 0P2, COFz and CO2. In the case of higher acids, still other by-product compounds are produced and the yield of the desired acid fluoride (corresponding to the starting compound) decreases rapidly with increase in number of carbon atoms. The situation is even more unfavorable in the case of polycarboxylate compounds. The yields when using hydrocarbon acids as starting compounds are materially lower than when using the anhydrides of the acids as starting compounds. These observations are based on a great many laboratory and pilot plant runs by Minnesota Mining & Manufacturing Company (St. Paul, Minnesota) wherein numerous operating variables and expedients have been studied in the attempt to improve yields.

The importance of this from the commercial production standpoint is apparent in view of the high prices which it has been necessary to charge for fluorocarbon compounds and which have seriously limited their acceptance except for special applications. charge from $10.00 per pound to $50.00 per pound and upwards for fluorocarbon acids.

It is evident, therefore, that any innovation Which can materially increase the yields of the electrochemical process is of great value in promoting the usage of fluorocarbon compounds, which are unique and have many fields of utility that could be served if not too expensive.

We have discovered a modification of the above-described process by whic as the result of using different starting compounds, the yields of trifluoroacetyl fluoride can be substantially doubled, and the yields of higher compounds can be improved in an even greater ratio, as compared with the yields obtained when using the anhydrides of hydrocarbon acids as starting compounds. The improvement is even more marked when comparison is made with the use of hydrocarbon acids as starting compounds. Furthermore, the acid fluoride product yield per unit of electrical energy (electrical efficiency) is more than doubled. A further advantage of the present procedure is that there is no formation of OF2 (oxygen fluoride), apart from what may be formed from impurities, and the formation of COFz (caronbyl fluoride) is markedly decreased.

In this new procedure, we employ as the starting compound the acid fluoride of the hydrocarbon carboxylic acid, that is, the hydrocarbon acyl fluoride, rather than It has been necessary to 3 the acid itself (or its anhydride). Despite the higher cost of the acid fluoride as compared with the acid (or its anhydride) there is a very substantial net economic gain because of the extent of gain in the yield and in the electrical efficiency.

According to one procedure the hydrocarbon acid fluoride starting compound is directly added to the liquid HF of the cell. It is converted by the electrochemical process to the corresponding fluorocarbon acid fluoride compound as indicated by:

where R is a hydrocarbon group (saturated or unsaturated), R: is the corresponding saturated fluorocarbon group (resulting from complete fluorination), and m is an integer.

Another procedure is to add the corresponding hydrocarbon acid chloride compound to the liquid HF (either before or after the latter is introduced into the cell), whereupon reaction occurs (even when no current is flowing) by which the chlorine atoms are replaced by fluorine atoms, with evolution of HCl, resulting in a solution of hydrocarbon acid fluoride in the liquid HP. The HCl is insoluble in the liquid HF and is released as fast as formed. As before, electrochemical fluorination then results in the production of the fluorocarbon acid fluoride product. These two steps can be indicated by:

This two step procedure has an advantage in many cases since the acid chloride compound may be more readily or cheaply prepared from the original source materials. In either case, the hydrocarbon acid fluoride compound is used as the actual starting compound (dissolved in the liquid HF) that is electrochemically fluorinated. Similarly, use can be made of the hydrocarbon acid bromide or iodide compound, which likewise react in liquid HF to yield the acid fluoride compound, releasing HBr or HI (which are low-boiling gases insoluble in the HF).

Thus the starting compound added to the liquid HF is in any case a hydrocarbon acid halide, R(COX)m, where X stands for F, Cl, Br or I. When the chloride, bromide or iodide is added it is converted to the fluoride, which is the actual starting compound for the electrochemical process.

