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

Description

TECHNICAL FIELD

[0001] The invention relates to a process for preparing a palladium-containing catalyst based on a zeolite of structural type AFX, the catalyst prepared or capable of being prepared by the process according to any 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 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 described in patent applications WO 2015 / 085303 A1 and WO2016 / 135465A1. However, neither of these patent applications highlights the advantage conferred by this structure, in particular depending on its mode of synthesis.

[0009] Patent application WO2019 / 224088 describes the synthesis of a zeolite of the AFX structural type with SiO 2 / Al 2 O 3 molar ratios of between 4 and 100, preferably between 6 and 80, using the organic structuring agents 1,5-bis(methylpiperidinium) pentane, 1,6-bis(methylpiperidinium) hexane or 1,7-bis(methylpiperidinium) heptane in their dihydroxide form and as a source of silicon and aluminum FAU type zeolites with SiO 2 / Al 2 O 3 molar ratios of between 6 and 200. Document FR-A-3 095 130 describes another method of synthesizing an AFX zeolite.

[0010] 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 [Rh(C 2 H 4 N 2 ) 3 Cl 3 -3H 2 O stabilized by amine ligands to obtain a zeolite of structural type AFX with a crystallization time ranging from 1 day to about 100 days.

[0011] The applicant has discovered that a new catalyst based on a zeolite of structural type AFX prepared according to a particular synthesis method and containing palladium not only has an increased total NOx storage capacity, but also a higher NOx desorption temperature compared to the prior art. 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 zeolite catalyst comprising a zeolite of AFX structural type and palladium, comprising at least the following steps: i) the mixture in aqueous medium of a zeolite of structural type FAU having a total molar ratio SiO 2(FAU) / Al 2 O 3(FAU) of between 6 and 100, 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 6 and 100, H 2 O / (SiO 2(FAU) ) of between 5 and 60, MPC6 / (SiO 2(FAU) ) of between 0.05 and 0.50, Na 2 O / (SiO 2 (FAU) ) of between 0.05 and 0.25, 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) the hydrothermal treatment of said precursor gel obtained at the end of step i) at a temperature of between 120°C and 220°C, for a period of between 12 hours and 15 days to obtain an AFX zeolite iii) a step of filtration, washing and drying of said AFX zeolite 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 zeolite, followed by calcination of said dried AFX zeolite 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 zeolite;iv) iv) at least one ion exchange step of said calcined AFX structural type zeolite obtained in step iii) comprising bringing said calcined AFX zeolite 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 period of between 1 hour and 2 days to obtain an AFX zeolite calcined in ammonium form, which is again dried at a temperature of between 60 and 120°C; v) depositing a palladium solution on said AFX zeolite calcined in ammonium form and dried. ;

[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 silicon in the oxide form SiO 2 denoted SiO 2 (c), so that the molar ratio SiO 2 (c) / SiO 2 (FAU) is between 0.001 and 0.01, limits included, the content of SiO 2 (FAU) in said ratio being the content provided by the zeolite of structural type FAU, the reaction mixture of step i) having the following molar composition: (SiO 2(c) + SiO 2 (FAU) ) / Al 2 O 3 (FAU) between 6 and 100 H 2 O / (SiO 2(c) + SiO 2 (FAU) ) between 5 and 60 MPC6 / (SiO 2(c) + SiO 2 (FAU) ) between 0.05 and 0.50 Na 2 O / (SiO 2(c) + SiO 2 (FAU) ) between 0.05 and 0.25, limits included.

[0015] The reaction mixture of step i) may comprise at least one additional source of aluminium in oxide form denoted: Al 2 O 3 (c), so that the molar ratio Al 2 O 3 (c) / Al 2 O 3 (FAU) is between 0.001 and 8, limits included, 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 6 and 100, limits included H 2 O / SiO 2 (FAU) between 5 and 60 MPC6 / SiO 2 (FAU) between 0.05 and 0.50 Na 2 O / SiO 2 (FAU) between 0.05 and 0.25, 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 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] In one embodiment, the reaction mixture of step i) contains: at least one additional source of an oxide SiO 2 (c) and at least one additional source of an oxide Al 2 O 3 (c), FAU zeolite representing between 65 and 85% by mass compared to the total quantity of SiO sources 2 and Al 2 O 3 in anhydrous form present in said mixture, and the reaction mixture having the following molar composition: (SiO 2(c) + SiO 2 (FAU) ) / (Al 2 O 3 (FAU) + Al 2 O 3(c) ) between 6 and 100 H 2 O / (SiO 2(c) + SiO 2 (FAU) ) between 5 and 60 MPC6 / (SiO 2(c) + SiO 2 (FAU) ) between 0.05 and 0.50 Na 2 O / (SiO 2 (c) + SiO 2 (FAU) ) between 0.05 and 0.25, limits included.

