Synthesis of a palladium-containing composite AFX-BEA zeolite catalyst for NOx adsorption

Description

TECHNICAL FIELD

[0001] The invention relates to a process for preparing a zeolitic composite catalyst composed of a mixture of zeolites of AFX structural type and of BEA structural type and containing palladium, the catalyst prepared or capable of being prepared by the process according to any one of its variants, and its use as a passive NOx adsorber, in particular on internal combustion engines. 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. Selective catalytic reduction, or SCR, is emerging as 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 reducing agent, usually ammonia, and can therefore be referred to as NH3-SCR. Transition metal-exchanged zeolites are notably used as catalysts for NH3-SCR applications in transportation.Small-pore zeolites, particularly copper-exchanged chabazites, are particularly suitable. For spark-ignition engines operating at stoichiometry, three-way catalysts (TWCs) are very effective in reducing NOx when their formulation is appropriate.

[0003] However, during a cold start or more generally when exhaust temperatures are below 180-200°C, neither system can effectively treat NOx emissions. Indeed, the decomposition of the aqueous urea solution which requires temperatures above 180°C limits the NH 3 -SCR efficiency and as for the TWC catalyst an optimization of the low temperature activity is still possible but still seems very difficult to achieve.

[0004] Devices capable of storing NOx at low temperatures and releasing it thermally offer the potential to address the inefficiency of SCR and TWC systems at low temperatures. While the concept is not new, proposed by Ford Global Technologies LLC (US Patent No. 6,182,443) and BASF Catalysts LCC (US Patent No. 6,471,924) in the late 1990s, recent work has shown it to be one of the most promising technologies for limiting NOx emissions at cold start. These systems are often referred to as passive NOx adsorbers (PNAs). Various materials have been evaluated recently for NOx adsorption and desorption.

[0005] As part of a cold start concept, Johnson Matthey Inc proposes an adsorber based on Palladium dispersed on a Cerium oxide, a mixed oxide or a Cerium-based composite oxide which can be thermally regenerated (US Patent No. 8,105,559).

[0006] US patent 2015 / 01580 19 A1 describes the use of a zeolite with a platinum group metal as an adsorber, in particular the use of a CHA or AEI type zeolite. Different zeolite structures have been explored. The passive NOx adsorber comprises a noble metal and a molecular sieve having an OFF type structure (US20190217269A1), a MAZ type structure (WO2016135465A1), an LTL type structure (WO2017 / 001828) or an STI type structure (WO 2019 / 186163 A1). Patent WO 2020039015 A1 describes a passive NOx adsorber which comprises a 12MR and 8MR zeolite such as the MOZ structure zeolite (ZSM-10) and which contains Palladium.

[0007] US Patent 2020 / 0061595A1 presents passive NOx adsorbers based on small-pore zeolite materials with transition metals atomically dispersed in the microporosity. High metal loadings atomically dispersed in the micropores of a small-pore zeolite, particularly SSZ-13 (CHA), impart significant adsorption properties.

[0008] AFX structural type zeolites appear in the list of small-pore zeolites that can be used for NOx adsorption in patent applications WO 2015 / 085303 A1 and WO2016135465A1. However, neither of these patent applications highlights the advantage conferred by this structure, in particular depending on its mode of synthesis.

[0009] In the publication Catal Lett 146, 1706-1711 (2016), the performance evaluation of a Pd / BEA catalyst shows good qualities in terms of NOx storage at 100°C but they degrade rapidly with increasing temperature. NOx desorption is observed from 200°C.

[0010] Patent applications WO2019 / 224090 and WO2019 / 224091 present methods for synthesizing a zeolite composite material composed of a mixture of zeolites of AFX structural type and BEA structural type.

[0011] The applicant has discovered that a new zeolitic composite catalyst composed of an intimate mixture of zeolites of AFX structural type and BEA structural type prepared according to a particular synthesis method and containing palladium (Pd) has not only an increased total NOx storage capacity, but also a higher NOx desorption temperature compared to the prior art. This desorption temperature can be modulated depending on the relative proportion of BEA and AFX zeolites in the composite material. In addition, this catalyst has a high hydrothermal stability which allows it to be used in the severe conditions of an automobile exhaust. SUMMARY OF THE INVENTION

[0012] The invention relates to a process for preparing a zeolitic composite catalyst comprising a mixture of zeolites of AFX structural type and BEA structural type and palladium, comprising at least the following steps: i) the mixture in aqueous medium of a zeolite or a mixture of zeolites of structural type FAU having a total molar ratio SiO 2 (FAU) / Al 2 O 3 (FAU) of between 20 and 60, limits included, of an organic nitrogen compound MPC6, MPC6 being 1,6-bis(methylpiperidinium)hexane dihydroxide, of at least one source of sodium cations, the reaction mixture having the following molar composition: (SiO 2 (FAU) ) / (Al 2 O 3 (FAU) ) of between 20 and 60, limits included, H 2 O / (SiO 2 (FAU) ) of between 5 and 60, MPC6 / (SiO 2 (FAU) ) of between 0.10 and 0.50, Na 2 O / (SiO 2 (FAU) ) of between 0.05 and 0.11, limits included, SiO 2 (FAU) designates the quantity of SiO 2 provided by the FAU zeolite(s), and Al 2 O 3 (FAU) designates the quantity of Al 2 O 3 provided by the FAU zeolite(s), until a homogeneous precursor gel is obtained;ii) hydrothermal treatment of said precursor gel obtained at the end of step i) at a temperature of between 160°C and 220°C, for a period of between 12 and 150 hours to obtain an AFX-BEA zeolite composite material; iii) a step of filtration, washing and drying of the AFX-BEA zeolite composite material obtained at the end of step ii), said drying being carried out at a temperature of between 60 and 120°C, for a period of between 5 and 24 hours to obtain a dried AFX-BEA zeolite composite material, followed by calcination of said dried AFX-BEA zeolite composite material at a temperature of between 500 and 700°C for a period of between 2 and 20 hours, the calcination possibly being preceded by a gradual increase in temperature, to obtain a calcined AFX-BEA zeolite composite material;iv) at least one ion exchange step of the calcined AFX-BEA zeolite composite material obtained in step iii) comprising contacting the calcined AFX-BEA zeolite composite material obtained in step iii), with a solution containing ammonium cations, preferably ammonium nitrate, with stirring at a temperature of between 20 and 95°C, preferably between 60 and 85°C for a period of between 1 hour and 2 days to obtain a calcined material in ammonium form, which is again dried at a temperature of between 60 and 120°C; v) depositing the palladium solution on the calcined and dried AFX-BEA zeolite composite material in ammonium form.

[0013] The palladium solution can be deposited in step v) by dry impregnation or by colloidal means.

[0014] The reaction mixture of step i) may comprise at least one additional source of aluminum in oxide form noted: Al 2 O 3 (C), the reaction mixture of step i) having the following molar composition: SiO 2 (FAU) / (Al 2 O 3 (FAU) + Al 2 O 3 (C) ) between 20 and 60, H 2 O / SiO 2 (FAU) between 5 and 60 MPC6 / SiO 2 (FAU) between 0.10 and 0.50 Na 2 O / SiO 2 (FAU) between 0.05 and 0.11, limits included,

[0015] SiO 2 (FAU) being the quantity of SiO 2 provided by the FAU zeolite, Al 2 O 3 (FAU) being the quantity of Al 2 O 3 provided by the FAU zeolite and Al 2 O 3 (C) being the quantity of Al 2 O 3 provided by the additional source of aluminum considered in its oxide form, MPC6 being the organic nitrogen compound 1,6-bis(methylpiperidinium)hexane in its dihydroxide form.

