US3454476A - Electrolytic process for preparation of chlorine pentafluoride - Google Patents

Description

'July 8, 1969 E. A. LAWTON ETAL 3,454,476

ELECTROLYTIC PROCESS FOR PREPARATION OF CHLORINE PENTAFLUORIDE Filed July 12, 1963 INVENTORS EMIL A. LAWTON ,BY HOWARD H. ROGERS ATTORNEY United States Patent Ofice 3,454,476 Patented July 8, 1969 3,454,476 ELECTROLYTIC PROCESS FOR PREPARATION OF CHLORINE PENTAFLUORIDE Emil A. Lawton and Howard H. Rogers, Woodland Hills,

Calif., assignors to North American Rockwell Corporation, a corporation of Delaware Filed July 12, 1963, Ser. No. 294,765 Int. Cl. B01k 1/00; C06b 15/00; C06d /02 US. Cl. 20459 4 Claims This invention relates to an electrolytic process for preparation of chlorine pentafluoride (C11 A method for the preparation of ClF by subjecting a mixture of fluorine and chlorine, for example, to a glow discharge is described in patent application Ser. No. 253,521, filed Jan. 21, 1963, by Walter Maya and Hans F. Bauer, now Patent 3,354,626. As therein mentioned, ClF is an extremely high-energy oxidizer of greater oxidizing potential than chlorine trifluoride which finds util- 1ty as an oxidizer for rocket propellant fuels. The boiling point of ClF is about 14 C., and the compound is stable to at least 300 C. in containers of stainless steel, for example.

By the above-mentioned glow discharge process, the yield of ClF is relatively low, whereas for the process of the instant invention the yields are substantially higher and more precisely predictable. Significantly the process of this invention involves chemical changes which occur with the reactants being in liquid phase whereas the reactions of the glow discharge process occur only in gaseous phase at low pressure, and as a consequence the 6thciency of the process of this invention with respect to apparatus and process operations is substantially greater than that of the glow discharge process.

Broadly stated, the invention comprises subjecting a solution of chlorine trifluoride in hydrogen fluoride and a conductivity additive, to an electric current in an electrolytic cell, and collecting the chlorine pentafluoride which is evolved from the cell. The chemical changes which occur at the anode may be represented by the following equations:

The chemical changes which occur at the cathode are apparently as follows:

The total chemical change for the cell may be represented by the equations:

2ClF +electrical energy ClF +ClF (5 5ClF +electrical energy 3ClF +Cl- (6) Thus, in addition to the chlorine pentafluoride, other resultants of the electrolysis are C1 ClF, and possibly F and when impure reactants are employed various contaminants are produced, such as C10 CIO F, ClO F, and SP The chlorine pentafluoride may be separated from the other resultants and from the contaminants by conventional procedures including vacuum fractionation.

With respect to the conductivity additive, any of the alkali metal halides are usable, preferably an alkali metal fluoride. Pure hydrogen fluoride being practically nondissociated, the conductivity additive furnishes the ions by which the electric current is carried through the electrolytic cell; however, the additive is not consumed in the overall process, the additive being continually regenerated by the fluorine of the chlorine trifluoride. In cases where the additive is a halide other than fluorine, mere substitution of the non-fluorine halogen by fluorine atoms from the hydrogen fluoride occurs with evolution of the corresponding hydrogen halide. It is to be noted further that the hydrogen fluoride is not consumed in the formation of the pentafluoride.

The invention is hereinafter illustrated by description with reference to the accompanying drawing, the single figure of which is a diagrammatic representation of a suitable laboratory apparatus for the electrochemical synthesis of chlorine pentafluoride according to the process of this invention.