[0017] 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.

[0018] The source of sodium cations can be sodium hydroxide.

[0019] Crystalline seeds of a zeolite of structural type AFX can be added to the reaction mixture of step i), in an amount of between 0.01 and 10% 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

[0020] 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.

[0021] 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 catalyst.

[0022] The invention also relates to a palladium-containing AFX zeolite catalyst obtained by the preparation method according to any of its variants.

[0023] Advantageously, the SiO 2 / Al 2 O 3 ratio of the catalyst is between 7 and 20, inclusive, preferably between 8 and 16, inclusive.

[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 chemisorption of CO 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 80 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) patterns of the zeolite-based catalyst of structural type AFX and containing palladium, Pd / AFX1 obtained according to Example 2. The Figure 2 represents the X-ray diffraction (XRD) patterns of the zeolite-based catalyst of BEA structural type and containing palladium, Pd / BEA obtained according to Example 3. The Figure 3 represents the X-ray diffraction (XRD) patterns of the zeolite-based catalyst of structural type SSZ-13 and containing palladium, Pd / SSZ-13 obtained according to Example 4. The Figure 4represents the NOx concentrations desorbed by the catalysts synthesized according to Example 2 (Pd-AFX1), Example 3 (Pd-BEA) and Example 4 (Pd-SSZ-13). The curve marked by circles, the one marked by crosses and the one marked by triangles correspond respectively to the tests carried out with the catalysts synthesized according to Example 2 (Pd-AFX1), Example 3 (Pd-BEA) and Example 4 (Pd-SSZ-13). Figure 5 represents the NOx concentrations desorbed by the catalysts aged for 4 hours at 750°C according to Example 6 and synthesized according to Example 2 (aged Pd-AFX1), Example 3 (aged Pd-BEA) and Example 4 (aged Pd-SSZ-13). The curve marked by circles, the one marked by crosses and the one marked by triangles correspond respectively to the tests carried out with the catalysts aged according to Example 6 and synthesized according to Example 2 (aged Pd-AFX1), Example 3 (aged Pd-BEA) and Example 4 (aged Pd-SSZ-13). 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 catalyst based on zeolite of AFX structural type containing palladium comprising at least the following steps.

[0036] Step i): the mixture in aqueous medium of a zeolite of structural type FAU having a molar ratio SiO 2 (FAU) / Al 2 O 3 (FAU) of between 6 and 100, 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 6 and 100 H 2 O / (SiO 2 (FAU) ) between 5 and 60 MPC6 / (SiO 2(FAU) ) between 0.05 and 0.50 Na 2 O / (SiO 2 (FAU) ) between 0.05 and 0.25, limits included, in which SiO 2 (FAU) denotes the quantity of SiO 2 provided by the FAU zeolite, and Al 2 O 3 (FAU) denotes the quantity of Al 2 O 3 provided by the FAU zeolite, until a homogeneous precursor gel is obtained;

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

[0038] In one embodiment, the reaction mixture of step i) may comprise at least one additional source of an oxide SiO 2 , denoted SiO 2 (c) so that the molar ratio SiO 2 (c) / SiO 2 (FAU) is between 0.001 and 0.01, limits included, the content of SiO 2 (FAU) in said ratio being the content provided by the zeolite of structural type FAU.

[0039] In this case, the reaction mixture from step i) may have the following molar composition: (SiO 2(c) + SiO 2 (FAU) ) / Al 2 O 3 (FAU) between 6 and 100 H 2 O / (SiO 2 (c) + SiO 2 (FAU) ) between 5 and 60 MPC6 / (SiO 2 (c) + SiO 2 (FAU) ) between 0.05 and 0.50 Na 2 O / (SiO 2 (c) + SiO 2 (FAU) ) between 0.05 and 0.25, limits included.

