HK1073100B

HK1073100B - Process for the production of fluoroethane and use of the produced fluoroethane

Info

Publication number: HK1073100B
Application number: HK05105757.7A
Authority: HK
Inventors: Kazunari Kaga; Hiromoto Ohno; Toshio Ohi
Original assignee: Showa Denko K.K.
Priority date: 2002-03-11
Filing date: 2003-03-07
Publication date: 2007-03-16

Description

Process for producing fluoroethane and use of the fluoroethane obtained

Cross Reference to Related Applications

This application is filed in accordance with 35u.s.c.111(a), claiming benefit from the filing date of provisional application 60/364,035 filed on 3, 15, 2002 in accordance with 35u.s.c.111(b) in accordance with 35u.s.c.119(e) (1).

Technical Field

The present invention relates to a process for producing pentafluoroethane, a process for producing hexafluoroethane using pentafluoroethane obtained by the production process, and uses of the obtained pentafluoroethane.

Background

Pentafluoroethane (CF)₃CHF₂) As cryo-refrigerants or as etching gases, and also for the production of hexafluoroethane (CF)₃CF₃) The raw materials of (1).

As a process for producing pentafluoroethane, various processes have been known.

For example, these methods are:

(1) chloroethylene (CCl) hydrofluoride₂＝CCl₂) Or a fluorinated product thereof (see Japanese unexamined patent publication No. 8-268932 and Japanese International application domestic publication No. 9-511515),

(2) reacting Chloropentafluoroethane (CF)₃CF₂Cl) (see Japanese patent 2,540,409), and

(3) a method of reacting fluorine gas with halogen-containing ethylene (see Japanese unexamined patent publication No. 1-38034).

In using these production methods, various impurities such as chlorofluorocarbon (CFC), Hydrochlorofluorocarbon (HCFC) and Hydrofluorocarbon (HFC) are contained in the target substance pentafluoroethane.

In order to obtain pentafluoroethane of high purity, it is necessary to remove these impurities as much as possible. Among these impurities, various purification methods have been proposed to remove chlorofluorocarbons not only for the purpose of achieving high purity but also for the purpose of preventing damage to the ozone layer. In particular, chloropentafluoroethane has a boiling point close to that of pentafluoroethane and is difficult to separate by distillation, so various purification methods have been proposed.

For example, these methods are:

(1) an extractive distillation method (see Japanese International application, domestic publication No. 9-508626),

(2) a method of subjecting chloropentafluoroethane to hydrogenolysis (see Japanese unexamined patent publication No. 8-301801), and

(3) a method for removing chloropentafluoroethane after fluorination of chloropentafluoroethane with Hydrogen Fluoride (HF) (see Japanese unexamined patent publication No. 2001-48816).

On the other hand, as for the method of separating impurities containing hydrochlorofluorocarbons or hydrofluorocarbons, only some methods have been proposed. For example, Japanese International application, national publication No. 9-508627 describes an extractive distillation purification process. Among hydrochlorofluorocarbons and hydrofluorocarbons, difluoromethane (CH) is known₂F₂) And 1, 1, 1-trifluoroethane (CF)₃CH₃) Form azeotropic mixtures with pentafluoroethane, and these compounds are difficult to separate from pentafluoroethane.

When pentafluoroethane is produced by a method including hydrogenolysis, 1, 1, 1-trifluoroethane is generally produced as a by-product due to an excessive hydrodehalogenation reaction, and the content of the compound in pentafluoroethane is large. For removing 1, 1, 1-trifluoroethane from pentafluoroethane, an extractive distillation method has been known. However, this method has a problem in that a lot of expensive equipment such as a distillation column is required, and the equipment cost is very high.

Disclosure of the invention

Under these circumstances, an object of the present invention is to provide an industrially advantageous process for producing high-purity pentafluoroethane, wherein pentafluoroethane can be used as a low-temperature refrigerant or as an etching gas or as a raw material for producing high-purity hexafluoroethane; a process for producing hexafluoroethane using pentafluoroethane obtained by the above process; and the use of the pentafluoroethane obtained.

As a result of intensive studies in order to solve the above-mentioned problems, the present inventors have found that these problems can be achieved by a process for producing pentafluoroethane, comprising (1) a step of fluorinating tetrachloroethylene to obtain crude pentafluoroethane containing impurities, and (2) a step of contacting the crude pentafluoroethane containing impurities with oxygen and/or an oxygen-containing compound in the presence of a catalyst. The present invention has been completed based on this finding.

