Synthesis of self-pillared pentasil zeolites without organic templates and seeds†

Abstract

Self-pillared pentasil (SPP) zeolites have received considerable interest due to their distinctive intergrowth structure, while the precise process and mechanism for the formation of SPP zeolites remain obscure. Herein, SPP zeolites (ZSM-5) have been successfully synthesized by pre-aging an Al-rich gel without employing any organic templates or seeds for the first time. The as-synthesized SPP zeolites possess a notably high external surface area while the micropores for Ar adsorption are partially blocked by excess Na⁺, which can be fully recovered by Mg²⁺ or H⁺ exchange. The crystallization process is monitored and the impacts of synthesis parameters are investigated. The results show that self-pillaring originates from the partial lattice distortion at the intersections of nanosheets, offering a new insight into the self-pillaring process. Typically, with decreasing SiO₂/Al₂O₃ ratio, more crossovers could be observed in the crystals, hinting that self-pillaring predominately occurs at the (101) plane of twins in the ZSM-5 precursor due to Al-rich lattice distortion. Finally, in the catalytic cracking of n-heptane, H-SPP zeolites exhibit superior performance to commercial H-ZSM-5 zeolites due to their abundant Brønsted acid sites arising from a low framework SiO₂/Al₂O₃ ratio of ∼21 and the short diffusion path originating from the house-of-cards structure.