Isotopic Tracer Studies of Thiophene Desulfurization Reactions Using Hydrogen from Alkanes on H-ZSM5 and Co/H-ZSM5

Reaction pathways for the desulfurization of thiophene using alkanes as hydrogen sources were probed by measuring the chemical and isotopic composition of products formed from 13C-labeled C3H8 and unlabeled C4H4S mixtures on H-ZSM5 and Co/H-ZSM5. Aliphatic hydrocarbons formed only from propane carbon atoms while aromatic molecules contained carbon atoms from both propane and thiophene. Hydrogen-deficient thiophene and thiophene-derived species react with surface hydrogen and alkenes formed from propane and desorb as unreactive aromatics, in steps that lead to the irreversible exit of alkenes from oligomerization-cracking cycles. These steps remove kinetic bottlenecks in thiophene desulfurization resulting from the irreversible formation of strongly adsorbed unsaturated species, which cannot desorb without reactions with hydrogen or hydrogen-rich surface species. This kinetic coupling between alkane and thiophene reactions leads to the observed concurrent increase in the rates of both propane aromatization and thiophene desulfurization compared with those achieved with each pure reactant. The scavenging of unsaturated intermediates formed via thiophene decomposition using hydrogen or hydrogen-rich intermediates formed from propane decreases the rate of bimolecular Diels–Alder reactions, which lead to larger organosulfur compounds and to low H2S selectivity. As a result, H2S selectivities are higher and deactivation rates are lower when propane is present as a co-reactant during reactions of thiophene.