US3037919A

US3037919A - Preparation of hexafluoroethane and higher fluorocarbons

Info

Publication number: US3037919A
Application number: US750270A
Authority: US
Inventors: Olstowski Franciszek; Iii John J Newport
Original assignee: Dow Chemical Co
Current assignee: Dow Chemical Co
Priority date: 1958-07-22
Filing date: 1958-07-22
Publication date: 1962-06-05
Anticipated expiration: 1979-06-05
Also published as: GB868020A

Description

States This invention relates to a process `for the preparation of hexauoroethane and higher molecular weight iluorocarbons, and more particularly, to the preparation of these fluorocarbons by the electrolysis of a non-volatile molten metal fluoride.

Presently the preparation of uorocarbons is mainly limited to iluorination of chlorinated or unsaturated hydrocarbons by use of uorinating agents, such as hydrogen iluoride or fluorinating of saturated hydrocarbons by elemental uorine. These processes involve handling hazardous materials which are expensive and require special equipment. n

In United States Letters Patent No. 785,961, a process for the preparation of carbon tetrafluoride by electrolysis of sodium or potassium uoride is disclosed. In the process disclosed, a solid carbon anode is used which is surrounded with carbonaceous material, such as charcoal or lampblack, to protect it from disintegration. The charcoal or lampblack is confined by a refractory non-conducting sleeve or container. Upon electrolysis of the particular uorides at a voltage `of 8 volts and a temperature of l000 C. only carbon tetraliuoride is obtained. The metal uorides are cheap raw materials. A process whereby hexailuoroethane and higher molecular weight tluorocarbons could be prepared by electrolysis of fused metal uorides would greatly reduce the production cost of these compounds.

It is, therefore, among the objects of this invention to provide a process for the preparation of hexauoroethane and higher molecular Weight fluorocarbons by the electrolysis of non-volatile molten metal fluorides.

The above and other objects are attained by electrolysis of the non-volatile metal fluoride at a temperature in the range of 600 to 925 C. using -an yanode consisting of carbonaceous material in particulate form with the individual particles being in electrical contact with each other and an insoluble cathode at -a sulicient anode density to liberate a gas at the anode containing uorocarbons of which at least 10 mole percent is hexafluoroethane and higher molecular weight iluorocarbons.

It has been discovered that when a non-volatile metal fluoride is electrolyzed at a temperature of 600 to 925 C. with an anode consisting of carbonaceous material in particulate form loosely confined at a suiliciently high anode density, hexatluoroethane and higher molecular weight uorocarbons are liberated at the anode as a product of the electrolysis.

The invention may be more easily understood when the detailed discussion is considered in conjunction with the drawings, in which:

FIGURE l diagrammatically illustrates an electrolytic cell in which the invention may be carried out, and

FIGURES 2, 3, 4, land 5 show the elect of the anode current density and temperature of the electrolyte on the composition of the anode gases obtained from the cell.

The electrolytic cell diagrammatically `shown in FIG- URE 1 comprises a metal tank 1 having a cover plate 2, an electrical non-conducting refractory cylindrical liner 3, such as made from alumina or zirconia, and a carbon lead 4 extending part way into the tank through an opening in cover plate 2. As shown, the cover plate is fastened to tank 1 =by means of multiplicity of screws 6 to form a gas-tight seal. Clamps or other means may be used instead of the screws. A pipe 7 is attached to cover plate 2 so that the pipe encompasses an opening 8 in the cover plate and provides a passageway through which the anode gas produced as a product inside of tank 1 may be withdrawn Vfrom the tank. Where the carbon lead 4 passes through `cover plate 2 an electrical insulating seal 9 is used so that a gas-tight seal is obtained. To the end of the carbon lead 4 not extending into tank 1, an electrical lead 10 is attached through which the current is supplied to the cell. Another lead 11 is electrically connected to the surface of the tank which through a mol-ten cathode 12 at the bottom of the tank completes the circuit for the current ilow in the electrolytic cell.

In the operation of the cell, electrolyte 13 is placed in the tank 1 and is heated to a temperature of 600 to 925 C. A metal, inert to the lluoride electrolyte and having a melting point below the electrolysis temperature, such as lead, is placed in the cell. The metal settles to the bottom of the cell contacting the surface of the tank and thus acts as a cathode. The inert metal to act as a cathode is only used where the metal -deposited at the cathode is lighter than the electrolyte. Even in this case a molten cathode would not have to be used, but an arrangement in the cell design would have to lbe made to collect the metal deposited from the top of the electrolyte. The metal fluoride used as electrolyte is added to the cell in an amount such that the -level of the electrolyte is below the top of the liner I3. The car-bon lead 4 is inserted into the tank through the cover plate until the end of the lead is above the electrolyte. Carbonaceous material 14 which is lighter than the molten electrolyte is placed within the liner 3 on the surface of the electrolyte so that it surrounds and contacts the carbon lead 4.

