Radical Cation Intermediates in Propane Dehydrogenation and Propene Hydrogenation over H-[Fe] Zeolites

Abstract

This report investigates the mechanistic relationship between the monomolecular propane dehydrogenation reaction and the reverse reaction, the propene hydrogenation, over H-[Fe]ZSM-5 catalysts. It is shown that the difference in the apparent activation energies of the forward and reverse reactions is equal to the reaction enthalpy (∼130 kJ mol–1) and that the rate constants of the reactions have an isokinetic relationship. The ratios of the rate constants of the forward to the reverse reactions are equal to the equilibrium constant (e.g., KP ≈ 0.033 bar at 773 K) even if the reactions occur separately, away from equilibrium. The results are consistent with the principle of microscopic reversibility and indicate that both forward and reverse reactions are structurally related and proceed through the same elementary steps and reaction intermediates. The pattern of selectivity, the activation energy, and the estimated enthalpy and entropy of formation of the transition states in H-[Fe]ZSM-5 are very different from the observed values for the isostructural H-[Al]ZSM-5, indicating that despite their structural similarities the reactions proceed through different mechanisms in each catalyst. Analysis of the energy change along the reaction coordinate, including the reaction enthalpy and the apparent activation energies, suggests that in H-[Fe]ZSM-5 the reaction proceeds through radical cation-like intermediates. Analysis of a putative reaction mechanism and the energetics of electron transfer in the zeolite channels shows that dehydrogenation of propane is kinetically favored (as observed) over cracking of propane because ethene radical cations are less stable than propene radical cations.