[0040] In another embodiment, the reaction mixture of step i) may comprise at least one additional source of an Al 2 O 3 oxide, denoted: Al 2 O 3 (C) so that the molar ratio Al 2 O 3(c) / Al 2 O 3 (FAU) is between 0.001 and 8, limits included, the content of Al 2 O 3 (FAU) in said ratio being the content provided by the zeolite of structural type FAU.

[0041] In this case, the reaction mixture of step i) may preferably have the following molar composition: SiO 2 (FAU) / (Al 2 O 3 (FAU) + Al 2 O 3 (c) ) between 6 and 100 H 2 O / SiO 2 (FAU) between 5 and 60 MPC6 / SiO 2 (FAU) between 0.05 and 0.5 Na 2 O / SiO 2 (FAU) between 0.05 and 0.25, limits included, SiO 2 (FAU) being the quantity of SiO 2 provided by the FAU zeolite, and Al 2 O 3 (FAU) being the quantity of Al 2 O 3 provided by the FAU zeolite, MPC6 being the organic nitrogen compound 1,6-bis(methylpiperidinium)hexane in its dihydroxide form.

[0042] In another embodiment, the reaction mixture of step i) may contain: at least one additional source of an oxide SiO 2 (c) and at least one additional source of an oxide Al 2 O 3(c), FAU zeolite representing between 65 and 85% by mass compared to the total quantity of SiO sources 2 and Al 2 O 3 in anhydrous form present in said mixture - that is to say of trivalent and tetravalent elements in oxide form SiO 2 (FAU) , SiO 2(c) , Al 2 O 3 (FAU) and Al 2 O 3(c) of the reaction mixture -, and the reaction mixture having the following molar composition: (SiO 2(c) + SiO 2 (FAU) ) / (Al 2 O 3 (FAU) + Al 2 O 3(c) ) between 6 and 100 H 2 O / (SiO 2(c) + SiO 2 (FAU) ) between 5 and 60 MPC6 / (SiO 2(c) + SiO 2 (FAU) ) between 0.05 and 0.50 Na 2 O / (SiO 2 (c) + SiO 2 (FAU) ) between 0.05 and 0.25, limits included.

[0043] Crystalline seeds of a zeolite of structural type AFX can be added to the reaction mixture of step i) in an amount of between 0.01 and 10% 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

[0044] 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.

[0045] Step ii): the hydrothermal treatment of said precursor gel obtained at the end of step i) is carried out at a temperature between 120°C and 220°C, for a duration between 12 hours and 15 days, preferably between 12 hours and 8 days.

[0046] The hydrothermal treatment of step ii) can be carried out under autogenous pressure.

[0047] Advantageously, the SiO 2 / Al 2 O 3 molar ratio of the AFX zeolite obtained is between 7 and 20, inclusive, and preferably between 8 and 16, inclusive.

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

[0049] In this step, the AFX structural type zeolite obtained at the end of step ii) is filtered, washed, and dried at a temperature between 60 and 120°C, for a period of between 5 and 24 hours to obtain a dried AFX structural type zeolite. The dried zeolite is then calcined at a temperature 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 structural type zeolite obtained in step iii) and comprises bringing the AFX structural type zeolite 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 AFX zeolite in ammonium form. At the end of the ion exchange step, the AFX structural type zeolite 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 said AFX zeolite 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 catalyst. Dry impregnation

[0054] In one embodiment according to the invention, the palladium solution is deposited by dry impregnation method. More particularly, the process for preparing the catalyst comprises the following steps: a) an aqueous solution comprising at least one palladium precursor salt is prepared; b) the dried AFX zeolite 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) preferably at room temperature; c) the catalyst precursor obtained at the end of step b) is dried to obtain a dried catalyst precursor;

[0055] The catalyst precursor is generally dried to remove all or part of the water introduced during 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.

[0056] 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.

[0057] d) optionally, the dried catalyst precursor obtained in step d) is calcined at a temperature of between 250 and 900°C to obtain a calcined catalyst precursor; 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 approximately 350°C and approximately 550°C. The calcination time is generally between 0.5 and 5 hours.

