SYNTHESIS OF A CATALYST COMPRISING AN IRON-CONTAINING AFX STRUCTURAL TYPE ZEOLITE FOR THE REDUCTION OF NOx AND N2O

Abstract

The invention relates to a process for preparing a catalyst based on an AFX zeolite and iron comprising at least: i) mixing in an aqueous medium, at least one source of SiO2, at least one source of Al2O3, a particular organic nitrogen compound R, at least one alkali metal and / or an alkaline-earth metal M of valence n, and a source of Fe, with the following molar composition: SiO2 / Al2O3 between 2 and 100, H2O / SiO2 between 5 and 60, R / SiO2 between 0.05 and 0.50, M2 / nO / SiO2 between 0.05 and 0.40, Fe2O3 / SiO2 between 0.01 and 0.10, ii) hydrothermal treatment of the precursor gel obtained iii) optionally at least one ion exchange with iron,iv) heat treatment. The invention also relates to the catalyst obtainable or directly obtainable by the process and the process for reducing NOx and N2O using the catalyst. Figure to be published: Figure 2

Description

Title of the invention: SYNTHESIS OF A CATALYST COMPRISING A ZEOLITE OF THE AFX STRUCTURAL TYPE CONTAINING IRON FOR THE REDUCTION OF NOx AND N2 O Technical field

[0001] The subject of the invention is a process for preparing a catalyst based on a zeolite of structural type AFX and Iron, the catalyst prepared or capable of being prepared by the process, and the process for reducing NOx and N2O using said catalyst in the presence of a reducing agent, in particular on combustions and industrial processes such as the production of nitric and adipic acid. Prior art

[0002] Nitrogen oxide (NOx) emissions resulting from combustion are a major concern for society as they are responsible for health problems, tropospheric ozone, acid rain and smog. Increasingly stringent standards are being implemented by government authorities to limit the impact on the environment and health. Highly efficient pollution control systems such as three-way catalysts or selective catalytic reduction catalysts, designated by the English acronym "SCR" for "Selective Catalytic Reduction", have therefore been developed to equip means of transport in order to achieve these objectives. However, it is not uncommon for the selectivity of these systems to induce nitrous oxide (N2O) emissions. Nitrous oxide is the third largest contributor to radiative forcing after carbon dioxide (CO2) and methane (CH4).In addition to affecting stratospheric ozone, a given amount of N2O in the atmosphere has 298 times more effect over 100 years on global warming than the same amount of CO2, according to the 4th IPCC report.

[0003] Ammonia is considered an important fuel for decarbonization. However, the combustion of ammonia (or an H2 / NH3 mixture) presents potentially significant emissions of N2O, NOx, NH3 at the exhaust.

[0004] Other sectors are the source of high NOx and N2O emissions: the nitric acid industry is one of the main sources of nitrous oxide (N2O) emissions. N2O is formed as a by-product of the oxidation of ammonia over a Pt / Rh catalyst. The NOx, which is the main product at the outlet of the Pt / Rh catalyst, is then absorbed in water to form nitric acid. The absorption step is not 100% efficient and results in NOx emissions at the outlet (100 to 500 ppm).

[0005] Concomitant treatment of NOx and N2O for applications related to decarbonization and industrial fumes is therefore necessary.

[0006] The treatment of NOx has already been the subject of numerous studies, whether in industry or transport.

[0007] For the treatment of N2O, several types of catalysts have been studied depending on the temperature at which they are effective. Precious metals have good efficiencies around 250°C, but are expensive and very often deactivated in the presence of inhibitors (CO, H2O, NOx, O2, etc.) which is not compatible with the constraints of nitric or adipic acid production. Zeolites, for their part, have a relatively low cost and a very large developed surface which makes it possible to obtain a good catalytic system by incorporating a transition metal, in particular iron. Sâdovskâ et al. (Sâdovskâ, G., Bernauer, M., Bemauer, B., Tabor, E., Vondrovâ, A., & Sobalfk, Z. (2018). On the mechanism of high-temperature N2O decomposition over Fe-FER in the presence of NO. Catalysis Communications, 112, 58-62) showed that iron-FER type zeolites exhibited very high performance in N2O decomposition and that there was a positive effect of NO for decomposition even at 600-900°C. The FER structure that presents Al pairs resists high temperature DeN2O conditions at 900°C (Tabor, E., Mlekodaj, K., Sâdovskâ, G., Bernauer, M., Klein, P., Sazama, P.,... & Sobalfk, Z. (2019). Structural stability of metal containing ferrierite under the conditions of HT-N2O decomposition. Mi-croporous and Mesoporous Materials, 281, 15-22.)]. The presence of cation, especially iron, favors the decomposition, but increasing the content beyond 1% does not seem to favor the activity for a Fe-BEA zeolite (Chen, B., Liu, N., Liu, X., Zhang, R., Li, Y., Li, Y., & Sun, X. (2011). Study on the direct decomposition of nitrous oxide over Fe-beta zeolites: From experiment to theory. Catalysis today, 175(1), 245-255). The redox behavior of Fe species in Fe-ZSM-5 catalysts for N2O and NH3-SCR decomposition of NOx was analyzed by Sazama et al. (Sazama, P., Wichterlovâ, B., Tâbor, E., Sfastnÿ, P., Sathu, NK, Sobalfk, Z.,... & Vondrovâ, A. (2014). Tailoring of the structure of Fe-cationic species in Fe-ZSM-5 by distribution of Al atoms in the framework for N2O decomposition and NH3-SCR-NOx. Journal of catalysis, 312, 123-138.). More recently, CHA type zeolites have been studied by Zhang et al. (Zhang, T., Qiu, Y., Liu, G., Chen, J., Peng, Y., Liu, B., & Li, J. (2020). Nature of active Fe species and reaction mechanism over high-efficiency Fe / CHA catalysts in catalytic decomposition of N2O. Journal of Catalysis, 392, 322-335.); a higher activity of Fe / CHA catalysts was observed compared to the current Fe / zeolite state, i.e., Fe / FER. In addition, Fe / CHA catalysts showed significant hydrothermal stability at high temperature. Wang et al. compared a Fe-CHA with a Fe-Beta zeolite. The Fe / . SSZ-13 showed higher reaction rates in catalytic decomposition of N2O while Fe / Beta showed higher reaction rates in N2O reduction by NH3 (Wang, A., Wang, Y., Walter, E.D., Kukkadapu, R.K., Guo, Y., Lu, G.,... & Gao, F. (2018). Catalytic N2O decomposition and reduction by NH3 over Fe / Beta and Fe / SSZ-13 catalysts. Journal of catalysis, 358, 199-210.) It is possible to combine NOx and N2O reduction in a single reactor. In the case of a nitric acid plant, Graves et al. (Michael CE Graves & Alexander Sasonow (2010) Uhde EnviNOx® technology for NOX and N2O abatement: a contribution to reducing emissions from nitric acid plants, Journal of Integrative Environmental Sciences, 7:S1, 211-222) tested different configurations: a N2O decomposition stage, associated with an NH3-SCR stage; a DeNOx stage followed by N2O reduction with ammonia or with hydrocarbons (methane or propane).

[0008] The use of AFX structural type zeolites for NH3-SCR applications is known (Fickel, DW, & Lobo, RF (2009), The Journal of Physical Chemistry C, 114(3), 1633-1640 or patent application WO2020 / 212354 of the Applicant), but no work has evaluated the efficiency of catalysts in DeN2O or simultaneous DeNOx / DeN2O operation. Summary of the invention

[0009] The applicant has discovered that a catalyst based on a zeolite of structural type AFX and iron as transition metal, prepared according to a particular synthesis method, exhibited simultaneous conversion performances of NOx and N2O in the presence of a reducing agent, in particular over the temperature range of 300 to 500°C. The properties of direct decomposition of N2O using this catalyst are also particularly interesting from 450°C. This catalyst also exhibits high thermal and hydrothermal stability which allows it to operate at high temperatures, above 800°C.

