Low-temperature synthesis of catalytic converters based on zeolite-AFX and their use in NH3-SCR

Description

TECHNICAL FIELD

[0001] The subject of the invention is a process for preparing a catalyst based on a zeolite of the AFX structural type synthesized at low temperature and at least one transition metal. PRIOR TECHNIQUE

[0002] Nitrogen oxide (NOx) emissions resulting from the combustion of fossil fuels are a major concern for society. Increasingly stringent standards are being implemented by government bodies to limit the impact of combustion emissions on the environment and health. For light vehicles in Europe under the Euro 6c regulation, NOx and particulate emissions must reach a very low level for all operating conditions. The new Worldwide Harmonized Light Vehicle Test Cycle (WLTC) and the Real Driving Emissions (RDE) regulations associated with the conformity factors require the development of a highly efficient emission control system to achieve these objectives.Selective catalytic reduction, or SCR, is an effective technology for removing nitrogen oxides from oxygen-rich exhaust gases typical of lean-burn diesel and spark-ignition engines. Selective catalytic reduction is achieved using a reductant, usually ammonia, and can therefore be referred to as NH3-SCR. The ammonia (NH3) involved in the SCR process is generally generated via the decomposition of an aqueous urea solution (AdBlue or DEF), and produces N2 and H2O upon reaction with NOx.

[0003] Transition metal-exchanged zeolites are used as catalysts for NH3-SCR applications in transport. Small-pore zeolites, particularly copper-exchanged chabazites, are particularly suitable. They are commercially available in the form of Cu-SAPO-34 silico-aluminophosphate and Cu-SSZ-13 (or Cu-SSZ-62) aluminosilicates. Their hydrothermal resistance and NOx conversion efficiency make them the current benchmarks. However, with increasingly stringent standards, catalyst performance still needs to be improved.

[0004] The use of AFX structural type zeolites for NH 3 -SCR applications is known, but few studies evaluate the effectiveness of catalysts using this zeolite.

[0005] Fickel et al. (Fickel, DW, & Lobo, RF, The Journal of Physical Chemistry C, 2010, 114(3), 1633-1640) investigates the use of a copper-exchanged SSZ-16 (AFX structural type) for NOx removal. This zeolite is synthesized according to US patent 5,194,235 (synthesis at 150°C for 6 days), in which copper is introduced by exchange using copper(II) sulfate at 80°C for 1 hour. Recent results (Fickel, DW, D'Addio, E., Lauterbach, JA, & Lobo, RF, Applied Catalysis B: Environmental, 2011, 102(3), 441-448) show excellent conversion and good hydrothermal performance at a loading of 3.78 wt% copper.

[0006] AFX structural type zeolites include in particular SSZ-16 zeolite, and SAPO-56 and MEAPSO-56 zeotypes. The AFX structural type zeolite has a three-dimensional system of pores delimited by eight tetrahedra and is formed by two types of cages: gmelinite (GME cage) and a large AFT cage (-8.3 x 13.0 Ǻ). Many methods for the synthesis of AFX structural type zeolites, in particular SSZ-16 zeolite, are known. SSZ-16 zeolite was synthesized using organic nitrogen species derived from 1,4-di(1-azoniabicyclo[2.2.2]octane)butyl dibromide and with a crystallization time typically greater than 3 days and at a temperature equal to or greater than 140°C (US. Patent No. 4,508,837). Chevron Research and Technology Company prepared SSZ-16 zeolite in the presence of DABCO-C n -diquat cations, where DABCO represents 1,4-diazabicyclo[2.2.2]octane and n is 3, 4 or 5 with a crystallization time typically greater than 3 days and at a temperature greater than 100°C, preferably greater than 130°C (US 5,194,235). SB Hong et al used the diquaternary alkylammonium ion Et6-diquat-n, where Et6-diquat represents N',N'-bis-triethylpentanediammonium and n is 5, as a structuring agent for the synthesis of SSZ-16 zeolite with a formation time of SSZ-16 zeolite between 7 and 14 days and at a temperature of 160°C, (Micropor. Mesopor. Mat., 60 (2003) 237-249). Mention may also be made of the use of 1,3-bis(adamantyl)imidazolium cations as a structuring agent for the preparation of AFX structural type zeolite with a crystallization time of between 7 and 10 days and generally between 2 and 15 days, and at a temperature above 100°C, preferably between 120 and 160°C (RH Archer et al., Microp. Mesopor. Mat., 130 (2010) 255-265, Johnson Matthey Company WO2016077667A1).Inagaki Satoshi et al. (JP2016169139A) used divalent N,N,N',N'-tetraarquirubicyclo[2.2.2]oct-7-ene-2,3:05,6-dipyrrolidium cations substituted with alkyl groups with a crystallization time generally between 20 and 400 hours and at temperatures between 100 and 200°C, preferably between 150 and 175°C, to prepare zeolite SSZ-16. Chevron USA (WO2017 / 200607 A1) proposes to carry out the synthesis of a SSZ-16 zeolite with a crystallization time of 1 to 28 days and at temperatures between 130 and 175°C using the dications: 1,1'-(1,4-cyclohexylenedimethylene)bis[1-methylpiperidinium], 1,1'-(1,4-cyclohexylenedimethylene)bis[1-methylpyrrolidinium], 1,1'-(1,4-cyclohexylenedimethylene)bis[1-ethylpyrrolidinium]. H.-Y. Chen et al.(Johnson Matthey Company, US2018 / 0093897) used a mixture of cations containing at least 1,3-bis(adamantyl)imidazolium and a neutral amine to prepare JMZ-10 zeolite of AFX structure type in the absence of alkali cations with a crystallization time between 1 and 20 days and at temperatures between 100 and 200°CH-Y. Chen et al. (Johnson Matthey Company, US2018 / 0093259) used a mixture of cations containing an organic molecule selected from 1,3-bis(adamantyl)imidazolium, N,N-dimethyl-3,5-dimethylpiperidinium, N,N-diethyl-cis-2,6-dimethylpiperidinium, N,N,N-1-trimethyladamantylammonium, N,N,N-dimethylethylcyclohexylammonium and at least one alkaline earth metal cation to obtain the JMZ-7 zeolite of AFX structural type which has Al sites close to a zeolite obtained by a synthesis using alkali cations.The time required to obtain this zeolite varies from 3 to 15 days at a temperature above 100°C, preferably between 120 and 180°C.

