Hydrodesulfurization of methyl-substituted dibenzothiophenes: fundamental study of routes to deep desulfurization

Four catalysts, Co-Mo and Ni-Mo, designed to cover a wide range of activity for aromatic ring hydrogenation and two catalysts, zeolite-Co-Mo, with different cracking activities were tested, in the hydrodesulfurization of dibenzothiophene (DBT) and 4,6-dimethyl-DBT (DMDBT) in a fixed-bed reactor at 360°C and a hydrogen pressure of 5.4 MPa. Increasing the hydrogenation activity of the catalysts increased the mole ratio, α, of cyclohexylbenzenes to biphenyls produced from both reactants. Although α had little effect on the hydrodesulfurization (HDS) rate of DBT, the HDS rate of DMDBT increased significantly with α, approching the same level as for DBT at α=2. Introduction of zeolite H-ZSM-5 to the Co-Mo -Al catalyst increased the HDS rate of DBT at a lower value of α and decreased that of DMDBT. No cracking reactions of DBT and DMDBT were detected. A Co-Mo-Al catalyst containing HY zeolite displayed significant cracking activity of DMDBT defined by two routes: demethylation of benzenic rings and scission of the C-C bond connecting the benzenic rings. It resulted in increasing HDS rates of DMDBT by about threefold relative to the Co-Mo-Al catalyst, yielding toluene and benzene, at a ratio of about 3, as the main HDS products. About 80% of the DMDBT desulfurized with HY-Co-Mo-Al catalyst was converted through "cracking" intermediates, 90% of those intermediates being produced through the scission of the C-C bond connecting the benzenic rings.