MOLECULAR MECHANICS STUDY OF THE INTERACTION OF THIOPHENE WITH A MOLYBDENUM-DISULFIDE CATALYST

Molecular mechanics calculations have been performed to investigate the structure of the active site of a molybdenum disulfide hydrodesulfurisation catalyst and the binding of thiophene. Active sites are created by removing bridging sulfur atoms, thus exposing pairs of molybdenum atoms. Relaxing the MoS2 hexagonal slabs using molecular mechanics shows that the environment of molybdenum and sulfur atoms at the active site is different from that in the bulk. Structures of the thiophene-molybdenum disulfide active site complex have been investigated. Stable structures can be produced with the sulfur atom ot thiophene attached to one or both of the coordinatively unsaturated (cus) molybdenum atoms. Our calculations show the stable configuration ot thiophene at these sites. Thiophene binds through sulfur to one cus molybdenum atom with the ring either parallel to the surface (more stable) or perpendicular (slightly less stable). When the ring is perpendicular the thiophene sulfur atom can bind to the second adjacent cus molybdenum thus forming a Mo-S(thiophene)-Mo bridge. When the thiophene is parallel to the surface, bound to o-e molybdenum atom through sulfur, then carbon atoms of the thiophene can bind to the adjacent cus molybdenum atom. The most likely scenario for the formation of the active-site complex is for the thiophene molecule to bind perpendicular to the surface, first to one molybdenum atom and then to a second, to form an Mo-S-Mo bridge.