Mechanistic insights into aluminosilicate zeolite synthesis mediated by multi-functional ionic liquids

Direct synthesis of aluminosilicate zeolites with good acidity via fluoride route remains challenging, since such systems generally favor high-silica frameworks and often restrict the incorporation of framework Al. To address this issue, a sustainable strategy for synthesizing aluminosilicate zeolites using hexafluorophosphate ([PF 6 ] – )-based ionic liquid as both structure-directing agent (SDA) and fluorine source (the mineralizing agent) was proposed. This approach enables the fabrication of MFI-, TON-, or ANA-type zeolites under solvent-free and vapor-assisted conditions. Mechanistic investigations using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), solid-state nuclear magnetic resonance (NMR), and two-dimensional (2D) correlation techniques reveal that [PF 6 ] – anions undergo in situ decomposition to release F – in a slow and controlled manner during crystallization. This controlled fluoride-release behavior regulates fluoride availability in the synthesis medium, which facilitated the incorporation of both silicon and aluminum into the zeolite framework, affording ZSM-5 zeolites with lower Si/Al ratios than those obtained using conventional fluoride sources as the mineralizing agent. Simultaneously, phosphorus species generated from [PF 6 ] – were uniformly dispersed within the ZSM-5 zeolite. These in situ -formed phosphorus species could interact with framework aluminum to create stabilizing P-O-Al linkages upon calcination, thereby reducing dealumination and enhancing zeolite acidity when the P content was appropriate. ZSM-5 with retained phosphorus species exhibited superior catalytic performance in n -hexane cracking, achieving high conversion and light olefins yield. This strategy based on multi-functional ionic liquid offers an alternative route to tailor zeolite framework and acid sites for catalytic applications.