FR2918982A1 - CATALYST SUPPORT BASED ON BETA-SIC WITH ALUMINUM LAYER - Google Patents

Abstract

The invention relates to a composite formed of a porous alumina layer deposited on a rigid support of beta-SiC. This composite can be used as a catalyst support. The alumina layer may contain catalytically active phases, and especially phases that do not adhere well to untreated beta-SiC, such as silver particles.

Description

Catalyst support based on a-SiC with a layer of alumina Domain

  The present invention relates to the field of f3-SiC-based catalyst supports, and more particularly to a process for functionalizing the surface of f3-SiC by depositing a layer of porous alumina for depositing active phases that do not readily adhere to [3-SiC as such. STATE OF THE ART Silicon carbide, and in particular f3-SiC with a high specific surface area, is known as a catalyst support. (3-SiC can be obtained by the reaction between SiO vapors with reactive carbon at a temperature of between 1100 ° C. and 1400 ° C. (Ledoux process, see EP 0 313 480 B1), or by a process in which a mixture a liquid or pasty prepolymer and a silicon powder is crosslinked, charred and carburized at a temperature of between 1000 ° C. and 1400 ° C. (Dubots process, see EP 0 440 569 B1 or EP 0 952 889 B1). in addition, (3-SiC) foams, which can be obtained by a variant of the Dubots process, comprising impregnating a polyurethane foam with a suspension of a silicon powder in an organic resin (Pr-in process, see EP 0 624 560 B1, EP 0 836 882 B1 or EP 1 007 207 A1).

  These different forms of SiC, and especially of R-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. It is known that SiC has excellent thermal conductivity compared to other catalyst supports, as well as good chemical resistance at high temperatures, which allows it to be used at elevated 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. 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 zeolite deposition on a support of (3 • -SiC The problem that the present invention seeks to solve is to propose a method for improving the adhesion on a support of 6 SiC porous of certain active phases, such as silver, which do not adhere well to untreated 6-SiC 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 an I3-SiC support By the formation of at least one catalytically active phase deposited, at least on the alumina layer, a catalyst is obtained which can be used to catalyze chemical reactions in gaseous or liquid phase.

  Another object of the present invention is a process for producing a composite formed of a layer of alumina on a (3-SiC) rigid support, in which (a) a (3-SiC) support is provided, preferably in the form of extrudates, foam or cellular foam, and a so-called precursor solution of oxide is prepared by dispersing X% by weight of yAIOOH in Y% by weight of water, preferably acidified with a mineral acid (b) optionally, said support is heated to a temperature greater than 600 ° C., and preferably between 800 ° C. and 1100 ° C. (c) said support is brought into contact with said oxide precursor solution; 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 of between 500 ° C. and 700 ° C. C, to form said composite. DESCRIPTION OF THE FIGURES FIG. 1 shows the typical macroporous distribution of a 6-SiC that can be used in the context of the present invention. The distribution shows a peak at 0.044 μm and a peak at 0.14 μm. Figure 2 shows a transmission electron micrograph of 6-SiC. It can be seen that the core is purely SiC while the surface is covered with an amorphous layer of SiOXCY.