[0058] e) optionally, a reducing treatment is carried out by contact with a reducing gas. Colloidal impregnation

[0059] 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 zeolite 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 zeolite 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 said AFX zeolite calcined in ammonium form and dried. c) maturation of the impregnated support during step b) After impregnation, the AFX zeolite 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).

[0060] The catalyst precursor is generally dried to remove all or part of the water introduced during 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.

[0061] 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.

[0062] e) calcination under combustion air of the dried catalyst obtained in step d) (optional step)

[0063] 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 approximately 350°C and approximately 550°C. The calcination time is generally between 0.5 and 5 hours.

[0064] The present invention also relates to a catalyst in the form of an AFX zeolite containing palladium obtained by the process according to any one of the embodiment variants of the invention.

[0065] The palladium content of the zeolite 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.

[0066] In the zeolite 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

[0067] 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 AFX structural type. The purity obtained is advantageously greater than 90%, preferably greater than 95%, very preferably greater than 99.8% by weight. More generally, at the end of 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 a catalyst based on AFX zeolite.

[0068] In other words, the catalyst according to the invention based on AFX zeolite 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 (the limits not being included). Very advantageously, the process of the invention leads to the formation of a catalyst based on AFX zeolite, free of any other crystalline or amorphous phase.

[0069] Advantageously, the catalyst obtained by the process according to the invention has the X-ray diffraction diagram including at least the lines listed in Table 1.

[0070] 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 (Å) are indicated. Each of these values ​​must be affected by the measurement error Δ(d hkl ) between ± 0.6Å and ± 0.01Å.

[0071] Table 1: Average values ​​of d hkl and relative intensities measured on an X-ray diffraction pattern of the calcined AFX structural type crystalline solid Table 1 2 theta (°) dhkl (Å) Irel 2 theta (°) dhkl (Å) Irel 7,51 11,77 f 26,09 3,41 mf 8,75 10,10 m 27,19 3,28 ff 11,72 7,54 FF 27,56 3,23 ff 12,98 6,81 m 28,23 3,15 m 14,99 5,90 ff 28,68 3,11 ff 15,65 5,66 f 30,23 2,95 mf 17,51 5,06 f 30,56 2,92 m 18,04 4,91 m 31,17 2,87 ff 19,56 4,53 ff 31,58 2,83 mf 19,86 4,47 f 31,91 2,80 ff 20,38 4,35 F 32,73 2,73 ff 21,84 4,07 FF 33,86 2,65 mf 22,28 3,99 ff 34,25 2,62 ff 22,55 3,94 ff 34,74 2,58 f 23,83 3,73 f 35,33 2,54 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.

[0072] 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.

[0073] 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.

[0074] 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

[0075] The applicant has discovered that the material obtained by the process according to the invention has characteristics different from the small-pore aluminosilicate materials comprising palladium known until now. In particular, the zeolite 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. 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 the palladium, which makes it possible to contribute to the improved catalytic performances in reducing NOx. Furthermore, the better dispersion of the 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.

[0076] Advantageously, the SiO 2 / Al 2 O 3 molar ratio of the AFX zeolite-based catalyst obtained is between 7 and 20, preferably between 8 and 16, inclusive.

[0077] The palladium content of the zeolite 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.

[0078] In the zeolite 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

[0079] 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 one 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.

[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, FeCrAl type steel.

[0081] 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.

[0082] 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.

[0083] 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).

[0084] 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.

[0085] 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.

[0086] 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 and between 150°C and 500°C for the desorption phase and preferably between 250°C and 400°C. ADVANTAGES OF THE INVENTION

[0087] The catalyst according to the invention, based on a zeolite of the AFX structural type 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 quantities of NOx between -20°C and 200°C.

[0088] It also exhibits better resistance to hydrothermal aging, guaranteeing high adsorption performance even after this aging. EXAMPLES

[0089] 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).

[0090] 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%).

[0091] 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).

[0092] 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 a catalyst containing a zeolite of structural type AFX and palladium according to the invention Pd-AFX1

[0093] 275.8 g of an aqueous solution of 1,6-bis(methylpiperidinium)hexane dihydroxide (18.36% by weight, MPC6) 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 , 58.55% by weight of Al 2 O 3 , Merck) are incorporated into the synthesis mixture, which is kept stirring for half an hour at room temperature. Then 70.88 g of a zeolite of structural type FAU (CBV780, SiO 2 / Al 2 O 3 = 90.54, Zeolyst, PAF = 8.52%) are poured in and the suspension obtained is kept stirring 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 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 14 hours at 180°C with a temperature rise of 5°C / min up to 180°C with stirring at 200 rpm to allow crystallization of the AFX structural type zeolite.The resulting crystallized product is filtered, washed with deionized water, and then dried overnight at 100°C. The loss on ignition of the dried solid 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 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. The resulting material is named AFX1.