[0016] Said additional aluminum source may be chosen from aluminum hydroxide or an aluminum salt, a sodium aluminate, an aluminum alkoxide, or alumina taken alone or in mixture, preferably said conventional aluminum source is aluminum hydroxide.

[0017] The source of sodium cations is advantageously sodium hydroxide.

[0018] Crystalline seeds of a zeolite of AFX structural type or of a zeolite of BEA structural type or of a mixture of the two may be added to the reaction mixture of step i), preferably in an amount of between 0.01 and 10% by weight relative to the total mass of the sources of SiO 2 and Al 2 O 3 in anhydrous form present in said mixture, said crystalline seeds not being taken into account in the total mass of the sources of SiO 2 and Al 2 O 3 .

[0019] 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.

[0020] The palladium content introduced by the deposition step v) may be between 0.5 and 5% by mass, preferably between 0.8 and 3% by mass, more preferably between 0.9 and 2% by mass, very advantageously around 1% by mass relative to the total mass of the anhydrous composite catalyst.

[0021] The invention also relates to a palladium-containing AFX-BEA zeolitic composite catalyst obtained by the preparation method according to any of its variants.

[0022] Advantageously, the AFX-BEA zeolitic composite catalyst according to the invention has a SiO 2 / Al 2 O 3 ratio of between 6 and 80, inclusive, preferably between 10 and 40, inclusive.

[0023] The mass ratio between the quantities of zeolites of AFX structural type and BEA type in said catalyst is between 0.9 and 5.7.

[0024] Advantageously, the palladium content in said catalyst is between 0.5 and 5% by mass, preferably between 0.8 and 3% by mass, more preferably between 0.9 and 2% by mass, very advantageously around 1% by mass relative to the total mass of the final anhydrous catalyst and the metallic dispersion of the palladium measured by CO chemisorption is between 40 and 100%, preferably between 50 and 100%.

[0025] The invention also relates to the use of the catalyst according to any one of the variants described or obtained by the process according to any one of the variants described, for the selective reduction of NO x by a reducing agent such as NH 3 or H 2 .

[0026] The catalyst can be formed by deposition as a coating on a honeycomb structure or a plate structure.

[0027] Said 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.

[0028] The quantity of catalyst deposited on said structure can be between 50 and 240 g / L for filter structures and between 50 and 320 g / L for structures with open channels.

[0029] 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 shaped by deposition in the form of a coating.

[0030] 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.

[0031] In another embodiment, said catalyst may be in extrudate form, containing up to 100% of said catalyst.

[0032] The structure coated with said catalyst or obtained by extrusion of said catalyst can be integrated into an exhaust line of an internal combustion engine. LIST OF FIGURES

[0033] Other characteristics and advantages of the process for preparing the catalyst 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. There Figure 1represents the X-ray diffraction (XRD) diagram of the zeolitic composite material composed of a mixture of zeolites of AFX structural type and BEA structural type containing Pd, Pd / AFX-BEA1 obtained according to example 2. The Figure 2 represents the X-ray diffraction (XRD) diagram of the zeolitic composite material composed of a mixture of zeolites of AFX structural type and BEA structural type containing Pd, Pd / AFX-BEA2 obtained according to example 3. The Figure 3 represents the X-ray diffraction (XRD) pattern of the AFX structural type zeolite containing Pd obtained according to Example 5. The Figure 4 represents the X-ray diffraction (XRD) pattern of the Pd-containing BEA zeolite (CP814E) used according to Example 4. The Figure 5 represents the X-ray diffraction (XRD) diagram of the mixture of zeolites of structural type AFX and structural type BEA (CP814E), Pd / AFX-BEA-meca1 obtained according to example 5. The Figure 6represents the NOx concentrations desorbed by the catalysts synthesized according to example 2 (Pd / AFX-BEA1, according to the invention), example 3 (Pd / AFX-BEA2, according to the invention), example 4 (Pd / BEA, comparative) example 5 (Pd / AFX-BEA-meca1, comparative) and example 6 (Pd / AFX-BEA-meca2, comparative). Figure 7 represents the NOx concentrations desorbed by the Pd / AFX-BEA1-aged, Pd / AFX-BEA2-aged, Pd / BEA-aged, Pd / AFX-BEA-meca1-aged and Pd / AFX-BEA-meca2-aged catalysts. DESCRIPTION OF THE EMBODIMENTS

[0034] Unless otherwise specified, throughout the description, the ranges of values ​​are understood to include the limits.

[0035] The invention relates more particularly to a process for preparing a zeolitic composite catalyst composed of an intimate mixture of zeolites of AFX structural type and of BEA structural type and containing palladium, comprising at least the following steps.

[0036] Step i): the mixture in aqueous medium of a zeolite or a mixture of zeolites of structural type FAU having a total molar ratio SiO 2 (FAU) / Al 2 O 3 (FAU) of between 20 and 60, limits included, of an organic nitrogen compound MPC6, MPC6 being 1,6-bis(methylpiperidinium)hexane dihydroxide, of at least one source of sodium cations, the reaction mixture having the following molar composition: (SiO 2 (FAU) ) / (Al 2 O 3 (FAU) ) between 20 and 60, H 2 O / (SiO 2 (FAU) ) between 5 and 60, MPC6 / (SiO 2 (FAU) ) between 0.10 and 0.50, Na 2 O / (SiO 2 (FAU) ) between 0.05 and 0.11, limits included, SiO 2 (FAU) designates the quantity of SiO 2 provided by the FAU zeolite(s), and Al 2 O 3 (FAU) designates the quantity of Al 2 O 3 provided by the FAU zeolite(s), until a homogeneous precursor gel is obtained;

[0037] Preferably, the source of sodium cations is sodium hydroxide.

[0038] In another embodiment, the reaction mixture of step i) may comprise at least one additional conventional source, i.e. not derived from an already formed zeolite, of aluminum considered in its oxide form noted: Al 2 O 3 (C).

[0039] In this case, the reaction mixture of step i) preferably has the following molar composition: SiO 2 (FAU) / (Al 2 O 3 (FAU) + Al 2 O 3 (C) ) between 20 and 60, limits included H 2 O / SiO 2 (FAU) between 5 and 60 MPC6 / SiO 2 (FAU) between 0.10 and 0.50 Na 2 O / SiO 2 (FAU) between 0.05 and 0.11, limits included, SiO 2 (FAU) being the quantity of SiO 2 provided by the FAU zeolite, Al 2 O 3 (FAU) being the quantity of Al 2 O 3 provided by the FAU zeolite and Al 2 O 3 (C) being the quantity of Al 2 O 3 provided by the conventional additional source of aluminum considered in its oxide form, MPC6 being the organic nitrogen compound 1,6-bis(methylpiperidinium)hexane in its dihydroxide form.

[0040] Preferably, the source of sodium cations is sodium hydroxide.

[0041] Crystalline seeds of a zeolite of structural type AFX or BEA or a mixture of the two can be added to the reaction mixture of step i), said seeds preferably being introduced in an amount of between 0.01 and 10% by weight relative to the total mass of the sources of SiO 2 and Al 2 O 3 in anhydrous form present in said mixture, said crystalline seeds not being taken into account in the total mass of the sources of SiO 2 and Al 2 O 3 .

[0042] The zeolite or mixture of zeolites of structural type FAU used in step i) of synthesis have a molar ratio SiO 2 / Al 2 O 3 of between 20 and 60, limits included.

