Effect of phase composition of the oxidic precursor on the HDS activity of the sulfided molybdates of Fe(II), Co(II), and Ni(II)

The catalytic HDS activities of unsupported sulfided molybdates of Fe(II), Co(II), and Ni(II) have been examined measuring the conversion of thiophene at 400 degrees C under atmospheric pressure, The oxidic precursors employed included the hydrates AMoO(4) . H2O and the alpha- and beta-AMoO(4) polymorphs (A = Fe, Co, or Ni). The previous finding that sulfided beta-NiMoO4 is a better HDS catalyst than alpha-NiMoO4 is now generalized to the other two molybdate systems, suggesting that the tetrahedral environment of Mo in the beta-isomorphs provides a more active A-Mo-S phase than the octahedral one in the alpha-molybdates. The reduced (nonpresulfided) molybdate samples showed lower HDS activities than those sulfided in pure H2S. Prereduction followed by sulfiding seems to be an optimum procedure for the highest HDS activity of the nickel molybdates and also of beta-FeMoO4. It was found that NiMoO4 . H2O renders a more active sulfided catalyst than the beta-phase synthesized by calcination at 760 degrees C, and this seems to be related to the in situ generation of beta-NiMoO4 with higher surface area during the activation of the hydrate at 400 degrees C. The hydrated phase of cobalt behaved similarly, but that of iron, on the contrary, was a poorer catalytic precursor than beta-FeMoO4. It is suggested that decomposition of the hydrated compound in the case of Fe could generate a more crystalline material upon sulfiding due to the possibility of changes in the oxidation state of the metals (i.e., Fe2+ reversible arrow Fe3+, Mo6+ reversible arrow Mo5+) during thermal transformations in presence of evolved water. (C) 1997 Academic Press.