Hydrogenolysis and β-elimination mechanisms for C-S bond scission of dibenzothiophene on CoMoS edge sites

Unraveling the mechanisms of hydrodesulfurization (HDS) of dibenzothiophene (DBT) and the corresponding active sites represents a scientific challenge to improve the intrinsic performances of Co-promoted MoS2 (CoMoS) catalysts. By using density functional theory calculations, we compare two historical mechanisms for the CAS bond scission of DBT (direct desulfurization): direct hydrogenolysis of DBT and beta-elimination of alpha,beta-dihydro-diobenzothiophene (alpha,beta-DHDBT) on four relevant sites of the two CoMoS M- and S-edges. On the Co promoted M-edge, the alpha,beta-DHDBT is formed through dihydrogenation which is kinetically competing with hydrogenolysis (both exhibiting activation free energies, Delta G double dagger, smaller than +1.24 eV). On the S-edge, both dihydrogenation and hydrogenolysis exhibit higher Delta G double dagger (>+1.78 eV). Interestingly, on the S-edge, the beta-elimination (E2 type) on the alpha,beta-DHDBT is found to be kinetically competing (Delta G double dagger = +1.14 eV). The elimination of H-beta atom involves a S-2 dimer close to the S-vacancy site where DHDBT is adsorbed. Since this leaving H-beta atom is distinct from the one added at dihydrogenation step, this may explain why direct desulfurization of 4,6-alkyl substituted DBT compounds is hampered according to the elimination mechanism. We finally discuss the possible synergy between the two edges of CoMoS for HDS of DBT. (C) 2021 Elsevier Inc. All rights reserved.

Automated summary

This study compares two historical mechanisms for the CAS bond scission of DBT and finds that dihydrogenation on the Co promoted M-edge is kinetically competing with hydrogenolysis, while beta-elimination on the S-edge exhibits higher activation free energies.