[0043] The starting zeolite or mixture of zeolites of FAU structural type having a SiO 2 / Al 2 O 3 molar ratio of between 20 and 60, inclusive, can be obtained by any method known to those skilled in the art, such as for example by steam treatment and acid washes on a zeolite of FAU structural type with a SiO 2 (FAU) / Al 2 O 3 (FAU) molar ratio of less than 6.00, or by mixing zeolites of FAU structural type with different SiO 2 (FAU) / Al 2 O 3 (FAU) ratios to obtain a SiO 2 (FAU) / Al 2 O 3 (FAU) molar ratio of between 20 and 60, inclusive. Sources of FAU include commercial zeolites CBV712, CBV720, CBV760 and CBV780 produced by Zeolyst, and commercial zeolites HSZ-350HUA, HSZ-360HUA and HSZ-385HUA produced by TOSOH.The starting FAU structural type zeolite can also be used in its sodium form or any other form or a partial or total exchange of sodium cations with ammonium cations followed or not by a calcination step.

[0044] The additional source of an Al 2 O 3 oxide 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 colloidal alumina, pseudoboehmite, gamma alumina, or alpha or beta trihydrate. Mixtures of the above sources may also be used. Preferably the additional source of an Al 2 O 3 oxide is aluminum hydroxide.

[0045] 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.

[0046] Step ii): the hydrothermal treatment of said precursor gel obtained at the end of step i) is carried out at a temperature between 160°C and 220°C, for a duration between 12 and 150 hours.

[0047] Advantageously, the SiO 2 / Al 2 O 3 molar ratio of the AFX-BEA zeolitic composite material obtained is between 6 and 80, inclusive, and preferably between 10 and 40, inclusive.

[0048] Step iii) filtration, washing, drying, and calcination;

[0049] In this step, the AFX-BEA zeolite composite material obtained at the end of step ii) is filtered, washed, and dried at a temperature of between 60 and 120°C, for a period of between 5 and 24 hours to obtain a dried AFX-BEA zeolite composite material. The dried AFX-BEA zeolite composite material is then calcined at a temperature of between 500 and 700°C for a period of between 2 and 20 hours, the calcination possibly being preceded by a gradual increase in temperature.

[0050] Step iv) of ion exchange: at least one ion exchange is carried out on the calcined AFX-BEA zeolite composite material obtained in step iii) and comprises bringing the calcined AFX-BEA zeolite composite material obtained in step iii) into contact with a solution containing ammonium cations, preferably ammonium nitrate, with stirring at temperatures between 20 and 95°C, preferably between 60 and 85°C for a period of between 1 hour and 2 days to obtain a calcined material in ammonium form. At the end of the ion exchange step, the calcined AFX-BEA zeolite composite material in ammonium form is dried again at a temperature between 60 and 120°C, preferably for a period of between 5 and 24 hours. This ion exchange step can be repeated several times in order to obtain a solid containing less than 0.01% Na by mass.

[0051] Step v) of palladium deposition: the deposition of the palladium solution, on the AFX-BEA zeolite composite material calcined in ammonium form and dried, can be carried out using any of the techniques known to those skilled in the art.

[0052] Preferably, the palladium solution is deposited by dry impregnation method or by colloidal means.

[0053] The palladium content introduced by the deposition step v) is advantageously between 0.5 and 5% by mass, preferably between 0.8 and 3% by mass, more preferably between 0.9 and 2% by mass, very advantageously around 1% by mass relative to the total mass of the anhydrous composite catalyst. Dry impregnation

[0054] In one embodiment according to the invention, the palladium solution is deposited by dry impregnation method. More particularly, the catalyst preparation process comprises the following steps: a) an aqueous solution comprising at least one palladium precursor salt is prepared; b) the dried AFX-BEA zeolite composite material in ammonium form obtained in step iv) of the preparation process according to the invention is impregnated with the aqueous solution obtained in step a) at room temperature. c) the catalyst precursor obtained at the end of step b) is dried to obtain a dried catalyst precursor; The catalyst precursor is generally dried in order to remove all or part of the water introduced during the impregnation, preferably at a temperature between 50°C and 250°C, more preferably between 70°C and 200°C. The drying time is preferably between 0.5 and 20 hours. Longer times are not excluded, but do not necessarily provide an improvement.The drying is generally carried out under combustion air of a hydrocarbon, preferably methane, or under heated air comprising between 0 and 80 g of water per kg of combustion air, an oxygen level of between 5% and 25% volume and a carbon dioxide level of between 0% and 10% volume. d) optionally, the dried catalyst precursor obtained in step c) is calcined at a temperature of between 250 and 900°C to obtain a calcined catalyst precursor; After drying, the catalyst can be calcined under air, preferably combustion air, and more preferably methane combustion air, comprising between 40 and 80 g of water per kg of air, an oxygen level of between 5% and 15% volume and a CO2 level of between 4% and 10% volume. The calcination temperature is generally between 250°C and 900°C, preferably between 350°C and 550°C. The calcination time is generally between 0.5 and 5 hours.e) optionally, a reducing treatment is carried out by contact with a reducing gas. Colloidal impregnation

[0055] In another embodiment according to the invention, the palladium solution is deposited by colloidal method. More particularly, the process for preparing the catalyst comprises the following steps: a) preparation of an aqueous solution comprising at least one palladium precursor salt In step a), an aqueous solution comprising at least one palladium precursor salt is prepared. The palladium precursor salt is preferably selected from sodium chloropalladate and palladium nitrate. b) preparation of an impregnated support The deposition of palladium on the AFX-BEA zeolitic composite material calcined in ammonium form and dried obtained in step iv) of the preparation process according to the invention is carried out by dry impregnation of the aqueous solution obtained in step a) on the AFX-BEA zeolitic composite material calcined in ammonium form and dried, the volume of said aqueous solution generally being between 0.9 and 1.1 times the pore volume of the AFX-BEA zeolitic composite material calcined in ammonium form and dried.c) maturation of the support impregnated during step b) After impregnation, the AFX-BEA zeolite composite material calcined in ammonium form and dried, is matured in the wet state for 0.5 to 40 hours, preferably for 1 to 30 hours. Longer times are not excluded, but do not necessarily provide an improvement. d) drying of the catalyst precursor obtained in step b) or c) The catalyst precursor is generally dried in order to remove all or part of the water introduced during the impregnation, preferably at a temperature between 50°C and 250°C, more preferably between 70°C and 200°C. The drying time is preferably between 0.5 and 20 hours. Longer times are not excluded, but do not necessarily provide an improvement.Drying is generally carried out under combustion air of a hydrocarbon, preferably methane, or under heated air comprising between 0 and 80 g of water per kg of combustion air, an oxygen level of between 5% and 25% volume and a carbon dioxide level of between 0% and 10% volume. e) calcination under combustion air of the dried catalyst obtained in step d) (optional step).

[0056] After drying, the catalyst can be calcined in air, preferably combustion air, and more preferably methane combustion air, comprising between 40 and 80 g of water per kg of air, an oxygen level of between 5% and 15% volume and a CO 2 level of between 4% and 10% volume. The calcination temperature is generally between 250°C and 900°C, preferably between 350°C and 550°C. The calcination time is generally between 0.5 and 5 hours.

[0057] The present invention also relates to a composite catalyst in the form of a zeolitic material composed of a mixture of zeolites of AFX structural type and BEA structural type, and containing palladium, which can be prepared by the process according to one embodiment of the invention, said composite catalyst having a mass ratio between the quantities of zeolite of AFX structural type and of zeolite of BEA type of between 0.9 and 5.7.

