A preparation process is described for a porous composite material formed from an amorphous core based on at least one silicon oxide on which crystals of EU-1 zeolite are dispersed, said process comprising 1) the impregnation of a solid comprising a silicon oxide and an aluminium oxide with an aqueous solution comprising a hexamethonium cation, 2) the hydrothermal treatment, implemented in an autoclave of volume V (ml) under steam and at a temperature T comprised between 120 and 220 DEG C., of said solid from stage 1), the quantity of water introduced beforehand into said autoclave being strictly greater than a volumetric quantity equal to V*[23.48*10-10*T3-48*10-8*T2+5*10-5T-0.002] and less than or equal to 0.25*V, and is such that said solid is not in direct contact with it, 3) the drying then calcination of the solid from stage 2). The preparation a catalyst from said material for its use in the isomerization of C8 aromatics is also described.