EP2464613A1

EP2464613A1 - Method for producing a ceramic foam having reinforced mechanical strength for use as a substrate for a catalyst bed

Info

Publication number: EP2464613A1
Application number: EP10752014A
Authority: EP
Inventors: Pascal Del-Gallo; Daniel Gary
Original assignee: Air Liquide SA; LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Current assignee: Air Liquide SA; LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Priority date: 2009-08-10
Filing date: 2010-07-15
Publication date: 2012-06-20
Also published as: US20120142526A1; CN102471172A; BR112012002863A2; FR2948935A1; WO2011018568A1; FR2948935B1

Abstract

The invention relates to a method for manufacturing a ceramic foam, including the following steps: a) a first step of impregnating a polymer foam having an open porosity with a first suspension of ceramic particles in a solvent; b) a first step of drying the impregnated polymer foam at a temperature between the ambient temperature and 200°C and/or for a duration of between 30 min and 24 hrs; c) a heat treatment of the dried polymer foam comprising: (i) a step of thermally decomposing the dried polymer foam at a temperature of between 150 and 700°C and/or for a duration of between 30 min and 48 hrs; (ii) a step of unbinding the organic compounds contained in the polymer foam from step (i), at a temperature of between 200 and 900°C and/or for a duration of between 30 min and 48 hrs; and (iii) presintering the ceramic particles contained in the polymer foam from step (ii), at a temperature of between 900 and 1400°C and/or for a duration of between 30 min and 6 hrs; d) a second step of impregnating the polymer foam from step c) with a second suspension of ceramic particles in a solvent; e) a second step of drying the polymer foam impregnated in step d); f) a step of sintering the ceramic particles contained in the polymer foam dried in step e), at a temperature of between 1200 and 2000°C and/or for a duration of between 30 min and 6 hrs; the size of the ceramic particles of the second suspension being smaller than the size of the ceramic particles of the first suspension.

Description

Process for producing a reinforced mechanical ceramic foam for use as a catalyst bed support

The subject of the present invention is a reinforced ceramic cellular architecture with reinforced mechanical strength, of foam type for example, its production process and its use as a catalyst support in the field of heterogeneous catalysis reactions.

The invention proposes a new method of manufacturing controlled cellular architecture, such ceramic foam type for example to enhance the mechanical properties thereof while maintaining an open porous structure (macroporosity).

By "open porous structure" is meant a structure having a maximum accessibility of fluids in the latter. In other words, the pore content (cells in this case) opening is maximum, more than 95% of these are not obstructed and are open.

The most widely used open macro porous ceramic foaming method consists of impregnating a polymeric foam (most often polyurethane or polyester), cut to the desired geometry, with a suspension of ceramic particles in an aqueous solvent or organic. The excess suspension is removed from the polymer foam by repeated application of compression or centrifugation, in order to keep only a thin layer of suspension on the strands of the polymer. After one or more impregnations of the polymeric foam by this method, it is dried so as to remove the solvent while maintaining the mechanical integrity of the deposited ceramic powder layer. The foam is then heated at high temperature in two stages. The first step called debinding consists in degrading the polymer and other organic substances possibly present in the suspension, by a slow and controlled temperature rise until complete elimination of the volatile substances (typically 500-900 ^° C.). The second step called sintering consists in consolidating the residual mineral structure by a high temperature heat treatment. This method of manufacture thus makes it possible to obtain an inorganic foam which is the replica of the initial polymer foam, at the sintering shrinkage. The final porosity allowed by this method covers a range of 30% to 95% for a pore size ranging from 0.2mm to 5mm. The final pore size (or open macroporosity) is derived from the macrostructure of the initial organic template (polymer foam, polyurethane usually). It generally varies from 60 to 5 ppi (ppi: pore per inch, from 50 μm to 5 mm).

