EP1007207A1

EP1007207A1 - Silicon carbide foam with high specific surface area and improved mechanical properties

Info

Publication number: EP1007207A1
Application number: EP98939711A
Authority: EP
Inventors: Marie Prin; Benoist Ollivier
Original assignee: Pechiney Recherche GIE
Current assignee: Centre National de la Recherche Scientifique CNRS
Priority date: 1997-07-25
Filing date: 1998-07-20
Publication date: 2000-06-14
Also published as: FR2766389B1; FR2766389A1; JP4647778B2; US6251819B1; AU8812998A; AU735809B2; CA2297766C; JP2001510729A; JP2010155241A; CA2297766A1; KR100518063B1; WO1999004900A1; KR20010021804A

Abstract

The invention concerns a silicon carbide foam for use as a catalyst support, with high specific surface area of at least 5 m2/g and improved mechanical properties, in particular compressive strength more than 0.2 MPa's; it is obtained by impregnating an organic foam with a silicon suspension in a resin to which a hardening agent has been added, incomplete cross-linking of the resin, carbonising the organic foam and the resin and carburizing the silicon.

Description

SILICON CARBIDE FOAM WITH HIGH SPECIFIC SURFACE AND IMPROVED MECHANICAL CHARACTERISTICS

TECHNICAL AREA

The invention relates to a silicon carbide foam with a specific surface and high porosity having improved mechanical characteristics, in particular crushing resistance, this foam essentially serving as a catalyst support, for example in the chemical or petrochemical industry and in exhaust pipes from internal combustion engines, or filters.

It also relates to its production process and its applications.

STATE OF THE ART

It is known from patent FR 2657603 to obtain catalyst supports, in particular SiC, with a high specific surface (greater than 15 m2 / g), having a bi-modal porosity in which a first family of pores of average diameter between 1 to 1 00 μm allows a gas to have access to a second family of pores with an average diameter of less than 0.1 μm responsible for the specific surface 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 with optionally adjuvants, shaping the mixture, crosslinking and polymerization of the resin, obtaining a skeleton porous carbon containing Si or its compound, by carbonization in a non-oxidizing atmosphere at a temperature between 500 and 1000 ° C, and finally carburizing the Si at a temperature between 1000 and 1400 ° C always in a non-oxidizing atmosphere.

Such a support has good crush resistance and has a rather high density, generally of the order of 0.6 to 0.8 g / cm 3, but it does not have the usually aerated appearance of a foam, but rather that of a more massive porous body; therefore it does not have sufficient permeability to process large volumes of gas per unit weight of support and sees its field of use limited. In other words, as soon as the support has large dimensions, its center is difficult to access by the gases to be treated and represents an unused dead mass.

Patent FR 2,684,092 describes an SiC foam obtained by carburetion reaction from a volatile Si compound with an activated carbon foam. This activated carbon foam can result from a polyurethane foam reinforced by impregnation with a resin, hardening of the resin, carbonization and activation.

The carbide foam obtained has a specific surface of at least 20 m2 / g thanks to macropores having edges whose lengths can vary from 50 to 500 μm and mainly to mesopores whose diameter is usually between 0.03 and 0.05 μm, this more generally being approximately three times larger than that of the pores of activated carbon foam.

Its density is between 0.03 and 0.1 g / cm3, on the other hand its relatively modest mechanical strength (compressive strength not exceeding about 0.02 MPa) can limit its field of use or require the use of special treatments to strengthen it if necessary.

It is also known from patent FR 2705340 a process for obtaining silicon carbide foam which consists of starting from a polyurethane foam, to impregnate it with a suspension of silicon in an oxygenated organic resin (usually furfuryl), polymerize the resin up to 250 ° C at a speed of 5 ° C / min, simultaneously carbonize the foam and the resin between 250 and 1000 ° C under an inert atmosphere, carburize the Si contained in the resulting carbon foam up to at a temperature between 1,300 and 1,600 ° C with maintaining this temperature for 2 h under an inert atmosphere and cooling the carbide obtained.

