WO2009034268A2 - Catalyst substrate containing β-sic with an alumina layer - Google Patents

Abstract

The invention relates to a composite comprising a layer of porous alumina deposited on a rigid substrate made of β-SiC. The composite can be used as a catalyst substrate. The alumina layer may include catalytically active phases, in particular phases that do not properly bind onto the non-treated β-SiC, such as silver particles.

Description

 Β-SiC catalyst support with a layer of alumina

Technical field of the invention

The present invention relates to the field of β-SiC-based catalyst supports, and more particularly to a process for functionalizing the β-SiC surface by the deposition of a porous alumina layer making it possible to deposit active phases which do not adhere easily to β-SiC as such.

State of the art

Silicon carbide, and in particular β-SiC with a high specific surface area, is known as a catalyst support. The β-SiC can be obtained by the reaction between SiO 2 vapors with reactive carbon at a temperature of between 1100 ^° C. and 1400 ^° C. (Ledoux process, see EP 0 313480 B1), or by a process in which a mixture a liquid or pasty prepolymer and a silicon powder is crosslinked, carbonized and carburized at a temperature of between 1000 ^° C. and 1400 ° C. (Dubots process, see EP 0 440569 B1 or EP 0 952 889 B1).

Furthermore, β-SiC foams, which can be obtained by a variant of the Dubots process, including the impregnation of a polyurethane foam with a suspension of a silicon powder in an organic resin (Prin process, see EP) are known. 0624560 B1, EP 0 836 882 B1 or EP 1 007207 A1).

These different forms of SiC, and in particular of β-SiC, can serve as a catalyst or as a catalyst support. In the latter case, a catalytically active phase is deposited on the support, generally from a precursor which is deposited by liquid or gaseous phase, and which must most often be activated, for example by reduction of the metal compound which it contains. SiC is known to have excellent thermal conductivity compared to other catalyst supports, as well as good chemical resistance at high temperatures, which allows it to be used at high temperatures.

The active phases used for the catalysis are most often transition metals (such as platinum or palladium) or their compounds; these metals are quite expensive. Some active phases do not adhere well to SiC. This decreases the life of the catalysts, and in addition, the diffusion of these elements in the products of the catalyzed chemical reaction can contaminate said products.

The functionalization of SiC by other materials for the deposition of a catalytically active phase has been mentioned with cerium oxide (EP 0 880 406), with carbon nanotubes (WO 03/048039) and with zeolites ( WO 98/06495). The patent application WO 03/059509 describes the deposition of zeolite on a β-SiC support.

The problem that the present invention seeks to solve is to propose a method for improving the adhesion to a porous β-SiC support of certain active phases, such as silver, which do not adhere well to the β- SiC untreated.

Objects of the invention

A first object of the present invention is a composite formed of a layer of alumina deposited on a rigid support, characterized in that said rigid support is a β-SiC support. By forming at least one catalytically active phase deposited at least on the alumina layer, a catalyst is obtained that can be used to catalyze chemical reactions in the gas or liquid phase.

Another subject of the present invention is a process for producing a composite formed of a layer of alumina on a rigid β-SiC support, in which

(a) supplying a β-SiC support, preferably in the form of extrudates, foam or cellular foam, and preparing a solution called precursor oxide solution by dispersing X% by weight Y-AIOOH in Y% by weight of water, preferably acidified with a mineral acid;

(b) optionally, said support is heated to a temperature greater than 600 ⁰ C, and preferably between 800 ⁰ C and 1100 ⁰ C;

(c) contacting said support with said oxide precursor solution;

(d) drying said support from step (c), preferably by steaming, optionally preceded by a drying step in ambient air and at room temperature;

said dried support is calcined, preferably at a temperature between

500 ⁰ C and 700 ⁰ C, to form said composite.

Description of figures

Figure 1 shows the macroporous distribution typical of a β-SiC used in the context of the present invention. The distribution shows a peak at 0.044 μm and a peak at 0.14 m.

Figure 2 shows a transmission electron microscopy of β-SiC. It can be seen that the core is purely SiC while the surface is covered with an amorphous layer of SiO _x C _y .

Figure 3 shows scanning electron micrographs of extruded γ-AI ₂ O ₃ ZSiC prepared according to the invention. Magnification is indicated at the bottom right of each micrograph by the length of the white bar.

