Structure and location of oxomolybdenum entities in zeolite-supported HDS oxidic precursors

A dissolution-precipitation phenomenon is identified during the preparation of zeolite-supported Mo oxidic precursors by impregnation with ammonium heptamolybdate (AHM). Extra-framework aluminium atoms (EFAL) are extracted to form the Anderson heteropolyanion AlMo6O24H63- (AlMo6), which is conserved during drying, and is located in the extra-granular porosity of the support. Calcination leads to a surface "Al-2(MoO4)(3)-type" phase, and AlMo6 is recovered on return to air. In the absence of EFAL, AHM precipitates on the support, and transforms into MoO3 upon calcination. MoO3 vapocondensation leads to dimeric oxomolybdenic species in the vicinity of the zeolitic structure.