Optimization of Hierarchical Structures for Beta Zeolites by Post-Synthetic Base Leaching

Abstract

Hierarchical structures of zeolite beta with Si/Al ratios in the range of 14–250 are optimized by post-synthetic base leaching, including direct alkaline treatments in NaOH solutions, fluorination–desilication sequences, and alkaline treatments in the presence of tetrapropylammonium cations (TPA+) as the pore-directing agent (PDA). Mesoporosity can be conveniently obtained by a single step of base leaching in NaOH (e.g., 0.2 M NaOH at 65 °C) for high Al beta zeolites (Si/Al 20), the alkaline treatments in the presence of PDA (e.g., TPA+) are crucial for creating mesopores without sacrificing the original zeolite framework. In the absence of TPA+, the high Si zeolite framework is very sensitive to NaOH leaching, resulting in severe structural amorphization. High Al zeolite beta can also be leached by alkaline treatments with TPA+, which features sm...