One-Pot Sequential Alkaline-Acid Route to ZSM-5 Nanosheets with Low Framework Defects and Suppressed Aluminum Zoning

Strongly alkaline synthesis of zeolites is efficient, yet often induces silanol defects and Al zoning with elevated external acidity, while acid- or near-neutral routes commonly depend on fluoride mineralization or preformed seeds to sustain crystallization. Here, we report a fluoride-free, seed-free, one-pot sequential alkaline-acid strategy that affords high-yield ZSM-5 nanosheets under near-neutral acidic conditions. In this route, the initial alkaline stage promotes the formation of abundant precursor nuclei, while the subsequent near-neutral stage enables near-complete crystallization, thereby overcoming sluggish spontaneous nucleation in nonalkaline media. The obtained nanosheets possess reduced silanol-related defects, enhanced framework Al incorporation, and weakened outer-region Al enrichment, accompanied by lower external/near-surface Brønsted acidity and a low fraction of proximate Al sites. Systematic variation of acidification pH and timing shows that lower synthesis alkalinity promotes sheet-like morphology, reduces silanol-related defects, and decreases external/near-surface Brønsted acidity, whereas delayed acidification after the growth inflection point leads to stronger Al expulsion and more pronounced outer-region Al enrichment. Crystallization tracking, together with NaOH etching, reveals preferential c-axis elongation and a weakened Al expulsion process under near-neutral acidic conditions, accounting for reduced outer-region Al enrichment and lower Al-pair fraction in the final nanosheets. Catalytic probing using 1,3,5-triisopropylbenzene cracking confirms reduced external/near-surface acidity, while lactic acid-to-lactide conversion tests show improved shape-selective performance with reduced coke formation. This work provides a practical strategy for simultaneously regulating silanol-related defects, framework Al distribution, and acid-site accessibility in ZSM-5 nanosheets.