Mechanism and site requirements for thiophene hydrodesulfurization on supported Re domains in metal or sulfide form

Abstract

The elementary steps and active structures involved in thiophene hydrodesulfurization (HDS) are examined here through structural and functional assessments of Re and ReSx catalysts prepared from ReOx precursors by treatment in H2 or H2S. These samples retain their respective bulk phases at sulfur chemical potentials prevalent during HDS, because nucleation barriers inhibit the interconversion of isotropic Re metal and lamellar ReSx layers. HDS turnover rates were much higher on ReSx than Re, but both phases showed similar kinetic effects of thiophene, H2, and H2S and binding constants for adsorbed thiophene and S-atoms, consistent with a common mechanism involving active sites that differ in number but not in binding properties. In such elementary steps, the surface consists of a template of refractory S-atoms that are bound irreversibly, known to form even at H2S/H2 ratios much lower than in HDS practice. Interstices within such templates can reversibly bind reactive intermediates, thus allowing catalytic turnovers, and act as HDS active sites. The number of such interstices depends on MS bond strength, which is lower for particles with ReSx than with Re bulk phases; their binding properties, however, are not dictated by the bulk phase, because they consist of those surface spaces that become capable of binding S-species weakly enough to allow their formation and removal as part of each catalytic turnover. On both Re and ReSx, thiophene conversion rates are limited by the addition of one H-atom to bound thiophene to form intermediate species that give tetrahydrothiophene (THT) and C4 hydrocarbons at a kinetic branch after this kinetically-relevant step. Thiophene pressures and H2S/H2 ratios do not influence THT/C4 product ratios, which decrease as residence time increases because of secondary CS cleavage in THT to form C4 products. Both products form in a single surface sojourn at similar site coverages by intermediates, as is also the case for secondary THT reactions. The effects of H2 on these primary and secondary events indicate that the kinetic branching occurs at a bound intermediate with the H-content of dihydrothiophene, from which the CS bond cleavage transition state is also formed. As in CC and CO cleavage, CS bond scission requires H-removal from saturated reactants (THT) by (i) increasing the bond order of its surface attachment; (ii) weakening the CX bond being cleaved (X = C, O, S); and (iii) evolving H2 to minimize entropy losses upon formation of the transition state.