Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
  • B01D53/34—Chemical or biological purification of waste gases
  • B01D53/92—Chemical or biological purification of waste gases of engine exhaust gases
  • B01D53/94—Chemical or biological purification of waste gases of engine exhaust gases by catalytic processes
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
  • B01D53/34—Chemical or biological purification of waste gases
  • B01D53/74—General processes for purification of waste gases; Apparatus or devices specially adapted therefor
  • B01D53/86—Catalytic processes
  • B01D53/864—Removing carbon monoxide or hydrocarbons
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
  • B01J23/38—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals
  • B01J23/40—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals of the platinum group metals
  • B01J23/42—Platinum
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J27/00—Catalysts comprising the elements or compounds of halogens, sulfur, selenium, tellurium, phosphorus or nitrogen; Catalysts comprising carbon compounds
  • B01J27/20—Carbon compounds
  • B01J27/22—Carbides
  • B01J27/224—Silicon carbide
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
  • B01J37/02—Impregnation, coating or precipitation
  • B01J37/0201—Impregnation
  • B01J37/0203—Impregnation the impregnation liquid containing organic compounds
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
  • B01J37/02—Impregnation, coating or precipitation
  • B01J37/0201—Impregnation
  • B01J37/0205—Impregnation in several steps
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J35/00—Catalysts, in general, characterised by their form or physical properties
  • B01J35/30—Catalysts, in general, characterised by their form or physical properties characterised by their physical properties
  • B01J35/396—Distribution of the active metal ingredient
  • B01J35/397—Egg shell like

Definitions

• the invention relates to the field of catalysts based on ⁇ -SiC, for heterogeneous catalysis, and in particular the biphasic impregnation of supports with high specific surface 0 with active phase precursors to form such a catalyst.
• the catalysts currently used in the fields of the chemical or petrochemical industry, or in the depollution of exhaust gases from motor vehicles are essentially in the form of grains, extrudates, barrels or monoliths to name only the most used forms. These materials play the role of active phase support or precursor of said active phase, an active phase being in the latter case deposited on said support to form the catalyst. This active phase often consists of metals or metal oxides.
• the deposition of the active phase on the supports currently used is carried out by means of an impregnation step, during which the solution containing a precursor of the active phase is deposited homogeneously over the entire surface of the support. Most often, this precursor is then subjected to an activation treatment.
• the precursor in question can be a salt or an organometallic compound.
• the precursor solution can be an aqueous solution or an organic solution.
• aqueous solution or an organic solution.
• Patent application WO 99/20390 (National Center for Scientific Research) describes the impregnation of a non-porous Si 3 N powder with a BET specific surface area of 8.8 m 2 / g with a solution of bis-acetyl- palladium (II) acetonate in toluene, as well as the manufacture, characterization and use of the catalyst thus obtained.
• the problem that the present invention seeks to solve is to present a new ⁇ -SiC catalyst comprising a support and at least one active phase with controlled localization, in which the influence of the phenomena of diffusion of the reagents towards the active sites and of retro diffusion of the products towards the surface of the catalyst over the kinetics of the reaction operated using said catalyst is lower than in the case of known catalysts.
• the first object of the present invention is a process for impregnating a ⁇ -SiC support having a specific surface, determined by the BET method of nitrogen adsorption at the temperature of liquid nitrogen according to standard NF X 11-621, at least equal to 1 m 2 / g and comprising comprising macropores with a size between 0.05 and 10 ⁇ m, and optionally in addition to the mesopores with a size between 4 and 40 nm, said process comprising at least the following steps:
• the active phase precursor preferably a metallic compound
• Said precursor can be advantageously chosen from salts and compounds organo-metallic of said elements.
• Yet another object of the present invention is the catalyst capable of being obtained by said process for the preparation of a catalyst.
• Yet another object of the present invention is the use of the catalyst obtained by said process for catalyzing chemical reactions, such as the oxidation of methane or other hydrocarbons, or the oxidation of carbon monoxide.
• FIG. 1 schematically shows two profiles of the macroscopic localization of the active phase relative to the support in catalysts according to the state of the art, (a): crust deposition, (b): deposition in the center, (c ): uniform deposit.