The carbide foam obtained has a specific surface of at least 5 m2 / g, which depends in particular on the final temperature reached. It has a bi-modal porosity comprising macropores with an average diameter between 100 and 150 μm and mesopores between 0.0275 and 0.035 μm.

This foam can be used as a catalyst support or as a diesel engine filter.

It gives satisfactory results in catalytic reactions. On the other hand, as before, its resistance to crushing or to abrasion proves to be insufficient when it must be subjected to high thermal and / or mechanical stresses in particular for use in exhaust pipes.

The Applicant has therefore tried to make the use of said SiC foam supports safer, in particular in exhaust pipes or for regeneration treatments, by significantly improving their mechanical properties without penalizing their catalytic properties. , in particular their specific surface or their bimodal porosity, which is not obvious because generally one is obtained at the expense of the other, while retaining their permeability.

The plaintiff therefore sought to strengthen the skeleton of the foam

DESCRIPTION OF THE INVENTION

The invention is a foam based on silicon carbide for catalytic applications, having a high specific surface, typically its BET surface is at least 5 m2 / g, characterized in that it has a compressive strength greater than 0.2 MPa (2 bar), but generally at least 0.4 MPa (4 bar).

The foam according to the invention generally has a bimodal porosity, measured with mercury, essentially comprising a family of pores whose average diameter is between 10 and 200 μm allowing easy access to the gases to be treated towards the mesoporosity whose pores have a diameter medium between 0.005 and 1 μm and which allows catalytic activity. This bimodal porosity is added to the porous structure of the foam which is typically in the form of a network that could be described as "fibrous" comprising kinds of communicating cages delimited by carbide edges (or bridges) , generally between 50 and 500 μm thick, linked together by knots. The mega pores of this network, visible to the naked eye, have dimensions which can be between 0.4 and 1.6 mm and correspond to a pore volume of

3 to 1 2 cm3 / g approximately. Therefore it has a non-Darcian air permeability of at least 10 ^'5 m at 20 ° C. This permeability makes it possible to measure the ease with which the gases to be catalytically treated can pass through it.

It is remarkable to note that the foam most often has a specific surface greater than 10 m2 / g.

Its density is typically between 0.06 and 0.2 and preferably between 0.08 and 0.1 5.

It is advantageously in the form of a monolithic part, but it can also be used in particulate form, that is to say pieces of stacked foam.

Compressive strength is measured by a hardness test well known in the field of material resistance. It consists in applying a force to a cylindrical punch of known plane section and to measuring the force necessary to make it penetrate into the foam over a height of 1 cm, the sample having at least two parallel flat faces distant by at least 5 cm.

The foam according to the invention also has very good resistance to thermal shock.

Thus, it resists at least one thermal shock consisting in bringing it to at least 800 ° C. and in brutally cooling it in air at ambient temperature, without its resistance to compression being reduced. 3

But it is even more remarkable to note that it resists a succession of several thermal shock cycles, each cycle comprising heating at high temperature FOLLOWED by brutal cooling in the air.

For example, it has been subjected to a succession of heating and cooling cycles carried out at temperature levels ranging from 800 ° C. to 950 ° C. and spaced at 25 ° C., two cycles being carried out at each temperature level, without noting any significant degradation of its compressive strength

During these thermal shocks, the brutal cooling takes place at an average speed of around 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% That of residual C does not exceed 3%, usually 2%; the latter can moreover be eliminated by oxidation in air at a controlled temperature of around 600 ° C. to 850 ° C.

To obtain this foam, an organic starting foam is impregnated, usually of polyurethane, using a suspension of a silicon powder in a resin; this resin contains oxygen, has a carbon yield greater than 30%, and is added with a crosslinking catalyst in a proportion of 1 to 10% (by weight), preferably 5%; in general it is a furfuryl resin and the crosslinker of hexamethylenetetramine, the weight ratio silicon to resin being between 0.6 and 1, 2. The weight ratio of the total mass of impregnated foam to the mass of the starting foam is greater than 10 and less than 20, which generally corresponds to a resin to foam weight ratio greater than 5 and not exceeding 1 1 to avoid the risk of blocking the porous structure of the foam. The impregnated foam is heat treated so that the resin is incompletely crosslinked at the time of degradation of the organic foam, then the organic foam and the resin are carbonized by bringing the temperature to 1200 ° C. under an inert atmosphere; the silicon is carburetted, always under an inert atmosphere, by bringing the temperature from 1,200 ° C. to 1,370 ° C. to obtain a carbide foam with a high specific surface or at a higher temperature when obtaining a very high specific surface area is less critical, for example when carbide foam is used as a filter in a diesel engine.