Pure anhydrous liquid HF is non-conductive. Hydrocarbon acid fluorides (acyl fluorides) such as acetyl fluoride (CHzCOF) and butyryl fluoride (CaHvCOF) are soluble but do not ionize therein, and therefore a pure anhydrous solution is non-conductive. This is in contrast to the corresponding acids and their anhydrides which even in pure form can be added to pure anhydrous liquid HF to provide conductive solutions. The necessary conductivity of the solution to permit of eflicient current flow in the electrochemical cell, can be provided by including a small amount (e. g., 0.1 to 5%) of sodium fluoride as a conductivity additive (carrier electrolyte). The use of conductivity additives in conjunction with non-conductive organic starting compounds was described in the aforesaid patentof I. H. Simons, No. 2,519,983 (see, especially, columns 9-11), and need not be elaborated upon. A small amount of acid or a trace of water can also be employed for this purpose, for example. In fact, the materials employed in practicing the present process will often be found to contain impurities (such as traces of acid or water or both) which will serve as carrier electrolytes and make unnecessary the deliberate addition of a conductivity additive.

As shown by Example 4, adequate conductivity has been obtained without using a conductivity additive in the case of starting compounds containing a substantial number of carbon atoms in the molecule. The scientific explanation is in doubt, since it may be that the higher compounds ionize sufliciently to provide adequate conductivity ductivity additive, since previously it had been the experience that the non-ionizable organic starting compounds (that require the use of an additive to provide a carrier electrolyte) cannot be electrochemically fluorinated in as high yields and efliciencies as can those which ionize in the IF and per se provide adequate conductivity. 3. H. Simons had published a negative report on evidence of formation of trifluor'oacetyl fluoride (CFsCOF) by the electrochemical process, using acetyl chloride (CH3COCI) as the starting compound in conjunction with sodium fluoride to provide conductivity (J. H. Simons et al.,

Journal of the Electrochemical Society, vol. 95, No. 2, February 1949, pp. 4767, see especially pp. 5354). The particular circumstances of his laboratory experiment were quite different from the operating conditions and procedures of the electrochemical process as employed in our work and in plant operations.

The reality of the substantial increase in yield obtained by our process, and of the reduction in manufacturing cost, has been demonstrated by many laboratory experiments and pilot plant runs wherein all other variables were kept as constant as possible in making comparisons. The improvement is of too high a magnitude to be explained by any variations in other conditions. Moreover, the improvement was found to be present even when large variations were made in operating conditions; such as variations in concentration of organic starting material ranging from /2% to 30%, variations in concentration of sodium fluoride (or other conductivity additive or carrier electrolyte), variations in temperature ranging from 10 to 40 C., etc. (It is not meant to imply that these are operative limit ranges; they merely indicate a range of experimentation which has established that the improvement is not limited to some particular combination of operating conditions.)

Experiments have been run using alkyl monocarboxylic acid fluoride starting compounds (C11H27L+1COF) having from two to ten carbon atoms in the molecule to produce the corresponding fluorocarbon acid fluorides (CnF21H-1COF) having from two to ten carbon atoms in the molecule; which were hydrolyzed to produce the corresponding acids (CnF2n+1COOH), ranging from trifluoroacetic acid (CFaCOOH, having a B. P. of 72 C.) to perfluorocapric acid (C9F19COOH, having a B. P. of 218 C.). Both the normal and the iso-perfluorobutyric acids were made in this way, using butyryl fluoride and isobutyryl fluoride, respectively, as starting compounds. The unsaturated starting compound crotonyl fluoride, CxHsCOF, was also used in making perfluorobutyryl acid fluoride, CsFwCOF, which was hydrolyzed to yield perfluorobutyric acid, CIZF'ICOOH.

In an experiment using phthalyl fluoride, CsH4(COF)2, as the starting compound, a mixture of two different acid fluoride product compounds was obtained, namely, perfluorocyclohexane-dicarboxylic acid fluoride, CsF1o(COF)2, and perfluorocyclohexanecarboxylic acid fluoride, CeFuCOF; and these were hydrolyzed to produce the corresponding acids, perfluorocyclohexane-dicarboxylic acid, CeF1o(COOH)z, and perfluorocyclohexane-carboxylic acid, CsFuCOOI-I.

Examples of aliphatic fluorocarbon polycarboxylic acids that have been made by employing the present invention are perfluorosuccinic acid, (CF2)2(COOH)2, produced from CFz) 2 (GOP) 2 using succinyl fluoride, (CH2)2(COF)2, as the starting compound; perfluoroadipic acid (CF2)4(CO0H)2, produced from (CF2)4(COF)2 using adipyl fluoride, (CH2)4(COF)2, as the starting compound; and perfiuorosebacic acid, (CF2)8(COOH)2, produced from (CF2)'8(COF)2 using sebacyl fluoride, (CH2)s(COF )2, as the starting compound. Perfluorosebacic acid was described and claimed in a copending application of R. A. Guenthner, since issued as Patent No. 2,606,206 (August 5, 1952 Our process has particularly notable commercial value in making acids containing from four to ten carbon atoms in the molecule, which constitute an important class as to utility and as to which the previously obtained low yields stood as a strong obstacle to extensive commercial use.