[0010] The invention relates to a process for preparing a catalyst based on a zeolite of AFX structural type and iron comprising at least the following steps: i) the mixture in an aqueous medium of at least one source of at least one silicon oxide SiO2, of at least one source of at least one aluminum oxide A12O3, of an organic nitrogen compound R, also called specific structuring agent chosen from 1,6-bis(methylpiperidinium)hexane dihydroxide and 1,5-bis(methylpiperidinium)pentane dihydroxide, of at least one alkali metal and / or one alkaline-earth metal M of valence n, n being an integer greater than or equal to 1 and a source of iron (Fe), the reaction mixture having the following molar composition:

[0011] SiO2 / Al2O3 between 2 and 100, preferably between 12 and 40 H2O / SiO2 between 5 and 60, preferably between 10 and 40 R / SiO2 between 0.05 and 0.50, preferably between 0.10 and 0.40 M2 / nO / SiO2 between 0.05 and 0.40, preferably between 0.15 and 0.30, Fe2O3 / SiO2 between 0.01 and 0.10, preferably between 0.01 and 0.05, in which M is one or more alkali and / or alkaline earth metal(s) chosen from lithium, sodium, potassium, calcium, magnesium and the mixture of at least two of these metals, very preferably M is sodium, step i) being carried out for a duration greater than or equal to 10 minutes, allowing the production of a homogeneous mixture called precursor gel;

[0012] ii) the hydrothermal treatment of said precursor gel obtained at the end of step i) at a temperature between 120°C and 250°C, preferably between 150°C and 230°C, for a period between 2 and 12 hours, preferably between 2 and 10 hours until said zeolite of AFX structural type is formed. iii) optionally at least one ion exchange comprising bringing said zeolite obtained at the end of the previous step into contact with a solution comprising at least iron, in solution in reactive form with stirring at a temperature between 40 and 95°C, preferably between 45 and 90°C, for a period between 1 hour and 2 days; iv) a heat treatment comprising drying said AFX zeolite obtained in step ii) or iii) at a temperature of between 20 and 150°C, preferably between 60 and 100°C, for a period of between 2 and 24 hours, followed by at least one calcination, in air, optionally dry, at a temperature of between 450 and 700°C, preferably between 500 and 600°C for a period of between 2 and 20 hours, preferably between 6 and 16 hours, more preferably between 8 and 13 hours, the flow rate of optionally dry air being preferably between 0.5 and 1.5 L / h / g of solid to be treated, more preferably between 0.7 and 1.2 L / h / g of solid to be treated.

[0013] At least one source of SiO2 and / or at least one source of Al2O3 may be a zeolite of structural type FAU having a molar ratio SiO2 / Al2O3 of between 2.00 and 100.

[0014] Steps iii) and iv) may be reversed, and possibly repeated.

[0015] The AFX zeolite obtained in step ii) can directly undergo a step iv) of heat treatment, then at least one ion exchange with an acid, or a compound such as ammonium chloride, sulfate or nitrate to obtain a calcined AFX zeolite in protonated form, before step iii) of ion exchange with iron.

[0016] Crystalline seeds of a zeolite of structural type AFX can be added to the reaction mixture of step i), in an amount of between 0.01 and 10% of the total mass of the sources of the tetravalent (Si) and trivalent (Al) elements in their oxide form (SiO2 and Al2O3) used in the reaction mixture, said crystalline seeds not being not included in the total mass of sources of tetravalent and trivalent elements.

[0017] Step i) may comprise a step of maturing the reaction mixture at a temperature of between 20 and 80°C, with or without stirring, for a period of between 30 minutes and 24 hours.

[0018] The AFX zeolite obtained in step ii) may undergo a heat treatment comprising drying the solid at a temperature of between 20 and 150°C, preferably between 60 and 100°C, for a period of between 2 and 24 hours, followed by at least one calcination, in air, optionally dry, at a temperature of between 450 and 700°C, preferably between 500 and 600°C for a period of between 2 and 20 hours, preferably between 6 and 16 hours, more preferably between 8 and 13 hours, the flow rate of optionally dry air being preferably between 0.5 and 1.5 L / h / g of solid to be treated, more preferably between 0.7 and 1.2 L / h / g of solid to be treated, then an ion exchange with an acid, or a compound such as ammonium chloride, sulfate or nitrate for obtain a calcined AFX zeolite in protonated form, before step iii) of ion exchange with iron.

[0019] The iron content introduced by synthesis step i) and optionally ion exchange step iii) may be between 0.5 and 6% by mass, preferably between 0.5 and 5% by mass, more preferably between 1 and 4% by mass, relative to the total mass of the final anhydrous catalyst.

[0020] The invention also relates to a catalyst based on an AFX zeolite and Fe for the decomposition or reduction of N2O or the simultaneous reduction of NOx and N2O capable of being obtained or directly obtained by the preparation process according to any one of its variants.

[0021] The total iron content of the catalyst may be between 0.5 and 6% by mass, preferably between 0.5 and 5% by mass, more preferably between 1 and 4% by mass, relative to the total mass of the final anhydrous catalyst.

[0022] The invention also relates to a process for decomposing N2O or reducing N2O or simultaneously reducing NOx and N2O by a reducing agent such as NH3 or H2, in which the gas to be treated is brought into contact with a catalyst according to any one of the variants described.

[0023] The catalyst may be formed by deposition in the form of a coating, on a honeycomb structure or a plate structure, or said catalyst is in the form of an extrudate or a bead, containing up to 100% of said catalyst.

[0024] The honeycomb structure may be formed of parallel channels open at both ends or comprise porous filtering walls for which the adjacent parallel channels are alternately blocked on either side of the channels.

[0025] The quantity of catalyst deposited on said structure can be between 50 and 250 g / L for filter structures and between 80 and 300 g / L for structures with open channels.

[0026] The catalyst may be combined with a binder such as ceria, zirconium oxide, alumina, non-zeolitic silica-alumina, titanium oxide, a mixed oxide of ceria-zirconia type, a tungsten oxide and / or a spinel to be shaped by deposition in the form of a coating, said coating preferably being able to be combined with another coating having capacities for adsorbing pollutants, in particular NOx, for reducing pollutants, in particular NOx, or promoting the oxidation of pollutants.

[0027] Said catalyst can be integrated: - in an exhaust line of an internal combustion engine operating on carbonaceous or non-carbonaceous fuels, or - in a reactor to treat industrial fumes. LIST OF FIGURES [Figure 1]

[0028] [Fig.l] represents the chemical formulas of the nitrogenous organic compounds R which can be chosen as the structuring agent used in the synthesis process according to the invention, 1,6-bis(methylpiperidinium)hexane dihydroxide and 1,5-bis(methylpiperidinium)pentane dihydroxide. [Figure 2]

[0029] [Fig.2] represents the X-ray diffraction diagram of the zeolite of structural type AFX obtained according to example 3.

