Effect of support on the acidity of NiMo/Al2O3-MgO and NiMo/TiO2-Al2O3 catalysts and on the resulting competitive hydrodesulfurization/hydrodenitrogenation reactions

The influence of Al2O3, MgO-Al2O3, and TiO2-Al2O3 supports on the activity of NiMoS catalysts was evaluated for the competitive hydrodesulfurization/hydrodenitrogenation (HDS/HDN) reactions using dibenzothiophene and quinoline as model compounds. Nitrogen physisorption results show that the Al2O3-MgO and TiO2-Al2O3 supports present specific surface areas of 362m(2)/g and 170m(2)/g, respectively. Also, Al2O3-MgO and TiO2-Al2O3 supports have net surface pH values respectively of 9.0 and 4.9. The supports were impregnated with Ni/Mo aqueous precursor solutions and characterized at their oxide state using UV-vis diffuse reflectance and Raman spectroscopies after drying and calcination steps. Results show that the NiMo/MgO-Al2O3 catalyst presents mainly Ni-Oh(2+)/MoO42- species while Ni-Oh(2+)/Mo7O246-/Mo8O264- species are found on NiMo/TiO2-Al2O3 catalyst. NiMo catalysts supported on Al2O3-MgO and TiO2-Al2O3 were characterized at the sulfide state mainly by infrared spectroscopy using pyridine as a probe. In this respect, the NiMo catalyst supported on Al2O3 shows the highest concentration of Lewis acid sites of all catalysts series. Finally, the sulfide catalysts were evaluated in the HDS/HDN competitive reaction using dibenzothiophene and quinoline compounds. The highest DBT HDS activity for the NiMo/Al2O3 catalyst was associated with Lewis acid properties. However, the HDS inhibition caused by quinoline addition depends on the nature of the support. The NiMo/MgO-Al2O3 catalyst exhibits lower inhibition than NiMo/Al2O3 and NiMo/TiO2-Al2O3 catalysts. This behavior was related to the fact that the pyridine adsorption enthalpy depends on the support nature. The NiMo catalyst supported on MgO-Al2O3 requires more energy to adsorb pyridine than the catalysts supported on Al2O3 or TiO2-Al2O3.