Kinetic Assessment of the Simultaneous Hydrodesulfurization of Dibenzothiophene and the Hydrogenation of Diverse Polyaromatic Structures

The elimination of sulfur from fossil fuels via hydrodesulfurization (HDS) is paramount to produce cleaner fuels. Ultradeep HDS refers to reducing sulfur in fuels below the 10 ppm level. Nevertheless, under such conditions, dibenzothiophenes (DBTs) are to be desulfurized in the presence of highly complex aromatic structures that possibly exert inhibitory effects. Therefore, this contribution presents a kinetic study of the inhibition effect of diverse aromatic structures: naphthalene (NP), fluorene (FL), and phenanthrene (PHE), on the HDS of dibenzothiophene over a sulfided NiMo/gamma-Al2O3 catalyst. Kinetic modeling was based on the Langmuir-Hinshelwood-Hougen-Watson (LHHW) formalism and was submitted to regression analyses with the reparametrized form of the Arrhenius and vant Hoff equations. Before addressing inhibition effects, the kinetics of the HDS of DBT was revisited. In this sense, observations were better fitted when considering that the two parallel pathways for the HDS of DBT, i.e., the so-called direct desulfurization (DDS) and hydrogenation-mediated desulfurization (HYD) routes, occur on two different types of active sites. The developed model was used as a basis for the kinetic modeling of the inhibition of aromatics on the HDS of DBT. The kinetic parameters for the aromatics were estimated on both catalytic sites and exhibited thermodynamic consistency. Kinetic modeling indicated the following: (i) aromatic compounds and their reaction products are adsorbed on both DDS and HYD sites; (ii) the hydrogenation of naphthalene occurs on both sites while fluorene and phenanthrene only react on HYD sites; (iii) the entropy values suggested that the mobility of the molecules is higher on HYD sites than on DDS sites, except for dibenzothiophene; and (iv) fluorene strongly inhibits HYD sites, because of its structure similarity with dibenzothiophene. These findings are important because they provide an insight into the inhibition effects of polyaromatic compounds of different chemical structures on ultradeep HDS.