Figure 4 shows extruded scanning electron micrographs of a) 12% AgO / SiC and b) 12% AgO / SiC treated under H ₂ to 900 ° C and c) 12% Ag Iγ -AI ₂ O ₃ / SiC and d) 12% Ag // - Al ₂ O ₃ / SiC having undergone treatment under H ₂ up to 900 ^° C. The magnification is indicated at the bottom right of each micrograph by the length of the white bar. Description of the invention

According to the invention, the problem is solved by depositing a porous oxide layer, and in particular a porous alumina layer, on the high surface area β-SiC support.

The deposition of a porous oxide layer, and more particularly of porous alumina, on the monolithic catalyst supports is known as such. Such layers are called in English "washcoat". This deposit is commonly used to increase the surface area of low surface area catalyst supports. In the context of the present invention, such a porous oxide layer is deposited on a porous support.

The silicon carbide used in the context of the present invention may be in any of the most diverse forms used in catalysis (extrudates, beads, grains, spheres, trilobes, monoliths, foams, foam foams, etc.). The β-SiC parts can be manufactured according to any of the known methods described in the patents cited above. The processes for producing β-SiC do not require the addition of a binder for shaping, whereas the α-SiC-based ceramic parts, which are generally obtained by shaping powders, must be held by a binder to be extruded.

According to the invention, the deposition of the alumina on the β-SiC can be done by impregnation of an alumina precursor solution followed by a heat treatment. The maintenance of the layer is done by Si-O-Al chemical interaction allowing excellent adhesion of the alumina. This composite material makes it possible to benefit from the surface chemistry of alumina (stabilizing effect) accompanied by the intrinsic properties of β-SiC.

The parts obtained by the process according to the invention are therefore constituted in a continuity of β-SiC. This has the advantage of giving a shaped ceramic of high thermal conductivity, resistant to oxidation, of high mechanical strength, stable over time and having a specific surface area of a few to more than a hundred m ² / g (without addition of porogens). In addition to the advantages listed above, β-SiC has been chosen as a backbone for alumina deposition for two other reasons. The first is that the surface of β-SiC naturally oxidizes forming an oxide passivation layer, characterized as being a mixture of silica (SiO ₂ ) and silicon oxycarbide (SiO _x Cy). This thin oxide layer (thickness typically between 2 - 5 nm) makes it possible to create an interface between the substrate (β-SiC) and the deposition of alumina. One of the advantages of β-SiC is that its synthetic process makes it possible to produce cellular structures (β-SiC foam) of large size. The foam foams, which are implemented in a particularly preferred embodiment of the present invention, have the advantage of having a large exchange surface, a continuity of material that allows to have good thermal conductivity and especially d generate a very low pressure drop when passing one or more fluids.

The alumina layer deposited on a support based on β-SiC is intended to stabilize certain noble metals capable of coalescing under a reaction flow. The combination of the chemical properties of the porous oxide layer (washcoat) and the physicochemical properties of the substrate (β-SiC) makes it possible to create a new class of catalyst support. Indeed, silicon carbide alone does not always stabilize some highly labile transition metals such as silver. At the same time, alumina is an insulating material, and the use of alumina supports promotes the appearance of hot spots in reaction, damaging the life of the catalyst, the selectivity and can create thermal runaway. It is therefore one of the most important advantages of the present invention to present a catalyst support that combines the specific qualities of each of the two materials that constitute it, without suffering from their specific disadvantages.

The deposition of alumina can be done using an alumina precursor, γ-AIOOH (Disperal), or Al (OH) ₃ which makes it possible to perform a deposition by simple immersion of β-SiC in a solution containing the precursor followed by a stoving and a heat treatment. In the case of β-SiC foams, in addition to immersion in the Disperal solution, the solution of Disperal through the alveolar structure. In addition, the viscosity of the Disperal solution can be controlled by addition of polyvinyl alcohol (PVA), which also acts as a glue.

The present invention consists in producing a β-SiC-based ceramic on which a layer of alumina is deposited, which makes it possible to combine the advantageous physical and physicochemical properties of alumina and silicon carbide. The support according to the invention makes it possible to stabilize silver particles thus limiting their sintering. Such a catalyst can be used for the partial oxidation of ethylene.

Detailed description of the invention

The present invention relates to the preparation of a β-SiC based catalyst support on which a layer of alumina is deposited. This support can be obtained by immersing the silicon carbide in a so-called "oxide precursor solution" containing an alumina precursor which disperses easily, such as aluminum hydroxide (Al (OH) ₃ ) or preferably, boehmite (γ-AIOOH). To modify the chemical properties of the alumina layer, other oxides can be added to this oxide precursor solution, such as ceria.