[0007] KG Strohmaier et al. (Exxon Mobil, WO2017202495A1) used the organic molecule 1,1'-(hexane-1,6-dyil)bis(1-methylpiperidinium) in the presence of a metal complex stabilized by amine ligands to obtain a zeolite of the AFX structural type with a crystallization time of 1 day to about 100 days and at temperatures between 100 and 200°C, preferably between 150 and 170°C.

[0008] The applicant has discovered that a catalyst based on a zeolite of AFX structural type prepared according to a particular synthesis method, i.e. at crystallization temperatures lower than or equal to 95°C and at least one transition metal, in particular copper, exhibited interesting NOx conversion performances and selectivity towards N2O. The NOx conversion performances, in particular at low temperature (T<250°C), are notably superior to those obtained with catalysts of the prior art, such as catalysts based on AFX structural type zeolite exchanged with copper, while retaining good selectivity towards nitrous oxide N2O. Another advantage of this method of synthesizing zeolite is that it is not necessary to use reactors which operate at a pressure higher than atmospheric. SUMMARY OF THE INVENTION

[0009] The invention relates to a process for preparing a catalyst based on AFX zeolite exchanged with at least one transition metal comprising at least the following steps: i) the mixture in an aqueous medium of at least one source of silicon (Si) in the form of SiO 2 oxide, at least one source of aluminum (AI) in the form of Al 2 O 3 oxide, of an organic nitrogen compound R, R being 1,6-bis(methylpiperidinium)hexane dihydroxide, of at least one source of at least one alkali metal chosen from lithium, potassium, or sodium, and the mixture of at least two of these metals, the reaction mixture having the following molar composition: SiO 2 / Al 2 O 3 between 4 and 60, preferably between 8 and 40 H 2 O / SiO 2 between 5 and 60, preferably between 10 and 40 R / SiO 2 between 0.05 to 0.50, preferably between 0.10 and 0.30 M 2 O / SiO 2 between 0.10 to 0.30, preferably between 0.15 and 0.25, until a homogeneous precursor gel is obtained;ii) hydrothermal treatment under atmospheric pressure of said precursor gel obtained at the end of step i) at a temperature between 75°C and 95°C, inclusive, for a period of between 40 and 100 hours, inclusive, to obtain a crystallized solid phase, called "solid"; said solid phase formed from a zeolite of AFX structural type being filtered, washed and then dried at the end of the reaction, iii) at least one ion exchange comprising bringing said solid obtained at the end of the previous step into contact with a solution comprising at least one species capable of releasing a transition metal, in solution in reactive form with stirring at room temperature for a period of between 1 hour and 2 days;wherein said at least one transition metal released into the exchange solution of step iii) is selected from the group consisting of the following elements: Ti, V, Mn, Mo, Fe, Co, Cu, Cr, Zn, Nb, Ce, Zr, Rh, Pd, Pt, Au, W, Ag, preferably from the group consisting of the following elements: Fe, Cu, Nb, Ce or Mn, more preferably from Fe or Cu and even more preferably said transition metal is Cu iv) heat treatment by drying the solid obtained at the end of the previous step at a temperature between 20 and 150°C followed by at least one calcination under air flow at a temperature between 400 and 700°C. ;

[0010] Steps iii) and iv) can be reversed, and possibly repeated.

[0011] The precursor gel obtained at the end of step i) advantageously has a molar ratio of the quantity of SiO2 to the quantity of Al2O3 of between 4.00 and 60.00, preferably between 8.00 and 40.00, limits included.

[0012] Crystalline seeds of a zeolite of structural type AFX may be added to the reaction mixture of step i), preferably in an amount of between 0.05 and 10% of the total mass of the sources of alumina and silica in anhydrous form used in the reaction mixture, said crystalline seeds not being taken into account in the total mass of the sources of alumina and silica.

[0013] Step i) may comprise a step of maturing the reaction mixture at a temperature of between 20 and 60°C, with or without stirring, for a period of time. The hydrothermal treatment of step ii) may preferably be carried out at a temperature of between 85 and 95°C, inclusive, for a period of time preferably of between 40 and 80 hours, very preferably of between 48 and 80 hours, inclusive.

[0014] Step iii) of ion exchange is carried out by bringing the solid into contact with a solution comprising a single species capable of releasing a transition metal or by successively bringing the solid into contact with different solutions each comprising at least one, preferably a single, species capable of releasing a transition metal, preferably the transition metals of the different solutions being different from each other.

