Process for preparing a copper-containing microporous aluminosilicate material with AFX structure, comprising at least the following steps: a) mixing, in an aqueous medium, of at least one aluminum source, at least one silicon source, sodium hydroxide, at least one copper source, an organic complexing agent OCPLX chosen from triethylenetetramine (TETA) or tetraethylenepentamine (TEPA) and a structuring agent 1,4-diazabicyclo[2.2.2]octane-C4-diquat dibromide (DABCO-C4), in order to obtain a gel of molar composition: a SiO 2: b Al 2 O 3: c Na 2 O: d DABCO-C4: e CuO: f OCPLX: g H 2 O a/b being between 100 and 40, c / b being between 25 and 50, d / b being between 3 and 10, e / b being between 0.05 and 0.1, f / e being between 1 and 1.5 and g / b being between 4000 and 1000; b) hydrothermal treatment of said gel at a temperature of between 130 and 180°C, under an autogenous reaction pressure, for a period of between 1 and 8 days with stirring in order to obtain the crystallization of said copper-comprising microporous aluminosilicate material with AFX structure; c) drying heat treatment of the solid obtained at the end of the preceding step at a temperature of between 80 and 120°C under a stream of inert gas followed by calcining under dry air at a temperature of between 400 and 600°C; d) ion exchange comprising bringing the solid obtained at the end of the preceding step into contact with a solution comprising a species capable of releasing copper in solution in reactive form with stirring at ambient temperature for a period of between 1 h and 2 d; e) drying heat treatment of the solid obtained at the end of the preceding step at a temperature of between 80 and 120°C followed by calcining under a stream of inert gas and then air at a temperature of between 400 and 600°C; the total amount of copper contained in the material obtained at the end of said preparation process being between 1.5 and 5.75% by weight relative to the total weight of said material in its anhydrous form. a) mixing, in an aqueous medium, of at least one aluminum source, at least one silicon source, sodium hydroxide, at least one copper source, an organic complexing agent OCPLX chosen from triethylenetetramine (TETA) or tetraethylenepentamine (TEPA) and a structuring agent 1,4-diazabicyclo[2.2.2]octane-C4-diquat dibromide (DABCO-C4), in order to obtain a gel of molar composition: a SiO 2: b Al 2 O 3: c Na 2 O: d DABCO-C4: e CuO: f OCPLX: g H 2 O a/b being between 100 and 40, c / b being between 25 and 50, d / b being between 3 and 10, e / b being between 0.05 and 0.1, f / e being between 1 and 1.5 and g / b being between 4000 and 1000; b) hydrothermal treatment of said gel at a temperature of between 130 and 180°C, under an autogenous reaction pressure, for a period of between 1 and 8 days with stirring in order to obtain the crystallization of said copper-comprising microporous aluminosilicate material with AFX structure; c) drying heat treatment of the solid obtained at the end of the preceding step at a temperature of between 80 and 120°C under a stream of inert gas followed by calcining under dry air at a temperature of between 400 and 600°C; d) ion exchange comprising bringing the solid obtained at the end of the preceding step into contact with a solution comprising a species capable of releasing copper in solution in reactive form with stirring at ambient temperature for a period of between 1 h and 2 d; e) drying heat treatment of the solid obtained at the end of the preceding step at a temperature of between 80 and 120°C followed by calcining under a stream of inert gas and then air at a temperature of between 400 and 600°C;