• Figure 2 shows schematically the location of the active phase in a catalyst.
• Catalyst according to the invention comprising a support with hydrophilic / hydrophobic properties, impregnated by the two-phase impregnation method.
• the active phase is found on the hydrophobic zones (outside the pores).
• FIG. 3 shows the conversion of CH 4 to CO 2 as a function of the reaction temperature for an hourly space velocity of 15,000 h ⁇ 1 on the Pd (0) / ⁇ -SiC catalysts prepared by conventional impregnation (black dots) and biphasic (open circles).
• Figure 4 shows an enlargement of Figure 3 highlighting the half-conversion temperatures.
• FIG. 5 shows the conversion of CH into CO 2 as a function of the reaction temperature for an hourly space velocity of 40,000 h -1 on the Pd (0) / ⁇ -SiC catalysts prepared by conventional impregnation (black dots) and biphasic (open circles).
• FIG. 6 shows an enlargement of Figure 5 highlighting the half-conversion temperatures.
• FIG. 7 shows the conversion of CH4 to CO 2 as a function of the reaction temperature for an hourly space velocity of 200,000 h ⁇ 1 on the Pd (0) / ⁇ -SiC catalysts prepared by conventional impregnation (black dots) and biphasic (open circles).
• Figure 8 shows an enlargement of Figure 7 highlighting the half-conversion temperatures.
• Figure 9 shows the size distribution of macropores in two ⁇ -SiC supports which are suitable for carrying out the invention.
• the problem posed is solved by means of an impregnation method called "two-phase impregnation".
• This impregnation method the principle of which is described in the documents US Pat. of the agent, either the hydrophilic zones or the hydrophobic zones of the support in order to be able to selectively deposit and thus locate the precursor compound forming the active phase, either on the hydrophobic zones, or on the hydrophilic zones depending on the intended reaction.
• the method thus makes it possible to control the localization of the active phase in a microscopic manner relative to the matrix of the support and not conventionally in a macroscopic manner as described above.
• polar agent is understood here to mean a molecule having a permanent dipole moment. Agent X is less polar than Agent Y, if the permanent dipole moment of Agent X is greater than that of Agent Y.
• water is a polar agent, and toluene is a less polar agent than water.
• the present invention applies to catalysts manufactured on a ⁇ -SiC support having two distinct surface functions: hydrophobic and hydrophilic.
• Any ⁇ -SiC catalyst support having these two functions may be suitable, provided that they have sufficient porosity and a specific surface, determined by the BET method of nitrogen adsorption, that is to say at least 1 m 2 / g, and preferably at minus 2 m 2 / g.
• the support has a specific surface of between 1 and 100 m 2 / g.
• the supports are preferred having a specific surface greater than 10 m 2 / g, and preferably greater than 20 m 2 / g. This specific surface is due to the presence of pores.
• micropores with an average size typically less than 4 nm there are three types of pores: micropores with an average size typically less than 4 nm, mesopores with a size typically between 4 and 50 nm, and macropores, which can form networks, and whose diameter is typically greater than 50 nm.
• supports are preferred whose total porosity, measured by nitrogen adsorption, essentially consists of mesopores between 4 and 40 nm and a macroporous system with an average diameter between 0, 05 and 100 ⁇ m, preferably between 0.05 and 10 ⁇ m, and even more preferably between 0.05 and 1 ⁇ m.
• the pore size distribution is demonstrated by penetration of mercury.
• the pores can also be directly observed by scanning electron microscopy.
• the size distribution of the macropores is between 0.06 and 0.4 ⁇ m, and even more preferably between 0.06 and 0.2 ⁇ m.
• a ⁇ -SiC silicon carbide is used in the form of extrudates or beads which is prepared using any of the synthesis techniques described in patent applications EP 0 313 480 A, EP 0440 569 A, EP 0 511 919 A, EP 0 543 751 A and EP 0 543 752 A.
• the surface of the silicon carbide ( ⁇ -SiC) prepared according to one of the references indicated above consists of two types of zones of different reactive nature.
