Process for perfluorocyclobutane purification

Details Process for perfluorocyclobutane purification Process for perfluorocyclobutane purification

2001-04-04

2001-12-25

Siegel, Alan (Department: 1621)

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

Organic compounds

Halogen containing

C570S177000

Reexamination Certificate

active

06333440

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to processes for the purification of perfluorocyclobutane by the removal of unsaturated and/or saturated halocarbons.
BACKGROUND OF THE INVENTION
Perfluorocyclobutane (c-318 or octafluorocyclobutane, b.p. −6° C.) is a valuable material which can be used as a propellant, etch gas, and fire extinguishant. This compound is typically made by cyclodimerization of tetrafluoroethene (TFE) or is recovered as a co-product from the manufacture of TFE. As a result c-318 may be contaminated with unsaturates such as E- and Z-perfluoro-2-butene (i.e., CF
3
CF═CFCF
3
or PFC-1318my). These compounds are toxic and must be removed prior to commercial use. PFC-1318's are difficult to separate from c-318 by distillation as the boiling points are similar.
Saturated by-products are also typically produced during the c-318 manufacturing process. These include isomers of compounds of the formula C
2
Cl
2
F
4
, C
2
HClF
4
and C
2
H
2
F
4
. Some of these impurities form azeotropes with the c-318 and some have similar boiling points. Hence, separation of c-318 from saturated by-products can also be challenging.
U.S. Pat. No. 5,001,287 discloses a process for treating an impure mixture consisting essentially of at least one olefinic impurity and at least one saturated halocarbon by contacting the mixture with a source of hydrogen in the presence of a hydrogenation catalyst (e.g., a catalyst containing a Group VIII metal or rhenium). Perfluorocyclobutane is included among the examples of saturated halocarbons.
U.S. Pat. No. 5,763,698 discloses a process for reducing the fluorine content of hydrochlorofluorocarbons and hydrofluorocarbons by reacting these compounds with HCl in the vapor phase at an elevated temperature in the presence of a catalyst.
U.S. Pat. No. 5,345,017 discloses a process for reducing the fluorine content of acyclic fluorocarbons and chlorofluorocarbons by reacting these compounds with HCl in the vapor phase at an elevated temperature in the presence of a catalyst. In Example 8, a linear perfluorocarbon, perfluoropropane is converted to chlorofluoropropanes.
There is a need for alternative methods of purification.
SUMMARY OF THE INVENTION
This invention provides a process for obtaining octafluorocyclobutane of increased purity from a mixture comprising (a) octafluorocyclobutane and (b) at least one halocarbon impurity which is difficult to separate from octafluorocyclobutane by distillation (e.g., azeotropes of octafluorocyclobutane with such halocarbons). The process comprises (1) contacting the mixture with a catalyst in the vapor phase in the presence of at least one hydrogen halide selected from the group consisting of HCl and HF at a temperature sufficient to react component (b) impurity with said hydrogen halide to provide a product mixture comprising halogenated product which is more easily separated from octafluorocyclobutane by distillation than the unreacted impurity; and (2) separating halogenated product obtained in (1) from octafluorocyclobutane by distillation.
DETAILS OF THE INVENTION
Octafluorocyclobutane is typically produced during the manufacture of tetrafluoroethylene and hexafluoropropylene by the pyrolysis of chlorodifluoromethane as discussed in U.S. Pat. No. 5,672,784. Octafluorocyclobutane can also be produced by the electrolysis of 1,1,2,2-tetrafluorocyclobutane as disclosed in European Patent Publication No. 0 455 399. During the electrolysis process, the major product (about 62 mole %) is octafluorocyclobutane. However, minor amounts (about 7 mole %) of product with an intermediate degree of fluorination are also found.
