Catalytic Hydrodesulfurization of Thiophene, Dibenzothiophene and 4,6-Dimethyldibenzothiophene on a CoMoS Catalyst

Previous studies have employed density functional theory (DFT) modeling to investigate hydrodesulfurization (HDS) pathways for heavy aromatic sulfides, typically focusing on hydrogenation to assist in C-S bond cleavage on both pristine and promoted MoS₂ catalysts. These investigations, which primarily examine the reduced Mo- and sulfur-terminated edges of MoS₂ slabs, generally categorize the reaction pathways into two types: direct desulfurization (DDS) and hydrogenation-desulfurization (HYD). Traditionally, these models assume that C-S bond cleavage occurs through interactions with edge sulfur atoms, with less attention given to the role of promoter metals like Co. However, our recent work indicates that Co atoms on the S-edges of MoS₂ slabs may play a crucial role in activating and dissociating C-S bonds, particularly through an α-carbon transfer. This process has been identified as key in the desulfurization of small thiols like methanethiol, prompting further investigation into its relevance for aromatic thiols such as thiophene, dibenzothiophene (DBT), and 4,6-dimethyldibenzothiophene (DMDBT). In the DFT calculations presented in this article, we demonstrate that the activation barrier for C-S bond cleavage to Co remains consistent at 1.0-1.1 eV/atom for the unsubstituted aromatic sulfides with a higher 1.67 eV for DMDBT. This oxidative addition mechanism of Co is strongly favored by the presence of dissociated hydrogen on adjacent sites and the aromatic nature of the molecule being desulfurized, while self-desulfurization through this pathway is found to be unfavorable. Our findings provide new insights into the chemistry of promoter atoms in the HDS of heavy aromatic sulfides.