[0030] Other characteristics and advantages of the synthesis process according to the invention, of the catalyst according to the invention and of the use according to the invention will appear on reading the following description of non-limiting examples of embodiments, with reference to the figures appended and described below. Description of the embodiments

[0031] The invention relates to a process for preparing a catalyst based on a zeolite of AFX structural type and iron as transition metal comprising at least the following steps: i) the mixture in an aqueous medium of at least one source of at least one silicon oxide SiO2, of at least one source of at least one aluminum oxide A12O3, and / or of at least one zeolite of structural type FAU having a molar ratio SiO2 / Al2O3 of between 2.00 and 100, of an organic nitrogen compound R, also called specific structuring agent chosen from 1,6-bis(methylpiperidinium)hexane dihydroxide and 1,5-bis(methylpiperidinium)pentane dihydroxide, of at least one alkali metal and / or a metal alkaline earth metal M of valence n, n being an integer greater than or equal to 1 and a source of iron Fe, the reaction mixture having the following molar composition: SiO2 / Al2O3 between 2 and 100, preferably between 12 and 40 H2O / SiO2 between 5 and 60, preferably between 10 and 40 R / SiO2 between 0.05 to 0.50, preferably between 0.10 and 0.40 M2 / nO / SiO2 between 0.05 to 0.40, preferably between 0.15 and 0.30, Fe2O3 / SiO2 between 0.01 to 0.10, preferably between 0.01 and 0.05, in which M is one or more alkali and / or alkaline earth metal(s) chosen from lithium, sodium, potassium, calcium, magnesium and the mixture of at least two of these metals, very preferably M is sodium, step i) being carried out for a duration greater than or equal to 10 minutes, allowing the production of a homogeneous mixture called precursor gel comprising all the components of the reaction mixture; ii) hydrothermal treatment of said precursor gel obtained at the end of step i) at a temperature between 120°C and 250°C, preferably between 150°C and 230°C, for a period between 2 and 12 hours, preferably between 2 and 10 hours until said zeolite of AFX structural type is formed. iii) optionally an ion exchange comprising bringing said AFX zeolite obtained at the end of the previous step into contact with a solution comprising iron in a form capable of releasing iron, in solution in reactive form with stirring at a temperature between 40 and 95°C for a period between 1 hour and 2 days; iv) heat treatment advantageously comprising drying the AFX zeolite obtained at the end of the previous step at a temperature of between 20 and 150°C, preferably between 60 and 100°C, for a period of between 2 and 24 hours, followed by at least one calcination, in air, optionally dry, at a temperature of between 450 and 700°C, preferably between 500 and 600°C for a period of between 2 and 20 hours, preferably between 6 and 16 hours, more preferably between 8 and 13 hours, the flow rate of optionally dry air being preferably between 0.5 and 1.5 L / h / g of solid to be treated, more preferably between 0.7 and 1.2 L / h / g of solid to be treated.

[0032] Steps iii) and iv) may be reversed, and possibly repeated.

[0033] In a preferred embodiment of this variant, the AFX zeolite obtained in step ii) directly undergoes a heat treatment according to step iv), then at least one ion exchange with an acid, or a compound such as ammonium chloride, sulfate or nitrate to obtain a calcined AFX zeolite in protonated form, before step iii) of ion exchange with iron.

[0034] Crystal seeds of a zeolite of structural type AFX can be added to the reaction mixture of step i), preferably in an amount of between 0.01 and 10% of the total mass of the sources of said tetravalent (Si) and trivalent (Al) elements in their oxide form (SiO2 and Al2O3) used in the reaction mixture, said crystalline seeds not being taken into account in the total mass of the sources of the tetravalent and trivalent elements.

[0035] Step i) may comprise a step of maturing the reaction mixture at a temperature of between 20 and 80°C, with or without stirring, for a period of between 30 minutes and 24 hours.

[0036] Step iii) of ion exchange is not necessarily necessary if the total quantity of iron source has been introduced into the synthesis.

[0037] The iron content introduced in the synthesis step i) and optionally by the ion exchange step iii) is advantageously between 0.5 and 6% by mass, preferably between 0.5 and 5% by mass, more preferably between 1 and 4% by mass, relative to the total mass of the final anhydrous catalyst.

[0038] The invention also relates to the catalyst based on an AFX zeolite and Fe capable of being obtained or directly obtained by the preparation process.

[0039] The iron content of the catalyst obtained is advantageously between 0.5 and 6% by mass, preferably between 0.5 and 5% by mass, more preferably between 1 and 4% by mass, relative to the total mass of the final anhydrous catalyst.

[0040] The invention also relates to the use of the catalyst described above or the use of the catalyst obtainable or directly obtainable by the preparation process for the direct decomposition of N2O, the reduction of N2O or the simultaneous reduction of NOx and N2O by a reducing agent such as NH3 or H2.

[0041] The catalyst can be shaped directly by extrusion in the form of pellets, balls, or any other form, or by deposition in the form of a coating, on a honeycomb structure or a plate structure.

[0042] The honeycomb structure may be formed of parallel channels open at both ends or may comprise porous filtering walls for which the adjacent parallel channels are alternately blocked on either side of the channels.

[0043] The quantity of catalyst deposited on said structure can advantageously be between 50 and 250 g / L for filtering structures and between 80 and 300 g / L for structures with open channels.

[0044] The catalyst may be combined with a binder such as ceria, zirconium oxide, alumina, non-zeolitic silica-alumina, titanium oxide, a mixed oxide of the ceria-zirconia type, a tungsten oxide and / or a spinel to be shaped by extrusion or by deposition in the form of a coating.

[0045] Said coating may be associated with another coating having pollutant adsorption capacities, in particular NOx, and pollutant reduction capacities, in particular particular NOx or promoting the oxidation of pollutants such as CO or hydrocarbons.

[0046] Said catalyst may be in the form of a bead or extrudate or any other form known to those skilled in the art, containing up to 100% of said catalyst.

[0047] The structure coated with said catalyst or obtained by extrusion of said catalyst can be integrated: - in an exhaust line of an internal combustion engine operating on carbon-based or non-carbon fuels, such as NH3, H2, etc. (non-exhaustively) - in a reactor to treat industrial fumes. In a nitric acid plant, it can be integrated in the tertiary stage, either to sequentially eliminate N2O by decomposition and NOx with the addition of a reducing agent such as NH3, or to concomitantly eliminate NOx and N2O with the addition of a reducing agent. The catalyst

[0048] The catalyst according to the invention comprises at least one AFX type zeolite, and iron (Fe) as additional transition metal.

[0049] The total iron content in the catalyst is advantageously between 0.5 and 6% by mass, preferably between 0.5 and 5% by mass, and even more preferably between 1 and 4% by mass, relative to the total mass of the final catalyst, in its anhydrous form.

[0050] The catalyst according to the invention may also comprise other elements, such as for example alkali and / or alkaline earth metals, for example sodium, originating in particular from the synthesis, in particular of the compounds of the reaction medium of step i) of the process for preparing said catalyst. Process for preparing the catalyst Step i) mixing

[0051] This step involves mixing in an aqueous medium at least one source of at least one silicon oxide SiO2, at least one source of at least one aluminum oxide A12O3 - at least one of these sources being able to be at least one zeolite of structural type FAU having a SiO2 / Al2O3 molar ratio of between 2.00 and 100 -, an organic nitrogen compound R, also called a specific structuring agent, chosen from 1,6-bis(methylpiperidinium)hexane dihydroxide and 1,5-bis(methylpiperidinium)pentane dihydroxide, at least one alkali metal and / or an alkaline earth metal M of valence n, n being an integer greater than or equal to 1 and a source of iron (Fe), the reaction mixture having the following molar composition: SiO2 / Al2O3 of between 2 and 100, preferably between 12 and 40 H2O / SiO2 between 5 and 60, preferably between 10 and 40 R / SiO2 between 0.05 to 0.50, preferably between 0.10 and 0.40 M2 / nO / SiO2 between 0.05 and 0.40, preferably between 0.15 and 0.30, Fe2O3 / SiO2 between 0.01 and 0.10, preferably between 0.01 and 0.05, in which M is one or more alkali and / or alkaline earth metal(s) chosen from lithium, sodium, potassium, calcium, magnesium and the mixture of at least two of these metals, very preferably M is sodium, step i) being carried out for a time allowing the obtaining of a homogeneous mixture called precursor gel, once all the components have been introduced into the reaction mixture.

