This invention relates to an electrolytic process for the replacement of hydrogen by fluorine in partially fluorinated organic compounds, in particular compounds containing the group -CH2 . Electrochemical fluorination using a liquid hydrogen fluoride electrolyte, which involves the replacement of hydrogen atoms in the starting material by fluorine is known and has been described for example in US 2,519,983. However, known electrochemical fluorinations of the type using a liquid hydrogen fluoride electrolyte have hitherto suffered from the disadvantage that the process is highly unselective. Thus, the products resulting from known electrochemical fluorination processes, for example the electrochemical fluorination of ethane, ethylene and 1 , 1-dichloroethane comprise a complex mixture of all the possible higher fluorinated products. For example, the products from the electrochemical fluorination of ethane as described i ^** - the "Bulletin of the Chemical Society of Japan,

Vol 39. , 1966, pages 219-222," comprises a mixture of mono-, di-, tri- , tetra-, penta- and per-fluoroethane . Furthermore, the predominant product is typically the perfluorinated derivative. When employing lower volatility substrates, for example those described in the Bulletin of Electrochemistry 6 (4) April 1990 at pages 471-477, the perfluorinated product may be formed exclusively.

We have now found that when certain partially fluorinated substrates are electrochemically fluorinated, higher partially fluorinated derivatives, rather than the expected perfluoro derivative may be produced with a high degree of selectivity .

According to the present invention there is provided a process for the production of a compound of formula:

R-0 _n - (R ¹ ) _m -CHF2 wherein R is hydrogen or a fluorine containing C _ alkyl group, Ri is an optionally fluorinated methylene group, and m and n are independently 0 or 1 provided that where n •= 0 , m = 0 and that where n=l, R is not hydrogen comprising subjecting a compound of formula: R-0 _n - (R ¹ ) _m -CH2 wherein R, R ¹ , m and n have the definitions above to electrochemical fluorination in a liquid hydrogen fluoride electrolyte.

We have found that when the compound of formula R-0 _n - (R-*- ) _m -CH2F is subjected to electrolysis in liquid hydrogen fluoride, the partially fluorinated derivative R-0 _n - (R ¹ _m -CHF2 may be produced with high selectivities which may be greater than 60Z and even up to 802, despite the presence of a further hydrogen atom available for substitution by fluorine to produce a compound of formula R-0 _n - (R ¹ ) _m -CF3.

The compound of formula R-0 _n - (R ¹ ) _m -CH2F ma-y comprise halogen atoms other than fluorine, for example chlorine, which may be unaffected by the electrolytic process but the compound usually comprises a fluorine containing compound having no other halogen atoms. The compounds are usually therefore hydrofluoroalkanes and fluorinated dialkyl ethers, which are characterised by containing the group -CH2 . Thus, where n = 0 , the compound of formula

R-0 _n - (R ¹ ) _m ^_CH 2 ^F (hereafter referred to as "the substrate") to be fluorinated comprises a compound of formula R-CH2 wherein R is hydrogen or a fluorine containing Cι_ alkyl group. Preferably it is a group comprising hydrogen, or a fluorine-containing alkyl

group having 1 or 2 carbon atoms. The substrate preferably comprises a hydrofluoro-methane , -ethane or -propane. Particularly preferred substrates include CH3F and CF3CH2F.

Where n = 1 , the substrate comprises a partially fluorinated dialkyl ether of formula R-0-R-*-CH2F where R is a fluorine-containing Cι_ alkyl group. The group R ¹ may comprise a fluorinated methylene group, that is -CH ₂ -1-CHF-, and -CF2- and the substrate may be a partially fluorinated ethyl ether. Preferably however m=0 and the substrate comprises a monofluoromethyl ether. The group R preferably also comprises a partially fluorinated methyl group, and preferred dialkyl ether substrates comprise partially fluorinated dimethyl ethers. Especially preferred substrates include CH2F-O-CH2F and CH3-O-CH ₂ F.

During the process of the invention, hydrogen atoms in the group R of the substrate may be replaced by fluorine such that in the product of the process, the group R will usually comprise a perfluorinated group, although where the group R in the substrate comprises a -CH2F group, the group R of the product of the process may comprise a -CF2H group.

The construction of the electrochemical cell in which the process is carried out may follow conventional technology and suitable cells are described for example in US 2,519,983, GB 741,399 and 758,492. However, we particularly prefer to employ the design of cell which is described in The Journal of Fluorine Chemistry, 32, 1986, pp 1003

-1134 comprising an electrode stack of alternating anodes and cathodes, separated by an insulating material .

The anode material may be for example iron or carbon but is preferably nickel. A particularly

preferred anode material due to its particularly high surface area per unit volume ratio is a nickel foam, commercially available under the trade name "Retimet" from Dunlop Ltd.

The cathode material may be for example iron or steel, but preferably the cathode is also constructed from nickel .

