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

Abstract

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.