[0052] In the molar composition of the reaction mixture above and throughout the description: SiO2 denotes the molar quantity of the tetravalent element silicon (Si) expressed in oxide form, and Al2O3 denotes the molar quantity of the trivalent element aluminum (Al) expressed in oxide form, H2O the molar quantity of water present in the reaction mixture, R the molar quantity of said organic nitrogen compound, M2 / nO the molar quantity expressed in the oxide form of M2 / nO by the source of alkali metal and / or alkaline earth metal. Fe2O3 the molar quantity expressed in oxide form by the source of Fe.

[0053] According to the invention, at least one source of SiO2 oxide is incorporated into the mixture for carrying out step (i) of the preparation process. The silicon source may be any of said sources commonly used for the synthesis of zeolites, for example powdered silica, silicic acid, colloidal silica, dissolved silica or tetraethoxysilane (TEOS). Among the powdered silicas, precipitated silicas may be used, in particular those obtained by precipitation from an alkali metal silicate solution, pyrogenic silicas, for example "CAB-O-SIL" and silica gels. Colloidal silicas with different particle sizes can be used, for example with an average equivalent diameter between 10 and 15 nm or between 40 and 50 nm, such as those marketed under registered trademarks such as "LUDOX".As a source of SiO2 oxide, it is also possible to use at least one zeolite of structural type FAU having a molar ratio SiO2 / A12O3 between 2.00 and 100 alone or in a mixture with other sources of SiO2.

[0054] Preferably, the silicon source is a zeolite of structural type FAU.

[0055] The aluminum source is preferably aluminum hydroxide or an aluminum salt, for example chloride, nitrate, or sulfate, a sodium aluminate, an aluminum alkoxide, or alumina itself, preferably in hydrated or hydratable form, such as for example colloidal alumina, pseudo-doboehmite, gamma alumina or alpha or beta trihydrate. Mixtures of the sources mentioned above can also be used.

[0056] As source of oxide A12O3 it is also possible to use at least one zeolite of structural type FAU having a molar ratio SiO2 / Al2O3 of between 2.00 and 100 alone or in a mixture with other sources of A12O3.

[0057] In accordance with the invention, R is an organic nitrogen compound chosen from 1,6-bis(methylpiperidinium)hexane dihydroxide and 1,5-bis(methylpiperidinium)pentane dihydroxide, said compound being incorporated into the reaction mixture for the implementation of step (i), as an organic structuring agent.

[0058] According to the invention, at least one source of at least one alkali and / or alkaline-earth metal M of valence n is used in the reaction mixture of step i), n being an integer greater than or equal to 1, M preferably being chosen from lithium, potassium, sodium, magnesium and calcium and the mixture of at least two of these metals. Very preferably, M is sodium.

[0059] Preferably, the source of at least one alkali and / or alkaline earth metal M is sodium hydroxide.

[0060] The iron source introduced during step i) is a species capable of dissociating in an aqueous medium, notably chosen from sulfates, nitrates, chlorides, oxalates, organometallic complexes of Fe such as Fe TEPA (tetraethylenepentamine) or Fe TETA (triethylenetetramine) or mixtures thereof. Preferably, the species capable of releasing a transition metal is Fe TEPA (tetraethylenepentamine) or Fe TETA (triethylenetetramine) or mixtures thereof.

[0061] It may be advantageous to add seeds of a zeolite of AFX structural type to the reaction mixture during said step i) of the process of the invention in order to reduce the time required for the formation of crystals of a zeolite of AFX structural type and / or the total crystallization time. Said crystal seeds also promote the formation of said zeolite of AFX structural type to the detriment of impurities. Such seeds comprise crystallized solids, in particular crystals of a zeolite of AFX structural type. The crystal seeds are generally added in a proportion of between 0.01 and 10% of the total mass of the sources of said tetravalent, silicon, and trivalent, aluminum elements, in oxide form used in the reaction mixture, said crystal seeds not being taken into account in the total mass of the sources of the tetravalent and trivalent elements.Said germs are also not taken into account for determining the composition of the reaction mixture and / or the gel, defined further, that is to say in the determination of the different molar ratios of the composition of the reaction mixture.

[0062] Step i) of mixing is carried out until a homogeneous mixture is obtained, preferably for a period greater than or equal to 10 minutes, preferably with stirring by any system known to those skilled in the art with low or high shear rate.

[0063] At the end of step i), a homogeneous precursor gel is obtained.

[0064] It may be advantageous to carry out a ripening of the reaction mixture before the hydrothermal crystallization during said step i) of the process of the invention in order to control the size of the crystals of a zeolite of AFX structural type. Said ripening also promotes the formation of said zeolite of AFX structural type to the detriment of impurities. The ripening of the reaction mixture during said step i) of the process of the invention may be carried out at room temperature or at a temperature between 20 and 80°C with or without stirring, for a duration advantageously between 30 minutes and 24 hours. Step ii) of hydrothermal treatment

[0065] In accordance with step ii) of the process according to the invention, the precursor gel obtained at the end of step i) is subjected to a hydrothermal treatment, preferably carried out at a temperature of between 120°C and 250°C for a period of between 2 and 12 hours, until said zeolite of AFX structural type (or “crystalline solid”) is formed.

[0066] The precursor gel is advantageously placed under hydrothermal conditions under an autogenous reaction pressure, optionally by adding gas, for example nitrogen, at a temperature preferably between 120°C and 250°C, preferably between 150°C and 230°C, until complete crystallization of a zeolite of AFX structural type.

[0067] The time required to obtain crystallization varies between 2 and 12 hours, preferably between 2 and 10 hours, and more preferably between 2 and 8 hours.

[0068] The reaction is generally carried out with stirring or without stirring, preferably with stirring. As stirring system, any system known to those skilled in the art can be used, for example, inclined blades with counterblades, stirring turbines, Archimedes screws.

[0069] It is also advantageous to obtain the protonated form of the zeolite of structural type AFX after step ii). Said protonated form can be obtained by carrying out at least one ion exchange with an acid, in particular a strong mineral acid such as hydrochloric, sulfuric or nitric acid, or with a compound such as ammonium chloride, sulfate or nitrate, before the ion exchange of step iii) with iron.

[0070] In this embodiment, the AFX structural type zeolite obtained at the end of step ii) directly undergoes a heat treatment (step iv) comprising drying at a temperature of between 20 and 150°C, preferably between 60 and 100°C, for a duration of between 2 and 24 hours, followed by at least one calcination, in air, optionally dry, at a temperature of between 450 and 700°C, preferably between 500 and 600°C for a duration of between 2 and 20 hours, preferably between 6 and 16 hours, more preferably between 8 and 13 hours, the flow rate of optionally dry air preferably being between 0.5 and 1.5 L / h / g of solid to be treated, more preferably between 0.7 and 1.2 L / h / g of solid to be treated. The dried and calcined AFX zeolite then undergoes an ion exchange with an acid, or a compound such as ammonium chloride, sulfate or nitrate to obtain a calcined AFX zeolite in protonated form, before step iii) of ion exchange with iron. Step iii) exchange

[0071] The process for preparing the catalyst according to the invention may comprise an ion exchange step, comprising bringing the crystallized solid obtained at the end of the previous step, i.e. the AFX zeolite obtained at the end of step ii) or the dried and calcined AFX zeolite obtained at the end of step iv) in the case where steps iii) and iv) are reversed, or the calcined AFX zeolite in protonated form, into contact with at least one solution comprising at least one species capable of releasing Fe in solution in reactive form, with stirring at a temperature of between 40 and 95°C for a period of between 1 hour and 2 days, advantageously for a period of between 6 and 12 hours, the concentration of said species capable of releasing Fe in said solution being a function of the quantity of Fe that it is desired to incorporate into said crystallized solid.

