Reaction mechanisms of the two distinct routes, namely hydrogenation (HYD) and direct desulfurization (DDS), of dibenzothiophene (DBT)-type compounds in hydrodesulfurization (HDS) process have been investigated by means of density functional theory (DFT). A series of methyl- and dimethyl-compounds substituted at different positions in DBT were compared to get a deeper insight into the electronic effects of the alkyl position substitution. The Gibbs energy of reaction for each route and the CS bond energies were calculated. The calculations showed that HYD route is more exergonic than DDS route. Refractory compounds exhibited less reaction energy values and high CS bond stability. Full chemical structures in the energetic profile of HYD and DDS routes were identified. We proposed a reaction mechanism for each HDS route based on our theoretical calculations. According to our proposed mechanisms, the HYD reaction route is favored compared to DDS reaction route because energetic barriers are higher on the DDS route. The effect of methyl groups is clearly observed. Refractory compounds have higher activation and lower formation energies. This systematic study trough DFT method provides an important contribution in the field of HDS because the proposed mechanisms are related with some experimental observations. © 2012 Wiley Periodicals, Inc.
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