Study by EXAFS, Raman, and NMR spectroscopies of the genesis of oxidic precursors of zeolite-supported HDS catalysts

A 3 wt % Mo zeolite-supported oxidic precursor prepared from ammonium heptamolybdate by incipient wetness impregnation was studied using Raman and Al-27 NMR spectroscopies, at the various steps of its preparation. The formation of an Anderson type aluminomolybdate anion, which was preserved during drying, was observed. This species led to the formation of aluminum molybdate Al-2(MoO4)(3) upon calcination and was partially reformed upon rehydration of the sample. XANES and EXAFS experiments at the Mo K-edge confirmed these results and showed the Anderson entity to be the majority species. The use of different loadings and supports showed that the Anderson entity is formed through extraction from the zeolite of nonframework aluminum atoms and that ammonium heptamolybdate precipitates when the Mo loading is too high. EXAFS, and XRD experiments applied to another sample prepared by MoO3 vapocondensation showed that the observed Anderson entity cannot be situated in the supercages of the zeolite.