[0072] According to the invention, by "species capable of releasing Fe" is meant a species capable of dissociating in an aqueous medium, such as for example sulfates, nitrates, chlorides, oxalates, organometallic complexes of Fe or their mixtures. Preferably, the species capable of releasing a transition metal is a sulfate or a nitrate of said transition metal.

[0073] At the end of the exchange, the solid obtained is advantageously filtered, washed and then dried to obtain said catalyst in powder form.

[0074] The total quantity of Fe (iron) contained in said final catalyst is between 0.5 and 6% by mass relative to the total mass of the catalyst in its anhydrous form.

[0075] According to one embodiment, the catalyst according to the invention is prepared by a process comprising a step iii) of ion exchange, the zeolite or the dried and calcined zeolite, optionally protonated, being brought into contact with a solution comprising a species capable of releasing Fe in solution in reactive form. Advantageously, the total amount of Fe contained in said final catalyst, i.e. at the end of the preparation process according to the invention, is between 0.5 and 6%, preferably between 1 and 4% by mass, all percentages being mass percentages relative to the total mass of the final catalyst according to the invention in its anhydrous form, obtained at the end of the preparation process.

[0076] The ion exchange step iii) is not necessarily necessary if the iron source has been introduced entirely during the synthesis. In one embodiment, all the iron is introduced during step i) of zeolite synthesis. Step iv) heat treatment

[0077] The preparation process according to the invention comprises at least one step iv) of heat treatment carried out at the end of the previous step, i.e. at the end of step ii) of hydrothermal treatment or, where appropriate, at the end of step iii) of ion exchange. Step iii) of the preparation process can advantageously be interchanged with step iv). Each of the two steps iii) and iv) can also optionally be repeated.

[0078] Said heat treatment step iv) comprises drying the solid at a temperature of between 20 and 150°C, preferably between 60 and 100°C, advantageously for a period of between 2 and 24 hours, followed by at least one calcination, in air, optionally dry, at a temperature advantageously of between 450 and 700°C, preferably between 500 and 600°C for a period of between 2 and 20 hours, preferably between 6 and 16 hours, more preferably between 8 and 13 hours, the flow rate of optionally dry air being preferably of between 0.5 and 1.5 L / h / g of solid to be treated, more preferably of between 0.7 and 1.2 L / h / g of solid to be treated. The calcination may be preceded by a gradual rise in temperature.

[0079] The catalyst obtained at the end of heat treatment step iv) is free of any organic species, in particular free of the organic structuring agent R.

[0080] In particular, the catalyst obtained by a process comprising at least steps i), ii), iii), and iv) previously described has improved properties for the conversion of N2O.

[0081] Characterization of the catalyst prepared according to the invention

[0082] The catalyst comprises a zeolite of AFX structure according to the classification of the International Zeolite Association (IZA), exchanged by at least one transition metal. This structure is characterized by X-ray diffraction (XRD).

[0083] The X-ray diffraction pattern (XRD) is obtained by radiocrystal-lographic analysis using a diffractometer using the classical powder method with Ka radiation, of copper (X = 1.5406Â). From the position of the diffraction peaks represented by the angle 20, the characteristic reticular equidistances dhki of the sample are calculated using the Bragg relation. The measurement error A(d hki) on dhki is calculated using the Bragg relation as a function of the absolute error A(20) assigned to the measurement of 20. An absolute error A(20) equal to + 0.02° is commonly accepted. The relative intensity Irei assigned to: each value of dhki is measured according to the height of the corresponding diffraction peak. Comparison of the diffraction pattern with the records in the ICDD (International Centre for Diffraction Data) database using software such as DIFFRACT.SUITE allows us to also allows the identification of the crystalline phases present in the material obtained.

[0084] The qualitative and quantitative analysis of the chemical species present in the materials obtained is carried out by X-ray fluorescence spectrometry (FX). This is a chemical analysis technique using a physical property of matter, X-ray fluorescence. The spectrum of X-rays emitted by the matter is characteristic of the composition of the sample; by analyzing this spectrum, the elemental composition, i.e. the mass concentrations of elements, can be deduced.

[0085] The loss on ignition (LAI) of the catalyst obtained after the drying step (and before calcination) or after the calcination step of step iv) of the process according to the invention is generally between 5 and 18% by weight. The loss on ignition of a sample, designated by the acronym LAI, corresponds to the difference in mass of the sample before and after a heat treatment at 1000°C for 2 hours. It is expressed in % corresponding to the percentage loss of mass. The loss on ignition generally corresponds to the loss of solvent (such as water) contained in the solid but also to the elimination of organic compounds contained in the mineral solid constituents.

[0086] Process for decomposing N2O or reducing NOx and N2O by a reducing agent such as NH3 using the catalyst according to the invention

[0087] The invention also relates to the use of the catalyst according to the invention, directly prepared or capable of being prepared by the process described above for the direct decomposition of N2O or the simultaneous reduction of NOx and N2O by a reducing agent such as NH3 or a hydrocarbon, advantageously formed by deposition in the form of a coating ("washcoat" according to English terminology) on a honeycomb structure mainly for mobile applications or a plate structure which is found particularly for stationary applications. The invention can also be formed in the form of extrudates or beads.

[0088] The honeycomb structure is formed of parallel channels open at both ends (flow-through in English) or comprises porous filtering walls and in this case the adjacent parallel channels are alternately blocked on either side of the channels in order to force the gas flow to pass through the wall (wall-flow monolith in English). Said honeycomb structure thus coated constitutes a catalytic bread. Said structure can be composed of cordierite, silicon carbide (SiC), aluminum titanate (AITi), alpha alumina, mullite or any other material whose porosity is between 30 and 70%. Said structure can be made of metal sheet, stainless steel containing chromium and aluminum, FeCrAl type steel.

[0089] The quantity of catalyst according to the invention deposited on said structure is between 50 and 250 g / L for the filtering structures and between 80 and 300 g / L for the structures with open channels.

[0090] The coating itself (“washcoat”) comprises the catalyst according to the invention, advantageously combined with a binder such as ceria, zirconium oxide, alumina, non-zeolitic silica-alumina, titanium oxide, a mixed oxide of ceria-zirconia type, a tungsten oxide, a spinel. Said coating is advantageously applied to said structure by a deposition method (washcoating in English) which consists of dipping the monolith in a suspension (slurry in English) of catalyst powder according to the invention in a solvent, preferably water, and potentially binders, metal oxides, stabilizers or other promoters. This dipping step can be repeated until the desired quantity of coating is reached. In certain cases the slurry can also be sprayed within the monolith. Once the coating is deposited, the monolith is calcined at a temperature of 300 to 600°C for 1 to 10 hours.

[0091] Said structure may be coated with one or more coatings. The coating comprising the catalyst according to the invention is advantageously associated with, i.e. covers one or is covered by, another coating having capacities for adsorbing pollutants, in particular NOx, for reducing pollutants, in particular NOx, or promoting the oxidation of pollutants, in particular that of ammonia.

[0092] Another possibility is to put the catalyst in the form of an extrudate or a bead or any other form known to those skilled in the art. In this case, the structure obtained can contain up to 100% of catalyst according to the invention.

[0093] The catalyst support used in the process according to the invention can advantageously be shaped by any technique known to those skilled in the art. The shaping can advantageously be carried out, for example, by extrusion, by pelletizing, by the oil-drop method, by granulation on a rotating plate or by any other method well known to those skilled in the art. The supports thus obtained can be in different shapes and sizes. Advantageously, the different constituents of the support or of the catalyst can be shaped by a mixing step to form a paste then extrusion of the paste obtained, or by mixing powders then pelletizing, or by any other known method of agglomerating a powder containing alumina. The supports thus obtained can be in different shapes and sizes. Preferably, the shaping is carried out by mixing and extrusion.

