An efficient approach for template free synthesis of Silicalite-1 zeolite: Effect of seed nature and NH4F

Silicalite-1 zeolite is considered as an ideal inorganic material for applications in adsorption, separation and catalysis. Herein, this study provides a new strategy for the hydrothermal synthesis with fluoride (F⁻/Si = 0.25) and seeds (5.6 wt%) in an organic-free template system based on the unique dual-functional properties of NH₄F (mineralizer and mixed-phase inhibitor). The interaction between seeds and fluoride is studied, and the factors controlling the zeolite growth are identified. The results show that NH₄F can bind to the five-membered ring of the seeds, which can effectively inhibit the generation of SiO₂ and promote the formation of highly crystalline Silicalite-1 zeolites. The synergistic effect of the two compounds provides the possibility of precisely regulating the morphology and particle size of zeolite. When the content of the seeds is higher than 5.6 wt%, the crystal morphology depends on the seeds, while when the F⁻/Si ratio is equal to or higher than 0.25, the crystal morphology is determined by NH₄F. Compared to traditional template-free methods, this study provides new insights into the low-cost, green and efficient preparation of Silicalite-1 zeolites, while also laying a theoretical foundation for the controlled synthesis of MFI-type zeolites.