An electrical potential is applied to leads 10` and 11 andv the anode gas produced is drawn from within tank 1 through pipe 7 after which the gas is fur-ther processed by known methods to recover and separate the particular iluorocarbons. The metal depositing out from the electrolyte is deposited in the molten cathode 12 and later recovered.

In the electrolysis of a molten electrolyte consisting essentially of -a metal uoride which is non-volatile and stable at the temperature of the electrolysis according to the invention, a series `of homologous iluorocarbons is obtained as an anode product. The anode product is a gas at the electrolysis temperature and contains uorocarbons, such as `carbon tetrailuoride, heX-aiiuoroethane, and higher molecular weight uorocarbons. With an anode comprising a particulate carbonaceous material with the individual particles being in an electrical contact with each other, hexafluoroethane and higher uorocarbons are obtained when the temperature of the electrolyte is 925 C. or below and a suiiiciently high anode current density is used.

The eilect of temperature is illustrated in FIGURES 2, 3, and 4 which show the composition of the lluorocarbon product obtained in the anode gas as a function of the temperature of the electrolyte at particular anode current densities. FIGURE 5 shows the effect of anode current density -upon the composition of the uorocarbon product obtained at the alnode at an electrolysis temperature of 700 C. The details of the tests and data upon which the figures are based are set forth in Example I below. In FIGURES 2, 3, and 4, the abscissa represents the temperature of a lithiumuoride-sodium uoride mixture electrolyte at which the electrolysis was effected and the ordinate represents the composition of the fluorocarbon product in the anode gas in mole percent.

FIGURE 2 illustrates the composition of the fluorocarbon product obtained as a function of temperature at an anode current density of l ampere per square inch. It lwill be noted that with the electrolyte at 700 C. the

as theV temperature Was increased over 900 C.

, Y 3 Y uorocanbon product obtained'contai-ned approximately 30 mole percent of hexalluoroethane. As the temperature of the electrolyte increased, the amount of hexaluoroethane decreased, while theamount of carbon tetra- Vfluoride increased until *at 800 C. approximately all of could onlybe detected at 800 C. v

Withan anode current Vdensity above 3 amperes'per square inch, a very pronounced increase in the higher molecular weight uorocarbons production was obtained as the temperature was increased fromr700 toY 800 C.

This, can be seen. in FIGURE 4 where the current density was approximately 5 amperes per square inch.V A fluorocarborrproduct containing a maximum amount of hexal fluoroethane, approximately 70 percent of the total iluorocarbon product, was obtained at 800 C., While the amount of carbon tetrafluoride obtained decreased to a minimum of around 3'0 mole percent. tion of the hexafluoroethane dropped olf very rapidly In FIGURE 5 the composition of the anode uorocarbon product obtained is shown as a function of the current density during the electrolysis of the lithium fluoride-sodium iluoride mixture at 700 C. 'Ilhe abscissa represents the anode current density in amperes per square inch and the ordinate the composition of the fluorocarbon product in mole percent. rThe amounts of carbon tetrauoride, hexauoroethane, and octaiiuoropropane are plotted as a function of the current density.Y

From the curves it may be seen that the amount of carbon tetrailuon'de. decreased as the anode current density was increased, while the amount of hexafluoroethane in- Y creased.v The maximum amount of octafluoropropane obtained was over 5 mole percent.

. While the particular amounts of carbonV tetrauorida hexafiuoroethane, and the higher molecular weight fluorocarbons will vary somewhat with the particular metal luonide or uorides employed as the electrolyte, similar results to those shown in the above gures are obtained. VA uorocarbon product containing` at least 10 mole percent of hexauoroethane and higher molecular weight fluorocarbons isrobtained at electrolysis temperatures of 600 to 925 C. with the amount of these fluoro- 'Canbons decreasing rapidly to practically nil as the temperature is increased above 925 C. MetalV fluorides which are non-volatile and stable at the electrolysis temperature may be used as the electrolyte in the production of these iluorocarbons. Alkali metal uorides, alkaline earth metal fluorides, earth metal luorides and mixtures thereof may be used. Illustrative examples of these metal fluorides `aremagnesfium uoride, aluminum uoride, sodium uoride,-barium uoride,strontium'uoride, calcium iluoride, lithium fluoride, yttrium iuoride, and cesium fluoride.

