Process for producing hexafluoroethane and use thereof

Details Process for producing hexafluoroethane and use thereof Process for producing hexafluoroethane and use thereof

2002-01-14

2002-12-03

Siegel, Alan (Department: 1621)

Organic compounds -- part of the class 532-570 series

Organic compounds

Halogen containing

C570S134000, C570S164000, C570S165000, C570S166000, C570S187000, C570S188000, C570S169000

Reexamination Certificate

active

06489523

ABSTRACT:

DETAILED DESCRIPTION OF THE INVENTION
1. Technical Field of the Invention
The present invention relates to a process for producing hexafluoroethane, comprising a step of reacting a gas mixture containing pentafluoroethane and a compound having chlorine atom with hydrogen fluoride in the gaseous phase in the presence of a fluorination catalyst to fluorinate the compound having chlorine atom and a step of reacting the gas mixture containing pentafluoroethane and the fluorinated compound with fluorine gas in the gaseous phase in the presence of a diluting gas, and also relates to the use thereof.
2. Background Art
Pentafluoroethane (hereinafter referred to as “CF
3
CHF
2
”) is used, for example, as a refrigerant for low-temperature use or a starting material for the production of hexafluoroethane (hereinafter referred to as “CF
3
CF
3
”).
For the production of CF
3
CHF
2
, for example, the following methods have been heretofore known:
(1) a method of fluorinating perchloroethylene (CCl
2
═CCl
2
) or a fluoride thereof with hydrogen fluoride (see, JP-A-5-97724 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”), JP-A-6-506221, JP-A-7-76534, JP-A-7-118182, JP-A-8-268932 and JP-A-9-511515),
(2) a method of performing hydrogenolysis of chloropentafluoroethane (CClF
2
CF
3
), and
(3) a method of reacting a fluorine gas with a halogen-containing ethylene (see, JP-A-1-38034).
When these methods for producing CF
3
CHF
2
are used, the objective CF
3
CHF
2
contains a compound having chlorine atom within the molecule as main impurities. The compound having chlorine atom within the molecule includes a compound having one carbon atom within the molecule, such as chloromethane, chlorodifluoromethane and chlorotrifluoromethane, a compound having two carbon atoms within the molecule, such as chloropentafluoroethane, dichlorotetrafluoroethane, chlorotetrafluoroethane and chlorotrifluoroethane, and an unsaturated compound such as chlorotrifluoroethylene.
In the case of producing CF
3
CF
3
by a direct fluorination reaction of reacting CF
3
CHF
2
with a fluorine gas (F
2
), if CF
3
CHF
2
contains the compound having chlorine atom within the molecule, chlorine, hydrogen chloride, chlorine fluoride or different kinds of chlorofluorocarbons are generated in the reaction with fluorine gas. Even when hydrofluorocarbons (HFC) or perfluorocarbons (PFC) are contained in CF
3
CHF
2
, there arises no particular problem, however, for example, chloromethane (CH
3
Cl) or chlorodifluoromethane (CHClF
2
) reacts with fluorine gas to produce chlorotrifluoromethane (CClF
3
). The objective CF
3
CF
3
and chlorotrifluoromethane form an azeotropic composition, therefore, CClF
3
is difficult to remove even by performing distillation, adsorption for purification, or the like. Accordingly, in the case of reacting CF
3
CHF
2
with a fluorine gas to produce CF
3
CF
3
, the amount of the compound having chlorine atom within the molecule contained in CF
3
CHF
2
should be reduced as much as possible.
According to conventional production methods for CF
3
CHF
2
, the total amount of the compound having chlorine atom within the molecule is sometimes as high as about 1 vol %. Therefore, a distillation operation is repeated for removing these compounds contained in CF
3
CHF
2
and elevating the purity of CF
3
CHF
2
, however, this has such a problem that the distillation cost increases, the distillation loss is caused, the profitability is bad and some compounds having chlorine atom within the molecule form an azeotropic mixture or an azeotrope-like mixture with CF
3
CHF
2
and are very difficult to remove only by the distillation operation. In particular, chloropentafluoroethane (hereinafter referred to as “CClF
2
CF
3