[0015] Said at least one transition metal released into the exchange solution of step iii) is selected from the group formed by the following elements: Ti, V, Mn, Mo, Fe, Co, Cu, Cr, Zn, Nb, Ce, Zr, Rh, Pd, Pt, Au, W, Ag, preferably from the group formed by the following elements: Fe, Cu, Nb, Ce or Mn, more preferably from Fe or Cu and even more preferably said transition metal is Cu.

[0016] The content of transition metal(s) introduced 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.

[0017] Step iv) of heat treatment may comprise 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 5 and 10 hours, even more preferably between 6 and 9 hours, the flow rate of optionally dry air being preferably between 0.5 and 1.5 L / h / g of solid to be treated, even more preferably between 0.7 and 1.2 L / h / g of solid to be treated.

[0018] The description also relates to a catalyst based on an AFX zeolite and at least one transition metal obtained by the process according to any one of the variants described.

[0019] The transition metal or metals may be selected from the group consisting of the following elements: Ti, V, Mn, Mo, Fe, Co, Cu, Cr, Zn, Nb, Ce, Zr, Rh, Pd, Pt, Au, W, Ag, preferably from the group consisting of the following elements: Fe, Cu, Nb, Ce or Mn, more preferably from Fe or Cu and even more preferably said transition metal is Cu.

[0020] The total content of transition metals is advantageously between 0.5 and 6% by weight, preferably between 0.5 and 5% by weight, more preferably between 1 and 4% by weight, relative to the total mass of the final anhydrous catalyst. In one embodiment, the catalyst comprises copper, alone, at a content of between 0.5 and 6% by weight, preferably between 0.5 and 5% by weight, very preferably between 1 and 4% by weight relative to the total mass of the final anhydrous catalyst.

[0021] The catalyst may comprise copper in association with at least one other transition metal chosen from the group formed by Fe, Nb, Ce, Mn, the copper content of the catalyst being between 0.05 and 2% by mass, preferably 0.5 and 2% by mass, the content of said at least one other transition metal being between 1 and 4% by mass relative to the total mass of the final anhydrous catalyst.

[0022] The catalyst may comprise iron in association with another metal chosen from the group formed by Cu, Nb, Ce, Mn, the iron content being between 0.05 and 2% by mass, preferably between 0.5 and 2% by mass, the content of said other transition metal being between 1 and 4% by mass, relative to the total mass of the final anhydrous catalyst.

[0023] The description also relates to the use of the catalyst as described above in any of its variants or obtained by the process according to any of its variants, for the selective reduction of NO x by a reducing agent such as NH 3 or H 2 .

[0024] The catalyst can be formed by deposition as a coating on a honeycomb structure or a plate structure.

[0025] The honeycomb structure may be formed of parallel channels open at both ends or may have porous filtering walls in which adjacent parallel channels are alternately blocked on either side of the channels.

[0026] The quantity of catalyst deposited on said structure can be between 50 and 180 g / L for filter structures and between 80 and 200 g / L for structures with open channels.

[0027] The catalyst may be combined with a binder such as ceria, zirconia, alumina, non-zeolitic silica-alumina, titanium oxide, a mixed oxide of the ceria-zirconia type, a tungsten oxide and / or a spinel to be formed by deposition in the form of a coating.

[0028] Said coating may 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.

[0029] Said catalyst may also be in extruded form, containing up to 100% of said catalyst.

[0030] Advantageously, the structure coated with said catalyst or obtained by extrusion of said catalyst is integrated into an exhaust line of an internal combustion engine. LIST OF FIGURES

[0031] [ Fig 1 ] There Figure 1 represents the chemical formula of the organic nitrogen compound which can be chosen as the structuring agent used in the synthesis process according to the invention. [ Fig 2 ] There figure 2 represents the X-ray diffraction pattern of the AFX zeolite obtained according to example 2.

[0032] Other characteristics and advantages of the catalyst preparation process will appear on reading the following description of non-limiting examples of embodiments, with reference to the figures appended and described below. DESCRIPTION OF EMBODIMENTS

[0033] The invention relates to a process for preparing a catalyst based on zeolite of AFX structural type and at least one transition metal comprising at least the following steps: i) the mixture in aqueous medium of at least one source of silicon (Si) in the form of SiO 2 oxide, at least one source of aluminum (AI) in the form of Al 2 O 3 oxide, of an organic nitrogen compound R, R being 1,6-bis(methylpiperidinium)hexane dihydroxide ( Figure 1 ),of at least one source of at least one alkali metal chosen from lithium, potassium, or sodium, and the mixture of at least two of these metals, the reaction mixture having the following molar composition: SiO 2 / Al 2 O 3 between 4 and 60, preferably between 8 and 40 H 2 O / SiO 2 between 5 and 60, preferably between 10 and 40 R / SiO 2 between 0.05 to 0.50, preferably between 0.10 and 0.30 M 2 O / SiO 2 between 0.10 to 0.30, preferably between 0.15 and 0.25, until a homogeneous precursor gel is obtained; ii) the hydrothermal treatment under atmospheric pressure of said precursor gel obtained at the end of step i) at a temperature between 75 and 95°C, preferably between 85°C and 95°C, inclusive, for a period between 40 and 100 hours. Advantageously, the SiO ratio 2 / Al 2 O 3 of the AFX zeolite obtained is between 4 and 60, preferably between 8 and 40, limits included.

[0034] Preferably, M is sodium.