[0094] Furthermore, the use of additives can advantageously be implemented to facilitate shaping and / or improve the final mechanical properties of the supports as is well known to those skilled in the art. As examples of additives, mention may in particular be made of cellulose, carboxymethylcellulose, carboxyethylcellulose, tall oil, xanthan gums, surfactants, flocculants such as polyacrylamides, carbon black, starches, stearic acid, polyacrylic alcohol, polyvinyl alcohol, biopolymers, glucose, polyethylene glycols, etc.

[0095] Water can advantageously be added or removed to adjust the viscosity of the paste to be extruded. This step can advantageously be carried out at any stage of the kneading step.

[0096] To adjust the solids content of the extrudable paste to make it extrudable, a predominantly solid compound, preferably an oxide or a hydrate, may also be added. A hydrate is preferably used, and even more preferably an aluminum hydrate. The loss on ignition of this hydrate is advantageously greater than 15%.

[0097] The extrusion of the paste resulting from the kneading step can advantageously be carried out by any conventional, commercially available tool. The paste resulting from the kneading is advantageously extruded through a die, for example using a piston or a single-screw or double-screw extrusion machine. The extrusion can advantageously be carried out by any method known to those skilled in the art.

[0098] The catalyst supports according to the invention are generally in the form of cylindrical or polylobed extrudates such as bilobed, trilobed, polylobed of straight or twisted shape, but can optionally be manufactured and used in the form of crushed powders, tablets, rings, balls and / or wheels. Preferably, the catalyst supports according to the invention have the form of spheres or extrudates. Advantageously, the support is in the form of extrudates with a diameter of between 0.5 and 5 mm and more particularly between 0.7 and 2.5 mm. The shapes can be cylindrical (which can be hollow or not) and / or twisted cylindrical and / or multi-lobed (2, 3, 4 or 5 lobes for example) and / or rings. The multi-lobed shape is advantageously used in a preferred manner. Advantages of the invention

[0099] The catalyst according to the invention, based on a zeolite of structural type AFX and iron (Fe), has improved properties compared to the catalysts of the prior art, in particular in terms of N2O and also NOx reduction performance. Furthermore, the hydrothermal resistance of the catalyst obtained makes it possible to envisage use at temperatures above 600°C. EXAMPLES

[0100] Example 1: Preparation of 1,6-bis(methylpiperidinium)hexane dihydroxide (structuring R).

[0101] 50 g of 1,6-dibromohexane (0.20 mol, 99%, Alfa Aesar) are added to a flask of 1 L containing 50 g of N-methylpiperidine (0.51 mol, 99%, Alfa Aesar) and 200 mL of ethanol. The reaction medium is stirred and refluxed for 5 hours. The mixture is then cooled to room temperature and filtered. The mixture is poured into 300 mL of cold diethyl ether, then the precipitate formed is filtered and washed with 100 mL of diethyl ether. The solid obtained is recrystallized from an ethanol / ether mixture. The solid obtained is dried under vacuum for 12 hours. 71 g of a white solid is obtained (i.e. a yield of 80%).

[0102] The product has the expected 'H NMR spectrum. 'H NMR (D2O, ppm / TMS): 1.27

[0103] (4H,m); 1.48 (4H,m); 1.61 (4H,m); 1.70 (8H,m); 2.85 (6H,s); 3.16 (12H,m).

[0104] 18.9 g of Ag2O (0.08 mole, 99%, Aldrich) are added to a Teflon beaker of 250 mL containing 30 g of the prepared structuring agent l,6-bis(methylpiperidinium)hexane dibromide (0.07 mol) and 100 mL of deionized water. The reaction medium is stirred away from light for 12 hours. The mixture is then filtered. The filtrate obtained is composed of an aqueous solution of l,6-bis(methylpiperidinium)hexane dihydroxide. The determination of this species is carried out by proton NMR using formic acid as a standard.

[0105] Example 2: preparation of a zeolite of AFX structural type according to the invention with 2% Fe.

[0106] In this example, a zeolite of structural type AFX is synthesized with incorporation of a portion of the Fe in step i) of zeolite synthesis using an Fe-Tetraethylenepentamine (TEPA) complex and the remainder by ion exchange.

[0107] Preparation of AFX zeolite containing part of the Fe:

[0108] 75.5 g of a zeolite of structural type FAU (CBV780, SiO2 / Al2O3= 82.83, Zeolyst,PAF = 17.5%) were mixed with 270.2 g of an aqueous solution of l,6-bis(methylpiperidinium)hexane dihydroxide (20% by weight) prepared according to Example 1. 438.8 g of deionized water were added to the previous mixture, the resulting preparation was kept stirring for 10 minutes. 12.2 g of sodium hydroxide (99.5% by weight, Aldrich) were added to the synthesis mixture and kept stirring for 15 minutes. Subsequently, 3.5 g of amorphous aluminum hydroxide gel (A1(OH)3 amorphous gel, 58.55% by weight of A12O3, Merck), were incorporated into the synthesis mixture, which was kept stirring for half an hour. Subsequently, 1.07 g of Fe(NO3)3*9H2O (99% by weight, Prolabo) and 0.51 g of TEPA (BASF) are incorporated into the synthesis mixture, which is kept stirring for half an hour.The molar composition of the mixture is as follows: 100 SiO2: 3.125 Al2O3: 16.7 R: 15 Na2O: 3673 H2O: 0.95 Fe2O3: 1.91 TEPA, i.e. a SiO2 / Al2O3 ratio of 32. To this mixture are added seeds of an AFX type zeolite (6.1 g of AFX zeolite with a SiO2 / Al2O3 molar ratio of 10). The precursor gel is then transferred, after homogenization, into a stainless steel reactor. 1000 mL equipped with a stirring system with four inclined blades. The reactor is closed and then heated for 24 hours at 170°C while stirring at 250 - 300 rpm. The crystallized product obtained is filtered, washed with deionized water and then dried overnight at 100°C. The loss on ignition of the dried solid is 14%. The solid is then introduced into a muffle furnace where a calcination step is carried out: the calcination cycle includes a temperature rise of 1.5°C / min up to 200°C, a hold at 200°C maintained for 2 hours, a temperature rise of 1°C / min up to 550°C followed by a hold at 550°C maintained for 8 hours and then a return to room temperature.

[0109] The calcined solid product was analyzed by X-ray diffraction and identified as consisting of a zeolite of structural type AFX (ICDD sheet, PDF 04-011-1869), with a purity greater than 99% by weight. The product has a SiO2 / Al2O3 molar ratio of 15 as determined by X-ray fluorescence, and a mass percentage of Fe of 0.2% as determined by FX.

[0110] The calcined AFX zeolite is then brought into contact with a 3 molar NH4NO3 solution for 1 hour with stirring at 80°C. The ratio between the volume of NH4NO3 solution and the mass of solid is 10. The solid obtained is filtered and washed and the exchange procedure is repeated two more times under the same conditions. The final solid is separated, washed and dried for 12 hours at 100°C. A DRX analysis shows that the product obtained is a pure AFX structural type zeolite (ICDD sheet, PDF 04-011-1869).

[0111] The AFX zeolite in ammoniacal form is treated under air flow at 550°C for 8 hours with a temperature rise ramp of 1°C / min. The product obtained is an AFX zeolite in protonated form.

[0112] Ionic exchange with Fe:

[0113] The calcined AFX zeolite in protonated form is brought into contact with a solution of Fe(NO3)3 (1.5*104 molar) for 16 hours with stirring at 80°C. The ratio between the volume of exchange solution (in mL) and the mass of zeolite to be treated (in grams) is 160. The final solid is separated, washed and dried for 12 hours at a temperature of 100°C.

