Substituent effects of 4,6-DMDBT on direct hydrodesulfurization routes catalyzed by Ni-Mo-S active nanocluster-A theoretical study

To study the substituent effects of 4,6-dimethyl-dibenzothiophene (4,6-DMDBT) on the direct hydrodesulfurization (DDS) routes catalyzed on a Ni-Mo-S nanocluster, a non-periodic computational Ni-Mo-S model is established, and density functional theory (DFT) is used to comparatively calculate the adsorption and the conversion of the dibenzothiophene (DBT) and 4,6-DMDBT on the Ni-S-edge, Ni-Mo-edge and corner sites of the Ni-Mo-S active nanocluster. The calculation results show that on the Ni-Mo-S active nanocluster, the Ni-S-edge could stably provide active hydrogen, whereas the C-S bond cleavage on this active site required higher activation energy. The Ni-Mo-edge could not stably provide active hydrogen, and this active site has the highest activity of C-S bond cleavage. The corner site shows favorable ability of hydrogen activation and C-S bond cleavage. The methyl group of 4,6-DMDBT weakens the Ni-S bonds caused by the adsorption on the Ni-S-edge, where the decrease of the adsorption energy is compensated by an additional dispersion provided by two methyl groups. The two methyl groups also decrease the adsorption angle between the 4,6-DMDBT and the CUS sites on the Ni-Mo-edge and corner sites. The flat adsorption of 4,6-DMDBT will further hinder the hydrogen activation and transfer on the Ni-Mo-edge. Moreover, the methyl group of 4,6-DMDBT decreases the energy difference between the reactant and the intermediate of the DDS route, increasing the reaction energy of the C-S bond cleavage. The dominant DDS route of the DBT and 4,6-DMDBT involves the interaction between active hydrogen and the a-C followed by the movement of the newly formed aromatic ring away from the S atom of the sulfur compounds. During this process, the methyl groups of the 4,6-DMDBT will hinder the C-S bond cleavage on all CUS sites on the Ni-S-edge, Ni-Mo-edge and corner sites of the Ni-Mo-S nanocluster. Therefore, the activation energy of the C-S bond cleavage of 4,6-DMDBT is higher than that of DBT, and this is the main reason for the lower DDS rate of the former compared to the latter.