Effect of experimental parameters on the relative reactivity of dibenzothiophene and 4-methyldibenzothiophene Original Research Article

Numerous studies concerning the hydrodesulfurization of crude oil have already been realized and have shown that substituted dibenzothiophenes, and especially the 4-methyldibenzothiophene and the 4,6-dimethyldibenzothiophene are refractory molecules of the process. The reaction mechanism of their transformation is still discussed. In this work, we have chosen to carry out the transformation of dibenzothiophene and 4-methyldibenzothiophene on various catalysts and under different experimental conditions. Attention has been particularly paid to the effect of H2S on the two different reaction pathways: the DDS (direct desulfurization) and the HDS (hydrogenation followed by hydrogenolysis) routes. The relative importance between the two routes is greatly modified by the presence of H2S: the DDS route appears to be more inhibited by this compound. An original approach to hydrodesulfurization, the competitive catalytic reaction kinetics, was also used in this study: it demonstrates that the dibenzothiophene and the 4-methyldibenzothiophene have the same adsorption equilibrium constant and suggests that the molecules are adsorbed flat on the catalyst surface. The lower reactivity of the 4-methyldibenzothiophene can then be due to a steric hindrance of the methyl group during the approach of the hydrogen molecule for the hydrogenolysis reaction.