The invention describes a heteropolycompound constituted by a nickel salt of a lacunary Keggin type heteropolyanion comprising tungsten in its structure, with formula: Nix+y/2AW11-yO39-5/2y, zH2O wherein Ni is nickel, A is selected from phosphorus, silicon and boron, W is tungsten, O is oxygen, y=0 or 2, x=3.5 if A is phosphorus, x=4 if A is silicon, x=4.5 if A is boron, and x=m/2+2 for the rest, and z is a number in the range 0 to 36, in which said heteropolycompound has no nickel atom in substitution for a tungsten atom in its structure, said nickel atoms being placed in the counter-ion position in the structure of said compound.