SOLID-STATE MO-95 NMR-STUDY OF HYDRODESULFURIZATION CATALYSTS .2. INVESTIGATION OF REDUCED SULFIDED MOLYBDENA-ALUMINA CATALYSTS AND THE EFFECT OF PROMOTER IONS ON FRESH AND REDUCED SULFIDED MOLYBDENA-ALUMINA CATALYSTS

Solid-state static-echo, MAS-echo, and spikelet-echo Mo-95 NMR was utilized to obtain information on reduced/sulfided molybdena-alumina catalysts, as well as the effect of addition of cobalt, cesium, and potassium to "fresh" molybdena-alumina and cobalt to reduced/sulfided molybdena-alumina. We also investigated a used catalyst exposed to thiophene for 24 h. The results obtained for the reduced/sulfided catalysts indicate that the Mo(VI)-O species present in the "fresh" catalyst undergo only partial reduction and sulfiding, with a portion of the tetrahedral Mo(IV)-O species remaining unchanged in the reduced and sulfided catalyst. The bulk MoO3 and octahedral molybdenum species, also present in the "fresh" catalyst, are reduced to Mo(IV) and sulfided to MoS2. An unidentified species, possibly MoOS, was also found to be present. Many other minor species are present on the surface which cannot be identified but which represent a high proportion of the molybdenum present on the surface. These could consist of a whole range of Mo(VI) and Mo(IV) mixed oxygen/sulfur compounds. There is no evidence for the presence of MoO2, except in one case. Simulation of the static powder line shapes allowed the deconvolution of the various components of the line shapes, thus yielding values of quadrupole parameters as well as relative intensity data. From the spikelet experiments it was ascertained that all species are present as both static adsorbed and dynamically active phases. The used catalyst shows the presence of the tetrahedral molybdenum-oxo species, along with a much reduced MoS2 resonance (relative to the "fresh" system), perhaps indicating that MoS2 is the active site in the thiophene HDS experiment and is degraded over time into a range of Mo(IV) and Mo(VI) oxo/sulfur species. In the case of the cobalt promoted "fresh" catalysts, the observed line widths indicated the presence of a paramagnetic coupling. This broadening was also found to be dependent on the method of impregnation of the promoter ion. Spikelet-echo experiments demonstrated the difference in surface chemistry between catalysts prepared by different impregnation techniques, namely, a much stronger Co-Mo interaction in the separately impregnated catalysts. Upon calcination, the usual increase in line width associated with polymerization of the surface molybdena was not observed for the promoted catalyst prepared by separate impregnation. The line width actually decreased for these catalysts, indicating either that the paramagnetic coupling occurred via the dipole coupling provided by the water molecules present on the uncalcined surface or that the cobalt was sequestered by the surface alumina to form CoAl2O4. In the promoted reduced/sulfided catalysts, one observes no MoS2 resonance in the separately impregnated promoted catalyst, while the MoS2 is observed in the coimpregnated catalysts. This indicates that the cobalt is closely associated with the MoS2 phase in the separately impregnated catalysts. In the case of the cesium and potassium promoted "fresh" catalysts, there is a marked increase in the line width of the uncalcined catalysts. The large alkali-metal ions may interact with the molybdenum in such a way as to distort the molybdena species, and thus produce a larger quadrupole interaction, or they donate electron density to the molybdenum thus creating a more shielded environment as well as perturbing the existing electric field gradient. In the calcined catalysts containing cesium or potassium, the line shapes are considerably narrowed, in comparison with the uncalcined catalysts. Comparison with the static and MAS spectra of Cs2MoO4 demonstrates the presence of predominantly Cs2MoO4 on the surface of high loading catalysts. K2MoO4 was also found to predominate at high loadings of potassium. Spikelet-echo experiments of the uncalcined cesium promoted catalysts revealed the presence of a homogeneous broadening mechanism which increases with increasing loading.