”) is usually contained in CF
3
CHF
2
in a concentration of thousands of ppm or more but since an azeotropic mixture is formed by CF
3
CHF
2
and CClF
2
CF
3
, the separation is hardly attained by distillation which is a commonly used separation and purification method.
For separating CClF
2
CF
3
contained in CF
3
CHF
2
, various methods have been proposed, for example,
(1) a method of adding a third component to a mixture of CF
3
CHF
2
and CClF
2
CF
3
and performing the extractive distillation (see, JP-A-6-510980, JP-A-7-133240, JP-A-7-258123, JP-A-8-3082, JP-A-8-143486 and JP-A-10-513190),
(2) a method of removing CClF
2
CF
3
contained in CF
3
CHF
2
using an adsorbent (see, JP-A-6-92879 and JP-W-8-508479 (the term “JP-W” as used herein means an “unexamined published international patent application”)), and
(3) a method of converting CClF
2
CF
3
contained in CF
3
CHF
2
into CF
3
CHF
2
in the presence of a hydrogenation catalyst (see, JP-A-7-509238, JP-A-8-40949, JP-A-8 -301801 and JP-A-10-87525).
However, these methods have a problem, that is, the method of (1) requires a step of recovering the third component from the mixture of CClF
2
CF
3
and the third component, the method of (2) requires a step of regenerating the adsorbent, and the method of (3) suffers from reduction in the catalytic life due to hydrogen chloride produced.
Problems to be Solved by the Invention
The present invention has been made under these circumstances and the object of the present invention is to provide a method for producing CF
3
CF
3
with good profitability using a gas mixture containing CF
3
CHF
2
and a compound having chlorine atom within the molecule in the method for producing CF
3
CF
3
which is used as an etching or cleaning gas in the process of producing a semiconductor device, and also provide a use thereof.
Means to Solve the Problems
As a result of extensive investigations to solve the above-described problems, the present inventors have found that in the method for producing CF
3
CF
3
, when a gas mixture containing CF
3
CHF
2
and a compound having chlorine atom within the molecule as impurities is reacted with hydrogen fluoride in the presence of a fluorination catalyst to convert CClF
2
CF
3
which is contained in the gas mixture, into CF
3
CF
3
and then performing a direct fluorination reaction of reacting the resulting gas mixture containing CF
3
CHF
2
and CF
3
CF
3
with a fluorine gas in the gaseous phase in the presence of a diluting gas, the above-described problems can be solved. The present invention has been accomplished based on this finding. The present invention provides a process for producing CF
3
CF
3
and use thereof, described in [1] to [19] below.
[1] A process for producing hexafluoroethane, comprising the following two steps:
(1) a step of reacting a gas mixture containing pentafluoroethane and a compound having chlorine atom with hydrogen fluoride in the gaseous phase in the presence of a fluorination catalyst to fluorinate the compound having chlorine atom; and
(2) a step of reacting the gas mixture containing pentafluoroethane and the fluorinated compound obtained in the step (1) with a fluorine gas in the gaseous phase in the presence of a diluting gas.
[2] The process for producing hexafluoroethane as described in [1], wherein the compound having chlorine atom is at least one compound selected from the group consisting of chloromethane, chlorotrifluoromethane, chloropentafluoroethane, dichlorotetrafluoroethane, chlorotetrafluoroethane, chlorotrifluoroethane and chlorotrifluoroethylene.
[3] The process for producing hexafluoroethane as described in [1] or [2], wherein the total amount of the compound having chlorine atom contained in the gas mixture of the step (1) is 1 vol % or less.
[4] The process for producing hexafluoroethane as described in [1] or [2], wherein the total amount of the compound having chlorine atom contained in the gas mixture of the step (1) is 0.5 vol % or less.
[5] The process for producing hexafluoroethane as described in any one of [1j to [4], wherein in the step (1), the fluorination catalyst is a bulk catalyst obtained by adding indium to an oxide of chromium.
[6&

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