[0094] 40.0 g of AFX1 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 was dried overnight at 100°C. The resulting material is named NH 4 -AFX1.

[0095] 0.25 g Pd(NH 3 ) 4 Cl 2 hydrate is diluted in 8.2 mL of mineralized water and then impregnated at 25°C (by the dry impregnation method) on 10 g of the zeolite NH 4 -AFX1 prepared above. The solid obtained is dried in air for 2 h at 120°C. The Pd-AFX1 catalyst obtained is dried in air at 120°C, then is 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.

[0096] The Pd-AFX1 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%.

[0097] The Pd-AFX1 catalyst was analyzed by X-ray diffraction and identified as consisting mainly of a zeolite of the AFX structural type with a purity greater than 99% by weight. The X-ray diffraction pattern performed on the Pd-AFX1 catalyst is given in Figure 1 The product has a SiO 2 / Al 2 O 3 molar ratio of 14.5 as determined by FX. Example 3 (Comparative) : preparation of a catalyst containing a zeolite of structural type BEA with Pd, Pd-BEA

[0098] A 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.

[0099] 0.25 g Pd(NH 3 ) 4 Cl 2 hydrate is diluted in 17 mL of mineralized water then impregnated at 25°C (by the dry impregnation method) on 10 g of zeolite NH 4 -BEA.

[0100] The obtained solid is dried in air for 2 h at 120°C. The obtained Pd-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.

[0101] 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%.

[0102] The Pd-BEA catalyst was analyzed by X-ray diffraction and identified as consisting of a BEA structural type zeolite with a purity greater than 99% by weight. The X-ray diffraction pattern performed on the Pd-BEA catalyst is given in Figure 2 . The Pd-BEA catalyst has a SiO 2 / Al 2 O 3 molar ratio of 25.16 as determined by FX. Example 4 (Comparative) : preparation of a Pd-SSZ-13 zeolite (CHA structural type)

[0103] 27.21 g of an aqueous solution of N,N,N-trimethyl-1-adamantammonium hydroxide (TMAdA, 20.11% by weight, SACHEM) were mixed with 24.43 g of deionized water. 1.35 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, 1.09 g of pseudo boehmite (Pural SB3, 74.20% Al 2 O 3 , Condea) were incorporated and the synthesis gel was kept stirring for 15 minutes. Finally, 25.93 g of colloidal silica (Ludox AS40, 40% SiO 2 by weight, Aldrich) were incorporated into the synthesis mixture which was kept for half an hour at room temperature under stirring (350 rpm). The molar composition of the precursor gel is as follows: 60 SiO 2 : 2.75 Al 2 O 3 : 9.0 TMAda: 6.0 Na 2 O: 1201.0 H 2 O, i.e. a SiO 2 / Al 2 O 3 ratio of 21.8.The precursor gel is then transferred, after homogenization, into a 160 mL stainless steel reactor equipped with a four-blade inclined stirring system. The reactor is closed, then heated for 120 hours with a temperature rise of 3°C / min up to 160°C while stirring at 200 rpm to allow the crystallization of the CHA structural type zeolite. The crystallized product obtained is filtered, washed with deionized water, and 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 8 hours and then a return to room temperature. The resulting material is named SSZ-13.

[0104] 5.0 g of SSZ-13 material was exchanged 3 times with 3M aqueous NH4NO3 solution at 80°C for 1 hour under stirring (300 rpm) and a solution volume to zeolite mass ratio equal to 10 (V / P). Then, the solid was dried overnight at 100°C. The obtained material is named NH4-SSZ-13. Pd impregnation

[0105] 0.25 g Pd(NH 3 ) 4 Cl 2 hydrate is diluted in 10 mL of mineralized water then impregnated at 25°C (by the dry impregnation method) on 10 g of the zeolite NH 4 -SSZ-13 prepared above.