[0035] Preferably, the source of at least one alkali metal is sodium hydroxide.

[0036] Crystalline seeds of a zeolite of structural type AFX can be added to the reaction mixture of step i), preferably in an amount of between 0.05 and 10% of the total mass of SiO 2 and Al 2 O 3 , said crystalline seeds not being taken into account in the total mass of the sources of the elements Si and Al.

[0037] Step i) may comprise a step of maturing the reaction mixture at a temperature of between 20 and 100°C, with or without stirring, for a period of between 30 minutes and 48 hours.

[0038] The hydrothermal treatment of step ii) is advantageously carried out under reflux at a temperature of between 75 and 95°C, preferably between 85°C and 95°C, limits inclusive, for a period of between 40 and 100 hours, preferably between 48 and 80 hours, said solid phase formed from a zeolite of structural type AFX being filtered, washed and then dried at the end of the reaction.

[0039] iii) at least one ion exchange comprising bringing said solid obtained at the end of the preceding step into contact with a solution comprising at least one species capable of releasing a transition metal, in solution in reactive form with stirring at room temperature for a period of between 1 hour and 2 days, in which said at least one transition metal released in the exchange solution of step iii) is selected from the group formed by the following elements: Ti, V, Mn, Mo, Fe, Co, Cu, Cr, Zn, Nb, Ce, Zr, Rh, Pd, Pt, Au, W, Ag, preferably from the group formed by the following elements: Fe, Cu, Nb, Ce or Mn, more preferably from Fe or Cu and even more preferably said transition metal is Cu.

[0040] iv) heat treatment by drying the solid obtained at the end of the previous step at a temperature between 20 and 150°C followed by at least one calcination under air flow at a temperature between 400 and 700°C.

[0041] Steps iii) and iv) can be reversed, and possibly repeated.

[0042] More particularly, the process according to the present invention comprises a step i) of mixing in an aqueous medium, at least one source of silicon (Si) in the form of SiO 2 oxide, at least one source of aluminum (AI) in the form of Al 2 O 3 oxide, of an organic nitrogen compound R, R being 1,6-bis(methylpiperidinium)hexane dihydroxide, of at least one source of at least one alkali metal chosen from lithium, potassium, or sodium, and the mixture of at least two of these metals, the reaction mixture having the following molar composition: SiO 2 / Al 2 O 3 between 4 and 60, preferably between 8 and 40 H 2 O / SiO 2 between 5 and 60, preferably between 10 and 40 R / SiO 2 between 0.05 to 0.50, preferably between 0.10 and 0.30 M 2 O / SiO 2 between 0.10 to 0.30, preferably between 0.15 and 0.25, until a homogeneous precursor gel is obtained; then a step ii) of hydrothermal treatment of said precursor gel obtained at the end of step i) at a temperature between 75°C and 95°C, preferably between 85°C and 95°C, limits inclusive, for a period between 40 and 100 hours, preferably between 48 and 80 hours, until said high-purity AFX structural type zeolite is formed.

[0043] In the molar composition of the reaction mixture above and throughout the description: SiO 2 denotes the molar quantity of silicon expressed in oxide form, and Al 2 O 3 denotes the molar quantity of aluminum expressed in oxide form, H 2 O the molar quantity of water present in the reaction mixture, R the molar quantity of said organic nitrogen compound, M 2 O the molar quantity expressed in oxide form of M 2 O by the alkali metal source.

[0044] An advantage of the present invention is therefore to provide a new preparation process allowing the formation of a pure AFX structural type zeolite at the end of step ii).

[0045] Another advantage of the present invention is to allow the preparation of a precursor gel of a zeolite of the AFX structural type thanks to the combination of a specific organic or structuring species comprising two quaternary ammonium functions, 1,6-bis(methylpiperidinium)hexane dihydroxide and very specific operating conditions, in particular a hydrothermal treatment at controlled temperature.

[0046] Step ii) comprises a hydrothermal treatment of said precursor gel obtained at the end of step i) which is carried out at a temperature of between 75°C and 95°C, preferably between 85°C and 95°C, limits inclusive, for a period of between 40 and 100 hours, preferably between 48 and 80 hours, until said zeolite of AFX structural type crystallizes. It is thus possible to carry out this step at atmospheric pressure, in particular in a reactor open to the atmosphere. According to the invention, at least one source of at least one SiO 2 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 "AEROSIL" and silica gels. Colloidal silicas having different particle sizes may be used, for example with an average equivalent diameter of between 10 and 15 nm or between 40 and 50 nm, such as those marketed under registered trademarks such as "LUDOX". Preferably, the source of silicon is LUDOX HS-40. According to the invention, at least one source of Al 2 O 3 is incorporated into the mixture for carrying out said step (i).The aluminum source is preferably aluminum hydroxide or an aluminum salt, for example chloride, nitrate, or sulfate, sodium aluminate, aluminum alkoxide, or alumina itself, preferably in hydrated or hydratable form, such as, for example, colloidal alumina, pseudoboehmite, gamma alumina, or alpha or beta trihydrate. Mixtures of the above sources may also be used.

[0047] According to the invention, R is a nitrogenous organic compound, 1,6-bis(methylpiperidinium)hexane dihydroxide, said compound being incorporated into the reaction mixture for the implementation of step (i), as an organic structuring agent. The anion associated with the quaternary ammonium cations present in the structuring organic species for the synthesis of a zeolite of structural type AFX according to the invention is the hydroxide anion.