As already mentioned, the starting organic foam is generally a shaped part. Advantageously, it can comprise a doping element making it possible to improve the resistance of the SiC foam to oxidation at high temperature, for example a powder of at least one easily oxidizable metal, such as Al, Ca, Y ..., or of an alloy containing these metals, this doping element being introduced into the mass of the foam, for example, during its manufacture. In addition, we unexpectedly notice that the addition of these dopants generally improves the mechanical characteristics of the final carbide foam, in particular its resistance to crushing.

If necessary, the permeability of said organic foam can be improved by a preliminary treatment, for example with sodium hydroxide when it is a polyurethane.

Instead of starting from a generally polyméπque and optionally doped organic foam, the invention also includes starting from the components making it possible to obtain the foam (for example monomeric or copolymeric agents, porogenic adjuvants, hardeners, crosslinking agents or others) possibly added with said doping element and optionally add to this mixture the suspension of silicon in the resin. This mixture can then be shaped by molding, injection. , before obtaining the foam and being heat treated.

The suspension of Si in an organic resin can contain various adjuvants: solvent (for example alcohol), filler (for example carbon black) to adjust the viscosity, plasticizer, surfactant, etc. In this case, a step of heating at moderate temperature to remove the solvents can be carried out, while maintaining the thermal regime under the conditions mentioned above.

The silicon powder generally has a grain size passing the sieve 50 μ and preferably has an average particle diameter of less than 10 μm; it can be introduced in the form of an alloy comprising said doping elements making it possible to improve the resistance to oxidation of the SiC foam; these can also be introduced in the form of a metallic powder or in the form of a decomposable salt mixed with said Si powder. The proportion of doping elements does not typically exceed 10% relative to the silicon introduced into the resin.

The polyméπsée resin typically contains at least 5% by weight of oxygen and preferably 1 5%.

It is essential to carry out an incomplete crosslinking of the resin before the degradation of the organic foam. Indeed, the remaining plasticity makes it possible to withstand the dimensional variations, the deformations and the stresses which can occur during the transformation of said foam into carbon occurring during subsequent heat treatments. Thus the risks of defects in the skeleton of the foam, resulting in the presence of voids in the bridges, defects in bonding of the bridges etc. , are significantly reduced and improve the mechanical properties. Likewise, the absence of constraints contributes to significantly improving the solidity of the carbide foam, in particular its resistance to thermal shock.

The rate of incomplete polymerization can be characterized by measuring the glass transition temperature (Tg) of the partially polymerized resin. In general, this temperature is less than 110 ° C. and corresponds to the appropriate degree of polymerization at the time of starting carbonization; it is also greater than 70 ° C so that the shaped part has sufficient hold during the heat treatment.

The controlled polymerization heat treatment can be carried out in different ways; it is generally adapted to the size of the parts treated.

One can, for example, heat the part by steaming at a temperature below 225 ° C, typically between 1 50 and 225 ° C and preferably about 200 ° C, for a period between 10 and 90 min, preferably between 60 and 90 min, then possibly cool before continuing the heat treatment. We can also operate more quickly at a higher temperature by introducing the part into an oven preheated to a temperature above the degradation temperature of the organic foam, for example at 300 ° C., and by limiting the residence time in the oven so that the degradation of said organic foam occurs before the complete polymerization of the resin.

The poiyméπsée resin typically contains at least 5% (by weight) of oxygen and preferably 1 5%.

It may also be noted that in combination with the controlled polymerization heat treatment, the high proportion of resin, and therefore of impregnation suspension, introduced into the organic foam contributes to the increase in mechanical characteristics, in particular of crushing, without that the specific surface, which characterizes the catalytic properties of carbide foam, be affected.