  Figure 3 shows scanning electron microscopy pictures of yy-Al2O3 / SiC extrudates 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 H2 to 900C and c) 12% Ag / y- AI203 / SiC and d) 12% Ag / y-Al2O3 / SiC treated under H2 up to 900 C. 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 13-SiC support with a high specific surface area. 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 washcoat in English. 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 (3-SiC) parts may be manufactured by any of the known methods described in the above-cited patents.The methods of making 13-SiC do not require the addition of a binder for shape, while the ceramic parts based on a-SiC, which are generally obtained by shaping powders, must be maintained by a binder to be extruded. If it can be done by an impregnation of a precursor solution of alumina followed by a heat treatment, the layer is maintained by Si-O-Al chemical interaction allowing excellent adhesion of the alumina. allows to benefit from the surface chemistry of alumina (stabilizing effect) accompanied by the intrinsic properties of 13-SiC The parts obtained by the process according to the invention are thus constituted in a continuity of (3-SiC. advantage of giving a shaped eraser of high thermal conductivity, resistant to oxidation, of high mechanical strength, 4 stable over time and having a specific surface area of a few to more than a hundred m2 / g (without addition of porogens). In addition to the advantages listed above, G3-SiC was chosen as a backbone for alumina deposition for two other reasons. The first is that the surface of (3-SiC naturally oxidizes forming an oxide passivation layer, characterized as being a mixture of silica (SiO2) and silicon oxycarbide (SiOXCy) .This thin oxide layer (thickness typically included between 2 - 5 nm) makes it possible to create an interface between the substrate (j3-SiC) and the deposition of alumina One of the advantages of R-SiC is that its synthetic process makes it possible to produce alveolar structures (foam of -SiC) of large size Foam foams, which are implemented in a particularly preferred embodiment of the present invention, have the advantage of having a large exchange surface, a material continuity that allows to have a good thermal conductivity and above all to generate a very low pressure drop when passing one or more fluids The alumina layer deposited on a support based on R-SiC is intended to stabilize certain noble metals likely to coa The combination of the chemical properties of the porous oxide layer (washcoat) and the physicochemical properties of the substrate (R-SiC) make it possible to produce 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 which 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, y-AIOOH (Disperal), or Al (OH) 3 which allows a deposit by simple immersion of (3-SiC in a solution containing the precursor followed by steaming and heat treatment In the case of R-SiC foams, in addition to immersion in the Disperal solution, the Disperal solution can be passed through 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 ceramic based on (3- SiC 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 thereby limiting sintering, such a catalyst can be used for for the partial oxidation of ethylene.

  Detailed Description of the Invention The present invention relates to the preparation of a 13-SiC based catalyst support on which a layer of alumina is deposited. This support can be obtained by dipping the silicon carbide into a so-called precursor solution of oxide, containing a precursor of alumina which disperses easily, such as aluminum hydroxide (Al (OH) 3), or preferred way, boehmite (y-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 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., resulting in the formation of γ-alumina. A preferred embodiment is based on the following method: i) 13-SiC is supplied in any form, including extrudates, foam or cellular foam. This R-SiC is either used as it is 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 13-SiC in air at a temperature above 600 C, and preferably above 800 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 γ-AOHOH in Y% by weight of distilled water. Advantageously, 10 <X <30 and 70 <Y <90. The solution is acidified with a mineral acid, preferentially 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 6-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 Fo-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-A 10 OH likely to clog the cells of the foamed 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 parts of / 3-SiC covered with γ-A 10 OH are steamed, typically at a temperature between 90 ° C. and 130 ° C., and preferably between 100 C and 120 C, and preferably about 110 C, for a sufficient time, which can be of the order of two hours. v) Then, said pieces are calcined to form the γ-alumina phase on the SiC surface. A calcination at a temperature of between 500 ° C. and 700 ° C. for a period of between 1 h and 5 hours may be suitable; treatment for 2 hours at 600 C gives good results. A new catalyst support is thus obtained in which the alumina layer is strongly bonded to the silicon carbide backbone by interaction between the alumina, the oxide layer (SiOXCy) naturally present on the SiC, and the core. 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 (3-SiC) This can be linked to the high roughness of the surface of 3-SiC and to its large surface area, which advantageously lies 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 (3-SiC whose specific surface is essentially formed of meso and macropores (FIG. macroporous distribution of [3-SiC extrudates obtained by mercury intrusion).