[0048] According to the invention, at least one source of at least one alkali metal is used in the reaction mixture of step i), M preferably being chosen from lithium, potassium, sodium and the mixture of at least two of these metals. Very preferably, M is sodium.

[0049] Preferably, the source of at least one alkali and / or alkaline earth metal M is sodium hydroxide.

[0050] 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 0.05 and 10% of the total mass of the sources of said element(s) Si and Al in anhydrous form used in the reaction mixture, said crystal seeds not being taken into account in the total mass of the sources of the elements Si and Al.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.

[0051] 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.

[0052] At the end of step i), a homogeneous precursor gel is obtained.

[0053] 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 60°C with or without stirring, for a duration advantageously between 30 min and 48 hours.

[0054] 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, carried out at a temperature between 75 and 95°C, preferably carried out at a temperature between 85 and 95°C, limits included for a period of between 40 and 100 hours, until said zeolite of structural type AFX is formed.

[0055] The time required to achieve crystallization varies between 40 and 100 hours, preferably between 48 and 80 hours.

[0056] The reaction is generally carried out with or without stirring, preferably with stirring. Any stirring system known to those skilled in the art can be used as the stirring system, for example, inclined blades with counterblades, stirring turbines, Archimedes screws.

[0057] At the end of the reaction, after carrying out said step ii) of the preparation process according to the invention, the solid phase formed from a zeolite of AFX structural type is filtered, washed and then dried. Drying is generally carried out at a temperature between 20 and 150°C, preferably between 60 and 100°C, for a period of between 5 and 24 hours.

[0058] It is also advantageous to obtain the protonated form of the AFX structural type zeolite obtained by the process according to the invention. Said protonated form can be obtained by carrying out an 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. The ion exchange can be carried out by suspending said AFX structural type zeolite in one or more times with the ion exchange solution. Said zeolite can be calcined before or after the ion exchange, or between two ion exchange steps. The zeolite is preferably calcined before the ion exchange, in order to eliminate any organic substance included in the porosity of the zeolite, since the ion exchange is thereby facilitated.

[0059] The process for preparing the catalyst according to the invention also comprises: iii) at least one ion exchange comprising bringing said solid obtained at the end of the previous step into contact with a solution comprising at least one species capable of releasing a transition metal, in solution in reactive form with stirring at room temperature for a period of between 1 hour and 2 days;

[0060] The transition metal or metals included in the catalyst is (are) selected from the group consisting of the following elements: Ti, V, Mn, Mo, Fe, Co, Cu, Cr, Zn, Nb, Ce, Zr, Rh, Pd, Pt, Au, W, Ag.

[0061] Preferably, the catalyst comprises copper, alone or combined with at least one other transition metal, chosen from the group of elements listed above; in particular Fe, Nb, Ce, Mn.

[0062] The total content of transition metals 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.

[0063] For the process according to the invention, in which the transition metal is copper alone, the copper content introduced by the ionic step iii) is between 0.5 and 6%, preferably between 0.5 and 5%, and more preferably between 1 and 4% by weight relative to the total mass of the final anhydrous catalyst.

[0064] For the process according to the invention, in which the transition metal is copper in association with at least one other transition metal chosen from the group formed by Fe, Nb, Ce, Mn, the copper content introduced by the ion exchange step iii) is between 0.05 and 2% by mass, preferably 0.5 and 2% by mass while that of the other transition metal is preferably between 1 and 4% by mass, the transition metal contents being given as a percentage by mass relative to the total mass of the final dry catalyst.

[0065] For catalysts containing only iron as the transition metal, the content is between 0.5 and 4% and preferably between 1.5 and 3.5% relative to the total mass of the final anhydrous catalyst.

[0066] For catalysts comprising iron and another element such as, preferably Cu, Nb, Ce, Mn, the iron content of the catalyst is between 0.05 and 2% by mass, preferably between 0.5 and 2% by mass, while that of the other transition metal is preferably between 1 and 4% by mass, the transition metal contents being given as a percentage by mass relative to the total mass of the final dry catalyst.

[0067] According to the invention, by "species capable of releasing a transition metal" is meant a species capable of dissociating in an aqueous medium, such as for example sulfates, nitrates, chlorides, oxalates, organometallic complexes of a transition metal or their mixtures. Preferably, the species capable of releasing a transition metal is a sulfate or a nitrate of said transition metal.

[0068] According to the invention, the solution with which the crystallized solid or the dried and calcined crystallized solid is brought into contact, comprises at least one species capable of releasing a transition metal, preferably a single species capable of releasing a transition metal, preferably iron or copper, preferentially copper.

[0069] The catalyst 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.

[0070] The preparation process also comprises a step iv) of heat treatment by drying the solid obtained at the end of the previous step at a temperature between 20 and 150°C followed by at least one calcination under air flow at a temperature between 450 and 700°C, steps iii) and iv) being able to advantageously be reversed, and possibly repeated if necessary.

[0071] 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 5 and 10 hours, more preferably between 6 and 9 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. The calcination may be preceded by a gradual increase in temperature.

[0072] 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.

[0073] The present description also relates to the catalyst comprising a zeolite of structural type AFX and at least one transition metal capable of being obtained or directly obtained by the method described above.

[0074] The description finally concerns the use of a catalyst for the selective catalytic reduction of NOx in the presence of a reducing agent.

