Enhancement of the olefin selectivity over ZnSAPO-34 zeolite toward methanol-to-olefins conversion and its mechanistic interpretation

The SAPO-34 zeolite serves as a crucial catalyst in the methanol-to-olefins (MTO) conversion to produce ethylene and propylene, yet suffering from the unsatisfied trade-off between long catalyst lifetime and high light olefins selectivity. Herein, we report a stable and efficient hierarchical ZnSAPO-34 zeolite prepared via hydrothermal method, which is featured with the successful introduction of Zn species into SAPO-34 framework. Impressively, the ZnSAPO-34 catalyst achieves a significantly enhanced ethylene selectivity of 60 % without obviously sacrificing catalyst lifetime. A combination study including online mass spectrometer (MS), ¹³C MAS NMR and in situ UV–visible (UV–vis) spectroscopy demonstrates that Zn species facilitate the initial generation of carbonyl species accompanied by the formation of hydrocarbon pool species, thus boosting the ethylene selectivity via the aromatic-based cycle. Meanwhile, the one-step introduction of Zn species leads to smaller crystal size and increases mesoporous in zeolite, enhancing the diffusion capacity of the ZnSAPO-34 sample, which accounts for the almost unchanged catalyst lifetime. The mechanistic interpretation provides a novel perspective to optimize zeolite catalysts by one-step introducing heteroatom, which shows potential applications in industrial MTO production.