The process for producing pentafluoroethane of the present invention includes the steps of:

(1) a step of fluorinating tetrachloroethylene to obtain crude pentafluoroethane, and

(2) a step of contacting the crude pentafluoroethane with oxygen and/or an oxygen-containing compound in the presence of a catalyst.

The crude pentafluoroethane used in the step (2) is preferably obtained by another step of contacting with hydrogen.

The temperature in step (2) is preferably 150 ℃ to 400 ℃.

The catalyst is preferably a supported or bulk catalyst comprising mainly trivalent chromium oxide.

The catalyst is also preferably a supported catalyst mainly containing at least one metal selected from the group consisting of palladium, rhodium, ruthenium, rhenium, platinum and gold.

The support used in the supported catalyst is preferably alumina, fluorided alumina or zeolite.

The crude pentafluoroethane may contain, as impurities, at least one compound selected from the group consisting of: fluoromethane, difluoromethane, fluoroethane, 1, 1-difluoroethane, 1, 2-difluoroethane, 1, 1, 1-trifluoroethane, and 1, 1, 2-trifluoroethane.

The total amount of impurities contained in the crude pentafluoroethane is 2 vol% or less.

In another embodiment, the process for producing pentafluoroethane of the present invention comprises contacting a crude pentafluoroethane with oxygen and/or an oxygen-containing compound at 150-400 ℃ in the presence of a catalyst mainly comprising trivalent chromium oxide, and then separating impurities by distillation.

In another embodiment, the process for producing pentafluoroethane of the present invention comprises contacting a crude pentafluoroethane with oxygen and/or an oxygen-containing compound at 150 ℃ and 400 ℃ in the presence of a supported catalyst mainly containing at least one metal selected from the group consisting of palladium, rhodium, ruthenium, rhenium, platinum and gold, and then separating impurities by distillation.

The crude pentafluoroethane may contain at least trifluoroethane as an impurity.

The concentration of oxygen and/or oxygen-containing compounds is preferably 0.1 to 20% by volume.

The present invention provides pentafluoroethane having a total amount of impurities of 500 ppm by volume or less, which is obtained by one of these production methods.

The content of trifluoroethane as an impurity in pentafluoroethane is preferably 100 ppm by volume or less.

The present invention also provides a refrigerant containing pentafluoroethane.

The process for producing hexafluoroethane of the present invention comprises the steps of:

(1) a step of fluorinating tetrachloroethylene to obtain crude pentafluoroethane,

(2) a step of contacting the crude pentafluoroethane with oxygen and/or an oxygen-containing compound in the presence of a catalyst to obtain pentafluoroethane, and

(3) reacting pentafluoroethane obtained by the step (2) with a fluorine gas to obtain hexafluoroethane.

Best Mode for Carrying Out The Invention

The present invention will be described in detail below.

As described above, pentafluoroethane can be produced by a method of fluorinating tetrachloroethylene or a fluorinated product thereof with Hydrogen Fluoride (HF) or a method of subjecting chloropentafluoroethane to hydrogenolysis. Regardless of the production method employed, pentafluoroethane obtained by a conventional purification step such as distillation contains chloropentafluoroethane, which is an impurity difficult to separate from pentafluoroethane. The chloropentafluoroethane must be separated so as to obtain high-purity pentafluoroethane, and from the viewpoint of preventing the destruction of the ozone layer, it is required not to contain chloropentafluoroethane.

As for the method of separating chloropentafluoroethane in pentafluoroethane, as described above, for example, a method using hydrogenolysis, a method using extractive distillation, and a method using absorption have been proposed. Among these methods, a method using hydrogenolysis can produce pentafluoroethane more inexpensively from the viewpoint of equipment cost. There is a problem that when a method including a hydrogenolysis step is selected as a method for producing or purifying pentafluoroethane, Hydrofluorocarbons (HFCs) such as 1, 1, 1-trifluoroethane, which are difficult to separate, are produced due to an excessive hydrogenation reaction. In particular, difluoromethane and 1, 1, 1-trifluoroethane are difficult to separate by conventional purification methods because these substances have boiling points very close to that of pentafluoroethane and form azeotropic mixtures. For the separation of Hydrofluorocarbons (HFCs) in pentafluoroethane, an extractive distillation method has been proposed, but this method has a problem that a plurality of expensive equipments such as distillation columns are required and the equipment cost is very high.

