Spatial confinement in zeolites dominates pentene monomolecular cracking pathway

Precise regulation of the competition between monomolecular and bimolecular reaction pathways in hydrocarbon conversion remains challenging. Pentene monomolecular cracking to ethylene and propylene (C₅⁼ → C₂⁼ + C₃⁼) is an ideal atom-economical reaction for the co-production of C₂⁼ and C₃⁼. Focusing on C₅⁼ cracking with competing monomolecular and bimolecular pathways, this work systematically investigated the regulation mechanisms of acidity of FER and MFI zeolites and reaction conditions on reaction pathways. Results show that C₅⁼ cracking over FER is consistently dominated by monomolecular pathway, with product distribution insensitive to acidity and process variations. In contrast, MFI exhibits competing pathways highly susceptible to acidity and reaction conditions. Combined with zeolite reaction space analysis, reaction performance, and molecular simulation, the distinction arises from reaction spatial confinement: the 10-MR channels of both FER and MFI matches the structure of C₅⁺ species, facilitating monomolecular cracking. However, the channel intersection cavities of MFI enable bimolecular reaction intermediates (C₁₀⁺) formation. Results of C₅⁼ cracking over zeolites with different reaction spaces further verify this viewpoint. Thus, the chemical basis for the dominance of the monomolecular pathway in C₅⁼ cracking is spatial confinement, and optimal zeolites feature 10-MR channels without reaction space exceeding 0.6 nm in Di. For zeolites with competing pathways, the monomolecular pathway can be enhanced by tuning acidity and process conditions. Accordingly, efficient C₅⁼ monomolecular cracking catalysts were developed: ZSM-22 zeolite with one-dimensional 10-MR channel and phosphorus-modified ZSM-5 (MFI) zeolite. These findings offer scientific support for designing efficient hydrocarbon conversion processes toward C₂⁼ and C₃⁼.