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) with good preservation of microporosity, which is attributed to the dual play of framework stability and Al inhibition. For high Si 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 smaller mesopores as compared to the ones desilicated by NaOH alone. According to the NH 3 -TPD characterization, the acidity is well preserved for the alkaline-treated samples with reference to the parent zeolite beta (Si/Al:14), with preservation of total amounts of acid sites up to 85% and similar NH 3 desorption activation energy. The sequential fluorination−desilication protocol, while effective for creating hierarchical high Al MFI-type zeolites, is not effective for zeolite beta under similar conditions due to the excessive framework activation upon fluorination that results in severe loss of microporosity and crystallinity.
Hierarchical zeolites can be synthesized by different strategies, resulting in the creation of mesoporosity with different configurations (inter-crystalline or intra-crystalline) in addition to unique porous and catalytic properties.
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