Pore-Confined and Diffusion-Dependent Olefin Catalytic Cracking for the Production of Propylene over SAPO Zeolites

Higher-olefin cracking into propylene is an ideal process to meet the increasing demand for propylene driven by polymer industries. However, this process is usually stuck in poor propylene selectivity owing to the complicated reaction routes for facile side reactions and evident catalyst deactivation from severe coking. In this work, various SAPO zeolites with moderate acidity were synthesized for the 1-hexene cracking reaction. Among them, SAPO-41 exhibited an excellent propylene selectivity of ∼90% at a super high 1-hexene conversion of ∼95% and superior stability. It is ascribed to the dominant monomolecular cracking mechanism derived from the pore-confined effect with elliptical channels (10-membered ring, 4.3 × 7.0 Å) and shorter diffusional distance with nanosheet-like morphology, which could effectively suppress the side reactions such as hydrogen transfer and coking. In contrast, fast deactivation and obviously lower propylene selectivity were found over SAPO-5 and SAPO-41/5 with larger circular channels (12-membered ring, 7.3 × 7.3 Å), resulting from much longer diffusional distance and enhanced bimolecular cracking route to give more undesired light alkanes, butenes, aromatics, and cokes. Especially, a systematic experimental investigation combined with molecular dynamics simulation demonstrates that medium chain length alkenes (C6–C8) are more suitable for a stable cracking process along with a high conversion level, owing to the synergistic effects between moderate diffusion ability and higher cracking activity.