The process for producing pentafluoroethane of the present invention includes the steps of: (1) a step of fluorinating tetrachloroethylene to obtain a crude pentafluoroethane, and (2) a step of contacting the crude pentafluoroethane with oxygen and/or an oxygen-containing compound in the presence of a catalyst. The method of step (1) is not particularly limited, and for example, tetrachloroethylene can be fluorinated in two steps using Hydrogen Fluoride (HF) in the presence of a catalyst to obtain crude pentafluoroethane.

In the present invention, impurities contained in pentafluoroethane, such as Hydrofluorocarbons (HFCs), are brought into contact with oxygen and/or oxygen-containing compounds in the presence of a catalyst in the gas phase at a temperature of 150-400 ℃, wherein any hydrofluorocarbon as an impurity is oxidized and converted to carbon dioxide or the like. For example, when difluoroethane or 1, 1, 1-trifluoroethane contained in pentafluoroethane is oxidized by oxygen, it is considered that a reaction represented by the following formula (a) or (b) proceeds:

formula (a)

Formula (b)

The primary oxidation product is carbon dioxide and by-product HF is produced.

Compounds that can be converted to carbon dioxide by this reaction include fluoromethane, difluoromethane, fluoroethane, 1, 1-difluoroethane, 1, 2-difluoroethane, 1, 1, 1-trifluoroethane, 1, 1, 2-trifluoroethane, and the like. In the case of a production or purification process involving a hydrogenolysis step, the total content ofthese compounds in pentafluoroethane is generally about several thousand ppm by volume. These impurities must be removed in order to obtain high-purity pentafluoroethane.

In the process for producing pentafluoroethane of the present invention, the total amount of impurities such as Hydrofluorocarbons (HFCs) contained in the crude pentafluoroethane is preferably 2% by volume or less, more preferably 0.5% by volume or less, further more preferably 0.3% by volume or less. If the content of impurities such as hydrofluorocarbons exceeds 2% by volume, the reaction temperature must be high and the life of the catalyst can be shortened.

The catalyst used for the reaction is preferably (i) a supported or bulk catalyst comprising predominantly trivalent chromium oxide or (ii) a supported catalyst comprising predominantly at least one metal selected from the group consisting of palladium, rhodium, ruthenium, rhenium, platinum and gold. The raw materials which can be used include, for example, these metals, and oxides and salts of these metals. Supports that may be used in supported catalysts include, for example, alumina, fluorided alumina, and zeolites.

The catalyst (i) mainly containing trivalent chromium oxide can be prepared, for example, as follows: adding a basic substance such as ammonia dropwise to an aqueous solution of a chromium metal salt to precipitate chromium hydroxide, washing/filtering/drying the precipitate, molding the resulting chromium hydroxide, and heat-treating the molded article in the presence of an inert gas such as nitrogen. The supported catalyst (ii) mainly containing palladium, rhodium, ruthenium, rhenium, platinum and/or gold can be prepared, for example, as follows: dissolving a salt of the metal in a water-soluble solvent such as water, methanol and acetone, immersing the carrier in the solution so as to absorb the necessary elements, distilling off the solvent, and reducing the carrier with hydrogen under heating.

The temperature in step (2) is preferably 150-400 deg.C, more preferably 180-370 deg.C. If the reaction temperature exceeds 400 ℃, the life of the catalyst is shortened and the kind and content of by-products not belonging to the main reaction are increased.

The concentration of oxygen and/or oxygen-containing compound contained in the reaction substrate gas is preferably 0.1 to 20% by volume. The oxygen may be high purity oxygen or air, but is preferably high purity oxygen. If the oxygen concentration is less than 0.1% by volume, the conversion is disadvantageously decreased due to the shortage of oxygen required for the reaction, but it depends on the kind and amount of hydrofluorocarbon as an impurity contained in pentafluoroethane. On the other hand, if the oxygen concentration exceeds 20% by volume, the excessive reaction causes decomposition reaction of pentafluoroethane, which is the main component of the reaction substrate gas, which is not preferable from the viewpoint of efficiency because loss of pentafluoroethane increases. Oxygen-containing compounds which may be used include, for example, Nitric Oxide (NO), nitrous oxide (N)₂O), nitrogen dioxide (NO)₂) And ozone (O)₃)。

The process for producing pentafluoroethane of the present invention may be carried out under the above-mentioned reaction conditions, but if the reaction product contains carbon dioxide, by-products other than pentafluoroethane, which are not main reactions, such as hydrofluorocarbons, and acidic substances such as HF, it is preferable to remove carbon dioxide and acidic substances.

