Organic compounds -- part of the class 532-570 series – Organic compounds – Halogen containing
Reexamination Certificate
2000-05-11
2001-11-27
Siegel, Alan (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Halogen containing
C570S151000
Reexamination Certificate
active
06323379
ABSTRACT:
The present invention relates to a process which allows to obtain in high yields very pure CHF
2
—CF
3
(HFC 125).
HFC 125 is an harmless fluorocarbon for the ozone layer, therefore meeting the requirements of the Montreal Treaty. For the commercial uses of the compound an high purity is required.
The possibility to obtain pure pentafluoroethane depends on the type of impurities which are formed during the synthesis. For example CFC 115 (chloropentafluoroethane CF
2
Cl—CF
3
) is an impurity which can be eliminated with difficult from HFC 125, therefore its presence does not allow to obtain the compound at a very pure level. In order to produce pentafluoroethane meeting these requirements, processes must be employed wherein CFC 115 does not form or is formed only in traces.
The processes used in the prior art to obtain HFC 125 do not allow to obtain the compound in high yield and purity.
In U.S. Pat. No. 5,345,014 in the name of the Applicant HFC 125 is obtained by a dismutation reaction of gaseous HCFC-124 (tetrafluorochloroethane C
2
HF
4
Cl) in the presence of supported Cr
2
O
3
, at temperatures in the range 150°-330° C., for contact times between 1 and 20 seconds. A mixture of HFC 125, HCFC 115 and 2,2-dichloro, 1,1,1,-trifluoroethane CF
3
—CHC1
2
(HCFC-123), together with lower amounts of other by-products, is formed. Tests carried out by the Applicant have shown that by this process it is not possible to combine an high yield in HFC 125, calculated as molar ratio between the obtained HFC 125/reacted HCFC 124, with the purity required for the product, since by using reaction conditions such as to increase the yield in HFC 125, at the same time also CFC 115 increases. It has been furthermore verified that the product obtained with the process according to this patent contains also impurities of dichlorotetrafluoroethane and of C
2
F
4
Cl
2
isomers. Besides it is to be considered that the starting compound HCFC 124 contains CFC 114 which is an impurity difficult to be separated since it has an high affinity towards HCFC 124 and when the unreacted tetrafluorochloroethane is recycled, in the reaction conditions CFC 114 can dismutate forming CFC 115.
In the European patent application EP 776,878 in the name of the Applicant a process for preparing pentafluoroethane with very low residual amounts of CFC 115, is described. The compound is obtained by dismutation in gaseous phase of tetrafluorochloroethane CF
3
CHClF (HCFC 124) to form HFC 125 and HCFC 123, in the presence of a catalyst formed of chromium oxide Cr
2
O
3
supported on AlF
3
, for contact times between 15 and 30 seconds at temperatures in the range 140° C.-180° C. The contact times decrease up to 0.1-1 seconds at temperatures in the range 260° C.-300° C. The Examples show that the CFC 115 amount is very low and in some cases negligible, but the yield in HFC 125 is unsatisfactory and the molar ratio HFC 125/HCFC 123 is always very close to the unit.
The need was felt to prepare HFC 125 substantially impurity-free, specifically CFC 115, with a combination of an improved yield and purity with respect to the prior art.
The solution of these technical problems has been reached by the process as described hereinbelow.
An object of the present invention is a process wherein pentafluoroethane HFC 125 is obtained by dismutation of gaseous tetrafluorochloroethane HCFC 124, at temperatures in the range 200° C.-300° C., preferably 250° C.-280° C. on a trivalent chromium oxide (Cr
2
O
3
) catalyst, preferably supported, preferably the support is aluminum fluoride, feeding a HF amount such that the HCFC 124/HF molar ratio is comprised in the range 10/1-1/1, preferably 2/1-1/1.
The contact time with the catalyst, determined as the ratio between the catalyst volume and that of the gas flow at the working temperature and pressure, is in the range 1-50 seconds, preferably 5-30 seconds.
The pressure is not critical, one commonly works at atmospheric pressure (1 atm) or higher. For example one can operate at pressures up to 5 atm or higher.
In the process optionally inert gases, such as for example nitrogen, as diluents of the reaction gases, can be used.
The supported catalyst is prepared by using a chromium chloride solution and an aluminum fluoride support, usually with the method known as “dry impregnation”, for example as described in EP 408,005, herein incorporated by reference.
At the end of the impregnation, the catalyst must be activated: the operation can be directly carried out in the reactor used for the dismutation, by calcining in inert gas current, at the temperature of about 400° C. for 4-8 hours and then by treating at 360° C. with anhydrous HF for 12-24 hours.
The aluminum fluoride used as a support is obtainable by alumina fluorination, and it has a fluorine content not lower than 90%, preferably not lower than 95%, with respect to the stoichiometric.
Generally the AlF
3
is mainly formed of gamma phase, as described in FR 1,383,927, and has a surface area generally in the range 25-35 m
2
/g. If the catalyst is used in a fluidized bed the support has the granulometry suitable for this kind of reactor, as well known to the skilled in the art. The chromium content in the supported catalyst is in the range 1-15% by weight.
According to the process object of the present invention it is possible to increase the yield, calculated as molar ratio between the obtained HFC 125/reacted HCFC 124, up to 70% and more, maintaining weight ratios CFC 115/HFC 125 and CFC 114/HCFC 124 lower than or equal to 1000 ppm.
The reaction is preferably carried out in a continuous way and the catalyst is used in a fluidized bed.
REFERENCES:
patent: 4129603 (1978-12-01), Bell
patent: 5345014 (1994-09-01), Cuzzato
patent: 117 444 (1976-01-01), None
patent: 0 130 532 A2 (1985-01-01), None
patent: 0 408 005 B1 (1991-01-01), None
patent: 0 569 832 A1 (1993-11-01), None
patent: 0 776 878 A2 (1997-06-01), None
patent: 1383927 (1963-11-01), None
Bragante Letanzio
Cuzzato Paolo
Rinaldi Francesco
Arent Fox Kintner & Plotkin & Kahn, PLLC
Ausimont S.p.A.
Siegel Alan
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