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 hy drogen sources were probed by measuring the chemical and isotopic compositi on of products formed from C-13-labeled C3H8 and unlabeled C4H4S mixtures o n H-ZSM5 and Co/H-ZSM5. Aliphatic hydrocarbons formed only from propane car bon 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 u nreactive aromatics, in steps that lead to the irreversible exit of alkenes from oligomerization-cracking cycles. These steps remove kinetic bottlenec ks in thiophene desulfurization resulting from the irreversible formation o f strongly adsorbed unsaturated species, which cannot desorb without reacti ons with hydrogen or hydrogen-rich surface species. This kinetic coupling b etween alkane and thiophene reactions leads to the observed concurrent incr ease in the rates of both propane aromatization and thiophene desulfurizati on 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 b imolecular Diels-Alder reactions, which lead to larger organosulfur compoun ds 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 duri ng reactions of thiophene. (C) 2001 Academic Press.