  As regards the physicochemical characteristics of this 13-SiC surface, the surface analyzes performed by induced photoelectron spectroscopy (XPS = X-ray F'photelectron Spectroscopy) and imaging (TEM = Transmission Electron Microscopy) show clearly that silicon carbide has a dual aspect. Indeed, the TEM analysis shows that the core of the ceramic is purely SiC while the surface has a much more oxidized character, with an amorphous layer of thickness of 2 to 5 nm approximately. Figure 2 shows a transmission electron microscopy snapshot of R-SiC. Surface analysis (XPS) indicates that this amorphous layer consists of Si, O and C, whose stoichiometry is not fully characterized. This SiOxCy oxycarbide layer serves as a natural pre-layer for attachment to the alumina layer, allowing better anchoring of the alumina. Furthermore, the oxide layer can be artificially increased by carrying out a heat treatment of [3-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 the / 3-SiC), a strong 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 to sintering, the possibility of using the macroscopic forms of foamed alveolar types with active phases which adhere poorly to the [3 -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 grains of (3-SiC covered with alumina (denoted Al2O3113-SiC) .This catalyst can be used as catalyst for partial oxidation of ethylene. catalytically active can be done directly on the support AI2O3 / SiC, in particular by one of the following methods: the precipitation method, the method of impregnation of the pore volume, 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, nickelf. Such a catalyst can be used to catalyze chemical reactions in the gas or liquid phase. By way of example, a silver-laden catalyst in the form of nanoparticles 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 catalytic reaction is the oxidation of H 2 S using 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 can be added to alumina. 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 carrier of (3-SiC) In this example, a support of silicon carbide in the form of extrusions of 2 mm diameter by 2 to 7 The colloidal solution of λ-AOHOH, loaded to 10% by mass of solid, was prepared in the following manner: 100 g of γ-AOHOH (Disperal, Sassol) were suspended in 1000 mL of distilled water, the acidity of the solution was obtained by adding 6.15 g of concentrated HNO3 (65% i, Fluka) .The disperse acid solution of γ-AOHOH was Stirring vigorously for 30 min of R-SiC extrudates was immersed in the Disperal solution for 30 minutes with stirring The extrudates were filtered and then parboiled at 100 ° C. for 12 hours (mass of the solid after steaming). After drying, the extrudates were calcined at 700 ° C. for 10 hours. in order to convert γ-AlOHOH to 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 impregnation step: The solid obtained from the procedure described in Example 1 was impregnated in the same way and in the same solution as that described in Example 1 (10% y-AOHOH). 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 (3-SiC) extrudates were heat-treated at 900 ° C. for two hours in order to increase the thickness of the silicon carbide passivation layer (SiOXCy). 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 baking at 100 ° C. for 12 hours. recovered 33.45 g of impregnated solid After calcination at 700 ° C. for 10 hours, 33.09 g of γ-Al 2 O 3 / SiC was recovered, ie an increase in mass of 10.3% relative to the SiC support. For example, the preparation of Ag / γ-Al 2 O 3 / SiC 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 y-Al 2 O 3 / SiC extrudates prepared according to 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 catalyst prepared according to Example 4 was subjected to a flow of hydrogen from room temperature to 900 ° C. For comparison, a silver catalyst supported on R-SiC extrudates without alumina layer underwent the same treatment. It has been found that the y-Al 2 O 3/13-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 / R-SiC catalyst according to the state of the art does not support this treatment and almost all of the metal phase is detached from the surface. Scanning electron micrographs of different impregnated substrates before and after heat treatment under H2 are shown in Figure 4. 12

Claims (10)

  1. Composite formed of a layer of alumina deposited on a rigid support, characterized in that said rigid support is a support (3-SiC.   2. Composite according to claim 1, characterized in that said rigid support has a BET specific surface area of at least 1 m2 / g, preferably between 10 m2 / g and 50 m2 / g, and even more preferably included between 20 m2 / g and 35 m2 / 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. A process for producing a composite formed of an alumina layer on a (3-SiC) rigid support, in which (a) a (3-SiC) support is provided, preferably in the form of extruded, foam or cellular foam, and a so-called precursor solution solution of oxide is prepared by dispersing X% by weight of y-AIOOH in Y% by weight of water, preferably acidified with a mineral acid; (b) optionally said substrate is heated to a temperature greater than 600 ° C., and preferably between 800 ° C. and 1100 ° C. (c) said support is brought into contact with said oxide precursor solution; step (c), preferably by steaming, optionally preceded by a step of drying in ambient air and at room temperature (e) calcining said dried support, preferably at a temperature of between 500 ° C. and 700 ° C. C, to form said composite.   7. The method of claim 6, further comprising a step of depositing a catalytically active phase, or a precursor of such a phase.   8. Process according to claim 7, characterized in that said catalytically active phase comprises silver.   9. Use of a composite according to claim 5 or capable of being prepared from the process according to claim 7 or 8 for catalyzing chemical reactions in gaseous or liquid phase.   Use according to claim 9 for effecting partial oxidation of ethylene.