Example 1' Use Was made of an iron-cathode nickel-anode pilot plant cell which had an anode surface area of about 110 square feet. (A photograph of this pilot plant cell appears on page 418 of the book Fluorine Chemistry, edited by J. H. Simons, published in 1950 by Academic Press Inc., New York City.) The cell was initially charged with 13 pounds of acetyl fluoride (CHzCOF) and about 330 pounds of anhydrous liquid HP, to which 7.5 pounds of sodium fluoride were added as a carrier electrolyte. The CHsCOF and HF were replenished from time to time during the run to substantially maintain the initial concentration. The cell was operated at a pressure of about 3 p. s. i. gauge (i. e., slightly above atmospheric pressure) and the cell temperature was about C. The average concentration of CI-IaCOF was about 4.5%. The average current value was 1955 amperes and the average voltage value was 5.45 volts. The average current density was 18 amperes/sq. ft. The duration of the run was 1145 hours.

The gas mixture from the cell was led through a series of low temperature condensers to condense out the bulk of the HF which was drained back to the cell. The exit gas mixture, after warming to room temperature, was passed through a packed tower countercurrently to a descending flow of water. The bottom eflluent was an aqueous solution containing the CFsCOOH and HF formed by hydrolysis of the CFaCOF. This was processed to recover the trifluoroacetic acid (CFzCOOH) in pure form.

During the run 1750.5 pounds of acetyl fluoride were consumed (this being the total amount added to the cell for replenishment). A total of 2276.9 pounds of trifluoroacetic acid was produced, determined from analysis of the eflluent solution from the tower. The conversion of the CFsCOF to CFaCOOH was essentially quantitative. The average production rate of CFaCOOI-I was 1.015 pounds per 1000 ampere hours. The yield of acid (and hence also the yield of CFaCOF) was 71% based upon the acetyl fluoride charged to the cell.

A closely similar run was made using acetic anhydride, (CI-I3CO)2O, as the starting compound charged to the cell, and provides a comparison. In this case the production rate of CFsCOOH 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 charged to the cell.

Example 2 Example 3 In this experiment the Example 1. pounds of procedure was similar to that of The pilot plant cell was charged with 13 n-butyryl fluoride, CH3(CH2)2COF, 330

pounds of anhydrous liquid HF, and 0.9 pound of NaF. The average concentration of the n-butyryl fluoride was 5% and a total of 444 pounds was added during the run of 711 hours. The current averaged 1670 amperes, the voltage averaged 5.9 volts, and the average current density was about 15 amperes per sq. ft.

A total of 416 pounds of crude acid was produced which analyzed 93% CF3(CF2)2COOH, and 7% CH3 (CFz zCOOH as the starting compound. A total of 907 pounds was added for replenishment during the run. A total of 522.8 pounds of crude acid was produced which analyzed 65% CF3(CF2)2COOH, and 35% CFsCFzCOOH and CFsCOOH. The average production rate of the heptafluorobutyric acid was 0.144 pound per 1000 ampere hours. The yield was 15.3% based on the n-butyric acid charged. to the cell.

This comparison shows how strikingly the present process increases the acid fluoride production rate per 1000 ampere hours (electrical efficiency), the relative yield of the desired acid versus lower by-product acids, and the yield of acid relative to starting compound employed.

Example 4 The same pilot plant cell was used in this run. The initial charge was 350 pounds of anhydrous liquid HF and 35 pounds of caprylyl chloride, CH3(CH2)sCOCl, both of which were replenished during the run of 537 hours. A total of 442 pounds of additional caprylyl chloride was added. The additions of caprylyl chloride were made slowly and the cell was vented to release the HCl which instantly forms and is insoluble in the HF, so as to avoid an explosive reaction. The cell was operated at a pressure of 6 p. s. i. gauge and at a temperature of about 24 C. An average current of 1650 amperes at 5.8 volts was passed through the cell. No conductivity additive was found necessary due, possibly, to the presence of impurities which performed the function of a carrier electrolyte, or, possibly, to ionization of this starting compound, or both.

