Deuterium isotopic tracer studies of thiophene desulfurization patheways using propane of dihydrogen as co-reactants

摘要

Isotopically labeled rectants (D_2 and C_3D_8)were used in order to probe H_2S formation pathways during thiophene desulfurization on Co/H-ZSM5 at 773K using H_2 or propane as co-reactants.With D_2/C_4H_4S or C_3D_8/C_4H_4S reactants,both D and H were present inthe hydrogen sulfide formed,suggesting that desulfurization can occur via both direct thiophene decomposition with intramolecular hydrogen transfer and deuterium additionfrom D_2 or C_3D_8.Thiophene,howeer,becomes readily deuterated via rapid exchange with D-containing intermediates formed from D_2 or C_3D_8.Thiophene,However,becomes readily deuterated via rapid exchange with D-containing intermediates formed from D_2 or C_3D_8,suggesting that thiophene protonation-deprotonation steps are fast.These isotopic equilibration processes prevent a defintive assessment of the contributions of these two pathways to hydrogen sulfide formation.The D-content in hydrogen sulfide and in hydrocarbons are higher with C_3D_8/C_4H_4S than with D_2/C_4H_4S reactants,indicating that propane is more effective than dihydrogen as a source of hydrogen for desulfurizatioin rection.C_3H_8/C_4H_4S and C_3H_8/C_4H_4S 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 C_3H_8/D_2/C_4H_4S,C_3H_8/C_4H_4S,and D_2/C_4H_4S mixtures confirmed that propane is a more effective source of hydrogen than H_2.The deuterium content in all products formed from C_3H_8/D_2/C_4H_4S mixtures was lower than in products formed from C_3H_8/D_2 reactants.These findings suggest that thiphene inhibits the hydrogen adsorption-desorption steps responsible for the incorporation of D-atoms into adsorbed intermediates from D_2 and for the desorption of dihydrogen during propane reactions.