Chemistry of inorganic compounds – Silicon or compound thereof – Binary compound
Reexamination Certificate
1994-07-20
2001-04-17
Hendrickson, Stuart (Department: 1754)
Chemistry of inorganic compounds
Silicon or compound thereof
Binary compound
C423S249000, C423S250000, C423S251000, C423S252000, C423S256000, C423S439000, C423S440000, C423S263000
Reexamination Certificate
active
06217841
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The invention relates to the technical field of heavy metal or silicon carbides having a large specific surface and intended to serve as catalysts or catalyst supports for the chemical or petrochemical industry or for silencers.
DESCRIPITON OF THE RELATED ART
The specific surface of a catalyst is due to three porosity types, namely a macroporosity due to the pores having an average diameter greater than 2 &mgr;m, a mesoporosity due to pores having an average diameter of 30 to 50 Angstroms and a microporosity due to pores having an average diameter of 5 to 15 Angstroms. A catalyst must have an adequate macroporosity in order to give the gases to be treated access to the micropores and particularly mesopores responsible for the actual catalytic activity.
French patent application 2,645,143 describes a process for the preparation of heavy metal carbides having a large specific surface and consisting of the vacuum reaction of a heavy metal compound, at a temperature such that said compound is gaseous, on a carbon having a large specific surface, e.g. active carbon. The carbide obtained retains the memory of the specific surface of the starting carbon and has a specific surface significantly larger than that of the prior art metal carbides, although smaller than that of the starting carbon.
In this carbide preparation process, the catalysts or supports obtained are in the form of a powder, or agglomerates in granule form for the formation of a fixed bed placed in a reactor. The gases to be treated flow, sometimes at high speed, through the said bed and consequently attrition phenomena occur with the production of fines, which clog the bed or seal the sintered parts defining said bed.
For certain catalytic applications, particularly for the catalysis of the oxidation of exhaust gases, it is necessary to choose the geometry of the support or catalyst and produce it in the form of monolithic parts, whilst ensuring the accessibility of the catalyst to the gases to be treated by the macroporosity thereof.
The problem of the invention is to obtain carbides having a high specific surface, in monolithic form and having a significant open macroporosity giving easy access to the reaction gases.
Thus, whereas in the process of French patent application 2,645,143, firstly the carbide was produced and then placed in a bed, in the process according to the invention firstly a porous solid carbon is produced and this is then transformed into metal carbide.
OBJECT OF THE INVENTION
The object of the invention is a process for the preparation of metal carbides having a large specific surface in monolithic form and a significant open macroporosity permitting easy access to the reaction gases, characterized in that the starting product is a carbon foam, which is activated to increase its specific surface and the activated carbon, under vacuum or inert gas scavenging, is subject to the action of a volatile compound of the metal in question. The use of certain organic precursors permitting by calcination the obtaining of carbon foams with interesting characteristics also forms part of the invention.
A second object of the invention is a monolithic catalyst support or a catalyst constituted by a silicon carbide or metal carbide foam having a significant open macroporosity, a large specific surface more particularly due to its mesoporosity and a small crystallite size, whilst having no residual metal or other metallic compound.
DESCRIPTION OF THE PRIOR ART
The aforementioned French patent application 2,645,143 describes the preparation principle of metal carbides having a large specific surface, as well as performance examples. The carbon having a large specific surface, e.g. active carbon, is mixed with a compound, which is volatile at the reaction temperature, of the metal whereof a carbide is to be obtained. The resulting product is placed in a crucible, which is heated to a temperature between 900 and 1400° C. under a vacuum below 13 hPa for between 1 and 7 hours, but usually at least 4 hours.
As a variant, it is also possible to vaporize the volatile compound of the metal in a first area and carry out the reduction and carburization in a second area, the temperatures of the two areas possibly being different.
French patent application 2,621,904 describes the preparation of silicon carbide SiC with a specific surface exceeding 100 m
2
/g by generation in a first area of SiO vapour by the reaction of SiO
2
on Si at a temperature between 1100 and 1400° C. and under a vacuum of 0.1 to 1.5 hPa and reaction, in a second area, of the SiO formed with carbon having a specific surface exceeding 200 m
2
/g and at a temperature between 1100 and 1400° C.
U.S. Pat. No. 2,431,326 (Heyroth) describes a product constituted by a continuous, crosslinked, crystalline SiC skeleton distributed in a uniform manner and whose interstices are filled with silicon or a silicon-rich compound. The skeleton can be produced by calcining wood, molasses, casein, dextrin, flour or other carbonizable substances. Preferably, use is made of the reaction of HCL or H
2
SO
4
on furfural, furfuryl alcohol or mixtures thereof. The carbon precipitates very rapidly and grows in dendritic form in order to form a sponge filling the entire receptacle. The thus obtained skeleton is contacted with silicon, e.g. by making it float on molten Si well above the melting point thereof or by exposing it to silicon vapours.
French Patent 1,484,116 (US Atomic Energy Commission) describes a process for the preparation of foams of carbon and carbides (particularly Si) by a method derived from the preparation of polyurethane foams. The reaction of an isocyanate on an alcohol or a phenol leads to a urethane and with diisocyanates and dialcohols straight-chain polyurethanes of formula:
(—OC—NH—R—NH—CO—O—R′—O—)
are obtained. In addition, the isocyanates react with water giving CO
2
and a urein.
Polymerization in the presence of water consequently makes it possible to form polyurethane foams which, by carbonization, give carbon foams. However, the carbon yield is low and the foams obtained are not solid.
Thus, the invention of said patent consists of on the one hand mixing a binder, whose C yield is high and partly polymerized furfuryl alcohol and on the other hand mixing powders of metals or metal oxides in order to obtain metal or metal carbide foams (W,Ta,Nb,Al,B,Si).
In the process described, the carbide is obtained in a single operation, at the time of calcining the mixture of the resins and the metal oxide or metal, which does not make it possible to control the specific surface of the carbide obtained. In the process of the present patent application, the carbide is obtained in two operations separated by the activation of the foam, which makes it possible to obtain a much larger specific surface of the carbide (20 to 100 m
2
/g), a smaller crystallite size (40 to 400 Angstroms=40 to 400.10
−10
m) and a carbide foam containing little or no residual metal (<0.1% by weight).
British Patent 2,137,974 (UK Atomic Energy Authority) describes the production of a silicon matrix reinforced by a silicon carbide network by the treatment of a carbon foam by contacting molten silicon.
SUMMARY OF THE INVENTION
The process according to the invention consists of the combination of the following successive stages:
optionally, preparing carbon foam by calcining a foam of an organic polymer,
producing, from the thus prepared carbon foam or a commercially available carbon foam, a part having the dimensions and shape of the catalyst support or catalyst which it is wished to obtain,
preferably, degassing the carbon foam part in vacuo or under the scavenging of an inert gas, such as argon,
activating the surface of said part by exposing it to an oxidation in carbon dioxide CO
2
,
exposing the thus activated part to the action of a volatile compound of the metal, whose carbide is to be prepared at a temperature (1000 to 1400° C.) and for a time (2 to 6 hours) such that the metal content of the compound is reduced and the me
Grindatto Bernard
Jourdan Alex
Prin Marie
Dennison, Scheiner Schultz & Wakeman
Hendrickson Stuart
Pechiney Recherche
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