After steaming of the silicon carbide parts impregnated with the precursor oxide solution, the material is calcined between 400 ^° C. and 800 ^° C., preferably between 550 ^° C. and 650 ^° C., leading to the formation of γ-alumina. .

A preferred embodiment is based on the following method:

i) It supplies β-SiC, in any form, and especially in the form of extruded foam or foam. This β-SiC is either used as such or, in one variant of the process, oxidized to increase the thickness of the oxycarbide surface layer. This oxidation can be done by heating the β-SiC in air at a temperature greater than 600 ⁰ C, and preferably greater than 800 ⁰ C; advantageously, a temperature of the order of 900 ^° C. is used.

ii) An oxide precursor solution is prepared by dispersing X% by weight of x-AlOOH in Y% by weight of distilled water. Advantageously, 10 <X <30 and 70 <Y <90. The solution is acidified with a mineral acid, preferably nitric acid HNO ₃ (for example: 0.94 and 1.25% by weight). The mixture is stirred, for example for half an hour, and if necessary adjusts the viscosity of the solution by modifying its pH or by adding polyvinyl alcohol (PVA).

iii) The β-SiC support is brought into contact with said oxide precursor solution, typically for a few minutes. In a particular embodiment, the β-SiC pieces are immersed in the oxide precursor solution. If the pieces are made of foam-SiC, it is also possible, in another embodiment, to pass the precursor oxide solution through the foam. In the latter case, the foams can be placed in a three-phase reactor in which the solution of γ-AOHOH is passed with air. In any case, the excess of γ-AIOOH likely to clog the cells of the cellular structure foam can be removed by the passage of air through the foam.

iv) After an optional step of drying in ambient air and at room temperature, the β-SiC parts coated with γ-AOHOH are steamed, typically at a temperature between 90 ^° C. and 130 ^° C., and preferably included between 100 ⁰ C and 120 ⁰ C, and preferably about 110 ⁰ C ₁ for a sufficient time, which can be of the order of two hours.

v) Then, said pieces are calcined to form the γ -alumine phase on the SiC surface. A calcination at a temperature of between 500 ^° C. and 700 ^° C. for a period of between 1 hour and 5 hours may be suitable; treatment for 2 hours at 600 ⁰ C gives good results. A new catalyst support is thus obtained in which the alumina heap is strongly bonded to the silicon carbide backbone by interaction between the alumina, the oxide layer (SiO _x Cy) naturally present on the SiC, and the heart in SiC. This new type of support is particularly suitable for applications at high temperatures and high space velocities, especially for foams. The high intrinsic thermal conductivity of silicon carbide allows the heat of reaction to be rapidly removed. This eliminates or at least limits the appearance of hot spots. Thus, the temperature of the catalytic bed is rapidly homogenized and the heat is rapidly released. As a result, many problems of thermal runaway, catalyst life, process selectivity encountered with prior art catalysts can be solved. Alumina alone does not overcome these problems because it is an insulating ceramic. As for silicon carbide, it has intrinsic surface properties which do not make it possible to stabilize certain metals under a reaction flow. The combination of the two materials makes it possible to combine the physical and physicochemical properties of SiC with the chemical properties of alumina.

The inventors have found that the alumina layer adheres particularly well to β-SiC. This can be related to the high roughness of the β-SiC surface and its large surface area. The latter is advantageously between 1 and

100 m ² / g, preferably between 10 and 50 m ² / g and more preferably between 20 and 35 m ² / g. It is preferred to use a β-SiC whose specific surface is essentially formed of meso and macropores (FIG. 1 represents the macroporous distribution of β-SiC extrusions obtained by mercury intrusion).

With regard to the physicochemical characteristics of this surface of the β-

SiC, surface analyzes performed by induced photoelectron spectroscopy (XPS = X-ray Photoelectron Spectroscopy) and imaging (TEM = Transmission Electron Microscopy) clearly show that silicon carbide has a dual aspect. Indeed, the TEM analysis shows that the core of the ceramic is "pure" SiC whereas the surface has a much more oxidized character, with an amorphous layer of thickness of 2 to 5 nm approximately. FIG. 2 represents a transmission electron microscopy of β-SiC. Surface Analysis (XPS) indicates that this amorphous layer consists of Si, O and C, whose stoichiometry is not fully characterized. This layer of oxycarbide SiO _x Cy serves as a natural pre-layer attachment to the alumina layer for better anchoring of the alumina. Furthermore, the oxide layer can be artificially increased by carrying out a thermal treatment of β-SiC, typically between 700 and 1200 ^° C., and more particularly at 900 ^° C. for about 2 hours.