Such a well crushable carbide foam can be used as a catalyst support in the divided form of stacked pieces; but it is particularly well suited to be used as a piece of monolithic shape, for example in exhaust pipes; it is sufficient to cover it with a deposit of the desired catalyst according to conventional methods.

These mechanical properties of the foam according to the invention also make it particularly suitable for being treated after use with a view to recovering the deposit of catalyst covering it by simple hydrometallurgical processes, and / or with a view to its recycling.

To improve the oxidation resistance of the silicon carbide foam, the process can also be completed by a stabilization heat treatment step, in an oxidizing atmosphere. This treatment can be carried out during the removal of the residual carbon; it is particularly advantageous to practice it when the foam contains a doping element II is usually carried out between 850 and 1,200 ° C for a period between 5 min and 24 h or preferably between 950 and 1,100 ° C for 1 5 min at 10 a.m., the longer the longer the temperature is low. It results in a coating of the foam with an oxide film comprising at least one of the silicon oxides or doping elements, the silicon oxide generally containing that of the doping elements.

To obtain good resistance to oxidation, the foam can also be impregnated, for example under vacuum, using a solution of a decomposable salt of at least one of said doping elements, heat treatment to decompose the salt, then advantageously complete with the previous stabilizing treatment to obtain the corresponding protective film.

The following examples illustrate the invention.

Example 1

This example relates to a silicon carbide foam obtained according to a process of the state of the art.

This process is of the type described in patent FR 2705340.

A piece of cylindrical polyurethane foam 14 cm in diameter and 8 cm high with a density of 0.028 was impregnated with a suspension containing Si powder with an average grain diameter of 5 μm in 95% d furfuryl alcohol and 5% hexamethylenetetramine serving as a polycondensation catalyst.

The ratio of the mass of silicon to that of the resin is 0.7.

After impregnation of the polyurethane foam with the suspension, the ratio of the weight of resin to the weight of said foam is 4.1, and the ratio of the total mass of impregnated foam to the mass of polyurethane is 7.8.

The polymerization was carried out by increasing the temperature to 250 ° C at a speed of 5 ° C / min for 45 min, with a plateau at 250 ° C with a duration of 5 min to polymerize the resin. The glass transition temperature (Tg) of this resin under these conditions is 1 1 8 ° C.

The carbonization was then carried out by bringing the temperature from 250 ° to 1000 ° C under an Ar atmosphere at a speed of 1 ° C / min.

The heat treatment continued by increasing the temperature to 1,350 ° C. at a speed of 3 ° C./min with a temperature level of 2 hours at 1,350 ° C., still under an inert atmosphere.

The resulting carbide foam was then treated at 800 ° C with pure air to destroy the residual carbon.

The BET specific surface is then 1 0.8 m2 / g and the crushing resistance measured by the hardness test is 0.08 MPa.

Example 2

This example illustrates the invention.

We started with a piece of polyurethane foam identical to that of Example 1

For the foam impregnation suspension, a silicon powder with an average grain diameter of 5 μm was used in furfuryl alcohol with 5% of crosslinking catalyst (hexamethylenetetramine).

The mass ratio of Si to resin mass is 0.7.

On the other hand, the ratio of the mass of impregnated foam to the mass of polyurethane is 1 6

The incomplete polymerization was carried out by steaming, bringing the impregnated foam to 200 ° C. with a temperature rise rate of 5 ° C./min. The duration did not exceed 35 min.

The Tg value is 103 ° C.

The hardened product was then introduced into an oven under an Ar atmosphere, the temperature of which was brought to 1,200 ° C. with a speed of 3 ° C./min., To carry out carbonization.

The heat treatment was continued by increasing the temperature to 1350 ° C. under the same conditions, with the final temperature held constant for 2 h to carry out the carburetion of the silicon.

The Si carbide foam shaped part has a BET specific surface area of 1 1, 2 m2 / g and a crushing strength of 0.6 MPa, which makes it particularly suitable for being impregnated with a catalyst for be used as an exhaust catalyst.

Example 3

This example illustrates the production of a carbide foam with dopant according to the invention.