[0058] The palladium content of the zeolitic composite catalyst according to the invention is advantageously between 0.5 and 5% by weight relative to the total mass of anhydrous catalyst, preferably between 0.8 and 3% by weight, very preferably between 0.9% and 2% by weight, very advantageously around 1% by weight.

[0059] In the zeolitic composite catalyst according to the invention, the dispersion of palladium measured by chemisorption of CO is advantageously between 40 and 100%, preferably between 50 and 100%. Characterizations of the catalyst according to the invention

[0060] At the end of the preparation steps ii), iii), iv), or v) of the catalyst according to the invention, X-ray diffraction makes it possible to verify that the solid obtained by the process according to the invention is indeed an AFX-BEA zeolitic composite material containing the zeolites of the AFX and BEA structural type. The purity obtained is advantageously greater than 90%, preferably greater than 95%, very preferably greater than 99.8% by weight.

[0061] In other words, the zeolitic composite catalyst according to the invention based on AFX / BEA zeolitic composite material and containing palladium comprises less than 5% by mass, preferably less than 1% by mass and even more preferably less than 0.2% by mass of impurities and / or crystalline or amorphous phase other than AFX and BEA (the limits not being included). Very advantageously, the process of the invention leads to the formation of a catalyst based on AFX-BEA zeolitic composite material, free of any other crystalline or amorphous phase.

[0062] This technique also makes it possible to determine the relative proportions of each zeolite, AFX and BEA, contained in said catalyst according to the invention or obtained by the process of the invention according to any one of its variants.

[0063] Advantageously, the solid obtained by the process according to the invention has the X-ray diffraction diagram including at least the lines listed in Table 1.

[0064] 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 obtained at the end of step ii) of the process according to the invention comprises at least the lines with the values ​​of d hkl given in Table 1 (Average values ​​of d hkl and relative intensities measured on an X-ray diffraction pattern of the zeolitic composite catalyst according to the invention). In the column of d hkl , the average values ​​of the inter-reticular distances in Angstroms (Â) are indicated. Each of these values ​​must be affected by the measurement error Δ(d hkl ) between ± 0.6Â and ± 0.01Â. Table 1 2 theta (°) dhkl (Å) Irel 2 theta (°) dhkl (Å) Irel 7,49 11,79 ff-f 26,11 3,41 fm 7,71 11,46 ff-f 26,94 3,31 Ff 8,71 10,14 mf-m 27,11 3,29 Ff 11,66 7,59 f-mf 27,61 3,23 Ff 12,97 6,82 f 28,04 3,18 mf-m 15,00 5,90 ff 28,68 3,11 Ff 15,40 5,75 ff 29,51 3,03 Ff 15,66 5,66 f-mf 30,19 2,96 ff-f 17,47 5,07 mf 30,58 2,92 Mf 17,90 4,95 f-mf 30,99 2,88 Ff 19,42 4,57 ff 31,59 2,83 f-mf 19,88 4,46 ff-f 32,50 2,75 Ff 20,38 4,36 F-FF 33,73 2,66 f-mf 21,08 4,21 f 34,29 2,61 Ff 21,31 4,17 ff-f 34,78 2,58 Ff 21,82 4,07 F-FF 35,11 2,55 Ff 22,19 4,00 fm 35,79 2,51 Ff 22,34 3,98 f-FF 37,56 2,39 Ff 22,54 3,94 ff-f 38,00 2,37 Ff 22,70 3,91 ff-f 39,18 2,30 Ff 23,67 3,76 mf 39,61 2,30 Ff 25,24 3,52 ff where FF = very strong; F = strong; m = medium; mf = medium weak; f = weak; ff = very weak. The relative intensity I rel is given in relation to a relative intensity scale where a value of 100 is assigned to the most intense line in the X-ray diffraction pattern: ff < 15; 15 ≤ f < 30; 30 ≤ mf < 50; 50 ≤ m < 65; 65 ≤ F < 85; FF ≥ 85; 1 ≤ ff-f < 30; 30 ≤ mf-m < 65; 15 ≤ f-mf < 50; 65 ≤ F-FF ≤ 100; 15 ≤ fm < 65; 15 ≤ f-FF ≤ 100.

[0065] According to the invention, the mass composition of the prepared composite material, in particular the relative mass fractions of the AFX structural type and BEA structural type zeolites present in said composite material-based catalyst, is advantageously determined using a method similar to ASTM D3906 03, by comparing the peak areas at angles (2θ) 20.38 ± 0.1 (hkl: 211); 23.67 ± 0.1 (hkl: 105); 26.1 ± 0.1 (hkl: 303) and 28.02 ± 0.1 (hkl: 106) of the X-ray diagrams obtained for the composite material according to the invention and a reference AFX structural type zeolite, preferably of high purity. The mass ratio of the two zeolites AFX and BEA in the composite material according to the invention is thus evaluated by comparing the sum of the peak areas at the angles (2θ) mentioned above obtained for the composite material prepared according to the invention with a reference sample of zeolite and using the following calculation formula: AFX / BEA = S AFXc / S AFXr − S AFXc where S AFXc is the sum of the areas of the peaks present at the angles (2θ) 20.38 ± 0.1 (hkl: 211); 23.67 ± 0.1 (hkl: 105); 26.1 ± 0.1 (hkl: 303) and 28.02 ± 0.1 (hkl: 106) of the diffractogram of the AFX-BEA composite material prepared according to the invention and S AFXr is the sum of the areas of the peaks present at the angles (2θ): 20.38 (hkl: 211); 23.67 (hkl: 105); 26.1 (hkl: 303) and 28.02 (hkl: 106) of the diffractogram of the pure AFX structural type zeolite, used as a reference. The pure AFX structural type zeolite, used as a reference, can, for example, be prepared according to the method illustrated in Example 5 of the present disclosure.

[0066] 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.

[0067] The loss on ignition (LOI) of the catalyst obtained after the drying step (and before calcination) or after the calcination step of step iii) of the process according to the invention is generally between 2 and 20% by weight. The loss on ignition of the catalyst sample, designated by the acronym LOI, 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.

[0068] Particle dispersion (D) is a unitless number, often expressed as a percentage. The smaller the particles, the greater the dispersion. It is defined in R. Van Hardeveld and F. Hartog, "The statistics of surface atoms and surface sites on metal crystals," Surface Science 15, 1969, 189-230. Dispersion can be measured by CO chemisorption. Catalyst according to the invention

[0069] The applicant has discovered that the catalyst obtained by the process according to the invention has characteristics different from the microporous aluminosilicate materials comprising palladium known from the prior art. In particular, the AFX-BEA zeolitic composite catalyst containing palladium, obtained by any of the variants of the process according to the invention, has an increased adsorption capacity and desorption temperature as well as superior hydrothermal resistance.

[0070] Without being bound by any theory, it would appear that the process for preparing the catalyst according to the invention allows in particular a better dispersion of palladium, which contributes to improved catalytic performances in reducing NOx. Furthermore, the better dispersion of palladium makes it possible to optimize the quantity of accessible and active palladium, and to use lower palladium contents while obtaining very good catalytic performances.

[0071] Advantageously, the SiO 2 / Al 2 O 3 molar ratio of the AFX-BEA composite zeolite-based catalyst obtained is between 6 and 80, inclusive, preferably between 10 and 40, inclusive.

