The invention relates to chemical compositions that can be used for hydrocarbon catalytic cracking processes with vanadium as a contaminant, including an active phase formed by different pyrophosphates M2P2O7 (M=Ba or Ca) supported on a mixture of magnesium and aluminium oxide, preferably magnesium aluminate in the spinel phase. The composition captures the metals originating from the charge, particularly vanadium, and thus protects the catalyst. Said composition is preferably used in the form of a separated particle in order to the control the addition thereof to the unit according to the metal content of the charge. The invention also relates to the method for preparing said composition, including synthesis of pyrophosphates, formation of a suspension of boehmite alumina, magnesium oxide or magnesium hydroxide, together with oxides M2P2O7, spray drying and calcination of the microspheres without generating any loss in the crystalline structure of oxides M2P2O7.