Effect of TiO2–Al2O3 Sol–Gel Supports on the Superficial Ni and Mo Species in Oxidized and Sulfided NiMo/TiO2–Al2O3 Catalysts: Influence on Dibenzothiophene Hydrodesulfurization

Abstract

The present work presents a comparative study of NiMo catalysts supported on sol-gel TiO2-Al2O3 mixed oxides with 5 and 95 mol% content of Al2O3. The DRX and N2 physisorption results showed that the sol-gel method allows preparation of TiO2-Al2O3 mixed oxides possessing high superficial area and an amorphous TiO2 structure. Results of f-potential showed that the net surface pH of the supports depends on their structure and compo- sition. According to UV-Vis and Raman spectra obtained from the solids after impregnation, catalysts with high content of Al2O3 showed Mo7O24 2- and Mo8O26 4- species displaying Mo-O-Mo stretching vibration modes. On the other hand, catalysts with high content of TiO2 showed Mo7O24 2- and Mo8O26 4- species with vibration modes corresponding to terminal Mo=Ot bonds. Therefore, it appears that impregnation of catalysts with a pH 9 solution allows a polymerization process of MoO4 2- and (Ni 2? 4O 2- ) solution species to Mo8O26 4- and Mo7O24 2- species with a close interaction with (Ni 2? 6O 2- ) species. However, these species have low interaction with the support. Thus, composition of the support appears to be more important than net surface pH in order to obtain a better distribution of superficial Mo species. XPS results suggest a higher proportion of ''NiMoS'' phase on the TiO2 rich support. The most active catalyst in the dibenzothiophene hydrodesulfurization was NiMo/TiO2- Al2O3 with 5 mol% Al2O3. This suggests that Mo7O24 2- and Mo8O26 4- in combination with (Ni 2? 6O 2- ) species produce a better Ni/(Ni ? Mo) ratio and NiMoS phase.