[0072] The palladium content of the zeolitic composite catalyst according to the invention is advantageously between 0.5 and 5% by weight relative to the total mass of anhydrous catalyst, preferably between 0.8 and 3% by weight, very preferably between 0.9% and 2% by weight, very advantageously around 1% by weight.

[0073] In the zeolite composite catalyst prepared according to the invention, the dispersion of palladium is advantageously between 40 and 100%, preferably between 50 and 100%. Use of the catalyst according to the invention

[0074] 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 according to any of its variants, for the adsorption of NOx at temperatures below 200°C, advantageously formed by deposition in the form of a coating ("washcoat" according to English terminology) on a honeycomb structure mainly for mobile applications.

[0075] 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, FeCrAl type steel.

[0076] The quantity of catalyst according to the invention deposited on said structure is between 50 and 240 g / L for filter structures and between 50 and 320 g / L for structures with open channels.

[0077] 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 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 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.

[0078] 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 or is covered by, another coating having pollutant reduction capacities, in particular NOx and / or promoting the oxidation of pollutants, in particular carbon monoxide (CO) and hydrocarbons (HC).

[0079] Another possibility is to put the catalyst in extruded form. In this case, the resulting structure can contain up to 100% of the catalyst according to the invention.

[0080] Said structure coated with the catalyst according to the invention is advantageously integrated into an exhaust line of an internal combustion engine. Under these engine operating conditions, the exhaust gases contain in particular the following pollutants: soot, unburned hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx). Said structure coated with the catalyst according to the invention is placed upstream of a NOx treatment catalyst which may be an NH 3 -SCR catalyst, a NOxTrap or a three-way catalyst. An oxidation catalyst whose function is to oxidize HC and CO as well as a filter for removing soot from the exhaust gases may be placed indifferently upstream or downstream of said structure.

[0081] The function of said coated structure being to adsorb NOx, its operating range being: between -20°C and 300°C for the adsorption phase, and preferably between -20°C and 200°C; between 150°C and 500°C for the desorption phase and preferably between 250°C and 400°C. ADVANTAGES OF THE INVENTION

[0082] The catalyst according to the invention, based on a zeolitic composite material composed of an intimate mixture of zeolites of AFX structural type and BEA structural type and containing palladium, has improved properties compared to the catalysts of the prior art. In particular, the use of the catalyst according to the invention makes it possible to adsorb NOx between - 20°C and 200°C.

[0083] It also exhibits better resistance to hydrothermal aging, guaranteeing high adsorption performance even after this aging. EXAMPLES

[0084] The invention is illustrated by the following examples which are in no way limiting. Example 1: preparation of 1,6-bis(methylpiperidinium)hexane dihydroxide (organic structuring agent MPC6).

[0085] 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%).

[0086] 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).

[0087] 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 (MPC6). The determination of this species is carried out by proton NMR using formic acid as a standard. Example 2: preparation of an AFX-BEA zeolite catalyst containing palladium according to the invention

[0088] 275.3 g of an aqueous solution of 1,6-bis(methylpiperidinium)hexane dihydroxide (18.36% by weight) prepared according to Example 1 are mixed with 390.61 g of deionized water, with stirring and at room temperature. 7.18 g of sodium hydroxide (98% by weight, Aldrich) are dissolved in the previous mixture with stirring and at room temperature. Subsequently, 6.32 g of amorphous aluminum hydroxide gel (Al(OH) 3 amorphous gel, 58.55% by mass of Al 2 O 3 , Merck), are incorporated into the synthesis mixture, which is kept stirring for half an hour at room temperature. As soon as the suspension obtained is homogeneous, we begin to pour 70.9 g of a zeolite of structural type FAU (CBV780, SiO 2 / Al 2 O 3 = 90.54, Zeolyst, PAF = 8.52%) and we keep stirring the suspension obtained for 30 minutes at room temperature.Then, 5.73 g of seeds (8.36% relative to the total mass of the SiO 2 and Al 2 O 3 sources in anhydrous form present in said mixture) of a calcined AFX structural type zeolite are added to the synthesis mixture which is kept stirring for 30 minutes. The molar composition of the precursor gel is as follows: 1 SiO 2 : 0.05 Al 2 O 3 : 0.167 MPC6: 0.093 Na 2 O: 36.73 H 2 O, i.e. a SiO 2 / Al 2 O 3 ratio of 20. The precursor gel is then transferred, after homogenization, into a 1000 mL stainless steel reactor equipped with a stirring system with four inclined blades. The reactor is closed, then heated for 18 hours with a temperature rise of 3°C / min up to 180°C while stirring at 200 rpm to allow the crystallization of a mixture of a zeolite of structural type AFX and a zeolite of structural type BEA. The crystallized product obtained is filtered, washed with deionized water, then dried for 12 hours at 100°C.The loss on ignition of the dried solid is 16.38%. 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 rise of 1°C / min up to 550°C followed by a hold at 550°C maintained for 12 hours and then a return to room temperature.

[0089] After calcination, 40.0 g of this mixture of zeolites were exchanged 3 times with a 3M aqueous solution of NH 4 NO 3 at 80 °C for 1 hour under stirring (300 rpm) and a volume of solution to mass of zeolite ratio equal to 10 (V / P). Then, the solid was filtered and dried overnight at 100 °C. It is named: NH 4 -AFX-BEA1.

[0090] 0.25 g Pd(NH 3 ) 4 Cl 2 hydrate is diluted in 12 ml of demineralized water then impregnated at 25°C (by the dry impregnation method) on 10 g of the solid NH 4 -AFX-BEA1 prepared above.

[0091] The obtained Pd-NH 4 -AFX-BEA1 catalyst is dried in air for 2 h at 120°C, then calcined for 2 hours at 550°C under a combustion air flow with a VVH of 3000 liters of combustion air per liter of catalyst per hour. The combustion air contains approximately 60 g of water per kg of dry air.

[0092] The Pd / AFX-BEA1 catalyst thus prepared comprises 1% by weight of palladium relative to the total weight of catalyst. The metallic dispersion of Pd obtained by CO chemisorption is 60%.

[0093] The Pd / AFX-BEA1 catalyst was analyzed by X-ray diffraction and identified as a zeolite composite material consisting of a mixture of approximately 78% by mass of an AFX structural type zeolite and 22% by mass of a BEA structural type zeolite, giving an AFX / BEA mass ratio of 3.55. The X-ray diffraction pattern performed on the Pd / AFX-BEA1 catalyst is given on the Figure 1The product has a SiO 2 / Al 2 O 3 molar ratio of 16.64 as determined by FX. Example 3: preparation of a catalyst containing a zeolite of structural type AFX-BEA with palladium according to the invention

[0094] 23.9 g of a zeolite of structural type FAU (CBV712 Zeolyst, SiO 2 / Al 2 O 3 = 11.42, PAF = 12.81%) were mixed with 495.2 g of deionized water. 57.3 g of a zeolite of structural type FAU (CBV780 Zeolyst, SiO 2 / Al 2 O 3 = 98.22, PAF = 8.52%) were added to the previous mixture, the preparation obtained was kept stirring for 10 minutes. 290.5 g of an aqueous solution of 1,6-bis(methylpiperidinium)hexane dihydroxide (20.91% by weight) prepared according to Example 1, were added to the previous mixture. The mixture was then kept stirring for 10 minutes. 33.0 g of a 20% by weight aqueous solution of sodium hydroxide (solution prepared from 98% by weight sodium hydroxide, Aldrich) are added to the mixture and kept stirring for 10 minutes. The molar composition of the precursor gel is as follows: 1 SiO 2 : 0.03 Al 2 O 3 : 0.167 MPC6: 0.072 Na 2 O: 36.73 H 2 O, i.e. a SiO 2 / Al 2 O 3 ratio of 33.3.The precursor gel is then transferred, after homogenization, into an autoclave. The autoclave is closed and then heated for 6 days at 180°C with stirring at 35 rpm using a rotating spit system. The resulting crystallized product is filtered, washed with deionized water and then dried overnight at 100°C.