Other methods of making ceramic foams exist. It is for example possible to introduce porosity in a ceramic part by adding to the ceramic powder, a second so-called pore-forming phase which is degradable during sintering. However, this method does not achieve high porosity (> 85%) and large pore size (> lmm). The direct production of a foam by emulsion of a ceramic suspension using surfactants makes it possible to obtain very large pore volume structures (up to

97%) retaining high mechanical properties. However, it remains difficult to control the size and distribution of porosity with this method. The maximum pore size is also smaller than that allowed by a polymeric foam impregnation technique.

The realization of a high volume foam porous (> 80%) and large pore size

(> lmm) is thus facilitated by the use of the previously described method of the replication of a polymeric foam. In addition, this method of manufacture is distinguished from other methods by its ease of implementation and the control of macroporosity over a wide range of size and pore volume.

The major disadvantage of the method of replication of a polymeric foam resides in the presence of a cavity in the heart of the ceramic foam instead of the initial polymer. This cavity, which retains the triangular shape typical of polymeric foam strands, is very often surrounded by microcracks and other microstructural defects such as porosities for example. The presence of these defects considerably lowers the mechanical properties of ceramic foams.

FIG. 1 illustrates, in the context of NiF or NiFeCrAlO-based metal foams obtained by impregnation of a polymeric foam, the presence of the triangular shape of the metal-core core foam.

US 4,610,832 claims the use of a mineral binder (hydrated alumina) in the initial ceramic suspension to promote the sintering of the ceramic foam and improve these mechanical properties. This method does not affect the porosity of the strands. The document EP 0369 098 describes the reinforcement of a pre-existing ceramic foam by a suspension of colloidal silica under vacuum and then a new heat treatment. A very small portion of the silica manages to enter the cavity of the foam strands and the deposited layer may be cracked due to the difference in coefficient of expansion between the latter and the material constituting the foam.

The document US Pat. No. 6,635,339 B1 proposes a method for strengthening the strands of a ceramic foam by partially or completely filling the defects of the strands (porosities, cracks, cavities). A suspension of a metal phase, a glass or a ceramic is deposited on the foam before or after sintering. The final heat treatment is performed at a temperature allowing the melting of the deposited phase (metal, glass or ceramic) while the ceramic foam remains intact. The molten phase then partially or completely filled the cavities of the strands. The difficulty of this method is to correctly choose the filler material which must have the same coefficient of expansion of the foam and not react too strongly with it. Another disadvantage is that the maximum temperature of use of the foam, especially as a catalyst support, is greatly reduced by the use of the fuse phase.

A scientific article Han, Y., et al., The effect of sintering temperatures on alumina foam strength. Ceramics International, 2002. 28 (7): p. 755-759 mentions the possibility of increasing the mechanical strength of ceramic foams by controlling the sintering temperature. The increase of the mechanical strength is here directly induced by the densification of the ceramic structure which increases with the sintering temperature. The optimum is reached when the sintering shrinkage is maximal and the ceramic microstructure is completely densified. Heat treatment at higher temperature or longer may eventually result in a slight decrease in mechanical strength if the grain size increases for example. This approach is well known in ceramic processes and the temperatures and sintering time are generally chosen to allow total densification of the microstructure without generating granular magnification.

The document EP1735122B1 for manufacturing Ni-based metal foam mentions an additional impregnation of a solution before or after the first treatment. thermal device containing metals allowing capillarity to fill the cavities (souls of the structure) formed.

Starting from there, a problem that arises is to provide a honeycomb ceramic architecture with controlled macroporosity and having a reinforced mechanical strength.