• a first sort of zone hydrophobic in nature, constitutes the external surface of the solid and lines the internal surface of the macropores. These zones are essentially constituted by planes of low Miller indices, stable and having a very low reactivity with respect to the oxygen of the air. In the presence of organic solvents, wetting is carried out essentially on these hydrophobic zones.
• the second kind of zone is hydrophilic in nature and essentially concerns the internal walls of the solid mesopores. These zones consist of atomic planes with high Miller indices, and therefore rich in structural defects.
• the use of the bi-impregnation technique therefore makes it possible to neutralize these mesopores for the deposition of an active phase in the macropores, or else to deposit first an active phase in the mesopores and then another active phase in the macropores.
• the first objective can be achieved in a particular embodiment by a preliminary heat treatment of the support under inert gas, which has the effect of reducing the mesoporosity.
• the support as described above is impregnated as described below, playing on its hydrophobic and hydrophilic properties, in order to be able to modify and control the location of the active phase with respect to the pore network, in order to improve the access of the reagents to the active sites and in order to keep the same reaction yield while reducing the residence time of the reagents and the porosity of the support.
• the biphasic impregnation mode consists of two successive impregnation steps, the first using a polar agent A (such as water), the second using a less polar agent B than agent A, and in particular of an apolar organic liquid.
• the agents can advantageously be liquids.
• Said liquids can be solutions, and can in particular contain metal salts.
• the first impregnation step consists of wetting the ⁇ -SiC support with water (preferably demineralized or distilled water).
• water preferably demineralized or distilled water.
• the volume of water is equal to or slightly greater than the total pore volume of the solid. This operation makes it possible to completely saturate the hydrophilic zones of the surface of the solid which are essentially located inside the pores of the material. The water thus remains trapped inside the pores leaving free the hydrophobic zones which constitute the external surface of the solid.
• a polar liquid containing one or more soluble compounds is used during this first impregnation step.
• Said soluble compound can be a metallic compound.
• Said metal compound can play the role of active phase or active phase precursor.
• the impregnated solid is dried superficially, in order to remove the moisture from the external surface of the body, while keeping the liquid in the pores.
• a temperature of 50 ° C at normal pressure is suitable for an aqueous liquid; the precise conditions (temperature and duration) for a given support can be easily determined using simple routine experiments.
• a precursor of the active phase is deposited on the solid, preferably in an essentially apolar organic solution.
• precursor of the active phase is understood to mean a compound of a metal, typically a salt of a metal or an organometallic compound, which forms, after a calcination treatment optionally followed by other treatments such as reduction , the active phase of the catalyst. It is desirable to select the solvent so that the affinity of the organic solvent with the hydrophobic zones allows perfect wetting of the latter, while the interior of the pores remains inaccessible due to the presence of the water previously impregnated. .
• the impregnated support is then dried, for example in air at room temperature and then at a temperature advantageously between 100 ° C. and 200 ° C. in an oven in order to vaporize the organic solvent.
• the dried solid is calcined in air at a temperature typically between 200 ° C and 500 ° C and more preferably between 300 ° C and 400 ° C for a period which depends on the charge of the furnace, the characteristics of the solvent and those of the support, in order to decompose the precursor of the active phase into its corresponding metal oxide.
• the calcined solid can be used as it is as a catalyst.
• a second, optional, activation step which is advantageously a treatment under reactive gas, and preferably a reduction.
• the oxide can either be reduced under a flow of hydrogen H 2 to obtain the corresponding metal, or treated with other gases in order to obtain the desired active phase.
• the active phase thus obtained is located essentially on the surfaces made up of the hydrophobic zones of the support, ie the surfaces outside the pores.
• the localization of the active phase can be diagrammatically compared the porosity of the support as shown in Figure 2.
• the active phase can consist of any metal having a salt soluble in a weakly polar solvent, or alternatively an organometallic compound which is sufficiently stable.
• these metals there may be mentioned in particular: Co, Ni, Fe, Cu, Pt, Pd, Rh, Ru, Ir.
