Kinetics and Reaction Pathways for Propane Dehydrogenation and Aromatization on Co/H-ZSM5 and H-ZSM5

Abstract

Co/H-ZSM5 catalyzes propane dehydrogenation and aromatization reactions. Initial product selectivities, product site-yields, and the 13 C content and distribution in the products of 2-13 C-propane show that propane undergoes two primary reactionssdehydrogenation to propene and H 2 and cracking to methane and ethene. Propene and ethene then form aromatics via oligomerization-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 dehydrogenation (k 1 ), ethene hydrogenation (k 3 ), and alkene dehydrocyclization (k 4 ) rate constants increased with increasing Co/Al 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-desorption steps, and in this manner, they increase the deuterium content in all products of C 3 H 8 -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.

Variational quantum approaches for computing vibrational energies of polyatomic molecules

“…[24] used methods most closely related to those described in this section. Yu has studied several molecules using similar ideas [159,160]. He contracts only the bend part and not the stretch part, but the greatest benefit is derived from the bend contraction.…”

“…It was first tested on HOOH where its efficiency was established [157]. Yu has contracted the bend but not the stretch basis to compute energy levels for several molecules [159,160,194]. Lee and Light contracted both bend and stretch bases [161,162].…”

Mathematical Methods in Quantum Molecular Dynamics

“…One way to deal with the basis-size problem is to use basis functions that are not products of functions of a single variable [27,28,29,30,23,24,20,31,32,33,21]. Another strategy is to use product functions but only a fraction of the full product basis [34,35,36,37,38,39,40,41,42]. Often many of the functions in a product basis have large zeroth order energies.…”

Calculating vibrational spectra with sum of product basis functions without storing full-dimensional vectors or matrices

“…First, prune a standard product basis by retaining only some of the functions. [19][20][21][22][23][24][25][26][27] Second, use contracted basis functions obtained by solving reduced-dimension eigenproblems. [28][29][30][31][32][33][34][35] Third, optimize 1D functions with the multi-configuration time dependent Hartree method (MCTDH).…”