Effect of TiO_2-Al_2O_3 Sol-Gel Supports on the Superficial Ni and Mo Species in Oxidized and Suffided NiMo/TiO_2-Al_2O_3 Catalysts:Influence on Dibenzothiophene Hydrodesulfurization

摘要

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 zeta-potential showed that the net surface pH of the supports depends on their structure and composition.According to UV-Vis and Raman spectra obtained from the solids after impregnation,catalysts with high content of Al2O3 showed Mo7O_(24)~(2-)and Mo8O_(26)~(4-)species displaying Mo-O-Mo stretching vibration modes.On the other hand,catalysts with high content of TiO2 showed Mo7O_(24)~(2-)and Mo8O_(26)~(4-)species with vibration modes corresponding to terminal Mo=O_t bonds.Therefore,it appears that impregnation of catalysts with a pH 9 solution allows a polymerization process of MoO_4~(2-)and[Ni~(2+)4O~(2-)]solution species to Mo8O_(26)~(4-)and MoO_(24)~(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 Mo7O_(24)~(2-)and Mo8O_(26)~(4-)in combination with[Ni~(2+)6O~(2-)]species produce a better Ni/(Ni+Mo)ratio and NiMoS phase.