In the drawing, reference numeral 10 designates an electrolytic cell of stainless steel comprising a doublewalled tank 12 having a circumferentially continuous flange 13 at its top, a cover 15 secured to the flange with a gasket 16 of Teflon between the cover and the flange. The tank has a drain 18 for emptying the cell and has an inlet 19 and an outlet 20 for continuous flow of a coolant through the space between the tank walls. Spaced apart within the tank by about one-half inch are two plate electrodes 22 and 23 (50 sq. cm. on each face) of a metal, e.g. nickel, which is not easily soluble in the reactants and does not form an insulating anodic film. The electrodes are suspended from the cover 15 by posts 25 electrically connected by leads 27 to a power source at 29, e.g. a continuously variable, full-wave rectified system including calibrated meters for measurement of the power.

An inert gas, e.g. helium, is preferably employed for purging the apparatus and to serve as a carrier for the product. There is a valve controlled line 32 extending through the cover of cell 10 and adapted to be connect d at its outer end to a cylinder (not shown) of helium under pressure. A gauge 33 connected to the line 32 indicates the pressure in the cell. For supplying the reactant, chlorine trifluoride, a flow tube 35, having a metering section 36, is connected to the cell 10 through the cell cover and is adapted at its outer end for connection to a cylinder (not shown) of liquid ClF under its own vapor pressure. A valved vent 38 on the flow tube 35 permits removal of air from the tube 35. For supplying hydrogen fluoride to the cell, there is a valve-controlled inlet tube 40 adapted to be connected at its upstream end to a supply of liquid hydrogen fluoride and at its downstream end to the inside of the electrolytic cell. A branch 41 on the line 40 serves to admit a solution of the conductivity additive in hydrogen fluoride to the cell.

Standing upright from the center of the cell 10 is a condenser 44 with its lower end extending through the cell cover. The upper end of the condenser is connected by a tube 45, controlled by a valve 46, to one end of an absorber column 48 filled with sodium fluoride pellets for absorbing any hydrogen fluoride gas which may be carried over from the condenser 44. A by-pass 49 controlled by a valve 50 is connected to the tube 45 upstream of the valve 46 for preliminary removal of gases. A train of U-tube traps is connected to the downstream end of the HF absorber 48, such train comprising a flow line 53, first and second U-tube traps 54 and 55 of PEP- Teflon, and a flow line 56 leading to a place for storage at 57 for the chlorine pentafluoride. A gauge 59 connected to the line 56 indicates the pressure in the train of traps. A purge outlet 60 is connected adjacent the downstream end of line 56. The valves of the apparatus are formed of Monel metal and except where otherwise explained above, the rest of the apparatus is formed of stainless steel.