[0075] At the end of said preparation step ii) of the AFX zeolite, X-ray diffraction makes it possible to verify that the solid obtained by the process according to the invention is indeed a zeolite of the AFX structural type. The purity obtained is advantageously greater than 90%, preferably greater than 95%, very preferably greater than 99.8% by weight.

[0076] This diffraction pattern is obtained by X-ray crystallographic analysis using a diffractometer using the classical powder method with Kα 1 radiation from copper (λ = 1.5406Å). From the position of the diffraction peaks represented by the angle 2θ, the characteristic reticular equidistances d hkl of the sample are calculated using the Bragg relation. The measurement error Δ(d hkl ) on d hkl is calculated using the Bragg relation as a function of the absolute error Δ(2θ) assigned to the measurement of 2θ. An absolute error Δ(2θ) equal to ± 0.02° is commonly accepted. The relative intensity I rel assigned to each value of d hkl is measured from the height of the corresponding diffraction peak. The X-ray diffraction pattern of the crystalline solid of structural type AFX according to the invention comprises at least the lines with the values ​​of d hkl given in Table 1.In the d hkl column, the average values ​​of the inter-reticular distances in Angstroms (Å) are indicated. Each of these values ​​must be affected by the measurement error Δ(d hkl ) between ± 0.6Å and ± 0.01Å.

[0077] Table 1: Average dhkl values ​​and relative intensities measured on an X-ray diffraction pattern of the crystalline solid of AFX structural type. [Table 1] 2 theta (°) dhkl (Å) Irel 2 theta (°) dhkl (Å) Irel 7,47 11,83 mf 22,52 3,95 f 8,56 10,32 f 26,06 3,42 m 8,67 10,19 mf 27,69 3,22 mf 11,59 7,63 f 27,76 3,21 f 12,96 6,82 mf 27,86 3,20 mf 14,99 5,91 ff 29,74 3,00 ff 15,60 5,67 f 30,22 2,95 mf 17,42 5,09 mf 30,49 2,93 mf 17,77 4,99 mf 31,48 2,84 mf 19,86 4,47 f 33,57 2,67 f 20,32 4,37 m 34,68 2,58 f 21,74 4,08 FF

[0078] X-ray fluorescence (FX) spectrometry is a chemical analysis technique that uses a physical property of matter, X-ray fluorescence. It allows the analysis of most chemical elements from Beryllium (Be) in concentration ranges from a few ppm to 100%, with accurate and reproducible results. X-rays are used to excite the atoms in the sample, causing them to emit X-rays with energies characteristic of each element present. The intensity and energy of these X-rays are then measured to determine the concentration of the elements in the material. Use of the catalyst

[0079] The description also relates to the use of the catalyst, directly prepared or capable of being prepared by the process described above for the selective reduction of NO x by a reducing agent such as NH 3 or H 2 , advantageously shaped 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 particularly found for stationary applications.

[0080] The honeycomb structure is formed of parallel channels open at both ends (flow-through in English) or has 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 (AlTi), 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, FeCrAI type steel.

[0081] The quantity of catalyst deposited on said structure is between 50 and 180 g / L for filter structures and between 80 and 200 g / L for structures with open channels.

[0082] The coating itself ("washcoat") comprises the catalyst, advantageously associated 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, 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 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 into the monolith. Once the coating is deposited, the monolith is calcined at a temperature of 300 to 600°C for 1 to 10 hours.

[0083] Said structure may be coated with one or more coatings. The coating comprising the catalyst 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.

[0084] Another possibility is to put the catalyst in extruded form. In this case, the resulting structure can contain up to 100% catalyst.

[0085] Said structure coated with the catalyst is advantageously integrated into an exhaust line of an internal combustion engine operating mainly in a lean mixture, that is to say in excess of air compared to the stoichiometry of the combustion reaction as is the case for diesel engines for example. Under these engine operating conditions, the exhaust gases contain in particular the following pollutants: soot, unburned hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx). Upstream of said structure coated with the catalyst can be placed an oxidation catalyst whose function is to oxidize the HC and the CO as well as a filter to remove the soot from the exhaust gases, the function of said coated structure being to remove the NOx, its operating range being between 100 and 900°C and preferably between 200°C and 500°C. ADVANTAGES OF THE INVENTION

[0086] The catalyst, based on a zeolite of structural type AFX and at least one transition metal, in particular copper, has improved properties and ease of preparation compared to prior art catalysts. In particular, the use of the catalyst makes it possible to obtain lower initiation temperatures for the NOx conversion reaction and better NOx conversion over the entire operating temperature range (150°C - 600°C), while maintaining good selectivity to N 2 O. It also has better resistance to hydrothermal aging, ensuring high performance even after this aging. EXAMPLES

[0087] The invention is illustrated by the following examples which are in no way limiting. Example 1: preparation of 1,6-bis(methylpiperidinium)hexane dihydroxide (structuring agent R).

[0088] 50 g of 1,6-dibromohexane (0.20 mol, 99%, Alfa Aesar) are added to a 1 L flask 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 (a yield of 80%).

[0089] The product has the expected 1< H NMR spectrum. 1< H NMR (D 2 O, ppm / TMS): 1.27 (4H,m); 1.48 (4H,m); 1.61 (4H,m); 1.70 (8H,m); 2.85 (6H,s); 3.16 (12H,m).

