US3373096A - Electrolytic preparation of chlorine pentafluoride - Google Patents

Description

March 12, 1968 E. A. LAWTON ETAL 3,373,096

ELECTROLYTIC PREPARATION OF CHLORINE PENTAFLUORIDE 2 Sheets-Shem 1 Filed Sept. 24, 1964 INVENTORS EMIL A. LAWTON HOWARD H. ROGERS FIG-- 2.

United States Patent O 3,373,096 ELECTROLYTIC 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 Sept. 24, 1964, Ser. No. 399,110 3 Claims. (Cl. 204-59) This invention relates to an electrolytic process for prepartion of chlorine pentafluoride, ClF from chlorine and hydrogen fluoride.

A method for the preparation of C11 by electrolysis of a solution of chlorine trifluoride and hydrogen fluoride is described in patent application Ser. No. 294,765, filed July 12, 1963, now Patent No. 3,303,401. As therein mentioned, C11 is an extremely high-energy oxidizer of greater oxidizing potential than chlorine trifluoride which finds utility as an oxidizer for rocket propellant fuels. The boiling point of C11 is about 14 C.

Compared with chlorine, chlorine trifluoride is expensive. Hydrogen fluoride is relatively inexpensive. Therefore, the use of the relatively inexpensive raw materials, chlorine and hydrogen fluoride, for synthesis of ClF as provided by this invention, offers a significant advantage.

Broadly stated, the invention comprises subjecting chlorine and hydrogen fluoride with 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 of the cell may be represented by the following equations:

In addition to the chlorine pentafluoride, other resultants of the electrolysis are ClF, HCl, and possibly F and when impure reactants are employed various contaminants are produced, such as C CIO F, and ClO F. The chlorine pentafluoride may be separated from chlo rine and from the other resultants and contaminants by conventional procedures including low temperature distillation.

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 hydrogen fluoride. 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.

The invention is hereinafter illustrated by descriptio with reference to the accompanying drawing, in which:

FIG. 1 is a diagrammatic representation of a suitable laboratory apparatus for the electrochemical synthesis of chlorine pentafluoride according to the process of this invention;

FIG. 2 is a detail section on an enlarge-d scale, through the electrolytic cell of the apparatus, taken upon a plane which is indicated on FIG. 1 by line 22;

FIGS. 3, 4, and 5 are graphs in which anode potential is plotted against time of operation of the cell for showing the behavior of the potential at the anode of the cell.

In FIG. 1 of the drawing, reference numeral 10 designates an electrolytic cell of stainless steel comprising a double-walled tank 12 having a circumferentially conlinuous flange 13 at its top, a cover 15 secured to the flange with a gasket 16 of Teflon between the cover and "ice the flange. The tank has a drain 18 for emptying the cell and has an inlet 19 and an outlet 29 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, anode 22 and cathode 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 current and voltage.

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 connected 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, a flow line 35, containing a valve 36, a flow meter 37, and a metering valve 37', extends through the cell cover 15 and is adapted at its outer end for connection to a cylinder (not shown) of chlorine. Within the cell, the chlorine inlet line 35 continues as a section 33 of Teflon having a leg portion 39 paralleling the lower edge of the anode plate 22. The leg 39 is closed at its end 40 and is provided with pinholes 41 for bubbling chlorine into the cell. The anode is preferably bent as along bend line 42 (FIG. 2), and the section 33 is correspondingly bent, whereby the bent portion of the anode plate 22 extends over the perforated leg portion 39 so that bubbles of chlorine will impinge against the anode plate instead of rising unobstructed in the cell.

For supplying hydrogen fluoride to the cell, there is a valve-controlled inlet tube 44 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 call. A branch 45 on the line 44 serves to admit a solution of the conductivity additive in hydrogen fluoride.

Standing upright from the center of the cell 10 is a condenser 47 with its lower end extending through the cell cover. The upper end of the condenser is connected by a tube 48, controlled by a valve 4-9, to one end of an absorber column 50 filled with sodium fluoride pellets for absorbing any hydrogen fluoride gas which may be carried over from the condenser 47. A bypass 51 controlled by a valve 52 is connected to the tube 48 upstream of the valve 49 for preliminary removal of gases. A train of U-tube traps is connected to the downstream end of the HF absorber 50, 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 49 is closed and the bypass 51 is opened thereby to remove contaminants, e.g., sulfur and oxygen compounds from the hydrogen fluoride, with the helium or other inert gas from line 32 as a carrier flowing over the surface of the liquid in the cell, thence up through the condenser 47 and out through the bypass 51. Thereafter, valve 49 is opened,

3 valve 52 is closed, branch line 57 leading to storage is closed, purge outlet 60 is opened, and chlorine is bubbled into the cell through the perforations of its inlet tube section 39 preferably at a rate of from about 60 to 240 cc. per min. The electric power source is then energized, and the gaseous products (including ClF pass into the condenser 47. The condenser is cooled as with methanol at from about --l0 C. to C. whereby most of the hydrogen fluoride vapors are refluxed back into the cell. The carrier gas, chlorine and the chlorine pentafluoride along with the other products of the electrolysis reactions, 1

i.e., F and ClF, then flow through the HF absorber 50, and thence through the train of cold traps 54-55. The first cold trap 54 is preferably cooled to 78 C. by envelopment in a hath (not shown) of Dry Ice and trichloroethylene to collect chlorine and most of the ClF The second cold trap 55 is preferably cooled to -l96 C. by a bath of liquid nitrogen for collecting ClF, ClF and the remainder of the chlorine. Thereafter, a purge outlet 66 is closed, the train of traps is closed off from the condenser and the cooling baths are removed whereby the collected products in the traps pass as gases to storage 57.