Although only one of the alkalimetal fluorides, alkaline earth metal il-uorides, or earth metal lluorides may be used as electrolyte, a mixtore of these metal uorides is often used to increase the conductivity or may have to be used -to lower the melting point of the bath to a temperature below 925 C.- For this purpose, lithium uoride and sodium iluoride'are most commonly added to the other metal luorides, but other mixtures and coml' b inations may also be used. When other fluorides are added to either increase the conductivity or lower the melting point of a4 particular metal fluoride bath, the uorides of the metals Which are higher in electromotive series or more electronegativc than the metal to be extracted at the cathode from the particular bath are pre- The concentra- Y Y 4 ferred. By using uorides of metals more electronegative, these metals will not deposit out at the cathode with the desired metal except at an exceptionally high cathode current density. Thus'the cathode product is not contaminated under normal operating conditions. Also in continuous operation of the cell, the metal fluoride added to the electrolyte is not depleted by the electrolysis and onlyY the uoride of the particular metal being deposited at the cathode has to be added continuously. For eX- ample, when lithium tluoride Yi-s added to a magnesium bath, the -lithium is more electronegative than magnesium and thus will not deposit out at the cathode. Once the Vlithium iluoride is added to the bath it wil-l not be depleted lby the electrolysis and only magnesium fluoride has to be added for continuous operation. Y

Some examples of the metal fluoride mixtures that may 'be used for the preparation of hexauoroethane and Vhigher.molecular weight fluorocarbons are shown in the y Metal Prefercntally Deposited at the Cathode Electrolyte Composition The term earth metals, as used herein, means the elements aluminum, scandium, yttrium, and lanthanum of the third group of the periodic system.

While generally the desired electrolysis Itemperature is the same for all of the metal iluoridesV or mixtures used, the optimum temperature Vfor particular electrolytes may vary to a certain degree. The temperature of the bath must not exceed 925 C. so that the electrolyte used in the cell must have a melting point below this temperature. Metal lluorides which -are operative in the cell but have melting points above 925 C. must be thus combined with other metal fluorides to lower the melting point of the bath. As may be seen from the plots of FIGURES 2-5, lower temperatures are preferred. The employment of lower temperature will result in the production of more of the higher molecular weight uorocarbons. While the maximum production of hexailuoroethane may be obtained at temperatures in the range of 750 yto 850 C., higher lluorocarbons such as Yoctailuoropropane are only obtained in appreciable amounts at a temperature below 800 C.Vv ',l'hus, for the production of higher molecular weight iluorocarbons than hexauoroethane a temperatureY of 600 to 700C. is desirable. The optimum production of hexauorocthane and higher uorocarbons is generally obtained for most of the metal fluoride elec-V trolytes at a temperature in the range of 725 toV 825 C.

vAs may be also seen from FIGURES 2-5, the anode current density necessary to produce a lluorocarbon productcontaining at least l0 percent of hexafluoroethane and higher molecular weight lluorocarbons varies with the temperature of the electrolyte. At 700 C. the anode product obtained at practically any anode current density contains-at least l0 percent hexauoroethane and higher molecular Weight uorocarbons. This is specically shown irrFIGURE 5. As the temperature of the bath or electrolyte is increased, the anode current density necessary to produ-ce a uorocarbon stream containing the desired amount of hexafluoroethane and higher fluorocarbons increases. At 800 C. an anode current density of approximately 1.5 amperes per square inch is required,

while at 900 C. the anode current density is approximately 3 amperes per square inch. The particular anode current density required at -a given temperature to obtain a stream containing at least percent of hexawhich do not have a vapor pressure in excess of 10 millimeters of mercury at the electrolysis temperature and which do not decompose due to heat `alone at the electrolysis temperature.