A solution of the invention is a method of manufacturing a ceramic foam, comprising the following steps:

a) a first step of impregnating an open porosity polymeric foam with a first suspension of ceramic particles in a solvent;

b) a first step of drying the impregnated polymeric foam at a temperature of between room temperature and 200 ^° C. and / or of a duration of between 30 minutes and 24 hours;

c) a heat treatment of the dried polymeric foam comprising:

(i) a step of thermal decomposition of the polymer foam dried at a temperature of between 150 and 700 ^° C. and / or of a duration of between 30 minutes and 48 hours,

(ii) a debinding step of the organic compounds contained in the polymeric foam, resulting from step (i), at a temperature of between 200 and 900 and / or of a duration of between 30 minutes and 48 hours, and

(iii) a pre-sintering of the ceramic particles contained in the polymeric foam resulting from step (ii) at a temperature of between 900 and 1400 ^° C. and / or of a duration of between 30 minutes and 6 hours,

d) a second step of impregnating the polymeric foam resulting from step c) with a second suspension of ceramic particles in a solvent;

e) a second step of drying the polymeric foam impregnated in step d);

f) a sintering step of the ceramic particles contained in the polymer foam dried in step e), at a temperature of between 1200 and 2000 ^° C. and / or of a duration between

30 min and 6 h.

The term "duration" means the duration related to the temperature rise ramp and the duration of the step at the ramp temperature. In the case of debinding and drying the ramps can be very slow (0, 1 ° C / min) from which a very long time while the time is only 1-2 hours. By "ambient temperature" is meant the temperature of the ambient air, generally between 18 and 25 ° C.

Different polymeric materials may be used in step a) such as polyurethane (PU), polyvinyl chloride (PVC), polystyrene (PS), polyethylene (PE), cellulose and latex but the ideal choice of foam is limited by stringent requirements. If not soaking, the polymeric foam must be elastic enough to recover its original form without irreversible deformation after being compressed during the impregnation process. The polymeric foam must have at least some hydrophobic / hydrophilic interactions with the solvent of the suspension. The polymeric material must not release toxic compounds, for example PVC is avoided because it can lead to the release of hydrogen chloride.

Polyurethane foams are available in a wide range of low cost porosity. In addition, they can be deformed and return to their original shape after impregnation. Different types of polyurethane exist, called ether polyurethane, ester polyurethane or ester-polyurethane polyurethane depending on the nature of the side chain of the polyol polymerized with isocyanate. Even though the polymer is generally hydrophobic, the polymer side chains have hydrophilic (ester) or hydrophobic (ether) properties. It should be noted that polyurethane can cause the release of NOx.

Any other foam (except polystyrene foams) is not commercially available. And the polystyrene is not good enough to be compressed during the impregnation step.

Suspensions of ceramic particles typically consist of ceramic particles, solvent and additives. The suspension must be sufficiently fluid to impregnate the polymeric foam, but it must be sufficiently viscous to be retained on the polymeric foam. The ceramic particles must be dispersed homogeneously in the suspension. The particle size must be fine enough to favor the sintering process.

In order to improve the formulation of the suspension, additives (dispersants, binders, wetting agents, flocculation agents) can be used. These additives can be added to allow:

a stabilization of the suspension; an even coating of the foam,

- better adhesion of the suspension ...

The first impregnation step makes it possible to cover the strands of the polymeric foam by homogeneous deposition of suspension while preserving the open porous structure of the foam.

The first drying step makes it possible to evacuate the solvent.

The thermal decomposition step makes it possible to burn the polymer matrix. The debinding step makes it possible to evacuate the volatile materials, including the polymeric foam and the organic additives introduced into the suspension.

The pre-sintering step makes it possible to give the material sufficient mechanical strength to handle it while maintaining a low density microstructure.

The second impregnation stage is intended to deposit on and in the hollow ceramic strands a new mineral material charge.

The second drying step makes it possible to evacuate the solvent.

The sintering step completes the heat treatment.

Preferably, the debinding step and the first heat treatment are conducted consecutively without intermediate handling of the foam.