The acidic substances can be removed, for example, by the following methods: the reaction product is contacted with a purifying agent, or the reaction product is contacted with water, an aqueous alkaline solution, or the like.The gas from which the acidic substances have been removed is preferably dehydrated with a dehydrating agent such as zeolite, followed by distillation to remove carbon dioxide and simultaneously remove by-products which are not the main reaction.

In another embodiment, the present invention provides a process for producing pentafluoroethane, comprising contacting crude pentafluoroethane with oxygen and/or an oxygen-containing compound at 150-400 ℃ in the presence of a catalyst mainly comprising trivalent chromium oxide, and then separating impurities by distillation.

The method of purification after the reaction is not particularly limited, and purification can be carried out by a conventional distillation. As for the distillation method, for example, the following method can be used.

After the crude pentafluoroethane was contacted with oxygen and/or an oxygen-containing compound at 150 ℃ and 400 ℃ in the reactor, the resultant gas was introduced into a distillation column. The internal pressure of the distillation column is preferably from atmospheric pressure to 2 MPa. If the internal pressure is lower than the atmospheric pressure, a decompression system device is disadvantageously required, whereas if the internal pressure exceeds 2MPa, a high-pressure system device is required, which is not preferable. For example, in the case of performing the above-described catalytic reaction using oxygen, a low boiling fraction containing oxygen is extracted from the top of the distillation column, and a high boiling fraction is extracted from the bottom of the distillation column. At this time, each component taken out from the top and the bottom sometimes contains pentafluoroethane as an object component. If this is the case, the components may be introduced into separate distillation columns for purification to recover pentafluoroethane. When the component separated here is an intermediate for producing pentafluoroethane, the component may be returned to the reaction step and reused.

By this purification operation, pentafluoroethane having a higher purity can be obtained. The content of impurities is 500 volume ppm or less. Pentafluoroethane having a purity of 99.95 vol% or more can be analyzed by Gas Chromatography (GC) using a TCD method or a FID method, or by gas chromatography-mass spectrometry (GC-MS).

The use of pentafluoroethane obtained by the production process of the present invention is described below.

High-purity pentafluoroethane can be used as Chlorodifluoromethane (CHF)₂Cl), the latter currently being the operating fluid for cryocoolers; they can also be used as starting materials for mixed refrigerants which are other alternatives to chlorodifluoromethane, for example difluoromethane/pentafluoroethane/1, 1, 1, 2-tetrafluoroethane and difluoromethane/pentafluoroethane.

In addition, high-purity pentafluoroethane can be used as a raw material for producing high-purity hexafluoroethane. Specifically, in the reaction of pentafluoroethane with fluorine gas (F)₂) In the process for producing hexafluoroethane, when high-purity pentafluoroethane is used as a raw material, it is possible to prevent generation of impurities which are difficult to separate from hexafluoroethane. The range of setting the fluorination reaction conditions can be expanded, the reaction can be stablycontrolled, and the purification step can be simplified.

Accordingly, the present invention provides a process for producing hexafluoroethane, comprising (1) a step of fluorinating tetrachloroethylene to obtain crude pentafluoroethane, (2) a step of contacting the crude pentafluoroethane with oxygen and/or an oxygen-containing compound in the presence of a catalyst, and (3) a step of reacting the pentafluoroethane obtained by the step (2) with a fluorine gas.

High purity pentafluoroethane or mixtures thereof with inert gases, e.g. He, N₂And Ar, HCl, O₂、H₂Etc. can be used as an etching gas in an etching step in the production of semiconductor devices. In the production of semiconductor devices such as LSIs, TFTs and organic EL devices, thin or thick films are formed using a CVD method, a sputtering method or a vapor deposition method, and circuit patterns are formed by etching, wherein a mixed gas containing pentafluoroethane can be used as an etching gas. The etching using pentafluoroethane may be performed under various anhydrous etching conditions such as plasma etching and microwave etching.

The present invention will be described in more detail below, but the present invention is not limited to these examples.

Starting example of pentafluoroethane

Tetrachloroethylene and hydrogen fluoride are introduced into a first reactor containing a catalyst to produce a gas containing mainly the intermediates 1, 1, 1-trifluoro-2, 2-dichloroethane and 1, 1, 1, 2-tetrafluoro-2-chloroethane. This gas is introduced into the second reactor together with HF to produce pentafluoroethane. The obtained pentafluoroethane was subjected to distillation to obtain pentafluoroethane containing 0.5% chloropentafluoroethane as an impurity.