uoroethane and higher fluorocarbons will vary somewhat 5 The following example further illustrates the invention with the particular electrolyte used in the bath. Howbut is not to be construed as limiting it thereto. ever,l with all of the electrolytes a lower anode current Exam. l 1 density may be used to obtain the higher iluorocarbons p e at lower temperatures. The cathode current density gen- A Series 0f runs WaS made Where an electrolyte conerally employed is in the range of 1 to 30 amperes per 10 sisting essentially of 48 percent of lithium fluorideand square inch. 52 weight percent sodium iluoride was subjected to elec- Since the anode in the cell comprises carbonaceous ma. trolysis at dilerent anode current densities and different terial in particulate form loosely conned, it is ydiiiicult temperatures A Cell similar t0 that SllOWn in FIGURE to accurately determine the area of the anode contacting Vl Was uSed- The tank Was aPPIOXlmatelY 6 inches ih the electrolyte. In determining the anode current dendiameter and an aluInina liner WaS inserted Which WaS 3 sity, as herein expressed, the anode area is considered inches in diameter and 5 inches high- The Carbon lead to be equal to the area of the electrolyte upon which the eXtendiug through the COVeI' 0f. the tank Was a 3A lhCh carbonaceous material oats. Since the carbonaceous graphite TOdy material is considerably lighter than the electrolyte, the ln the OPeratiOh 0f the Cell, lead Which Was t0 yast as a bed of the carbonaceous material remains substantially niOlten CathOde and 100@ vgrains 0f a IniXtufe Containing on the surface of the electrolyte with very little of the 48 Weight Percent Of lithium fluoride and 52 Weight Pel" carbonaceous material being immersed in the electrolyte. Cent 0f Sodium lluOIide Were Placed in the Celi inside Of Thus, in determining the anode current density, the total the alumina liner and heated to melt the electrolyte. The cell current in amperes is divided by the area ofthe elec- 3%: inch graphite md Was eXtehded ihtO the Cell Until it trolyte subjected to the carbonaceous material in square alInOSt tOuChed the SurfaCe 0f the eleCtfOlyte- The Carinches. bonaceous material was placed on top of the electrolyte to Practically any carbonaceous material in particulate form a bed 2 inches thick within the alumina liner and form may be used. Charcoal, coke, lampblack, powdered surrounding the 3%; inch graphite rod. 11n one of the runs carbon, and powdered ygraphite are illustrative examples lampblack was used as a cai'bonaceous material and in of some of the carbonaceous materials which are operathe remaining runs petroleum coke passing through Ai tive. Due to its availability, coke in particulate form is inch mesh screen and being retained on a number 40 mesh preferred. Generally, particles of carbonaceous matestandard screen was used. The cover was placed on the rial larger than 1 inch are not used except in a large unit tank and tightened to obtain a gas-tight seal. where a large bed is employed. Particles as small as The cell was then placed in the furnace Where the those passing through a No. l0() standard mesh screen and 35 metal uoride electrolyte and lead cathode were heated being retained on No. 300 mesh screen or lampblack are to -a predetermined temperature. When the electrolyte operative. However, generally it is preferred to use carreached the desired temperature, current Was passed bonaceous material which will pass through a No. 6 standthrough the cell at a given voltage to produce the desired ard mesh screen and be retained on a No. 40 standard anode current density. The anode product being promesh screen. 40 duced was drawn off from inside the cell through a gas The voltage used is such that sufficient anode current exit line in the cover of the tank similar to that indicated density is obtained to give a product containing at least as number 7 in FIGURE 1. The cell was operated for l0 percent hexauoroethane and higher molecular weight 1 hour during which time the `anode gas issuing from the fluorocarbons. The actual voltage necessary to obtain cell was collected. It was analyzed by infrared analysis the desired anode current density will vary with the temor by vapor phase chromatography from which the comperature, the carbonaceous material, anode to cathode sepposition of the uorooarbon product obtained was deteraration and the metal uoride or mixtures of metal uomined. rides used in the electrolysis. However, a higher voltage The results obtained and the pertinent data of the is used at a higher electrolysis temperature, since a larger runs made are shown in the table below. The results of anode current density is required. At a bath temperature these runs were used to obtain the curves shown in FIG- of around 700 C. a voltage of approximately 5 volts may URES 2, 3, 4, and 5.

Fluorocarbon Analysis Electrolyte Anode Cell Cell Based on Total Flu- Iempera- Anode Current Current, Potential, orocarbons, mole perture, C. Material Density, Amps. Volts cent rampa/in.t

GF4 CzF C3Fs 700 Coke i/B" to i 7 8.5 60 31 -40 mesh. 700 do 5 30 20 43.5 55.4 3.5 25 20 46 48.6 i 8 4.5 10o 2 15 83.4 11.2 3 20 7.5 5o 5o 4 30 20 36.4 62.7 5 33-37 10.5 30.5 69.5 3.5 25 i8 48.2 51.5 2.1 15 7 97.3 2.7 2 11-15 8 100 5 35 9.5 74.2 25.8 do 6.5 50 10.5 76.0 24.2

Lampblack 0.5 4 8.0 100 be sufficient to give the required anode current density, while at 900 C. a voltage of from 9 volts to as high as 30 volts may have to be used.

The term non-volatile and stable, as herein used in reference to the metal fluoride, means metal uorides In a manner similar to that above a iluorocarbon product from the anode may be obtained containing at least 10 mole percent of hexauoroethane and higher nuorocarbons by electrolysis of other alkali metal lluorides, alkaline earth metal luorides, and earth metal fluoxides which are nonvolatile and stable at the temperature of 4the electrolysis as well as mixtures of these metal fluorides.