[0095] The solid is then introduced into a muffle furnace where a calcination step is carried out under air flow: 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.

[0096] After calcination, 40.0 g of this mixture of zeolites were exchanged 3 times with a 3M aqueous solution of NH 4 NO 3 at 80 °C for 1 hour under stirring (300 rpm) and a volume ratio of solution to mass of zeolite equal to 10 (V / P). Then, the solid was filtered and dried overnight at 100 °C. It is named: NH 4 -AFX-BEA2.

[0097] 0.25 g of hydrated Pd(NH 3 ) 4 Cl 2 is diluted in 13 ml of demineralized water and then impregnated at 25°C (by the dry impregnation method) on 10 g of the zeolite NH 4 -AFX-BEA2 prepared above.

[0098] The obtained Pd-NH 4 -AFX-BEA2 catalyst is dried in air for 2 h at 120°C, then calcined for 2 hours at 550°C under a combustion air flow with a VVH of 3000 liters of combustion air per liter of catalyst per hour. The combustion air contains approximately 60 g of water per kg of dry air.

[0099] The Pd / AFX-BEA2 catalyst thus prepared comprises 1% by weight of palladium relative to the total weight of catalyst. The metallic dispersion of Pd obtained by CO chemisorption is 58%.

[0100] The Pd / AFX-BEA2 catalyst was analyzed by X-ray diffraction and identified as a zeolitic composite material consisting of a mixture of approximately 50% by mass of an AFX structural type zeolite and 50% by mass of a BEA structural type zeolite, which gives an AFX / BEA mass ratio of 1. The X-ray diffraction pattern performed on the Pd / AFX-BEA2 catalyst is given on the Figure 2 . Example 4 (Comparative) : preparation of a catalyst containing a BEA structural type zeolite with Pd

[0101] Commercial zeolite NH 4 -BEA (CP814E, SiO 2 / Al 2 O 3 = 25.16, Zeolyst), was used as a support for the impregnation of the palladium precursor.

[0102] 0.25 g of hydrated Pd(NH 3 ) 4 Cl 2 is diluted in 17 ml of demineralized water and then impregnated at 25°C (by the dry impregnation method) on 10 g of zeolite NH 4 -BEA (CP814E, SiO 2 / Al 2 O 3 = 25.16, Zeolyst).

[0103] The obtained solid is dried in air for 2 h at 120°C. The obtained Pd-NH 4 -BEA catalyst is dried in air at 120°C, then calcined for 2 hours at 550°C under a combustion air flow with a VVH of 3000 liters of combustion air per liter of catalyst per hour. The combustion air contains approximately 60 g of water per kg of dry air.

[0104] The Pd / BEA catalyst thus prepared comprises 1% by weight of palladium relative to the total weight of catalyst. The metallic dispersion of Pd obtained by CO chemisorption is 28%. The X-ray diffraction pattern carried out on the Pd / BEA catalyst is given on the Figure 4 . Example 5 (Comparative) : preparation of a catalyst containing a mechanical mixture of zeolites of structural type AFX and BEA (78 / 22) with palladium 1°) preparation of a catalyst containing a zeolite of structural type AFX with palladium

[0105] 275.8 g of an aqueous solution of 1,6-bis(methylpiperidinium)hexane dihydroxide (18.36% by weight) prepared according to Example 1 are mixed with 391.23 g of deionized water, with stirring and at room temperature. 7.18 g of sodium hydroxide (98% by weight, Aldrich) are dissolved in the previous mixture with stirring and at room temperature. Subsequently, 6.33 g of amorphous aluminum hydroxide gel (Al(OH) 3 amorphous gel, 58.55% by mass of Al 2 O 3 , Merck), are incorporated into the synthesis mixture, which is kept stirring for half an hour at room temperature. As soon as the suspension obtained is homogeneous, we begin to pour 70.88 g of a zeolite of structural type FAU (CBV780, SiO 2 / Al 2 O 3 = 90.54, Zeolyst, PAF = 8.52%) and we keep stirring the suspension obtained for 30 minutes at room temperature.In order to promote the formation of an AFX structural type zeolite, 6.14 g of seeds (8.7% relative to the mass of CBV780 zeolite) of a calcined AFX structural type zeolite are added to the synthesis mixture which is kept stirring for 5 minutes. Then, the reaction mixture undergoes a maturing step for 24 hours at room temperature with stirring (200 rpm). The molar composition of the precursor gel is as follows: 1 SiO 2 : 0.05 Al 2 O 3 : 0.167 R: 0.093 Na 2 O: 36.73 H 2 O, i.e. a SiO 2 / Al 2 O 3 ratio of 20. The precursor gel is then transferred, after homogenization, into a 1000 mL stainless steel reactor equipped with a stirring system with four inclined blades. The reactor is closed, then heated for 14 hours with a temperature rise of 5°C / min up to 180°C while stirring at 200 rpm to allow the crystallization of the AFX structural type zeolite.The crystallized product obtained is filtered, washed with deionized water, then dried overnight at 100°C. 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 rise of 1°C / min up to 550°C followed by a hold at 550°C maintained for 12 hours and then a return to room temperature.

[0106] After calcination, 40.0 g of this zeolite were exchanged 3 times with a 3M aqueous solution of NH 4 NO 3 at 80°C for 1 hour under stirring (300 rpm) and a volume of solution to mass of zeolite ratio equal to 10 (V / P). Then, the solid NH 4 -AFX was filtered and dried overnight at 100°C.

[0107] 0.25 g of hydrated Pd(NH 3 ) 4 Cl 2 is diluted in 8.2 ml of demineralized water and then impregnated at 25°C (by the dry impregnation method) on 10 g of the NH 4 -AFX zeolite prepared above.

[0108] The obtained Pd-NH 4 -AFX catalyst is dried in air for 2 h at 120°C, then calcined for 2 hours at 550°C under a combustion air flow with a VVH of 3000 liters of combustion air per liter of catalyst per hour. The combustion air contains approximately 60 g of water per kg of dry air.

[0109] The Pd / AFX catalyst thus prepared comprises 1% by weight of palladium relative to the total weight of catalyst. The metallic dispersion of Pd obtained by CO chemisorption is 55%.

[0110] The Pd / AFX catalyst was analyzed by X-ray diffraction and identified as consisting of a zeolite of the AFX structural type with a purity greater than 99% by weight. The X-ray diffraction pattern performed on the calcined solid is given in Figure 3 . The product has a SiO 2 / Al 2 O 3 molar ratio of 14.46 as determined by FX. 2°) preparation of a Pd AFX-BEA catalyst obtained by mechanical mixing (comparative)

[0111] 156 mg of the palladium AFX structural type zeolite as prepared and 44 mg of a palladium BEA structural type zeolite prepared according to Example 4 are mixed (i.e. an AFX / BEA mass ratio of 3.55). The material obtained is named Pd / AFX-BEA-meca1.

[0112] The Pd / AFX-BEA-meca1 catalyst thus prepared comprises 1% by weight of palladium relative to the total weight of catalyst. The metallic dispersion of Pd obtained by CO chemisorption is 50%.