[0106] The resulting solid is dried in air for 2 hours 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.

[0107] The Pd-SSZ-13 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 47%.

[0108] The Pd-SSZ-13 catalyst was analyzed by X-ray diffraction and identified as consisting mainly of a CHA structural type zeolite with a purity greater than 99% by weight. The X-ray diffraction pattern performed on the Pd-SSZ-13 catalyst is given in Figure 3 . Example 6 : Hydrothermal aging stage

[0109] 550 mg of each of the samples synthesized according to Example 2 (Pd-AFX1), Example 3 (Pd-BEA) and Example 4 (Pd-SSZ-13) 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.

[0110] 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. Example 7 :

[0111] The sample synthesized according to Example 2 and aged under the conditions of Example 6 is named aged Pd-AFX1.

[0112] The sample synthesized according to Example 3 and aged under the conditions of Example 6 is named aged Pd-BEA.

[0113] The sample synthesized according to Example 4 and aged under the conditions of Example 6 is named aged Pd-SSZ-13. Example 8 : NOx adsorption and desorption tests

[0114] To highlight the adsorption capacities of the synthesized materials, an adsorption test of NO at 120°C followed by a temperature ramp from 120°C to 600°C at 10°C / min is carried out with example 2 (Pd-AFX1, according to the invention), example 3 (Pd-BEA) and example 4 (Pd-SSZ-13). 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 carried out under the same mixture up to 120°C.

[0115] The 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 increase 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.

[0116] An FTIR analyzer can measure the concentration of the species NO, NO2, NH3, N2O, CO, CO2, H2O, O2 at the reactor outlet.

[0117] There Figure 4shows the desorbed NOx concentrations during the temperature ramp. The curve marked by circles, the one marked by crosses and the one marked by triangles correspond respectively to the tests carried out with the catalysts synthesized according to Example 2 (Pd-AFX1), Example 3 (Pd-BEA) and Example 4 (Pd-SSZ-13). 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-AFX1 83,3 395 Pd-BEA 63,9 333 Pd-SSZ-13 62,6 310

[0118] The Pd-AFX1 catalyst synthesized according to the invention provides much higher performance than the Pd-BEA and Pd-SSZ-13 catalysts synthesized according to the prior art in terms of adsorbed quantity and desorption temperature. The desorption of NOx only occurs around 250°C for the Pd / AFX1 catalyst according to the invention. This desorption of NOx above 250°C presents a notable advantage for the control of NOx emissions. Example 9 : NOx adsorption and desorption tests after hydrothermal aging

[0119] An NO adsorption test at 120°C followed by a temperature ramp at 10°C / min up to 600°C is carried out with Example 2 aged according to Example 6 (Pd-AFX1 aged, according to the invention), Example 3 aged according to Example 6 (Pd-BEA aged) and Example 4 aged according to Example 6 (Pd-SSZ-13 aged). For the test of each sample, 523 mg of catalyst in powder form are placed in a quartz reactor. A preconditioning is carried out, it consists of a temperature rise from ambient (20°C) up 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. The 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.

[0120] 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

[0121] There Figure 5shows the desorbed NOx concentrations during the temperature ramp. The curve marked by circles, the one marked by crosses and the one marked by triangles correspond respectively to the tests carried out with the catalysts aged according to Example 6 and synthesized according to Example 2 (aged Pd-AFX1), Example 3 (aged Pd-BEA) and Example 4 (aged Pd-SSZ-13). 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] Aged Pd-AFX1 74,3 350 Aged Pd-BEA 40,9 340 Aged Pd-SSZ-13 64,1 310

[0122] The aged Pd-AFX1 catalyst synthesized according to the invention provides much higher performance than the aged Pd-BEA and aged Pd-SSZ-13 catalysts synthesized according to the prior art in terms of adsorbed quantity and desorption temperature. NOx desorption only occurs around 250°C for the aged Pd / AFX1 catalyst according to the invention. This NOx desorption above 250°C presents a significant advantage for controlling NOx emissions.

