Fluorinated hydrotreatment catalysts: Characterization and Hydrodesulphurization Activity of Fluorine—Nickel—Molybdenum/γ-Alumina Catalysts

Abstract

The oxidic and sulphided state of a series of F Ni Mo/γ-Al 2O 3 catalysts containing constant MoO 3 and NiO loadings (11.5% and 7.3%, respectively) and various amounts of F −, 0.0–2.0%, introduced after the deposition of the Mo(VI) and Ni(II) were characterized using X-ray photoelectron spectroscopy (XPS), X-ray powder analysis, diffuse reflectance spectroscopy, nitric oxide adsorption, B.E.T. and pore-volume distribution measurements. The results were correlated with the catalytic hydrodesulphurization activity of the specimens, performed in the temperature range 255–325°C using a differential fixed-bed reactor operating under atmospheric pressure. Concerning the oxidic precursor state, the formation of the following species takes place: (i) a supported molybdate phase in which a fraction of the supported Ni(II) has been incorporated in octahedral sites; (ii) α-NiMoO 4 and (iii) NiAl 2O 4. The relative concentrations of these phases are independent of the fluoride content. Low fluoride concentration (≤ 0.8%) has little effect on the texture of the solids. Further increase in the concentration of the modifier results in a marked decrease in the specific surface area which can be partly attributed to the breaking of the inner walls of the relatively thin pores and the creation of larger ones in the range 15–50Å. The XPS spectra of the oxide precursors showed that fluorination does not change the structure of the supported phase. No AlF 3 was detected. The surface coverage of the active phase was estimated in the sulphided state by the total amount of nitric oxide chemisorbed (per m 2 of the catalyst) at 25°C. This decreases slightly up to 0.3 wt% F − and then increases linearly with the fluoride content. The XPS spectra of the sulphided specimens showed that fluorination inhibits the extent of sulphidation and/or reduction of Mo(VI) and the sulphidation of Ni(II) as well. The intensity of both effects increased with the fluoride content. The intrinsic hydrodesulphurization activity was found to increase with the fluoride content. The variation in the activity with the surface coverage of the active phase in the sulphided state showed that the promoting action of F − is mainly due to the fact that these ions increase the surface coverage of the supported Mo and Ni and, therefore, the number of active sites per unit surface area of catalyst. The increase in the surface coverage was attributed to both the increase in the dispersion and the decrease in the specific surface area.