In order that the anode potential may be monitored during anode conditioning and substrate fluorination, the cell may also comprise a reference electrode which may also comprise nickel, for example a nickel wire. The nickel wire may be sheathed in an insulating material, for example polytetrafluoroethylene . The process may be carried out at temperatures in the range from about -80°C to about 30°C, preferably at temperatures in the range from about -10°C to about 10°C and at atmospheric pressure although superatmospheric or subatmospheric pressures m y be employed if desired. Preferably, the operating temperature and pressure are chosen such that the products of the reaction distill from the cell.

The substrates of the invention may be in the liquid or vapour phase depending upon the particular substrate and the operating conditions.

Where the substrate is a vapour under the operating conditions of the process the substrate may be fed to the cell as a vapour, for example bubbled into the cell through an inlet in the bottom of the cell. The substrate vapour feed may be diluted with an inert gas, for example nitrogen, if desired.

The feed flow rate may vary within a wide range depending inter alia upon the capacity of the cell but will usually be in the range from about 10

ml/minute to about lOOml/minute for an electrochemical cell having a capacity of 1.5 litres.

Where the substrate is a liquid under the operating conditions of the process the liquid may be charged to the cell and the cell may be provided with means for example a pump, for circulating the hydrogen fluoride and liquid substrate within or to and from the cell.

The proportion of substrate to liquid hydrogen fluoride may vary within a wide range but we have found that the higher partially fluorinated products are formed with high selectivity where substantially more substrate is employed relative to hydrogen fluoride than would be employed following conventional practice. Thus we prefer to employ the substrate and hydrogen fluoride in a molar ratio from about 1:2 to about 1:10, preferably from about 1:3 to about 1:5.

In carrying out the process of the invention, liquid hydrogen fluoride is charged to the cell. The hydrogen fluoride is preferably anhydrous and if desired a potential may be applied across the electrodes of the cell in order to dry the hydrogen fluoride before the substrate is added. The anode surface is preferably conditioned following the removal of water from the hydrogen fluoride. Conditioning of the anode surface may be carried out by applying an anode potential prior to feeding the substrate to the cell. The conditioning potential may be in the range from about 3V to about 8V , and preferably from about 4V to about 6V.

Following conditioning of the anode, the substrate may be fed to the cell whilst an electrode potential is applied across the cell. The gaseous products of fluorination may be recovered through

vents in the top of the cell and may be passed through various, conventional scrubbers in order to remove any hydrogen fluoride and OF2 which may have become entrained in the product stream. Liquid products of fluorination may be drained from the bottom of the cell. Where the substrate is a liquid, a portion of the dried hydrogen fluoride may be carefully withdrawn from the cell (so as to avoid contamination with water), the liquid substrate may be mixed with the hydrogen fluoride and the mixture may be returned to the cell.

The process may be operated with a direct current anode potential in the range from about 3V to about 7V, preferably from about 4V to about 6V. However, we have found that the selectivity with which the group -CH2F is fluorinated to the group -CHF2 is substantially increased by pulsing the anode potential, known also as applying a periodic potential. This may be achieved, for example, by superimposing on a fixed direct current potential a periodic, fluctuating potential in a pulsating ,- and preferably alternating waveform, that is a pulsating waveform in which the maximum and minimum potentials of the pulse are of opposite polarity. We particularly prefer to apply an alternating waveform which may have a square or rectangular waveform.

We have also found that the increase in selectivity achieved by pulsing the anode potential is dependent, inter alia, upon the difference between the maximum and minimum potentials of the pulsed potential. It is preferred that the difference in potential between the maximum and minimum potentials of the pulse is at least 3V, preferably at least 4V and more preferably at least 5V.

The invention is illustrated with reference to Figure 1 which is a vertical cross section through an electrochemical cell in which the process of the invention may be effected.

In Figure 1, a 1.5 litre capacity electrochemical cell 1 comprises a Monel cell body 2 having channels 3 for circulation of a coolant fluid. A Monel cell head 4 is provided having a series of ports 5 for a nickel reference electrode (not shown), thermocouple (not shown), hydrogen fluoride and nitrogen feeds (not shown) , a nickel anode connection 6 and a nickel cathode connection 7 and product vent port 8. A Monel cell base 9 is provided having a substrate feed port 10 and drain ports 11.

Within the cell body 2, an electrode stack 12 is provided comprising alternating nickel mesh cathodes 13 and nickel foam anodes 14 separated by polytetrafluoroethylene spacer rings 15. A nickel anode annulus 16 and nickel cathode annulus 17 are provided and the stack is supported on a polytetrafluoroethylene support ring 18. Electrical contact is made between the electrodes and the electrode connections with nickel tape 19 partially insulated with polytetrafluoroethylene sleeving.

The electrode stack is held together by nickel tie rods 20 and nickel end plates 21.