In operation, the apparatus is preferably first flushed by flowing helium from the inlet 32 to the purge outlet 60. Hydrogen fluoride and the conductivity additive, e.g. sodium fluoride, are then introduced into the cell to a level covering the plate portions of the electrodes. It is preferred to purify the hydrogen fluoride, if not purified when introduced, and for that purpose electric-current is passed between the electrodes 22 and 23, the valve 46 is closed and the by-pass 49 is opened thereby to remove contaminants, e.g. sulfur and oxygen compounds from the hydrogen fluoride, with the helium or other inert gas set forth, appears to have little, if any, effect on the percent yield of chlorine pentafluoride. Chlorine trifium ride boils .at 11 C. Gaseous ClF is partially soluble in hydrogen fluoride. Liquid ClF is miscible with HP. The lowest concentration of ClF with respect to HF set forth from line 32 as a carrier flowing over the surface of the 5 in the above table is a molar ratio of 550 (HF) :1(ClF liquid in the cell, thence up through the condenser 44 At lower concentrations of CIF it appears that the conand out through the by-pass 49. Thereafter, valve 46 is centration of GE in the area adjacent the electrodes beopened, valve 50 is close-d, branch line 57 leading to comes insutficient for the process of this invention'and storage is closed, purge outlet 60 is opened, a measured fluorine and hydrogen are evolved with resultant exploamount of chlorine trifluoride is added to the liquid HF sions. As the concentration of ClF is increased to below in the cell through its inlet 35, the electric power source a molar ratio of 1:1 (HF:CIF the vapor pressure of is energized, and the gaseous products (including C11 the CIF;, becomes so high that, with laboratory apparatus, pass into the condenser 44. The condenser is cooled as a practical problem of removing the greater volumes of with methanol at from about -l0 C. to 20 C. 15 ClF gas presents itself. It is conjectured that in a comwhereby most of the hydrogen fluoride and chlorine trimercial process, recycling of the ClF may not be objecfluoride vapors are refluxed back into the cell. The carrier tionable. Even at an HP to ClF molar ratio of about gas and the chlorine pentafluoride along with the other 15:1, with the use of the laboratory apparatus described products of the electrolysis reactions, i.e. F ClF, and C1 hereinabove, a large amount of ClF was carried over by then flow through the HF absorber 48, and thence through the helium. A preferred concentration is about 40:1. the train of cold traps 54-55. The first cold trap 54 is Another facet of the matter of concentration of the preferably cooled to 78 C. by envelopment in a bath components of the solution in the electrolytic cell is that (not shown) of dry ice and trichlorethylene to collect of including other chemical compounds. Obviously, such ClF and most of the ClF The second cold trap 55 is contaminants which contain oxygen and through compreferably cooled to -196 C. by a bath of liquid bination with fluorine produce undesirable products such nitrogen for collecting ClF and ClF Thereafter, purge as 0P CIO F and ClO F, should be avoided. Bromine outlet 60 is closed, the train of traps is closed ofi from would interfere with the efficiency of the reaction in that the condenser and the cooling baths are removed whereby it is more easily fluorinated than chlorine trifluoride. the collected products in the traps pass as gases to stor- Silver fluoride and thalium fluoride, which are soluble age 57. in hydrogen fluoride, would lend conductivity to the The following table sets forth particulars of operating solution in the cell; however, it is expected that silver conditions for several examples of the practice of the would plate out at the cathode. process of this invention using the apparatus described Another variable to be considered as having an effect above: on the process of this invention is that of the pressure in Current Voltage Time Products other than 011% and 012 Test Mol. ratio in in in Temp. 0! No HF:MX:ClF.-l amps volts min. cell in C. ClF ClOz C1021? 01031? SE, 1 16:0.39z1 0.5 4.5-4.4 60 -13 to 11 X X X 3.0 6.8-6.0 30 1620.4:1 0.72 4. 8-4.4 105 -14 X X X 5621.5:1 3.0 6.0-6.8 162 --10 X X X 103:26:1 1.0 4.3-5.8 360 --15 X X X 550112.:1 1.0 5.0-5.8 1 -13 to 10 X X 25 1? i3 5 32% $23 X X X 52021351 213 815-710 90 X X X 6221.621 2.3 5.8-6.8 245 11 X X X 59:1.6z1 2.3 6.2-7.5 170 6 X X X In tests No. 1 the MX was KF. In the remaining tests the MX was NaF.

Percent yields of from about 15 to 20 were realized on a continuous operational basis for all of the examples in the table. The term percent yield, as used herein, is calculated as 100 times the quotient of the actual yield of C11 in grams divided by the theoretical yield, with reference to formula No. 1 above. The theoretical yield in grams equals the number of coulombs passed multiplied by the gram equivalent weight of ClF i.e. 130.5 divided by 193,000 coulombs.

The efliciency of the process of this invention is affected by the relative concentrations of the components in the solution in the electrolytic cell. In the examples of the above table, the molar ratio of hydrogen fluoride to the alkali metal halide is about 40 to 1. 'When such ratio was increased to about 80 to l, the process of this invention proceeded quantitatively, however a substantial increase in voltage was required to attain the same amount of electric current, whereby the efliciency of the process was substantially reduced from a practical standpoint. Saturated solutions of the alkali metal halide in the hydrogen fluoride may be employed.