Depending on the case, the method according to the invention may have one or more of the above-mentioned characteristics:

the ceramic particles of the first suspension are of the same nature as the ceramic particles of the second suspension;

the second suspension has a lower viscosity than the first suspension;

the size of the ceramic particles of the second suspension is smaller than the size of the ceramic particles of the first suspension;

the second impregnation step is carried out under vacuum; The three features mentioned above facilitate the insertion of the second suspension into the hollow of the ceramic strands. Indeed, it requires a very liquid suspension, reduced mineral load and small initial particle size to be inserted into the slots present after the pre-sintering. The fact of using the vacuum then favors this diffusion. This second impregnation step consists in filling the hollows of the strands, thus improving the mechanical properties in the end. the ceramic particles of the first and second suspensions are chosen from alumina (Al ₂ O 3) and / or doped alumina (La (1 to 20% by weight) -Al ₂ O ₃ , Ce- (I to 20 wt.% By weight) -Al ₂ O ₃ , Zr (I at 20% by weight) -Al ₂ O ₃ ), magnesia (MgO), spinel (MgA12O4), Hydrotalcite, CaO, oxide zinc, cordierite, mullite, aluminum titanate, silico-limestone (Si _x Ca _y O _z ), silico-aluminous, (Si _x Al _y O _z ) CaO-Al 2 O 3 bases, carbides and nitrates and zircon (ZrSiO4).

the ceramic particles of the first and second suspensions are chosen from ceria (CeO ₂ ), zirconium (ZrO ₂ ), stabilized ceria (Gd ₂ O ₃ between 3 and 10 mol% ceria) and stabilized zirconium (Y ₂ O ₃ between 3 and 10 mol% zirconium) and the mixed oxides of formula (I):

where O <x <1 and δ ensures the electrical neutrality of the oxide,

or the doped mixed oxides of formula (II):

This ₍ i- _x - _y) Zr _x D _y O ₂ -δ (H),

wherein D is selected from magnesium (Mg), yttrium (Y), strontium (Sr), lanthanum (La), presidium (Pr), samarium (Sm), gadolinium (Gd), Erbium (Er) or Ytterbium (Yb), where O <x <1, 0 <y <0; 5 and δ assures the electrical neutrality of the oxide.

The subject of the present invention is also a ceramic foam that can be obtained by a process according to the invention, comprising a porosity of between 10 and 90% and a pore size of between 2 and 60 ppi (ppi = pore per inch). , characterized in that said foam has strands at least partially filled by the ceramic particles of the second suspension.

The strands of the foam are preferably filled to more than 50%, more preferably to more than 80%.

The ceramic foams obtained by the process according to the invention exhibit an increase in the mechanical properties compared with foams made according to the conventional method and have a quantity of microstructural defects (pores, cracks, etc.) that is significantly lower than foams made in the same conditions according to the conventional method.

Ceramic foams according to the invention can in particular be used as a catalyst support in a heterogeneous catalysis. FIG. 2 is a scanning electron micrograph with an xl20 magnification of an alumina foam made by a conventional impregnation method. It illustrates the presence of a triangular cavity in all the strands which corresponds to the impression left by the replicated polymeric foam.

FIG. 2 is a scanning electron micrograph with x250 magnification of an alumina foam produced by the process according to the invention. It illustrates the microstructural modification of the strands which are partially or completely filled by the impregnation phase occurring after the pre-sintering.

FIG. 3 is a graph showing the evolution of the mechanical strength (mean and standard deviation) of two sets of foams as a function of their apparent porosity. The series A corresponds to the production of alumina foams by the conventional protocol previously illustrated in FIG. 1. The B series corresponds to the production of alumina foams by the process according to the invention and previously illustrated in FIG. 2. Apart from this difference, the sintering temperatures of the two series and other operating parameters are exactly the same.

This increase in the mechanical properties of the foam does not come at the expense:

- The refractory nature of the foam by the use of a fuse material at low or high temperature;

a property intrinsic to the main material constituting the foam by the use of one or more filler materials;

maintaining a high and open porous volume;

- Preservation of a small loss of load.

On the other hand, this increase in mechanical properties does not necessarily take place by the use of a different chemical phase of the main material constituting the foam.

The invention will be described in more detail in Examples 1 to 3.

