Process for catalytic chemical reaction and catalyst

Details Process for catalytic chemical reaction and catalyst Process for catalytic chemical reaction and catalyst

2000-04-18

2002-09-10

Clarke, Sara (Department: 3743)

Combustion

Process of combustion or burner operation

In a porous body or bed, e.g., surface combustion, etc.

C502S326000

Reexamination Certificate

active

06447285

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a process for a high-temperature catalytic chemical reaction which uses a solid catalyst comprising an active metal phase and a non-oxidized refractory catalytic support on which the active phase is deposited.
More particularly, the catalytic support is based on silicon nitride (Si
3
N
4
).
Silicon nitride exists in two crystalline forms, the &agr; form (&agr;-Si
3
N
4
), known as the low temperature form (<1200° C.), and the &bgr; form (&bgr;-Si
3
N
4
), known as the high temperature form (>1500° C.). The two forms coexist between 1200° C. and 1500° C. The structures of each of these forms are essentially distinguished by a different arrangement of the SiN
4
tetrahedra.
V.I. Simagina et al. [1] have described the use, for the oxidation reaction of methane, of a catalyst comprising, as catalytic support, porous &bgr;-Si
3
N
4
on which nickel oxide is dispersed. In order to obtain a satisfactory conversion yield, the reaction temperature has to reach 850° C. This is because the stability of the methane molecule requires high temperatures for initiating the reaction. However, the use of high temperatures results in the undesirable production of contaminants, such as carbon monoxide, resulting from the decomposition of carbon dioxide and/or from the partial oxidation of methane, and nitrogen oxides, originating from the oxidation of atmospheric nitrogen.
SUMMARY OF THE INVENTION
The authors of the present invention have looked for a catalyst which is sufficiently active to catalyze reactions of the abovementioned type but which makes it possible to lower the temperature thereof in order to eliminate the abovementioned disadvantages while having high stability and high reactivity under particularly severe conditions, namely oxidizing atmosphere and high exothermicity.
Thus, they have discovered, for catalytic chemical reactions which normally take place at high temperature, that a solid catalyst comprising an active metal phase and a refractory catalytic support comprising mainly silicon nitride in its &agr; form makes it possible to lower the reaction temperature to and to control it at a value of between 200° C. and 800° C. and preferably between 200° C. and 600° C.
The silicon nitride is advantageously in the substantially non-porous state.
Before describing the present invention in more detail, certain terms employed are defined hereinbelow.
“Reaction which normally takes place at high temperature” comprises reactions which only take place under or the yield of which is improved only under high temperature conditions (so-called “operating temperature”) and also highly exothermic reactions, the operating temperature of which is not necessarily very high but which release thermal energy which raises the local operating temperature with the possible creation of hot spots.
“Substantially non-porous” is understood to mean a material which does not exhibit a hysteresis cycle in nitrogen adsorption/desorption isotherms at the temperature of liquid nitrogen.
According to the invention, the support comprises mainly silicon nitride in its &agr; form; this means that it is composed of 90% at least of said form, it being possible for the remainder to be in particular the &bgr; form of silicon nitride.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The preferred alternative forms of the process of the invention are presented hereinbelow:
the support is advantageously in the discrete form with a particle size of between 0.1 and 1 &mgr;m;
the active metal phase, that is to say the catalytic phase, is chosen in particular from transition metals and preferably consists of palladium;
the process of the invention is particularly suitable for the catalysis of chemical reactions which take place in an oxidizing atmosphere, such as oxidations; thus, the combustion reaction of methane, in an oxidizing atmosphere, which is illustrated in the examples which will follow, is advantageously carried out according to the process described hereinabove,
the support exhibits a specific surface preferably of between 5 and 20 m
2
/g,
the ratio of the proportion by weight of the metal of the active metal phase and in particular of palladium to that of the silicon of the support is advantageously between 0.5 and 2% by weight,
the catalyst is preferably obtained by impregnating the support with metal acetylacetonate salts and then activating according to a protocol which will be described later.
Another subject-matter of the invention is a catalyst comprising an active metal phase and a refractory catalytic support comprising mainly silicon nitride in its &agr; form. Preferably, the support is substantially non-porous.


REFERENCES:
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patent: 4940684 (1990-07-01), Okutani et al.
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Y. Soma-Noto et al., “Infrared Spectra Carbon Monoxide Absorbed on Supported Palladium and Palladium-Silver Alloys,”Journal of Catalysis, 32, 315-324, 1974.
N. Kundo et al., “Catalyst for oxidn, of methane with air oxygen—consists of nickelous oxide active component on beta,”STN International, Wpindix File Search Results, Sep. 1996 (Abstract).
Dwyer, “Catalysis for Control of Automotive Emissions”, Marcel Dekker, Inc., 1972.
“Physical Constants of Organic Compounds,”CRC Handbook of Chemistry and Physics, 70thEdition, 1989-1990, CRC Press, Inc.
Boudart et al., “La Cinétique Des Réactions En Catalyse Hétérogéne,” Masson, 1982.
L. Lozzi et al., “Surface Stoichiometry Determination of SiOxNyThin Films by Means of XPS,”,Surface and Interface Analysis, vol. 22, 190-192, 1994.
K. Le Bas, “Preparation de Catalyseurs Pd-Cu/SiO2a Partir D'Acetylacetonates, Reactivite vis A vis De L'Hydrogenation Du Butadiene-1-3, ET DE La Reduction de No Par Co,” Doctoral Thesis, Devant, Devant l'Universite, Claude Bernard—Lyon-I, Sep. 1994.
Brunauer et al., “Adsorption of Gases in Multimolecular Layers,”Contribution from the Bureau of Chemistry And Soils and George Washington University, vol. 60, pp. 309-320, Feb. 1938.
Taylor et al., “Chemical Reactions of N2+Ion Beams with Group IV Elements and Their Oxides,”Journal of Electron Spectroscopy and Related Phenomena, 13, pp. 435-444, 1978.

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