GB2135669A - Electrolytic production of tetrafluoroethylene - Google Patents

Abstract

Tetrafluoroethylene is made by the electrolytic reduction of an electrolyte solution containing dichlorodifluoromethane in the cathode compartment of an electrolytic cell having a separator between anode and cathode compartments.

Description

SPECIFICATION Manufacture of tetrafluoroethylene This invention relates to a process for the manufacture of tetrafluoroethylene, and more particularly to a process based on electrolysis and using chlorofluorocarbons as the feedstock.

It is known, from an article in Journal of Electroanalytical Chemistry (volume 100 (1979) pages 217-223) by Heinz P. Fritz and Wolfgang Kornrumpf, to subject difluorodibromomethane to electrolytic reduction. The reduction is postulated as producing the unstable difluorocarbene, which is not detected or isolated, but is converted by reaction in situ with olefins to form difluorocyclopropane derivatives. There is also mention of a polymer (stated to be similar to polytetrafluoroethylene) being observed to be formed on the electrodes.

This procedure is not a convenient one for making the polytetrafluoroethylene, and there is no indication that the difluorocarbene could be made to dimerise so as to form tetrafluoroethylene monomer, and then for this monomer to remain in the monomeric form and be recovered as such.

It has now been discovered that this electrolytic system may be adapted to form the basis for an electrolytic method for producing tetrafluoroethylene monomer in good yield, using the readily available material dichlorodifluoromethane as feedstock.

Thus, according to the invention, there is provided a process for the manufacture of tetrafluoroethylene which comprises the electrolytic reduction of an electrolyte solution containing dichlorodifluoromethane in the cathode compartment of an electrolytic cell having a separator between anode and cathode compartments.

The cell may utilise as separator any material which is sufficiently permeable to the electrolytes used to enable electrolysis to proceed while preventing appreciable mixing of the catholyte and anolyte.

Suitable materials include glass fibre, porous alumina and porous plastics materials provided they are insoluble in the solvents used, for example polyvinyl chloride, polyethylene, polypropylene and, especially, polytetrafluoroethylene.

The electrolyte solution to be used in the cathode compartment (the catholyte) of the cell needs to comprise (a) dichlorodifluoromethane (b) an organic liquid which is a solvent for the dichlorodifluoromethane, (c) an electrolyte salt which is soluble in the solvent (b) and renders it electrolytically conducting.

The dichlorodifluoromethane may be the usual commercially available material.

The organic liquid solvent may be any liquid which is substantially inert towards the dichlorodifluoromethane and towards difluorocarbene, and in which the electrolyte salt is sufficiently soluble to impart the desired degree of electrolytic conductivity. For this purpose, the solvent should be substantially free of olefinic unsaturation and acidic hydrogens which can react with the intermediate radicals and anions formed during the reduction. The solvent should itself be stable to electrolytic reduction.

Very suitable groups of compounds useful as the solvent include nitriles, halogenated hydrocarbons and carbonic esters and mixtures thereof. Examples of such compounds include chlorinated hydrocarbons of the run than and ethane selves (for example methylene chloride and ethylene dichloride); aliphatic nitriles (for example acetonitrile), and alkylene carbonates (for example diethyl carbonate and propylene carbonate).

The particular solvent used may be selected for its solvent properties (as indicated above) and also for its boiling point, which should be such as to allow easy handling, recovery, and separation from the product formed and any unused feedstock which needs to be recovered. The preferred solvent is methylene chloride, or a mixture of this with a co-solvent.

The electrolyte salt may be any salt which is sufficiently soluble (preferably at least 0.1 molar) in the organic solvent to impart the desired degree of electrolytic conductivity, and which is also inert towards the other components of the electrolyte and towards difluorocarbene. Usually these salts will be alkylammonium salts, especially tetra-alkylammonium salts.

The anion of such salts is conveniently a haiide ion (for example a bromide, iodide or, especially, a chloride ion) but need not necessarily be so. Alternative anions include, for example tetrafluoborate ions.

The proportions of the components may be varied over a wide range but it is convenient to use an electrolyte solution in which the electrolyte salt, for example tetra-alkylammonium halide, is present at a concentration of at least 0.1 molar, preferably about 0.2 molar and in which the concentration of the dichlorodifluoromethane is as high as possible. The solubility of the latter is increased by operating the cell at low temperatures, for example at 0--5 OC.

