Our recent mechanistic studies of thiophene hydrodesulfurization (HDS) on various transition metal and metal sulfide catalysts have suggested the importance of vacancy formation as a determinant of reactivity. Structural characterization via transmission electron microscopy (TEM), S-content, and X-ray absorption spectroscopy (XAS) has indicated that silica-supported Pt, Ru, and Re clusters retain their metallic bulk during reaction while exhibiting a surface nearly saturated with a S-monolayer, which sufficiently weakens the S-metal binding energy at these coverages to form S-vacancies required for HDS. In contrast to behaviors exhibited in the Pt and Ru systems, layered ReSx structures do not appear to undergo phase transformation to its metal analog upon exposure to identical conditions, allowing for rigorous comparisons of these distinct phases, which showed that stable turnover rates on ReSx were nearly two orders of magnitude greater than those on Re metal. HDS turnover rates increase in the order from Re, ReSx, Ru, to Pt and range over four orders of magnitude in value under similar hydrogen (1-3 MPa), thiophene (1-10 kPa), and H2S (0.2-15 kPa) pressures and temperatures (573-623 K).Tetrahydrothiophene (THT), butene, and butane were the major products observed on all catalysts studied. Kinetic evidences strongly suggest the formation of a common thiophene-derived surface intermediate that either hydrogenates further to form THT or cleaves the C-S bond to form the C4 products. This intermediate exhibits a degree of unsaturation necessary to activate the C-S bond, analogous to mechanistic features observed for C-C and C-O activation in the hydrogenolysis of alkanes and oxygenates respectively. Such features can describe the kinetic behaviors observed, independent of catalyst identity or phase, indicating that large differences in the overall reaction rates between these materials reflect their intrinsic properties, in particular their S-metal binding energies, which consequently determine the density of required vacancies during catalytic reaction.

Hydrodesulfurization on transition metal and metal sulfide surfaces: Elementary steps and site requirements

248th National Meeting (2014)

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