In operation of the cell, the cell is charged with hydrogen fluoride and a potential is applied across the cell to first dry the hydrogen fluoride and to condition the anode surfaces.

The substrate is then fed to the cell through inlet 10 in the base of the cell and product and unreacted gas is collected through product vent port 8 in the head of the cell.

The invention is further illustrated by the following examples.

Example 1. Using the cell shown in Figure 1, 1.5 litres of hydrogen fluoride were charged to the cell and the hydrogen fluoride was dried electrochemically by applying an anode potential of 6V (with respect to the Nickel reference electrode). The anode potential was then reduced to 3.8V and the anode surface was conditioned for 48 hours.

After this time, 55ml/minute 1 , 1 , 1 , 2-tetrafluoroethane was fed continuously to the cell, the anode potential was increased to 5.2V, and the cell was run for a period of 8 hours. During this time, the off gases were passed through a condensor to return hydrogen fluoride to the cell, scrubbed with sodium hydroxide and rubber chippings to remove and entrained OF2 and hydrogen fluoride, collected in a liquid nitrogen cold trap and analysed by gas chromatography . Over the 8 hour period, the conversion of 1 , 1 , 1 , 2-tetrafluoroethane was about 302 and the pentafluoroethane selectivity was 602.

Examples 2-5.

In the following examples, a 10ml laboratory cell was employed comprising a polytetrafluoroethylene ring sandwiched between two nickel sheet end plates to form a cylindrical cell body sealed with "0" rings constructed from hexafluoropropylene /vinylidene fluoride copolymer. Electrical contact was made from a source of electrical power to each of the nickel end plates, one of which acted as a cathode. The anode consisted of a nickel foam cylinder (30mm

diameter and 7mm thickness having a surface area of 5600cm ² /cm ³ ) in ^' electrical contact with the other nickel end plate through a nickel washer which served to form a small cavity behind the anode. The substrate was fed into this cavity via a transfer pot connected by a vacuum line and fitted with a nitrogen purge. The transfer pot was filled with hydrogen fluoride which acted as a pre-saturator and so as to minimise electrolyte loss from the cell.

Example 2.

The cell was charged with hydrogen fluoride, the hydrogen fluoride was dried and the anode surface conditioned by applying an anode potential of between 4V and 5V for 3-4 hours without a 1 , 1 , 1 , 2-tetrafluoroethane feed.

Undiluted 1 ,1 , 1 , 2-tetrafluoroethane was then fed to the cell with a flow rate of about 2.5ml/minute at an anode potential of 4.5V for a period of 8 hours. The off gases from the cell were scrubbed with (i) potassium carbonate solution and (ii) water, to remove hydrogen fluoride and scrubbed with rubber chippings to remove OF2. The gaseous products were collected in a liquid nitrogen cold trap and analysed by gas chromatography . The average yield of pentafluoroethane over the 8 hour period was 402.

Example 3.

The procedure of example 2 was repeated except that the anode potential was 6V and it was maintained for 3 hours. The yield of pentafluoroethane (excluding start-up time of 1 hours operation) was 602.

Example 4.

The procedure of example 2 was repeated except that after 2 hours operation at a steady anode potential of 6V, a series of different pulsed anode potentials were applied across the cell. The pulse waveforms were as follows:

(i) 0.2 seconds at 5V followed by 0.2 seconds at -IV,

(ii) 0.2 seconds at 5.5V followed by 0.2 seconds at -0.5V,

(iii) 0.2 seconds at 5.5V followed by 0.2 seconds at 3.5V ,

(iv) 1.0 second at 5.5V followed by 0.2 seconds at 3.5V.

Each pulsed anode potential was applied for about 2 hours .

The results are shown graphically in Figure 2, (the units of the y-axis are 2) in which the periods- of time during which each pulsed waveform was applied are indicated by the vertical columns on the figure, each of which is labelled (i)-(iv) corresponding to the pulsed waveforms described previously.

Example 5.

8ml of 1 , 2-difluorodimethylether was charged to the transfer pot and 8ml of hydrogen fluoride were distilled into it. 10ml of the mixture in the transfer pot was distilled into the cell and an anode potential of 6V was applied whilst continuously feeding nitrogen into the bottom of the cell after sparging through the hydrogen fluoride / 1 , 2-difluorodimethylether mixture in the

transfer pot. The vent gases were scrubbed with (i) potassium carbonate solution and (ii) water, to remove hydrogen fluoride and scrubbed with rubber chippings to remove fluorine. The gaseous products were collected in a liquid nitrogen cold trap. The products were analysed by gas chromatography and found to comprise the following fluorinated materials :

2v/v

1,1,2, 2-tetrafluorodimethylether 50 pentafluorodimethylether 19 perfluorodimethylether 15 difluoromethane 1 trifluoromethane 14 tetrafluoromethane 1

The conversion of 1 , 2-difluorodimethylether was 622