Pentafluoroethane was reacted with hydrogen in the presence of an industrially available hydrogenation catalyst (reaction pressure: 0.35MPa, reactor temperature: 280 ℃ C., H₂The molar ratio of chloropentafluoroethane is 5). The acidic substances contained in the resulting mixed gas are removed by a known method, and the residue is purified by distillation, thereby obtaining a distillate mainly containing pentafluoroethane. The distillate was analyzed by gas chromatography, and the mixed gas was found to have the composition shown in Table 1.

TABLE 1

Components	Concentration (% by volume)
Components	Concentration (% by volume)	CF₃CHF₂	99.7171
CF₃CF₂Cl	0.0005	CF₃CHF₂	99.7171
CF₃CF₂Cl	0.0005	CF₃CH₂F	0.0201
CF₃CH₃	0.2621	CF₃CH₂F	0.0201
CF₃CH₃	0.2621	CHF₃	0.0002

Production example of catalyst (catalyst 1)

Chromium nitrate nonahydrate was dissolved in water and mixed with a 28 wt% aqueous ammonia solution while stirring to obtain a chromium hydroxide slurry. This was isolated by filtration, washed thoroughly with water and then dried at 120 ℃. The obtained cake was pulverized, mixed with graphite, and pelletized with a pelletizer. The resulting pellets were tested at 400 ℃ in N₂The stream was baked for 4 hours to give catalyst 1, which mainly contained trivalent chromium oxide.

Production example of catalyst (catalyst 2)

Chloroplatinic acid was dissolved in water, and a spherical alumina support of 3mm diameter was immersed in the resulting solution to absorb platinum salt. Then, the solvent was distilled off at a temperature of 100 ℃ and the residue was baked in air at 300 ℃ and then subjected to hydrogen reduction at 350 ℃. The percentage of platinum supported in the resulting platinum catalyst 2 was 0.25%.

Example 1

The catalyst (catalyst 1) (100ml) was packed in a reactor made of Inconel 600 and having an inner diameter of 1 inch and a length of 1m, and maintained at 300 ℃ while passing nitrogen gas. Then, oxygen gas was supplied at a flow rate of 2.0NL/hr, a gas having the composition shown in Table 1 was supplied at a flow rate of 38.0NL/hr, and then the supply of nitrogen gas was stopped to start the reaction. After 2 hours, the gas withdrawn from the reactor was washed with an aqueous potassium hydroxide solution to remove contained acidic substances, and then contacted with molecular sieve 3A (produced by union showa k.k.) and dried. The resulting dry gas mainly containing pentafluoroethane was collected by cooling and purified by distillation. After purification, the gas was analyzed by gas chromatography, and it was found that the gas had the composition shown in table 2.

TABLE 2

Components	Concentration (% by volume)
Components	Concentration (% by volume)	CF₃CHF₂	99.9665
CF₃CF₂Cl	0.0004	CF₃CHF₂	99.9665
CF₃CF₂Cl	0.0004	CF₃CH₂F	0.0126
CF₃CH₃	0.0204	CF₃CH₂F	0.0126
CF₃CH₃	0.0204	CHF₃	0.0001

Example 2

Pentafluoroethane was obtained in the same manner as in example 1, except that the catalyst 2 was used. After purification, the gas was analyzed by gas chromatography, and it was found that the gas had the composition shown in table 3.

TABLE 3

Components	Concentration (% by volume)
Components	Concentration (% by volume)	CF₃CHF₂	99.9840
CF₃CF₂Cl	0.0004	CF₃CHF₂	99.9840
CF₃CF₂Cl	0.0004	CF₃CH₂F	0.0101
CF₃CH₃	0.0054	CF₃CH₂F	0.0101
CF₃CH₃	0.0054	CHF₃	0.0001

Example 3

A nickel reactor (using a heating system,using an electric heater; the reactor having been inerted with fluorine gas at 500 ℃ C.) having an inner diameter of 1 inch and a length of 50cm was supplied with nitrogen gas at a total flow rate of 30NL/hr through two gas inlets, and the reactor was maintained at 420 ℃. Then, HF was fed from the above two gas inlets at a total flow rate of 50NL/hr, and the mixed gas mainly containing pentafluoroethane obtained in example 1 was supplied from one gas inlet at a flow rate of 3.5 NL/hr. Also, fluorine gas was supplied from another gas inlet at a flow rate of 3.85 NL/hr. Thereby carrying out the reaction. After 3 hours, the gas withdrawn from the reactor was contacted with an aqueous potassium hydroxide solution and an aqueous potassium iodide solution to remove HF and unreacted fluorine gas. The gas is then contacted with a dehydrogenating agent to be dried, and the resulting dried gas is collected by cooling and purified by distillation. After purification, the gas was analyzed by TCD, FID and ECED methods of gas chromatography and GC-MS method, and the results are shown in table 4.