Example 1

A ceramic suspension (suspension A) is obtained by mixing a fine particle size alumina powder (d50 <1 μm) with deionized water and an acrylic binder and a ammonium polyacrylate used as a dispersant of alumina. The proportion by volume of the mineral phase is 30-40 vol%, the binder portion is 5-10 vol%.

The suspension is used to impregnate a cylinder of polyurethane foam of dimension D50mm H50mm and porosity lOppi. The recovery of the polyurethane strands by the suspension homogeneously is achieved by the repeated application of compression either manually or using a machine with adjustable double roll air gap. The excess suspension is evacuated until the mass of the foam covered by the suspension is 24g. The foam is dried in an oven and then placed in an oven where it undergoes a heat treatment comprising a temperature rise ramp of the ambient at 600 ^° C. in 26 eures then a second ramp of temperature rise of 600 ^° C. to 1250 ^° C. in 8 hours followed by a plateau at 1250 ^° C. for 30 minutes (so-called pre-sintering stage).

After cooling the foam is white, without polyurethane residue, its mechanical strength is sufficient for easy handling. A new suspension (suspension B) is used to cover the foam with a new layer of alumina either by a dipping method or by a casting method. Suspension B is carried out by dilution of suspension A, its charge rate is reduced to 15-25 vol%.

After a new drying phase, the foam is placed in an oven where it undergoes heat treatment at 1560 ^° C. for 1 h (so-called sintering step).

The mechanical resistance to compression of the foam thus produced is 2.2 MPa ± 0.3 MPa for a porosity of 90% and a linear pressure drop of 6000-8000 Pa / m (air, 3m / s, 20 ⁰ C). In comparison, a foam made according to a conventional protocol that does not use the partial sintering and second impregnation stages has a compressive strength of 0.8 MPa ± 0.2 MPa for a porosity of 88%. Example 2

A ceramic suspension (suspension A) is obtained by mixing a fine particle size alumina powder (d50 <1 μm) with deionized water and an acrylic binder and an ammonium polyacrylate used as an alumina dispersant. The proportion by volume of the mineral phase is 30-40 vol%, the binder portion is 5-10 vol%.

The suspension is used to impregnate a cylinder of polyurethane foam of dimension D50mm H50mm and porosity lOppi. The recovery of the polyurethane strands by the Homogeneously suspension is achieved by the repeated application of compression either manually or using a machine with adjustable double air gap rollers. The excess suspension is evacuated until the mass of the foam covered by the suspension is 26 g. The foam is dried in an oven and then placed in an oven where it undergoes a heat treatment (thermal decomposition of the polymer matrix + debinding + pre-sintering) comprising a temperature rise ramp of the ambient at 600 ^° C. in 26 hours. (Thermal decomposition + partial debinding) then a second ramp temperature rise of 600 ⁰ C to 1200 ⁰ C in 8 hours (Debinding + total thermal decomposition) followed by a plateau at 1200 ⁰ C 30 minutes (pre-sintering).

After a new drying phase, the foam is placed in an oven where it undergoes a heat treatment at 1630 ⁰ C for Ih (sintering step).

The mechanical compressive strength of the foam thus produced is 3.8 MPa ± 0.6 MPa for a porosity of 87%. Example 3

The suspension is used to impregnate a cylinder of polyurethane foam of dimension D50mm H50mm and 5ppi porosity. The recovery of the polyurethane strands by the suspension homogeneously is achieved by the repeated application of compression either manually or using a machine with adjustable double roll air gap. The excess suspension is evacuated until the mass of the foam covered by the suspension is 31 g. The foam is dried in an oven and then placed in an oven where it undergoes a heat treatment (thermal decomposition + debinding + pre-sintering) comprising a ramp of rise in ambient temperature to 600 ⁰ C in 26 hours and a second ramp temperature rise of 600 ⁰ C to 1250 ⁰ C in 8 hours followed by a plateau at 1250 ⁰ C 30 minutes. After cooling the foam is white, without polyurethane residue, its mechanical strength is sufficient for easy handling. A new suspension (suspension B) is used to cover the foam with a new layer of alumina either by a dipping method or by a casting method. Suspension B is carried out by dilution of suspension A, its charge rate is reduced to 15-25 vol%.

