Catalyst – solid sorbent – or support therefor: product or process – Miscellaneous
Reexamination Certificate
1999-07-27
2001-02-06
Griffin, Steven P. (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Miscellaneous
C502S178000
Reexamination Certificate
active
06184178
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a silicon carbide based catalyst support with a high specific surface area in the form of particles having improved mechanical characteristics through improved crystallinity.
DESCRIPTION OF RELATED ART
From patent FR 2657603 it is known how to obtain catalyst supports, particularly in SiC, with a high specific surface area (more than 15 m2/g) coming from a first group of pores whose average diameter is between 1 and 100 &mgr;m giving the gas access to a second group of pores with an average diameter of less than 0.1 &mgr;m responsible for the specific surface area and catalytic activity.
This support is obtained by mixing a powder of Si or one of its reducible compounds in a polymer or polymerizable organic resin and possibly with additives, giving the mixture its form, reticulating and polymerizing the resin, obtaining a porous frame of carbon and Si or its compound by carbonizing in a non-oxidising atmosphere at a temperature of between 500 and 1000° C., and finally carburizing Si at a temperature of between 1000 and 1400° C. again in a non-oxidising atmosphere.
It is also known from patent FR 2684091 how to obtain a metallic carbide, in particular of Si, by causing to react in an oven in a flow of inert gas at atmospheric pressure a mixture of carbon having a specific surface area of at least 200 m2/g and a volatile Si compound at a temperature of between 900 and 1400° C. in order to reduce and carburize said compound. With activated charcoal, whose porosity comprises macropores between nodules with an average diameter of 2 to 5 &mgr;m, mesopores between particles with an average diameter of 0.003 and 0.005 &mgr;m and microporosity of particles with an average diameter of between 0.0005 and 0.015 &mgr;m, a carbide is obtained whose macroporosity has been preserved, the size of the mesopores has been multiplied by approximately 3 and whose microporosity has disappeared.
Patent FR 2684092 describes a SiC foam obtained by a method of similar type to the preceding method, through reaction of a volatile Si compound on a foam of activated carbon. This activated carbon foam may be produced using a polyurethane foam reinforced by impregnation with a resin and setting of the resin, the reinforced foam being subsequently carbonized to give a carbon foam which is activated.
The carbide foam obtained has a specific surface area of at least 20 m2/g attributable in particular to macropores comprising edges whose lengths may vary from 50 to 500 &mgr;m, and chiefly to mesopores whose diameter, as previously, has been multiplied by a factor of approximately 3 in relation to the pore diameter of the activated carbon foam which lies between 0.002 and 0.02 &mgr;m.
Its specific mass is between 0.03 and 0.1 g/cm3.
Finally from patent FR 2705340 a method is known of how to obtain a silicon carbide foam as a catalyst support, resembling the method of the first patent described above FR 2657603. It consists of using a polyurethane foam, impregnating the foam with a suspension of Si in an oxygenated organic resin, of polymerizing the resin, carbonizing simultaneously the foam and the resin at between 250 and 1000° C. in an inert atmosphere and of carburizing the Si up to a temperature of between 1300 and 1400° C. again in an inert atmosphere.
The foam catalyst support has a specific surface area of more than 10 m2/g and bimodal porosity comprising macropores with an average diameter of between 100 and 150 &mgr;m and mesopores of between 0.0275 and 0.035 &mgr;m.
Also, a foam is described which may be used as a diesel engine filter which has only maintained macropores of between 100 and 150 &mgr;m and whose mesoporosity is very low after carburization at a higher final temperature of between 1300° and 1600° C.
The catalyst supports described above may be used in granular form in particular for chemical or petrochemical catalytic reactions, for example hydrogenation, dehydrogenation, isomerization, decyclization of hydrocarbides with good results.
However, during their industrial use, these particle supports are subjected to considerable mechanical stress due for example to the fact that they are used in the form of beds or are repeatedly handled or stored.
The applicant has therefore set out to improve the mechanical characteristics of these supports in granular form so that they can resist against the above-mentioned stresses without impairing their catalytic properties.
SUMMARY OF THE INVENTION
The invention is a catalyst support in granular form essentially made up of SiC&bgr; crystallites, having a high specific surface area, that is typically of at least 5 m2/g, and having improved mechanical characteristics, characterized in that its porosity essentially comprises pores whose average diameter is between 0.001 and 1 &mgr;m, preferably 0.5 &mgr;m, and in that its crystallinity is determined by:
a Full Width at Half Maximum of diffraction X rays, corresponding to plane [2 2 0] of the SiC&bgr; crystallites, that is between 0.15 and 0.60°, angle 2&thgr; of Bragg's law
a bidimensional peak height h[1 0], corresponding to directions [1 0] normalized by the integrated intensity of the peak of plane [2 2 0], that is between 0.15 and 0.40.
These types of measurement are known and have been used according to the methods described by
(1) P. J. Schields: Testing a thermostatistical theory of stacking fault abundance and distribution in silicon carbide using SPRD, HRTEM and NMR PhD thesis, Arizona State University, 1994
(2) M M J Treacy, J M Nowsam and M W Deom: A general recursion method for calculating diffracted intensities from crystals containing planar faults. Proc Roy Soc London, A433, 499-520 (1991).
The full width at half maximum of rays [2 2 0] of the SiC&bgr; cubic crystals is given in 2&thgr; degrees generally corresponding to the K&agr; radiation of copper (CuK&agr;); it is representative of the size of the coherent areas of the product's crystallites.
The full widths at half maximum of the diffraction peaks are typically determined under the following measurement conditions: aperture and antiscattering slits of 1°; detector diaphragms of 0.06°. The heights at half width given above are not adjusted for instrument widening.
For full width at half maximum values below the limit of 0.15°, cyrstallite size becomes too great, the specific surface area of the support is lost and the mechanical resistance of the granules strongly decreases.
One explanation for this loss of mechanical resistance could be lack of bonds between the SiC particles owing to the fact that during the heat treatment described below the carbon frame which serves as link has not been converted into SiC but on the contrary may have been partly converted into CO to react with the grains of Si dispersed in said frame.
For values of more than 0.6° corresponding to crystallinity that is too low, the specific surface area may be high but the mechanical resistance of the granules is again insufficient. In this case the explanation may be that this insufficient mechanical resistance is due to insufficient SiC crystallite size.
The height of the bidimensional peak normalized by the integrated intensity of the peak of plane [2 2 0], noted [h(10)/I (220)], is representative of the stacking fault abundance in the cubic structure of the SiC. When the height of this peak is too short, the specific surface area is lost, and when it is too great it is found that mechanical resistance becomes insufficient, probably due to lack of coherency in the stacking of the crystallites connected with the specific surface area obtained.
The specific surface area of the granules is at least 5 m2/g but is usually more than 10 m2/g and in practice between 10 and 50 m2/g.
The non-packed density of the granules typically lies between 0.5 and 0.9, preferably between 0.6 and 0.8.
Granule size may vary to a large extent; it is generally less than 5 mm in diameter for the support to be efficient, and more than 0.4 mm to give good particle a
Baluais Gerard
Ollivier Benoist
Dennison, Scheiner Schultz & Wakeman
Griffin Steven P.
Ildebrando Christina
Pechiney Recherche
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