Catalyst based on a group VI metal and a group VIII metal at least partially present in the form of heteropolyanions in the oxide precursor

A catalyst containing at least one group VIII element and at least molybdenum and/or tungsten, said elements being present at least in part in the catalyst in the dry state in the form of at least one heteropolyanion having a structural formula MxAB6O24H6C(3-2x).tH2O(I);MxAB6O24H6C(4-2x).tH2O(I′);

• MxA2B10O38H4C(6-2x).tH2O;
• MxA2B10O38H4C(8-2x).tH2O(I′″);
• or MxA2B10O38H4C(7-2x).tH2O(I″″);
• wherein M is cobalt, nickel, iron, copper, zinc, or mixtures thereof, A is an element of group VIII of the periodic table for formulae I and I′ or one or elements of group VIII of the periodic table for formulae I″, I′″, and I″″, B is molybdenum and/or tungsten and C is an H+ ion and/or a (NR1R2R3R4)+ type ammonium ion, wherein R1, R2, R3 and R4, may be identical or different, and correspond either to a hydrogen atom or to an alkyl group, cesium, potassium, sodium or mixtures thereof, t is a number from 0 to 15 and x is 0 to 3/2 in (I), 0 to 2 in (I′), 0 to 3 in (I″), 0 to 4 in (I′″) and 0 to 7/2 in (I″″) and wherein the number of bonds connecting the group VIII element or elements with the molybdenum and/or tungsten having a length of 3.6 angstroms or less is greater than 2.