[0114] The exchanged solid obtained after contact with the Fe(NO3)3 solution is calcined under air flow at 550°C for 8 hours.

[0115] The calcined solid product is analyzed by X-ray diffraction and identified as a zeolite of structural type AFX (ICDD file, PDF 04-011-1869).

[0116] The product has a SiO2 / Al2O3 molar ratio of 15 and a mass percentage of Fe of 2% as determined by FX.

[0117] The catalyst obtained is denoted FeAFX2.

[0118] Example 3: preparation of a zeolite of structural type AFX according to the invention 3% Fe

[0119] In this example, a zeolite of structural type AFX is synthesized with incorporation of Fe in the synthesis of the zeolite using a Fe-Tetraethylenepentamine (TEPA) complex.

[0120] 10.1 g of a zeolite of structural type FAU (CBV720, SiO2 / Al2O3= 33.47, Zeolyst, PAF = 13.5%) were mixed with 16.02 g of an aqueous solution of l,6-bis(methylpiperidinium)hexane dihydroxide (21.9% by weight) prepared according to Example 1. 69.6 g of deionized water are added to the previous mixture, the preparation obtained is kept stirring for 10 minutes. 2.7 g of sodium hydroxide (99.5% by weight, Aldrich) are added to the synthesis mixture and kept stirring for 15 minutes. Subsequently, 1.42 g of Fe(NO3)3*9H2O (99% by weight, Prolabo) and 0.68 g of TEPA (BASF), are incorporated into the synthesis mixture, which is kept stirring for half an hour. The molar composition of the mixture is as follows: 100 SiO2: 2.99 A12O3: 8 R: 24.1 Na2O: 3401 H2O: 1.27 Fe2O3: 2.54 TEPA, i.e. a SiO2 / Al2O3 ratio of 33.4. To this mixture are added seeds of an AFX type zeolite (0.83 g of AFX zeolite with a SiO2 / Al2O3 molar ratio of 10).The precursor gel is then transferred, after homogenization, into a 160 mL stainless steel reactor equipped with a four-blade inclined stirring system. The reactor is closed and then heated for 26 hours at 180°C with stirring at 250-300 rpm. The crystallized product obtained is filtered, washed with deionized water and then dried overnight at 100°C. The loss on ignition of the dried solid is 13%. The solid is then introduced into a muffle furnace where a calcination step is carried out: the calcination cycle includes a temperature rise of 1.5°C / min up to 200°C, a hold at 200°C maintained for 2 hours, a temperature rise of 1°C / min up to 550°C followed by a hold at 550°C maintained for 8 hours and then a return to room temperature.

[0121] The calcined solid product was analyzed by X-ray diffraction and identified as consisting of a zeolite of structural type AFX (ICDD sheet, PDF 04-011-1869), with a purity greater than 99% by weight. The product has a SiO2 / Al2O3 molar ratio of 13 and a mass percentage of Fe of 3% as determined by FX. The diffraction pattern carried out on this solid is given in [Fig.2]. The catalyst obtained is denoted FeAFX3. Example 4 (comparative)

[0122] In this example, an Fe-exchanged SSZ-16 zeolite is synthesized according to the prior art. In this example, the Fe is introduced by ion exchange.

[0123] Preparation of SSZ-16 zeolite:

[0124] 17.32 g of sodium hydroxide are dissolved in 582.30 g of deionized water, under stirring (300 rpm) and at room temperature. 197.10 g of sodium silicate are added to this solution and the whole is homogenized under stirring (300 rpm at room temperature). 9.95 g of NaY CBV100 zeolite are then added under stirring (300 rpm) and continue until the zeolite dissolves. 43.67 g of the DABCO-C4 structuring agent are dissolved in the solution obtained and homogenized under stirring (450 rpm) for 30 minutes at room temperature.

[0125] The reaction mixture has the following molar composition: 100 SiO2: 1.67 Al 2O3: 50 Na2O: 10 DABCO-C4: 4000 H2O

[0126] The reaction mixture obtained in the mixing step is kept at room temperature with stirring for 24 hours.

[0127] The gel obtained is introduced into a reactor and heated to a temperature of 150°C for 6 days with stirring (200 rpm). The crystals obtained are separated and washed with deionized water until the pH of the washing water is lower than 8. The washed crystallized solid is dried for 12 hours at 100°C. The loss on ignition (LAI) is 18% by weight.

[0128] A DRX analysis shows that the product obtained is a raw, pure, synthetic SSZ-16 zeolite of AFX structural type (ICDD file, PDF 04-011-1869).

[0129] The crude SSZ-16 zeolite is calcined under a flow of dry air at 550°C for 12 hours. The calcined SSZ-16 zeolite is brought into contact with a 3 molar NH4NO3 solution for 5 hours under stirring at room temperature. The ratio between the volume of NH4NO3 solution and the mass of solid is 10. The solid obtained is filtered and washed and the exchange procedure is repeated once again under the same conditions. The final solid is separated, washed and dried for 12 hours at 100°C.

[0130] The SSZ-16 zeolite in ammoniacal form (NH4-SSZ-16) is treated under a flow of dry air at 550°C for 8 hours with a temperature rise ramp of 1°C / min. The product obtained is an SSZ-16 zeolite in protonated form (H-SSZ-16).

[0131] Ionic exchange with Fe

[0132] The calcined SSZ-16 zeolite in protonated form is brought into contact with a solution of Fe(NO3)3 (1.5*104 molar) for 16 hours with stirring at 80°C. The ratio between the volume of exchange solution (in mL) and the mass of zeolite to be treated (in grams) is 200. The final solid is separated, washed and dried for 12 hours at a temperature of 100°C.

[0133] The exchanged solid obtained after contact with the Fe(NO3)3 solution is calcined under air flow at 550°C for 8 hours.

[0134] The calcined solid product is analyzed by X-ray diffraction and identified as a zeolite of structural type AFX (ICDD file, PDF 04-011-1869).

[0135] The product has a SiO2 / Al2O3 molar ratio of 13 and a mass percentage of Fe of 2% as determined by FX. The catalyst obtained is denoted FeSSZl6.

[0136] Example 5: Reduction of NOx and N2O by NH3

[0137] A catalytic test for the reduction of nitrogen oxides (NOx) and nitrous oxide (N2O) by ammonia (NH3) in the presence of oxygen (O2) is carried out at different temperatures. operating temperatures for the catalysts synthesized according to Example 2 (FeAFX2), Example 3 (FeAFX3), and Example 4 (FeSSZ16).

[0138] For the test of each sample, 200 mg of catalyst in powder form is placed in a quartz reactor. 145 L / h of a gas mixture having the following molar composition is fed into the reactor: 200 ppm NO, 200 ppm NO2, 200 ppm N2O, 800 ppm NH3, 8.5% O2, 9% CO2, 10% H2O, qpc N2.

[0139] An FTIR analyzer is used to measure the concentration of NO, NO2, NH3, N2O, CO, CO2, H2O, O2 species at the reactor outlet. The conversions of NOx and N2O are calculated as follows: NOx conversion = (NOx in - NOx out) / NOx in Conversion N2O = (N2O in - N2O out) / N2O in

[0140] In these formulas, the input and output indices indicate the content before and after catalytic reduction respectively.

[0141] The NOx conversion results are shown in the following Table 1. [Tables 1] 300°C 400°C 450°C 500°C FeAFX2 88% 100% 100% 100% FeAFX3 45% 95% 100% 100% FeSSZlô (comparative) 76% 100% 100% 100%

[0142] The N2O conversion results are shown in the following Table 2. [Tables2] 300°C 400°C 450°C 500°C FeAFX2 0% 8% 18% 52% FeAFX3 0% 6% 12% 51% FeSSZlô (comparative) 0% 0% 5% 21%

[0143] It appears that the catalysts according to the invention make it possible to convert NOx and N2O.