[0113] The Pd / AFX-BEA-meca1 catalyst was analyzed by X-ray diffraction and identified as a zeolitic composite material consisting of a mixture of approximately 78% by mass of an AFX structural type zeolite and 22% by mass of a BEA structural type zeolite, which gives an AFX / BEA mass ratio of 1. The X-ray diffraction pattern performed on the Pd / AFX-BEA-meca1 catalyst is given on the Figure 5 . Example 6 (Comparative) : preparation of a catalyst containing a mechanical mixture of zeolites of structural type AFX and BEA (50 / 50) with palladium

[0114] 100 mg of a palladium AFX structural type zeolite prepared according to Example 5 and 100 mg of a palladium BEA structural type zeolite prepared according to Example 4 are mixed (i.e. an AFX / BEA mass ratio of 1). The material obtained is named Pd / AFX-BEA-meca2.

[0115] The Pd / AFX-BEA-meca2 catalyst thus prepared comprises 1% by weight of palladium relative to the total weight of catalyst. The metallic dispersion of Pd obtained by CO chemisorption is 48%. Example 7 : Hydrothermal aging stage

[0116] 550 mg of each of the samples synthesized according to Example 2 (Pd / AFX-BEA1), Example 3 (Pd / AFX-BEA2), Example 4 (Pd / BEA), Example 5 (Pd / AFX-BEA-meca1) and Example 6 (Pd / AFX-BEA-meca2) are placed in powder form in a quartz reactor. They are crossed by a flow rate of 150 L / h of a mixture of the following molar composition: 10% H 2 O, 20% O 2 and N 2 in addition.

[0117] The samples are subjected to these conditions for 4 hours at a temperature of 750°C. They are then cooled to room temperature under a flow of N2.

[0118] The sample synthesized according to Example 2 and aged under the conditions of Example 7 is named Pd / AFX-BEA1-aged.

[0119] The sample synthesized according to Example 3 and aged under the conditions of Example 7 is named Pd / AFX-BEA2-aged.

[0120] The sample synthesized according to Example 4 and aged under the conditions of Example 7 is named Pd / BEA-aged.

[0121] The sample synthesized according to Example 5 and aged under the conditions of Example 7 is named Pd / AFX-BEA-meca1-aged.

[0122] The sample synthesized according to Example 6 and aged under the conditions of Example 7 is named Pd / AFX-BEA-meca2-aged. Example 8 : NOx adsorption and desorption tests

[0123] To demonstrate the adsorption capacities, a NOx adsorption test at 120°C followed by a temperature ramp from 120°C to 600°C at 10°C / min is carried out with the catalysts synthesized according to Example 2 (Pd / AFX-BEA1, according to the invention), Example 3 (Pd / AFX-BEA2, according to the invention), Example 4 (Pd / BEA), Example 5 (Pd / AFX-BEA-meca1) and Example 6 (Pd / AFX-BEA-meca2). For the test of each sample, 523 mg of catalyst in powder form are placed in a quartz reactor. Preconditioning is carried out, it consists of a temperature rise from ambient (20°C) to 550°C with a ramp of 10°C / min under a mixture composed of 10% O 2 , 5% CO 2 , 10% H 2 O and N 2 in addition. The temperature decrease is done under the same mixture up to 120°C. Adsorption is carried out at 120°C under the following mixture: 200 ppm NO + 300 ppm CO + 5% CO 2 + 10% H 2 O and N 2 in addition. The duration of this adsorption phase is 10 min. To desorb the adsorbed NOx, the sample undergoes a temperature rise up to 550°C following a ramp of 10°C / min under a mixture of 10% CO 2 + 10% H 2 O and N 2 in addition.

[0124] An FTIR analyzer can measure the concentration of the species NO, NO2, NH3, N2O, CO, CO2, H2O, O2 at the reactor outlet.

[0125] There Figure 6shows the desorbed NOx concentrations during the temperature ramp. The curve marked by squares, the one marked by diamonds, the one marked by triangles, the one marked by crosses and the one marked by circles correspond respectively to the tests carried out with the catalysts synthesized according to Example 2 (Pd / AFX-BEA1, according to the invention), Example 3 (Pd / AFX-BEA2, according to the invention), Example 4 (Pd / BEA), Example 5 (Pd / AFX-BEA-meca1) and Example 6 (Pd / AFX-BEA-meca2). The quantities of desorbed NOx per mass of catalyst during the temperature ramp as well as the maximum desorption temperature are given below: Table 2 NOx [µmol / gcat] Temp. [°C] Pd / AFX-BEA1 79,6 360 Pd / AFX-BEA2 68,2 335 Pd / BEA 63,9 333 Pd / AFX-BEA-meca1 76,4 370 Pd / AFX-BEA-meca2 66,6 340

[0126] The Pd / AFX-BEA1 and Pd / AFX-BEA2 catalysts synthesized according to the invention provide superior adsorption performances to the Pd / BEA, Pd / AFX-BEA-meca1 and Pd / AFX-BEA-meca2 catalysts in terms of adsorbed quantity. Unlike the Pd / BEA, Pd / AFX-BEA-meca1 and Pd / AFX-BEA-meca2 catalysts which exhibit NOx desorption from 150°C, NOx desorption only appears around 250°C for the Pd / AFX-BEA1 and Pd / AFX-BEA2 catalysts according to the invention. These NOx desorptions above 250°C present a notable advantage for controlling NOx emissions. Example 10 : NOx adsorption and desorption tests after hydrothermal aging

[0127] A NOx adsorption test at 120°C followed by a temperature ramp at 10°C / min up to 600°C is carried out with the aged catalysts according to example 7: Pd / AFX-BEA1-aged, Pd / AFX-BEA2-aged, Pd / BEA-aged, Pd / AFX-BEA-meca1-aged and Pd / AFX-BEA-meca2-aged.

[0128] For testing each sample, 523 mg of catalyst in powder form is placed in a quartz reactor. Preconditioning is carried out, it consists of a temperature rise from ambient (20°C) to 550°C with a ramp of 10°C / min under a mixture composed of 10% O 2 , 5% CO 2 , 10% H 2 O and N 2 in addition. The temperature decrease is done under the same mixture up to 120°C. Adsorption is carried out at 120°C under the following mixture: 200 ppm NO + 300 ppm CO + 5% CO 2 + 10% H 2 O and N 2 in addition. The duration of this adsorption phase is 10 min. To desorb the adsorbed NOx, the sample undergoes a temperature rise up to 550°C following a ramp of 10°C / min under a mixture of 10% CO 2 + 10% H 2 O and N 2 in addition.

[0129] An FTIR analyzer can measure the concentration of the species NO, NO 2 , NH 3 , N 2 O, CO, CO 2 , H 2 O, O 2 at the reactor outlet

[0130] There Figure 7shows the desorbed NOx concentrations during the temperature ramp. The curve marked by squares, the one marked by diamonds, the one marked by triangles, the one marked by crosses and the one marked by circles correspond respectively to the tests carried out with the Pd / AFX-BEA1-aged, Pd / AFX-BEA2-aged, Pd / BEA-aged, Pd / AFX-BEA-meca1-aged and Pd / AFX-BEA-meca2-aged catalysts. The quantities of desorbed NOx per mass of catalyst during the temperature ramp as well as the maximum desorption temperature are given below: Table 3 NOx [µmol / gcat] Temp. [°C] Pd / AFX-BEA1-aged 68,4 330 Pd / AFX-BEA2-aged 62,4 335 Pd / BEA-aged 40,9 340 Pd / AFX-BEA-meca1-aged 62,5 350 Pd / AFX-BEA-meca2-aged 51,1 348

[0131] The Pd / AFX-BEA1-aged and Pd / AFX-BEA2-aged catalysts synthesized according to the invention give superior adsorption performances to the Pd / BEA-aged catalysts, and to the catalysts made by mechanical mixing Pd / AFX-BEA-meca1-aged and Pd / AFX-BEA-meca2-aged synthesized according to the prior art in terms of adsorbed quantity. The Pd / BEA-aged, Pd / AFX-BEA-meca1-aged and Pd / AFX-BEA-meca2-aged catalysts exhibit NOx desorption from 150°C, for the Pd / AFX-BEA1-aged and Pd / AFX-BEA2-aged catalysts according to the invention the desorption of NOx only appears around 250°C.