After a new drying phase, the foam is placed in an oven where it undergoes heat treatment at 1560 ^° C. for 1 h (sintering).

The mechanical compressive strength of the foam thus produced is 1, 4 MPa ± 0.4 MPa for a porosity of 87% and a linear pressure drop of 3000-5000 Pa / m (air, 3 m / s, 20 ⁰ C). .

Claims

claims

A method of manufacturing a ceramic foam, comprising the steps of:

c) a heat treatment of the dried polymeric foam comprising:

(i) a step of thermal decomposition of the dried polymeric foam at a temperature of between 150 and 700 ^° C. and / or of a duration of between 30 minutes and 48 hours,

(iii) a pre-sintering of the ceramic particles contained in the polymeric foam, resulting from step (ii), at a temperature of between 900 and 1400 ^° C. and / or of a duration of between 30 minutes and 6 hours,

e) a second step of drying the polymeric foam impregnated in step d);

f) a step of fπttage ceramics particles contained in the polymer foam dried in step e), at a temperature between 1200 and 2000 ⁰ C and / or a duration of between 30 min and 6 h;

the size of the ceramic particles of the second suspension being smaller than the size of the ceramic particles of the first suspension.

2. The manufacturing method according to claim 1, characterized in that the ceramic particles of the first suspension are of the same nature as the ceramic particles of the second suspension.

3. The manufacturing method according to one of claims 1 or 2, characterized in that the second suspension has a lower viscosity than the first suspension.

4. Manufacturing process according to one of claims 1 to 3, characterized in that the second impregnation step is carried out under vacuum.

5. Manufacturing process according to one of claims 1 to 4, characterized in that the ceramic particles of the first and second suspension are selected from alumina (Al2θ3) and / or doped alumina (La (1). 20% by weight) -Al ₂ O ₃ , Ce- (I to 20 wt.% by weight) - Al ₂ O ₃ , Zr (I to 20% by weight) -Al ₂ O ₃ ), magnesia (MgO), spinel (MgA12O4), Hydrotalcite, CaO, zinc oxide, cordierite, mullite, aluminum titanate, silico-limestone (Si _x Ca _y O _z ), silico-aluminous, (Si _x Al _y O _z ) bases CaO-Al2O3, carbides and nitrates and zircon (ZrSiO4).

6. Method according to one of claims 1 to 5, characterized in that the ceramic particles of the first and second suspension are selected from cerine (CeO ₂ ), zirconium (ZrO ₂ ), ceria stabilized (Gd ₂ O ₃ between 3 and 10 mol% in cerine) and stabilized zirconium (Y ₂ O ₃ between 3 and 10 mol% in zirconium) and the mixed oxides of formula (I):

This (i- _X ) Zr _x O ( _2- δ) (I),

where 0 <x <1 and δ ensures the electrical neutrality of the oxide,

or the doped mixed oxides of formula (II):

This ₍ i_ _x _ _y) Zr _x D _y O ₂ _ _δ (II),

wherein D is selected from magnesium (Mg), tetratrium (Y), strontium (Sr), lanthanum (La), presidium (Pr), samarium (Sm), gadolinium (Gd), erbium ( Er) or Ytterbium (Yb), where 0 <x <1, 0 <y <0; 5 and δ assures the electrical neutrality of the oxide.

7. Ceramic foam obtainable by a method according to one of claims 1 to 6, comprising a porosity of between 10 and 90% and a pore size of between 2 and 60 ppi, characterized in that said foam has strands at least partially filled by the ceramic particles of the second suspension.

Ceramic foam according to claim 7, characterized in that the strands are more than 80% filled.

9. Use of a ceramic foam according to one of claims 7 or 8 as a catalyst support in a heterogeneous catalysis.