Suzanne Harris

Thiophene hydrodesulfurization on MoS2; Theoretical aspects

Hydrodesulfurization (HDS), the removal of sulfur in the form of H2S from petroleum, is a crucial step in the industrial refinement process. Using the extended Huckel tight binding method, we have examined the nature of the active site and the mechanism of desulfurization in the case of thiophene on MoS2. The η5-bound sites, in which the thiophene ring lies parallel to the surface, are particularly advantageous to weakening the SC bond. η1-bound sites are less active. The removal of sorrounding surface sulfurs increases the HDS potential of the η5 geometry, but is ineffective at promoting the poorer adsorption modes. Surface reconstruction has also been examined; the possibility of MoMo pairing is suggested. The effect of poisons and promoters is considered. The role they play in HDS catalysis may be determined by the position they occupy in the MoS2 lattice. Metals which replace a surface molybdenum tend to poison HDS, whereas those which “pseudo-intercalate” between the SS layers can serve to promote the reaction.

Understanding the binding and activation of thiophenic molecules in transition metal complexes and clusters

Abstract Organometallic complexes and clusters provide examples of binding modes of thiophenic molecules, activation of the CS bond in thiophene, and actual desulfurization of thiophenic molecules. Our molecular orbital calculations on these complexes are aimed toward understanding how the binding and reactivity of the thiophenic ligands are influenced by the electronic structures of the various complexes. Results of Fenske-Hall molecular orbital calculations for complexes incorporating both η5 and η1 thiophene ligands show that although the thiophene ligand acts primarily as a donor, thiophene can and does act as a π acceptor under certain conditions. These results suggest how binding of a thiophenic ligand and/or activation of the CS bond might be optimized in η5 or η1 complexes. Results for metal-inserted or ring opened complexes show distinct differences in the electronic structure of saturated and unsaturated complexes. The orbital structures and charge distributions provide explanations for the different reactivities of a number of these complexes. Comparisons of the electronic structures and reactivities of the metal-inserted complexes and the butterfly cluster (Cp′)2Mo2 Co2S3(CO)4, which is known to remove sulfur from thiophene, suggest similarities in the mechanism of binding and activation of a thiophene ring in the complexes and the butterfly cluster.