The electrolysis may be carried out using a cell made of any conventional materials which are stable or inert towards the electrolyte, and using (a) as cathode material, mercury or, preferably, lead although platinum, copper or nickel may be used when the solvent is substantially anhydrous.

(b) as anode material, platinum, TiO2/Ru02, graphite, or other material which is corrosion-resistant, insoluble in the solvent and has a low over potential for chlorine (c) a separator as described above.

(d) a catholyte comprising an electrolyte solution as described above, and (e) an anolyte comprising the solvent and the electrolyte salt.

The electrolysis may suitably be carried out at a cathode potential between -1.6 and -2.0 volts (vs Ag/Ag+), a current density of at least 5 mA/cm2 and at a temperature between -20 and 200 C, preferably 0--50C.

The electrolysis may be carried out by conventional means, and the process may be continued as long as desired by appropriate addition of more feedstock, with or without circulation of the catholyte.

In a batchwise electrolysis, the process would normally be taken up to about 50% conversion as indicated by a halving of the cell current at a constant cathode potential.

The tetrafluoroethylene product may be recovered by distillation outside the cell.

The process has the advantage over the commonly used pyrolytic processes for making tetrafluoroethylenE, of leading to a more pure product. In particular, the process of the invention permits the production of tetrafluoroethylene free from trifluoroethylene since it can be ensured, by appropriate selection of the cathode potential, that any chlorodifluoromethane present in the electrolyte solution is not reduced. By contrast, tetrafluoroethylene obtained by the conventional process of pyrolysing chlorodifluoromethane must be subjected to an extremely rigorous fractional distillation to reduce the trifiuoroethylene to an acceptable level. Furthermore, the chlorine obtained as by-product in the present process can be re-used in the production of dichlorodifluoromethane whereas in the conventional pyroiysis, the by-product is the less valuable hydrogen chloride.

The invention is illustrated but not limited by the following examples which are presented in tabular form.

Cathode Current Coulombs Moles of TFE % Current Example Anode Anolyth Separator Catholyte Cathode Potential Density Passed Produced Efficiency 1 Pt 0,1M A12 glass 0,1M A12 Mercury -2,1v 15 mA/cm2 200 8 x 10-5 15,4 0,2M TAA fibre 0,2M TAA 50/50 50/50 THF/PC THF/PC 2 Pt 0,1M A12 glass 0,1M A12 Mercury -2.1V 15 mA/cm2 408 33 x 10-5 31 0,2M TAA flbre 0,2M TAA EDC EDC 3 Pt Saturated glass Saturated Mercury -2,3V 15 mA/cm2 697 20 x 10-5 11 A12 flbre A12 0,2M TAA 0,2M TAA 50/50 50/50 PC/EDC PC/EDC 4 Pt Saturated glass Saturated Mercury -2,3V 15 mA/cm2 813 21,4 x 10-5 10.2 A12 fibre A12 0,2M TAM 0,2M TAM 50/50 50/50 PC/EDC PC/EDC 5 Pt Setureted glass Saturated Mercury -1,8V 15 mA/cm2 800 10 x 10-8 4.8 A12 fibre A12 0,1M TEA 0,1M TEA CH3CN CH3CN

A12 = difluorodichloromethane EDC = 1,2-dichloroethabe TAA = Tetrabutylammonium tetrafluoroborate TAM = Tetrabutylemmonium methylsulphonate THF = Tetrahydrofuran TEA = Tetraethylammonium chloride PC = Propylene carbonate

Claims (5)

1. A process for the manufacture of tetrafluoroethylene which comprises the electrolytic reduction of an electrolyte solution containing dichlorodifluoromethane in the cathode compartment of an electrolytic cell having a separator between anode and cathode compartments.

2. A process according to claim 1 wherein the electrolyte solution contains an organic nitrile, halogenated hydrocarbon or carbonic acid ester as solvent.

3. A process according to claim 1 or claim 2 wherein the electrolyte solution contains a tetraalkylammonium salt.

4. A process according to claim 1 substantially as hereinbefore described with reference to the foregoing Examples.

5. Tetrafluoroethylene whenever manufactured by a process claimed in any one of the preceding claims.