Unveiling the Structural Evolution and Deposition Behavior of Leached Al Species during Alkaline Treatment of ZSM-5 Zeolites

Alkaline treatment is a key strategy for constructing hierarchical zeolites, yet its dealumination mechanism and effects on acidity and pore structure remain unclear. Using ZSM-5 as a model, samples with varied dealumination degrees were prepared by adjusting treatment time. Advanced solid-state nuclear magnetic resonance (NMR) techniques including 1H–1H double-quantum single-quantum (DQ-SQ), triple-quantum single-quantum (TQ-SQ), and hyperpolarized (HP) 129Xe NMR were employed to track the structural evolution of framework Al. These results reveal that alkaline treatment progressively removes framework Al, leading to a decrease in Brønsted acid site (BAS) density. Extracted Al initially forms hydroxylated tetracoordinated Al–OH (AlIV), not directly octahedral extra-framework species. HP 129Xe NMR reveals altered micropore environments from Al removal and redeposition. Alkaline treatment prolongs the methanol to olefin reaction lifetime to 70 h via synergistic mass transport and acidity effects. This work elucidates framework Al evolution during alkaline treatment, offering insights for precise zeolite modification and catalyst design.