Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Inorganic carbon containing
Reexamination Certificate
2000-02-15
2001-06-26
Wood, Elizabeth D. (Department: 1755)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Inorganic carbon containing
C501S080000, C501S081000, C501S082000, C501S083000, C501S084000, C501S085000, C501S088000, C502S177000, C502S182000, C502S200000, C502S439000, C502S527140, C502S527150, C502S527240
Reexamination Certificate
active
06251819
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a silicon carbide foam with high specific surface area and high porosity, and improved mechanical properties (particularly compression strength), this foam being used mainly as a catalyst support, for example in the chemical or petrochemical industry and in exhaust silencers for internal combustion motors, or filters.
It also relates to its manufacturing process and its applications.
DESCRIPTION OF RELATED ART
Patent FR 2657603 describes how to obtain catalyst supports, particularly made of SiC, with a high specific area (greater than 15 m
2
/g) with a dual mode porosity in which a first pore family with an average diameter of between 1 and 100 &mgr;m enables a gas to access a second pore family with average diameters smaller than 0.1 &mgr;m responsible for the specific area and the catalytic activity.
This support is obtained by mixing an Si powder or one of its reducible compounds in a polymeric or polymerizable organic resin, possibly with additives, forming of the mixture, cross-linking and polymerization of the resin, obtaining a porous carbon skeleton containing Si or a compound of Si, by carbonation in a non-oxidizing atmosphere at a temperature of between 500 and 1000° C., and finally carbonation of Si at a temperature of between 1000 and 1400° C., still under a non-oxidizing atmosphere.
This type of support has good compression strength and a fairly high density, usually of the order of 0.6 to 0.8 g/cm
3
, but it looks more like a solid porous body rather than having the normally aerated appearance of a foam; consequently, its permeability is not sufficient to process large volumes of gas per unit weight of support, and its range of applications is limited. In other words, if the dimensions of the support are large it becomes difficult for treated gasses to reach its center which is consequently unused dead mass.
Patent FR 2684092 describes an SiC foam obtained by a carburization reaction starting from a volatile compound of Si with an activated carbon foam. This activated carbon foam may result from a polyurethane foam reinforced by impregnation using a resin, setting up the resin, carbonation and activation.
The carbide foam obtained has a specific area of not less than 20 m
2
/g due to macrbpores containing edges with lengths varying from 50 to 500 &mgr;m, and mainly mesopores for which the diameter is usually between 0.03 and 0.05 &mgr;m, which is generally about three times larger than the diameter of the pores in the activated carbon foam.
Its density is between 0.03 and 0.1 g/cm
3
, however its relatively modest mechanical strength (the compression strength does not exceed about 0.02 MPa) may limit its field of use or may necessitate specific treatments to strengthen it if necessary.
Patent FR 2705340 describes a process for making a silicon carbide foam that consists of starting from a polyurethane foam, impregnating it with a silicon suspension in an oxygenated organic resin (usually furfurylic), polymerizing the resin up to 250° C. at a rate of 5° C./min., simultaneously carbonizing the foam and the resin between 250 and 1000° C. under an inert atmosphere, carbonizing the Si contained in the resulting carbon foam up to a temperature of between 1300 and 1600° C., and maintaining this temperature for 2 h under an inert atmosphere and cooling the carbide obtained.
The carbide foam obtained has a specific area of not less than 5 m
2
/g, which in particular depends on the maximum temperature reached. It has a two mode porosity comprising macropores with an average diameter of between 100 and 150 &mgr;m and mesopores between 0.0275 and 0.035 &mgr;m.
This foam may be used as a catalyst support or as a diesel engine filter.
It gives satisfactory results in catalytic reactions. However as before, its compression strength and its resistance to abrasion are insufficient when severe thermal and/or mechanical loads are applied to it, particularly for use in exhaust silencers.
Thus the petitioner has attempted to make the use of the said SiC foam supports more reliable, particularly in exhaust silencers or for regeneration treatment, by significantly improving their mechanical properties without penalizing their catalytic properties, particularly their specific area or dual mode porosity (which is not easy since usually one is obtained at the detriment of the other) while maintaining their permeability.
Therefore, the petitioner attempted to improve the foam skeleton.
SUMMARY OF THE INVENTION
The invention is a foam based on silicon carbide for catalytic applications with a high specific area (its BET area is typically at least 5 m
2
/g), characterized in that it has a compression strength higher than 0.2 MPa (2 bars), but generally at least 0.4 MPa (4 bars).
DETAILED DESCRIPTION OF THE INVENTION
The foam according to the invention usually has a bimodal porosity, measured with mercury, comprising essentially a family of pores with an average diameter of between 10 and 200 &mgr;m enabling easy access of the gasses to be processed towards mesoporosity in which the average diameter of the pores is between 0.005 and 1 &mgr;m, and which enables the catalytic activity.
This bimodal porosity is additional to the porous structure of the foam which is typically in the form of a network that could be qualified as “fibrous” comprising sorts of communicating cages delimited by carbide edges (or bridges) usually with a thickness of between 50 and 500 &mgr;m, connected to each other by nodes. The megapores of this network, visible to the naked eye, have dimensions that may be between 0.4 and 1.6 mm and correspond to a porous volume of about 3 to 12 cm
3
/g. Consequently, its non darcian permeability to air is at least 10
−5
m at 20° C. This permeability measures the ease with which gasses to be catalytically treated can pass through it.
It is remarkable to note that the specific area of the foam usually exceeds 10 m
2
/g.
Its density is typically between 0.06 and 0.2 and preferably between 0.08 and 0.15.
It is beneficially in the form of a monolithic part, but it may also be used in particular form, in other words as stacked pieces of foam.
The compression strength is measured by a hardness test, well known in the strength of materials field. It consists of applying a force to a cylindrical punch with a known plane section and measuring the force necessary to make it penetrate into the foam by a distance of 1 cm, the sample having at least two plane parallel surfaces at a separating distance of at least 5 cm.
The foam according to the invention also has very good resistance to thermal shocks.
Thus, it resists at least one thermal shock consisting of increasing it to at least 800° C. and then suddenly cooling it in air to ambient temperature, without reducing the compression strength.
But it is even more remarkable to note that it resists a succession of several thermal shock cycles, each cycle including heating to high temperature followed by sudden cooling in air. For example, it was subjected to a sequence of heating and cooling cycles carried out at temperatures varying from 800° C. to 950° C. at 25° C. intervals, two cycles being carried out at each temperature, without noting any significant reduction in its compression strength.
Sudden cooling in thermal shocks takes place at an average rate of about 60° C./min.
The SiC content of the foam is typically greater than 95%, or better 98%, the residual Si content generally not exceeding 0.1%. The residual C content does not exceed 3%, and normally does not exceed 2%; the residual C content can be eliminated by oxidation in air at a temperature of about 600° C. to 850° C.
This foam is obtained by impregnating an initial organic foam, usually polyurethane, using a suspension of a silicon powder in a resin; this resin contains oxygen with a carbon yield exceeding 30%, and a cross-linking catalyst is added to it in the proportion of 1 to 10% (by weight), and preferably 5%; it is usually furfurylic resin and the cross-linking agent is hexamethylenetetramine, the proportion of silicon
Ollivier Benoist
Prin Marie
Dennison, Scheiner Schultz & Wakeman
Pechiney Recherche
Wood Elizabeth D.
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