[0090] 18.9 g of Ag 2 O (0.08 mol, 99%, Aldrich) are added to a 250 ml Teflon beaker containing 30 g of the prepared 1,6-bis(methylpiperidinium)hexane dibromide structurant (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 resulting filtrate is composed of an aqueous solution of 1,6-bis(methylpiperidinium)hexane dihydroxide. The determination of this species is carried out by proton NMR using formic acid as a standard. Example 2: preparation of a catalyst containing a zeolite of structural type AFX according to the invention. Preparation of AFX zeolite

[0091] 49.83 g of an aqueous solution of 1,6-bis(methylpiperidinium)hexane dihydroxide (18.36% by weight) prepared according to Example 1 were mixed with 0.466 g of deionized water. 2.1 g of sodium hydroxide (solid, purity 98% by weight, Aldrich) were added to the previous mixture, the resulting preparation was kept stirring for 10 minutes. Subsequently, 1.66 g of sodium aluminate (53.17% Al 2 O 3 by weight, Strem Chemicals) were incorporated and the synthesis gel was kept stirring for 15 minutes. In the end, 25.96 g of colloidal silica (Ludox HS40, 40% SiO 2 by weight, Aldrich) and 1.038 g of seeds of a zeolite of structural type AFX obtained by any method known to those skilled in the art were incorporated into the synthesis mixture. The molar composition of the mixture without taking into account the seeds is as follows: 100 SiO 2 : 5 Al 2 O 3 : 16.7 R: 22.36 Na 2 O: 1836 H 2 O, i.e. a SiO 2 / Al 2 O 3 ratio of 10.The precursor gel is then transferred, after homogenization, into a reactor equipped with a reflux. The reactor is then heated with a temperature rise of 5°C / min up to 95°C for 40 hours while stirring at 200 rpm with a 4 inclined blade system. The crystallized product obtained is filtered, washed with deionized water and then dried overnight at 100°C. The loss on ignition of the product obtained after drying is 15%. 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.

[0092] The calcined solid product was analyzed by X-ray diffraction and identified as consisting of an AFX structural type zeolite with a purity greater than 99.8%. The diffraction pattern performed on the calcined AFX structural type solid is given in Figure 2 The product has a SiO 2 / Al 2 O 3 molar ratio of 12 as determined by X-ray fluorescence.

[0093] The calcined AFX zeolite is then brought into contact with a 3 molar NH 4 NO 3 solution for 1 hour under stirring at 80°C. The ratio between the volume of NH 4 NO 3 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. 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. Cu ion exchange

[0094] The calcined AFX zeolite in protonated form is brought into contact with a solution of [Cu(NH 3 ) 4 ](NO 3 ) 2 for 1 day under stirring at room temperature. The final solid is separated, washed and dried for 12 hours at a temperature of 100°C.

[0095] The Cu-AFX exchanged solid obtained after contact with the [Cu(NH 3 ) 4 ](NO 3 ) 2 solution is calcined under air flow at 550°C for 8 hours.

[0096] The solid obtained has a SiO 2 / Al 2 O 3 molar ratio of 12 and a mass percentage of Cu of 3% as determined by X-ray fluorescence.

[0097] The resulting catalyst is denoted CuAFX. Example 3 : Conversion of NOx to Standard SCR

[0098] A catalytic test for the reduction of nitrogen oxides (NOx) by ammonia (NH3) in the presence of oxygen (O2) under Standard SCR conditions is carried out at different operating temperatures for the catalyst obtained according to Example 2 (CuAFX, according to the invention). For the test of each sample, 200 mg of catalyst in powder form are placed in a quartz reactor. 145 L / h of a load representative of a mixture of exhaust gases from a Diesel engine are fed into the reactor.

[0099] This charge has the following molar composition: 400 ppm NO, 400 ppm NH 3 , 8.5% O 2 , 9% CO 2 , 10% H 2 O, qpc N 2 .

[0100] An FTIR analyzer is used to measure the concentration of NO, NO 2 , NH 3 , N 2 O, CO, CO 2 , H 2 O, O 2 species at the reactor outlet. NOx conversions are calculated as follows: Conversion = NOx entrée − NOx sortie / NOx entrée

[0101] The catalyst initiation temperatures are given below for Standard-SCR conditions: [Table 2] T50 T80 T90 T100 CuAFX 180°C 212°C 230°C 320°C

[0102] T50 corresponds to the temperature at which 50% of the NOx in the gas mixture is converted by the catalyst. T80 corresponds to the temperature at which 80% of the NOx in the gas mixture is converted by the catalyst. T90 corresponds to the temperature at which 90% of the NOx in the gas mixture is converted by the catalyst. T100 corresponds to the temperature at which 100% of the NOx in the gas mixture is converted by the catalyst. Example 5 : Conversion of NOx to Fast SCR

[0103] A catalytic test for the reduction of nitrogen oxides (NOx) by ammonia (NH 3 ) in the presence of oxygen (O 2 ) under Fast SCR conditions is carried out at different operating temperatures for the catalyst synthesized according to the invention (example 2)

[0104] 200 mg of catalyst in powder form is placed in a quartz reactor. 218 l / h of a feedstock representative of a diesel engine exhaust gas mixture is fed into the reactor. This feedstock has the following molar composition: 200 ppm NO, 200 ppm NO 2 , 400 ppm NH 3 , 8.5% O 2 , 9% CO 2 , 10% H 2 O, qpc N 2 for Fast SCR conditions.

