Understanding selectivity changes during hydrodesulfurization of dibenzothiophene on Mo2C/carbon catalysts

Abstract

The activity and stability of Mo2C supported on an activated carbon derived from petroleum coke (Mo2C/APC), is reported for the hydrodesulfurization (HDS) of dibenzothiophene (DBT). Mo2C/APC catalysts with Mo loading of 2%, 5%, and 10% were synthesized by carbothermal hydrogen reduction (CHR). XPS analysis of the fresh catalysts indicated ∼65% of Mo species were present as Mo carbide or oxycarbide, invariant with Mo loading; whereas, Mo2C particle size increased from 6.1 nm to 10.3 nm with increased Mo loading. The initial turnover frequency (TOF) for the HDS of DBT increased marginally with increased Mo2C particle size but DBT conversion declined rapidly with time-one-stream. A corresponding change in selectivity from >60% hydrogenation products (cyclohexylbenzene and bicyclohexane) to >70% biphenyl, the direct desulfurization product, also occurred. Characterization of the used catalysts showed the presence of MoS2 overlayers and an increase in Mo2C particle size, compared to the fresh catalysts. Density functional theory calculations were used to assess the impact of S on the Mo2C surface and the DBT adsorption geometry, showing that S enhances vertical DBT adsorption and consequently selectivity to the direct desulfurization product, biphenyl.