Catalyst comprising a mixture of an AFX-structure zeolite and a BEA-structure zeolite and at least one transition metal for selective reduction of NOx

A process for preparing a catalyst comprising a mixture of AFX-structure and BEA-structure zeolites, and at least one transition metal, comprising at least the following steps:

• i) mixing, in aqueous medium, a first FAU zeolite having an SiO2/Al2O3 molar ratio of between 30 and 100 and at least one second FAU-structure zeolite having an SiO2/Al2O3 molar ratio of greater than or equal to 2 and less than 30, and wherein the mathematical parameter, Pze, corresponding to the mass percentage of the FAU zeolite with an SiO2/Al2O3 molar ratio of between 30 and 100, in its anhydrous form (expressed in %) in the mixture of FAU zeolites, multiplied by the SiO2/Al2O3 molar ratio of the same FAU zeolite with an SiO2/Al2O3 molar ratio of between 30 and 100, is such that:
• 3250<Pze<7200, of at least one nitrogenous organic compound R, chosen from 1,5-bis(methylpiperidinium)pentane dihydroxide, 1,6-bis(methylpiperidinium)hexane dihydroxide, 1,7-bis(methylpiperidinium)heptane dihydroxide, and mixtures thereof, and of at least one source of at least one alkali metal and/or alkaline-earth metal M of valence n, n being an integer greater than or equal to 1, the reaction mixture having the following molar composition:
• (SiO2(FAU))/(Al2O3(FAU)) of between 30 and 80,
• H2O/(SiO2(FAU)) of between 1 and 100,
• R/(SiO2(FAU)) of between 0.01 and 0.6,
• M2/nO/(SiO2(FAU)) of between 0.005 and 0.45,
• wherein SiO2(FAU) is the molar amount of SiO2 provided by all the FAU-structure zeolites introduced into the mixture, Al2O3(FAU) is the molar amount of Al2O3 introduced by all the FAU-structure zeolites introduced into the mixture, H2O the molar amount of water present in the reaction mixture, R the molar amount of the nitrogenous organic compound, M2/nO being the molar amount of M2/nO provided by all the FAU zeolites and by the source of alkali metal and/or alkaline-earth metal,
• until a precursor gel is obtained;
• ii) hydrothermal treatment of the 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 a solid crystalline phase, termed “solid”;
• iii) at least one ion exchange comprising bringing the solid obtained at the end of the previous step into contact with at least one solution comprising at least one species that is capable of releasing a transition metal, in solution in reactive form, with stirring at ambient temperature for a period of between 1 hour and 2 days;
• iv) heat treatment by drying the solid obtained at the end of the previous step at a temperature of between 20 and 150° C. for a period of between 2 and 24 hours, followed by calcination under a stream of air at a temperature of between 450 and 700° C. for a period of between 2 and 20 hours.