An anion-promoted increase of the SiO₂/Al₂O₃ ratio of zeolites†

In this work, we reported a facile strategy to improve the SiO₂/Al₂O₃ ratio of zeolites by introducing NaI as an efficient additive. The SiO₂/Al₂O₃ ratio of MOR was increased by 0.87 with the aid of NaI. The experimental results indicated that the I⁻ anion played a key role in increasing the SiO₂/Al₂O₃ ratio of MOR. Density functional theory calculations (DFT) indicated that I⁻ facilitated the formation of Si-O-Si bonds (significantly decreasing free energy barriers for the formation of Si-O-Si bonds by 35.5 kJ mol⁻¹) and hindered the formation of Si-O-Al bonds (increasing free energy barriers for the formation of Si-O-Al bonds by 18.6 kJ mol⁻¹), thus leading to an increase of the SiO₂/Al₂O₃ ratio of zeolites and accelerating the crystallization of zeolites. With this method, we could not only increase the SiO₂/Al₂O₃ ratio of MOR, but also increase the SiO₂/Al₂O₃ ratio of NaY (FAU) and ZSM-5 (MFI).