Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Halogenous component
Reexamination Certificate
2002-03-21
2004-01-13
Silverman, Stanley S. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Halogenous component
C423S24000R, C423S266000, C423S628000, C588S253000, C588S249000
Reexamination Certificate
active
06676913
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an aluminum oxide based catalyst compositions and a method to abate perfluorinated compounds and/or hydrofluorocarbons using the aluminum oxide based catalyst. The present invention further relates to a catalyst and a catalytic process for the conversion of volatile perfluorinated compounds (PFC's) and volatile hydrofluorocarbons (HFC's) into elemental oxides and mineral acids.
BACKGROUND OF THE INVENTION
Perfluorinated compounds (PFC's) are used extensively in the manufacture of semiconductor materials, such as for dry chemical etching and chamber cleaning processes. For the purposes of this invention, PFC's are defined as compounds composed of nitrogen, carbon, or sulfur atoms, or mixtures thereof, and fluorine atoms that do not contain double or triple bonds. Typically the compounds consist of only the carbon, nitrogen or sulfur atom(s) and the fluorine atom(s). However the PFC's may also comprise compounds containing carbon and/or nitrogen and/or sulfur plus fluorine. Examples of PFC's include nitrogentrifluoride (NF
3
), tetrafluoromethane (CF
4
), hexafluoroethane (C
2
F
6
), sulfurhexafluoride (SF
6
), octafluoropropane (C
3
F
8
), decafluorobutane (C
4
F
10
) and octafluorocyclobutane (c-C
4
F
8
). The global warming potential of these compounds has been estimated to be many times greater than that of CO
2
resulting in a desire for economical technologies for achieving emissions control requirements. Hydrofluorocarbons (HFC's) are also used in the manufacture of semiconductor material, and are generated as by-products during semiconductor manufacture. HFC's are defined as compounds composed entirely of carbon, hydrogen and fluorine, and containing at least one of each element. Examples of HFC's include trifluoromethane (CHF
3
) and 1,1,1,2-tetrafluoroethane (C
2
H
2
F
4
). Like PFC's, HFC's are believed to contribute to global warming. Other applications for PFC's and HFC's include uses as polymer blowing agents and as refrigerants.
Catalytic technologies have been and continue to be widely used as an “end-of-the-pipe” means of controlling industrial emissions. This technology involves passing a contaminated stream over a catalyst in the presence of oxygen and/or water at an elevated temperature to convert the pollutants in the emissions stream to carbon dioxide, water and mineral acids, should halogens be associated with the parent compounds. This technology offers many advantages over thermal incineration as a means of controlling emissions. The principle advantages are related to the use of the catalyst, which reduces the temperature required to decompose the pollutants by several hundreds of degrees Celsius. These advantages include energy savings (which translates into lower operating costs), lower capital costs, small foot print of resulting abatement unit, a more controllable process, and no generation of thermal NO
x
.
A primary contributor to the success of any catalytic abatement unit is the catalyst. The catalytic destruction of PFC's and HFC's results in the formation of highly corrosive fluorine-containing products, such as F
2
, HF and/or COF
2
. In order for a catalyst to effectively decompose these PFC's and HFC's, the catalyst must be able to maintain its integrity in the highly corrosive environment. Many typical catalytic materials will not maintain their integrity in this reaction environment due to fluorine attack.
Titania and impregnated titania catalysts (anatase phase) have been reported to decompose selected fluorine-containing compounds. Karmaker and Green, in an article entitled “An investigation of CFCl
2
(CCl
2
F
2
) decomposition on TiO
2
catalyst,”
J. Catal
., p. 394 (1995), report the use of a TiO
2
catalyst to destroy dichlorodifluoromethane at reaction temperatures between 200 and 400° C. in streams of humid air. Although the authors report that the catalyst is stable, a review of the data reveals the conversion of dichlorodifluoromethane to decrease from 93 to 84% over the duration of the 100 hour test. The authors also report a decrease in the surface area of the catalyst, from 170 to 40 m
2
/g over the duration of the experiment. The authors also present evidence that the catalyst has undergone some degree of fluorination. Based on these results, it is doubtful that the TiO
2
catalyst will possess the required lifetime to be considered in a commercial application. The ability of the TiO
2
catalyst to destroy PFC's, such as CF
4
, C
2
F
6
, etc. was not reported in this paper. However, TiO
2
(anatase phase) is known to convert to the low surface area rutile phase at temperatures greater than about 450° C. (LeDuc, C. A., Campbell, J. M. and Rossin, J. A.; “
Effect of Lanthana as a Stabilizing Agent in Titanium Dioxide Support
,” Ind. Eng. Chem. Res. 35, (1996) 2473).
Fan and Yates, in an article entitled “Infrared Study of the Oxidation of Hexafluoropropene on TiO
2
,” J. Phys. Chem.
, p. 10621 (1994), report the destruction of hexafluoropropylene (C
3
F
6
) over TiO
2
. Although the catalyst was able to destroy hexafluoropropylene (C
3
F
6
), the loss of titanium, as TiF
4
, was evident. The formation of TiF
4
would undoubtedly lead to deactivation of the catalyst and would prevent it from being employed in commercial applications.
Campbell and Rossin, in a paper entitled “Catalytic Oxidation of Perfluorocyclobutene over a Pt/TiO
2
Catalyst,” presented at the 14th N. Am. Catal. Soc. Meeting (1995), report the use of a Pt/TiO
2
catalyst to destroy perfluorocyclobutene (c-C
4
F
6
) at reaction temperatures between 320 and 410° C. The authors reported some loss of reactivity over the duration of the near 100 hour reaction exposure. The authors also note the beneficial effects of water on improving the stability of the catalyst. The authors note that even at a reaction temperature of 550° C., the catalyst could not decompose perfluorocyclobutane (c-C
4
F
8
), a PFC used in the manufacture of semiconductor material. Results presented in this study demonstrate that perfluoroalkanes are significantly more difficult to destroy than the corresponding perfluoroalkene.
Aluminum oxide, particularly of the high surface area gamma phase, is widely used as a support for catalytically active metals. Aluminum oxide offers a combination of high surface area and excellent thermal stability, being able to maintain its integrity at temperatures of approximately 800° C. for short periods of time. Aluminum oxides; however, do not fare well as catalyst supports for the destruction of fluorine-containing compounds. Farris et al., in an article entitled “Deactivation of a Pt/Al
2
O
3
Catalyst During the Oxidation of Hexafluoropropylene,”
Catal. Today,
p. 501 (1992), report the destruction of hexafluoropropylene over platinum supported on a high surface area aluminum oxide catalyst. It is not reported whether the platinum or the aluminum oxide is responsible for the destruction of hexafluoropropylene. The catalyst could readily destroy hexafluoropropylene at reaction temperatures between 300 and 400° C.; however, severe deactivation of the catalyst was noted. Over the course of the experiment (less than 100 hours), the aluminum oxide was converted to aluminum trifluoride, which resulted in a severe loss of catalytic activity. This transformation of the aluminum oxide to aluminum trifluoride indicates that aluminum oxide will not be able to maintain its integrity in a fluorine environment for an extended period of operation.
Oxides of aluminum and zirconium are able to decompose PFC's and HFC's, and oxides of titanium are able to decompose HFC's. However, all these materials are rapidly deactivated during exposure to PFC's and/or HFC's and are therefore not suitable for commercial applications. Aluminum oxide is rapidly deactivated during the destruction of PFC's and HFC's due to the aluminum oxide being fluorinated, ultimately being transformed into aluminum trifluoride. Zir
Guild Associates, Inc.
Silverman Stanley S.
Vanoy Timothy C.
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