Significant Effects of EDTA^4–/EDTA^3– in ZSM-5 Zeolites: From Crystallization Mechanism to Product Morphology and Catalytic Performance

The application of EDTA exerted a governing effect on the synthesis of ZSM-5 zeolites. Analytical results revealed that EDTA presented as EDTA^4- and EDTA^3- in alkaline systems. Herein, EDTA^4- served as the mineralized agent for dissolving amorphous gel into soluble Si-Al species, while EDTA^3- acted as an electrostatic cross-linker, assembling with <i>n</i>-butylamine⁺ and soluble Si-Al species into "Inorganic-Organic Hybrid Sphere." Significantly, the objective of controlling the crystallization process and product morphology can be achieved by adjusting the EDTA^4-/EDTA^3- ratio. The appearance of sole EDTA^4- (pH ≥ 13) altered the crystallization mechanism from the solid hydrogel one into a solution-mediated one, shifting the zeolite morphology from aggregates into large crystals. Under the cooperativity of EDTA^4-/EDTA^3- (pH 8.5-10.5), the synthesis gel assembled into an "Inorganic-Organic Hybrid Sphere" and then crystallized into a hollow ZSM-5 capsule. Next, when tested in the methanol-to-aromatics (MTA) reaction, the enhanced stability (186h) and improved aromatics selectivity (40.2%) were directly attributed to the hollow structure, which not only elevated the coke-admitting capacity and suppressed the coke deposition rate but also increased the contact time of the intermediate products with the ZSM-5 shell. These findings established structure-property relationships governing zeolite crystallization mechanisms, product morphologies, and catalytic performances, as well as simplified the preparation procedure of hollow capsules.