Kinetics and reaction pathways for propane dehydrogenation and aromatization on Co/H-ZSM5 and H-ZSM5

Co/H-ZSM5 catalyzes propane dehydrogenation and aromatization reactions. Initial product selectivities, product site-yields, and the C-13 content and distribution in the products of 2-C-13-propane show that propane undergoes two primary reactions-dehydrogenation to propene and H-2 and cracking to methane and ethene. Propene and ethene then form aromatics via ofigomerization-cracking reactions and both ethene and propene hydrogenate to form ethane and propane, respectively. via both hydrogen transfer from coadsorbed intermediates and dissociative adsorption of H-2. These reaction pathways resemble those occurring on H-ZSM5, but Co cations provide an alternate pathway for the removal of hydrogen atoms in adsorbed intermediates as H-2. A kinetic model was used to describe experimental rates based on these observations and to obtain rate constants for individual reaction steps as a function of Co content and H-2 concentration. H-2 inhibits propane dehydrogenation to propene and alkene conversion to aromatics, and increases the rate and the rate constant for ethene hydrogenation, Rate constants were obtained for propane reactions on H-ZSM5 and Co/H-ZSM5 (Co/Al = 0.05-0.22). Propane dehydrogenadon (k(1)), ethene hydrogenation (k(3)), and alkene dehydrocyclization (k(4)) rate constants increased with increasing COW ratio, because Co cations catalyze both the recombinative desorption of H-2 and its microscopic reverse, the dissociative adsorption of H-2. Co cations increase the reversibility of hydrogen adsorption-de-sorption steps, and in this manner, they increase the deuterium content in all products of C3H8-D-2 reactions. Propane cracking rate constants (k(2)) were not influenced by Co cations, because cracking occurs on acid sites and its stoichiometry and mechanism do not require hydrogen.