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