Hydrodesulfurization of 4-methyl-dibenzothiophene: a detailed mechanistic study

A mechanism for the transformation of alkyl-dibenzothiophenes by hydrodesul furization (HDS) on a NiMo catalyst is proposed. 4-Methyl-dibenzothiophene (4-MDBT) (1) has been chosen as a model for this study because this non-sym metrical molecule leads to the formation of a great variety of intermediate s. Thanks to the synthesis of these compounds, a detailed study of the reac tion course by the observation of the relative concentrations of intermedia te products was realized. The results are consistent with adsorption of the molecules via the aromatic system, parallel to the catalyst surface, as pr oposed previously. The formation of a dihydrogenated intermediate is follow ed by successive hydrogenation or elimination steps. The rates of these rea ctions are dependent on the location of the methyl substituent and hence on the generated sterical hindrance. The relative amount observed in each com pound is in total accordance with this mechanistic proposal. (C) 1999 Elsev ier Science B.V. All rights reserved.