β-scission of olefins on acidic zeolites: A periodic PBE-D study in H-ZSM-5

Eight β-scission modes involving C₆ and C₈ olefin isomers are investigated using dispersion-corrected density functional theory (i.e., PBE-D) calculations. Potential energy surfaces are evaluated within an acidic H-ZSM-5 supercell containing a single, isolated active site. Minimum energy pathways are localized using the nudged elastic band method. The relative enthalpic barriers of β-scission steps can be described by the substitution order of the carbocationic carbon atom present in the reactant and transition states. Specifically, the total charge on the hydrocarbon fragment containing the β C atom increases going from the physi- or chemisorbed reactant state to the β-scission transition state; the magnitude of this change (+0.37e-0.97e) is found to correlate nearly monotonically with the activation energy (89-233 kJ mol^-1). A comparison of 1 to 3 (E ₁) and 3 to 1 (E₂) β-scission modes as well as 2 to 3 (B₁) and 3 to 2 (B₂) β-scission modes reveals that the barrier heights depend on the substitution order of the β C, indicating that a subcategorization of β-scission modes is required based on the substitution order of the β C atom. Isomerization reactions, which are fast with respect to β-scission, enable reactant hydrocarbons to explore and find low-barrier β-scission pathways. Selectivities predicted on the basis of the relative barrier heights of β-scission modes accessible to C ₆ and C₈ species indicate agreement with experimental observations.