• the concentration of said active phase can be included in a relatively wide range of around ten ppm (parts per million , relative to the mass) up to several tens of percent (relative to the mass), depending on the intended reaction.
• it is between 0.1 and 5% relative to the mass of the catalyst.
• Another variant taking advantage of the advantage offered by this impregnation method concerns the deposition of two different compounds on the same support, using for each of them a solvent of adequate polarity and thus allowing precise control of their respective localization.
• the two-phase impregnation method described can also be applied to successively deposit two compounds each forming a active phase, used separately because of their different and particular catalytic properties, but located on the same support.
• the catalyst thus prepared can be used under different conditions and in various reaction media. More particularly, it can be used for reactions having a very rapid rate of passage of reagents, ie for the depollution of exhaust gases from motor vehicles, or for reactions where the overall selectivity could be affected by secondary reactions between the products and one of the excess reagents, during the diffusion of the products from the active sites to the external surface of the support.
• the improved accessibility of active sites for reactants in the gas phase or in the liquid phase allows a significant increase in the overall yield of the reaction. Indeed, when the active phase is deposited directly on the external surface of the support or in a macroporosity, the accessibility of the active sites for the reactants in gas or liquid phase can be considerably increased; this increases the yield of the reaction.
• the extremely short residence time between the active phase and the reactants as well as the reaction products makes it possible to appreciably reduce the formation of the secondary products.
• the duration of migration of the products through the porosity of the support to reach the gas or liquid phase, or more generally escaping from the catalyst grain can be a very important factor in the overall selectivity of the reaction.
• the fact of locating the active phase on a hydrophobic zone has a certain advantage when one of the products of the reaction is water.
• Water can induce undesirable modifications to the active phase due to its oxidizing character.
• the water formed can adsorb on the active sites of the catalyst and oxidize the latter, while water adsorption is avoided when the active phase is located on the hydrophobic areas.
• This advantage is also significant when the reaction is carried out in the presence of water or any other solvent which interacts strongly with the hydrophilic zones and which has oxidizing properties in the reaction medium.
• the deposition method according to the invention also makes it possible to considerably reduce the content of the active phase necessary compared to that used in the crust impregnations. Therefore, the loss of the active phase, which can take place either by sintering or by attrition during the impregnation or more generally during the phase or phases of preparation or activation of the catalyst as well as during the catalytic test, is considerably reduced; this increases the service life of the catalyst.
• the catalysts prepared in the context of the present invention combine the advantages acquired on conventional macroscopic supports with those of better accessibility of the reagents to the active sites and better evacuation of the products. They thus allow a non-negligible gain in the yields of the various reactions while maintaining a maximum dispersion of the active phase so as not to harm the overall yield of the reaction.
• the preferred embodiment using extruded supports, beads or pellets of ⁇ -SiC also makes it possible to benefit from the specific advantages associated with this type of support.
• the catalyst according to the invention can be used in various fields, such as the chemical or petrochemical industry. For example, it can catalyze the oxidation of methane or the oxidation of carbon monoxide. It can also be used in the depollution reactions of exhaust gases from internal combustion engines (in particular those operating with liquid fuels such as petrol or diesel) where it makes it possible to obtain a better efficiency, thanks to. a very short contact time, and very good accessibility of the active sites of the catalyst by the reagents.
• a series of examples illustrating the invention is given below, without implied limitation.
• This example illustrates in a detailed manner the impregnation of an active phase based on platinum by the two-phase impregnation method on a support based on extruded silicon carbide ( ⁇ -SiC).
• the support based on extruded silicon carbide ⁇ -SiC is previously impregnated with a solution of distilled water whose volume is equal to the pore volume of the support, in order to block the entry of the pores of the solid. . 5 g of silicon carbide (BET specific surface 25 m 2 / g) are therefore first pre-impregnated with 3 mL of distilled water and then dried for 5 minutes at 50 ° C.
• the material is in a second step impregnated according to the drop method with a solution of platinum bis-acetylacetonate in toluene (non-polar solvent) at a rate of 2% by weight of platinum relative to the silicon carbide support (i.e.
