Optimization of Hierarchical Structures for Beta Zeolites by Post-Synthetic Base Leaching
2016
Zhang, Ke | Fernández, Sergio | Kobaslija, Sarah | Pilyugina, Tatiana | O’Brien, Jeremy | Lawrence, John A. | Ostraat, Michele L.
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 NH₃-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 NH₃ 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.
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