What is claimed is:

1. A process fordthe preparation of a uorocarbon Y anode product containing hexailuoroethane and higher molecular Weight iluorocarbons, which consists of passing an electric current atavoltage up to as Vhigh as 30 volts through a molten electrolyte at aY temperature inthe range of 600 to 925 C., between an anode consisting of can bonaceous material in particulate form, theV individual particles being in electrical contact with each other, and

insoluble cathode ata suiiicient anode current density, to liberate a gas at the anode containing iluorocarbons of Iwhich uorocar-bons at leastV 10 mole percent is hexailuoroethane and higher molecular Weight uorocarbons such that for electrolyte temperatures `above 700 C. the

' amideV current density is increased with, an in- 8 .ture is in the range'of 725 toA 825 C. and the carbonaceous material is petroleum coke.

4. A process for the preparation of a tluorocarbon anode product containing hexalluoroethane and higher uorocarbons, which consists of passing an electric current ata voltage up to as high as 30 volts through a molten electrolyte consisting essentially of a mixture of sodium ilu-orideiand lithium uoride at a temperature in Y the range of 600 to 925 C., between an anode consisting of carbonaceous material in particulate form, the

' individual particles being in electrical contact with each crease in temperature of the electrolyte so that at 8009 C. the minimum `anode current density is 1.5 amperes per square inch and at 900 C. the minimum anode current density is 3 amperes per square inch, said molten electrolyte consisting essentially of at least one metal tluoride whichisstable and non-volatile at the electrolysis Atemperature selected from the group consisting Vof alkali metal fluorides, alkaline earth metal fluorides, and earth Vmetal iluorides.

2. A process according to claim l wherein the molten electrolyte consists .essentially of at least one alkali metal 'Y uoride which is stable and non-volatile at the electrolysis temperature. l y Y 3. A process according to claim 1 wherein the tempera- Vrent density is 3 vamperes per square inch.

5. A process according to claim 4 wherein the temperatureV is -in the range of 725 to 825 C. Vand the anode current density is from 3 to 5 amperes per square inch.

6. A process according to f claim 4 wherein the carbonaceous material is petroleum coke.

References Cited in the file of this patent UNITED STATES PATENTS Lyons et a1., y Mar. 28, 1965 Radimer c July l, 1958 fa: l

Claims

1. A PROCESS FOR THE PREPARATION OF A FLUOROCARBON ANODE PRODUCT CONTAINING HEXAFLUOROETHANE AND HIGHER MOLECULAR WEIGTH FLUOROCARBONS, WHICH CONSISTS OF PASSING AN ELECTRIC CURRENT AT A VOLTAGE UP TO AS HIGH AS 30 VOLTS THROUGH A MOLTERN ELECTROLYTE AT A TEMPERATURE IN THE RANGE OF 600* TO925*C., BETWEEN AN ANODE CONSISTING OF CARBONACEOUS MATERIAL IN PARTICULTE FORM, THE INDIVIDUAL PARTICLES BEING IN ELECTRICAL CONTACT WITH EACH OTHER, AND AN INSOLUBLE CATHODE AT A SUFFICIENT ANODE CURRENT DENSITY TO LIBERATE A GAS AT THE ANODE CONTAINING FLUOROCARBONS OF WHICH FLUOROCARBONS AT LEAST 10 MOLE PERCENT IS HEXAFLUROOETHANE AND HIGHER MOLECULAR WEIGHT FLUOROCARBONS SUCH THAT FOR ELECTROLYTE TEMPERATURES ABOUT 700*C. THE MINIMUM ANODE CURRENT DENSITY IS INCREASED WITH AN INCREASE IN TEMPERATURE OF THE ELECTROLYTE SO THAT AT 800* C. THE MINIMUM ANODE CURRENT DENSITY IS 1.5 AMPERES PER SQUARE INCH AND AT 900*C. THE MINIMUM ANODE CURRENT DRNSITY IS 3 AMPERES PER SQUARE INCH, SAID MOLTEN ELECTROLYTE CONSISTING ESSENTIALLY OF AT LEAST ONE METAL FLUORIDE WHICH IS STABLE AND NON-VOLATILE AT THE ELECTTROLYSIS TEMPERATURE SELECTED FROM THE GROUP CONSISTING OF ALKALI METAL FLUORIDES, ALKALINE EARTH METAL FLUORIDES, AND EARTH METAL FLUORIDES.