The process according to the invention makes it possible to deposit the alumina layer in a uniform manner and strongly bonded to the silicon carbide support. Such a support makes it possible to disperse metals for use in catalytic reactions. The advantages offered by this new material are: a high mechanical strength compared to alumina ceramics, excellent thermal conduction (that of β-SiC), a high dispersion of the active phase in the alumina layer, a strong adhesion of the active phase on the support, a good resistance of the active phase vis-à-vis the sintering, the possibility of using the macroscopic forms of foams alveolar types with active phases that adhere poorly on the β-SiC (like money)

Such a support can accommodate a catalytically active phase. In particular, it can accommodate transition metals, and stabilize them under a reaction stream. The inventors have in particular deposited silver particles on β-SiC grains coated with alumina (denoted AfeCVβ-SiC). This catalyst can be used as a catalyst for partial oxidation of ethylene. The catalytically active phase can be deposited directly on the Al ₂ O ₃ ZSiC support, in particular by one of the following methods: the precipitation method, the porous volume impregnation method, the use of a microemulsion.

After steaming, calcination under air or protective gas optionally followed by reduction under reducing gas, good dispersion of the particles which constitute the catalytically active phase can be obtained. This phase may comprise at least one transition metal, and in particular at least one metal selected from the group consisting of silver, palladium, rhodium, platinum, iron, cobalt and nickel.

Such a catalyst can be used to catalyze chemical reactions in the gas or liquid phase. For example, a catalyst loaded with silver under the Nanoparticle form can be used to effect partial oxidation of ethylene. The advantage of such a catalyst is that the alumina surface stabilizes the silver particles (especially at reaction temperatures> 200 ° C). The thermal conductivity of the silicon carbide core meanwhile makes it possible to homogenize the temperature in the catalytic bed.

Another reaction that can be catalyzed is the oxidation of H ₂ S, in particular and preferably with the aid of an active phase comprising iron oxide.

In the context of the present invention, the alumina may comprise additives of other oxides or other elements or compounds; thus forming non-stoichiometric mixed oxides. For example, cerium oxide and / or lanthanum oxide may be added to alumina. In the case of lanthanum, the content of lanthanum, expressed in weight of lanthanum oxide relative to the total mass of alumina + lanthanum oxide is advantageously between 1.5% and 15%, preferably between 2.5% and 10%, and even more preferably between 3% and 7.5%.

The following examples represent various embodiments of the present invention and thus illustrate the invention, but they do not limit it.

Examples

Example 1

This example describes the deposition of a porous alumina layer on a porous β-SiC support. In this example, a silicon carbide support in the form of extrudates 2 mm in diameter by 2 to 7 mm in length having a specific surface area of 30 m ² / g was used. The colloidal solution of γ-AOHOH, loaded to 10% by mass of solid, was prepared in the following manner:

100 g of γ-AIOOH (Disperal, Sassol) were suspended in 1000 ml of distilled water, the acidity of the solution was obtained by adding 6.15 g of concentrated HNO ₃ (65%, Fluka) . The disperse acid solution of γ-AOHOH was stirred vigorously for 30 minutes. 30 g of β-SiC extrudates were immersed in the Disperal solution for 30 minutes with stirring. The extents were filtered and then cured at 100 ^° C. for 12 hours (mass of the solid after steaming 31.87 g). After stoving, the extrudates were calcined at 700 ^° C. for 10 hours in order to transform the κ-AIOOH into γ-Al ₂ O ₃ (mass of the solid after calcination: 31.48 g). A mass increase of 2.8% was found in relation to the initial mass of the support. The scanning electron micrographs of the support prepared according to Example 1 are shown in FIG.

Example 2

This example describes a second successive step of impregnation: The solid obtained from the procedure described in Example 1 was impregnated in the same manner and in the same solution as that described in Example 1 (10% of γ -AIOOH). Then, the same baking and calcination as described in Example 1 were carried out. The solid after calcination showed a mass increase of 2.9%.

