Dynamic Features of Transition States for β-Scission Reactions of Alkenes over Acid Zeolites Revealed by AIMD Simulations

Zeolite-catalyzed alkene cracking is key to optimize the size of hydrocarbons. The nature and stability of intermediates and transition states (TS) are, however, still debated. We combine transition path sampling and blue moon ensemble density functional theory simulations to unravel the behavior of C(7)alkenes in CHA zeolite. Free energy profiles are determined, linking pi-complexes, alkoxides and carbenium ions, for B-1 (secondary to tertiary) and B-2(tertiary to secondary) beta-scissions. B-1 is found to be easier than B-2. The TS for B-1 occurs at the breaking of the C-C bond, while for B-2 it is the proton transfer from propenium to the zeolite. We highlight the dynamic behaviors of the various intermediates along both pathways, which reduce activation energies with respect to those previously evaluated by static approaches. We finally revisit the ranking of isomerization and cracking rate constants, which are crucial for future kinetic studies.