Rapid Crystallization of Zeolites with Controllable Defects: Disentangling Fluoride Concentration and pH Using NH4F

Abstract

Zeolite synthesis is typically conducted either under basic conditions or in neutral fluoride media using hydrofluoric acid (HF). While basic (OH^–) conditions generally result in faster zeolite crystallization, they can also increase the likelihood of framework defects and crystal intergrowths. In contrast, synthesis in neutral fluoride media tends to produce fewer defects because fluoride balances positive charges from structure-directing agents. However, this method often requires significantly longer crystallization times and involves the handling of dangerous HF. In the present study, we pursue the best of both synthesis conditions, rapid syntheses with controllable defect concentrations, by disentangling of mineralizing agent and charge-balancing agent using ammonium fluoride (NH₄F) as an alternative to HF. We have investigated the use of NH₄F in the syntheses of siliceous and aluminum-containing zeolite A (LTA, small pore), ZSM-5 (MFI, medium pore), and siliceous Beta(*BEA, large pore). The crystallization times of all four zeolites decreased substantially with an increasing NH₄F concentration. Crystallization times were reduced from 24 to 4 h (Si-LTA), 96 to 36 h (Al-LTA), 240 to 6 h (ZSM-5), and 24 to 3 h (Si-*BEA). Additionally, increasing the NH₄F concentration in the synthesis mixtures decreases the defect densities of siliceous zeolites. Raman spectroscopy, along with ²⁹Si MAS NMR, ¹⁹F MAS NMR, ¹³C MAS NMR, and fluorine elemental analysis of Si-LTA samples confirms that the reduction in charged defects (Si–O^–) is due to the higher incorporation of F^– within the double four-membered ring (D4R) present in the LTA samples. We show that the accelerated crystallization is due to the role of F^– in enhancing the silica mineralization rate (formation of silicon hexafluoride species) and stabilizing D4Rs under basic conditions. Combining basic and fluoride-mediated synthesis could therefore be advantageous for faster zeolite production and improved control over structural properties for a wide variety of zeolite structures.