Adsorption structure and reaction path of heterocyclic sulfur compounds and the effect of methyl group on their reactivity: Ab initio study

Adsorption structures of benzothiophene and 7-methylbenzothiophene on MoS2 catalysts were studied by ab initio molecular orbital (Becke3LYP) method. The pi-bonded parallel configuration was more favored than the perpendicular adsorption through S, and the adsorption energies of both compounds were almost equal, i.e. the methyl group does not retard the adsorption. Also, the reaction pathways of hydrogenation supposed to be the first step of the desulfurization were studied in vacuo, and the insertion of H-2 to S was found to be hindered by the methyl group. From the analyses of the molecular orbitals as well as the experimental facts that the methyl group(s) drastically lower the reactivity of heterocyclic sulfur compounds, we suggest that the function of catalysts is the promotion of the hydrogenation on S, and this step is hampered by the methyl group. (C) 1997 Elsevier Science B.V.