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

Eight beta-scission modes involving C-6 and C-8 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 beta-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 beta C atom increases going from the physi- or chemisorbed reactant state to the beta-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 degrees to 3 degrees (E-1) and 3 degrees to 1 degrees (E-2) beta-scission modes as well as 2 degrees to 3 degrees (B-1) and 3 degrees to 2 degrees (B-2) beta-scission modes reveals that the barrier heights depend on the substitution order of the beta C, indicating that a subcategorization of beta-scission modes is required based on the substitution order of the beta C atom. Isomerization reactions, which are fast with respect to beta-scission, enable reactant hydrocarbons to explore and find low-barrier beta-scission pathways. Selectivities predicted on the basis of the relative barrier heights of beta-scission modes accessible to C-6 and C-8 species indicate agreement with experimental observations.