The starting polyurethane foam is impregnated using the same suspension as in Example 2, containing Si in furfuryl alcohol with crosslinking catalyst; however, aluminum nitrate monohydrate was added thereto in a proportion such that 0.75% (weight) of AI was obtained relative to the weight of final SiC.

The heat treatments are the same as those of Example 2.

The SiC foam obtained has a specific surface of 11.7 m ² / g, of the same order of magnitude as that of Example 2; on the other hand, the crush resistance of 0.9 MPa is significantly higher.

The SiC foam piece was separated into two pieces. One of them underwent a stabilization treatment at 1000 ° C for 2 h in air; however both have then subjected to an oxidation resistance test by exposure to air at 1100 ° C. for 5 h.

For the unstabilized piece the weight gain is 9.3%, while for the stabilized piece it is 1.6%.

By way of comparison, the same undoped foam (Example 2) exhibits, under the same conditions, a weight gain of 15.8% when it is not stabilized and of 6.7% when it is stabilized.

Claims

/ Foam based on silicon carbide for catalytic applications having a high BET specific surface characterized in that it has a compressive strength greater than 0.2 MPa.

/ Foam according to claim 1 characterized in that its BET specific surface is at least 5m2 / g and preferably at least 10m2 / g.

/ Foam according to any one of claims 1 or 2 characterized in that it has a bimodal porosity, adding to the porous structure of the foam, comprising a first family of macropores whose average diameter is between 1 0 and 200 μm and a second family of mesopores whose average diameter is between 0.005 and 1 μm.

/ Foam according to any one of claims 1 to 3 characterized in that the compressive strength is retained after having undergone at least one thermal shock at least 800 ° C.

/ Foam according to any one of claims 1 to 4 characterized in that it comprises doping elements improving its resistance to oxidation and its mechanical characteristics.

/ Foam according to claim 5 characterized in that the doping elements are easily oxidizable elements, preferably Al, Ca, Y.

/ Foam according to any one of claims 1 to 6 characterized in that it comprises a coating with an oxide layer to improve its resistance to oxidation.

/ Foam according to claim 7 characterized in that the oxide layer comprises at least one of the silicon oxides or doping elements. / A method for obtaining the foam of any one of claims 1 to 8 in which an organic foam is impregnated, having sufficient open permeability for the intended use, using a powder suspension of silicon in a polymerizable resin added with a crosslinking catalyst, the resin comprising oxygen and having a carbon yield of at least

30%, the weight ratio of the total mass of the impregnated foam to the mass of the starting foam being between 10 and 20, the impregnated foam is heat treated so that the resin is not fully crosslinked at the time of degradation organic foam, said organic foam and the resin are simultaneously carbonized by bringing the temperature to 1200 ° C. under an inert atmosphere and the silicon is carburized by bringing the temperature to at least 1370 ° C. to obtain a lower Si content at 0.1%.

/ A method according to claim 9 characterized in that the organic foam is a polyurethane foam.

/ Method according to any one of claims 9 or 1 0 characterized in that the organic foam contains at least one doping element.

/ Method according to any one of claims 9 to 1 1 characterized in that the polyméπsée resin contains at least 5% (by weight) of oxygen and has a carbon yield of at least 30% and that it is preferably a furfuryl resin.

/ Method according to any one of claims 9 to 1 2 characterized in that the resin contains at least one doping element.

/ Method according to any one of claims 9 to 1 3 characterized in that the incomplete polymerization is carried out so that the glass transition temperature of the resin is at most 1 10 ° C at the time of starting carbonization . / A method according to any one of claims 9 to 14 characterized in that the silicon has a particle size less than 50 microns.

/ Method according to any one of claims 9 to 1 5 characterized in that the silicon is introduced in the form of an alloy containing at least one doping element.

/ A method according to any one of claims 9 to 1 6 characterized in that it comprises an additional stabilization treatment step.

/ Method according to claim 1 7 characterized in that the stabilization treatment is carried out in an oxidizing atmosphere at a temperature between 850 and 1200 ° C for a period between 5 min to 24 h, preferably between 950 and 1100 ° C for 1 5 min to 10 h.