A DFT study of the effects of oxygen on the hydrodesulfurization of sulfur macromolecules during the direct hydrodesulfurization process

Abstract Periodic density functional theory (DFT) calculations were employed to study the effects of oxygen on the direct desulfurization (DDS) performance of Co-Mo-S catalyst in this work. The structural differences between Co-Mo-S and oxygen-containing Co-Mo-S (Co-Mo-O-S), as well as the oxygen effects on dibenzothiophene (DBT) and 4,6-dimethyl-dibenzothiophene (4,6-DMDBT) hydrotreatment during DDS, were calculated. The calculation results show that oxygen reduces the electronic quantity in metal Co and Mo atoms. During the DDS process, Co-Mo-S has stronger adsorption capacity for the reactants than Co-Mo-O-S. The dissociation of hydrogen was promoted by oxygen, which helps to provide active hydrogen more easily during the reaction, but oxygen slightly increases the activation energy of hydrogen transfer. Due to the asymmetric structure, two possible reaction pathways exist for the C S bond scission on Co-Mo-O-S. On the S-edge and M-edge of Co-Mo-O-S, C S bond cleavage is more inclined to react on the oxygen-rich side. Meanwhile, oxygen changes the priority site of the reaction. On Co-Mo-S, the S-edge is the priority reaction site. On Co-Mo-O-S, C S bond cleavage is favorable on the M-edge. This work paves a new pathway for improving HDS catalytic activity via synergistic structure and charge characteristics.