[0105] An FTIR analyzer is used to measure the concentration of NO, NO 2 , NH 3 , N 2 O, CO, CO 2 , H 2 O, O 2 species at the reactor outlet. NOx conversions are calculated as follows: Conversion = NOx entrée − NOx sortie / NOx entrée

[0106] Catalyst initiation temperatures are given below for Fast-SCR conditions [Table 3] T50 T80 T90 T100 CuAFX 144°C 178°C 198°C 255°C

[0107] T50 corresponds to the temperature at which 50% of the NOx in the gas mixture is converted by the catalyst. T80 corresponds to the temperature at which 80% of the NOx in the gas mixture is converted by the catalyst. T90 corresponds to the temperature at which 90% of the NOx in the gas mixture is converted by the catalyst. T100 corresponds to the temperature at which 100% of the NOx in the gas mixture is converted by the catalyst.

[0108] Nitrous oxide (N2O) emissions, in the case of the CuAFX catalyst according to the invention, remain low over the entire temperature range tested (<15ppm between 150 and 550°C).

Claims

1. Process for preparing a catalyst based on an AFX zeolite exchanged with at least one transition metal, comprising at least the following steps: i) mixing, in an aqueous medium, at least one source of silicon (Si) in SiOz oxide form, at least one source of aluminium (Al) in Al2O3 oxide form, a nitrogenous organic compound R, R being 1,6-bis(methylpiperidinium)hexane dihydroxide, and at least one source of at least one alkali metal M chosen from lithium, potassium or sodium, and the mixture of at least two of these metals, the reaction mixture having the following molar composition: SiO2 / Al2O3between 4 and 60, preferably between 8 and 40H2O / SiO2between 5 and 60, preferably between 10 and 40R / SiO2between 0.05 and 0.50, preferably between 0.10 and 0.30M2O / SiO2between 0.10 and 0.30, preferably between 0.15 and 0.25, until a homogeneous precursor gel is obtained; ii) hydrothermal treatment, under atmospheric pressure, of said precursor gel obtained at the end of step i) at a temperature of between 75°C and 95°C, limits included, for a period of between 40 and 100 hours, limits included, to obtain a solid crystalline phase, termed "solid"; said solid phase formed by a an AFX-structure zeolite being filtered, washed and then dried at the end of the reaction; iii) at least one ion exchange comprising bringing said solid obtained at the end of the previous step into contact with a solution comprising at least one species capable of releasing a transition metal, in solution in reactive form, with stirring at ambient temperature for a period of between 1 hour and 2 days; in which said at least one transition metal released in the exchange solution of step iii) is selected from the group made up of the following elements: Ti, V, Mn, Mo, Fe, Co, Cu, Cr, Zn, Nb, Ce, Zr, Rh, Pd, Pt, Au, W, Ag, preferably from the group made up of the following elements: Fe, Cu, Nb, Ce or Mn, more preferably from Fe or Cu, and even more preferably said transition metal is Cu; iv) heat treatment by drying the solid obtained at the end of the previous step at a temperature of between 20 and 150°C, followed by at least one calcination under a stream of air at a temperature of between 400 and 700°C.

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

3. Process according to either of the preceding claims, in which the precursor gel obtained at the end of step i) has a molar ratio of the total amount, expressed as oxides, of tetravalent elements to the total amount, expressed as oxides, of trivalent elements of between 4.00 and 60.00, preferably between 8.00 and 40.00, limits included.

4. Process according to one of the preceding claims, in which seed crystals of an AFX-structure zeolite are added to the reaction mixture of step i), preferably in an amount of between 0.05 and 10% of the total mass of the sources of alumina and silica in anhydrous form used in the reaction mixture, said seed crystals not being taken into account in the total mass of the sources of alumina and silica.

5. 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 60°C, with or without stirring, for a period of between 30 minutes and 48 hours.

6. Process according to one of Claims 1 to 5, in which the hydrothermal treatment of step ii) is carried out at a temperature of between 85 and 95°C, limits included, for a period of preferably between 40 and 80 hours, very preferably between 48 and 80 hours, limits included.

7. Process according to one of Claims 1 to 6, in which the ion-exchange step iii) is carried out by bringing the solid into contact with a solution comprising just one species capable of releasing a transition metal or by bringing the solid into contact successively with various solutions, each comprising at least one, preferably just one, species capable of releasing a transition metal, the transition metals of the various solutions preferably being different from one another.

8. Process according to one of the preceding claims, in which the content of transition metal(s) introduced by the 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 anhydrous final catalyst.

9. Process according to one of the preceding claims, in which the heat treatment step iv) comprises 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 air, 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 5 and 10 hours, even more preferably between 6 and 9 hours, the flow rate of optionally dry air being preferably between 0.5 and 1.5 l / h / g of solid to be treated, even more preferably between 0.7 and 1.2 l / h / g of solid to be treated.

10. Process according to one of the preceding claims, in which the metal is copper, alone, and the copper content introduced by the ion-exchange step iii) is between 0.5 and 6% by weight, preferably between 0.5 and 5% by weight, very preferably between 1 and 4% by weight, relative to the total mass of the anhydrous final catalyst.

11. Process according to one of the preceding claims, in which the transition metal is copper in combination with at least one other transition metal chosen from the group made up of Fe, Nb, Ce, Mn, the copper content introduced by the ion-exchange step iii) being between 0.05 and 2% by mass, preferably between 0.5 and 2% by mass, the content of said at least one other transition metal being between 1 and 4% by mass, relative to the total mass of the anhydrous final catalyst.