Claims

1. Process for preparing a zeolite catalyst comprising a zeolite of AFX structural type and palladium, comprising at least the following steps: i) the mixing, in an aqueous medium, of a zeolite of FAU structural type having a total SiO2 (FAU) / Al2O3 (FAU) molar ratio of between 6 and 100, 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 6 and 100, H2O / (SiO2 (FAU)) of between 5 and 60, MPC6 / (SiO2(FAU)) of between 0.05 and 0.50, Na2O / (SiO2 (FAU)) of between 0.05 and 0.25, 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 120°C and 220°C, for a time of between 12 h and 15 days, in order to obtain an AFX zeolite; iii) a step of filtration, washing and drying of said AFX zeolite 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 zeolite, followed by calcination of said dried AFX zeolite 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 zeolite; iv) at least one step of ion exchange of said calcined zeolite of AFX structural type obtained in step iii), comprising bringing said calcined AFX zeolite 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 AFX zeolite in ammonium form, which is again dried at a temperature of between 60 and 120°C; v) deposition of a palladium solution on said ammonium-form calcined and dried AFX zeolite.

2. Process according to Claim 1, wherein the 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) comprises at least one additional source of silicon in oxide form SiO2, denoted SiO2 (c), such that the SiO2 (c) / SiO2 (FAU) molar ratio is between 0.001 and 0.01, limits included, the content of SiO2 (FAU) in said ratio being the content provided by the zeolite of FAU structural type, the reaction mixture of step i) having the following molar composition: (SiO2(c) + SiO2 (FAU)) / Al2O3(FAU) of between 6 and 100, H2O / (SiO2 (c) + SiO2 (FAU)) of between 5 and 60, MPC6 / (SiO2 (c) + SiO2 (FAU)) of between 0.05 and 0.50, Na2O / (SiO2 (c) + SiO2 (FAU)) of between 0.05 and 0.25, limits included.

4. Process according to either of Claims 1 and 2, wherein the reaction mixture of step i) comprises at least one additional source of aluminum in oxide form, denoted: Al2O3 (C), such that the Al2O3 (c) / Al2O3 (FAU) molar ratio is between 0.001 and 8, limits included, the reaction mixture of step i) having the following molar composition: SiO2 (FAU) / (Al2O3(FAU) + Al2O3 (C)) of between 6 and 100, limits included, H2O / SiO2 (FAU) of between 5 and 60, MPC6 / SiO2 (FAU) of between 0.05 and 0.50, Na2O / SiO2 (FAU) of between 0.05 and 0.25, 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.

5. Process according to either of Claims 1 and 2, wherein the reaction mixture of step i) contains: - at least one additional source of an oxide SiO2 (c), - and at least one additional source of an oxide Al2O3 (c), the FAU zeolite representing between 65% and 85% by mass relative to the total amount of the sources of SiO2 and Al2O3 in anhydrous form present in said mixture, and the reaction mixture having the following molar composition: (SiO2(c) + SiO2 (FAU)) / (Al2O3 (FAU) + Al2O3 (c)) of between 6 and 100, H2O / (SiO2 (c) + SiO2 (FAU)) of between 5 and 60, MPC6 / (SiO2(c) + SiO2 (FAU)) of between 0.05 and 0.50, Na2O / (SiO2 (c) + SiO2 (FAU)) of between 0.05 and 0.25, limits included.

6. Process according to Claim 4 or 5, 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.

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

8. Process according to one of the preceding claims, wherein seed crystals of a zeolite of AFX structural type 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.

9. 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.

10. 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 catalyst.

11. AFX zeolite catalyst containing palladium, obtained by the process according to one of Claims 1 to 10.

12. AFX zeolite catalyst according to Claim 11, the SiO2 / Al2O3 ratio of which is between 7 and 20, limits included, preferably between 8 and 16, limits included.

13. AFX zeolite catalyst according to either of Claims 11 and 12, 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%.

14. Use of the catalyst according to one of Claims 11 to 13 or obtained by the process according to any one of Claims 1 to 10, for the selective reduction of NOx by a reducing agent such as NH3 or H2.

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

16. Use according to Claim 15, 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.

17. Use according to Claim 16, for which the amount of catalyst that is deposited on said structure is between 50 to 240 g / L for filtering structures and between 80 and 320 g / L for structures with open channels.

18. Use according to one of Claims 15 to 17, 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.

19. Use according to one of Claims 15 to 18, 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.

20. Use according to Claim 14, for which said catalyst is in the form of an extrudate, containing up to 100% of said catalyst.

21. Use according to one of Claims 14 to 20, 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.