• the solid obtained is dried in air at room temperature and then at 150 ° C in an oven for 2 hours. It is then calcined in air at 350 ° C for 2 hours to transform the platinum salt into its corresponding oxide, then reduced to 400 ° C under a stream of hydrogen for 2 hours to form the metallic platinum.
• the metallic platinum Pt is then located outside the pores of the silicon carbide.
• This example illustrates in the case of the total oxidation of methane to carbon dioxide the influence of the impregnation mode, namely purely aqueous to locate the active phase in the pores of the support, and two-phase to locate said active phase outside the mesoporosity of the support.
• Two catalysts based on palladium metal supported on grains of silicon carbide ( ⁇ -SiC, grains of diameter between 0.4 mm and 1 mm, specific surface 25 m 2 / g) are prepared with a palladium metal content of 1% in mass of the silicon carbide support: for one of the two catalysts, the purely aqueous impregnation method is used according to the state of the art, while for the other, the two-phase impregnation method is used according to the invention.
• the purely aqueous impregnation is carried out by impregnating the grains of silicon carbide ( ⁇ -SiC) with an aqueous solution of Pd ⁇ (NO 3 ) .H 2 O. After drying in air at room temperature, the solid is placed at 1 oven at 100 ° C for 2 hours. The dried solid is then calcined in air at 350 ° C. for 2 hours in order to form the palladium oxide PdO.
• the palladium metal catalyst supported on silicon carbide is obtained by reduction of its oxide counterpart at 400 ° C. under hydrogen for 2 hours. This purely aqueous impregnation leads to obtaining the palladium phase Pd (0) localized in the porosity of the support based on silicon carbide.
• the two-phase impregnation of the silicon carbide support is carried out by first impregnating the support with an aqueous solution of a volume equal to the pore volume of said support. After drying at 50 ° C for 5 minutes, 1% by mass of palladium is then deposited on the support in the form of palladium acetylacetonate (C 10 H 4 ⁇ Pd) in toluene. The material is then subjected to the same treatments as the catalyst prepared by traditional aqueous impregnation. The palladium oxide is then reduced to metallic palladium by heat treatment under hydrogen at 400 ° C. for 2 hours. The metallic palladium particles are then located outside the pores of the silicon carbide.
• the hourly space velocity (in English "gas hourly space velocity") is defined as being the ratio between the total flux and the volume of catalyst.
• Table 2 shows the half-conversion temperatures obtained on the two catalysts. function of the hourly space volume velocity of the flow containing methane and oxygen at a rate of 1% and 4% by volume respectively.
• the half-conversion temperature on the catalyst prepared by the two-phase impregnation method is 300 ° C. compared to 316 ° C. on the catalyst prepared by the method d "classic impregnation. This difference increases when the hourly space velocity increases, and the temperature difference is 23 ° C. for a space velocity of 40,000 h " .
• This gap of temperature reaches 57 ° C. when the total oxidation of methane is carried out at very high hourly space volume velocity, namely 200,000 h "1 .
• Table 2 Half-conversion temperatures obtained according to the method of impregnation of the catalyst as a function of the hourly space volume velocity of the reaction.
• the catalyst prepared by two-phase impregnation according to the invention has better performance, that is to say a significantly lower half-conversion temperature than the catalyst prepared by single-phase aqueous impregnation according to the state of the technical.
• This improved performance of the catalyst according to the invention prepared by two-phase impregnation can be attributed to the presence of palladium on the external surface of the support; palladium, which forms the active phase of the catalyst, thus has better accessibility vis-à-vis the reagent to be transformed.
• This example shows the distribution of the macropores in two supports of ⁇ -SiC which are very suitable for carrying out the invention, see FIG. 9. They are extraded into ⁇ -SiC.
• Support Z1 was made from Si -f C + resin, support Z2 with the addition of ethanol. It can be seen that the support Z1 has a distribution centered around around 0.06 ⁇ m, while the support Z2 has a distribution centered around around 0.11 ⁇ m.

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  • 2002-07-09 FR FR0208635A patent/FR2842125B1/fr not_active Expired - Fee Related
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