Deuterium Isotopic Tracer Studies of Thiophene Desulfurization Pathways Using Propane or Dihydrogen as Co-reactants

Isotopically labeled reactants (D2 and C3D8) were used in order to probe H2S formation pathways during thiophene desulfurization on Co/H-ZSM5 at 773 K using H2 or propane as co-reactants. With D2/C4H4S or C3D8/C4H4S reactants, both D and H were present in the hydrogen sulfide formed, suggesting that desulfurization can occur via both direct thiophene decomposition with intramolecular hydrogen transfer and deuterium addition from D2 or C3D8. Thiophene, however, becomes readily deuterated via rapid exchange with D-containing intermediates formed from D2 or C3D8, suggesting that thiophene protonation-deprotonation steps are fast. These isotopic equilibration processes prevent a definitive assessment of the contributions of these two pathways to hydrogen sulfide formation. The D-content in hydrogen sulfide and in hydrocarbons are higher with C3D8/C4H4S than with D2/C4H4S reactants, indicating that propane is more effective than dihydrogen as a source of hydrogen for desulfurization reaction. C3H8/C4H4S and C3D8/C4H4S reactants gave similar reactions rates for hydrocarbon formation, suggesting that C–H bond activation steps do not limit hydrocarbon formation rates. A normal kinetic isotope effect was observed for thiophene desulfurization, indicating that the abstraction of fragments from propane-derived intermediates for reactions with thiophene is more difficult for deuterated intermediates. A comparison of thiophene desulfurization rates with C3H8/D2/C4H4S, C3H8/C4H4S, and D2/C4H4S mixtures confirmed that propane is a more effective source of hydrogen than H2. The deuterium content in all products formed from C3H8/D2/C4H4S mixtures was lower than in products formed from C3H8/D2 reactants. These findings suggest that thiophene inhibits the hydrogen adsorption–desorption steps responsible for the incorporation of D-atoms into adsorbed intermediates from D2 and for the desorption of dihydrogen during propane reactions.