Coupling alkane dehydrogenation reactions with hydrogenation reactions on cation-modified H-ZSM5.

摘要

Hydrogen removal steps limit propane dehydrogenation rates on cation-modified H-ZSM5 and cause the surface to act for all kinetic purposes as if it were in equilibrium with H₂ pressures greater than in the prevalent gas phase. The hydrogen content of adsorbed reactive intermediates is controlled by the relative rates of C-H activation of propane and of H-H activation of H₂ and the hydrogen provided from both sources is kinetically indistinguishable. The surface hydrogen chemical potential, rigorously described as a measurable virtual pressure, influences reaction rates, product selectivities, and deactivation rates during propane reactions.; We exploited the presence of these high hydrogen chemical potentials by coupling propane dehydrogenation with thiophene desulfurization, a stoichiometric reaction requiring hydrogen. Thiophene desulfurization was achieved using hydrogen and alkenes formed from propane co-reactants on cation-exchanged H-ZSM5 to form H₂S without using gas phase H₂. 2-13C-C₃H₈/C₄H ₄S reactant mixtures were used to probe the reaction pathways. Thiophene scavenges hydrogen and alkenes formed from propane, increasing exit rates from oligomerization-cracking cycles and aromatics formation rates. The kinetic coupling between these reactions leads to the concurrent increase in propane aromatization and thiophene desulfurization rates, relative to those obtained with each pure reactant. C₃D₈/C₄H₄S reactant mixtures were used to probe H₂S formation routes. Both D and H were present in the hydrogen sulfide formed, suggesting that desulfurization occurs via both direct thiophene decomposition involving intramolecular hydrogen transfer and via deuterium addition from C₃D₈.; In situ infrared studies showed that acidic OH groups and Co cations were largely free of propane- or thiophene-derived intermediates during thiophene-propane reactions on H-ZSM5 and Co/H-ZSM5. In addition, Co K-edge X-ray absorption studies showed that the local structure of Co2+ cations did not change and that the cations were not reduced or sulfided during reaction.; Lastly, propane aromatization reaction pathways on Co/H-ZSM5 were shown to be similar to those on H-ZSM5, with Co cations providing a H₂ removal pathway. H₂ inhibits propane dehydrogenation and dehydrocyclization steps and increases ethene hydrogenation rates. Co cations increase propane dehydrogenation, ethene hydrogenation, and alkene dehydrocyclization rates, by catalyzing both recombinative desorption of hydrogen atoms and dissociative adsorption of H₂.