Mixed oxides having formula (I): Ni a Y b Z c O d . pC (I) possibly formed without a binder, wherein Y is a mixture of Mo and W in a molar ratio Mo/W greater than 0.1 and less than 10, Z is selected from Si, Al and mixtures thereof, O is oxygen, C is an organic component selected from a nitrogenated compound N, an organic residue R containing carbon and nitrogen, a mixture of the residue R and nitrogenated compound N, said nitrogenated compound N being an amine having formula (A) R 1 R 2 R 3 N (A) wherein R 1 is a linear, branched or cyclic alkyl, containing from 4 to 12 carbon atoms, and R 2 and R 3 , equal to or different from each other, are selected from H and linear, branched or cyclic alkyl, containing from 4 to 12 carbon atoms, said alkyl possibly being equal to or different from R 1 , a, c and d are the number of moles of Ni, Z, O, respectively, b is the sum of the moles of W and Mo p is the weight percentage of C with respect to the total weight of the compound having formula (I), a, b, c, d are greater than 0 a/b is greater than or equal to 0.6 and lower than or equal to 1.5, (a+b)/c is greater than or equal to 0.3 and lower than or equal to 10, and preferably ranges from 0.8 to 10 d = 2 a + 6 b + Hc / 2 wherein H = 4 when Z = Si H = 3 when Z = Al and p is greater than or equal to 0 and lower than or equal to 40%, wherein said mixed oxides of formula (I) are characterised in that their diffraction pattern show the presence of an amorphous phase and a wolframite isostructural monoclinic crystalline phase, the crystallinity degree of said mixed oxides being greater than 3 and lower than 90%. possibly formed without a binder, wherein Y is a mixture of Mo and W in a molar ratio Mo/W greater than 0.1 and less than 10, Z is selected from Si, Al and mixtures thereof, O is oxygen, C is an organic component selected from a nitrogenated compound N, an organic residue R containing carbon and nitrogen, a mixture of the residue R and nitrogenated compound N, said nitrogenated compound N being an amine having formula (A) R 1 R 2 R 3 N (A) wherein R 1 is a linear, branched or cyclic alkyl, containing from 4 to 12 carbon atoms, and R 2 and R 3 , equal to or different from each other, are selected from H and linear, branched or cyclic alkyl, containing from 4 to 12 carbon atoms, said alkyl possibly being equal to or different from R 1 , R 1 is a linear, branched or cyclic alkyl, containing from 4 to 12 carbon atoms, and R 2 and R 3 , equal to or different from each other, are selected from H and linear, branched or cyclic alkyl, containing from 4 to 12 carbon atoms, said alkyl possibly being equal to or different from R 1 , a, c and d are the number of moles of Ni, Z, O, respectively, b is the sum of the moles of W and Mo p is the weight percentage of C with respect to the total weight of the compound having formula (I), a, b, c, d are greater than 0 a/b is greater than or equal to 0.6 and lower than or equal to 1.5, (a+b)/c is greater than or equal to 0.3 and lower than or equal to 10, and preferably ranges from 0.8 to 10 d = 2 a + 6 b + Hc / 2 wherein H = 4 when Z = Si H = 3 when Z = Al wherein H = 4 when Z = Si H = 3 when Z = Al and p is greater than or equal to 0 and lower than or equal to 40%, wherein said mixed oxides of formula (I) are characterised in that their diffraction pattern show the presence of an amorphous phase and a wolframite isostructural monoclinic crystalline phase, the crystallinity degree of said mixed oxides being greater than 3 and lower than 90%. <EOS>