The following table sets forth particulars of operating conditions for several examples of the practice of the process of this invention using the apparatus described above:

cell when helium instead of chlorine wasbubbled from the tube section 39, and thereafter when C1 was substituted for the helium. No potential rise occurred and no ClF was produced until chlorine was bubbled into the cell. The degree of voltage rise is, of course, a function of the cell geometry and temperature of electrolyte.

Each of the tests 3, 6, and 7 in the hereinabove table were conducted at 0.5 amp on a day following an overnight rest of the cell which had theretofore been operated at 1.0 amp. The graph of FIG. 5 shows production of ClF under conditions of operation Where the cell had previously experienced a rise in anode potential.

Though chlorine was bubbled into the cell at different rates of 60, 120, and 240 cc./min. no significant difference in yield of ClF occurred. So long as chlorine is present, ClF will be produced. To avoid high concentrations of chlorine appearing in the collection traps 54 and 55, the rate of chlorine additive should be controlled to reduce the extent of chlorine bubbles at the surface of the liquid in the cell. When liquid chlorine was added to the cell at about 38 C., ClF was produced but in reduced yields.

Another variable to be considered as having an effect on the process of this invention is that of the pressure in the electrolytic cell. For the examples in the above table, pressures of about 1 atmosphere were employed. Lower Test Current Voltage Time in Temp. Percent The term percent yield, which appears in the above table, is calculated as 100 times the quotient of the actual yield of ClF in grams divided by the theoretical yield, with reference to Formula No. l hereinabove. 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 482,500 coulombs.

In the examples of the above table, the molar ratio of hydrogen fluoride to sodium fluoride as the alkali metal halide was initially about 40 to 1. If such ratio is increased to about 80 to 1, for example, the process of this invention will proceed but a substantial increase in voltage Will be required to attain the same amount of electric current, whereby the efl'iciency of the process will be substantially reduced from a practical standpoint. Decreasing such ratio decreases the voltage required to attain the same current. Saturated solutions of the alkali metal halide in the hydrogen fluoride may be employed.

The amount of current employed aifects the efliciency of operation of the process of this invention. Generally, the efliciency of the process increases with an increase of current density (amps per unit area of effective electrode surface). With the above described laboratory apparatus, currents of one-half to two amps were employed; h-owever, the graphs of FIGS. 3, 4 and 5 show that the current density is not as significant a factor to the synthesis of ClF as is the incidence of a potential rise at the anode when operating at a constant current. Referring to FIG. 3, it shows that when the illustrated cell was operated at a constant current of 0.5 amp (6 ma/cm?) for a period of 60 min, no ClF was produced, and the anode potential, as measured by a mercury-mercurous fluoride reference electrode, remained fairly constant at slightly above 4 vol-ts. Thereafter, the current Was increased to a density of 12 rna./cm. whereupon the anode potential increased steadily throughout :a period of about minutes to slightly above 6 volts and GE was noted as being pressures lower the boiling point of HF and thereby increase the amount of HF carryover. An increase in pressure raises the boiling point of HF and increases the concentration of chlorine in the electrolyte and therefore permits operation at higher temperatures. 7

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 l4 C.. may be set as a limited range, at atmosphericpressure. Operation at below l4 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 for convenient laboratory handling. It is important to note however, 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 l0 C.

Contaminants which contain oxygen and through combination with fluorine produce undesirable products such as C10 and C10 should'be avoided. Sulfur would terfere with the efliciency of the reaction as it is more easily fluorinated than chlorine. Silver fluoride and thalium fluoride, which are soluble in hydrogen fluoride, would lend conductivity to the solution in the cell; however,.it is expected that silver would plate out at the cathode.

In the above described laboratory preparation of C11 5 according to the process of this invention, the GR in storage 57 may be separated from the other components which were evolved from the cold traps 5'4 and 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 pentafluoride comprising the steps of passing an electric current between spaced electrodes in an electrolyte comprising chlorine, hydrogen 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 laboratory apparatus is employed comprising an electrolytic cell having two plate electrodes spaced apart by a distance of about one-half inch, each electrode being of about 50 sq. cm. per plate face, the apparatus being operated under the 01- lowing conditions: the molar ratio of HFzalkali metal halide is from about 80:1 to saturation of the alkali metal halide in HF; 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 initially maintained References Cited UNITED STATES PATENTS 5/1967 Yodis 20459 10/ 1966 Donohue et al. 204-101 HOWARD S. WILLIAMS, Primary Examiner.

Claims (1)

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

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