Benzothiophene hydrodesulfurization over NiMo/alumina catalysts modified by citric acid. Effect of addition stage of organic modifier

NiMo/Al2O3 catalysts were obtained by one-pot simultaneous impregnation of Mo, Ni and P (at 12, 3 and 1.6 wt%, respectively) over alumina. Citric acid (CA) was added (Ni/CA=1 mol ratio) to determine its influence over molybdenum species, as nickel precursor used was complexated Ni acetate. Three different preparation methodologies were used: (a) modification of calcined (400 °C) NiMo/Al2O3 by CA impregnation, (b) CA deposition directly onto Al2O3 carrier, prior to Ni-Mo-P impregnation, (c) simultaneous Ni-Mo-P-CA deposition on alumina support. Materials were characterized by N2 physisorption, infrared and UV-vis spectroscopies, temperature-programmed reduction and thermal analysis (TG-DTG), and tested in liquid-phase benzothiophene (BT) conversion in batch reactor. CA deposition over calcined NiMo/Al2O3 resulted in diminished proportion of refractory Mo6+(t) tetrahedral species but also in octahedral Mo6+(o) reducible at higher temperature, as to those over non-modified NiMo/alumina. That was reflected in lower hydrodesulfurizating (HDS) ability. The highest BT HDS activity (70% increase, as to the conventional calcined formulation with no organic additive) was found when CA was impregnated over bare alumina prior to Ni-Mo-P deposition. In that case, decreased interaction between deposited molybdenum and nickel species and the support was evidenced, that due to surface "passivation" from citric adsorption/decomposition on alumina surface. On the other hand, simultaneous Ni-Mo-P-CA deposition, where low pH of impregnating solution (pH~1.4) devoided citric acid ionization, (then, no Mo-citrate complex formation) was ineffective in providing catalyst of enhanced HDS activity.