[0144] The FeAFX2 catalyst and the FeAFX3 catalyst synthesized according to the invention give similar performances to the Fe-SSZ16 catalyst synthesized according to the prior art in terms of NOx conversion. For the conversion of N2O, the FeAFX2 and FeAFX3 catalysts have superior performances to the FeSSZ16 catalyst with initiation at lower temperature, in particular in the 400 to 500°C range which is close to the operating temperatures of the tertiary stage of nitric acid plants.

[0145] Example 6: decomposition of N2O

[0146] A catalytic test of decomposition of nitrous oxide (N2O) is carried out at different operating temperatures for the catalysts synthesized according to example 2 (FeAFX2), example 3 (FeAFX3) and example 4 (FeSSZ16, comparative).

[0147] For the test of each sample, 200 mg of catalyst in powder form is placed in a quartz reactor. 60 L / h of a gas mixture having the following molar composition: 600 ppm N2O, 8.5% O2, 10% H2O, qpc N2 are fed into the reactor.

[0148] An FTIR analyzer is used to measure the concentration of NO, NO2, NH3, N2O, CO, CO2, H2O, O2 species at the reactor outlet. The N2O conversions are calculated as follows: Conversion N2O = (N2O in - N2O out) / N2O in In this formula, the input and output indices indicate the content before and after catalytic reduction respectively.

[0149] The N2O conversion results are shown in the following Table 3: [Tables3] 400°C 425°C 450°C 475°C 500°C 550°C FeAFX2 0% 3% 10% 20% 34% 80% FeAFX3 2% 6% 13% 24% 41% 82% FeSSZlô (comparative) 0% 1% 6% 13% 26% 75%

[0150] It appears that the catalysts according to the invention make it possible to thermally decompose N2O. The FeAFX2 and FeAFX3 catalysts exhibit a higher N2O decomposition activity at iso temperature than the Fe-SSZ16 catalyst.

Claims

Claims

1. Process for preparing a catalyst based on a zeolite of structural type AFX and iron comprising at least the following steps: i) mixing in an aqueous medium, at least one source of at least one silicon oxide SiO2, at least one source of at least one aluminum oxide A12O3, at least one source of SiO2 and / or at least one source of A12O3 being a zeolite of structural type FAU having a SiO2 / Al2O3 molar ratio of between 2.00 and 100, of an organic nitrogen compound R, also called a specific structuring agent chosen from 1,6-bis(methylpiperidinium)hexane dihydroxide and 1,5-bis(methylpiperidinium)pentane dihydroxide, of at least one alkali metal and / or an alkaline-earth metal M of valence n, n being an integer greater than or equal to 1 and a source of Fe, the reaction mixture having the following molar composition: SiO2 / Al2O3 between 2 and 100, preferably between 12 and 40 H2O / SiO2 between 5 and 60,preferably between 10 and 40 R / SiO2 between 0.05 and 0.50, preferably between 0.10 and 0.40 M2 / nO / SiO2 between 0.05 and 0.40, preferably between 0.15 and 0.30, Fe2O3 / SiO2 between 0.01 and 0.10, preferably between 0.01 and 0.05, in which M is sodium, step i) being carried out for a period greater than or equal to 10 minutes, allowing the production of a homogeneous mixture called precursor gel;, ii) hydrothermal treatment of said precursor gel obtained at the end of step i) at a temperature between 120°C and 250°C, preferably between 150°C and 230°C, for a period of between 2 and 12 hours, preferably between 2 and 10 hours until said zeolite of AFX structural type is formed; iii) at least one ion exchange comprising bringing said zeolite obtained at the end of the previous step into contact with a solution comprising at least iron, in solution in reactive form with stirring at a temperature of between 40 and 95°C, preferably between 45 and 90°C, for a period of between 1 hour and 2 days; and iv) heat treatment comprising drying said AFX zeolite obtained in step ii) or iii) at a temperature of between 20 and 150°C, preferably between 60 and 100°C, for a period of between 2 and 24 hours, followed by at least one calcination, in air, possibly actually dry, at a temperature between 450 and 700°C, preferably between 500 and 600°C for a duration between 2 and 20 hours, preferably between 6 and 16 hours, more preferably between 8 and 13 hours, the flow rate of optionally dry air being preferably between 0.5 and 1.5 L / h / g of solid to be treated, more preferably between 0.7 and 1.2 L / h / g of solid to be treated.

2. Method according to claim 1 in which steps iii) and iv) are reversed, and optionally repeated.

3. Preparation process according to claim 2 in which the AFX zeolite obtained in step ii) directly undergoes a step iv) of heat treatment, then at least one ion exchange with an acid, or a compound such as chloride, sulfate or ammonium nitrate to obtain a calcined AFX zeolite in protonated form, before step iii) of ion exchange with iron.

4. Preparation process according to one of the preceding claims, in which crystalline seeds of a zeolite of structural type AFX are added to the reaction mixture of step i), in an amount of between 0.01 and 10% of the total mass of the sources of the tetravalent (Si) and trivalent (Al) elements in their oxide form (SiO2 and Al2O3) used in the reaction mixture, said crystalline seeds not being taken into account in the total mass of the sources of the tetravalent and trivalent elements.

5. Preparation process according to one of the preceding claims in which step i) comprises a step of maturing the reaction mixture at a temperature of between 20 and 80°C, with or without stirring, for a period of between 30 minutes and 24 hours.

6. Preparation process according to one of the preceding claims in which the iron content introduced by synthesis step i) and optionally ion exchange step iii) is between 0.5 and 6% by mass, preferably between 0.5 and 5% by mass, more preferably between 1 and 4% by mass, relative to the total mass of the final anhydrous catalyst.

7. A process for the decomposition of N2O or the reduction of N2O or the simultaneous reduction of NOx and N2O by a reducing agent such as NH3 or H2, in which the gas to be treated is brought into contact with a catalyst prepared according to one of claims 1 to 6.

8. A method of decomposing N2O or reducing N2O or simultaneously reducing NOx and N2O according to claim 7 wherein said catalyst is formed by deposition in the form of a coating, on a honeycomb structure or a plate structure, or said catalyst is prepared in the form of an extrudate or bead, containing up to 100% of said catalyst.

9. A method of decomposing N2O or reducing N2O or simultaneously reducing NOx and N2O according to claim 8 wherein the honeycomb structure is formed of parallel channels open at both ends or comprises porous filtering walls for which the adjacent parallel channels are alternately blocked on either side of the channels.

10. A method for decomposing N2O or reducing N2O or for simultaneously reducing NOx and N2O according to claim 9 wherein the amount of catalyst deposited on said structure is between 50 and 250 g / L for filter structures and between 80 and 300 g / L for structures with open channels.

11. Process for decomposing N2O or reducing N2O or for simultaneously reducing NOx and N2O according to one of claims 7 to 10 in which the catalyst is associated with a binder such as ceria, zirconium oxide, alumina, non-zeolitic silica-alumina, titanium oxide, a mixed oxide of ceria-zirconia type, a tungsten oxide and / or a spinel to be shaped by deposition in the form of a coating, said coating preferably being able to be associated with another coating having capacities for adsorbing pollutants, in particular NOx, for reducing pollutants, in particular NOx, or promoting the oxidation of pollutants.

12. Process for decomposing N2O or reducing N2O or for simultaneously reducing NOx and N2O according to one of claims 7 to 11 in which said catalyst is integrated: - in an exhaust line of an internal combustion engine operating from carbonaceous or non-carbonaceous fuels, or - in a reactor for treating industrial fumes.