The relative concentration of chlorine trifluoride, with respect to hydrogen fluoride, within the limits hereinafter the electrolytic cell. For the examples in the above table, pressures of about 1 atmosphere were employed. Lower pressures lower the boiling point of ClF and thereby increase the amount of CIF carryover. An increase in pressure tends to decrease volatilization of the 01B, and HF and therefore permits advantageous operation at higher temperatures.

Turning now to the factor of temperature as an operating condition for the process of this invention, for the laboratory apparatus illustrated in the drawing and described above, a range of temperature of from 0 to -14 C. may be set as a limited range, at atmospheric pressure. Operation at below -14 C. would result in a batch process instead of the advantageous continuous process by requiring periodic distillation for recovery of ClF Increasing the solution temperature in the cell to above 0 C. produces too much volatilization of HF and ClF for convenient laboratory handling. It is important to notehowever, that for an electrolytic process, an increase in temperature is advantageous because it increases the conductivity of the electrolyte whereby less voltage and hence less power is required for the same current density. A preferred temperature is about 10 C.

The amount of current employed affects the efliciency of operation of the process of this invention. Generally,

the efficiency of the process increases with an increase of current density (amps per unit area of effective electrode surface). With the above deescribed laboratory ap aratus, currents of one-half to 3 amps were employed. Use of currents below that range was not possible with the detecting apparatus employed. At currents above 3 amps, explosions occurred and the increase in heat from increased current resulted in temperatures above the range of useable temperatures explained above for illustrated laboratory apparatus. The decomposition potential of the HF is about 3.9 volts at about C. It is possible, though not practical, by the process of this invention to obtain ClF with voltages as low as 4.4 volts. A preferred current is about 2 amps.

In the above described laboratory preparation of CIF according to the process of this invention, the ClF in storage 57 may be separated from the other components which were evolved from the cold traps 54 and 55 in any conventional separation operation, e.g. passing the contents of storage 57 through a low temperature fractional distillation column to recover the ClF It will be understood that it is intended to cover all changes and modifications of the examples of the invention herein chosen for the purpose of illustration which do not constitute departures from the spirit and scope of the invention.

Having described the invention, what is claimed is:

1. A process for synthesizing chlorine pentafiuoride comprising the steps of passing an electric current between spaced electrodes in a solution of chlorine trifiuoride, hy-

drogen fluoride and an alkali metal halide whereby chlorine pentafluoride is evolved, and collecting the evolved chlorine pentafluoride.

2. The process of claim 1 in which said alkali metal halide is a fluoride.

3. The process of claim 1 in which a percent yield of from about 15 to about 20 is obtained when employing a laboratory apparatus comprising an electrolytic cell having two plate electrodes spaced apart by a distance of about one-half inch, each electrode being of about sq. cm. per plate face, the apparatus being operated under the following conditions: the molar ratio of HF: alkali metal halide is from about :1 to saturation of the alkali metal halide in HF the molar ratio of HF:ClF is from about 550:1 to about 1:1; the pressure in the cell is about atmospheric; the temperature of the solution in the cell is from about 0 to about -14 C.; and the current is from about one-half to 3 amps.

4. The process of claim 3, wherein said ratio of HF: alkali metal halide is about 40:1; said ratio of HF:CIF is about 40:1; said temperature is about 10 C.: and said current is about 2 amps.

No references cited.

REUBEN EPSTEIN, Primary Examiner.

U.S. Cl. X.R. 1491

Claims (1)

1. A PROCESS FOR SYNTHESIZING CHLORINE PENTAFLUORIDE COMPRISING THE STEPS OF PASSING AN ELECTRIC CURRENT BETWEEN SPACED ELECTRODES IN A SOLUTION OF CHLORINE TRIFLUORIDE, HYDROGEN FLUORIDE AND AN ALKALI METAL HALIDE WHEREBY CHLORINE PENTAFLUORIDE IS EVOLVED, AND COLLECTING THE EVOLVED CHLORINE PENTAFLUORIDE.

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