Example 3

In this example, the β-SiC extrudates have previously undergone heat treatment at 900 ^° C. for two hours in order to increase the thickness of the silicon carbide passivation layer (SiO _× C _y ). An acidic solution loaded to 10% by weight of γ-AOHOH was prepared according to Example 1. 30 g of heat-treated extrudates were placed in the solution for 30 minutes with stirring. After filtration and parboiling at 100 ^° C. for 12 hours, 33.45 g of impregnated solid were recovered. After calcination at 700 ^° C. for 10 hours, 33.09 g of λ-Al ₂ O ₃ ZSiC is recovered, ie an increase in mass of 10.3% relative to the SiC support.

Example 4

This example describes the preparation of the AgZ 2 -Al ₂ O ₃ ZSiC catalyst according to the porous volume impregnation method. 1.07 g of silver nitrate (Prolabo) was dissolved in 4 ml of distilled water and 1 ml of glycerol (Prolabo). 5 g of extrudates of γ-Al ₂ CVSiC prepared according to the example

1 were impregnated with the solution containing the silver nitrate. After stoving at 80 ^° C., the solid was dried at 100 ^° C. for 12 hours and then calcined at 300 ^° C. under air for two hours.

Example 5

In this example, the prepared catalyst according to Example 4 was subjected to a flow of hydrogen from room temperature to 900 ^° C. For comparison, a silver catalyst supported on non-layered β-SiC extrudates of alumina has undergone the same treatment.

It has been found that the γ-Al ₂ O ₃ / β-SiC support according to the invention has a better interaction with silver since the metal remains attached to the support after this treatment, whereas the Ag / β-SiC catalyst according to the invention the state of the art does not support this treatment and almost all of the metal phase is detached from the surface.

The scanning electron micrographs of the various impregnated supports before and after heat treatment under H ₂ are presented in FIG. 4.

Claims

1. Composite formed of a layer of alumina deposited on a rigid support, characterized in that said rigid support is a support of β-SiC. 2. Composite according to claim 1, characterized in that said rigid support has a BET specific surface area of at least 1 m ² / g, preferably between 10 m ² / g and 50 m ² / g, and still more preferably more preferably between 20 m ² / g and 35 m ² / g. 3. Composite according to claim 1 or 2, characterized in that said support is in the form of extrusions, grains, monolith, foam or foam. 4. Composite according to any one of claims 1 to 3, characterized in that it further comprises at least one catalytically active phase deposited at least on the alumina layer. 5. The composite according to claim 4, characterized in that said at least one catalytically active phase comprises at least one metal selected from the group consisting of silver, palladium, rhodium, platinum, iron, cobalt, nickel. 6. Composite according to any one of claims 1 to 5, characterized in that the alumina comprises cerium oxide and / or lanthanum. 7. Process for producing a composite formed of a layer of alumina on a rigid β-SiC support, in which (a) supplying a β-SiC support, preferably in the form of extrudates, foam or cellular foam, and preparing a so-called precursor solution of oxide by dispersing X% by mass of γ-AlOOH in Y % by weight of water, preferably acidified with a mineral acid; (b) optionally, said support is heated to a temperature greater than 600 ⁰ C, and preferably between 800 ⁰ C and 1100 ⁰ C; (c) contacting said support with said oxide precursor solution; (d) drying said support from step (c), preferably by steaming, optionally preceded by a drying step in ambient air and at room temperature; (e) calcining said dried support, preferably at a temperature between 500 ⁰ C and 700 ⁰ C, to form said composite. The method of claim 7 wherein 10 <X <30 and 70 <Y <90. 9. The method of claim 7 or 8, wherein step (d) comprises a step of baking at a temperature between 90 ⁰ C and 130 ⁰ C, and preferably between 100 ⁰ C and 120 ⁰ C. 10. Process according to any one of claims 7 to 9, wherein the calcination step is carried out at a temperature between 500 ⁰ C and 700 ⁰ C for a period of between 1 and 5 hours. 11. Method according to any one of claims 7 to 10, further comprising a step of depositing a catalytically active phase, or a precursor of such a phase. 12. The method of claim 11, characterized in that said catalytically active phase comprises silver. 13. Use of a composite according to claim 5 or 6 or capable of being prepared from the process according to claim 11 or 12 for catalyzing chemical reactions in gaseous or liquid phase. 14. Use according to claim 13 for effecting partial oxidation of ethylene. 15. Use of a composite according to claim 5 or 6 for catalyzing the oxidation of H ₂ S ₁ preferably with the aid of an active phase comprising iron oxide.