Mechanistic Insights into the Pore Confinement Effect on Bimolecular and Monomolecular Cracking Mechanisms of N-Octane over HY and HZSM-5 Zeolites: A DFT Study

Bimolecular and monomolecular cracking mechanisms of alkanes simultaneously occur and have a competitive relationship, which strongly influences the product distribution. In this work, the density functional theory (DFT) calculation is first carried out to elucidate two cracking mechanisms in HZSM-5 and HY zeolites. It is found that the overall apparent reaction barrier for the monomolecular cracking reaction at 750 K in the HZSM-5 zeolite is 5.30 kcal/mol, much lower than that (23.12 kcal/mol) for bimolecular cracking reaction, indicating that the monomolecular mechanism is predominant in the HZSM-5 zeolite. In contrast, the bimolecular mechanism is predominant in the HY zeolite because of a lower apparent reaction barrier energy barrier (6.95 kcal/mol) for bimolecular cracking reaction than that (24.34 kcal/mol) for the monomolecular cracking reaction. Moreover, the intrinsic reason for the different mechanisms is further elucidated. The confinement effect can effectively decrease the energy barrier whe...