The liquid product, consisting of a mixture of fluorocarbons and of fully fluorinated acid fluorides, was drained from the cell and from traps located beneath the low-temperature condensers, and totalled 745.1 pounds. This material was treated with water to hydrolyze the acid fluorides, and the perfluorocaprylic acid CF; (CF2)eCOOH was obtained when using adipyl chloride, (CH2)4(COC1)2, rather than adipic acid, (CH2)4(COOH)2, to charge the cell.

We claim: 1. An electrochemical process of making fluorocarbon acid fluoride derivatives by electrolyzing, in a cell containing an electrode pack having nickel anodes, a currentconducting solution comprising anhydrous liquid hydrogen fluoride mixed with an appropriate organic starting compound, the cell being operated at an average temperature which is not greatly below 0 C. and at an average voltage which does not exceed approximately 6 volts, such that a fluorocarbon acid fluoride product is obtained in a useful yield, and converting the fluorocarbon acid fluoride product to a derivative thereof which is recovered, characterized by electrolyzing a hydrocarbon acid fluoride starting compound.

2. An electrochemical process of making fluorocarbon acid fluoride derivatives by electrolyzing, in a cell containing an electrode pack having nickel anodes, a currentconducting solution comprising anhydrous liquid hydrogen fluoride mixed with an appropriate organic starting compound, the cell being operated at an average temperature which is not greatly below 0 C. and at an average voltage which does not exceed approximately 6 volts, 0

such that a fluorocarbon acid fluoride product is obtained in a useful yield, and converting the fluorocarbon acid fluoride product to a derivative thereof which is recovered, characterized by adding to the liquid hydrogen fluoride a hydrocarbon acid halide to provide a dissolved hydrocarbon acid fluoride and electrolyzing it as substantially the only organic starting material employed.

5 gen fluoride mixed with an appropriate organic starting compound, the cell being operated at an average temperature which is not greatly below 0 C. and at an average voltage which does not exceed approximately 6 volts, such that a fluorocarbon acid fluoride product is obtained 10 in a useful yield, and converting the fluorocarbon acid fluoride product to a derivative thereof which is recovered, characterized by electrolyzing a hydrocarbon acid fluoride starting compound having from four to ten carbon atoms in the molecule.

Cited in the file of this patent UiIiTED STATES PATENTS 2,519,983 Simons Aug. 22, 1950 2,567,011 Diesslin et al Sept. 4, 1951 2,606,206 Guenthner Aug. 5, 1952 OTHER REFERENCES Simons et al.: Journal Electrochemical Society, vol. 95

(February 1949), pp. 53-54.

Kauck et al.: Industrial and Engineering Chemistry,

vol. 43 (October 1951), pp. 2332-2334.

Claims (1)

1. AN ELECTROCHEMICAL PROCESS OF MAKING FLUOROCARBON ACID FLUIDIZE DERIVATIVES BY ELECTROLYZING, IN A CELL CONTAINING AN ELECTRODE PACK HAVING A NICKEL ANODES, A CURRENTCONDUCTING SOLUTION COMPRISING ANHYDROUS LIQUID HYDROGEN FLUORIDE MIXED WITH AN APPROPRIATE ORGANIC STARTING COMPOUND, THE CELL BEING OPERATED AT AN AVERAGE TEMPERATURE WHICH IS NOT GREATLY BELOW 0* C. AND AT AN AVERAGE VOLTAGE WHICH DOES NOT EXCEED APPROXIMATELY 6 VOLTS, SUCH THAT A FLUOROCARBON ACID FLUORIDE PRODUCT IS OBTAINED IN A USEFUL YIELD, AND CONVERTING THE FLUOROCARBON ACID FLUORIDE PRODUCT TO A DERIVATIVE THEREOF WHICH IS RECOVERED, CHARACTERIZED BY ELECTROLYZING A HYDROCARBON ACID FLUORIDE STARTING COMPOUND.