TABLE 4

Components	Concentration (% by volume)
Components	Concentration (% by volume)	CF₃CF₃	＞99.9998％
CF₄	Less than 0.4 ppm by volume	CF₃CF₃	＞99.9998％
CF₄	Less than 0.4 ppm by volume	CF₃CF₂Cl	Less than 0.1 ppm by volume
CF₃CHF₂	Less than 0.5 ppm by volume	CF₃CF₂Cl	Less than 0.1 ppm by volume
CF₃CHF₂	Less than 0.5 ppm by volume	SF₆	Less than 0.4 ppm by volume

As can be seen from the analysis results shown in table 4, hexafluoroethane contains almost no other impurities, and hexafluoroethane of high purity is obtained.

As described above, pentafluoroethane of high purity can be obtained according to the present invention. The pentafluoroethane obtained by the present invention can be used as a low-temperature refrigerant, an etching gas, or a raw material for producing high-purity hexafluoroethane.

Claims

1. A process for producing pentafluoroethane, comprising the steps of:

(1) a step of fluorinating tetrachloroethylene to obtain a crude pentafluoroethane, wherein the crude pentafluoroethane contains at least one compound selected from the group consisting of fluoromethane and fluoroethane, and

(2) a step of contacting the crude pentafluoroethane with oxygen and/or an oxygen-containing compound in the presence of a catalyst, wherein the catalyst is a supported or bulk catalyst mainly containing trivalent chromium oxide or a supported catalyst mainly containing at least one metal selected from the group consisting of palladium, rhodium, ruthenium, rhenium, platinum and gold.

2. The process according to claim 1, wherein the crude pentafluoroethane used in the step (2) is obtained by another step of contacting the crude pentafluoroethane obtained in the step (1) with hydrogen.

3. The process according to claim 1 or 2, wherein the temperature in step (2) is 150-400 ℃.

4. The process according to claim 1, wherein the carrier used in the supported catalyst is alumina, fluorided alumina or zeolite.

5. The process according to claim 1 or 2, wherein the crude pentafluoroethane obtained in the step (1) contains, as impurities, at least one compound selected from the group consisting of: difluoromethane, 1, 1-difluoroethane, 1, 2-difluoroethane, 1, 1, 1-trifluoroethane, and 1, 1, 2-trifluoroethane.

6. The process according to claim 1 or 2, wherein the total amount of impurities contained in the crude pentafluoroethane obtained in the step (1) is 2% by volume or less.

7. A process for producing pentafluoroethane, comprising contacting crude pentafluoroethane with oxygen and/or an oxygen-containing compound at 150-400 ℃ in the presence of a catalyst mainly comprising trivalent chromium oxide, and then separating impurities by distillation.

8. A process for producing pentafluoroethane, comprising contacting crude pentafluoroethane with oxygen and/or an oxygen-containing compound at 150 ℃ and 400 ℃ in the presence of a supported catalyst mainly containing at least one metal selected from the group consisting of palladium, rhodium, ruthenium, rhenium, platinum and gold, and then separating impurities by distillation.

9. The process according to claim 7 or 8, wherein the crude pentafluoroethane contains at least trifluoroethane as an impurity.

10. A process according to claim 7 or 8, wherein the concentration of oxygen and/or oxygen-containing compound is 0.1 to 20% by volume.

11. A process for producing hexafluoroethane comprising the steps of:

(1) a step of fluorinating tetrachloroethylene to obtain a crude pentafluoroethane containing at least one compound selected from the group consisting of fluoromethane and fluoroethane,

(2) a step of contacting the crude pentafluoroethane with oxygen and/or an oxygen-containing compound in the presence of a catalyst, to obtain pentafluoroethane, wherein the catalyst is a supported or bulk catalyst mainly containing trivalent chromium oxide or a supported catalyst mainly containing at least one metal selected from the group consisting of palladium, rhodium, ruthenium, rhenium, platinum and gold, and

(3) a step of reacting the pentafluoroethane obtained by the step (2) with a fluorine gas to obtain hexafluoroethane.

12. The process according to claim 11, wherein the crude pentafluoroethane used in the step (2) is obtained by another step of contacting the crude pentafluoroethane obtained in the step (1) with hydrogen.