Controlled-atmosphere 13C MAS NMR study of the initial stages of propane activation over H-ZSM-5

Abstract

13C MAS NMR was performed under controlled atmosphere to investigate the early stages of propane activation over H-MFI catalysts. Propane 2-13C was used as labelled reactant. The scrambling of the 13 C label in the very early stages of propane conversion suggests cyclic type transition states: protonated cyclopropane or C-ethanemethonium ion, pointing, thereby on carbenium or carbonium mechanistic pathways, respectively. The study of the role of, protonic and aprotonic acid sites and their strength, and the effects of catalyst pretreatment and coadsorption of various probe-molecules (H2O, CO and H2) on the initial stages of propane activation concludes to monofunctional mechanism involving propane protonation on the strong Bronsted sites of H-MFI and formation of carbonium ion type transition states. Adding of small amounts of propylene or benzene leads to classical bimolecular initiation via hydride transfer and carbenium ion type intermediates. The mechanism of the main reaction pathways leading to 13C scrambling in propane, cracking to ethane and methane and formation of butanes is discussed.