Processes used to produce c-318 may simultaneously produce a variety of halogenated impurities in the c-318 product stream (see for example PCT International Publication No. WO 99/44973). Examples of halogenated impurities that may be found in a c-318 production stream include linear and cyclic, saturated and unsaturated, perfluorocarbons (PFCs), chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), and hydrochlorocarbons (HCCs). Representative examples from these classes of halogenated impurities include: PFC-31-10 (normal iso-C
4
F
10
, perfluorobutane isomers), PFC-41-12 (C
5
F
12
, perfluoropentane isomers), PFC-1318my (cis and trans-CF
3
CF═CFCF
3
), PFC-1318c (CF
3
CF
2
CF═CF
2
), PFC-1216 (HFP or CF
3
CF═CF
2
), PFC-1114 (TFE or CF
2
═CF
2
) perfluoroisobutene (CF
2
═C(CF
3
)
2
), CFC-114 (CF
2
ClCF
2
Cl), CFC-114a (CFCl
2
CF
3
), CFC-216ba (CF
3
CFClCF
2
Cl), CFC-217ba (CF
3
CClFCF
3
), CFC-1113 (CClF═CF
2
), HCFC-22 (CHClF
2
), HCFC-21 (CHCl
2
F), HCFC-124 (CHFCICF
3
), HCFC-124a (CClF
2
CHF
2
), HFC-134 (CHF
2
CHF
2
), HFC-134a (CH
2
FCF
3
), HFC-152a (CH
3
CF
2
H), HFC-125 (CF
3
CF
2
H), HFC-227ca (CF
3
CF
2
CHF
2
), HFC-227ea (CF
3
CHFCF
3
), HFC-1225zc (CF
3
CH═CF
2
), HFC-236ca (CHF
2
CF
2
CHF
2
), HFC-236ea (CHF
2
CHFCF
3
), HFC-236fa (CF
3
CH
2
CF
3
), HCC-30 (CH
2
Cl
2
), HCC-40 (CH
3
Cl), and HCC-160 (CH
3
CH
2
Cl).
Compounds having a boiling point within about 15° C. of the c-318 boiling point are difficult to separate from c-318 by distillation, especially within about 10° C., and more especially within about 5° C. Compounds which form azeotropes or azeotrope-like compositions with c-318 are also difficult to separate by distillation.
Among the unsaturated impurities which are difficult to separate from c-318 and which may be reacted with hydrogen halides are the acyclic compounds selected from C
2
HClF
2
, C
2
H
2
ClF, C
3
ClF
5
, C
3
HF
5
, C
4
F
8
, C
4
H
2
F
6
, C
4
F
6
and the cyclic compounds selected from c-C
4
F
6
and c-C
5
F
8
. This invention provides a process for obtaining c-318 of increased purity from a mixture comprising c-318 and at least one of said unsaturated impurities. Of note is a process for obtaining c-318 of increased purity from a mixture comprising c-318 and C
4
F
8
. Also of note is a process for obtaining c-318 of increased purity from a mixture comprising c-318 and C
3
ClF
5
. Also of note is a process for obtaining c-318 of increased purity from a mixture comprising c-318 and C
3
HF
5
. Of particular note are processes where HCl is reacted with these unsaturated impurities.
Among the saturated impurities which are difficult to separate from c-318 and which may be reacted with hydrogen halides are the acyclic compounds selected from CHCl
2
F, C
2
Cl
2
F
4
, C
2
HClF
4
, C
2
H
2
F
4
, C
2
H
4
F
2
, C
3
ClF
7
, C
3
HF
7
, C
3
HClF
6
, C
3
HClF
6
, C
3
H
2
F
6
and C
4
F
10
. This invention provides a process for recovering c-318 from a mixture comprising c-318 and at least one of said saturated impurities. Of note is a process for obtaining c-318 of increased purity from a mixture comprising c-318 and C
2
Cl
2
F
4
. Also of note is a process for obtaining c-318 of increased purity from a mixture comprising c-318 and C
2
HClF
4
. Also of note is a process for obtaining c-318 of increased purity from a mixture comprising c-318 and C
2
H
2
F
4
. Of particular note are processes where HCl is reacted with C
2
Cl
2
F
4
, C
2
HClF
4
and/or C
2
H
2
F
4
and processes where HF is reacted with C
2
HClF
4
.
It will be evident that not all of the component (b) impurity must be reacted in order to obtain increased c-318 purity. Generally, where reaction of the component (b) impurity results in a halogenated product that boils at a lower temperature than c-318, this low-boiling halogenated product may be selectively distilled from the product mixture to leave an octafluorocyclobutane-containing composition of increased c-318 purity. Generally, where reaction of the component (b) impurity results in a halogenated product that boils at a higher temperature than c-318, an octafluorocyclobutane-containing composition of increased c-318 purity may be selectively distilled from the product mixture to leave a composition rich in this high-boiling halogenated product. In any case, it is often desirable to react at least 50 mole percent of the component (b) impurity in accordance wi

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