Claims

1. Process for preparing a zeolite composite catalyst comprising a mixture of zeolites of AFX structural type and of BEA structural type and palladium, comprising at least the following steps: i) the mixing, in an aqueous medium, of a zeolite or a mixture of zeolites of FAU structural type having a total SiO2 (FAU) / Al2O3 (FAU) molar ratio of between 20 and 60, limits included, an organic nitrogenous compound MPC6, MPC6 being 1,6-bis(methylpiperidinium)hexane dihydroxide, at least one source of sodium cations, the reaction mixture having the following molar composition: (SiO2 (FAU)) / (Al2O3 (FAU)) of between 20 and 60, H2O / (SiO2 (FAU)) of between 5 and 60, M PC6 / (SiO2 (FAU)) of between 0.10 and 0.50, Na2O / (SiO2 (FAU)) of between 0.05 and 0.11, limits included, SiO2 (FAU) denotes the amount of SiO2 provided by the FAU zeolite(s) and Al2O3 (FAU) denotes the amount of Al2O3 provided by the FAU zeolite(s), until a homogeneous precursor gel is obtained; ii) the hydrothermal treatment of said precursor gel obtained on conclusion of step i) at a temperature of between 160°C and 220°C, for a time of between 12 and 150 hours, in order to obtain an AFX-BEA zeolite composite material; iii) a step of filtration, washing and drying of the AFX-BEA zeolite composite material obtained on conclusion of step ii), said drying being carried out at a temperature of between 60 and 120°C, for a time of between 5 and 24 hours, in order to obtain a dried AFX-BEA zeolite composite material, followed by calcination of said dried AFX-BEA zeolite composite material at a temperature of between 500 and 700°C for a time of between 2 and 20 hours, the calcination possibly being preceded by a gradual increase in temperature, in order to obtain a calcined AFX-BEA zeolite composite material; iv) at least one step of ion exchange of the calcined AFX-BEA zeolite composite material obtained in step iii), comprising bringing the calcined AFX-BEA zeolite composite material obtained in step iii) into contact with a solution containing ammonium cations, preferably ammonium nitrate, with stirring at a temperature of between 20 and 95°C, preferably between 60 and 85°C, for a time of between 1 hour and 2 days, in order to obtain a calcined material in ammonium form, which is again dried at a temperature of between 60 and 120°C; v) deposition of a palladium solution on the ammonium-form calcined and dried AFX-BEA zeolite composite material.

2. Process according to Claim 1, wherein said palladium solution is deposited in step v) by dry impregnation or via a colloidal route.

3. Process according to either of Claims 1 and 2, wherein the reaction mixture of step i) may comprise at least one additional source of aluminum in oxide form, denoted: Al2O3 (C), the reaction mixture of step i) having the following molar composition: SiO2 (FAU) / (Al2O3 (FAU) + Al2O3 (C)) of between 20 and 60, limits included, H2O / SiO2 (FAU) of between 5 and 60, MPC6 / SiO2 (FAU) of between 0.10 and 0.50, Na2O / SiO2 (FAU) of between 0.05 and 0.11, limits included, SiO2 (FAU) being the amount of SiO2 provided by the FAU zeolite, Al2O3 (FAU) being the amount of Al2O3 provided by the FAU zeolite and Al2O3 (C) being the amount of Al2O3 provided by the additional source of aluminum considered in its oxide form, MPC6 being the organic nitrogenous compound 1,6-bis(methylpiperidinium)hexane in its dihydroxide form.

4. Process according to Claim 3, wherein said additional source of aluminum is selected from aluminum hydroxide or an aluminum salt, a sodium aluminate, an aluminum alkoxide, or alumina, taken alone or in a mixture, preferably said conventional source of aluminum is aluminum hydroxide.

5. Process according to one of the preceding claims, wherein the source of sodium cations is sodium hydroxide.

6. Process according to one of the preceding claims, wherein seed crystals of a zeolite of AFX structural type or of a zeolite of BEA structural type or of a mixture of the two are added to the reaction mixture of step i), in an amount of between 0.01% and 10% by weight relative to the total mass of the sources of SiO2 and Al2O3 in anhydrous form present in said mixture, said seed crystals not being taken into account in the total mass of the sources of SiO2 and Al2O3.

7. Process according to one of the preceding claims, wherein step i) comprises 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.

8. Process according to one of the preceding claims, wherein the content of palladium introduced by the deposition step v) is between 0.5% to 5% by mass, preferably between 0.8% and 3% by mass, more preferably between 0.9% and 2% by mass, very advantageously around 1% by mass, relative to the total mass of the anhydrous composite catalyst.

9. AFX-BEA zeolite composite catalyst containing palladium, obtained by the process according to one of Claims 1 to 8.

10. AFX-BEA zeolite composite catalyst according to Claim 9, the SiO2 / Al2O3 ratio of which is between 6 and 80, limits included, preferably between 10 and 40, limits included.

11. AFX-BEA zeolite composite catalyst according to Claim 9 or 10, wherein the mass ratio between the amounts of zeolites of AFX structural type and of BEA type is between 0.9 and 5.7.

12. AFX-BEA zeolite composite catalyst according to one of Claims 9 to 11, wherein the palladium content is between 0.5% to 5% by mass, preferably between 0.8% and 3% by mass, more preferably between 0.9% and 2% by mass, very advantageously around 1% by mass, relative to the total mass of the anhydrous final catalyst, and the metallic dispersion of the palladium, measured by CO chemisorption, is between 40% and 100%, preferably between 50% and 100%.

13. Use of the catalyst according to any one of Claims 9 to 12 or obtained by the process according to any one of Claims 1 to 8, for the selective reduction of NOx by a reducing agent such as NH3 or H2.

14. Use according to Claim 13, for which the catalyst is formed by deposition in the form of a coating on a honeycomb structure or a plate structure.

15. Use according to Claim 14, for which the honeycomb structure is formed by parallel channels open at both ends or comprises porous filtering walls for which the adjacent parallel channels are alternately blocked at either end of the channels.

16. Use according to Claim 15, for which the amount of catalyst that is deposited on said structure is between 50 to 240 g / L for filtering structures and between 50 and 320 g / L for structures with open channels.

17. Use according to one of Claims 14 to 16, for which the catalyst is 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 in order to be formed by deposition in the form of a coating.

18. Use according to one of Claims 14 to 17, for which said coating is combined with another coating having the capacity to adsorb pollutants, in particular NOx, to reduce pollutants, in particular NOx, or to promote the oxidation of pollutants.

19. Use according to Claim 13, for which said catalyst is in the form of an extrudate, containing up to 100% of said catalyst.

20. Use according to one of Claims 13 to 19, for which the structure coated with said catalyst or obtained by extrusion of said catalyst is integrated into an exhaust line of an internal combustion engine.